Sintered ore production method

Abstract

Provided is a method for producing sintered ore using a Dwight-Lloyd type sintering machine, which includes an ignition furnace for initial ignition and a reignition furnace for reignition arranged at a predetermined distance downstream of the ignition furnace and which advances sintering by downward suction, the method including: using, as bonding agents for a blended material, a low combustible carbonaceous material with a combustion start temperature exceeding 550 degrees C. and a highly combustible carbonaceous material with a combustion start temperature of 550 degrees C. or less, and a ratio of the highly combustible carbonaceous material with a particle size of 2.8 mm or more being 30 mass % or more and 80 mass % or less.

Description

TECHNICAL FIELD

[0001] The present invention relates to a method for producing sintered ore used as a material in a blast furnace.BACKGROUND ART

[0002] The main material for manufacture of pig iron in a blast furnace is sintered ore. In recent years, a reignition sintering method (Patent Literature 1 below) has been proposed as a method for producing sintered ore, in which two-stage (two times) ignition is performed for the purpose of improving the sintering yield. The reignition sintering method is a technique in which, after a first ignition is completed, the atmosphere is sucked in an atmosphere suction area for a predetermined period of time, and then a second ignition (reignition) is performed. In addition, as improved reignition sintering techniques, there are proposed a method in which segregation of carbonaceous materials (bonding agents) is strengthened in a layer thickness direction of a sinter material packed bed (Patent Literature 2 below), and a method in which char (highly combustible carbonaceous material described later), which is obtained by carbonizing raw coal having a Roga index of 10 or less, is used as the bonding agent (Patent Literature 3 below).

[0003] Regarding single-stage ignition sintering that involves a normal single-stage (one-time) ignition, various techniques using highly combustible carbonaceous materials (carbonaceous materials of which combustion start temperature is lower than that of low combustible carbonaceous materials such as coke and anthracite) have already been known. For example, a technique related to an appropriate blending ratio of the highly combustible carbonaceous material and the low combustible carbonaceous material (Patent Literature 4 below), a technique related to a preferred particle size of the highly combustible carbonaceous material (Patent Literature 5 below), and a method of addition in a granulation step (Patent Literatures 4 and 6 below) have been proposed.

[0004] Patent Literature 1 discloses a method (reignition sintering method) for producing sintered ore using a Dwight-Lloyd (DL) type sintering machine, the machine including: multiple pallets installed successively in a traveling direction from upstream to downstream, into which a sintering material is to be charged; an igniter that ignites, from above, a sinter material packed bed in the pallet located upstream in the traveling direction among the multiple pallets; a wind box that sucks in the atmosphere from under the multiple pallets; a flame heating device that is disposed downstream and spaced from the igniter and heats an entire width of an upper surface of the sinter material packed bed with a flame; and an atmosphere suction area formed between the igniter and the flame heating device where the atmosphere is sucked by downward suction and direct heating is not performed from the upper surface. Patent Literature 1 describes that it is possible to improve the yield and cold strength of sintered ore while maintaining reducibility.

[0005] Patent Literature 2 discloses a method for producing sintered ore using a Dwight-Lloyd type sintering machine in which a sinter material packed bed having a lower layer and a surface layer is formed in each pallet that circulates and moves in a traveling direction from upstream to downstream, and a concentration of a solid combustible in the lower layer is lower than that of the solid combustible in the surface layer, the method including: performing ignition, from above, on the sinter material packed bed in the pallet located upstream in the traveling direction; and after moving an ignited part of the sinter material packed bed downstream in the traveling direction, performing reignition on the reignited part. Patent Literature 2 describes that it is possible to increase the firing speed and improve the yield, thereby further improving productivity by increasing the concentration of the solid combustible in the surface layer while reducing the amount of unburned solid combustible and enabling it to be effectively utilized as heat.

[0006] Patent Literature 3 discloses a technique in which a Dwight-Lloyd (DL) type sintering machine including an igniter and a flame heating device that is disposed downstream and spaced from the igniter and that heats an upper surface of a sinter material packed bed with a flame is used, a blended material to be charged is obtained by removing only a part or all of a carbonaceous material, adding moisture, and adding, during or after granulation, the removed carbonaceous material to the material from which only the part or all of the carbonaceous material has been removed, and a carbonaceous material post-added is char (coal char) obtained by carbonizing raw coal with a Roga index of 10 or less. Patent Literature 3 describes that the yield and productivity are improved by post-adding coal char in the granulation step of the reignition sintering method.

[0007] Patent Literature 4 discloses, as a technique of improving sintering yield, a method for producing sintered ore in which a low combustible carbonaceous material made from at least one of coke breeze or anthracite and a highly combustible carbonaceous material of which combustion start temperature is lower than that of the low combustible carbonaceous material are used as bonding agents of the sintering material, a carbon content of the highly combustible carbonaceous material is in a range from 25 mass % to 75 mass % relative to a carbon content of the bonding agents, and at least one of the low combustible carbonaceous material or the highly combustible carbonaceous material is added in a latter half of a granulation step of the sintering material.

[0008] Patent Literature 5 discloses a method for producing sintered ore including: performing segregation charging of pseudo-particles that are obtained by granulating sintering materials made from iron ore, flux, return ore, and a solid carbonaceous material, into pallets of a sintering machine; and performing firing while creating a carbon concentration difference in a height of a material layer, in which when palm kernel shell charcoal, which is a solid carbonized material produced by a heat treatment of palm kernel shell, is blended as a part or all of the solid carbonaceous material in the sintering materials, the palm kernel shell charcoal with an average particle size adjusted to 2.7 mm to 6.0 mm is blended. Using the palm kernel shell charcoal as the solid carbonaceous material for sintering reduces the emission of carbon dioxide (global greenhouse gas), and using appropriate operating techniques improves sintering productivity. In Patent Literature 5, when the palm kernel shell charcoal is blended as a part of the solid carbonaceous material in the sintering materials, it is preferable to blend the palm kernel shell charcoal whose average particle size is adjusted to be coarser in a range from 1.0 mm to 4.5 mm than the average particle size of the solid carbonaceous material, which is coke powder or anthracite.

[0009] Patent Literature 6 discloses a method for producing a carbonaceous material for sintering including: calculating a Roga index of coal and producing the carbonaceous material for sintering using coal having a Roga index of 10 or less, and also discloses that at least a part of the carbonaceous material for sintering is post-added in a predetermined blend amount during or at the end of a granulation step of a material for sintering. Patent Literature 6 describes that it is possible to reduce the amount of NOx discharged during the production of sintered ore.CITATION LISTPatent Literature(S)Patent Literature 1: JP 2020-2457 A

[0011] Patent Literature 2: JP 2020-29603 A

[0012] Patent Literature 3: JP 2020-84204 A

[0013] Patent Literature 4: JP 2022-33594 A

[0014] Patent Literature 5: JP 2014-218713 A

[0015] Patent Literature 6: JP 2020-56086 ASUMMARY OF THE INVENTIONProblem(s) to be Solved by the Invention

[0016] The highly combustible carbonaceous material is a carbonaceous material with high combustibility, i.e. a carbonaceous material with a fast combustion speed. Regarding the combustion speed of carbonaceous materials (bonding agents), various measurement methods and results have already been disclosed. However, unless the measurements are performed in a well-controlled manner, the results will vary greatly. For this reason, the combustion start temperature (ignition temperature), which has a substantive correspondence relationship with the combustion speed, is used as an index to represent the combustion speed. In the invention, the low combustible carbonaceous material refers to a carbonaceous material with a high combustion start temperature such as coke and anthracite (a carbonaceous material with a combustion start temperature exceeding 550 degrees C.), and the highly combustible carbonaceous material refers to a carbonaceous material with a lower combustion start temperature than the low combustible carbonaceous material (a carbonaceous material with a combustion start temperature of 550 degrees C. or less).

[0017] As described above, it is known that using the highly combustible carbonaceous material in the single-stage ignition sintering improves the yield and production rate by increasing the combustion speed of the carbonaceous material in the sinter material packed bed, and it is also known that using coal char (highly combustible carbonaceous material) improves the yield and production rate in the reignition sintering method. However, until now, there has been no detailed study on the preferable conditions for using the highly combustible carbonaceous material in the reignition sintering method. As a result of extensive investigation into these preferable conditions, the inventors found that the production rate could be further improved under specific conditions, and completed the invention. An object of the invention is to provide a method for producing sintered ore that further improves the production rate when the reignition sintering technique is used.Means for Solving the Problems

[0018] According to an aspect(s) of the invention, the followings are provided.

[0019] [1] A method for producing sintered ore using a Dwight-Lloyd type sintering machine, which includes an ignition furnace for initial ignition and a reignition furnace for reignition arranged at a predetermined distance downstream of the ignition furnace and which advances sintering by downward suction, the method including: using, as bonding agents for a blended material, a low combustible carbonaceous material with a combustion start temperature exceeding 550 degrees C. and a highly combustible carbonaceous material with a combustion start temperature of 550 degrees C. or less, in which a ratio of the highly combustible carbonaceous material with a particle size of 2.8 mm or more is 30 mass % or more and 80 mass % or less.

[0020] [2] The method for producing sintered ore according to [1], in which a crushed product obtained by crushing a compression-molded product, which is provided by compressing and molding an aggregate of carbonized wood, is used as the highly combustible carbonaceous material.

[0021] [3] The method for producing sintered ore according to [2], in which the crushed product of the compression-molded product provided by compressing and molding the aggregate of the carbonized wood is produced by: carbonizing a wood material to produce the carbonized wood; crushing the produced carbonized wood as necessary to produce carbonized wood particles, and kneading the carbonized wood particles alone or with a binder to produce the aggregate of the carbonized wood; compressing and molding the aggregate to produce the compression-molded product; and crushing the compression-molded product.

[0022] [4] The method for producing sintered ore according to any one of [1] to [3], in which a mass ratio of a carbon content of the highly combustible carbonaceous material to a carbon content of the bonding agents is 25 mass % or more and 75 mass % or less.

[0023] [5] The method for producing sintered ore according to [4], in which an average particle size of the low combustible carbonaceous material is in a range from 0.8 mm to 1.2 mm.

[0024] [6] The method for producing sintered ore according to [5], in which a segregation-strengthened type charging device is used as a charging device for the blended material.

[0025] [7] The method for producing sintered ore according to [5], in which, of the bonding agents, only the low combustible carbonaceous material is added in a latter half of a granulation process.

[0026] [8] The method for producing sintered ore according to any one of [1] to [3], in which a downward suction air volume is reduced only in a section up to an outlet of the reignition furnace on a sinter strand upstream side.

[0027] [9] The method for producing sintered ore according to [8], in which an average superficial air volume of the atmosphere sucked in the section up to the outlet of the reignition furnace on the sinter strand upstream side is set to 60% or more and 80% or less of an average superficial air volume of the atmosphere sucked in a section downstream of the outlet of the reignition furnace.

[0028]

[10] The method for producing sintered ore according to [8], in which an average negative pressure in a wind box or a wind leg in the section up to the outlet of the reignition furnace on the sinter strand upstream side is set to 40% or more and 70% or less of an average negative pressure in a wind box or a wind leg in the section downstream of the outlet of the reignition furnace.

[0029]

[11] The method for producing sintered ore according to [8], in which a separation time, which is a time required for a pallet to pass through a section between the ignition furnace and the reignition furnace, is set to 30 seconds or more and 2 minutes or less.

[0030]

[12] The method for producing sintered ore according to any one of [1] to [3], in which a separation time, which is a time required for a pallet to pass through a section between the ignition furnace and the reignition furnace, is set to 1 minute or more, and an oxygen concentration of a suction gas sucked downward from a surface layer side of a sintered layer in the section is set to 30 volume % or more.

[0031]

[13] The method for producing sintered ore according to

[12] , in which the separation time is set to 5 minutes or less and the oxygen concentration of the suction gas is set to 40 volume % or less.

[0032]

[14] The method for producing sintered ore according to any one of [1] to [3], in which oxygen enrichment of a suction gas sucked downward from a surface layer side of a sintered layer is started after the reignition is completed, an oxygen enrichment time from the start of the oxygen enrichment to a completion of the oxygen enrichment is 30 seconds or more, and an oxygen concentration of the suction gas sucked downward during the oxygen enrichment time is set to 30 volume % or more.

[0033]

[15] The method for producing sintered ore according to

[14] , in which the oxygen enrichment time is set to 2 minutes or less and the oxygen concentration of the suction gas is set to 40 volume % or less.

[0034]

[16] The method for producing sintered ore according to

[14] , in which the oxygen enrichment is started at a timing more than 0 seconds and within 30 seconds after a point of time when the reignition is completed.

[0035]

[17] The method for producing sintered ore according to

[16] , in which the oxygen enrichment is started at a timing more than 0 seconds and within 10 seconds after the point of time when the reignition is completed.

[0036]

[18] The method for producing sintered ore according to any one of [1] to [3], in which a pallet, into which the blended material is to be charged, is provided with a support member having a sinter cake supporting surface, the support member being erected on a grate bar to be embedded in a sinter material packed bed.

[0037] According to the aspect(s) of the invention, there is provided a method for producing sintered ore using a Dwight-Lloyd type sintering machine, which includes an ignition furnace for initial ignition and a reignition furnace for reignition arranged at a predetermined distance downstream of the ignition furnace and which advances sintering by downward suction, in which a low combustible carbonaceous material and a highly combustible carbonaceous material are both used as bonding agents, and a ratio of the highly combustible carbonaceous material with a particle size of 2.8 mm or more is set to 30 mass % or more and 80 mass % or less. This method further improves the production rate.BRIEF DESCRIPTION OF DRAWINGS

[0038] FIG. 1 schematically illustrates an exemplary DL-type sintering machine used in a method for producing sintered ore (reignition sintering method) according to a first exemplary embodiment.

[0039] FIG. 2 schematically illustrates an example of a segregation-strengthened type charging device.

[0040] FIG. 3 is a graph illustrating a particle size distribution of a sintering material in each layer (first to fifth layers) of a sinter material packed bed for which segregation-type charging is performed.

[0041] FIG. 4 is a graph illustrating a ratio of an average particle size in each layer (first to fifth layers) to an overall average particle size in the sinter material packed bed for which the segregation-type charging is performed.

[0042] FIG. 5 is a graph illustrating a carbon concentration distribution in each layer (first to fifth layers) of the sinter material packed bed for which the segregation-type charging is performed.

[0043] FIG. 6 schematically illustrates an exemplary air volume control in the DL-type sintering machine used in the method for producing sintered ore (reignition sintering method) according to a second exemplary embodiment.

[0044] FIG. 7 schematically illustrates an example of oxygen enrichment in a DL-type sintering machine used in the method for producing sintered ore (reignition sintering method) according to a third exemplary embodiment.

[0045] FIG. 8 schematically illustrates another example of oxygen enrichment in a DL-type sintering machine used in the method for producing sintered ore (reignition sintering method) according to a fourth exemplary embodiment.

[0046] FIG. 9 schematically illustrates an exemplary pallet for sintering that is used for producing sintered ore through a stand-support sintering technique according to a fifth exemplary embodiment.

[0047] FIG. 10 is a graph illustrating a relationship between the ratio of a highly combustible carbonaceous material with a particle size of +2.8 mm and the production rate according to Example 1.

[0048] FIG. 11 is a graph illustrating a relationship between the production rate and the mass ratio of the carbon content of the highly combustible carbonaceous material to the carbon content of a bonding agent(s) according to Example 1.

[0049] FIG. 12 is a graph illustrating a relationship between the average particle size of a low combustible carbonaceous material (coke breeze) and the production rate according to Example 1.

[0050] FIG. 13 is a graph illustrating a relationship between the superficial air volume ratio and the production rate according to Example 2.

[0051] FIG. 14 is a graph illustrating a relationship between the oxygen concentration and the production rate according to Example 3.

[0052] FIG. 15 is a graph illustrating a relationship between the oxygen concentration and the production rate according to Example 4 (Test 1).

[0053] FIG. 16 is a graph illustrating a relationship between the oxygen enrichment time and the production rate according to Example 4 (Test 2).

[0054] FIG. 17 is a graph illustrating a relationship between the separation time and the production rate according to Example 5 (highly combustible carbonaceous material blended at 50 mass % (carbon content mass ratio)).

[0055] FIG. 18 is a graph illustrating a relationship between the separation time and the production rate according to Example 5 (highly combustible carbonaceous material blended at 0 mass %).DESCRIPTION OF EMBODIMENT(S)

[0056] Referring to the drawings, the invention and preferred embodiments thereof will be described in detail below. In the specification and the drawings, components or configurations having substantially the same functions are given the same or similar names or the same or similar reference numerals, and redundant explanations are omitted.First Exemplary Embodiment

[0057] First, a description will be made on a Dwight-Lloyd (DL) type sintering machine used in a reignition sintering method and a method for producing sintered ore using the DL-type sintering machine. The DL-type sintering machine used in the reignition sintering method includes a reigniter that performs a second ignition. The reigniter is located at a predetermined interval (corresponding to a “separation distance” described later) downstream in a pallet traveling direction of an igniter that performs a first ignition. The reigniter is a flame heater that heats with flame an upper surface (surface) of a sinter material packed bed after the first ignition is completed.

[0058] FIG. 1 schematically illustrates an exemplary Dwight-Lloyd (DL) type sintering machine used in a reignition sintering method. In the following description, an ore supply side (left side in FIG. 1) is defined as an upstream side, and an ore discharge side (right side in FIG. 1) is defined as a downstream side, based on a pallet traveling direction 5x. As illustrated in FIG. 1, a DL-type sintering machine 101 includes a reignition furnace 4, which is located downstream of an ignition furnace 3 at a predetermined interval (separation distance). As illustrated in FIG. 1, the ignition furnace 3 includes an igniter 31 that performs the first ignition (initial ignition) and a hood 32 that covers the igniter 31. The reignition furnace 4 includes a reigniter 41 that performs the second ignition (reignition) and a hood 42 that covers the reigniter 41. For example, the ignition furnace 3 is an ignition furnace provided with a burner and the like, which is usable in a single-stage ignition sintering method, and the reignition furnace 4 can be the same ignition furnace used in the single-stage ignition sintering method. As illustrated in FIG. 1, the hood 32 of the ignition furnace 3 and the hood 42 of the reignition furnace 4 are independent of each other. An atmosphere suction area 7 is provided between the ignition furnace 3 and the reignition furnace 4, which are provided at a predetermined interval (separation distance) in the pallet traveling direction 5x.

[0059] The atmosphere suction area 7 is a section (area) where an upper surface of the sinter material packed bed 10 in a pallet is not heated by a burner or the like and the atmosphere (the air) sucked in by downward suction 6x is supplied into the sinter material packed bed 10. As illustrated in FIG. 1, a section between a partition wall 32a located downstream of the hood 32 of the ignition furnace 3 and a partition wall 42a located upstream of the hood 42 of the reignition furnace 4 is the atmosphere suction area 7. In this specification, a distance between the partition wall 32a and the partition wall 42a in the pallet traveling direction 5x, i.e., a distance of the atmosphere suction area 7 in the pallet traveling direction 5x, is referred to as the separation distance. A time required for each pallet (pallets referring to multiple connected pallet carriages moving on an endless track, not illustrated in the drawings) of the DL-type sintering machine 101 to pass through the separation distance (time required to pass through the atmosphere suction area) is called a separation time. In this specification, the sinter material packed bed 10 refers to layers of a blended material formed on the pallet, regardless of the presence or absence of ignition (including initial ignition) or reignition. The sinter material packed bed 10 includes a combustion zone 10A in which a sintering reaction is progressing due to ignition, and a sinter cake 10B in which the sintering reaction is completed. In the following description, the sinter material packed bed 10 after the initial ignition is performed (after the start of firing) is also referred to as a sintered layer.

[0060] A lower limit (minimum value) in an appropriate range for the separation time described above is a limit at which the expansion of the combustion zone 10A can be sufficiently achieved. An upper limit (maximum value) in the appropriate range for the separation time is determined by the cooling of an upper part of the sintered layer after the initial ignition. Thus, the appropriate range for the separation time depends on the exemplary embodiment (first to fifth exemplary embodiments) (details will be described later), and can be in a range from 0.5 minutes to 6 minutes. If the separation time is less than the lower limit in the appropriate range, sufficient oxygen cannot be supplied to the combustion zone 10A in the upper part of the sintered layer, and the expansion of the combustion zone 10A is inhibited. If the separation time exceeds the upper limit in the appropriate range, the temperature of the upper part of the sintered layer decreases to the sintering reaction temperature or less, making it difficult to achieve the effect of improving the production rate of the reignition technique. The separation distance is determined by multiplying the separation time by a transport speed (pallet speed) of the sinter material packed bed 10 using the pallet. If a pallet speed of 3 m / min for a standard commercial sintering machine is used, an appropriate range for the separation distance is in a range from 1.5 m to 18 m. Obviously, the appropriate separation distance changes depending on the pallet speed, and thus the separation distance depends on the sintering machine.

[0061] A method for producing sintered ore through reignition sintering is a technique in which a reignition step is added to the single-stage ignition sintering method (details will be described later). Examples of materials for sintered ore (sintering materials), which are usable as appropriate, include iron materials such as iron ore (powder), iron-containing miscellaneous materials such as scale and steelmaking dust, MgO-containing flux such as peridotite, CaO-containing flux such as limestone, return ore, and bonding agents (carbonaceous materials) that serve as fuel for sintering (bonding). As illustrated in FIG. 1, each sintering material is stored in the corresponding one (one of 11 to 1x) of sinter material bins 1, and is dispensed and blended in a predetermined ratio. The blended material is fed into a drum mixer 2 and granulated to produce pseudo-particles. The blended material having been granulated (hereinafter, the blended material after granulation is also referred to as blended material granules) is loaded from a sinter mixture surge hopper 81 onto a pallet covered with bedding ore (not illustrated in the drawings), thus forming the sinter material packed bed 10.

[0062] The sinter material packed bed 10 moves continuously in the pallet traveling direction 5x as the pallet moves. When the sinter material packed bed 10 moves to below the ignition furnace 3, the carbonaceous material on a surface of the sinter material packed bed 10 is ignited by the flame of the igniter 31, starting sintering of the sinter material packed bed 10. A downward suction device 6 (see FIG. 6) is provided below the pallet moving in the pallet traveling direction 5x, and sucks in the atmosphere (the air) from below the pallet. The downward suction 6x supplies oxygen into the sinter material packed bed 10, the combustion of the bonding agents in the sinter material packed bed 10 (combustion zone 10A) progresses from top to bottom, and the sinter material packed bed 10 is gradually fired by the combustion heat of the bonding agents. When the sinter material packed bed 10 passes through the atmosphere suction area 7 and moves to below the reignition furnace 4, the sinter material packed bed 10 is reignited by the flame of the reigniter 41. The sinter cake 10B obtained by sintering the sinter material packed bed 10 is discharged at a downstream end of the DL-type sintering machine 101 in the pallet traveling direction 5x, and is sized by crushing, sieving, or the like, so that sintered ore having a particle size that can be charged into blast furnaces becomes a material for blast furnace iron production.

[0063] As described above, in the reignition sintering method, the sinter material packed bed 10 is ignited by the ignition furnace 3 and passes through the atmosphere suction area 7, and after a predetermined time interval (corresponding to the above-described separation time), the sinter material packed bed 10 is reignited by the reignition furnace 4. In the atmosphere suction area 7 downstream of the ignition furnace 3, heating from above, i.e., combustion by a flame burner such as an igniter, is not performed, so sufficient oxygen is supplied into the sinter material packed bed 10, and the combustion of the bonding agents in the combustion zone 10A progresses. In addition, since the atmosphere suction area 7 is provided between the ignition furnace 3 and the reignition furnace 4, the combustion of the bonding agents progresses further downward due to an increase in superficial velocity. This increases a thickness of the combustion zone 10A in an upper part of the sinter material packed bed 10. After the thickness of the combustion zone 10A has increased in the atmosphere suction area 7, the upper surface of the sinter material packed bed 10 is reignited by the flame of the reignition furnace 4 provided downstream of the atmosphere suction area 7. The reignition allows uncombusted bonding agents (residual bonding agents) that were not ignited and combusted by the ignition furnace 3 to be combusted without leaving any residue, and allows gas heated by the reignition and combustion of the residual bonding agents to be sucked into the sinter material packed bed 10. This increases a time during which the upper part of the sinter material packed bed 10 is held at high temperature (e.g., a time during which the temperature is held at 1,200 degrees C. or more), promoting the sintering reaction and improving the yield. As described above, the separation time is a time required for each pallet to move through the atmosphere suction area 7, i.e., a time after the initial ignition is completed until the reignition is performed. The separation time in the exemplary embodiment can be, for example, 0.5 minutes or more and 3.5 minutes or less. The separation time is preferably 30 seconds or more and 2 minutes or less by facilitating the combustion using a highly combustible carbonaceous material in combination.

[0064] As in the single-stage ignition sintering, a charging device provided with a segregation mechanism is used in the reignition sintering, when the blended material granules are charged onto the pallet. For example, a gradient plate chute-type charging device 8 as illustrated in FIG. 1 is generally used. The gradient plate chute-type charging device 8 includes the sinter mixture surge hopper 81 and a gradient plate chute 82. The sinter mixture surge hopper 81 stores the blended material granules, and the gradient plate chute 82 is inclined downward in the opposite direction to the pallet traveling direction 5x. The blended material granules in the sinter mixture surge hopper 81 are charged onto the pallet using the gradient plate chute 82, thereby forming a slope 10x on an upstream side of the sinter material packed bed 10. The blended material granules roll over the slope 10x and are classified, resulting in particle size segregation in a layer thickness (layer height) direction of the sinter material packed bed 10. Specifically, the granules with smaller particle sizes tend to be charged in the upper part of the sinter material packed bed 10, while the granules with larger particle sizes tend to be charged in the lower part of the sinter material packed bed 10.

[0065] Here, the granulation process of the blended material described above is aimed at ensuring the permeability of the sinter material packed bed 10. Generally, as described in the 3rd Edition of the Iron and Steel Handbook II, Iron and Steelmaking, p. 84 (FIG. 2.4) [Oct. 15, 1979], fine powder materials with a particle size of less than 0.25 mm, which are a main target of the granulation process, form pseudo-particles by adhering to the periphery of particles of 1.00 mm or more as nuclear particles, but particles of an intermediate size of 0.25 mm or more and less than 1.00 mm are difficult to granulate and do not easily become pseudo-particles. For that reason, the particle size of the bonding agents to be blended is adjusted even when the granulation process is performed, and then charging is performed by the gradient plate chute-type charging device 8, so that it is possible to adjust the distribution of the bonding agents in the height direction of the sinter material packed bed.

[0066] Coke or anthracite is typically used as the bonding agent. Coke for sintering is made by crushing coke with a particle size unsuitable for use in blast furnaces (typically 40 mm or less) into a particle size of less than 10 mm, which is suitable for sintering, during the process of producing coke for blast furnaces. However, since sieving may not be performed after crushing, some particles having a particle size of 10 mm or more may remain. In contrast to lump coke used in blast furnaces, coke for sintering is also called coke breeze. Anthracite is one of the classifications given to coal (lignite, bituminous coal, and anthracite), and is the most carbonized coal. Coal with a fuel ratio (fixed carbon / volatile matter content (mass ratio)) of 4 or more, or more simply, coal with a carbon content of 90 mass % or more, is classified as anthracite. Anthracite is also crushed to have a particle size of approximately less than 10 mm, similar to coke breeze.

[0067] The inventors focused on the following two points in the above-described reignition sintering method. The first point is that by using the highly combustible carbonaceous material in addition to a low combustible carbonaceous material as the bonding agents, and by coarsening the highly combustible carbonaceous material and distributing it concentratedly in a lower part of the sinter material packed bed 10, the effect of reducing airflow resistance due to the coarsening can be obtained from the start to the end of sintering. The second point is that if the coarsening target is the highly combustible carbonaceous material, its high combustibility would largely prevent the reduction in sinter ore production that may otherwise be caused by the delay in the combustion completion due to coarsening. The inventors focused on these two points and conducted extensive research, which led to the completion of the invention. The inventors also investigated influential factors applicable to the invention (type of highly combustible carbonaceous material, particle size of low combustible carbonaceous material, segregation charging method, etc.).

[0068] The invention relates to a method for producing sintered ore using a Dwight-Lloyd type sintering machine, which includes an ignition furnace for initial ignition and a reignition furnace for reignition arranged at a predetermined distance downstream of the ignition furnace and which advances sintering by downward suction, the method including: using, as bonding agents for a blended material, a low combustible carbonaceous material with a combustion start temperature exceeding 550 degrees C. and a highly combustible carbonaceous material with a combustion start temperature of 550 degrees C. or less, and a ratio of the highly combustible carbonaceous material with a particle size of 2.8 mm or more being 30 mass % or more and 80 mass % or less. The initial ignition means that the bonding agents on the surface of the sinter material packed bed 10 charged in the pallet are first ignited (first ignition). The reignition means that ignition is again performed after the initial ignition is completed (second ignition). The downward suction refers to sucking in the atmosphere (the air) from below the pallet. Oxygen-containing gas above the sinter material packed bed 10 is sucked into the sinter material packed bed 10 to supply oxygen into the sinter material packed bed 10. First, the low combustible carbonaceous material and the highly combustible carbonaceous material used as the bonding agents will be described below. In this specification, the term “particle size” refers to a value (particle diameter) measured by sieving using a sieve in accordance with JIS Z8801-1:2019.Low Combustible Carbonaceous Material and Highly Combustible Carbonaceous Material

[0069] The bonding agents (carbonaceous materials) for the sintering material are classified into the low combustible carbonaceous material and the highly combustible carbonaceous material.

[0070] Examples of the low combustible carbonaceous material include coke and anthracite. The highly combustible carbonaceous material includes carbonaceous materials that are more combustible than the low combustible carbonaceous material (carbonaceous materials of which combustion start temperature is lower than that of the low combustible carbonaceous material). Specifically, the low combustible carbonaceous material and the highly combustible carbonaceous material are classified based on the combustion start temperature obtained by differential thermogravimetry analysis. The combustion start temperature of the low combustible carbonaceous material exceeds 550 degrees C., and the combustion start temperature of the highly combustible carbonaceous material is 550 degrees C. or less. The combustion start temperature (ignition temperature) is defined as a temperature at which a rapid weight loss begins, based on a temperature-weight change curve obtained by thermogravimetry (TG) in an air stream using a termogravimetry-differential thermal analysis and mass spectrometry (TG-DTA / MS).

[0071] Examples of the highly combustible carbonaceous material include coal char and biomass charcoal (palm kernel shell charcoal, carbonized wood produced by carbonizing wood, etc.). The combustion start temperatures (ignition temperatures) of coke and anthracite, which are the low combustible carbonaceous materials, are approximately 670 degrees C. and 690 degrees C., respectively. In contrast, the combustion start temperature of the highly combustible carbonaceous material is low. The combustion start temperature of coal char (semi-coke, lignite char, and subbituminous coal char) is 430 degrees C. or more and 550 degrees C. or less, the combustion start temperature of palm kernel shell charcoal is approximately 470 degrees C., and the combustion start temperature of carbonized wood is approximately 400 degrees C. or more and 450 degrees C. or less. The coal char and biomass charcoal have similar combustibility because they have roughly the same ignition temperatures. Although details will be described later, a compression-molded product made primarily from carbonized wood, which is biomass charcoal (biochar), is also a highly combustible carbonaceous material, and a crushed product obtained by crushing the compression-molded product is also usable. The combustion start temperature of the compression-molded product is low, approximately 250 degrees C. or more and 450 degrees C. or less. The same applies to the crushed product of the compression-molded product. It is known that the combustion speed of the highly combustible carbonaceous material is 1.03 to 30.00 times that of coke.

[0072] The coal char is, for example, a carbonaceous material (char) produced by carbonizing raw coal, such as low-caking bituminous coal, lignite, and subbituminous coal, at a temperature of 700 degrees C. or more and 900 degrees C. or less. The carbonaceous materials produced by carbonization of low-caking bituminous coal, lignite, and subbituminous coal are called semi-coke, lignite char, and subbituminous char, respectively. These carbonaceous materials are produced by carbonizing raw coal (including blended coal) in a pyrolysis furnace (e.g., a rotary kiln).

[0073] The biomass charcoal is, for example, a carbonaceous material produced by heat-treating (carbonizing) biological resources (biomass), such as palm kernel shells and wood, as materials. The palm kernel shell charcoal (PKS charcoal) is a solid carbide produced by heat treatment (carbonization) of palm kernel shell. Since the method for producing PKS charcoal can be implemented by referring to documents such as the above-mentioned Patent Literature 5, a detailed description thereof will be omitted here.

[0074] As described above, a crushed product of a compression-molded product made primarily from carbonized wood, which is biomass charcoal (biochar), may be used as the bonding agent for the sintering material. This compression-molded product is a compression-molded product (hereinafter, also referred to simply as a carbonized-wood compression-molded product for convenience) obtained by compressing and molding an aggregate of carbonized wood, and a crushed product (hereinafter, also referred to simply as a carbonized-wood compression-molding crushed product for convenience) obtained by crushing the carbonized-wood compression-molded product is used as the bonding agent. Here, “carbonized wood” refers to a carbonized material obtained by heat treatment (carbonization) of “wood”. The above-mentioned “wood” refers to trunks and branches of trees, or materials made from them, including, for example, construction waste. The term “aggregate of carbonized wood” refers to an aggregate of carbonized wood particles or an aggregate of carbonized wood particles bonded together with a binder, and there are no limitations on the size or shape of the carbonized wood particles. The term “compressing and molding” refers to molding that involves compression, and includes not only compression molding, but also extrusion molding in which pressure is applied during extrusion, for example. The phase “made primarily from” is used for a solid material having the largest proportion (by mass) of the total solid materials. The term “crushing” refers to a process of reducing the particle size using a crusher (e.g. rod mill, hammer crusher, roll crusher, super sander, jaw crusher, and fret mill).

[0075] The crushed product of the carbonized-wood compression-molded product is produced as follows. First, a wood material is obtained and then carbonized to produce carbonized wood (carbonized material producing step). Next, the produced carbonized wood is crushed or otherwise processed as necessary to produce carbonized wood particles, and these carbonized wood particles are kneaded alone or with a binder or the like to produce an aggregate of carbonized wood (hereinafter referred to as a carbonized wood aggregate) (aggregate producing step). A binder is used to provide a strong aggregate. Next, the carbonized wood aggregate is compressed and molded to produce a compression-molded product (carbonized-wood compression-molded product) (compression step). Then, the carbonized-wood compression-molded product is crushed to produce a crushed product of the carbonized-wood compression-molded product (carbonized-wood compression-molding crushed product) (compression product crushing step).

[0076] In the carbonized material producing step, the carbonized wood is produced using wood as a material in a carbonization device (external combustion rotary kiln, internal combustion rotary kiln, fluidized bed reactor, moving bed reactor (shaft furnace), etc.) by appropriately setting carbonization conditions (temperature, time, etc.). The volatile matter content of the produced carbonized wood (measured in accordance with JIS M8812:2006) is preferably 15 mass % or less. For example, carbonizing cedar wood chips at 800 degrees C. for one hour can reduce the volatile matter content of the carbonized material (carbonized wood) to 4.8 mass %.

[0077] In the aggregate producing step, the carbonized wood produced in the carbonized material producing step is crushed as needed (e.g., to an average particle size of 1 mm or less). The carbonized wood particles are kneaded alone, together with a binder, together with a binder and water, or together with a binder, water and an additive to produce a carbonized wood aggregate. Examples of the binder include cornstarch (starch), bentonite, coal tar, biomass tar, petroleum pitch, and cement, and an additive such as alkali or acid is added to some binders (such as cornstarch) to produce a strong molded product. For kneading, a device also capable of crushing the carbonized wood at the same time (e.g., an extruder) may be used. The amount of binder added is preferably 1 mass % or more and 10 mass % or less (not included in total mass) when the carbonized wood is taken as 100 mass %.

[0078] In the compression step, the carbonized wood aggregate (kneaded product) produced in the aggregate producing step is compressed and molded to produce a carbonized-wood compression-molded product that is a compression-molded product. The compression molding method may use a compression molding machine or an extrusion molding machine. For example, a roll press method using a roll rotation type compression molding machine (ring die type, flat die type, etc.) or a tableting method using a biaxial compression molding machine (screw type extrusion molding machine) may be used. The carbonized-wood compression-molded product may have any shape, for example, a pellet (cylindrical shape) or a briquette (pillow shape).

[0079] In the compression product crushing step, the carbonized-wood compression-molded product produced in the compression step is crushed by a crusher to produce a crushed product of the carbonized-wood compression-molded product (carbonized-wood compression-molding crushed product). Examples of the crusher include a rod mill, hammer crusher, roll crusher, super sander, jaw crusher, and fret mill. After crushing, the crushed product is sieved using a sieve conforming to JIS Z8801-1:2019, for example, and the crushed product having a predetermined particle size is used as the highly combustible carbonaceous material. For example, the crushed product has a particle size (particle diameter) of less than 10 mm (size passing through a sieve with an opening size of 10 mm), more preferably has a particle size (particle diameter) less than 5 mm (size passing through a sieve with an opening size of 5 mm).

[0080] In the compression step, it is preferable to produce a carbonized-wood compression-molded product having the following properties. The volatile matter content and apparent density of the carbonized-wood compression-molded product do not change before and after the crushing in the compression product crushing step, and the crushed product of the carbonized-wood compression-molded product has similar properties.

[0081] The volatile matter content of the carbonized-wood compression-molded product (measured in accordance with JIS M8812:2006) is preferably 20 mass % or less. Normally, when sintered ore is produced, the volatile matter content of the bonding agents is controlled to be a predetermined value (e.g., 10 mass %) or less in order to prevent breakdown of an exhaust gas electric dust collector that collects the exhaust gas generated during production. It is thus preferable to specify an upper limit of the volatile matter content of the carbonized-wood compression-molded product. An increase in volatile matter content of the carbonized-wood compression-molded product relative to the volatile matter content of carbonized wood is due to the use of a binder in the aggregate producing step. In the invention, the carbonized-wood compression-molded product (highly combustible carbonaceous material) is used in combination with the low combustible carbonaceous material (coke breeze, anthracite). Here, the volatile matter content of coke breeze is often much lower than the above-mentioned predetermined value (10 mass %), and the volatile matter content of anthracite is also low at around 5 mass %. Taking these points into consideration, the upper limit of the volatile matter content of the carbonized-wood compression-molded product is set to 20 mass %, which is higher than 10 mass %. Setting the upper limit high makes it possible to use materials with a high volatile matter content as the material for the carbonized-wood compression-molded product. It is also preferable that the blending ratio of the carbonized-wood compression-molded product and the low combustible carbonaceous material (coke breeze and / or anthracite) be determined based on the volatile matter content of the carbonized-wood compression-molded product and the volatile matter content of the low combustible carbonaceous material used, so as not to exceed a predetermined value for the volatile matter content according to the capacity of the exhaust gas electric dust collector.

[0082] The apparent density of the carbonized-wood compression-molded product is preferably 0.6 g / cm3 or more, and more preferably 0.7 g / cm3 or more. The apparent density is measured by a bead volume displacement method. The bead volume displacement method is a measurement method adopted by Micromeritics, which is a volume displacement method that uses DryFlo (pseudo fluid), highly fluid beads, as a measurement sample. Specifically, the volume of only the beads put in a sample chamber is first measured, then the sample is put on a layer of beads in the sample chamber and the volume is measured; the volume of the measured sample, including pores and cavities, is calculated from the difference between the two volumes. The apparent density is a value obtained by dividing the mass of the measured sample by the calculated volume. By making the apparent density of the carbonized-wood compression-molded product 0.6 g / cm3 or more, the atmospheric temperature in the vicinity of the combustion of the bonding agents increases when producing sintered ore. This improves the yield and therefore the sintering production rate. However, if the apparent density exceeds 1.3 g / cm3, the combustion speed of the carbonized-wood compression-molded product decreases, and the sintering speed also decreases.Particle Size of Highly Combustible Carbonaceous Material

[0083] The particle size of the highly combustible carbonaceous material is determined as follows. After drying the highly combustible carbonaceous material at 105 degrees C. for 2 hours or more, the material is shaken by head tapping for 5 minutes using a Ro-Tap shaker equipped with a sieve with 2.8 mm mesh size as specified in JIS Z8801-1:2019, and the ratio of a particle size of 2.8 mm or more on the sieve is examined. In the invention, it is used a highly combustible carbonaceous material in which the ratio of a particle size of 2.8 mm or more is 30 mass % or more and 80 mass % or less. If the ratio of a particle size of 2.8 mm or more is less than 30 mass %, the yield decreases and therefore the production rate decreases. If the ratio of a particle size of 2.8 mm or more exceeds 80 mass %, uneven sintering results in a decrease in production rate.

[0084] According to the invention, the highly combustible carbonaceous material coarsened is concentratedly distributed in the lower part, thereby making it possible to improve the permeability in the lower part. The improved permeability in the lower part increases the sintering speed during sintering, resulting in an improved production rate. Here, the bonding agent subjected to coarsening is only the highly combustible carbonaceous material. Since the highly combustible carbonaceous material has a high combustion speed, the impact of the decrease in combustion speed due to coarsening is small. Therefore, the sintering speed is not likely to decrease. Further, the increased sintering speed increases the cooling speed of the combustion zone 10A. This fines hematite (Fe2O3) particles that crystallize from a liquid phase produced by a high-temperature sintering reaction. During the reduction of hematite to magnetite (Fe3O4), crystals of blast furnace sintered ore expand. At that time, the resulting cracks cause the sintered ore to break down into powder. This degradation phenomenon is called reduction degradation. As mentioned above, the finer hematite (Fe2O3) particles reduce the amount of crystal expansion, providing the effect of inhibiting the reduction degradation of sintered ore.Use of Crushed Product of Carbonized-Wood Compression-Molded Product

[0085] In the invention, it is preferable to use a crushed product of the carbonized-wood compression-molded product as the highly combustible carbonaceous material. The reason for this is as follows: compressing and molding carbonized wood or a crushed product of carbonized wood once and then crushing it for use improves the product yield due to the increase in apparent density, and therefore improves the production rate, rather than using the carbonized wood or the crushed product of carbonized wood as is. The carbonized wood or the crushed product of carbonized wood is porous (typically with an apparent density of less than 0.6 g / cm3) and therefore has excellent combustibility, but the sintering speed increases too much, which actually reduces the yield and production rate of sintered ore.Carbon Content Mass Ratio

[0086] In the invention, the mass ratio of the carbon content of the highly combustible carbonaceous material to the carbon content of all bonding agents is preferably 25 mass % or more and 75 mass % or less. The mass ratio of the carbon content of the highly combustible carbonaceous material to the carbon content of all bonding agents is adjusted based on the carbon content of each carbonaceous material (low combustible carbonaceous material, highly combustible carbonaceous material) from the industrial analysis of the low combustible carbonaceous material and the highly combustible carbonaceous material. This is based on the following thought: if the blending ratio of the highly combustible carbonaceous material is less than 25 mass %, the effect of improving the sintering speed (flame front speed), which is the effect of blending the highly combustible carbonaceous material, cannot be obtained; and if the blending ratio of the highly combustible carbonaceous material exceeds 75 mass %, the product yield will decrease due to the high-speed combustion characteristic of the highly combustible carbonaceous material.Average Particle Size of Low Combustible Carbonaceous Material

[0087] In the invention, the average particle size of the low combustible carbonaceous material is also preferably in a range from 0.8 mm to 1.2 mm. Fining the average particle size of the low combustible carbonaceous material increases the number of carbonaceous material particles, which makes the heat source supply uniform in a cubic region having a side length of several mm to 10 mm in each part of the combustion zone 10A, thereby improving the product yield. In addition, segregation charging increases the amount of the bonding agents in the upper part of the sinter material packed bed insufficient in heat, further improving the product yield. As a result, the effect of improving the production rate is accelerated.

[0088] The average particle size of the low combustible carbonaceous material is determined as follows. After drying the low combustible carbonaceous material at 105 degrees C. for 2 hours or more, the material is classified using five sieves with different opening sizes by head tapping for 5 minutes to shake the material with a Ro-Tap shaker, and a sample mass wi of each particle size category i is measured. As shown in Table 1, the particle sizes (0.5 mm, 1.0 mm, 2.8 mm, 4.76 mm, 10.0 mm) that are boundary values for the particle size category are the opening sizes of the sieves used for classification. For example, a particle size category 2 “0.5-1.0” refers to sizes remained on the sieve when sieved through a sieve with an opening size of 0.5 mm and passing through the sieve when sieved through a sieve with an opening size of 1.0 mm.TABLE 1Particle size category i12345Particle size (mm)0-0.50.5-1.01.0-2.82.8-4.764.76-10.0Representative value0.250.751.93.87.5xi

[0089] The average particle size (mm) is an arithmetic mean diameter calculated by weighting a representative value xi (≈median) of the particle size category by a mass fraction (mass ratio) for each particle size category, as represented by the following formula (1).Average⁢ particle⁢ size=Σ⁢wixi / Σ⁢wiFormula⁢ (1)xi: representative value of particle size category i

[0091] wi: sample mass of particle size category iSegregation-Strengthened Type Charging Device

[0092] In the invention, it is also preferable to use a segregation-strengthened type charging device instead of the normal charging device 8. Consider a case where the blended material granules are charged onto the pallet using the segregation-strengthened type charging device, and the sinter material packed bed after charging is divided into five parts at equal intervals in the layer thickness (layer height) direction. In that case, also preferably, the average particle size of the materials in the uppermost part of the five parts of the sinter material packed bed is 0.3 to 0.5 times the average particle size of the materials in the lowermost part. In addition, also preferably, segregation charging is performed so that the carbon ratio in the uppermost part of the five parts of the sinter material packed bed is 1.10 to 1.16 times the carbon ratio in all layers of the sinter material packed bed, when the sinter material packed bed after charging is divided into five parts in the layer thickness (layer height) direction.

[0093] The segregation-strengthened type charging device is a charging device capable of increasing particle size segregation in the layer thickness direction of the sinter material packed bed compared to the gradient plate chute 82 illustrated in FIG. 1. Examples of the segregation-strengthened type charging device include: a slit-bar type charging device as illustrated in FIG. 2 (One et al., CAMP-ISIJ 10 (1997), p. 191, The Iron and Steel Institute of Japan); a slit wire type charging device (Takai et al., CAMP-ISIJ 6 (1993), p. 916); an intensified sifting feeder (ISF) type charging device which is an arranged dispersion type (Nagai et al., CAMP-ISIJ 29 (2016), p. 563); a hybrid type magnetic segregation charging device (Oyama et al., CAMP-ISIJ 11 (1998), p. 225); and a wind segregation device (Shibata et al., CAMP-ISIJ 14 (2001), p. 193). As illustrated in FIG. 2, a sieve member of a slit-bar type charging device 8A is provided such that the space between wires 82A (or rods) parallel to a pallet width direction is larger from the upper part toward the lower part of the pallet (Yoshinaga et al., Tetsu-to-Hagane (1987) Vol. 73, Abstracts of the 114th Lecture Meeting of the Iron and Steel Institute of Japan, S846). The wires (or rods) may be arranged parallel in an up-down direction of the pallet. A sieve member of the arranged dispersion type charging device is provided such that many bars are arranged along the material flow and the difference in height between adjacent bars increases from the upstream to the downstream of the material flow (Inazumi et al., Tetsu-to-Hagane 77 (1991), pp. 63-70). The configuration of each segregation-strengthened type charging device is described in each document described above, and the device can be used by referring to each document, and therefore a detailed description thereof will be omitted here.

[0094] Here, there are shown exemplary results of investigating the material particle size distribution and carbon concentration distribution in the height direction of the sinter material packed bed in a case where a blended material (see Table 6) used in Example 1-10 described later was charged into an actual machine using a slit-bar type charging device, which is a segregation-strengthened type charging device (One et al., CAMP-ISIJ 10 (1997), p. 191, The Iron and Steel Institute of Japan). A sample was collected in the height direction of the sinter material packed bed using a sampling device (described in JP 2018-044188 A). Specifically, blended material granules were charged on a bottom lid placed on a bedding layer, and then a cylindrical sampling tube was driven thereinto from directly above the bottom lid and the sample was collected with the bottom lid closed. Since the sample can be collected by referring to the above document (JP 2018-044188 A), detailed explanation thereof will be omitted here.

[0095] The collected sample was divided into five parts at equal intervals in the height direction of the sinter material packed bed (first to fifth layers from the top), and each layer was subjected to analysis. Table 2 shows the analysis results for the respective layers (first to fifth layers). As shown in Table 2, from the top layer (first layer), the masses of the respective layers were 4.30 kg, 5.16 kg, 4.88 kg, 4.62 kg, and 5.33 kg, the mass ratios of the respective layers were 18%, 21%, 20%, 19%, and 22%, and the recovered masses in the respective layers were almost equal. The mass fraction in Table 2 was obtained by taking 300 g of blended material granules of each layer (each of the first to fifth layers), drying them at 105 degrees C. for at least 2 hours, shaking them for 15 seconds without head tapping in a Ro-Tap shaker using a sieve with each mesh size shown on the horizontal axis (particle size category), and classifying them. This procedure allows a user to understand the particle size distribution of the sintering material before granulation, and the mass ratio for each particle size category of the sintering material in each layer was expressed as the abundance ratio (mass %). The average particle size in each layer in Table 2 is an arithmetic mean diameter calculated similarly to the formula (1), based on the representative value of the particle size category and the mass fraction of each particle size category. In Table 2, the particle size category “+8.0” refers to sizes remained on the sieve when sieved through a sieve with an opening size of 8.0 mm. The particle size category “to 4.0” refers to sizes remained on the sieve when sieved through a sieve with an opening size of 4.0 mm, and passing through the sieve when sieved through the sieve with an opening size of 8.0 mm indicated by the particle size category “+8.0” in the left column. The same applies to the particle size categories “to 2.0”, “to 1.0”, “to 0.5”, “to 0.25”, and “to 0.125”. The particle size category “−0.125” refers to sizes passing through the sieve when sieved through a sieve with an opening size of 0.125 mm.TABLE 2Average particleMass fraction (mass %) for each particle size category (mm)Averagesize of eachUpper section: Particle size category, Lower section: Representative valueparticlelayer / AverageCarbon+8.0to 4.0to 2.0to 1.0to 0.5to 0.25to 0.125−0.125sizeparticle size of allMassconcentration12631.50.750.3750.18750.0625(mm)layers (%)(kg)(mass %)First layer0.22.834.334.918.07.12.10.61.910.554.305.06Second0.818.536.622.312.46.42.30.72.760.795.164.73layerThird layer9.327.728.516.210.15.42.20.63.981.144.884.22Fourth8.823.231.418.810.15.21.90.63.771.084.623.88layerFifth layer17.327.025.014.18.85.12.20.54.751.365.334.55All layers————————3.48——4.48

[0096] FIGS. 3 to 5 are graphs created based on Table 2. FIG. 3 illustrates a particle size distribution of the sintering material before granulation in each layer (first to fifth layers) of the sinter material packed bed. FIG. 4 illustrates a ratio of the average particle size in each layer (first to fifth layers) to the average particle size in all layers. FIG. 5 illustrates a carbon concentration distribution in each layer (first to fifth layers) of the sinter material packed bed. This carbon concentration is a free carbon value involved in combustion, excluding carbon contained in carbonates, etc., and is calculated based on the combustion-infrared absorption method (JIS G1211-3:2018).

[0097] As illustrated in FIG. 3, the layer(s) in the lower part of the sinter material packed bed had a higher mass fraction of the coarse sintering material, i.e., the particle size category “+8.0 mm” (a particle size of 8.0 mm or more) and the particle size category “to 4.0 mm” (a particle size of less than 8.0 mm and 4.0 mm or more) compared to the layer(s) in the upper part of the sinter material packed bed. As a result, as shown in Table 2, the average particle size of the blended material in the lowermost layer (fifth layer) was in a range from 3 mm to 5 mm, and the average particle size in the uppermost layer (first layer) was 2 mm or less. In addition, as illustrated in FIG. 4, there was a difference in average particle size between the respective layers, with the average particle size of the first layer being 0.4 times that of the fifth layer. As illustrated in FIG. 5, the carbon concentration was the highest in the uppermost layer (first layer) at 5.06 mass %, which was 1.13 times the average carbon concentration of all layers. Thus, by charging the blended material, in which the highly combustible carbonaceous material is coarsened so that the ratio of a particle size of 2.8 mm or more is set to 30 mass % or more and the low combustible carbonaceous material is made finer so that its average particle size is in a range from 0.8 mm to 1.2 mm, using a segregation-strengthened type charging device, the bonding agents with a particle size of 0.25 mm or more and less than 1.00 mm segregate in the upper layer(s) which becomes brittle after sintering, increasing the carbon concentration. As a result, as shown in Example 1-10 described later, the amount of heat required for the sintering reaction is supplied to the upper layer(s), which facilitates an improvement in product yield and therefore the production rate. If the ratio of the highly combustible carbonaceous material with a particle size of 2.8 mm or more increases too much, the number of bonding agent particles will decrease, causing temperature unevenness in the sinter material packed bed. It is thus preferable to set the upper limit of the ratio of a particle size of 2.8 mm or more to 80 mass %. Since the segregation-strengthened type charging device was used here, the segregation of coarse and fine particles was evident. However, similar segregation of coarse and fine particles would probably occur in segregation charging using an ordinary gradient plate chute.Post-Addition Method

[0098] In the invention, it is also preferable that the entire amount of only the low combustible carbonaceous material, which is a part of sintering materials, is added in the latter half of the granulation step. Here, the “latter half of the granulation step” refers to the latter half of a time period when a “total granulation time” described below is divided into two parts (former and latter parts). In the granulation step in which the sintering materials are granulated using a granulator, the sintering materials, excluding the low combustible carbonaceous material, discharged from the sinter material bins are first charged into the granulator, mixed, and the humidity is adjusted before granulation begins (hereinafter, the granulated sintering materials excluding the low combustible carbonaceous material are referred to as pre-granules). After a certain period of time, the low combustible carbonaceous material (hereinafter also referred to as a post-added bonding agent) is added to the granulator (during the granulation step) to produce blended material granules. By adding the post-added bonding agent at a proper later timing, the pre-granules are coated with the post-added bonding agent, that is, the post-added bonding agent is not encapsulated in the pre-granules. The post-added bonding agent is either attached to the surface layer of the pre-granules or exists as independent granules unattached thereto. By using the blended material granules coated with the post-added bonding agent, it is possible to adjust the timing of the start of combustion of the post-added bonding agent during the sintering step.

[0099] The post-addition in the granulation step is performed, for example, as follows. For granulation, a piston-flow type cylindrical granulator with a central axis inclined downward toward the downstream side is used, and a small transport conveyor is inserted from a downstream outlet to a predetermined position inside the granulator. When the sintering materials excluding the post-addition bonding agent (hereinafter also referred to as sintering materials before post-addition) are fed from an upstream inlet, the sintering materials before post-addition fed are mixed, water is then added, and they move downstream while being granulated. The post-addition bonding agent is placed on the transport conveyor and conveyed from the downstream outlet of the granulator to the inner upstream side. The post-added bonding agent is added at a predetermined position to the pre-granules that have been moved from the upstream side in the granulator while being granulated, and all sintering materials (pre-granules and post-added bonding agent) are further granulated and discharged from the downstream outlet as blended material granules.

[0100] The granulation time of the sintering materials before post-addition that elapses before the addition of the post-addition bonding agent is preferably 80% or more and 96% or less of the total granulation time of the sintering materials, in other words, the post-addition is preferably performed at a timing 80% to 96% of the total granulation time of the sintering materials in the granulation step. For example, when the total granulation time is 240 seconds, the blended material excluding the low combustible carbonaceous material (sintering materials before post-addition) may be granulated for 225 seconds, and then the low combustible carbonaceous material may be added and granulated for 15 seconds. If the post-addition is performed at a timing earlier than 80% of the total granulation time, it is not possible to sufficiently prevent the post-added bonding agent from being encapsulated in the granules. If the post-addition is performed at a timing later than 96% of the total granulation time, the post-added bonding agent is insufficiently granulated. Here, the total granulation time in the granulation step does not include a time during which the sintering materials are simply mixed (mixing time) from when they are charged into the granulator until water is added. For sintering, a drum mixer is mainly used, which is of a constant speed piston flow type. Therefore, the post-addition position can be set by making the above-described granulation time correspond to a distance in a longitudinal direction of the drum mixer.

[0101] As described above, when the post-addition of the low combustible carbonaceous material is performed, the granules of the blended material are coated with the low combustible carbonaceous material, accelerating the reaction with atmospheric oxygen supplied to the sinter material packed bed. In addition, the low combustible carbonaceous material receives a sufficient amount of heat from the heated atmosphere and combusts. This improves the combustion speed of the bonding agent, resulting in higher production rate. If the low combustible carbonaceous material is encapsulated due to bulk granulation, it is difficult for the low combustible carbonaceous material to receive the oxygen and heat required for combustion. On the other hand, the highly combustible carbonaceous material for which the post-addition is not performed is encapsulated in blended material granules, but the highly combustible carbonaceous material has a low combustion start temperature and a fast combustion speed. By controlling the timing of the start of combustion of the bonding agents to start with the low combustible carbonaceous material coating the blended material granules therewith and then the highly combustible carbonaceous material encapsulated in the blended material granules and utilizing the difference in combustion properties therebetween, the overall combustion speed is increased and the production rate is further improved.Second Exemplary Embodiment

[0102] A preferred example in which an air volume control technique is used in the invention will be described below as a second exemplary embodiment. As described in the first exemplary embodiment, the production rate is improved by using a predetermined amount (30 mass % or more and 80 mass % or less) of the coarse highly combustible carbonaceous material in the invention. However, since the blended material contains the highly combustible carbonaceous material with a particle size of less than 2.8 mm, the highly combustible carbonaceous material (especially fine particles) is present in the upper part of the sinter material packed bed. Since the highly combustible carbonaceous material has a fast combustion speed, the temperature of the upper part of the sintered layer (up to a depth of about 100 mm from the surface layer) rapidly drops in the section from the first ignition to the start of reignition, reducing the yield and decreasing the effect of improving the production rate. The inventors have considered that using the air volume control technique would be effective to solve this problem. In the following, the air volume control technique will be described first to state the inventors' aims.Air Volume Control Technique

[0103] FIG. 6 schematically illustrates an exemplary air volume control in a DL-type sintering machine used in a reignition sintering method. As illustrated in FIG. 6, the downward suction device 6 (not illustrated in FIG. 1) for causing the sintering reaction to proceed downward is provided below pallets of the DL-type sintering machine 101. The downward suction device 6 controls a downward suction air volume in a sinter strand direction (the same as the pallet traveling direction 5x). Specifically, the downward suction device 6 includes pairs of wind boxes 61 and wind legs 62 successively installed in the sinter strand direction below the sinter material packed bed 10. The wind boxes 61 and the wind legs 62 are each connected to a blower 65 via a duct 64. A damper 63 is provided at the connection between the wind box 61 and the wind leg 62 for each pair or in the middle of each wind leg 62. By adjusting the opening of the damper 63, the suction air volume by each wind box 61 can be adjusted, and the distribution of the suction air volume in the sinter strand direction (pallet traveling direction) can be controlled.

[0104] Regarding the above-mentioned problem of the invention using the reignition sintering method, the inventors considered that by limiting an air volume reduction area to an area up to a reignition furnace outlet located on a sinter strand upstream side, and not reducing the air volume after reignition is completed, it would be possible to inhibit a decrease in the sintering speed in the upper part of the sintered layer, significantly improve the product yield, and obtain the effect of improving the production rate. The inventors repeated sintering experiments and found the following preferred embodiment. In addition, the inventors also considered a configuration for optimizing the pressure and the air volume reduced in the air volume reduction area, and the timing of reignition (separation time). Those will be described below.

[0105] When implementing the invention, it is preferable to reduce the downward suction air volume only in a section from a sinter strand upstream end to the outlet of the reignition furnace on the sinter strand upstream side (section S in FIG. 6). This is because the production rate is improved by performing, in the section S up to the outlet of the reignition furnace on the sinter strand upstream side, downward suction while reducing the air volume more than in a section downstream of the reignition furnace outlet (see Example 2 described later). As illustrated in FIG. 6, in a case where the most upstream wind box 61 is installed immediately below the inlet of the ignition furnace 3 in the sinter strand direction (in a case where an upstream end of the most upstream wind box 61 is positioned immediately below an upstream end of the ignition furnace 3), the section S corresponds to a section of the sinter strand from the inlet of the ignition furnace 3 to the outlet of the reignition furnace 4.

[0106] Preferred examples of the air volume reduction include the followings: an average superficial air volume of the atmosphere sucked in the section S up to the outlet of the reignition furnace on the sinter strand upstream side is set to 60% or more and 80% or less of an average superficial air volume of the atmosphere sucked in the section downstream of the outlet of the reignition furnace; and an average negative pressure in the wind boxes 61 or the wind legs 62 in the section S is set to 40% or more and 70% or less of an average negative pressure in the wind boxes 61 or the wind legs 62 in the section downstream of the outlet of the reignition furnace.

[0107] In the wind boxes 61 or the wind legs 62, the superficial air volume is measured, for example, by a Pitot tube, and the average superficial air volume is calculated. It should be noted that the measurement environment in the wind boxes 61 or the wind legs 62 is not necessarily good because the gas contains dust. The measurement may thus be performed using a hot wire anemometer or the like for a surface of the sinter material packed bed 10. The reason why the average superficial air volume of the atmosphere sucked in the section S is set to 60% to 80% of that after reignition is completed is as follows: if that average superficial air volume is less than 60%, the sintering speed drops significantly and the production rate decreases, and if that average superficial air volume exceeds 80%, it is not possible to obtain the production rate improving effect provided by the yield improvement through the air volume reduction.

[0108] Measuring the superficial air volume is difficult in some cases. In that case, the air volume can be controlled by the average negative pressure in each section. A relationship between the air volume passing through the sinter material packed bed and the negative suction pressure is represented by equation (2) of Japanese Permeability Unit (JPU) below. JPU is an airflow resistance index of the sinter material packed bed, indicating the permeability (ease of gas passing) of the sinter material packed bed.JPU=(Q / A)·(H / P)0.6(2)Q: Air volume (suction air flow rate) (Nm3 / min)

[0110] A: Suction area (m2)

[0111] H: Thickness of packed bed (m)

[0112] P: Negative pressure (mH2O)

[0113] From the equation (2), it can be seen that an air volume of 60% (40% reduction) corresponds to a negative pressure of 40% (60% reduction), and an air volume of 80% (20% reduction) corresponds to a negative pressure of 70% (30% reduction).

[0114] In the exemplary embodiment, the separation time, which is a time required for the pallet to pass through the section (atmosphere suction area 7) between the ignition furnace 3 and the reignition furnace 4, may be, for example, in a range from 0.5 minutes to 3.5 minutes, and is more preferably in a range from 0.5 minutes to 2.0 minutes (in a range from 30 seconds to 2 minutes). If the separation time is less than 0.5 minutes, it is not possible to supply sufficient oxygen to the combustion zone 10A in the upper part of the sintered layer. If the separation time exceeds 2.0 minutes, the temperature of the upper part of the sintered layer drops below the sintering reaction temperature, making it difficult to obtain the effect of improving the production rate of the reignition technique. The phrase “the pallet to pass through the section” refers to movement of the pallet between a position and another position.Third Exemplary Embodiment

[0115] A preferred example in which an oxygen enrichment technique is used in the invention will be described below as a third exemplary embodiment. In the invention using the reignition sintering method, the production rate may be further improved by extending a high-temperature retention time of the sintered layer. In that case, if an attempt is made to extend the separation time, which is a time from the completion of initial ignition until reignition is performed (a time required for the pallet to move through the section between the ignition furnace and the reignition furnace), the temperature of the upper part of the sintered layer would drop too much before reignition is performed, and so there has been a limit to how long the separation time can be extended. In particular, when using the highly combustible carbonaceous material, even if the particles thereof are coarse (with a particle size of 2.8 mm or more being 30 mass % to 80 mass %), the combustion time is short and the cooling speed increases. In order to solve this problem, the inventors have considered that using the oxygen enrichment technique would be effective in place of extending the separation time. In the following, the oxygen enrichment technique will be described to state the inventors' aims.Oxygen Enrichment Technique

[0116] The oxygen enrichment technique is a technique in which oxygen is enriched in the air to be sucked downward and the enriched gas is supplied to the sinter material packed bed. Specifically, for example, an oxygen enrichment gas supplying unit 9 is provided above the sinter material packed bed 10 as illustrated in FIGS. 7 and 8. The oxygen enrichment gas supplying unit 9 supplies an oxygen enrichment gas with a higher oxygen concentration than the air supplied during an ordinary operation, as the gas to be sucked downward (suction gas). By increasing the oxygen concentration, the amount of bonding agents combusted per unit time can be increased.

[0117] Regarding the above-mentioned problem of the invention, the inventors have considered that the high-temperature retention time of the sintered layer can be extended to improve the production rate by supplying the oxygen enrichment gas in the atmosphere suction area 7 for the reignition sintering method. The inventors repeated sintering experiments and found the following preferred example. In addition, the inventors also considered a configuration for optimizing the separation time and the oxygen concentration of the oxygen enrichment gas supplied in the atmosphere suction area 7. Those will be described below.

[0118] FIG. 7 schematically illustrates an exemplary DL-type sintering machine according to the exemplary embodiment (oxygen enrichment). As illustrated in FIG. 7, a DL-type sintering machine 103 includes the oxygen enrichment gas supplying unit 9 above the sinter material packed bed 10 in addition to the components of the DL-type sintering machine 101 described above. The oxygen enrichment gas supplying unit 9 supplies an oxygen enrichment gas with a higher oxygen concentration than the air (such as a gas mixture of the air and oxygen) into the sinter material packed bed 10. The oxygen enrichment gas supplying unit 9 includes, for example, a hood 91 and a gas pipe 92 through which the oxygen enrichment gas is supplied into the hood 91. The oxygen enrichment gas supplying unit 9 is configured to continuously supply the oxygen enrichment gas. The supplied oxygen enrichment gas is guided into the sinter material packed bed 10 by the downward suction of the downward suction device 6, where the oxygen enrichment gas advances the sintering reaction in the combustion zone 10A, and is then recovered as exhaust gas by the downward suction device 6 (wind box 61). By adjusting the supply amount of oxygen gas injected from the gas pipe 92, the oxygen concentration of the oxygen enrichment gas can be set to a predetermined concentration that has a higher oxygen content than the atmosphere.

[0119] As illustrated in FIG. 7, the oxygen enrichment gas supplying unit 9 is installed in the section between the ignition furnace 3 and the reignition furnace 4 (atmosphere suction area 7, see FIG. 6). The oxygen enrichment gas supplying unit 9 supplies the oxygen enrichment gas from a surface layer side of the sinter material packed bed 10 (sintered layer) passing through the atmosphere suction area 7, and the supplied gas is sucked downward. That is, in the third exemplary embodiment, the atmosphere suction area 7 where the atmosphere is supplied in the first and second exemplary embodiments serves as an oxygen enrichment gas suction area 7x (see FIGS. 6 and 7) where the oxygen enrichment gas is supplied. FIG. 7 illustrates an example in which a small safety interval is provided between the hood 91 of the oxygen enrichment gas supplying unit 9 provided in the atmosphere suction area 7 and the hood 32 (partition wall 32a) of the ignition furnace 3 and between the hood 91 and the hood 42 (partition wall 42a) of the reignition furnace 4. In this case, the hood 91 of the oxygen enrichment gas supplying unit 9 is installed in the oxygen enrichment gas suction area 7x, but if there are no safety issues, it is preferable to arrange the hoods 32, 42, 91 without such intervals therebetween, or to consecutively arrange the hood 91 of the oxygen enrichment gas supplying unit 9 and the hood 32 of the ignition furnace 33 and / or the hood 42 of the reignition furnace 4. The distance between the ignition furnace 3 (downstream partition wall 32a) and the oxygen enrichment gas supplying unit 9 (upstream wall of the hood 91), and the distance between the oxygen enrichment gas supplying unit 9 (downstream wall of the hood 91) and the reignition furnace 4 (upstream partition wall 42a) should be provided only when necessary for safety reasons, and even in that case, it is preferable that the distance not be longer than the minimum required distance. This is because the effect of improving the production rate decreases as the oxygen enrichment time decreases.

[0120] The amount of oxygen gas introduced into the suction gas supplied to the oxygen enrichment gas suction area 7x can be controlled by a method for measuring and adjusting the oxygen concentration on a surface of the sintered layer by installing a sampling tube on the surface of the sintered layer in the oxygen enrichment gas suction area 7x, or an adjustment method in accordance with the air volume of the wind box 61 of the DL-type sintering machine 103. The method for introducing the oxygen gas is, for example, as follows: oxygen (e.g., industrial oxygen) is supplied directly into the hood 91 installed above the oxygen enrichment gas suction area 7x and it is supplied, into the sinter material packed bed 10, together with the atmosphere (hereinafter also referred to as the atmosphere outside the hood) sucked in from around the hood 91 (outside the hood 91). At that time, it is desirable to supply the oxygen gas from multiple points in a width direction of the DL-type sintering machine 103 (direction orthogonal to the pallet traveling direction 5x). Alternatively, an oxygen enrichment gas having a predetermined oxygen concentration, in which oxygen and the air are mixed in advance, may be supplied into the hood 91 through the gas pipe 92. In addition, it is possible to use the industrial oxygen that is produced in oxygen plants within steelworks.

[0121] In implementing the invention, the time required for the pallet to pass through the section between the ignition furnace 3 and the reignition furnace 4 (separation time) can be set to, for example, 0.5 minutes or more and 6 minutes or less (see Table 8 described later). Preferably, the separation time is set to 1 minute or more and the oxygen concentration of the suction gas sucked downward from the surface layer side of the sintered layer in this section is set to 30 volume % or more. Here, the separation time includes a time required to pass through the above-mentioned safety interval (e.g., more than 0 seconds and 2 seconds or less). As shown in Examples described later, if the oxygen concentration of the suction gas is less than 30 volume % or if the separation time is less than 1 minute, the amount of oxygen supplied is insufficient and a sufficient effect of improving the production rate cannot be obtained.

[0122] The separation time is preferably 6 minutes or less. If the separation time exceeds 6 minutes, the effect of reignition will not be obtained even with the oxygen enrichment because the sintered layer is cooled by the suction gas. Further, the oxygen concentration of the suction gas is preferably 50 volume % or less. In a method in which pure oxygen is supplied into the sinter material packed bed 10 together with the atmosphere outside the hood as the suction gas, when the oxygen concentration of the suction gas is 50 volume %, the ratio of the amount of gas of the pure oxygen to the amount of gas of the atmosphere outside the hood reaches 60 volume % (not included in total mass). The amount of gas sucked into the sinter material packed bed is determined by the permeability of the sinter material packed bed and the power of a sintering machine blower (blower 65), so the amount of suction gas varies in minutes or seconds. Therefore, as the ratio of pure oxygen supply in the suction gas increases, the oxygen concentration of the suction gas is more greatly affected by fluctuations in the amount of the suction gas (the amount of pure oxygen supply is adjusted according to changes in the amount of the suction gas, but it becomes difficult to control). As a result of the difficulty in control, it becomes difficult to provide a stable oxygen concentration of the suction gas to be supplied to the sintered layer.

[0123] More preferably, the oxygen concentration of the suction gas is 40 volume % or less and the separation time is 5 minutes or less. This is because, if the oxygen concentration exceeds 40 volume % or the separation time exceeds 5 minutes, the increase in production rate accompanying the increase in oxygen supply becomes slow and the effect of oxygen enrichment is no longer effective, as shown in Examples below.

[0124] As supported by Examples described below, the oxygen enrichment in the oxygen enrichment gas suction area 7x of the invention has the effect of improving product yield and production rate despite an increase in sintering speed, and the effect of extending the proper separation time. The reason for the improvement in production rate includes that reignition immediately after the oxygen enrichment supplies heat to a coke combustion field that has been activated by the oxygen enrichment, which has the effect of maintaining and continuing the activation of the coke combustion field. Thus, even a short period of oxygen enrichment can lead to an improved product yield and production rate. The reason for the extension of the proper separation time includes that the sintered layer temperature increases due to the activated coke combustion caused by the oxygen enrichment. It is necessary to perform reignition before the sintered layer cools down, but since the oxygen enrichment performed in the oxygen enrichment gas suction area 7x makes the sintered layer less likely to cool down, the effect is exhibited even with a long separation time. A long separation time extends the high-temperature retention time of sintered ore, resulting in an improvement in yield. In contrast, oxygen enrichment without reignition only serves to inhibit a decrease in production rate due to a common sintering speed increase.Fourth Exemplary Embodiment

[0125] Another preferred example in which the oxygen enrichment technique is used in the invention will be described below as a fourth exemplary embodiment. The inventors have considered that it is also effective to perform the oxygen enrichment technique immediately after reignition, in place of performing it in the atmosphere suction area 7. The inventors' aims will be described below, and then a preferred example will be described.

[0126] In the reignition method, an upper part of a high temperature zone (zone roughly 1,000 degrees C. or more) of the sintered layer formed by the initial ignition is heated again by reignition, and the bonding agents (carbonaceous materials) that remained unburned during the initial ignition combust. As illustrated in FIG. 8 described later, combustion reactions occur at two locations in a bed height direction due to 1) the combustion of remaining bonding agents in the combustion zone formed by reignition (hereinafter also referred to as a reignition combustion zone 10A2) and 2) the combustion of bonding agents in the combustion zone formed by the initial ignition (hereinafter also referred to as an initial ignition combustion zone 10A1). In the sintering machine, the atmosphere containing oxygen necessary for combustion is sucked downward and flows vertically from the top to the bottom of the sintered layer in the bed height direction. In order to promote the combustion reactions in the above two locations, a large amount of oxygen is required. Especially when the highly combustible carbonaceous material is used, even if the particles are coarse (with a particle size of 2.8 mm or more being 30 mass % or more and 80 mass % or less), the combustion speed is fast and more oxygen is required.

[0127] Immediately after the end of reignition, in addition to the initial ignition combustion zone 10A1, the reignition combustion zone 10A2 is generated at another location in the bed height direction, and as a result, the high temperature zone expands in the bed height direction. In the reignition combustion zone 10A2, heat propagates through the sinter cake, which has already been sintered by the initial ignition and has a high void ratio, and thus the surface area of the solid (sinter cake) in contact with the gas sucked downward is small. In contrast, in the initial ignition combustion zone 10A1, heat propagates through the sintering material containing fine powder with a particle size of less than 1 mm, and thus the surface area of the solid (sintering material) in contact with the gas sucked downward is large. Therefore, the flame front speed of the reignition combustion zone 10A2 is faster than that of the initial ignition combustion zone 10A1, and the reignition combustion zone 10A2 merges with the initial ignition combustion zone 10A1 at an early stage after the end of reignition (see FIG. 8). The high temperature zone in sintering includes a combustion zone from when the bonding agents start to combust until the combustion is completed, and a zone where the materials are cooled to a temperature (approximately 1,000 degrees C.) at which the metallurgical reaction subsequently continues.

[0128] As described above, since a large amount of oxygen is required in the reignition sintering method, the sintering speed decreases when the downward suction of the atmosphere is normally performed. The inventors thus have considered that implementing the oxygen enrichment technique immediately after reignition in the invention using the reignition method can improve the sintering speed and therefore the production rate. The inventors repeated sintering experiments and found the following preferred example. In addition, the inventors also considered a configuration for optimizing the timing at which the oxygen enrichment starts, the oxygen enrichment time, and the oxygen concentration of the oxygen enrichment gas supplied in an oxygen enrichment area. Those will be described below.

[0129] FIG. 8 schematically illustrates another exemplary DL-type sintering machine according to the exemplary embodiment (oxygen enrichment). The above phenomenon will be described in detail below with reference to FIG. 8. In producing sintered ore using the reignition sintering method, the atmosphere suction area 7 is provided between the ignition furnace 3 and the reignition furnace 4 to supply sufficient oxygen (the atmosphere) to the combustion zone 10A formed on the surface layer of the sinter material packed bed 10 by the initial ignition, and the subsequent reignition causes the combustion of the bonding agents remaining on the surface layer without being ignited. In addition to the merging of the two combustion zones (initial ignition combustion zone 10A1 and reignition combustion zone 10A2) mentioned above, the combustion of the remaining bonding agents due to reignition increases the temperature of the gas sucked in from above and passing through the initial ignition combustion zone 10A1. This promotes the combustion of the bonding agents directly below the initial ignition combustion zone 10A1, and the width of the combustion zone 10A and the high temperature zone further expand. Furthermore, by performing reignition before the sinter cake 10B is cooled by the supply of the atmosphere in the atmosphere suction area 7, the high temperature retention time (e.g., a time retained at 1,200 degrees C. or more) is increased in the upper part of the sinter material packed bed 10. The high temperature retention time can be increased by performing reignition at an appropriate timing to supply heat, which can lead to improved product yield and production rate.

[0130] In the exemplary embodiment, the area where the oxygen enrichment gas is supplied to the surface of the sinter material packed bed 10 (sintered layer) and is sucked downward (oxygen enrichment gas suction area) is a predetermined section after the end of reignition. Thus, a DL-type sintering machine 104 of the exemplary embodiment includes, in addition to the components of the DL-type sintering machine 101 described above, the oxygen enrichment gas supplying unit 9 having a configuration similar to that described in the third exemplary embodiment, downstream of the reignition furnace 4 in the pallet traveling direction. In the exemplary embodiment, the separation time can be set to 0.5 minutes or more and 3.5 minutes or less, for example.

[0131] In FIG. 8, a small interval is provided between the reignition furnace 4 and the oxygen enrichment gas supplying unit 9. However, if there are no safety issues, the reignition furnace 4 and the oxygen enrichment gas supplying unit 9 may be arranged consecutively. The oxygen enrichment gas supplying unit 9 is configured to continuously supply the oxygen enrichment gas for a predetermined time, which is defined as the oxygen enrichment time. Preferably, the time required for the pallet to move through the distance between the reignition furnace 4 (downstream partition wall 42b) and the oxygen enrichment gas supplying unit 9 (upstream wall of the hood 91), i.e., the time from a point of time when reignition is completed to a point of time when oxygen enrichment starts is short. For example, even taking safety issues into consideration, the above-mentioned time is preferably more than 0 seconds and 30 seconds or less, more preferably more than 0 seconds and 10 seconds or less, and still more preferably, oxygen enrichment should be started immediately after the end of reignition (more than 0 seconds and 2 seconds or less). If the above-mentioned time exceeds 30 seconds, a time period during which oxygen enrichment is effective decreases, and the resulting effect decreases (see Test 2 in Example 4 described later).

[0132] When implementing the invention, in the oxygen enrichment gas suction area (the section in the pallet travel direction 5x where the oxygen enrichment gas supplying unit 9 is installed), it is preferable that the oxygen concentration of the suction gas sucked in from the surface layer side of the sintered layer by downward suction is 30 volume % or more, and that the oxygen enrichment time by the oxygen enrichment gas supplying unit 9 after the end of reignition is 30 seconds or more. Here, the oxygen enrichment time refers to a time required for the pallet to move through the oxygen enrichment gas suction area, i.e., a time from the start of oxygen enrichment to the end of oxygen enrichment when the oxygen enrichment gas is supplied after the completion of reignition. If the oxygen concentration of the suction gas is less than 30 volume % or if the oxygen enrichment time is less than 30 seconds, the amount of oxygen supplied is insufficient and neither the sufficient sintering speed nor the effect of improving the production rate is achieved (see Test 2 in Example 4 described later).

[0133] The oxygen concentration of the suction gas in the oxygen enrichment gas suction area is preferably 40 volume % or less. The time for oxygen enrichment by the oxygen enrichment gas supplying unit 9 is preferably 2 minutes or less. This is because, if the oxygen concentration exceeds 40 volume % or the oxygen enrichment time exceeds 2 minutes, the increase in sintering speed and production rate accompanying the increase in oxygen supply becomes slow and the effect of oxygen enrichment is no longer effective. The timing at which the two combustion zones (initial ignition combustion zone 10A1 and reignition combustion zone 10A2) merge was unknown until now. However, based on the proper oxygen enrichment time obtained from the test results (see Test 1 in Example 4 described later), it is estimated that the merging occurs at a timing more than 2 minutes and within 3 minutes after a point of time when reignition is completed.

[0134] In the exemplary embodiment, the sintering speed is significantly increased by the oxygen enrichment after reignition, as supported in Example 4 described later. This significant increase is greater than the increase in sintering speed with the oxygen enrichment without reignition, as described in previous examples (Tetsu-to-Hagane Vol. 92 (2006), pp. 417-426). The reason why the sintering speed is significantly increased is that oxygen is effectively utilized due to the presence of two combustion zones activated by the oxygen enrichment after the end of reignition. Thus, even a short period of oxygen enrichment improves the sintering speed and therefore the production rate.Fifth Exemplary Embodiment

[0135] In the second to fourth exemplary embodiments, the inventors focused on the effect of improving the production rate in the upper part of the sintered layer, whereas the effect of improving the production rate in a lower part of the sintered layer is extremely small. In the exemplary embodiment, as an effect on the lower part of the sintered layer, attention is focused on a stand-support sintering technique effective to increase the firing speed in the lower part of the sintered layer. In the following, the stand-support sintering technique will be described to state the inventors' aims for a preferred example.

[0136] With the stand-support sintering technique, the sinter cake formed in the upper part of the sinter material packed bed is supported by a stand, which reduces the load from above on the lower part of the sinter material packed bed (hereinafter also referred to simply as the lower layer part) during sintering thereof, thereby ensuring voids in the lower layer part. As a result, the air flow resistance in the lower layer part is reduced, and the sintering speed is improved (see JPH04-168234 A). The inventors considered that the production rate of the entire sintered ore is improvable by applying, to the invention, the stand-support sintering technique that has the effect of increasing the production rate of the lower layer part. However, the increase in a combustion zone width of the upper layer part according to the invention also affects the sintering conditions in the lower layer part. As a result of repeated sintering experiments by the inventors, it revealed that a synergistic effect beyond expectations was obtained between the invention and the stand-support sintering technique, as shown in Example 5 described later. In addition, the appropriate range of the separation distance (the distance between the ignition furnace and the reignition furnace) was also examined in a case where the invention was used in combination with the stand-support sintering technique. Those will be described below.Stand-Support Sintering Technique

[0137] FIG. 9 schematically illustrates an exemplary pallet used for the exemplary embodiment (stand-support sintering technique). As illustrated in FIG. 9, a pallet 5 includes a main frame 52 on which a grate bar 51 is disposed, and pallet side walls 53 erected on both opposing end portions of the main frame 52. Further, the pallet 5 has a single stand 16 (support member) disposed in a center portion of an upper surface of the grate bar 51. The stand 16 is a substantially plate-shaped member having a substantially isosceles trapezoidal shape. The stand 16 has a sinter cake supporting surface 16a. The stand 16 is installed parallel to the pallet traveling direction 5x, vertically to be embedded in the sinter material packed bed 10. In the middle of the firing of the sinter material packed bed 10, the upper layer part of the sintered layer (sinter material packed bed 10) in the pallet 5 is the sinter cake 10B where sintering is completed, while the lower layer part (the lower part of the combustion zone 10A) remains as unsintered sintering material, as illustrated in FIG. 9. Here, the upper surface portion (sinter cake supporting surface 16a) of the stand (supporting member) 16 supports the sinter cake 10B in the upper layer part and inhibits compaction of the sintering material of the lower layer part. By inhibiting the compaction, the airflow resistance of the sintered layer during the firing of the lower layer part is reduced, and the sintering speed is improved. Inhibiting the compaction also has the effect of inhibiting uneven flow of gas flowing through the sintered layer, reducing the amount of unfired parts and improving the sintering yield.

[0138] In implementing the invention, the stand-support technique described above is preferably used. That is, for the pallet 5 into which the blended material is to be charged, it is preferable that the support member (stand 16) having the sinter cake supporting surface 16a is erected on the grate bar 51 so as to be embedded in the sinter material packed bed 10. In the exemplary embodiment, any arrangement of the support member (stand 16) is usable as long as the support member can support the sinter cake 10B formed in the upper layer part, and for example, two rows of support stands may be installed in the pallet width direction. By providing multiple support members (stands 16), the sinter cake 10B formed in the upper layer part is supported and the load on the unfired lower layer part is reduced during the sintering of the lower layer part. This ensures voids in the lower layer part and uniformalizes the gas flow speed in the pallet width direction.

[0139] The effect of applying the stand-support sintering technique to the invention using the reignition sintering method is considered to be as follows.Product Yield

[0140] As features of the stand-support sintering technique, the improvement in sintering speed and product yield is limited to the lower layer part. In the exemplary embodiment, the time (separation time) required for each pallet to pass through the section between the ignition furnace 3 and the reignition furnace 4 (atmosphere suction area 7) can be set, for example, to 0.5 minutes or more and 3.5 minutes or less, and preferably less than 3.0 minutes (see Table 12 described later). In combination with the reignition sintering method that has the yield improvement effect on the upper layer part, it is possible to obtain a yield improvement effect on the entire part from the upper part to the lower part, provided that the separation time is within the above predetermined range (less than 3.0 minutes). From the above, additivity holds in terms of product yield. When the separation time exceeds 3.0 minutes, the effect of the yield improvement provided by reignition slows down, and the product yield starts to deteriorate due to the shortened sintering time by using the stand support.Sintering Speed

[0141] In the reignition sintering method, the combustion zone width of the sintered layer is increased by performing ignition twice. Increasing the combustion zone width of the sintered layer leads to improved product yield in the upper part of the sintered layer. However, the combustion zone width is sufficiently secured even with single-stage ignition sintering in the lower part of the sintered layer, so no further improvement can be expected in the lower part. On the contrary, an increase in the combustion zone width in the lower part of the sintered layer leads to an increase in airflow resistance, decreasing the sintering speed (BTS: Burn through speed). In particular, when using the highly combustible carbonaceous material, even if the particles are coarse (with a particle size of 2.8 mm or more being 30 mass % or more and 80 mass % or less), the combustion start temperature is low and the combustion speed is fast, and thus the combustion zone width is large. Here, the use of the stand-support sintering technique allows the sinter cake in the upper part to be supported, reducing the increase in airflow resistance during the sintering of the lower part and improving the sintering speed in the lower part. The increase in the sintering speed in the lower part inhibits the increase in the combustion zone width in the lower part, thereby improving the sintering speed acceleratingly. From the above, a synergistic effect is achieved in terms of sintering speed.Production Rate

[0142] The production rate is proportional to the product of the product yield and the sintering speed. The additive effect in product yield and the synergistic effect in sintering speed as described above provide a synergistic effect that exceeds expectations between the reignition sintering method and the stand-support sintering technique (see Example 5 described later).EXAMPLES

[0143] Tests relating to the above-described invention and preferred configurations, each applicable to the invention, will be described below. Examples 1 to 5 correspond to the first to fifth exemplary embodiments, respectively. In addition, Example 4 is described as being divided into two tests (Test 1 and Test 2).

[0144] In all Examples (Examples 1 to 5), the blending ratio of the sintering materials was the same except for the bonding agents (see Table 4 described later). Regarding the downward suction conditions (suction pressure or air volume) during sintering, the measured value under the pot was constant at 1,300 mmAq (12.7 kPa) in Examples 1 and 5, and the air volume (exhaust gas) was constant at 1.80 Nm3 / min in Examples 3 and 4. In Example 2, the air volume (exhaust gas) was basically set at 1.80 Nm3 / min, and the air volume control was performed according to the test conditions described later.Example 1Test Cases

[0145] Tests relating to the conditions for the composition ratio of the carbonaceous materials (low combustible carbonaceous material and highly combustible carbonaceous material), the average particle size of the low combustible carbonaceous material, and the particle size of the highly combustible carbonaceous material described in the first exemplary embodiment will be described below. In the following tests, only coke breeze is used as the low combustible carbonaceous material, but the same effect can be obtained by using both anthracite and coke, or only anthracite, as the low combustible carbonaceous material. In the following tests, only semi-coke or only the carbonized-wood compression-molded product (crushed product) is used as the highly combustible carbonaceous material. The invention, however, is not limited thereto, and the same effect can be obtained, for example, by using any one of the other highly combustible carbonaceous materials or by mixing multiple types of highly combustible carbonaceous materials.

[0146] In Examples, the tests were performed using a method commonly known as the sinter pot test. In the sinter pot test, sintering materials (blended material) containing the bonding agents as fuel are charged into a container of a specified size, and the sintering process is promoted by performing ignition from above and performing suction from below. Although a device for the sinter pot test does not use a pallet to move the sinter material packed bed as in the Dwight-Lloyd (DL) type sinter machine, it can simulate sintering performed using the DL-type sinter machine. As shown in Table 6 below, 17 tests were performed, including Comparatives 1-0 to 1-3 and Examples 1-1 to 1-13. First, the materials and methods for the tests will be described, and then the test results will be described.Materials

[0147] Table 3 shows the results of industrial analysis and elemental analysis of the low combustible carbonaceous material and highly combustible carbonaceous materials used in the tests. As shown in Table 3, coke breeze was prepared as the low combustible carbonaceous material, and semi-coke and the carbonized-wood compression-molded product (crushed) were prepared as the highly combustible carbonaceous materials.TABLE 3Industrial analysis (mass %-dry)Apparent AshVolatile matterFixedElemental analysis (mass %)densitycontentcontentcarbonCHNTotal-S(g / cm3)LowCoke breeze12.51.186.484.2<0.101.30.61.3combustiblecarbonaceousmaterialHighlySemi-coke5.55.189.588.60.81.10.21.2combustibleCarbonized-wood2.68.289.290.71.370.750.020.9carbonaceouscompression-moldedmaterialproductBlend of Materials

[0148] Tables 4 and 5 show the blending ratios (mass %) of the sintering materials for each blended material used in the tests. As shown in Table 4, the proportions of new materials (iron ore, limestone, quicklime, and olivine) and return ore were kept constant for all test cases. Iron ores A to E are mutually different brands (production areas) of iron ore. The blending ratio of the return ore was set to 15.0 mass % (not included in total mass) with the new materials (iron ore, limestone, quicklime, and olivine) at 100 mass %. The blending ratios of the materials, excluding the bonding agents, are the same in Examples 1 to 5.

[0149] Table 5 shows the blending ratios (mass %) of the bonding agents in the blended material used in the tests. Coke breeze was used as the low combustible carbonaceous material in all test cases. As the highly combustible carbonaceous material, the crushed carbonized-wood compression-molded product was used in Examples 1-12 and 1-13, and semi-coke was used in the other test cases. In the test cases (Comparatives 1-0 and 1-2) in which only the low combustible carbonaceous material (coke breeze) was used as the bonding agent, the blending ratio of the bonding agent (coke breeze) was set to 4.5 mass % (not included in total mass) with the new materials at 100 mass %. In the test cases (except for Comparatives 1-0 and 1-2) in which the highly combustible carbonaceous material (semi-coke or the carbonized-wood compression-molded product) was blended, using the blending amount of the bonding agent (coke breeze 4.5 mass %) in Comparatives 1-0 and 1-2 as a basis, the blending ratios (middle part of Table 5) of the bonding agents (low combustible carbonaceous material and highly combustible carbonaceous material) to the new materials were adjusted so that the fixed carbon content (carbon content) of all bonding agents (coke breeze, semi-coke, the carbonized-wood compression-molded product) contained in the blended material and determined by industrial analysis was kept constant in all test cases, based on the mass ratio (upper part of Table 5) of the carbon content of the low combustible carbonaceous material (coke breeze) and the highly combustible carbonaceous material (semi-coke, the carbonized-wood compression-molded product) and the fixed carbon content (Table 3) of the low combustible carbonaceous material (coke breeze) and the highly combustible carbonaceous material (semi-coke, the carbonized-wood compression-molded product). The blending ratios of the low combustible carbonaceous material and the highly combustible carbonaceous material to the total amount of bonding agents are shown in the lower part of Table 5.TABLE 4New materialsReturn oreIron ore(non-Bonding agentsABCDELimestoneQuicklimePeridotiteTotalinclusive)(non-inclusive)Brands14.719.619.613.814.712.72.941.96100.015.0See middle part ofTable 5TABLE 5Com-Com-Comparative 1-3parativesparativeExample Examples 1-2, ExampleExampleExample1-0, 1-21-11-11-4 to 1-111-31-121-13Low combustible carbonaceous materialCoke breeze—Coke breezeCoke breezeCoke breezeCoke breezeCoke breezeHighly combustible carbonaceous material—Semi-cokeSemi-cokeSemi-cokeSemi-cokeCarbonized-Carbonized-woodwoodcompression-compression-molded molded productproductCarbon content mass Low combustible10002550758075ratio (mass %)carbonaceous(Carbon equivalentmaterialblending ratio)Highly combustible01007550252025carbonaceousmaterialBlending ratio of bondingLow combustible4.5001.132.253.383.603.38agent(s) to new materialscarbonaceous(non-inclusive, mass %)materialHighly combustible04.343.262.171.090.871.09carbonaceousmaterialTotal (sum total)4.504.344.384.424.464.474.46Blending ratio toLow combustible100025.750.975.780.575.6all bonding agentscarbonaceous(mass %)materialHighly combustible010074.349.124.319.524.4carbonaceousmaterialThe test conditions are shown in the upper part of Table 6 below. The carbon content mass ratio in Table 6 is the same as the carbon content mass ratio in the upper part of Table 5. As shown in Tables 5 and 6, Comparative 1-0 is a case in which no reignition was performed and only the low combustible carbonaceous material (coke breeze) was used as the bonding agent without blending the highly combustible carbonaceous material. In addition, Examples 1-12 and 1-13 are cases in which the crushed carbonized-wood compression-molded product was used as the highly combustible carbonaceous material.

[0151] As shown in Table 6, the average particle size of the low combustible carbonaceous material (see paragraph 0046) was 0.6 mm, 0.8 mm, 1.2 mm, and 1.4 mm in Examples 1-6 to 1-9, respectively, 1.2 mm in Examples 1-12 and 1-13, and 1.0 mm in the other test cases. The ratio of the highly combustible carbonaceous material with a particle size of 2.8 mm or more (the ratio of the highly combustible carbonaceous material with a particle size of 2.8 mm or more to the whole highly combustible carbonaceous material) was 20 mass %, 50 mass %, and 70 mass % in Comparative 1-3, Examples 1-4, and Examples 1-5, respectively, 80 mass % in Examples 1-12 and 1-13, and 30 mass % in the other test cases. In Examples 1-11 and 1-13, the entire amount of only the low combustible carbonaceous material out of the sintering materials was added in the latter half of the granulation step of the sintering materials. In addition, in Examples 1-10 and 1-13, the segregation-strengthened type charging device was used to charge the blended material after the granulation process.TABLE 6Comp. Comp.Comp.Comp.1-01-1Ex. 1-1Ex. 1-2Ex. 1-31-21-3Ex. 1-4Ex. 1-5Low combustibleCarbon content1000255075100505050carbonaceousmass ratiomaterial(mass %)(coke breeze)Average particle1.0—1.01.01.01.01.01.01.0size (mm)GranulationIn bulkIn bulkIn bulkIn bulkIn bulkIn bulkIn bulkIn bulkIn bulkmethodHighly combustibleCarbon content01007550250505050carbonaceousmass ratiomaterial(mass %)(semi-coke,+2.8 mm—30303030—205070carbonized-wood(mass %)compression-moldedproduct only inExamples 1-12 and1-13)Charging deviceNormalNormalNormalNormalNormalNormalNormalNormalNormalReignition-separation time (min)(no 1.01.01.01.01.01.01.01.0reignition)—Sintering speed (BTS) (mm / min)16.218.818.217.817.716.017.018.318.7Production rate (t / (Dm2))26.929.031.030.530.327.629.331.131.8Product yield (mass %)70.966.873.773.873.973.574.373.573.5Reduction degradation (mass %)34.029.727.827.528.033.031.927.827.8Ex. Ex. Ex. Ex. 1-6Ex. 1-7Ex. 1-8Ex. 1-9Ex. 1-101-111-121-13Low combustibleCarbon content5050505050508075carbonaceousmass ratiomaterial(mass %)(coke breeze)Average particle0.60.81.21.41.01.01.21.2size (mm)GranulationIn bulkIn bulkIn bulkIn bulkIn bulkPost-In bulkPost-methodadditionadditionHighly combustibleCarbon content5050505050502025carbonaceousmass ratiomaterial(mass %)(semi-coke,carbonized-wood+2.8 mm3030303030308080compression-molded(mass %)product only inExamples 1-12 and1-13)Charging deviceNormalNormalNormalNormalSegregationNormalNormalSegregationReignition-separation time (min)1.01.01.01.01.01.01.01.0Sintering speed (BTS) (mm / min)17.317.617.818.318.319.217.719.4Production rate (t / (Dm2))29.830.230.330.331.433.130.533.5Product yield (mass %)74.574.073.272.074.174.374.374.3Reduction degradation (mass %)28.127.627.327.927.627.126.926.9Granulation Method

[0152] The granulator used was a cylindrical batch-type drum mixer (rotation speed: 25 rpm) with a diameter of 600 mm and a length of 800 mm. In the test cases performing the bulk granulation other than Examples 1-11 and 1-13, all sintering materials including the new materials, the bonding agents, and returned ore were put into the granulator and mixed for 4 minutes, after which water was added (humidity adjustment) to reach a target moisture value, and the mixture was mixed (granulated) for another 4 minutes. In Examples 1-11 and 1-13 (cases in which only the low combustible carbonaceous material was post-added), the sintering materials excluding the low combustible carbonaceous material were put into the granulator and mixed for 4 minutes, after which water was added (humidity adjustment) to reach a target moisture value and mixed (granulated) for 3 minutes 45 seconds. The granulator was then stopped once, and the low combustible carbonaceous material (coke breeze) was added and mixed (granulated) for another 15 seconds (see paragraph 0056). The final target moisture value was set to 7.5 mass % (not included in total mass) based on the mass of the whole sintering material including all bonding agents and return ore.Charging Method

[0153] The pot test device used was a cylindrical pot with a diameter of 300 mm and a height of 500 mm. In Examples 1-10 and 1-13 (cases of segregation strengthened charging), as described below, charging was performed according to a method using a slit-bar type classifier described in CAMP-ISIJ 24 (2011), p. 795 (Hara et al.), simulating segregation strengthened charging using the above-mentioned segregation-strengthened type charging device (slit-bar type charging device, see paragraph 0051). In the test cases of normal charging other than Examples 1-10 and 1-13, a flat plate was used instead of the slit-bar type classifier.Segregation Strengthened Charging

[0154] In Examples 1-10 and 1-13, blended material granules classified using a slit-bar type classifier were charged into a sinter pot. Specifically, the blended material granules were placed in the slit-bar type classifier, and the blended material granules that fell from between slit bars were collected in four collection boxes arranged from upstream to downstream directly below the slit bars, and the blended material granules that slid on the slit bars were collected in another collection box. For the blended material granules classified and collected in the five collection boxes, the blended material granules that had slid on the slit bars were first dropped into the pot from thereabove, and then the blended material granules that had fallen from between the slit bars were dropped into the pot from thereabove in order from the most downstream collection box.Firing Conditions

[0155] After the blended material granules were charged, a surface of the sinter material packed bed was initially ignited (initial ignition) for 1.0 minute using an ignition device (flame heating). After 1.0 minute had elapsed from the end of ignition, reignition was performed for 1.0 minute. The separation time of 1.0 minute from the end of ignition to the start of reignition corresponds to 4% of the length of the entire strand when converted into the actual machine. The ignition time and the reignition time were each 1.0 min (heat amount: as sensible heat of the suction gas, 25 MJ / ton of blended material). In addition, since heat loss is large in the pot test, approximately 100 MJ / ton of blended material is required for the heating amount of LPG gas. The negative suction pressure during sintering was adjusted by changing the valve opening on the suction side of the blower so that it was constant at 1,300 mmAq (12.7 kPa) as measured under the pot.

[0156] Along with the pressure, the temperature was also measured using a thermocouple under the pot. In sintering, when the combustion zone reaches the lowermost part of the sinter material packed bed, the exhaust gas temperature under the pot starts to rise, eventually reaches a peak, and then decreases as the coke combustion is completed. Three minutes after the exhaust gas temperature peaked, the blower suction was stopped. The sintering time was determined as a time from a point of time when ignition started to a point of time when the exhaust gas temperature peaked. The sintering speed (BTS: burn through speed) was calculated by dividing the thickness of the material layer by the sintering time.Product Yield

[0157] After sintering, the resulting sinter cake was dropped four times from a height of 2 m, and the sintered ore with a particle size of +5 mm (exceeding 5 mm) excluding bedding ore was collected as a sintered product, and its mass was calculated and determined as the mass of the sintered product. A value obtained by dividing the mass of the sintered product by the mass of the sinter cake excluding bedding ore, was defined as the product yield.Production Rate

[0158] The production rate was calculated by dividing the mass of the sintered product (product amount (ton)) by the above-described sintering time (converted into days) and the firing area (pot bottom area (m2)), as represented by the following formula (3).Production⁢ Rate⁢ (t / (Dm2))=Product⁢ Amount⁢ (t) / {Sintering⁢ Time⁢ (Day)·Pot⁢ Bottom⁢ Area⁢ (m2)}Formula⁢ (3)Reduction Degradation

[0159] Reduction degradation of sintered ore indicates a degree of degradation of sintered ore under conditions simulating a low-temperature reduction zone of a blast furnace, and was performed using a method defined in the JIS method (JIS M8720:2017 “Iron ores-Determination of low-temperature reduction-disintegration”). That is, 500 g of the obtained sintered ore having a particle size of more than 15 mm and less than 20 mm was reduced for 30 minutes at 550 degrees C. by passing a gas of CO (30 vol %)-N2 (70 vol %) composition at 15 liters / min. Then, it was rotated at 30 rpm for 30 minutes in a cylindrical rotary drum (130 mmφ×200 mmL), and the ratio of the particle size-2.8 mm (mass ratio of iron ore under the sieve of 2.8 mm opening size) was used as an index of reduction degradation (RDI: reduction degradation index).Test Results

[0160] The test results are shown in the lower columns of Table 6. The cases in which the production rate was 29.5 t / (Dm2) or more in the test results are Examples of Example 1.

[0161] FIG. 10 is a graph illustrating the test results of Comparative 1-1 (using only the highly combustible carbonaceous material as the bonding agent), Examples 1-1 to 1-3 (carbon content mass ratio of the highly combustible carbonaceous material: 75 mass %, 50 mass %, and 25 mass %), Comparative 1-3 (carbon content mass ratio of the highly combustible carbonaceous material: 50 mass %), Examples 1-4 and 1-5 (carbon content mass ratio of the highly combustible carbonaceous material: 50 mass %), and Examples 1-12 and 1-13 (carbon content mass ratio of the highly combustible carbonaceous material: 20 mass % and 25 mass %, highly combustible carbonaceous material: carbonized-wood compression-molded product) in Table 6, the graph illustrating a relationship between the ratio of the highly combustible carbonaceous material with the particle size+2.8 mm and the production rate. FIG. 11 is a graph illustrating the test results of Comparative 1-0 (single-stage ignition), Comparatives 1-1 and 1-2 (using only the highly combustible carbonaceous material as the bonding agent, and using only the low combustible carbonaceous material as the bonding agent), Comparative 1-3 (ratio of semi-coke with a particle size of 2.8 mm or more: less than 30 mass %), Examples 1-1 to 1-5 (average particle size of coke breeze: 1.0 mm, highly combustible carbonaceous material: semi-coke), and Examples 1-12 and 1-13 (average particle size of coke breeze: 1.2 mm, highly combustible carbonaceous material: carbonized-wood compression-molded product) in Table 6, the graph illustrating a relationship between the mass ratio (mass %) of the carbon content of the highly combustible carbonaceous material to the carbon content of the bonding agent(s) and the production rate (t / (Dm2)). FIG. 12 is a graph illustrating the test results of Examples 1-6 to 1-11 in Table 6 (carbon content mass ratio of semi-coke: 50 mass %, ratio of semi-coke with a particle size of 2.8 mm or more: 30 mass %), the graph illustrating a relationship between the average particle size (mm) of the coke breeze and the production rate (t / (Dm2)).

[0162] As shown in Table 6 and FIGS. 10, 11, and 12, the production rate was improved in Examples 1-1 to 1-13, in which the mass ratio of the carbon content of the highly combustible carbonaceous material to the carbon content of all bonding agents was in a range from 25 mass % to 75 mass %, and the ratio of the highly combustible carbonaceous material (semi-coke) with a particle size of 2.8 mm or more was in a range from 30 mass % to 80 mass %. Since the carbonized-wood compression-molded product has a low combustion start temperature, the production rate increases similarly to semi-coke. Further, since the combustion start temperature of the carbonized-wood compression-molded product is lower than that of semi-coke, even when the mass ratio of the carbon content of the highly combustible carbonaceous material to the carbon content of all bonding agents was low at 20 mass % (Example 1-12), the production rate was equivalent to that of semi-coke at 25 mass % (Example 1-3). In addition, as shown in Table 6 and FIG. 12, in Examples 1-7 and 1-8 in which the average particle size of the coke breeze (low combustible carbonaceous material) was 0.8 to 1.2 mm, the production rate and reduction degradation were further improved (compared with Examples 1-6 and 1-9). In Example 1-10 in which segregation-strengthened charging was performed, the product yield was greatly improved, and therefore the production rate was further improved. In Example 1-11 in which coke breeze was post-added, the flame front speed (sintering speed) and the product yield were significantly improved, and therefore the production rate was significantly improved. By comparing Comparative 1-0 (single-stage ignition sintering method) with Comparative 1-2 (reignition sintering method), it was confirmed that performing the reignition improved the product yield while maintaining the sintering speed, thereby improving the production rate.Example 2Test Cases

[0163] In Example 2, the same sinter pot test as in Example 1 was used to verify the applicability of the air volume control technique to the invention. The test conditions and the like different from those of Example 1 for test cases of Example 2 (Comparatives 2-1 to 2-2, and Examples 2-1 to 2-3) will be described below with reference to Table 7 described later. In the following description, duplicated descriptions of the conditions, test methods, and the like that are same as those in Example 1 will be omitted as appropriate.Blending of Materials, etc.

[0164] The materials blended were the same as those in Example 1 (Table 4), except for the bonding agents. The blending of the bonding agents (carbonaceous materials) in the sintering material and the particle size of the bonding agents (the average particle size of the low combustible carbonaceous material and the ratio of the highly combustible carbonaceous material with a particle size of 2.8 mm or more) were the same as those in Comparative 1-2 or Example 1-2 of Example 1. Specifically, as shown in Tables 5 to 7, in Comparative 2-1 to 2-2, as with the former (Comparative 1-2), only 4.5 mass % (not included in total mass) of coke breeze (low combustible carbonaceous material) was used relative to the new materials (carbon content mass ratio of bonding agents: 0 mass % for semi-coke and 100 mass % for coke breeze), and the average particle size of coke breeze was 1.0 mm. In Examples 2-1 to 2-3, as with the latter (Example 1-2), the carbon content mass ratio was 50 mass % coke breeze (low combustible carbonaceous material) and 50 mass % semi-coke (highly combustible carbonaceous material), the average particle size of coke breeze was 1.00 mm, and the ratio of the highly combustible carbonaceous material with a particle size of 2.8 mm or more was 30 mass %. In all test cases of Example 2, the granulation method was bulk granulation and the charging method was normal charging.Firing Conditions

[0165] In all test cases of Example 2, the ignition time for ignition (corresponding to ignition by the ignition furnace 3 (initial ignition)) and the ignition time for reignition (corresponding to reignition by the reignition furnace 4) were each 1 minute (heat amount: as sensible heat of the suction gas, 25 MJ / ton of blended material). As shown in the column for the interval between two ignitions in Table 7, the point of time when reignition was started was set to 1 minute after the point of time when ignition was completed (separation time: 1 minute).

[0166] The suction air volume was adjusted by changing the valve opening on the suction side of the blower so that the superficial air volume inside the pot was one of Condition 0 to Condition 2 below.

[0167] Condition 0: Air volume control OFF, constant air volume of 1.80 Nm3 / min

[0168] Condition 1: Air volume control ON1, air volume of 1.35 Nm3 / min until completion of reignition, then air volume of 1.80 Nm3 / min

[0169] Condition 2: Air volume control ON2, air volume of 1.20 Nm3 / min until completion of reignition, then air volume of 1.80 Nm3 / min

[0170] As shown in the column of air volume control in Table 7, Comparatives 2-1 and 2-2 were set to Condition 0 (OFF) and Condition 2 (ON2), respectively. Also, the test was performed under Condition 0 (OFF) in Example 2-1, Condition 1 (ON1) in Example 2-2, and Condition 2 (ON2) in Example 2-3.TABLE 7Test conditionsMass ratio of a: Superficialcarbon content of air volumeb: SuperficialTest resultsbonding agentsInterval (until air volumeSintering(mass %)betweenAir completion(after completionspeedProductProduction(Coke breeze:ignition andvolumeof reignition)of reignition)(BTS)yieldrateSemi-coke)reignitioncontrol(Nm3 / min)(Nm3 / min)a / b(mm / min)(mass %)(t / (Dm2))Comp. 2-1100:01 minuteOFF1.801.801.019.476.035.1Comp. 2-2100:01 minuteON21.201.800.718.978.935.4Ex. 2-150:501 minuteOFF1.801.801.020.776.236.9Ex. 2-250:501 minuteON11.351.800.820.478.237.2Ex. 2-350:501 minuteON21.201.800.720.279.137.3Test Results

[0171] The right columns of Table 7 show the test results (sintering speed, product yield, and production rate for each test case). The cases in which the production rate was 36.0 t / (Dm2) or more in the test results are Examples of Example 2. The sintering speed, product yield, and production rate were determined in the same manner as in Example 1. FIG. 13 is a graph illustrating the test results, illustrating a relationship between the superficial air volume ratio (a / b) and the production rate (t / (Dm2)).

[0172] As shown in Table 7 and FIG. 13, Examples 2-1 to 2-3 in which the highly combustible carbonaceous material was used, had a higher production rate than Comparatives 2-1 and 2-2 in which no highly combustible carbonaceous material was used. In addition, in Example 2-2 (superficial air volume: 1.35 Nm3 / min) and Example 2-3 (superficial air volume: 1.20 Nm3 / min) where the air volume was reduced in the section up to the completion of reignition, the product yield was improved significantly, which compensated for the decrease in sintering speed more than Example 2-1 where no air volume reduction was performed. The production rate of Examples 2-2 and 2-3 were increased by 0.3 t / (Dm2) and 0.4 t / (Dm2), respectively.Example 3Test Cases

[0173] In Example 3, the same sinter pot test as in Example 1 was used to verify the applicability of the oxygen enrichment technique to the invention. The test conditions and the like different from those of Example 1 for test cases of Example 3 (Examples 3-1 to 3-10) will be described below with reference to Table 8 described later. In the following description, duplicated descriptions of the conditions, test methods, and the like that are same as those in Example 1 will be omitted as appropriate.Blending of Materials, etc.

[0174] The materials blended were the same as those in Example 1 (Table 4), except for the bonding agents. In all test cases of Example 3 (except for Comparative 2-1 shown for reference in Table 8), the blending of the bonding agents (carbonaceous materials) in the blended material and the particle size of the bonding agents (the average particle size of the low combustible carbonaceous material and the ratio of the highly combustible carbonaceous material with a particle size of 2.8 mm or more) were the same as those in Example 1-2 of Example 1. In all test cases of Example 3, the granulation method was bulk granulation and the charging method was normal charging. Semi-coke was used as the highly combustible carbonaceous material.Firing Conditions

[0175] In all test cases of Example 3, the ignition time for ignition (corresponding to ignition by the ignition furnace 3 (initial ignition)) and the ignition time for reignition (corresponding to reignition by the reignition furnace 4) were each 1 minute (heat amount: as sensible heat of the suction gas, 25 MJ / ton of blended material). As shown in Table 8, for each test case (Examples 3-1 to 3-10), the time (separation time) between ignition and reignition (corresponding to the movement through the oxygen enrichment gas suction area) and the oxygen concentration of the suction gas were changed to the levels indicated as the test conditions in Table 8.

[0176] During firing after ignition, the suction air volume was kept constant and the air volume was adjusted to 1.80 Nm3 / min of exhaust gas. As in Examples 1 and 2, suction by the blower was stopped 3 minutes after the exhaust gas temperature reached its peak, and firing was terminated. The sintering time was determined as a time from the point of time when ignition started to the point of time when the exhaust gas temperature peaked.

[0177] In Examples 3-1 and 3-4, oxygen enrichment was not performed, and the atmosphere (the air) was used as the suction gas. An oxygen injection method in Examples 3-2 to 3-3 and 3-5 to 3-10 was as follows: the atmosphere and oxygen were mixed in a gas blender to provide a predetermined oxygen concentration (oxygen concentration in Table 8), and then this mixed gas was supplied for a predetermined time (oxygen enrichment time in Table 8) using a hood placed over the pot. The supply and suction of oxygen enrichment gas was limited to a period from immediately after the completion of ignition (more than 0 seconds and 2 seconds or less after the point of time when ignition was completed) to immediately before the reignition (more than 0 seconds and 2 seconds or less before the point of time when reignition was started). Specifically, when ignition was completed, the hood was set and the oxygen enrichment gas with a predetermined oxygen concentration was supplied, and this oxygen enrichment gas was suctioned. After a predetermined time, the hood was immediately removed and reignition was performed. Here, the flow rate was determined so that an inlet-side gas volume (=suction gas volume) was all mixed gas via the gas blender. The inlet-side gas volume is determined from a nitrogen concentration of the inlet-side gas, a nitrogen concentration of the exhaust gas, and a volume of the exhaust gas. This is because the amounts of nitrogen gas on the inlet and outlet sides are equal. Furthermore, the nitrogen concentration of the exhaust gas can be calculated by subtraction (N2≈100−(CO+CO2+O2)) from the exhaust gas analysis (CO, CO2, O2). In Example 3, the volume of the exhaust gas was kept constant, so that the suction gas was easily adjusted.TABLE 8Test conditionsMass ratio of carbon contentSeparation timeof bonding agents (mass %)(= oxygenTest results(Low combustible carbonaceousenrichmentOxygenSinteringProductProductionmaterial:Highly combustibletime)concentrationspeedyieldratecarbonaceous material)(min)(vol. %)(mm / min)(mass %)(t / (Dm2))Ex. 3-150:500.52120.676.737.0Ex. 3-250:500.53020.877.837.9Ex. 3-350:500.54020.977.938.2Ex. 3-450:501.02120.776.236.9Ex. 3-550:501.03021.078.538.7Ex. 3-650:502.03021.778.740.2Ex. 3-750:502.04022.379.041.3Ex. 3-850:505.04022.679.141.5Ex. 3-950:505.05022.579.141.4Ex. 3-1050:506.04022.679.041.5Comp. 2-1100:01.02119.476.035.1Test Results

[0178] The right columns of Table 8 show the test results (sintering speed, product yield, and production rate) for each test case. The cases in which the production rate was 36.0 t / (Dm2) or more in the test results are Examples of Example 3. The sintering speed, product yield, and production rate were determined in the same manner as in Example 1. FIG. 14 is a graph illustrating the test results, illustrating a relationship between the oxygen concentration (vol. %) of the suction gas during the separation time (corresponding to a time period for the pallet to pass through the oxygen enrichment gas suction area 7x of the actual machine) and the production rate (t / (Dm2)).

[0179] As shown in the test results of Examples 3-1 to 3-3, when the separation time was set to 0.5 minutes, the increase in sintering speed, product yield, and production rate provided by the increase in oxygen concentration (oxygen concentration: Example 3-1<Example 3-2<Example 3-3) was slow. This is probably because the separation time was short, and the oxygen enrichment time was short, only 0.5 minutes.

[0180] On the other hand, as shown in the test results of Example 3-4 and Example 3-5, when the separation time was set to 1.0 minute, the product yield and production rate were significantly increased by increasing the oxygen concentration from 21 volume % to 30 volume % (oxygen concentration: Example 3-4<Example 3-5).

[0181] As shown in the test results of Example 3-5 and Example 3-6, the sintering speed, product yield, and production rate were significantly increased by increasing the separation time from 1.0 minute to 2.0 minutes (separation time: Example 3-5<Example 3-6) at the same oxygen concentration (30 volume %). This is the effect of increasing the oxygen enrichment time associated with the increase in separation time.

[0182] As shown in the test results of Example 3-6 and Example 3-7, when the separation time was set to 2.0 minutes, the sintering speed and production rate were significantly increased by increasing the oxygen concentration from 30 volume % to 40 volume % (oxygen concentration: Example 3-6<Example 3-7).

[0183] Furthermore, as shown in the test results of Example 3-7 and Example 3-8, the sintering speed, product yield, and production rate were improved by increasing the separation time from 2 minutes to 5 minutes (separation time: Example 3-7<Example 3-8).

[0184] However, as shown in the test results of Example 3-8 to Example 3-10, when the oxygen concentration exceeded 40 volume % (oxygen concentration: Example 3-8<Example 3-9), or when the separation time exceeded 5 minutes (separation time: Example 3-8<Example 3-10), the improvements in sintering speed, product yield, and production rate reached a plateau.Example 4Test Cases

[0185] In Example 4, the same sinter pot test as in Example 1 was used to verify the applicability of the oxygen enrichment technique to the invention. A description will be made below about the test conditions and test results of Test 1 (a total of 15 test cases, Examples 4-1 to 4-15, see Table 9) and Test 2 (a total of 6 test cases, Examples 4-16 to 4-21, see Table 11) using Tables 9 to 11 described later. Example 4-1 in Table 9 described later is the same test case as Example 2-1. In addition, Table 9 shows Comparative 2-1 of Example 2 for reference, and Table 11 shows Example 4-10 of Test 1 for reference. In the following description, duplicated descriptions of the conditions, test methods, and the like that are same as those in Example 1 will be omitted as appropriate.Test 1Blending of Materials, etc.

[0186] The materials blended were the same as those in Example 1 (Table 4), except for the bonding agents. In all test cases of Example 4 (Test 1 and Test 2 (Examples 4-1 to 4-21)), the blending of the bonding agents (carbonaceous materials) in the blended material and the particle size of the bonding agents (the average particle size of the low combustible carbonaceous material and the ratio of the highly combustible carbonaceous material with a particle size of 2.8 mm or more) were the same as those in Example 1-2 of Example 1. In all test cases of Example 4, the granulation method was bulk granulation and the charging method was normal charging.Firing Conditions

[0187] In all test cases of Example 4, the combustion time for ignition (corresponding to ignition by the ignition furnace 3 (initial ignition)) and the combustion time for reignition (corresponding to reignition by the reignition furnace 4) were each 1 minute (heat amount: as sensible heat of the suction gas, 25 MJ / ton of blended material). As shown in Table 9, the interval between ignition and reignition (separation time: corresponding to the time required to move through the atmosphere suction area 7) was set to 1 minute. The oxygen enrichment time and oxygen concentration were changed to the levels indicated as test conditions, with oxygen enrichment starting immediately after the completion of reignition (more than 0 seconds and 2 seconds or less after the point of time when reignition was completed).

[0188] During firing after ignition, the air volume was kept constant and the air volume was adjusted to 1.80 Nm3 / min of exhaust gas. As in Examples 1 to 3, suction by the blower was stopped 3 minutes after the exhaust gas temperature reached its peak, and firing was terminated. The sintering time was determined as a time from the point of time when ignition started to the point of time when the exhaust gas temperature peaked.

[0189] In Example 4-1, oxygen enrichment was not performed, and the atmosphere (the air) was used as the suction gas. An oxygen injection method in Examples 4-2 to 4-15 was as follows: the atmosphere and oxygen were mixed in a gas blender to provide a predetermined oxygen concentration (oxygen concentration in Table 9) as in Example 3, and then this mixed gas was supplied for a predetermined time (oxygen enrichment time in Table 9) using a hood placed over the pot. Specifically, when reignition was completed, the hood was set and the oxygen enrichment gas with a predetermined oxygen concentration was supplied, and this oxygen enrichment gas was suctioned. After a predetermined time, the hood was immediately removed. Here, the flow rate was determined so that the inlet-side gas volume (=suction gas volume) was all mixed gas via the gas blender.TABLE 9Test conditionsOxygenenrichment timeMass ratio of carbon content(immediately Test resultsof bonding agents (mass %)afterSinteringCommon condition(Low combustible carbonaceouscompletion ofOxygenspeedProductProductionSeparation time:material:Highly combustiblereignition)concentration(mm / yieldrate1.0(min)carbonaceous material)(min)(vol. %)min)(mass %)(t / (Dm2))Ex. 4-1 (Ex. 2-1)50:500.002120.776.236.9Ex. 4-250:500.253020.976.437.5Ex. 4-350:500.254021.076.337.6Ex. 4-450:500.503021.576.638.5Ex. 4-550:500.504021.976.439.2Ex. 4-650:501.003022.376.339.9Ex. 4-750:501.003522.676.140.3Ex. 4-850:501.004022.876.040.6Ex. 4-950:501.005022.975.840.7Ex. 4-1050:501.503522.876.040.6Ex. 4-1150:502.003022.876.540.9Ex. 4-1250:502.004023.276.141.3Ex. 4-1350:502.005022.875.740.5Ex. 4-1450:503.003023.176.741.4Ex. 4-1550:504.003023.276.141.3Comp. 2-1100:00.002119.476.035.1Test Results

[0190] The right columns of Table 9 show the test results (sintering speed, product yield, and production rate) for each test case. The cases in which the production rate was 36.0 t / (Dm2) or more in the test results are Examples of Example 4 (Test 1). The sintering speed, product yield, and production rate were determined in the same manner as in Example 1.Sintering Speed

[0191] Effect of oxygen concentration: The sintering speed increased until the oxygen concentration reached 40 vol. % (volume %), but tended to plateau at an oxygen concentration of 50 vol. % (Examples 4-9 and 4-13).

[0192] Effect of oxygen enrichment time: When the oxygen concentration was 30 vol. %, the effect was enhanced up to 2.0 minutes (Example 4-11), but the effect plateaued even when the time was extended to 3.0 minutes (Example 4-14) or 4.0 minutes (Example 4-15).Product Yield

[0193] The reignition method without oxygen enrichment (Example 4-1) yielded 76.2 mass %, while the reignition method with oxygen enrichment yielded 76.2±0.5 mass % for all test cases. Therefore, no effect on product yield was observed.Production Rate

[0194] FIG. 15 is a graph illustrating the test results, illustrating a relationship between the oxygen concentration (vol. %) and the production rate (t / (Dm2)). As illustrated in FIG. 15, the results are as follows.

[0195] Effect of oxygen concentration: The production rate increased until the oxygen concentration reached 40 vol. % (volume %), but tended to plateau at an oxygen concentration of 50 vol. % (Examples 4-9 and 4-13).

[0196] Effect of oxygen enrichment time: When the oxygen concentration was 30 vol. %, the effect was enhanced up to 2.0 minutes (Example 4-11), but the effect plateaued even when the time was extended to 3.0 minutes (Example 4-14) or 4.0 minutes (Example 4-15).

[0197] The results of calculating an oxygen utilization ratio and an oxygen consumption speed for Example 4-1 and Example 4-14 are shown in Table 10. Table 10 shows the test conditions and results for Example 4-14 (oxygen concentration 30 vol. %, oxygen enrichment time 3.0 minutes) that had the greatest effect on improving the production rate, and the test conditions and results for Comparatives 4-1 and 4-2 using single-stage ignition that were conducted for the comparison and evaluation for Example 4-14. In Comparative 4-2, oxygen enrichment was performed for 3.0 minutes immediately after the completion of ignition. The time period during which the oxygen utilization ratio and the oxygen consumption speed were evaluated was 3 minutes immediately after the completion of ignition in Comparative 4-1 and Comparative 4-2 (single-stage ignition sintering). In Example 4-1 and Example 4-14 (reignition sintering method), that time period was set to 3 minutes immediately after the completion of reignition. That is, for Comparative 4-2 and Example 4-14, the oxygen utilization ratio and oxygen consumption speed during the oxygen enrichment time period are shown. The oxygen release speed in exhaust gas (b) shown in Table was calculated from the results of exhaust gas analysis using a magnetic oxygen analyzer. The oxygen consumption speed (a) was calculated by subtracting the oxygen release speed in exhaust gas (b) from an oxygen flow speed sucked into the sintered layer. The oxygen flow speed sucked into the sintered layer can be 10 calculated by multiplying an inlet-side gas flow speed by the oxygen concentration.TABLE 10Comp.Ex.Comp.Ex.4-14-14-24-14Single-stage ignition sintering / Single-Reig-Single-Reig-Reignition sinteringstagenitionstagenitionOxygen enrichmentNoNoYes(*)Yes(*)((*) . . . Oxygen concentration30 vol. %, Oxygen enrichmenttime 3 minutes)Oxygen consumptionNm3 / min0.170.230.240.31speed (a)Oxygen release speedNm3 / min0.170.110.230.15in exhaust gas (b)Oxygen utilization%51685168ratio (a / (a + b))

[0198] As shown in Table 10, when comparing the test cases without oxygen enrichment (Example 4-1 and Comparative 4-1), the oxygen consumption speed and oxygen utilization ratio were higher in the three minutes immediately after the completion of reignition than in the three minutes immediately after the completion of ignition in single-stage ignition. Comparing the test cases in which reignition was performed (Example 4-1 and Example 4-14), in Example 4-14 where oxygen enrichment was performed for 3 minutes immediately after the completion of reignition, the oxygen utilization ratio was maintained, and the oxygen consumption speed increased in proportion to the increase in oxygen concentration.Test 2

[0199] In this test, only the point of time when oxygen enrichment was started was changed, with reference to Example 4-10 in Test 1, in 10-second increments from the point of time when reignition was completed to examine the effects. In other words, the test conditions for Examples 4-16 to 4-21 were the same as those for Example 4-10, except for the point of time when oxygen enrichment was started. Table 11 shows the test conditions and test results for Example 4-10 and Examples 4-16 to 4-21. The oxygen enrichment start time in Table 11 indicates an elapsed time (in seconds) from the point of time when reignition was completed to the point of time when reignition was started. In Example 4-10 where oxygen enrichment was started immediately after the completion of reignition, “0 seconds” indicates a time more than 0 seconds and 2 seconds or less, including an operation time from the completion of reignition to the start of oxygen enrichment. In addition, “10 seconds” in Example 4-16 indicates a time of more than 10 seconds and 12 seconds or less, including an operation time from the completion of reignition to the start of oxygen enrichment. The number of seconds for the oxygen enrichment start time in each of Examples 4-17 to 4-21 was defined similarly to Example 4-10 and Example 4-16.TABLE 11Common conditions①Mass ratio of carbonOxygen enrichmentcontent of start timebonding agents(Based on whenOxygen(mass %) = 50:50reignition wasenrichmentOxygenSinteringProductProduction②Separation time:completed)timeconcentrationspeedyieldrate1.0(min)(sec)(min)(vol. %)(mm / min)(mass %)(t / (Dm2))Ex. 4-1001.53522.876.040.6Ex. 4-16101.53522.977.241.5Ex. 4-17201.53522.476.440.1Ex. 4-18301.53522.576.640.4Ex. 4-19401.53521.678.339.6Ex. 4-20501.53521.875.338.4Ex. 4-21601.53521.776.438.7Test Results

[0200] The right columns of Table 11 show the test results (sintering speed, product yield, and production rate) for each test case. The cases in which the production rate was 36.0 t / (Dm2) or more in the test results are Examples of Example 4 (Test 2). The sintering speed, product yield, and production rate were determined in the same manner as in Example 1. FIG. 16 is a graph illustrating the test results in the right columns of Table 4, illustrating a relationship between the oxygen enrichment time and the production rate. FIG. 16 and the test results (sintering speed and production rate) in the right columns of Table 4 show that oxygen enrichment is preferably started within 10 seconds after the point of time when reignition was completed, but a significant effect can be obtained even if oxygen enrichment is started within 30 seconds after the point of time when reignition was completed. Even after 60 seconds, the effect is greater than those of Examples 4-1 to 4-3 shown in Table 9.Example 5Test Cases

[0201] In Example 5, the same sinter pot test as in Example 1 was used to verify the applicability of the stand-support sintering technique to the invention. A description will be made below about the test conditions and test results of test cases of Example 5 (Comparatives 5-1 to 5-2 and Examples 5-1 to 5-6) using Table 12 described later. In the following description, duplicated descriptions of the conditions, test methods, and the like that are same as those in Example 1 will be omitted as appropriate.Blending of Materials, Etc.

[0202] The materials blended were the same as those in Example 1 (Table 4), except for the bonding agents. The blending of the bonding agents (carbonaceous materials) in the blended material and the particle size of the bonding agents (the average particle size of the low combustible carbonaceous material and the ratio of the highly combustible carbonaceous material with a particle size of 2.8 mm or more) were the same as those in Comparative 1-2 or Example 1-2 of Example 1. In Comparatives 5-1 and 5-2, as with the former (Comparative 1-2), only 4.5 mass % (not included in total mass) of coke breeze (low combustible carbonaceous material) was used relative to the new materials (carbon content mass ratio of bonding agents: 0 mass % for semi-coke and 100 mass % for coke breeze), and the average particle size of coke breeze was 1.0 mm. In Examples 5-1 to 5-6, as with the latter (Example 1-2), the carbon content mass ratio was 50 mass % coke breeze (low combustible carbonaceous material) and 50 mass % semi-coke (highly combustible carbonaceous material), the average particle size of coke breeze was 1.00 mm, and the ratio of the highly combustible carbonaceous material with a particle size of 2.8 mm or more was 30 mass %. In all test cases of Example 5, the granulation method was bulk granulation and the charging method was normal charging.Firing Conditions

[0203] In all test cases of Example 5, the ignition time for ignition (corresponding to ignition by the ignition furnace 3 (initial ignition)) and the ignition time for reignition (corresponding to reignition by the reignition furnace 4) were each 1 minute (heat amount: 25 MJ / ton of blended material). As shown in the column for separation time in Table 12, the separation time was set to either 0.5 minutes, 1 minute, 2.5 minutes, or 3.5 minutes. As with Example 1, the suction pressure was adjusted by changing the valve opening on the suction side of the blower so that the measured value under the pot was constant at 1,300 mmAq (12.75 kPa) in all test cases of Example 5.

[0204] Table 12 shows the test conditions and test results for each test case in Example 5. As shown in Table 12, for cases where semi-coke (highly combustible carbonaceous material) was blend at 50 mass %, the effect of the stand-support sintering technique was evaluated at three levels of separation time (0.5 minutes, 2.5 minutes, and 3.5 minutes) (Examples 5-1 to 5-6). For comparison, the test cases in which semi-coke was blended at 0 mass % were evaluated with a separation time of 0.5 minutes (Comparatives 5-1 and 5-2). For reference, Comparative 1-2 and Example 1-2 of Example 1 are also shown.TABLE 12OxygenMass ratio of carbon content ofconcentrationbonding agentsbetween(mass %)ignition(Low combustible carbonaceous Reignition-furnace andSinteringmaterial:HighlySeparationreignitionspeedProductProductioncombustible carbonaceoustimefurnace(BTS)yieldratematerial)Stand-support(min)(vol. %)(mm / min)(mass %)(t / (Dm2))Comp. 5-1100:0—0.52115.073.526.5Comp. 5-2100:0Yes0.52116.971.728.5Comp. 1-2100:0—1.02116.073.527.6Ex. 1-250:50—1.02117.873.830.5Ex. 5-150:50—0.52117.873.730.5Ex. 5-250:50—2.52117.773.230.0Ex. 5-350:50—3.52117.573.529.8Ex. 5-450:50Yes0.52119.873.934.7Ex. 5-550:50Yes2.52119.573.633.9Ex. 5-650:50Yes3.52118.973.032.4Test Results

[0205] Table 12 shows the test results (sintering speed, product yield, and production rate) for each test case. The cases in which the production rate was 29.5 t / (Dm2) or more in the test results are Examples of Example 5. The sintering speed, product yield, and production rate were determined in the same manner as in Example 1. FIG. 17 (semi-coke blend: 50 mass %) and FIG. 18 (semi-coke blend: 0 mass %) are each a graph illustrating the test results of Table 12, illustrating a relationship between the separation time (min) and the production rate (t / (Dm2)).

[0206] As shown in Table 12 and FIG. 17, in the test cases where semi-coke was blended at 50 mass %, when the stand-support sintering technique was applied with a separation time of 0.5 or 2.5 minutes, the production rate had a high value, and the sintering speed, product yield, and production rate were all significantly improved.

[0207] On the other hand, as shown in Table 12 and FIG. 18, when the semi-coke was blended at 0 mass %, the effect of the stand-support sintering technique was reduced in all of the sintering speed, product yield, and production rate. Thus, the synergistic effect beyond expectation between the blending of the highly combustible carbonaceous material (semi-coke) and the stand-support sintering technique was confirmed in the reignition sintering method.

[0208] In the test cases (Examples 1 to 5) of the invention, the ignition time and reignition time were each 1 minute (heat amount: as sensible heat of the suction gas, 25 MJ / ton of blended material), but the exemplary embodiments of the invention are not limited thereto. This is because the ignition time in the test cases was set taking into account the heat loss in the pot test. In an actual machine (commercial sintering machine), if the ignition time is, for example, 30 seconds, it is not necessary to set the ignition time to 1 minute. The reignition sintering method can be performed by maintaining the ignition time of the actual operation. Similarly, the reignition time does not have to be 1 minute on the actual machine.

[0209] Although the preferred exemplary embodiments and Examples of the invention are described in detail above with reference to the accompanying drawings, the invention is not limited thereto. It is clear that a person with ordinary knowledge in the technical field to which the invention pertains can reach various modified or altered examples within the scope of the technical ideas recited in Claims, and it is understood that those also naturally fall within the technical scope of the invention.EXPLANATION OF CODES

[0210] 1 . . . sinter material bins, 2 . . . drum mixer, 3 . . . ignition furnace, 31 . . . igniter, 32 . . . hood of ignition furnace, 32a . . . partition wall (downstream side), 4 . . . reignition furnace, 41 . . . reigniter, 42 . . . hood of reignition furnace, 42a . . . partition wall (upstream side), 42b . . . partition wall (downstream side), 5 . . . pallet, 5x . . . pallet traveling direction, 51 . . . grate bar, 52 . . . main frame, 53 . . . pallet side wall, 6 . . . downward suction device, 6x . . . downward suction, 61 . . . wind box, 62 . . . wind leg, 63 . . . damper, 64 . . . duct, 65 . . . blower, 7 . . . atmosphere suction area, 7x . . . oxygen enrichment gas suction area, 8 . . . gradient plate chute-type charging device, 81 . . . sinter mixture surge hopper, 82 . . . gradient plate chute, 9 . . . oxygen enrichment gas supplying unit, 91 . . . hood of oxygen enrichment gas supplying unit, 92 . . . gas pipe, 10 . . . sinter material packed bed, 10x . . . slope, 10A . . . combustion zone, 10A1 . . . initial ignition combustion zone, 10A2 . . . reignition combustion zone, 10B . . . sinter cake, 16 . . . stand, 16a . . . sinter cake supporting surface, 101,103,104 . . . . DL-type sintering machine, S . . . section.

Examples

first exemplary embodiment

[0057]First, a description will be made on a Dwight-Lloyd (DL) type sintering machine used in a reignition sintering method and a method for producing sintered ore using the DL-type sintering machine. The DL-type sintering machine used in the reignition sintering method includes a reigniter that performs a second ignition. The reigniter is located at a predetermined interval (corresponding to a “separation distance” described later) downstream in a pallet traveling direction of an igniter that performs a first ignition. The reigniter is a flame heater that heats with flame an upper surface (surface) of a sinter material packed bed after the first ignition is completed.

[0058]FIG. 1 schematically illustrates an exemplary Dwight-Lloyd (DL) type sintering machine used in a reignition sintering method. In the following description, an ore supply side (left side in FIG. 1) is defined as an upstream side, and an ore discharge side (right side in FIG. 1) is defined as a downstream side, bas...

second exemplary embodiment

[0102]A preferred example in which an air volume control technique is used in the invention will be described below as a second exemplary embodiment. As described in the first exemplary embodiment, the production rate is improved by using a predetermined amount (30 mass % or more and 80 mass % or less) of the coarse highly combustible carbonaceous material in the invention. However, since the blended material contains the highly combustible carbonaceous material with a particle size of less than 2.8 mm, the highly combustible carbonaceous material (especially fine particles) is present in the upper part of the sinter material packed bed. Since the highly combustible carbonaceous material has a fast combustion speed, the temperature of the upper part of the sintered layer (up to a depth of about 100 mm from the surface layer) rapidly drops in the section from the first ignition to the start of reignition, reducing the yield and decreasing the effect of improving the production rate. ...

third exemplary embodiment

[0115]A preferred example in which an oxygen enrichment technique is used in the invention will be described below as a third exemplary embodiment. In the invention using the reignition sintering method, the production rate may be further improved by extending a high-temperature retention time of the sintered layer. In that case, if an attempt is made to extend the separation time, which is a time from the completion of initial ignition until reignition is performed (a time required for the pallet to move through the section between the ignition furnace and the reignition furnace), the temperature of the upper part of the sintered layer would drop too much before reignition is performed, and so there has been a limit to how long the separation time can be extended. In particular, when using the highly combustible carbonaceous material, even if the particles thereof are coarse (with a particle size of 2.8 mm or more being 30 mass % to 80 mass %), the combustion time is short and the ...

TECHNICAL FIELD

[0001] The present invention relates to a method for producing sintered ore used as a material in a blast furnace.BACKGROUND ART

[0002] The main material for manufacture of pig iron in a blast furnace is sintered ore. In recent years, a reignition sintering method (Patent Literature 1 below) has been proposed as a method for producing sintered ore, in which two-stage (two times) ignition is performed for the purpose of improving the sintering yield. The reignition sintering method is a technique in which, after a first ignition is completed, the atmosphere is sucked in an atmosphere suction area for a predetermined period of time, and then a second ignition (reignition) is performed. In addition, as improved reignition sintering techniques, there are proposed a method in which segregation of carbonaceous materials (bonding agents) is strengthened in a layer thickness direction of a sinter material packed bed (Patent Literature 2 below), and a method in which char (highly combustible carbonaceous material described later), which is obtained by carbonizing raw coal having a Roga index of 10 or less, is used as the bonding agent (Patent Literature 3 below).

[0003] Regarding single-stage ignition sintering that involves a normal single-stage (one-time) ignition, various techniques using highly combustible carbonaceous materials (carbonaceous materials of which combustion start temperature is lower than that of low combustible carbonaceous materials such as coke and anthracite) have already been known. For example, a technique related to an appropriate blending ratio of the highly combustible carbonaceous material and the low combustible carbonaceous material (Patent Literature 4 below), a technique related to a preferred particle size of the highly combustible carbonaceous material (Patent Literature 5 below), and a method of addition in a granulation step (Patent Literatures 4 and 6 below) have been proposed.

[0004] Patent Literature 1 discloses a method (reignition sintering method) for producing sintered ore using a Dwight-Lloyd (DL) type sintering machine, the machine including: multiple pallets installed successively in a traveling direction from upstream to downstream, into which a sintering material is to be charged; an igniter that ignites, from above, a sinter material packed bed in the pallet located upstream in the traveling direction among the multiple pallets; a wind box that sucks in the atmosphere from under the multiple pallets; a flame heating device that is disposed downstream and spaced from the igniter and heats an entire width of an upper surface of the sinter material packed bed with a flame; and an atmosphere suction area formed between the igniter and the flame heating device where the atmosphere is sucked by downward suction and direct heating is not performed from the upper surface. Patent Literature 1 describes that it is possible to improve the yield and cold strength of sintered ore while maintaining reducibility.

[0005] Patent Literature 2 discloses a method for producing sintered ore using a Dwight-Lloyd type sintering machine in which a sinter material packed bed having a lower layer and a surface layer is formed in each pallet that circulates and moves in a traveling direction from upstream to downstream, and a concentration of a solid combustible in the lower layer is lower than that of the solid combustible in the surface layer, the method including: performing ignition, from above, on the sinter material packed bed in the pallet located upstream in the traveling direction; and after moving an ignited part of the sinter material packed bed downstream in the traveling direction, performing reignition on the reignited part. Patent Literature 2 describes that it is possible to increase the firing speed and improve the yield, thereby further improving productivity by increasing the concentration of the solid combustible in the surface layer while reducing the amount of unburned solid combustible and enabling it to be effectively utilized as heat.

[0006] Patent Literature 3 discloses a technique in which a Dwight-Lloyd (DL) type sintering machine including an igniter and a flame heating device that is disposed downstream and spaced from the igniter and that heats an upper surface of a sinter material packed bed with a flame is used, a blended material to be charged is obtained by removing only a part or all of a carbonaceous material, adding moisture, and adding, during or after granulation, the removed carbonaceous material to the material from which only the part or all of the carbonaceous material has been removed, and a carbonaceous material post-added is char (coal char) obtained by carbonizing raw coal with a Roga index of 10 or less. Patent Literature 3 describes that the yield and productivity are improved by post-adding coal char in the granulation step of the reignition sintering method.

[0007] Patent Literature 4 discloses, as a technique of improving sintering yield, a method for producing sintered ore in which a low combustible carbonaceous material made from at least one of coke breeze or anthracite and a highly combustible carbonaceous material of which combustion start temperature is lower than that of the low combustible carbonaceous material are used as bonding agents of the sintering material, a carbon content of the highly combustible carbonaceous material is in a range from 25 mass % to 75 mass % relative to a carbon content of the bonding agents, and at least one of the low combustible carbonaceous material or the highly combustible carbonaceous material is added in a latter half of a granulation step of the sintering material.

[0008] Patent Literature 5 discloses a method for producing sintered ore including: performing segregation charging of pseudo-particles that are obtained by granulating sintering materials made from iron ore, flux, return ore, and a solid carbonaceous material, into pallets of a sintering machine; and performing firing while creating a carbon concentration difference in a height of a material layer, in which when palm kernel shell charcoal, which is a solid carbonized material produced by a heat treatment of palm kernel shell, is blended as a part or all of the solid carbonaceous material in the sintering materials, the palm kernel shell charcoal with an average particle size adjusted to 2.7 mm to 6.0 mm is blended. Using the palm kernel shell charcoal as the solid carbonaceous material for sintering reduces the emission of carbon dioxide (global greenhouse gas), and using appropriate operating techniques improves sintering productivity. In Patent Literature 5, when the palm kernel shell charcoal is blended as a part of the solid carbonaceous material in the sintering materials, it is preferable to blend the palm kernel shell charcoal whose average particle size is adjusted to be coarser in a range from 1.0 mm to 4.5 mm than the average particle size of the solid carbonaceous material, which is coke powder or anthracite.

[0009] Patent Literature 6 discloses a method for producing a carbonaceous material for sintering including: calculating a Roga index of coal and producing the carbonaceous material for sintering using coal having a Roga index of 10 or less, and also discloses that at least a part of the carbonaceous material for sintering is post-added in a predetermined blend amount during or at the end of a granulation step of a material for sintering. Patent Literature 6 describes that it is possible to reduce the amount of NOx discharged during the production of sintered ore.CITATION LISTPatent Literature(S)Patent Literature 1: JP 2020-2457 A

[0011] Patent Literature 2: JP 2020-29603 A

[0012] Patent Literature 3: JP 2020-84204 A

[0013] Patent Literature 4: JP 2022-33594 A

[0014] Patent Literature 5: JP 2014-218713 A

[0015] Patent Literature 6: JP 2020-56086 ASUMMARY OF THE INVENTIONProblem(s) to be Solved by the Invention

[0016] The highly combustible carbonaceous material is a carbonaceous material with high combustibility, i.e. a carbonaceous material with a fast combustion speed. Regarding the combustion speed of carbonaceous materials (bonding agents), various measurement methods and results have already been disclosed. However, unless the measurements are performed in a well-controlled manner, the results will vary greatly. For this reason, the combustion start temperature (ignition temperature), which has a substantive correspondence relationship with the combustion speed, is used as an index to represent the combustion speed. In the invention, the low combustible carbonaceous material refers to a carbonaceous material with a high combustion start temperature such as coke and anthracite (a carbonaceous material with a combustion start temperature exceeding 550 degrees C.), and the highly combustible carbonaceous material refers to a carbonaceous material with a lower combustion start temperature than the low combustible carbonaceous material (a carbonaceous material with a combustion start temperature of 550 degrees C. or less).

[0017] As described above, it is known that using the highly combustible carbonaceous material in the single-stage ignition sintering improves the yield and production rate by increasing the combustion speed of the carbonaceous material in the sinter material packed bed, and it is also known that using coal char (highly combustible carbonaceous material) improves the yield and production rate in the reignition sintering method. However, until now, there has been no detailed study on the preferable conditions for using the highly combustible carbonaceous material in the reignition sintering method. As a result of extensive investigation into these preferable conditions, the inventors found that the production rate could be further improved under specific conditions, and completed the invention. An object of the invention is to provide a method for producing sintered ore that further improves the production rate when the reignition sintering technique is used.Means for Solving the Problems

[0018] According to an aspect(s) of the invention, the followings are provided.

[0019] [1] A method for producing sintered ore using a Dwight-Lloyd type sintering machine, which includes an ignition furnace for initial ignition and a reignition furnace for reignition arranged at a predetermined distance downstream of the ignition furnace and which advances sintering by downward suction, the method including: using, as bonding agents for a blended material, a low combustible carbonaceous material with a combustion start temperature exceeding 550 degrees C. and a highly combustible carbonaceous material with a combustion start temperature of 550 degrees C. or less, in which a ratio of the highly combustible carbonaceous material with a particle size of 2.8 mm or more is 30 mass % or more and 80 mass % or less.

[0020] [2] The method for producing sintered ore according to [1], in which a crushed product obtained by crushing a compression-molded product, which is provided by compressing and molding an aggregate of carbonized wood, is used as the highly combustible carbonaceous material.

[0021] [3] The method for producing sintered ore according to [2], in which the crushed product of the compression-molded product provided by compressing and molding the aggregate of the carbonized wood is produced by: carbonizing a wood material to produce the carbonized wood; crushing the produced carbonized wood as necessary to produce carbonized wood particles, and kneading the carbonized wood particles alone or with a binder to produce the aggregate of the carbonized wood; compressing and molding the aggregate to produce the compression-molded product; and crushing the compression-molded product.

[0022] [4] The method for producing sintered ore according to any one of [1] to [3], in which a mass ratio of a carbon content of the highly combustible carbonaceous material to a carbon content of the bonding agents is 25 mass % or more and 75 mass % or less.

[0023] [5] The method for producing sintered ore according to [4], in which an average particle size of the low combustible carbonaceous material is in a range from 0.8 mm to 1.2 mm.

[0024] [6] The method for producing sintered ore according to [5], in which a segregation-strengthened type charging device is used as a charging device for the blended material.

[0025] [7] The method for producing sintered ore according to [5], in which, of the bonding agents, only the low combustible carbonaceous material is added in a latter half of a granulation process.

[0026] [8] The method for producing sintered ore according to any one of [1] to [3], in which a downward suction air volume is reduced only in a section up to an outlet of the reignition furnace on a sinter strand upstream side.

[0027] [9] The method for producing sintered ore according to [8], in which an average superficial air volume of the atmosphere sucked in the section up to the outlet of the reignition furnace on the sinter strand upstream side is set to 60% or more and 80% or less of an average superficial air volume of the atmosphere sucked in a section downstream of the outlet of the reignition furnace.

[0028]

[10] The method for producing sintered ore according to [8], in which an average negative pressure in a wind box or a wind leg in the section up to the outlet of the reignition furnace on the sinter strand upstream side is set to 40% or more and 70% or less of an average negative pressure in a wind box or a wind leg in the section downstream of the outlet of the reignition furnace.

[0029]

[11] The method for producing sintered ore according to [8], in which a separation time, which is a time required for a pallet to pass through a section between the ignition furnace and the reignition furnace, is set to 30 seconds or more and 2 minutes or less.

[0030]

[12] The method for producing sintered ore according to any one of [1] to [3], in which a separation time, which is a time required for a pallet to pass through a section between the ignition furnace and the reignition furnace, is set to 1 minute or more, and an oxygen concentration of a suction gas sucked downward from a surface layer side of a sintered layer in the section is set to 30 volume % or more.

[0031]

[13] The method for producing sintered ore according to

[12] , in which the separation time is set to 5 minutes or less and the oxygen concentration of the suction gas is set to 40 volume % or less.

[0032]

[14] The method for producing sintered ore according to any one of [1] to [3], in which oxygen enrichment of a suction gas sucked downward from a surface layer side of a sintered layer is started after the reignition is completed, an oxygen enrichment time from the start of the oxygen enrichment to a completion of the oxygen enrichment is 30 seconds or more, and an oxygen concentration of the suction gas sucked downward during the oxygen enrichment time is set to 30 volume % or more.

[0033]

[15] The method for producing sintered ore according to

[14] , in which the oxygen enrichment time is set to 2 minutes or less and the oxygen concentration of the suction gas is set to 40 volume % or less.

[0034]

[16] The method for producing sintered ore according to

[14] , in which the oxygen enrichment is started at a timing more than 0 seconds and within 30 seconds after a point of time when the reignition is completed.

[0035]

[17] The method for producing sintered ore according to

[16] , in which the oxygen enrichment is started at a timing more than 0 seconds and within 10 seconds after the point of time when the reignition is completed.

[0036]

[18] The method for producing sintered ore according to any one of [1] to [3], in which a pallet, into which the blended material is to be charged, is provided with a support member having a sinter cake supporting surface, the support member being erected on a grate bar to be embedded in a sinter material packed bed.

[0037] According to the aspect(s) of the invention, there is provided a method for producing sintered ore using a Dwight-Lloyd type sintering machine, which includes an ignition furnace for initial ignition and a reignition furnace for reignition arranged at a predetermined distance downstream of the ignition furnace and which advances sintering by downward suction, in which a low combustible carbonaceous material and a highly combustible carbonaceous material are both used as bonding agents, and a ratio of the highly combustible carbonaceous material with a particle size of 2.8 mm or more is set to 30 mass % or more and 80 mass % or less. This method further improves the production rate.BRIEF DESCRIPTION OF DRAWINGS

[0038] FIG. 1 schematically illustrates an exemplary DL-type sintering machine used in a method for producing sintered ore (reignition sintering method) according to a first exemplary embodiment.

[0039] FIG. 2 schematically illustrates an example of a segregation-strengthened type charging device.

[0040] FIG. 3 is a graph illustrating a particle size distribution of a sintering material in each layer (first to fifth layers) of a sinter material packed bed for which segregation-type charging is performed.

[0041] FIG. 4 is a graph illustrating a ratio of an average particle size in each layer (first to fifth layers) to an overall average particle size in the sinter material packed bed for which the segregation-type charging is performed.

[0042] FIG. 5 is a graph illustrating a carbon concentration distribution in each layer (first to fifth layers) of the sinter material packed bed for which the segregation-type charging is performed.

[0043] FIG. 6 schematically illustrates an exemplary air volume control in the DL-type sintering machine used in the method for producing sintered ore (reignition sintering method) according to a second exemplary embodiment.

[0044] FIG. 7 schematically illustrates an example of oxygen enrichment in a DL-type sintering machine used in the method for producing sintered ore (reignition sintering method) according to a third exemplary embodiment.

[0045] FIG. 8 schematically illustrates another example of oxygen enrichment in a DL-type sintering machine used in the method for producing sintered ore (reignition sintering method) according to a fourth exemplary embodiment.

[0046] FIG. 9 schematically illustrates an exemplary pallet for sintering that is used for producing sintered ore through a stand-support sintering technique according to a fifth exemplary embodiment.

[0047] FIG. 10 is a graph illustrating a relationship between the ratio of a highly combustible carbonaceous material with a particle size of +2.8 mm and the production rate according to Example 1.

[0048] FIG. 11 is a graph illustrating a relationship between the production rate and the mass ratio of the carbon content of the highly combustible carbonaceous material to the carbon content of a bonding agent(s) according to Example 1.

[0049] FIG. 12 is a graph illustrating a relationship between the average particle size of a low combustible carbonaceous material (coke breeze) and the production rate according to Example 1.

[0050] FIG. 13 is a graph illustrating a relationship between the superficial air volume ratio and the production rate according to Example 2.

[0051] FIG. 14 is a graph illustrating a relationship between the oxygen concentration and the production rate according to Example 3.

[0052] FIG. 15 is a graph illustrating a relationship between the oxygen concentration and the production rate according to Example 4 (Test 1).

[0053] FIG. 16 is a graph illustrating a relationship between the oxygen enrichment time and the production rate according to Example 4 (Test 2).

[0054] FIG. 17 is a graph illustrating a relationship between the separation time and the production rate according to Example 5 (highly combustible carbonaceous material blended at 50 mass % (carbon content mass ratio)).

[0055] FIG. 18 is a graph illustrating a relationship between the separation time and the production rate according to Example 5 (highly combustible carbonaceous material blended at 0 mass %).DESCRIPTION OF EMBODIMENT(S)

[0056] Referring to the drawings, the invention and preferred embodiments thereof will be described in detail below. In the specification and the drawings, components or configurations having substantially the same functions are given the same or similar names or the same or similar reference numerals, and redundant explanations are omitted.First Exemplary Embodiment

[0057] First, a description will be made on a Dwight-Lloyd (DL) type sintering machine used in a reignition sintering method and a method for producing sintered ore using the DL-type sintering machine. The DL-type sintering machine used in the reignition sintering method includes a reigniter that performs a second ignition. The reigniter is located at a predetermined interval (corresponding to a “separation distance” described later) downstream in a pallet traveling direction of an igniter that performs a first ignition. The reigniter is a flame heater that heats with flame an upper surface (surface) of a sinter material packed bed after the first ignition is completed.

[0058] FIG. 1 schematically illustrates an exemplary Dwight-Lloyd (DL) type sintering machine used in a reignition sintering method. In the following description, an ore supply side (left side in FIG. 1) is defined as an upstream side, and an ore discharge side (right side in FIG. 1) is defined as a downstream side, based on a pallet traveling direction 5x. As illustrated in FIG. 1, a DL-type sintering machine 101 includes a reignition furnace 4, which is located downstream of an ignition furnace 3 at a predetermined interval (separation distance). As illustrated in FIG. 1, the ignition furnace 3 includes an igniter 31 that performs the first ignition (initial ignition) and a hood 32 that covers the igniter 31. The reignition furnace 4 includes a reigniter 41 that performs the second ignition (reignition) and a hood 42 that covers the reigniter 41. For example, the ignition furnace 3 is an ignition furnace provided with a burner and the like, which is usable in a single-stage ignition sintering method, and the reignition furnace 4 can be the same ignition furnace used in the single-stage ignition sintering method. As illustrated in FIG. 1, the hood 32 of the ignition furnace 3 and the hood 42 of the reignition furnace 4 are independent of each other. An atmosphere suction area 7 is provided between the ignition furnace 3 and the reignition furnace 4, which are provided at a predetermined interval (separation distance) in the pallet traveling direction 5x.

[0059] The atmosphere suction area 7 is a section (area) where an upper surface of the sinter material packed bed 10 in a pallet is not heated by a burner or the like and the atmosphere (the air) sucked in by downward suction 6x is supplied into the sinter material packed bed 10. As illustrated in FIG. 1, a section between a partition wall 32a located downstream of the hood 32 of the ignition furnace 3 and a partition wall 42a located upstream of the hood 42 of the reignition furnace 4 is the atmosphere suction area 7. In this specification, a distance between the partition wall 32a and the partition wall 42a in the pallet traveling direction 5x, i.e., a distance of the atmosphere suction area 7 in the pallet traveling direction 5x, is referred to as the separation distance. A time required for each pallet (pallets referring to multiple connected pallet carriages moving on an endless track, not illustrated in the drawings) of the DL-type sintering machine 101 to pass through the separation distance (time required to pass through the atmosphere suction area) is called a separation time. In this specification, the sinter material packed bed 10 refers to layers of a blended material formed on the pallet, regardless of the presence or absence of ignition (including initial ignition) or reignition. The sinter material packed bed 10 includes a combustion zone 10A in which a sintering reaction is progressing due to ignition, and a sinter cake 10B in which the sintering reaction is completed. In the following description, the sinter material packed bed 10 after the initial ignition is performed (after the start of firing) is also referred to as a sintered layer.

[0060] A lower limit (minimum value) in an appropriate range for the separation time described above is a limit at which the expansion of the combustion zone 10A can be sufficiently achieved. An upper limit (maximum value) in the appropriate range for the separation time is determined by the cooling of an upper part of the sintered layer after the initial ignition. Thus, the appropriate range for the separation time depends on the exemplary embodiment (first to fifth exemplary embodiments) (details will be described later), and can be in a range from 0.5 minutes to 6 minutes. If the separation time is less than the lower limit in the appropriate range, sufficient oxygen cannot be supplied to the combustion zone 10A in the upper part of the sintered layer, and the expansion of the combustion zone 10A is inhibited. If the separation time exceeds the upper limit in the appropriate range, the temperature of the upper part of the sintered layer decreases to the sintering reaction temperature or less, making it difficult to achieve the effect of improving the production rate of the reignition technique. The separation distance is determined by multiplying the separation time by a transport speed (pallet speed) of the sinter material packed bed 10 using the pallet. If a pallet speed of 3 m / min for a standard commercial sintering machine is used, an appropriate range for the separation distance is in a range from 1.5 m to 18 m. Obviously, the appropriate separation distance changes depending on the pallet speed, and thus the separation distance depends on the sintering machine.

[0061] A method for producing sintered ore through reignition sintering is a technique in which a reignition step is added to the single-stage ignition sintering method (details will be described later). Examples of materials for sintered ore (sintering materials), which are usable as appropriate, include iron materials such as iron ore (powder), iron-containing miscellaneous materials such as scale and steelmaking dust, MgO-containing flux such as peridotite, CaO-containing flux such as limestone, return ore, and bonding agents (carbonaceous materials) that serve as fuel for sintering (bonding). As illustrated in FIG. 1, each sintering material is stored in the corresponding one (one of 11 to 1x) of sinter material bins 1, and is dispensed and blended in a predetermined ratio. The blended material is fed into a drum mixer 2 and granulated to produce pseudo-particles. The blended material having been granulated (hereinafter, the blended material after granulation is also referred to as blended material granules) is loaded from a sinter mixture surge hopper 81 onto a pallet covered with bedding ore (not illustrated in the drawings), thus forming the sinter material packed bed 10.

[0062] The sinter material packed bed 10 moves continuously in the pallet traveling direction 5x as the pallet moves. When the sinter material packed bed 10 moves to below the ignition furnace 3, the carbonaceous material on a surface of the sinter material packed bed 10 is ignited by the flame of the igniter 31, starting sintering of the sinter material packed bed 10. A downward suction device 6 (see FIG. 6) is provided below the pallet moving in the pallet traveling direction 5x, and sucks in the atmosphere (the air) from below the pallet. The downward suction 6x supplies oxygen into the sinter material packed bed 10, the combustion of the bonding agents in the sinter material packed bed 10 (combustion zone 10A) progresses from top to bottom, and the sinter material packed bed 10 is gradually fired by the combustion heat of the bonding agents. When the sinter material packed bed 10 passes through the atmosphere suction area 7 and moves to below the reignition furnace 4, the sinter material packed bed 10 is reignited by the flame of the reigniter 41. The sinter cake 10B obtained by sintering the sinter material packed bed 10 is discharged at a downstream end of the DL-type sintering machine 101 in the pallet traveling direction 5x, and is sized by crushing, sieving, or the like, so that sintered ore having a particle size that can be charged into blast furnaces becomes a material for blast furnace iron production.

[0063] As described above, in the reignition sintering method, the sinter material packed bed 10 is ignited by the ignition furnace 3 and passes through the atmosphere suction area 7, and after a predetermined time interval (corresponding to the above-described separation time), the sinter material packed bed 10 is reignited by the reignition furnace 4. In the atmosphere suction area 7 downstream of the ignition furnace 3, heating from above, i.e., combustion by a flame burner such as an igniter, is not performed, so sufficient oxygen is supplied into the sinter material packed bed 10, and the combustion of the bonding agents in the combustion zone 10A progresses. In addition, since the atmosphere suction area 7 is provided between the ignition furnace 3 and the reignition furnace 4, the combustion of the bonding agents progresses further downward due to an increase in superficial velocity. This increases a thickness of the combustion zone 10A in an upper part of the sinter material packed bed 10. After the thickness of the combustion zone 10A has increased in the atmosphere suction area 7, the upper surface of the sinter material packed bed 10 is reignited by the flame of the reignition furnace 4 provided downstream of the atmosphere suction area 7. The reignition allows uncombusted bonding agents (residual bonding agents) that were not ignited and combusted by the ignition furnace 3 to be combusted without leaving any residue, and allows gas heated by the reignition and combustion of the residual bonding agents to be sucked into the sinter material packed bed 10. This increases a time during which the upper part of the sinter material packed bed 10 is held at high temperature (e.g., a time during which the temperature is held at 1,200 degrees C. or more), promoting the sintering reaction and improving the yield. As described above, the separation time is a time required for each pallet to move through the atmosphere suction area 7, i.e., a time after the initial ignition is completed until the reignition is performed. The separation time in the exemplary embodiment can be, for example, 0.5 minutes or more and 3.5 minutes or less. The separation time is preferably 30 seconds or more and 2 minutes or less by facilitating the combustion using a highly combustible carbonaceous material in combination.

[0064] As in the single-stage ignition sintering, a charging device provided with a segregation mechanism is used in the reignition sintering, when the blended material granules are charged onto the pallet. For example, a gradient plate chute-type charging device 8 as illustrated in FIG. 1 is generally used. The gradient plate chute-type charging device 8 includes the sinter mixture surge hopper 81 and a gradient plate chute 82. The sinter mixture surge hopper 81 stores the blended material granules, and the gradient plate chute 82 is inclined downward in the opposite direction to the pallet traveling direction 5x. The blended material granules in the sinter mixture surge hopper 81 are charged onto the pallet using the gradient plate chute 82, thereby forming a slope 10x on an upstream side of the sinter material packed bed 10. The blended material granules roll over the slope 10x and are classified, resulting in particle size segregation in a layer thickness (layer height) direction of the sinter material packed bed 10. Specifically, the granules with smaller particle sizes tend to be charged in the upper part of the sinter material packed bed 10, while the granules with larger particle sizes tend to be charged in the lower part of the sinter material packed bed 10.

[0065] Here, the granulation process of the blended material described above is aimed at ensuring the permeability of the sinter material packed bed 10. Generally, as described in the 3rd Edition of the Iron and Steel Handbook II, Iron and Steelmaking, p. 84 (FIG. 2.4) [Oct. 15, 1979], fine powder materials with a particle size of less than 0.25 mm, which are a main target of the granulation process, form pseudo-particles by adhering to the periphery of particles of 1.00 mm or more as nuclear particles, but particles of an intermediate size of 0.25 mm or more and less than 1.00 mm are difficult to granulate and do not easily become pseudo-particles. For that reason, the particle size of the bonding agents to be blended is adjusted even when the granulation process is performed, and then charging is performed by the gradient plate chute-type charging device 8, so that it is possible to adjust the distribution of the bonding agents in the height direction of the sinter material packed bed.

[0066] Coke or anthracite is typically used as the bonding agent. Coke for sintering is made by crushing coke with a particle size unsuitable for use in blast furnaces (typically 40 mm or less) into a particle size of less than 10 mm, which is suitable for sintering, during the process of producing coke for blast furnaces. However, since sieving may not be performed after crushing, some particles having a particle size of 10 mm or more may remain. In contrast to lump coke used in blast furnaces, coke for sintering is also called coke breeze. Anthracite is one of the classifications given to coal (lignite, bituminous coal, and anthracite), and is the most carbonized coal. Coal with a fuel ratio (fixed carbon / volatile matter content (mass ratio)) of 4 or more, or more simply, coal with a carbon content of 90 mass % or more, is classified as anthracite. Anthracite is also crushed to have a particle size of approximately less than 10 mm, similar to coke breeze.

[0067] The inventors focused on the following two points in the above-described reignition sintering method. The first point is that by using the highly combustible carbonaceous material in addition to a low combustible carbonaceous material as the bonding agents, and by coarsening the highly combustible carbonaceous material and distributing it concentratedly in a lower part of the sinter material packed bed 10, the effect of reducing airflow resistance due to the coarsening can be obtained from the start to the end of sintering. The second point is that if the coarsening target is the highly combustible carbonaceous material, its high combustibility would largely prevent the reduction in sinter ore production that may otherwise be caused by the delay in the combustion completion due to coarsening. The inventors focused on these two points and conducted extensive research, which led to the completion of the invention. The inventors also investigated influential factors applicable to the invention (type of highly combustible carbonaceous material, particle size of low combustible carbonaceous material, segregation charging method, etc.).

[0068] The invention relates to a method for producing sintered ore using a Dwight-Lloyd type sintering machine, which includes an ignition furnace for initial ignition and a reignition furnace for reignition arranged at a predetermined distance downstream of the ignition furnace and which advances sintering by downward suction, the method including: using, as bonding agents for a blended material, a low combustible carbonaceous material with a combustion start temperature exceeding 550 degrees C. and a highly combustible carbonaceous material with a combustion start temperature of 550 degrees C. or less, and a ratio of the highly combustible carbonaceous material with a particle size of 2.8 mm or more being 30 mass % or more and 80 mass % or less. The initial ignition means that the bonding agents on the surface of the sinter material packed bed 10 charged in the pallet are first ignited (first ignition). The reignition means that ignition is again performed after the initial ignition is completed (second ignition). The downward suction refers to sucking in the atmosphere (the air) from below the pallet. Oxygen-containing gas above the sinter material packed bed 10 is sucked into the sinter material packed bed 10 to supply oxygen into the sinter material packed bed 10. First, the low combustible carbonaceous material and the highly combustible carbonaceous material used as the bonding agents will be described below. In this specification, the term “particle size” refers to a value (particle diameter) measured by sieving using a sieve in accordance with JIS Z8801-1:2019.Low Combustible Carbonaceous Material and Highly Combustible Carbonaceous Material

[0069] The bonding agents (carbonaceous materials) for the sintering material are classified into the low combustible carbonaceous material and the highly combustible carbonaceous material.

[0070] Examples of the low combustible carbonaceous material include coke and anthracite. The highly combustible carbonaceous material includes carbonaceous materials that are more combustible than the low combustible carbonaceous material (carbonaceous materials of which combustion start temperature is lower than that of the low combustible carbonaceous material). Specifically, the low combustible carbonaceous material and the highly combustible carbonaceous material are classified based on the combustion start temperature obtained by differential thermogravimetry analysis. The combustion start temperature of the low combustible carbonaceous material exceeds 550 degrees C., and the combustion start temperature of the highly combustible carbonaceous material is 550 degrees C. or less. The combustion start temperature (ignition temperature) is defined as a temperature at which a rapid weight loss begins, based on a temperature-weight change curve obtained by thermogravimetry (TG) in an air stream using a termogravimetry-differential thermal analysis and mass spectrometry (TG-DTA / MS).

[0071] Examples of the highly combustible carbonaceous material include coal char and biomass charcoal (palm kernel shell charcoal, carbonized wood produced by carbonizing wood, etc.). The combustion start temperatures (ignition temperatures) of coke and anthracite, which are the low combustible carbonaceous materials, are approximately 670 degrees C. and 690 degrees C., respectively. In contrast, the combustion start temperature of the highly combustible carbonaceous material is low. The combustion start temperature of coal char (semi-coke, lignite char, and subbituminous coal char) is 430 degrees C. or more and 550 degrees C. or less, the combustion start temperature of palm kernel shell charcoal is approximately 470 degrees C., and the combustion start temperature of carbonized wood is approximately 400 degrees C. or more and 450 degrees C. or less. The coal char and biomass charcoal have similar combustibility because they have roughly the same ignition temperatures. Although details will be described later, a compression-molded product made primarily from carbonized wood, which is biomass charcoal (biochar), is also a highly combustible carbonaceous material, and a crushed product obtained by crushing the compression-molded product is also usable. The combustion start temperature of the compression-molded product is low, approximately 250 degrees C. or more and 450 degrees C. or less. The same applies to the crushed product of the compression-molded product. It is known that the combustion speed of the highly combustible carbonaceous material is 1.03 to 30.00 times that of coke.

[0072] The coal char is, for example, a carbonaceous material (char) produced by carbonizing raw coal, such as low-caking bituminous coal, lignite, and subbituminous coal, at a temperature of 700 degrees C. or more and 900 degrees C. or less. The carbonaceous materials produced by carbonization of low-caking bituminous coal, lignite, and subbituminous coal are called semi-coke, lignite char, and subbituminous char, respectively. These carbonaceous materials are produced by carbonizing raw coal (including blended coal) in a pyrolysis furnace (e.g., a rotary kiln).

[0073] The biomass charcoal is, for example, a carbonaceous material produced by heat-treating (carbonizing) biological resources (biomass), such as palm kernel shells and wood, as materials. The palm kernel shell charcoal (PKS charcoal) is a solid carbide produced by heat treatment (carbonization) of palm kernel shell. Since the method for producing PKS charcoal can be implemented by referring to documents such as the above-mentioned Patent Literature 5, a detailed description thereof will be omitted here.

[0074] As described above, a crushed product of a compression-molded product made primarily from carbonized wood, which is biomass charcoal (biochar), may be used as the bonding agent for the sintering material. This compression-molded product is a compression-molded product (hereinafter, also referred to simply as a carbonized-wood compression-molded product for convenience) obtained by compressing and molding an aggregate of carbonized wood, and a crushed product (hereinafter, also referred to simply as a carbonized-wood compression-molding crushed product for convenience) obtained by crushing the carbonized-wood compression-molded product is used as the bonding agent. Here, “carbonized wood” refers to a carbonized material obtained by heat treatment (carbonization) of “wood”. The above-mentioned “wood” refers to trunks and branches of trees, or materials made from them, including, for example, construction waste. The term “aggregate of carbonized wood” refers to an aggregate of carbonized wood particles or an aggregate of carbonized wood particles bonded together with a binder, and there are no limitations on the size or shape of the carbonized wood particles. The term “compressing and molding” refers to molding that involves compression, and includes not only compression molding, but also extrusion molding in which pressure is applied during extrusion, for example. The phase “made primarily from” is used for a solid material having the largest proportion (by mass) of the total solid materials. The term “crushing” refers to a process of reducing the particle size using a crusher (e.g. rod mill, hammer crusher, roll crusher, super sander, jaw crusher, and fret mill).

[0075] The crushed product of the carbonized-wood compression-molded product is produced as follows. First, a wood material is obtained and then carbonized to produce carbonized wood (carbonized material producing step). Next, the produced carbonized wood is crushed or otherwise processed as necessary to produce carbonized wood particles, and these carbonized wood particles are kneaded alone or with a binder or the like to produce an aggregate of carbonized wood (hereinafter referred to as a carbonized wood aggregate) (aggregate producing step). A binder is used to provide a strong aggregate. Next, the carbonized wood aggregate is compressed and molded to produce a compression-molded product (carbonized-wood compression-molded product) (compression step). Then, the carbonized-wood compression-molded product is crushed to produce a crushed product of the carbonized-wood compression-molded product (carbonized-wood compression-molding crushed product) (compression product crushing step).

[0076] In the carbonized material producing step, the carbonized wood is produced using wood as a material in a carbonization device (external combustion rotary kiln, internal combustion rotary kiln, fluidized bed reactor, moving bed reactor (shaft furnace), etc.) by appropriately setting carbonization conditions (temperature, time, etc.). The volatile matter content of the produced carbonized wood (measured in accordance with JIS M8812:2006) is preferably 15 mass % or less. For example, carbonizing cedar wood chips at 800 degrees C. for one hour can reduce the volatile matter content of the carbonized material (carbonized wood) to 4.8 mass %.

[0077] In the aggregate producing step, the carbonized wood produced in the carbonized material producing step is crushed as needed (e.g., to an average particle size of 1 mm or less). The carbonized wood particles are kneaded alone, together with a binder, together with a binder and water, or together with a binder, water and an additive to produce a carbonized wood aggregate. Examples of the binder include cornstarch (starch), bentonite, coal tar, biomass tar, petroleum pitch, and cement, and an additive such as alkali or acid is added to some binders (such as cornstarch) to produce a strong molded product. For kneading, a device also capable of crushing the carbonized wood at the same time (e.g., an extruder) may be used. The amount of binder added is preferably 1 mass % or more and 10 mass % or less (not included in total mass) when the carbonized wood is taken as 100 mass %.

[0078] In the compression step, the carbonized wood aggregate (kneaded product) produced in the aggregate producing step is compressed and molded to produce a carbonized-wood compression-molded product that is a compression-molded product. The compression molding method may use a compression molding machine or an extrusion molding machine. For example, a roll press method using a roll rotation type compression molding machine (ring die type, flat die type, etc.) or a tableting method using a biaxial compression molding machine (screw type extrusion molding machine) may be used. The carbonized-wood compression-molded product may have any shape, for example, a pellet (cylindrical shape) or a briquette (pillow shape).

[0079] In the compression product crushing step, the carbonized-wood compression-molded product produced in the compression step is crushed by a crusher to produce a crushed product of the carbonized-wood compression-molded product (carbonized-wood compression-molding crushed product). Examples of the crusher include a rod mill, hammer crusher, roll crusher, super sander, jaw crusher, and fret mill. After crushing, the crushed product is sieved using a sieve conforming to JIS Z8801-1:2019, for example, and the crushed product having a predetermined particle size is used as the highly combustible carbonaceous material. For example, the crushed product has a particle size (particle diameter) of less than 10 mm (size passing through a sieve with an opening size of 10 mm), more preferably has a particle size (particle diameter) less than 5 mm (size passing through a sieve with an opening size of 5 mm).

[0080] In the compression step, it is preferable to produce a carbonized-wood compression-molded product having the following properties. The volatile matter content and apparent density of the carbonized-wood compression-molded product do not change before and after the crushing in the compression product crushing step, and the crushed product of the carbonized-wood compression-molded product has similar properties.

[0081] The volatile matter content of the carbonized-wood compression-molded product (measured in accordance with JIS M8812:2006) is preferably 20 mass % or less. Normally, when sintered ore is produced, the volatile matter content of the bonding agents is controlled to be a predetermined value (e.g., 10 mass %) or less in order to prevent breakdown of an exhaust gas electric dust collector that collects the exhaust gas generated during production. It is thus preferable to specify an upper limit of the volatile matter content of the carbonized-wood compression-molded product. An increase in volatile matter content of the carbonized-wood compression-molded product relative to the volatile matter content of carbonized wood is due to the use of a binder in the aggregate producing step. In the invention, the carbonized-wood compression-molded product (highly combustible carbonaceous material) is used in combination with the low combustible carbonaceous material (coke breeze, anthracite). Here, the volatile matter content of coke breeze is often much lower than the above-mentioned predetermined value (10 mass %), and the volatile matter content of anthracite is also low at around 5 mass %. Taking these points into consideration, the upper limit of the volatile matter content of the carbonized-wood compression-molded product is set to 20 mass %, which is higher than 10 mass %. Setting the upper limit high makes it possible to use materials with a high volatile matter content as the material for the carbonized-wood compression-molded product. It is also preferable that the blending ratio of the carbonized-wood compression-molded product and the low combustible carbonaceous material (coke breeze and / or anthracite) be determined based on the volatile matter content of the carbonized-wood compression-molded product and the volatile matter content of the low combustible carbonaceous material used, so as not to exceed a predetermined value for the volatile matter content according to the capacity of the exhaust gas electric dust collector.

[0082] The apparent density of the carbonized-wood compression-molded product is preferably 0.6 g / cm3 or more, and more preferably 0.7 g / cm3 or more. The apparent density is measured by a bead volume displacement method. The bead volume displacement method is a measurement method adopted by Micromeritics, which is a volume displacement method that uses DryFlo (pseudo fluid), highly fluid beads, as a measurement sample. Specifically, the volume of only the beads put in a sample chamber is first measured, then the sample is put on a layer of beads in the sample chamber and the volume is measured; the volume of the measured sample, including pores and cavities, is calculated from the difference between the two volumes. The apparent density is a value obtained by dividing the mass of the measured sample by the calculated volume. By making the apparent density of the carbonized-wood compression-molded product 0.6 g / cm3 or more, the atmospheric temperature in the vicinity of the combustion of the bonding agents increases when producing sintered ore. This improves the yield and therefore the sintering production rate. However, if the apparent density exceeds 1.3 g / cm3, the combustion speed of the carbonized-wood compression-molded product decreases, and the sintering speed also decreases.Particle Size of Highly Combustible Carbonaceous Material

[0083] The particle size of the highly combustible carbonaceous material is determined as follows. After drying the highly combustible carbonaceous material at 105 degrees C. for 2 hours or more, the material is shaken by head tapping for 5 minutes using a Ro-Tap shaker equipped with a sieve with 2.8 mm mesh size as specified in JIS Z8801-1:2019, and the ratio of a particle size of 2.8 mm or more on the sieve is examined. In the invention, it is used a highly combustible carbonaceous material in which the ratio of a particle size of 2.8 mm or more is 30 mass % or more and 80 mass % or less. If the ratio of a particle size of 2.8 mm or more is less than 30 mass %, the yield decreases and therefore the production rate decreases. If the ratio of a particle size of 2.8 mm or more exceeds 80 mass %, uneven sintering results in a decrease in production rate.

[0084] According to the invention, the highly combustible carbonaceous material coarsened is concentratedly distributed in the lower part, thereby making it possible to improve the permeability in the lower part. The improved permeability in the lower part increases the sintering speed during sintering, resulting in an improved production rate. Here, the bonding agent subjected to coarsening is only the highly combustible carbonaceous material. Since the highly combustible carbonaceous material has a high combustion speed, the impact of the decrease in combustion speed due to coarsening is small. Therefore, the sintering speed is not likely to decrease. Further, the increased sintering speed increases the cooling speed of the combustion zone 10A. This fines hematite (Fe2O3) particles that crystallize from a liquid phase produced by a high-temperature sintering reaction. During the reduction of hematite to magnetite (Fe3O4), crystals of blast furnace sintered ore expand. At that time, the resulting cracks cause the sintered ore to break down into powder. This degradation phenomenon is called reduction degradation. As mentioned above, the finer hematite (Fe2O3) particles reduce the amount of crystal expansion, providing the effect of inhibiting the reduction degradation of sintered ore.Use of Crushed Product of Carbonized-Wood Compression-Molded Product

[0085] In the invention, it is preferable to use a crushed product of the carbonized-wood compression-molded product as the highly combustible carbonaceous material. The reason for this is as follows: compressing and molding carbonized wood or a crushed product of carbonized wood once and then crushing it for use improves the product yield due to the increase in apparent density, and therefore improves the production rate, rather than using the carbonized wood or the crushed product of carbonized wood as is. The carbonized wood or the crushed product of carbonized wood is porous (typically with an apparent density of less than 0.6 g / cm3) and therefore has excellent combustibility, but the sintering speed increases too much, which actually reduces the yield and production rate of sintered ore.Carbon Content Mass Ratio

[0086] In the invention, the mass ratio of the carbon content of the highly combustible carbonaceous material to the carbon content of all bonding agents is preferably 25 mass % or more and 75 mass % or less. The mass ratio of the carbon content of the highly combustible carbonaceous material to the carbon content of all bonding agents is adjusted based on the carbon content of each carbonaceous material (low combustible carbonaceous material, highly combustible carbonaceous material) from the industrial analysis of the low combustible carbonaceous material and the highly combustible carbonaceous material. This is based on the following thought: if the blending ratio of the highly combustible carbonaceous material is less than 25 mass %, the effect of improving the sintering speed (flame front speed), which is the effect of blending the highly combustible carbonaceous material, cannot be obtained; and if the blending ratio of the highly combustible carbonaceous material exceeds 75 mass %, the product yield will decrease due to the high-speed combustion characteristic of the highly combustible carbonaceous material.Average Particle Size of Low Combustible Carbonaceous Material

[0087] In the invention, the average particle size of the low combustible carbonaceous material is also preferably in a range from 0.8 mm to 1.2 mm. Fining the average particle size of the low combustible carbonaceous material increases the number of carbonaceous material particles, which makes the heat source supply uniform in a cubic region having a side length of several mm to 10 mm in each part of the combustion zone 10A, thereby improving the product yield. In addition, segregation charging increases the amount of the bonding agents in the upper part of the sinter material packed bed insufficient in heat, further improving the product yield. As a result, the effect of improving the production rate is accelerated.

[0088] The average particle size of the low combustible carbonaceous material is determined as follows. After drying the low combustible carbonaceous material at 105 degrees C. for 2 hours or more, the material is classified using five sieves with different opening sizes by head tapping for 5 minutes to shake the material with a Ro-Tap shaker, and a sample mass wi of each particle size category i is measured. As shown in Table 1, the particle sizes (0.5 mm, 1.0 mm, 2.8 mm, 4.76 mm, 10.0 mm) that are boundary values for the particle size category are the opening sizes of the sieves used for classification. For example, a particle size category 2 “0.5-1.0” refers to sizes remained on the sieve when sieved through a sieve with an opening size of 0.5 mm and passing through the sieve when sieved through a sieve with an opening size of 1.0 mm.TABLE 1Particle size category i12345Particle size (mm)0-0.50.5-1.01.0-2.82.8-4.764.76-10.0Representative value0.250.751.93.87.5xi

[0089] The average particle size (mm) is an arithmetic mean diameter calculated by weighting a representative value xi (≈median) of the particle size category by a mass fraction (mass ratio) for each particle size category, as represented by the following formula (1).Average⁢ particle⁢ size=Σ⁢wixi / Σ⁢wiFormula⁢ (1)xi: representative value of particle size category i

[0091] wi: sample mass of particle size category iSegregation-Strengthened Type Charging Device

[0092] In the invention, it is also preferable to use a segregation-strengthened type charging device instead of the normal charging device 8. Consider a case where the blended material granules are charged onto the pallet using the segregation-strengthened type charging device, and the sinter material packed bed after charging is divided into five parts at equal intervals in the layer thickness (layer height) direction. In that case, also preferably, the average particle size of the materials in the uppermost part of the five parts of the sinter material packed bed is 0.3 to 0.5 times the average particle size of the materials in the lowermost part. In addition, also preferably, segregation charging is performed so that the carbon ratio in the uppermost part of the five parts of the sinter material packed bed is 1.10 to 1.16 times the carbon ratio in all layers of the sinter material packed bed, when the sinter material packed bed after charging is divided into five parts in the layer thickness (layer height) direction.

[0093] The segregation-strengthened type charging device is a charging device capable of increasing particle size segregation in the layer thickness direction of the sinter material packed bed compared to the gradient plate chute 82 illustrated in FIG. 1. Examples of the segregation-strengthened type charging device include: a slit-bar type charging device as illustrated in FIG. 2 (One et al., CAMP-ISIJ 10 (1997), p. 191, The Iron and Steel Institute of Japan); a slit wire type charging device (Takai et al., CAMP-ISIJ 6 (1993), p. 916); an intensified sifting feeder (ISF) type charging device which is an arranged dispersion type (Nagai et al., CAMP-ISIJ 29 (2016), p. 563); a hybrid type magnetic segregation charging device (Oyama et al., CAMP-ISIJ 11 (1998), p. 225); and a wind segregation device (Shibata et al., CAMP-ISIJ 14 (2001), p. 193). As illustrated in FIG. 2, a sieve member of a slit-bar type charging device 8A is provided such that the space between wires 82A (or rods) parallel to a pallet width direction is larger from the upper part toward the lower part of the pallet (Yoshinaga et al., Tetsu-to-Hagane (1987) Vol. 73, Abstracts of the 114th Lecture Meeting of the Iron and Steel Institute of Japan, S846). The wires (or rods) may be arranged parallel in an up-down direction of the pallet. A sieve member of the arranged dispersion type charging device is provided such that many bars are arranged along the material flow and the difference in height between adjacent bars increases from the upstream to the downstream of the material flow (Inazumi et al., Tetsu-to-Hagane 77 (1991), pp. 63-70). The configuration of each segregation-strengthened type charging device is described in each document described above, and the device can be used by referring to each document, and therefore a detailed description thereof will be omitted here.

[0094] Here, there are shown exemplary results of investigating the material particle size distribution and carbon concentration distribution in the height direction of the sinter material packed bed in a case where a blended material (see Table 6) used in Example 1-10 described later was charged into an actual machine using a slit-bar type charging device, which is a segregation-strengthened type charging device (One et al., CAMP-ISIJ 10 (1997), p. 191, The Iron and Steel Institute of Japan). A sample was collected in the height direction of the sinter material packed bed using a sampling device (described in JP 2018-044188 A). Specifically, blended material granules were charged on a bottom lid placed on a bedding layer, and then a cylindrical sampling tube was driven thereinto from directly above the bottom lid and the sample was collected with the bottom lid closed. Since the sample can be collected by referring to the above document (JP 2018-044188 A), detailed explanation thereof will be omitted here.

[0095] The collected sample was divided into five parts at equal intervals in the height direction of the sinter material packed bed (first to fifth layers from the top), and each layer was subjected to analysis. Table 2 shows the analysis results for the respective layers (first to fifth layers). As shown in Table 2, from the top layer (first layer), the masses of the respective layers were 4.30 kg, 5.16 kg, 4.88 kg, 4.62 kg, and 5.33 kg, the mass ratios of the respective layers were 18%, 21%, 20%, 19%, and 22%, and the recovered masses in the respective layers were almost equal. The mass fraction in Table 2 was obtained by taking 300 g of blended material granules of each layer (each of the first to fifth layers), drying them at 105 degrees C. for at least 2 hours, shaking them for 15 seconds without head tapping in a Ro-Tap shaker using a sieve with each mesh size shown on the horizontal axis (particle size category), and classifying them. This procedure allows a user to understand the particle size distribution of the sintering material before granulation, and the mass ratio for each particle size category of the sintering material in each layer was expressed as the abundance ratio (mass %). The average particle size in each layer in Table 2 is an arithmetic mean diameter calculated similarly to the formula (1), based on the representative value of the particle size category and the mass fraction of each particle size category. In Table 2, the particle size category “+8.0” refers to sizes remained on the sieve when sieved through a sieve with an opening size of 8.0 mm. The particle size category “to 4.0” refers to sizes remained on the sieve when sieved through a sieve with an opening size of 4.0 mm, and passing through the sieve when sieved through the sieve with an opening size of 8.0 mm indicated by the particle size category “+8.0” in the left column. The same applies to the particle size categories “to 2.0”, “to 1.0”, “to 0.5”, “to 0.25”, and “to 0.125”. The particle size category “−0.125” refers to sizes passing through the sieve when sieved through a sieve with an opening size of 0.125 mm.TABLE 2Average particleMass fraction (mass %) for each particle size category (mm)Averagesize of eachUpper section: Particle size category, Lower section: Representative valueparticlelayer / AverageCarbon+8.0to 4.0to 2.0to 1.0to 0.5to 0.25to 0.125−0.125sizeparticle size of allMassconcentration12631.50.750.3750.18750.0625(mm)layers (%)(kg)(mass %)First layer0.22.834.334.918.07.12.10.61.910.554.305.06Second0.818.536.622.312.46.42.30.72.760.795.164.73layerThird layer9.327.728.516.210.15.42.20.63.981.144.884.22Fourth8.823.231.418.810.15.21.90.63.771.084.623.88layerFifth layer17.327.025.014.18.85.12.20.54.751.365.334.55All layers————————3.48——4.48

[0096] FIGS. 3 to 5 are graphs created based on Table 2. FIG. 3 illustrates a particle size distribution of the sintering material before granulation in each layer (first to fifth layers) of the sinter material packed bed. FIG. 4 illustrates a ratio of the average particle size in each layer (first to fifth layers) to the average particle size in all layers. FIG. 5 illustrates a carbon concentration distribution in each layer (first to fifth layers) of the sinter material packed bed. This carbon concentration is a free carbon value involved in combustion, excluding carbon contained in carbonates, etc., and is calculated based on the combustion-infrared absorption method (JIS G1211-3:2018).

[0097] As illustrated in FIG. 3, the layer(s) in the lower part of the sinter material packed bed had a higher mass fraction of the coarse sintering material, i.e., the particle size category “+8.0 mm” (a particle size of 8.0 mm or more) and the particle size category “to 4.0 mm” (a particle size of less than 8.0 mm and 4.0 mm or more) compared to the layer(s) in the upper part of the sinter material packed bed. As a result, as shown in Table 2, the average particle size of the blended material in the lowermost layer (fifth layer) was in a range from 3 mm to 5 mm, and the average particle size in the uppermost layer (first layer) was 2 mm or less. In addition, as illustrated in FIG. 4, there was a difference in average particle size between the respective layers, with the average particle size of the first layer being 0.4 times that of the fifth layer. As illustrated in FIG. 5, the carbon concentration was the highest in the uppermost layer (first layer) at 5.06 mass %, which was 1.13 times the average carbon concentration of all layers. Thus, by charging the blended material, in which the highly combustible carbonaceous material is coarsened so that the ratio of a particle size of 2.8 mm or more is set to 30 mass % or more and the low combustible carbonaceous material is made finer so that its average particle size is in a range from 0.8 mm to 1.2 mm, using a segregation-strengthened type charging device, the bonding agents with a particle size of 0.25 mm or more and less than 1.00 mm segregate in the upper layer(s) which becomes brittle after sintering, increasing the carbon concentration. As a result, as shown in Example 1-10 described later, the amount of heat required for the sintering reaction is supplied to the upper layer(s), which facilitates an improvement in product yield and therefore the production rate. If the ratio of the highly combustible carbonaceous material with a particle size of 2.8 mm or more increases too much, the number of bonding agent particles will decrease, causing temperature unevenness in the sinter material packed bed. It is thus preferable to set the upper limit of the ratio of a particle size of 2.8 mm or more to 80 mass %. Since the segregation-strengthened type charging device was used here, the segregation of coarse and fine particles was evident. However, similar segregation of coarse and fine particles would probably occur in segregation charging using an ordinary gradient plate chute.Post-Addition Method

[0098] In the invention, it is also preferable that the entire amount of only the low combustible carbonaceous material, which is a part of sintering materials, is added in the latter half of the granulation step. Here, the “latter half of the granulation step” refers to the latter half of a time period when a “total granulation time” described below is divided into two parts (former and latter parts). In the granulation step in which the sintering materials are granulated using a granulator, the sintering materials, excluding the low combustible carbonaceous material, discharged from the sinter material bins are first charged into the granulator, mixed, and the humidity is adjusted before granulation begins (hereinafter, the granulated sintering materials excluding the low combustible carbonaceous material are referred to as pre-granules). After a certain period of time, the low combustible carbonaceous material (hereinafter also referred to as a post-added bonding agent) is added to the granulator (during the granulation step) to produce blended material granules. By adding the post-added bonding agent at a proper later timing, the pre-granules are coated with the post-added bonding agent, that is, the post-added bonding agent is not encapsulated in the pre-granules. The post-added bonding agent is either attached to the surface layer of the pre-granules or exists as independent granules unattached thereto. By using the blended material granules coated with the post-added bonding agent, it is possible to adjust the timing of the start of combustion of the post-added bonding agent during the sintering step.

[0099] The post-addition in the granulation step is performed, for example, as follows. For granulation, a piston-flow type cylindrical granulator with a central axis inclined downward toward the downstream side is used, and a small transport conveyor is inserted from a downstream outlet to a predetermined position inside the granulator. When the sintering materials excluding the post-addition bonding agent (hereinafter also referred to as sintering materials before post-addition) are fed from an upstream inlet, the sintering materials before post-addition fed are mixed, water is then added, and they move downstream while being granulated. The post-addition bonding agent is placed on the transport conveyor and conveyed from the downstream outlet of the granulator to the inner upstream side. The post-added bonding agent is added at a predetermined position to the pre-granules that have been moved from the upstream side in the granulator while being granulated, and all sintering materials (pre-granules and post-added bonding agent) are further granulated and discharged from the downstream outlet as blended material granules.

[0100] The granulation time of the sintering materials before post-addition that elapses before the addition of the post-addition bonding agent is preferably 80% or more and 96% or less of the total granulation time of the sintering materials, in other words, the post-addition is preferably performed at a timing 80% to 96% of the total granulation time of the sintering materials in the granulation step. For example, when the total granulation time is 240 seconds, the blended material excluding the low combustible carbonaceous material (sintering materials before post-addition) may be granulated for 225 seconds, and then the low combustible carbonaceous material may be added and granulated for 15 seconds. If the post-addition is performed at a timing earlier than 80% of the total granulation time, it is not possible to sufficiently prevent the post-added bonding agent from being encapsulated in the granules. If the post-addition is performed at a timing later than 96% of the total granulation time, the post-added bonding agent is insufficiently granulated. Here, the total granulation time in the granulation step does not include a time during which the sintering materials are simply mixed (mixing time) from when they are charged into the granulator until water is added. For sintering, a drum mixer is mainly used, which is of a constant speed piston flow type. Therefore, the post-addition position can be set by making the above-described granulation time correspond to a distance in a longitudinal direction of the drum mixer.

[0101] As described above, when the post-addition of the low combustible carbonaceous material is performed, the granules of the blended material are coated with the low combustible carbonaceous material, accelerating the reaction with atmospheric oxygen supplied to the sinter material packed bed. In addition, the low combustible carbonaceous material receives a sufficient amount of heat from the heated atmosphere and combusts. This improves the combustion speed of the bonding agent, resulting in higher production rate. If the low combustible carbonaceous material is encapsulated due to bulk granulation, it is difficult for the low combustible carbonaceous material to receive the oxygen and heat required for combustion. On the other hand, the highly combustible carbonaceous material for which the post-addition is not performed is encapsulated in blended material granules, but the highly combustible carbonaceous material has a low combustion start temperature and a fast combustion speed. By controlling the timing of the start of combustion of the bonding agents to start with the low combustible carbonaceous material coating the blended material granules therewith and then the highly combustible carbonaceous material encapsulated in the blended material granules and utilizing the difference in combustion properties therebetween, the overall combustion speed is increased and the production rate is further improved.Second Exemplary Embodiment

[0102] A preferred example in which an air volume control technique is used in the invention will be described below as a second exemplary embodiment. As described in the first exemplary embodiment, the production rate is improved by using a predetermined amount (30 mass % or more and 80 mass % or less) of the coarse highly combustible carbonaceous material in the invention. However, since the blended material contains the highly combustible carbonaceous material with a particle size of less than 2.8 mm, the highly combustible carbonaceous material (especially fine particles) is present in the upper part of the sinter material packed bed. Since the highly combustible carbonaceous material has a fast combustion speed, the temperature of the upper part of the sintered layer (up to a depth of about 100 mm from the surface layer) rapidly drops in the section from the first ignition to the start of reignition, reducing the yield and decreasing the effect of improving the production rate. The inventors have considered that using the air volume control technique would be effective to solve this problem. In the following, the air volume control technique will be described first to state the inventors' aims.Air Volume Control Technique

[0103] FIG. 6 schematically illustrates an exemplary air volume control in a DL-type sintering machine used in a reignition sintering method. As illustrated in FIG. 6, the downward suction device 6 (not illustrated in FIG. 1) for causing the sintering reaction to proceed downward is provided below pallets of the DL-type sintering machine 101. The downward suction device 6 controls a downward suction air volume in a sinter strand direction (the same as the pallet traveling direction 5x). Specifically, the downward suction device 6 includes pairs of wind boxes 61 and wind legs 62 successively installed in the sinter strand direction below the sinter material packed bed 10. The wind boxes 61 and the wind legs 62 are each connected to a blower 65 via a duct 64. A damper 63 is provided at the connection between the wind box 61 and the wind leg 62 for each pair or in the middle of each wind leg 62. By adjusting the opening of the damper 63, the suction air volume by each wind box 61 can be adjusted, and the distribution of the suction air volume in the sinter strand direction (pallet traveling direction) can be controlled.

[0104] Regarding the above-mentioned problem of the invention using the reignition sintering method, the inventors considered that by limiting an air volume reduction area to an area up to a reignition furnace outlet located on a sinter strand upstream side, and not reducing the air volume after reignition is completed, it would be possible to inhibit a decrease in the sintering speed in the upper part of the sintered layer, significantly improve the product yield, and obtain the effect of improving the production rate. The inventors repeated sintering experiments and found the following preferred embodiment. In addition, the inventors also considered a configuration for optimizing the pressure and the air volume reduced in the air volume reduction area, and the timing of reignition (separation time). Those will be described below.

[0105] When implementing the invention, it is preferable to reduce the downward suction air volume only in a section from a sinter strand upstream end to the outlet of the reignition furnace on the sinter strand upstream side (section S in FIG. 6). This is because the production rate is improved by performing, in the section S up to the outlet of the reignition furnace on the sinter strand upstream side, downward suction while reducing the air volume more than in a section downstream of the reignition furnace outlet (see Example 2 described later). As illustrated in FIG. 6, in a case where the most upstream wind box 61 is installed immediately below the inlet of the ignition furnace 3 in the sinter strand direction (in a case where an upstream end of the most upstream wind box 61 is positioned immediately below an upstream end of the ignition furnace 3), the section S corresponds to a section of the sinter strand from the inlet of the ignition furnace 3 to the outlet of the reignition furnace 4.

[0106] Preferred examples of the air volume reduction include the followings: an average superficial air volume of the atmosphere sucked in the section S up to the outlet of the reignition furnace on the sinter strand upstream side is set to 60% or more and 80% or less of an average superficial air volume of the atmosphere sucked in the section downstream of the outlet of the reignition furnace; and an average negative pressure in the wind boxes 61 or the wind legs 62 in the section S is set to 40% or more and 70% or less of an average negative pressure in the wind boxes 61 or the wind legs 62 in the section downstream of the outlet of the reignition furnace.

[0107] In the wind boxes 61 or the wind legs 62, the superficial air volume is measured, for example, by a Pitot tube, and the average superficial air volume is calculated. It should be noted that the measurement environment in the wind boxes 61 or the wind legs 62 is not necessarily good because the gas contains dust. The measurement may thus be performed using a hot wire anemometer or the like for a surface of the sinter material packed bed 10. The reason why the average superficial air volume of the atmosphere sucked in the section S is set to 60% to 80% of that after reignition is completed is as follows: if that average superficial air volume is less than 60%, the sintering speed drops significantly and the production rate decreases, and if that average superficial air volume exceeds 80%, it is not possible to obtain the production rate improving effect provided by the yield improvement through the air volume reduction.

[0108] Measuring the superficial air volume is difficult in some cases. In that case, the air volume can be controlled by the average negative pressure in each section. A relationship between the air volume passing through the sinter material packed bed and the negative suction pressure is represented by equation (2) of Japanese Permeability Unit (JPU) below. JPU is an airflow resistance index of the sinter material packed bed, indicating the permeability (ease of gas passing) of the sinter material packed bed.JPU=(Q / A)·(H / P)0.6(2)Q: Air volume (suction air flow rate) (Nm3 / min)

[0110] A: Suction area (m2)

[0111] H: Thickness of packed bed (m)

[0112] P: Negative pressure (mH2O)

[0113] From the equation (2), it can be seen that an air volume of 60% (40% reduction) corresponds to a negative pressure of 40% (60% reduction), and an air volume of 80% (20% reduction) corresponds to a negative pressure of 70% (30% reduction).

[0114] In the exemplary embodiment, the separation time, which is a time required for the pallet to pass through the section (atmosphere suction area 7) between the ignition furnace 3 and the reignition furnace 4, may be, for example, in a range from 0.5 minutes to 3.5 minutes, and is more preferably in a range from 0.5 minutes to 2.0 minutes (in a range from 30 seconds to 2 minutes). If the separation time is less than 0.5 minutes, it is not possible to supply sufficient oxygen to the combustion zone 10A in the upper part of the sintered layer. If the separation time exceeds 2.0 minutes, the temperature of the upper part of the sintered layer drops below the sintering reaction temperature, making it difficult to obtain the effect of improving the production rate of the reignition technique. The phrase “the pallet to pass through the section” refers to movement of the pallet between a position and another position.Third Exemplary Embodiment

[0115] A preferred example in which an oxygen enrichment technique is used in the invention will be described below as a third exemplary embodiment. In the invention using the reignition sintering method, the production rate may be further improved by extending a high-temperature retention time of the sintered layer. In that case, if an attempt is made to extend the separation time, which is a time from the completion of initial ignition until reignition is performed (a time required for the pallet to move through the section between the ignition furnace and the reignition furnace), the temperature of the upper part of the sintered layer would drop too much before reignition is performed, and so there has been a limit to how long the separation time can be extended. In particular, when using the highly combustible carbonaceous material, even if the particles thereof are coarse (with a particle size of 2.8 mm or more being 30 mass % to 80 mass %), the combustion time is short and the cooling speed increases. In order to solve this problem, the inventors have considered that using the oxygen enrichment technique would be effective in place of extending the separation time. In the following, the oxygen enrichment technique will be described to state the inventors' aims.Oxygen Enrichment Technique

[0116] The oxygen enrichment technique is a technique in which oxygen is enriched in the air to be sucked downward and the enriched gas is supplied to the sinter material packed bed. Specifically, for example, an oxygen enrichment gas supplying unit 9 is provided above the sinter material packed bed 10 as illustrated in FIGS. 7 and 8. The oxygen enrichment gas supplying unit 9 supplies an oxygen enrichment gas with a higher oxygen concentration than the air supplied during an ordinary operation, as the gas to be sucked downward (suction gas). By increasing the oxygen concentration, the amount of bonding agents combusted per unit time can be increased.

[0117] Regarding the above-mentioned problem of the invention, the inventors have considered that the high-temperature retention time of the sintered layer can be extended to improve the production rate by supplying the oxygen enrichment gas in the atmosphere suction area 7 for the reignition sintering method. The inventors repeated sintering experiments and found the following preferred example. In addition, the inventors also considered a configuration for optimizing the separation time and the oxygen concentration of the oxygen enrichment gas supplied in the atmosphere suction area 7. Those will be described below.

[0118] FIG. 7 schematically illustrates an exemplary DL-type sintering machine according to the exemplary embodiment (oxygen enrichment). As illustrated in FIG. 7, a DL-type sintering machine 103 includes the oxygen enrichment gas supplying unit 9 above the sinter material packed bed 10 in addition to the components of the DL-type sintering machine 101 described above. The oxygen enrichment gas supplying unit 9 supplies an oxygen enrichment gas with a higher oxygen concentration than the air (such as a gas mixture of the air and oxygen) into the sinter material packed bed 10. The oxygen enrichment gas supplying unit 9 includes, for example, a hood 91 and a gas pipe 92 through which the oxygen enrichment gas is supplied into the hood 91. The oxygen enrichment gas supplying unit 9 is configured to continuously supply the oxygen enrichment gas. The supplied oxygen enrichment gas is guided into the sinter material packed bed 10 by the downward suction of the downward suction device 6, where the oxygen enrichment gas advances the sintering reaction in the combustion zone 10A, and is then recovered as exhaust gas by the downward suction device 6 (wind box 61). By adjusting the supply amount of oxygen gas injected from the gas pipe 92, the oxygen concentration of the oxygen enrichment gas can be set to a predetermined concentration that has a higher oxygen content than the atmosphere.

[0119] As illustrated in FIG. 7, the oxygen enrichment gas supplying unit 9 is installed in the section between the ignition furnace 3 and the reignition furnace 4 (atmosphere suction area 7, see FIG. 6). The oxygen enrichment gas supplying unit 9 supplies the oxygen enrichment gas from a surface layer side of the sinter material packed bed 10 (sintered layer) passing through the atmosphere suction area 7, and the supplied gas is sucked downward. That is, in the third exemplary embodiment, the atmosphere suction area 7 where the atmosphere is supplied in the first and second exemplary embodiments serves as an oxygen enrichment gas suction area 7x (see FIGS. 6 and 7) where the oxygen enrichment gas is supplied. FIG. 7 illustrates an example in which a small safety interval is provided between the hood 91 of the oxygen enrichment gas supplying unit 9 provided in the atmosphere suction area 7 and the hood 32 (partition wall 32a) of the ignition furnace 3 and between the hood 91 and the hood 42 (partition wall 42a) of the reignition furnace 4. In this case, the hood 91 of the oxygen enrichment gas supplying unit 9 is installed in the oxygen enrichment gas suction area 7x, but if there are no safety issues, it is preferable to arrange the hoods 32, 42, 91 without such intervals therebetween, or to consecutively arrange the hood 91 of the oxygen enrichment gas supplying unit 9 and the hood 32 of the ignition furnace 33 and / or the hood 42 of the reignition furnace 4. The distance between the ignition furnace 3 (downstream partition wall 32a) and the oxygen enrichment gas supplying unit 9 (upstream wall of the hood 91), and the distance between the oxygen enrichment gas supplying unit 9 (downstream wall of the hood 91) and the reignition furnace 4 (upstream partition wall 42a) should be provided only when necessary for safety reasons, and even in that case, it is preferable that the distance not be longer than the minimum required distance. This is because the effect of improving the production rate decreases as the oxygen enrichment time decreases.

[0120] The amount of oxygen gas introduced into the suction gas supplied to the oxygen enrichment gas suction area 7x can be controlled by a method for measuring and adjusting the oxygen concentration on a surface of the sintered layer by installing a sampling tube on the surface of the sintered layer in the oxygen enrichment gas suction area 7x, or an adjustment method in accordance with the air volume of the wind box 61 of the DL-type sintering machine 103. The method for introducing the oxygen gas is, for example, as follows: oxygen (e.g., industrial oxygen) is supplied directly into the hood 91 installed above the oxygen enrichment gas suction area 7x and it is supplied, into the sinter material packed bed 10, together with the atmosphere (hereinafter also referred to as the atmosphere outside the hood) sucked in from around the hood 91 (outside the hood 91). At that time, it is desirable to supply the oxygen gas from multiple points in a width direction of the DL-type sintering machine 103 (direction orthogonal to the pallet traveling direction 5x). Alternatively, an oxygen enrichment gas having a predetermined oxygen concentration, in which oxygen and the air are mixed in advance, may be supplied into the hood 91 through the gas pipe 92. In addition, it is possible to use the industrial oxygen that is produced in oxygen plants within steelworks.

[0121] In implementing the invention, the time required for the pallet to pass through the section between the ignition furnace 3 and the reignition furnace 4 (separation time) can be set to, for example, 0.5 minutes or more and 6 minutes or less (see Table 8 described later). Preferably, the separation time is set to 1 minute or more and the oxygen concentration of the suction gas sucked downward from the surface layer side of the sintered layer in this section is set to 30 volume % or more. Here, the separation time includes a time required to pass through the above-mentioned safety interval (e.g., more than 0 seconds and 2 seconds or less). As shown in Examples described later, if the oxygen concentration of the suction gas is less than 30 volume % or if the separation time is less than 1 minute, the amount of oxygen supplied is insufficient and a sufficient effect of improving the production rate cannot be obtained.

[0122] The separation time is preferably 6 minutes or less. If the separation time exceeds 6 minutes, the effect of reignition will not be obtained even with the oxygen enrichment because the sintered layer is cooled by the suction gas. Further, the oxygen concentration of the suction gas is preferably 50 volume % or less. In a method in which pure oxygen is supplied into the sinter material packed bed 10 together with the atmosphere outside the hood as the suction gas, when the oxygen concentration of the suction gas is 50 volume %, the ratio of the amount of gas of the pure oxygen to the amount of gas of the atmosphere outside the hood reaches 60 volume % (not included in total mass). The amount of gas sucked into the sinter material packed bed is determined by the permeability of the sinter material packed bed and the power of a sintering machine blower (blower 65), so the amount of suction gas varies in minutes or seconds. Therefore, as the ratio of pure oxygen supply in the suction gas increases, the oxygen concentration of the suction gas is more greatly affected by fluctuations in the amount of the suction gas (the amount of pure oxygen supply is adjusted according to changes in the amount of the suction gas, but it becomes difficult to control). As a result of the difficulty in control, it becomes difficult to provide a stable oxygen concentration of the suction gas to be supplied to the sintered layer.

[0123] More preferably, the oxygen concentration of the suction gas is 40 volume % or less and the separation time is 5 minutes or less. This is because, if the oxygen concentration exceeds 40 volume % or the separation time exceeds 5 minutes, the increase in production rate accompanying the increase in oxygen supply becomes slow and the effect of oxygen enrichment is no longer effective, as shown in Examples below.

[0124] As supported by Examples described below, the oxygen enrichment in the oxygen enrichment gas suction area 7x of the invention has the effect of improving product yield and production rate despite an increase in sintering speed, and the effect of extending the proper separation time. The reason for the improvement in production rate includes that reignition immediately after the oxygen enrichment supplies heat to a coke combustion field that has been activated by the oxygen enrichment, which has the effect of maintaining and continuing the activation of the coke combustion field. Thus, even a short period of oxygen enrichment can lead to an improved product yield and production rate. The reason for the extension of the proper separation time includes that the sintered layer temperature increases due to the activated coke combustion caused by the oxygen enrichment. It is necessary to perform reignition before the sintered layer cools down, but since the oxygen enrichment performed in the oxygen enrichment gas suction area 7x makes the sintered layer less likely to cool down, the effect is exhibited even with a long separation time. A long separation time extends the high-temperature retention time of sintered ore, resulting in an improvement in yield. In contrast, oxygen enrichment without reignition only serves to inhibit a decrease in production rate due to a common sintering speed increase.Fourth Exemplary Embodiment

[0125] Another preferred example in which the oxygen enrichment technique is used in the invention will be described below as a fourth exemplary embodiment. The inventors have considered that it is also effective to perform the oxygen enrichment technique immediately after reignition, in place of performing it in the atmosphere suction area 7. The inventors' aims will be described below, and then a preferred example will be described.

[0126] In the reignition method, an upper part of a high temperature zone (zone roughly 1,000 degrees C. or more) of the sintered layer formed by the initial ignition is heated again by reignition, and the bonding agents (carbonaceous materials) that remained unburned during the initial ignition combust. As illustrated in FIG. 8 described later, combustion reactions occur at two locations in a bed height direction due to 1) the combustion of remaining bonding agents in the combustion zone formed by reignition (hereinafter also referred to as a reignition combustion zone 10A2) and 2) the combustion of bonding agents in the combustion zone formed by the initial ignition (hereinafter also referred to as an initial ignition combustion zone 10A1). In the sintering machine, the atmosphere containing oxygen necessary for combustion is sucked downward and flows vertically from the top to the bottom of the sintered layer in the bed height direction. In order to promote the combustion reactions in the above two locations, a large amount of oxygen is required. Especially when the highly combustible carbonaceous material is used, even if the particles are coarse (with a particle size of 2.8 mm or more being 30 mass % or more and 80 mass % or less), the combustion speed is fast and more oxygen is required.

[0127] Immediately after the end of reignition, in addition to the initial ignition combustion zone 10A1, the reignition combustion zone 10A2 is generated at another location in the bed height direction, and as a result, the high temperature zone expands in the bed height direction. In the reignition combustion zone 10A2, heat propagates through the sinter cake, which has already been sintered by the initial ignition and has a high void ratio, and thus the surface area of the solid (sinter cake) in contact with the gas sucked downward is small. In contrast, in the initial ignition combustion zone 10A1, heat propagates through the sintering material containing fine powder with a particle size of less than 1 mm, and thus the surface area of the solid (sintering material) in contact with the gas sucked downward is large. Therefore, the flame front speed of the reignition combustion zone 10A2 is faster than that of the initial ignition combustion zone 10A1, and the reignition combustion zone 10A2 merges with the initial ignition combustion zone 10A1 at an early stage after the end of reignition (see FIG. 8). The high temperature zone in sintering includes a combustion zone from when the bonding agents start to combust until the combustion is completed, and a zone where the materials are cooled to a temperature (approximately 1,000 degrees C.) at which the metallurgical reaction subsequently continues.

[0128] As described above, since a large amount of oxygen is required in the reignition sintering method, the sintering speed decreases when the downward suction of the atmosphere is normally performed. The inventors thus have considered that implementing the oxygen enrichment technique immediately after reignition in the invention using the reignition method can improve the sintering speed and therefore the production rate. The inventors repeated sintering experiments and found the following preferred example. In addition, the inventors also considered a configuration for optimizing the timing at which the oxygen enrichment starts, the oxygen enrichment time, and the oxygen concentration of the oxygen enrichment gas supplied in an oxygen enrichment area. Those will be described below.

[0129] FIG. 8 schematically illustrates another exemplary DL-type sintering machine according to the exemplary embodiment (oxygen enrichment). The above phenomenon will be described in detail below with reference to FIG. 8. In producing sintered ore using the reignition sintering method, the atmosphere suction area 7 is provided between the ignition furnace 3 and the reignition furnace 4 to supply sufficient oxygen (the atmosphere) to the combustion zone 10A formed on the surface layer of the sinter material packed bed 10 by the initial ignition, and the subsequent reignition causes the combustion of the bonding agents remaining on the surface layer without being ignited. In addition to the merging of the two combustion zones (initial ignition combustion zone 10A1 and reignition combustion zone 10A2) mentioned above, the combustion of the remaining bonding agents due to reignition increases the temperature of the gas sucked in from above and passing through the initial ignition combustion zone 10A1. This promotes the combustion of the bonding agents directly below the initial ignition combustion zone 10A1, and the width of the combustion zone 10A and the high temperature zone further expand. Furthermore, by performing reignition before the sinter cake 10B is cooled by the supply of the atmosphere in the atmosphere suction area 7, the high temperature retention time (e.g., a time retained at 1,200 degrees C. or more) is increased in the upper part of the sinter material packed bed 10. The high temperature retention time can be increased by performing reignition at an appropriate timing to supply heat, which can lead to improved product yield and production rate.

[0130] In the exemplary embodiment, the area where the oxygen enrichment gas is supplied to the surface of the sinter material packed bed 10 (sintered layer) and is sucked downward (oxygen enrichment gas suction area) is a predetermined section after the end of reignition. Thus, a DL-type sintering machine 104 of the exemplary embodiment includes, in addition to the components of the DL-type sintering machine 101 described above, the oxygen enrichment gas supplying unit 9 having a configuration similar to that described in the third exemplary embodiment, downstream of the reignition furnace 4 in the pallet traveling direction. In the exemplary embodiment, the separation time can be set to 0.5 minutes or more and 3.5 minutes or less, for example.

[0131] In FIG. 8, a small interval is provided between the reignition furnace 4 and the oxygen enrichment gas supplying unit 9. However, if there are no safety issues, the reignition furnace 4 and the oxygen enrichment gas supplying unit 9 may be arranged consecutively. The oxygen enrichment gas supplying unit 9 is configured to continuously supply the oxygen enrichment gas for a predetermined time, which is defined as the oxygen enrichment time. Preferably, the time required for the pallet to move through the distance between the reignition furnace 4 (downstream partition wall 42b) and the oxygen enrichment gas supplying unit 9 (upstream wall of the hood 91), i.e., the time from a point of time when reignition is completed to a point of time when oxygen enrichment starts is short. For example, even taking safety issues into consideration, the above-mentioned time is preferably more than 0 seconds and 30 seconds or less, more preferably more than 0 seconds and 10 seconds or less, and still more preferably, oxygen enrichment should be started immediately after the end of reignition (more than 0 seconds and 2 seconds or less). If the above-mentioned time exceeds 30 seconds, a time period during which oxygen enrichment is effective decreases, and the resulting effect decreases (see Test 2 in Example 4 described later).

[0132] When implementing the invention, in the oxygen enrichment gas suction area (the section in the pallet travel direction 5x where the oxygen enrichment gas supplying unit 9 is installed), it is preferable that the oxygen concentration of the suction gas sucked in from the surface layer side of the sintered layer by downward suction is 30 volume % or more, and that the oxygen enrichment time by the oxygen enrichment gas supplying unit 9 after the end of reignition is 30 seconds or more. Here, the oxygen enrichment time refers to a time required for the pallet to move through the oxygen enrichment gas suction area, i.e., a time from the start of oxygen enrichment to the end of oxygen enrichment when the oxygen enrichment gas is supplied after the completion of reignition. If the oxygen concentration of the suction gas is less than 30 volume % or if the oxygen enrichment time is less than 30 seconds, the amount of oxygen supplied is insufficient and neither the sufficient sintering speed nor the effect of improving the production rate is achieved (see Test 2 in Example 4 described later).

[0133] The oxygen concentration of the suction gas in the oxygen enrichment gas suction area is preferably 40 volume % or less. The time for oxygen enrichment by the oxygen enrichment gas supplying unit 9 is preferably 2 minutes or less. This is because, if the oxygen concentration exceeds 40 volume % or the oxygen enrichment time exceeds 2 minutes, the increase in sintering speed and production rate accompanying the increase in oxygen supply becomes slow and the effect of oxygen enrichment is no longer effective. The timing at which the two combustion zones (initial ignition combustion zone 10A1 and reignition combustion zone 10A2) merge was unknown until now. However, based on the proper oxygen enrichment time obtained from the test results (see Test 1 in Example 4 described later), it is estimated that the merging occurs at a timing more than 2 minutes and within 3 minutes after a point of time when reignition is completed.

[0134] In the exemplary embodiment, the sintering speed is significantly increased by the oxygen enrichment after reignition, as supported in Example 4 described later. This significant increase is greater than the increase in sintering speed with the oxygen enrichment without reignition, as described in previous examples (Tetsu-to-Hagane Vol. 92 (2006), pp. 417-426). The reason why the sintering speed is significantly increased is that oxygen is effectively utilized due to the presence of two combustion zones activated by the oxygen enrichment after the end of reignition. Thus, even a short period of oxygen enrichment improves the sintering speed and therefore the production rate.Fifth Exemplary Embodiment

[0135] In the second to fourth exemplary embodiments, the inventors focused on the effect of improving the production rate in the upper part of the sintered layer, whereas the effect of improving the production rate in a lower part of the sintered layer is extremely small. In the exemplary embodiment, as an effect on the lower part of the sintered layer, attention is focused on a stand-support sintering technique effective to increase the firing speed in the lower part of the sintered layer. In the following, the stand-support sintering technique will be described to state the inventors' aims for a preferred example.

[0136] With the stand-support sintering technique, the sinter cake formed in the upper part of the sinter material packed bed is supported by a stand, which reduces the load from above on the lower part of the sinter material packed bed (hereinafter also referred to simply as the lower layer part) during sintering thereof, thereby ensuring voids in the lower layer part. As a result, the air flow resistance in the lower layer part is reduced, and the sintering speed is improved (see JPH04-168234 A). The inventors considered that the production rate of the entire sintered ore is improvable by applying, to the invention, the stand-support sintering technique that has the effect of increasing the production rate of the lower layer part. However, the increase in a combustion zone width of the upper layer part according to the invention also affects the sintering conditions in the lower layer part. As a result of repeated sintering experiments by the inventors, it revealed that a synergistic effect beyond expectations was obtained between the invention and the stand-support sintering technique, as shown in Example 5 described later. In addition, the appropriate range of the separation distance (the distance between the ignition furnace and the reignition furnace) was also examined in a case where the invention was used in combination with the stand-support sintering technique. Those will be described below.Stand-Support Sintering Technique

[0137] FIG. 9 schematically illustrates an exemplary pallet used for the exemplary embodiment (stand-support sintering technique). As illustrated in FIG. 9, a pallet 5 includes a main frame 52 on which a grate bar 51 is disposed, and pallet side walls 53 erected on both opposing end portions of the main frame 52. Further, the pallet 5 has a single stand 16 (support member) disposed in a center portion of an upper surface of the grate bar 51. The stand 16 is a substantially plate-shaped member having a substantially isosceles trapezoidal shape. The stand 16 has a sinter cake supporting surface 16a. The stand 16 is installed parallel to the pallet traveling direction 5x, vertically to be embedded in the sinter material packed bed 10. In the middle of the firing of the sinter material packed bed 10, the upper layer part of the sintered layer (sinter material packed bed 10) in the pallet 5 is the sinter cake 10B where sintering is completed, while the lower layer part (the lower part of the combustion zone 10A) remains as unsintered sintering material, as illustrated in FIG. 9. Here, the upper surface portion (sinter cake supporting surface 16a) of the stand (supporting member) 16 supports the sinter cake 10B in the upper layer part and inhibits compaction of the sintering material of the lower layer part. By inhibiting the compaction, the airflow resistance of the sintered layer during the firing of the lower layer part is reduced, and the sintering speed is improved. Inhibiting the compaction also has the effect of inhibiting uneven flow of gas flowing through the sintered layer, reducing the amount of unfired parts and improving the sintering yield.

[0138] In implementing the invention, the stand-support technique described above is preferably used. That is, for the pallet 5 into which the blended material is to be charged, it is preferable that the support member (stand 16) having the sinter cake supporting surface 16a is erected on the grate bar 51 so as to be embedded in the sinter material packed bed 10. In the exemplary embodiment, any arrangement of the support member (stand 16) is usable as long as the support member can support the sinter cake 10B formed in the upper layer part, and for example, two rows of support stands may be installed in the pallet width direction. By providing multiple support members (stands 16), the sinter cake 10B formed in the upper layer part is supported and the load on the unfired lower layer part is reduced during the sintering of the lower layer part. This ensures voids in the lower layer part and uniformalizes the gas flow speed in the pallet width direction.

[0139] The effect of applying the stand-support sintering technique to the invention using the reignition sintering method is considered to be as follows.Product Yield

[0140] As features of the stand-support sintering technique, the improvement in sintering speed and product yield is limited to the lower layer part. In the exemplary embodiment, the time (separation time) required for each pallet to pass through the section between the ignition furnace 3 and the reignition furnace 4 (atmosphere suction area 7) can be set, for example, to 0.5 minutes or more and 3.5 minutes or less, and preferably less than 3.0 minutes (see Table 12 described later). In combination with the reignition sintering method that has the yield improvement effect on the upper layer part, it is possible to obtain a yield improvement effect on the entire part from the upper part to the lower part, provided that the separation time is within the above predetermined range (less than 3.0 minutes). From the above, additivity holds in terms of product yield. When the separation time exceeds 3.0 minutes, the effect of the yield improvement provided by reignition slows down, and the product yield starts to deteriorate due to the shortened sintering time by using the stand support.Sintering Speed

[0141] In the reignition sintering method, the combustion zone width of the sintered layer is increased by performing ignition twice. Increasing the combustion zone width of the sintered layer leads to improved product yield in the upper part of the sintered layer. However, the combustion zone width is sufficiently secured even with single-stage ignition sintering in the lower part of the sintered layer, so no further improvement can be expected in the lower part. On the contrary, an increase in the combustion zone width in the lower part of the sintered layer leads to an increase in airflow resistance, decreasing the sintering speed (BTS: Burn through speed). In particular, when using the highly combustible carbonaceous material, even if the particles are coarse (with a particle size of 2.8 mm or more being 30 mass % or more and 80 mass % or less), the combustion start temperature is low and the combustion speed is fast, and thus the combustion zone width is large. Here, the use of the stand-support sintering technique allows the sinter cake in the upper part to be supported, reducing the increase in airflow resistance during the sintering of the lower part and improving the sintering speed in the lower part. The increase in the sintering speed in the lower part inhibits the increase in the combustion zone width in the lower part, thereby improving the sintering speed acceleratingly. From the above, a synergistic effect is achieved in terms of sintering speed.Production Rate

[0142] The production rate is proportional to the product of the product yield and the sintering speed. The additive effect in product yield and the synergistic effect in sintering speed as described above provide a synergistic effect that exceeds expectations between the reignition sintering method and the stand-support sintering technique (see Example 5 described later).EXAMPLES

[0143] Tests relating to the above-described invention and preferred configurations, each applicable to the invention, will be described below. Examples 1 to 5 correspond to the first to fifth exemplary embodiments, respectively. In addition, Example 4 is described as being divided into two tests (Test 1 and Test 2).

[0144] In all Examples (Examples 1 to 5), the blending ratio of the sintering materials was the same except for the bonding agents (see Table 4 described later). Regarding the downward suction conditions (suction pressure or air volume) during sintering, the measured value under the pot was constant at 1,300 mmAq (12.7 kPa) in Examples 1 and 5, and the air volume (exhaust gas) was constant at 1.80 Nm3 / min in Examples 3 and 4. In Example 2, the air volume (exhaust gas) was basically set at 1.80 Nm3 / min, and the air volume control was performed according to the test conditions described later.Example 1Test Cases

[0145] Tests relating to the conditions for the composition ratio of the carbonaceous materials (low combustible carbonaceous material and highly combustible carbonaceous material), the average particle size of the low combustible carbonaceous material, and the particle size of the highly combustible carbonaceous material described in the first exemplary embodiment will be described below. In the following tests, only coke breeze is used as the low combustible carbonaceous material, but the same effect can be obtained by using both anthracite and coke, or only anthracite, as the low combustible carbonaceous material. In the following tests, only semi-coke or only the carbonized-wood compression-molded product (crushed product) is used as the highly combustible carbonaceous material. The invention, however, is not limited thereto, and the same effect can be obtained, for example, by using any one of the other highly combustible carbonaceous materials or by mixing multiple types of highly combustible carbonaceous materials.

[0146] In Examples, the tests were performed using a method commonly known as the sinter pot test. In the sinter pot test, sintering materials (blended material) containing the bonding agents as fuel are charged into a container of a specified size, and the sintering process is promoted by performing ignition from above and performing suction from below. Although a device for the sinter pot test does not use a pallet to move the sinter material packed bed as in the Dwight-Lloyd (DL) type sinter machine, it can simulate sintering performed using the DL-type sinter machine. As shown in Table 6 below, 17 tests were performed, including Comparatives 1-0 to 1-3 and Examples 1-1 to 1-13. First, the materials and methods for the tests will be described, and then the test results will be described.Materials

[0147] Table 3 shows the results of industrial analysis and elemental analysis of the low combustible carbonaceous material and highly combustible carbonaceous materials used in the tests. As shown in Table 3, coke breeze was prepared as the low combustible carbonaceous material, and semi-coke and the carbonized-wood compression-molded product (crushed) were prepared as the highly combustible carbonaceous materials.TABLE 3Industrial analysis (mass %-dry)Apparent AshVolatile matterFixedElemental analysis (mass %)densitycontentcontentcarbonCHNTotal-S(g / cm3)LowCoke breeze12.51.186.484.2<0.101.30.61.3combustiblecarbonaceousmaterialHighlySemi-coke5.55.189.588.60.81.10.21.2combustibleCarbonized-wood2.68.289.290.71.370.750.020.9carbonaceouscompression-moldedmaterialproductBlend of Materials

[0148] Tables 4 and 5 show the blending ratios (mass %) of the sintering materials for each blended material used in the tests. As shown in Table 4, the proportions of new materials (iron ore, limestone, quicklime, and olivine) and return ore were kept constant for all test cases. Iron ores A to E are mutually different brands (production areas) of iron ore. The blending ratio of the return ore was set to 15.0 mass % (not included in total mass) with the new materials (iron ore, limestone, quicklime, and olivine) at 100 mass %. The blending ratios of the materials, excluding the bonding agents, are the same in Examples 1 to 5.

[0149] Table 5 shows the blending ratios (mass %) of the bonding agents in the blended material used in the tests. Coke breeze was used as the low combustible carbonaceous material in all test cases. As the highly combustible carbonaceous material, the crushed carbonized-wood compression-molded product was used in Examples 1-12 and 1-13, and semi-coke was used in the other test cases. In the test cases (Comparatives 1-0 and 1-2) in which only the low combustible carbonaceous material (coke breeze) was used as the bonding agent, the blending ratio of the bonding agent (coke breeze) was set to 4.5 mass % (not included in total mass) with the new materials at 100 mass %. In the test cases (except for Comparatives 1-0 and 1-2) in which the highly combustible carbonaceous material (semi-coke or the carbonized-wood compression-molded product) was blended, using the blending amount of the bonding agent (coke breeze 4.5 mass %) in Comparatives 1-0 and 1-2 as a basis, the blending ratios (middle part of Table 5) of the bonding agents (low combustible carbonaceous material and highly combustible carbonaceous material) to the new materials were adjusted so that the fixed carbon content (carbon content) of all bonding agents (coke breeze, semi-coke, the carbonized-wood compression-molded product) contained in the blended material and determined by industrial analysis was kept constant in all test cases, based on the mass ratio (upper part of Table 5) of the carbon content of the low combustible carbonaceous material (coke breeze) and the highly combustible carbonaceous material (semi-coke, the carbonized-wood compression-molded product) and the fixed carbon content (Table 3) of the low combustible carbonaceous material (coke breeze) and the highly combustible carbonaceous material (semi-coke, the carbonized-wood compression-molded product). The blending ratios of the low combustible carbonaceous material and the highly combustible carbonaceous material to the total amount of bonding agents are shown in the lower part of Table 5.TABLE 4New materialsReturn oreIron ore(non-Bonding agentsABCDELimestoneQuicklimePeridotiteTotalinclusive)(non-inclusive)Brands14.719.619.613.814.712.72.941.96100.015.0See middle part ofTable 5TABLE 5Com-Com-Comparative 1-3parativesparativeExample Examples 1-2, ExampleExampleExample1-0, 1-21-11-11-4 to 1-111-31-121-13Low combustible carbonaceous materialCoke breeze—Coke breezeCoke breezeCoke breezeCoke breezeCoke breezeHighly combustible carbonaceous material—Semi-cokeSemi-cokeSemi-cokeSemi-cokeCarbonized-Carbonized-woodwoodcompression-compression-molded molded productproductCarbon content mass Low combustible10002550758075ratio (mass %)carbonaceous(Carbon equivalentmaterialblending ratio)Highly combustible01007550252025carbonaceousmaterialBlending ratio of bondingLow combustible4.5001.132.253.383.603.38agent(s) to new materialscarbonaceous(non-inclusive, mass %)materialHighly combustible04.343.262.171.090.871.09carbonaceousmaterialTotal (sum total)4.504.344.384.424.464.474.46Blending ratio toLow combustible100025.750.975.780.575.6all bonding agentscarbonaceous(mass %)materialHighly combustible010074.349.124.319.524.4carbonaceousmaterialThe test conditions are shown in the upper part of Table 6 below. The carbon content mass ratio in Table 6 is the same as the carbon content mass ratio in the upper part of Table 5. As shown in Tables 5 and 6, Comparative 1-0 is a case in which no reignition was performed and only the low combustible carbonaceous material (coke breeze) was used as the bonding agent without blending the highly combustible carbonaceous material. In addition, Examples 1-12 and 1-13 are cases in which the crushed carbonized-wood compression-molded product was used as the highly combustible carbonaceous material.

[0151] As shown in Table 6, the average particle size of the low combustible carbonaceous material (see paragraph 0046) was 0.6 mm, 0.8 mm, 1.2 mm, and 1.4 mm in Examples 1-6 to 1-9, respectively, 1.2 mm in Examples 1-12 and 1-13, and 1.0 mm in the other test cases. The ratio of the highly combustible carbonaceous material with a particle size of 2.8 mm or more (the ratio of the highly combustible carbonaceous material with a particle size of 2.8 mm or more to the whole highly combustible carbonaceous material) was 20 mass %, 50 mass %, and 70 mass % in Comparative 1-3, Examples 1-4, and Examples 1-5, respectively, 80 mass % in Examples 1-12 and 1-13, and 30 mass % in the other test cases. In Examples 1-11 and 1-13, the entire amount of only the low combustible carbonaceous material out of the sintering materials was added in the latter half of the granulation step of the sintering materials. In addition, in Examples 1-10 and 1-13, the segregation-strengthened type charging device was used to charge the blended material after the granulation process.TABLE 6Comp. Comp.Comp.Comp.1-01-1Ex. 1-1Ex. 1-2Ex. 1-31-21-3Ex. 1-4Ex. 1-5Low combustibleCarbon content1000255075100505050carbonaceousmass ratiomaterial(mass %)(coke breeze)Average particle1.0—1.01.01.01.01.01.01.0size (mm)GranulationIn bulkIn bulkIn bulkIn bulkIn bulkIn bulkIn bulkIn bulkIn bulkmethodHighly combustibleCarbon content01007550250505050carbonaceousmass ratiomaterial(mass %)(semi-coke,+2.8 mm—30303030—205070carbonized-wood(mass %)compression-moldedproduct only inExamples 1-12 and1-13)Charging deviceNormalNormalNormalNormalNormalNormalNormalNormalNormalReignition-separation time (min)(no 1.01.01.01.01.01.01.01.0reignition)—Sintering speed (BTS) (mm / min)16.218.818.217.817.716.017.018.318.7Production rate (t / (Dm2))26.929.031.030.530.327.629.331.131.8Product yield (mass %)70.966.873.773.873.973.574.373.573.5Reduction degradation (mass %)34.029.727.827.528.033.031.927.827.8Ex. Ex. Ex. Ex. 1-6Ex. 1-7Ex. 1-8Ex. 1-9Ex. 1-101-111-121-13Low combustibleCarbon content5050505050508075carbonaceousmass ratiomaterial(mass %)(coke breeze)Average particle0.60.81.21.41.01.01.21.2size (mm)GranulationIn bulkIn bulkIn bulkIn bulkIn bulkPost-In bulkPost-methodadditionadditionHighly combustibleCarbon content5050505050502025carbonaceousmass ratiomaterial(mass %)(semi-coke,carbonized-wood+2.8 mm3030303030308080compression-molded(mass %)product only inExamples 1-12 and1-13)Charging deviceNormalNormalNormalNormalSegregationNormalNormalSegregationReignition-separation time (min)1.01.01.01.01.01.01.01.0Sintering speed (BTS) (mm / min)17.317.617.818.318.319.217.719.4Production rate (t / (Dm2))29.830.230.330.331.433.130.533.5Product yield (mass %)74.574.073.272.074.174.374.374.3Reduction degradation (mass %)28.127.627.327.927.627.126.926.9Granulation Method

[0152] The granulator used was a cylindrical batch-type drum mixer (rotation speed: 25 rpm) with a diameter of 600 mm and a length of 800 mm. In the test cases performing the bulk granulation other than Examples 1-11 and 1-13, all sintering materials including the new materials, the bonding agents, and returned ore were put into the granulator and mixed for 4 minutes, after which water was added (humidity adjustment) to reach a target moisture value, and the mixture was mixed (granulated) for another 4 minutes. In Examples 1-11 and 1-13 (cases in which only the low combustible carbonaceous material was post-added), the sintering materials excluding the low combustible carbonaceous material were put into the granulator and mixed for 4 minutes, after which water was added (humidity adjustment) to reach a target moisture value and mixed (granulated) for 3 minutes 45 seconds. The granulator was then stopped once, and the low combustible carbonaceous material (coke breeze) was added and mixed (granulated) for another 15 seconds (see paragraph 0056). The final target moisture value was set to 7.5 mass % (not included in total mass) based on the mass of the whole sintering material including all bonding agents and return ore.Charging Method

[0153] The pot test device used was a cylindrical pot with a diameter of 300 mm and a height of 500 mm. In Examples 1-10 and 1-13 (cases of segregation strengthened charging), as described below, charging was performed according to a method using a slit-bar type classifier described in CAMP-ISIJ 24 (2011), p. 795 (Hara et al.), simulating segregation strengthened charging using the above-mentioned segregation-strengthened type charging device (slit-bar type charging device, see paragraph 0051). In the test cases of normal charging other than Examples 1-10 and 1-13, a flat plate was used instead of the slit-bar type classifier.Segregation Strengthened Charging

[0154] In Examples 1-10 and 1-13, blended material granules classified using a slit-bar type classifier were charged into a sinter pot. Specifically, the blended material granules were placed in the slit-bar type classifier, and the blended material granules that fell from between slit bars were collected in four collection boxes arranged from upstream to downstream directly below the slit bars, and the blended material granules that slid on the slit bars were collected in another collection box. For the blended material granules classified and collected in the five collection boxes, the blended material granules that had slid on the slit bars were first dropped into the pot from thereabove, and then the blended material granules that had fallen from between the slit bars were dropped into the pot from thereabove in order from the most downstream collection box.Firing Conditions

[0155] After the blended material granules were charged, a surface of the sinter material packed bed was initially ignited (initial ignition) for 1.0 minute using an ignition device (flame heating). After 1.0 minute had elapsed from the end of ignition, reignition was performed for 1.0 minute. The separation time of 1.0 minute from the end of ignition to the start of reignition corresponds to 4% of the length of the entire strand when converted into the actual machine. The ignition time and the reignition time were each 1.0 min (heat amount: as sensible heat of the suction gas, 25 MJ / ton of blended material). In addition, since heat loss is large in the pot test, approximately 100 MJ / ton of blended material is required for the heating amount of LPG gas. The negative suction pressure during sintering was adjusted by changing the valve opening on the suction side of the blower so that it was constant at 1,300 mmAq (12.7 kPa) as measured under the pot.

[0156] Along with the pressure, the temperature was also measured using a thermocouple under the pot. In sintering, when the combustion zone reaches the lowermost part of the sinter material packed bed, the exhaust gas temperature under the pot starts to rise, eventually reaches a peak, and then decreases as the coke combustion is completed. Three minutes after the exhaust gas temperature peaked, the blower suction was stopped. The sintering time was determined as a time from a point of time when ignition started to a point of time when the exhaust gas temperature peaked. The sintering speed (BTS: burn through speed) was calculated by dividing the thickness of the material layer by the sintering time.Product Yield

[0157] After sintering, the resulting sinter cake was dropped four times from a height of 2 m, and the sintered ore with a particle size of +5 mm (exceeding 5 mm) excluding bedding ore was collected as a sintered product, and its mass was calculated and determined as the mass of the sintered product. A value obtained by dividing the mass of the sintered product by the mass of the sinter cake excluding bedding ore, was defined as the product yield.Production Rate

[0158] The production rate was calculated by dividing the mass of the sintered product (product amount (ton)) by the above-described sintering time (converted into days) and the firing area (pot bottom area (m2)), as represented by the following formula (3).Production⁢ Rate⁢ (t / (Dm2))=Product⁢ Amount⁢ (t) / {Sintering⁢ Time⁢ (Day)·Pot⁢ Bottom⁢ Area⁢ (m2)}Formula⁢ (3)Reduction Degradation

[0159] Reduction degradation of sintered ore indicates a degree of degradation of sintered ore under conditions simulating a low-temperature reduction zone of a blast furnace, and was performed using a method defined in the JIS method (JIS M8720:2017 “Iron ores-Determination of low-temperature reduction-disintegration”). That is, 500 g of the obtained sintered ore having a particle size of more than 15 mm and less than 20 mm was reduced for 30 minutes at 550 degrees C. by passing a gas of CO (30 vol %)-N2 (70 vol %) composition at 15 liters / min. Then, it was rotated at 30 rpm for 30 minutes in a cylindrical rotary drum (130 mmφ×200 mmL), and the ratio of the particle size-2.8 mm (mass ratio of iron ore under the sieve of 2.8 mm opening size) was used as an index of reduction degradation (RDI: reduction degradation index).Test Results

[0160] The test results are shown in the lower columns of Table 6. The cases in which the production rate was 29.5 t / (Dm2) or more in the test results are Examples of Example 1.

[0161] FIG. 10 is a graph illustrating the test results of Comparative 1-1 (using only the highly combustible carbonaceous material as the bonding agent), Examples 1-1 to 1-3 (carbon content mass ratio of the highly combustible carbonaceous material: 75 mass %, 50 mass %, and 25 mass %), Comparative 1-3 (carbon content mass ratio of the highly combustible carbonaceous material: 50 mass %), Examples 1-4 and 1-5 (carbon content mass ratio of the highly combustible carbonaceous material: 50 mass %), and Examples 1-12 and 1-13 (carbon content mass ratio of the highly combustible carbonaceous material: 20 mass % and 25 mass %, highly combustible carbonaceous material: carbonized-wood compression-molded product) in Table 6, the graph illustrating a relationship between the ratio of the highly combustible carbonaceous material with the particle size+2.8 mm and the production rate. FIG. 11 is a graph illustrating the test results of Comparative 1-0 (single-stage ignition), Comparatives 1-1 and 1-2 (using only the highly combustible carbonaceous material as the bonding agent, and using only the low combustible carbonaceous material as the bonding agent), Comparative 1-3 (ratio of semi-coke with a particle size of 2.8 mm or more: less than 30 mass %), Examples 1-1 to 1-5 (average particle size of coke breeze: 1.0 mm, highly combustible carbonaceous material: semi-coke), and Examples 1-12 and 1-13 (average particle size of coke breeze: 1.2 mm, highly combustible carbonaceous material: carbonized-wood compression-molded product) in Table 6, the graph illustrating a relationship between the mass ratio (mass %) of the carbon content of the highly combustible carbonaceous material to the carbon content of the bonding agent(s) and the production rate (t / (Dm2)). FIG. 12 is a graph illustrating the test results of Examples 1-6 to 1-11 in Table 6 (carbon content mass ratio of semi-coke: 50 mass %, ratio of semi-coke with a particle size of 2.8 mm or more: 30 mass %), the graph illustrating a relationship between the average particle size (mm) of the coke breeze and the production rate (t / (Dm2)).

[0162] As shown in Table 6 and FIGS. 10, 11, and 12, the production rate was improved in Examples 1-1 to 1-13, in which the mass ratio of the carbon content of the highly combustible carbonaceous material to the carbon content of all bonding agents was in a range from 25 mass % to 75 mass %, and the ratio of the highly combustible carbonaceous material (semi-coke) with a particle size of 2.8 mm or more was in a range from 30 mass % to 80 mass %. Since the carbonized-wood compression-molded product has a low combustion start temperature, the production rate increases similarly to semi-coke. Further, since the combustion start temperature of the carbonized-wood compression-molded product is lower than that of semi-coke, even when the mass ratio of the carbon content of the highly combustible carbonaceous material to the carbon content of all bonding agents was low at 20 mass % (Example 1-12), the production rate was equivalent to that of semi-coke at 25 mass % (Example 1-3). In addition, as shown in Table 6 and FIG. 12, in Examples 1-7 and 1-8 in which the average particle size of the coke breeze (low combustible carbonaceous material) was 0.8 to 1.2 mm, the production rate and reduction degradation were further improved (compared with Examples 1-6 and 1-9). In Example 1-10 in which segregation-strengthened charging was performed, the product yield was greatly improved, and therefore the production rate was further improved. In Example 1-11 in which coke breeze was post-added, the flame front speed (sintering speed) and the product yield were significantly improved, and therefore the production rate was significantly improved. By comparing Comparative 1-0 (single-stage ignition sintering method) with Comparative 1-2 (reignition sintering method), it was confirmed that performing the reignition improved the product yield while maintaining the sintering speed, thereby improving the production rate.Example 2Test Cases

[0163] In Example 2, the same sinter pot test as in Example 1 was used to verify the applicability of the air volume control technique to the invention. The test conditions and the like different from those of Example 1 for test cases of Example 2 (Comparatives 2-1 to 2-2, and Examples 2-1 to 2-3) will be described below with reference to Table 7 described later. In the following description, duplicated descriptions of the conditions, test methods, and the like that are same as those in Example 1 will be omitted as appropriate.Blending of Materials, etc.

[0164] The materials blended were the same as those in Example 1 (Table 4), except for the bonding agents. The blending of the bonding agents (carbonaceous materials) in the sintering material and the particle size of the bonding agents (the average particle size of the low combustible carbonaceous material and the ratio of the highly combustible carbonaceous material with a particle size of 2.8 mm or more) were the same as those in Comparative 1-2 or Example 1-2 of Example 1. Specifically, as shown in Tables 5 to 7, in Comparative 2-1 to 2-2, as with the former (Comparative 1-2), only 4.5 mass % (not included in total mass) of coke breeze (low combustible carbonaceous material) was used relative to the new materials (carbon content mass ratio of bonding agents: 0 mass % for semi-coke and 100 mass % for coke breeze), and the average particle size of coke breeze was 1.0 mm. In Examples 2-1 to 2-3, as with the latter (Example 1-2), the carbon content mass ratio was 50 mass % coke breeze (low combustible carbonaceous material) and 50 mass % semi-coke (highly combustible carbonaceous material), the average particle size of coke breeze was 1.00 mm, and the ratio of the highly combustible carbonaceous material with a particle size of 2.8 mm or more was 30 mass %. In all test cases of Example 2, the granulation method was bulk granulation and the charging method was normal charging.Firing Conditions

[0165] In all test cases of Example 2, the ignition time for ignition (corresponding to ignition by the ignition furnace 3 (initial ignition)) and the ignition time for reignition (corresponding to reignition by the reignition furnace 4) were each 1 minute (heat amount: as sensible heat of the suction gas, 25 MJ / ton of blended material). As shown in the column for the interval between two ignitions in Table 7, the point of time when reignition was started was set to 1 minute after the point of time when ignition was completed (separation time: 1 minute).

[0166] The suction air volume was adjusted by changing the valve opening on the suction side of the blower so that the superficial air volume inside the pot was one of Condition 0 to Condition 2 below.

[0167] Condition 0: Air volume control OFF, constant air volume of 1.80 Nm3 / min

[0168] Condition 1: Air volume control ON1, air volume of 1.35 Nm3 / min until completion of reignition, then air volume of 1.80 Nm3 / min

[0169] Condition 2: Air volume control ON2, air volume of 1.20 Nm3 / min until completion of reignition, then air volume of 1.80 Nm3 / min

[0170] As shown in the column of air volume control in Table 7, Comparatives 2-1 and 2-2 were set to Condition 0 (OFF) and Condition 2 (ON2), respectively. Also, the test was performed under Condition 0 (OFF) in Example 2-1, Condition 1 (ON1) in Example 2-2, and Condition 2 (ON2) in Example 2-3.TABLE 7Test conditionsMass ratio of a: Superficialcarbon content of air volumeb: SuperficialTest resultsbonding agentsInterval (until air volumeSintering(mass %)betweenAir completion(after completionspeedProductProduction(Coke breeze:ignition andvolumeof reignition)of reignition)(BTS)yieldrateSemi-coke)reignitioncontrol(Nm3 / min)(Nm3 / min)a / b(mm / min)(mass %)(t / (Dm2))Comp. 2-1100:01 minuteOFF1.801.801.019.476.035.1Comp. 2-2100:01 minuteON21.201.800.718.978.935.4Ex. 2-150:501 minuteOFF1.801.801.020.776.236.9Ex. 2-250:501 minuteON11.351.800.820.478.237.2Ex. 2-350:501 minuteON21.201.800.720.279.137.3Test Results

[0171] The right columns of Table 7 show the test results (sintering speed, product yield, and production rate for each test case). The cases in which the production rate was 36.0 t / (Dm2) or more in the test results are Examples of Example 2. The sintering speed, product yield, and production rate were determined in the same manner as in Example 1. FIG. 13 is a graph illustrating the test results, illustrating a relationship between the superficial air volume ratio (a / b) and the production rate (t / (Dm2)).

[0172] As shown in Table 7 and FIG. 13, Examples 2-1 to 2-3 in which the highly combustible carbonaceous material was used, had a higher production rate than Comparatives 2-1 and 2-2 in which no highly combustible carbonaceous material was used. In addition, in Example 2-2 (superficial air volume: 1.35 Nm3 / min) and Example 2-3 (superficial air volume: 1.20 Nm3 / min) where the air volume was reduced in the section up to the completion of reignition, the product yield was improved significantly, which compensated for the decrease in sintering speed more than Example 2-1 where no air volume reduction was performed. The production rate of Examples 2-2 and 2-3 were increased by 0.3 t / (Dm2) and 0.4 t / (Dm2), respectively.Example 3Test Cases

[0173] In Example 3, the same sinter pot test as in Example 1 was used to verify the applicability of the oxygen enrichment technique to the invention. The test conditions and the like different from those of Example 1 for test cases of Example 3 (Examples 3-1 to 3-10) will be described below with reference to Table 8 described later. In the following description, duplicated descriptions of the conditions, test methods, and the like that are same as those in Example 1 will be omitted as appropriate.Blending of Materials, etc.

[0174] The materials blended were the same as those in Example 1 (Table 4), except for the bonding agents. In all test cases of Example 3 (except for Comparative 2-1 shown for reference in Table 8), the blending of the bonding agents (carbonaceous materials) in the blended material and the particle size of the bonding agents (the average particle size of the low combustible carbonaceous material and the ratio of the highly combustible carbonaceous material with a particle size of 2.8 mm or more) were the same as those in Example 1-2 of Example 1. In all test cases of Example 3, the granulation method was bulk granulation and the charging method was normal charging. Semi-coke was used as the highly combustible carbonaceous material.Firing Conditions

[0175] In all test cases of Example 3, the ignition time for ignition (corresponding to ignition by the ignition furnace 3 (initial ignition)) and the ignition time for reignition (corresponding to reignition by the reignition furnace 4) were each 1 minute (heat amount: as sensible heat of the suction gas, 25 MJ / ton of blended material). As shown in Table 8, for each test case (Examples 3-1 to 3-10), the time (separation time) between ignition and reignition (corresponding to the movement through the oxygen enrichment gas suction area) and the oxygen concentration of the suction gas were changed to the levels indicated as the test conditions in Table 8.

[0176] During firing after ignition, the suction air volume was kept constant and the air volume was adjusted to 1.80 Nm3 / min of exhaust gas. As in Examples 1 and 2, suction by the blower was stopped 3 minutes after the exhaust gas temperature reached its peak, and firing was terminated. The sintering time was determined as a time from the point of time when ignition started to the point of time when the exhaust gas temperature peaked.

[0177] In Examples 3-1 and 3-4, oxygen enrichment was not performed, and the atmosphere (the air) was used as the suction gas. An oxygen injection method in Examples 3-2 to 3-3 and 3-5 to 3-10 was as follows: the atmosphere and oxygen were mixed in a gas blender to provide a predetermined oxygen concentration (oxygen concentration in Table 8), and then this mixed gas was supplied for a predetermined time (oxygen enrichment time in Table 8) using a hood placed over the pot. The supply and suction of oxygen enrichment gas was limited to a period from immediately after the completion of ignition (more than 0 seconds and 2 seconds or less after the point of time when ignition was completed) to immediately before the reignition (more than 0 seconds and 2 seconds or less before the point of time when reignition was started). Specifically, when ignition was completed, the hood was set and the oxygen enrichment gas with a predetermined oxygen concentration was supplied, and this oxygen enrichment gas was suctioned. After a predetermined time, the hood was immediately removed and reignition was performed. Here, the flow rate was determined so that an inlet-side gas volume (=suction gas volume) was all mixed gas via the gas blender. The inlet-side gas volume is determined from a nitrogen concentration of the inlet-side gas, a nitrogen concentration of the exhaust gas, and a volume of the exhaust gas. This is because the amounts of nitrogen gas on the inlet and outlet sides are equal. Furthermore, the nitrogen concentration of the exhaust gas can be calculated by subtraction (N2≈100−(CO+CO2+O2)) from the exhaust gas analysis (CO, CO2, O2). In Example 3, the volume of the exhaust gas was kept constant, so that the suction gas was easily adjusted.TABLE 8Test conditionsMass ratio of carbon contentSeparation timeof bonding agents (mass %)(= oxygenTest results(Low combustible carbonaceousenrichmentOxygenSinteringProductProductionmaterial:Highly combustibletime)concentrationspeedyieldratecarbonaceous material)(min)(vol. %)(mm / min)(mass %)(t / (Dm2))Ex. 3-150:500.52120.676.737.0Ex. 3-250:500.53020.877.837.9Ex. 3-350:500.54020.977.938.2Ex. 3-450:501.02120.776.236.9Ex. 3-550:501.03021.078.538.7Ex. 3-650:502.03021.778.740.2Ex. 3-750:502.04022.379.041.3Ex. 3-850:505.04022.679.141.5Ex. 3-950:505.05022.579.141.4Ex. 3-1050:506.04022.679.041.5Comp. 2-1100:01.02119.476.035.1Test Results

[0178] The right columns of Table 8 show the test results (sintering speed, product yield, and production rate) for each test case. The cases in which the production rate was 36.0 t / (Dm2) or more in the test results are Examples of Example 3. The sintering speed, product yield, and production rate were determined in the same manner as in Example 1. FIG. 14 is a graph illustrating the test results, illustrating a relationship between the oxygen concentration (vol. %) of the suction gas during the separation time (corresponding to a time period for the pallet to pass through the oxygen enrichment gas suction area 7x of the actual machine) and the production rate (t / (Dm2)).

[0179] As shown in the test results of Examples 3-1 to 3-3, when the separation time was set to 0.5 minutes, the increase in sintering speed, product yield, and production rate provided by the increase in oxygen concentration (oxygen concentration: Example 3-1<Example 3-2<Example 3-3) was slow. This is probably because the separation time was short, and the oxygen enrichment time was short, only 0.5 minutes.

[0180] On the other hand, as shown in the test results of Example 3-4 and Example 3-5, when the separation time was set to 1.0 minute, the product yield and production rate were significantly increased by increasing the oxygen concentration from 21 volume % to 30 volume % (oxygen concentration: Example 3-4<Example 3-5).

[0181] As shown in the test results of Example 3-5 and Example 3-6, the sintering speed, product yield, and production rate were significantly increased by increasing the separation time from 1.0 minute to 2.0 minutes (separation time: Example 3-5<Example 3-6) at the same oxygen concentration (30 volume %). This is the effect of increasing the oxygen enrichment time associated with the increase in separation time.

[0182] As shown in the test results of Example 3-6 and Example 3-7, when the separation time was set to 2.0 minutes, the sintering speed and production rate were significantly increased by increasing the oxygen concentration from 30 volume % to 40 volume % (oxygen concentration: Example 3-6<Example 3-7).

[0183] Furthermore, as shown in the test results of Example 3-7 and Example 3-8, the sintering speed, product yield, and production rate were improved by increasing the separation time from 2 minutes to 5 minutes (separation time: Example 3-7<Example 3-8).

[0184] However, as shown in the test results of Example 3-8 to Example 3-10, when the oxygen concentration exceeded 40 volume % (oxygen concentration: Example 3-8<Example 3-9), or when the separation time exceeded 5 minutes (separation time: Example 3-8<Example 3-10), the improvements in sintering speed, product yield, and production rate reached a plateau.Example 4Test Cases

[0185] In Example 4, the same sinter pot test as in Example 1 was used to verify the applicability of the oxygen enrichment technique to the invention. A description will be made below about the test conditions and test results of Test 1 (a total of 15 test cases, Examples 4-1 to 4-15, see Table 9) and Test 2 (a total of 6 test cases, Examples 4-16 to 4-21, see Table 11) using Tables 9 to 11 described later. Example 4-1 in Table 9 described later is the same test case as Example 2-1. In addition, Table 9 shows Comparative 2-1 of Example 2 for reference, and Table 11 shows Example 4-10 of Test 1 for reference. In the following description, duplicated descriptions of the conditions, test methods, and the like that are same as those in Example 1 will be omitted as appropriate.Test 1Blending of Materials, etc.

[0186] The materials blended were the same as those in Example 1 (Table 4), except for the bonding agents. In all test cases of Example 4 (Test 1 and Test 2 (Examples 4-1 to 4-21)), the blending of the bonding agents (carbonaceous materials) in the blended material and the particle size of the bonding agents (the average particle size of the low combustible carbonaceous material and the ratio of the highly combustible carbonaceous material with a particle size of 2.8 mm or more) were the same as those in Example 1-2 of Example 1. In all test cases of Example 4, the granulation method was bulk granulation and the charging method was normal charging.Firing Conditions

[0187] In all test cases of Example 4, the combustion time for ignition (corresponding to ignition by the ignition furnace 3 (initial ignition)) and the combustion time for reignition (corresponding to reignition by the reignition furnace 4) were each 1 minute (heat amount: as sensible heat of the suction gas, 25 MJ / ton of blended material). As shown in Table 9, the interval between ignition and reignition (separation time: corresponding to the time required to move through the atmosphere suction area 7) was set to 1 minute. The oxygen enrichment time and oxygen concentration were changed to the levels indicated as test conditions, with oxygen enrichment starting immediately after the completion of reignition (more than 0 seconds and 2 seconds or less after the point of time when reignition was completed).

[0188] During firing after ignition, the air volume was kept constant and the air volume was adjusted to 1.80 Nm3 / min of exhaust gas. As in Examples 1 to 3, suction by the blower was stopped 3 minutes after the exhaust gas temperature reached its peak, and firing was terminated. The sintering time was determined as a time from the point of time when ignition started to the point of time when the exhaust gas temperature peaked.

[0189] In Example 4-1, oxygen enrichment was not performed, and the atmosphere (the air) was used as the suction gas. An oxygen injection method in Examples 4-2 to 4-15 was as follows: the atmosphere and oxygen were mixed in a gas blender to provide a predetermined oxygen concentration (oxygen concentration in Table 9) as in Example 3, and then this mixed gas was supplied for a predetermined time (oxygen enrichment time in Table 9) using a hood placed over the pot. Specifically, when reignition was completed, the hood was set and the oxygen enrichment gas with a predetermined oxygen concentration was supplied, and this oxygen enrichment gas was suctioned. After a predetermined time, the hood was immediately removed. Here, the flow rate was determined so that the inlet-side gas volume (=suction gas volume) was all mixed gas via the gas blender.TABLE 9Test conditionsOxygenenrichment timeMass ratio of carbon content(immediately Test resultsof bonding agents (mass %)afterSinteringCommon condition(Low combustible carbonaceouscompletion ofOxygenspeedProductProductionSeparation time:material:Highly combustiblereignition)concentration(mm / yieldrate1.0(min)carbonaceous material)(min)(vol. %)min)(mass %)(t / (Dm2))Ex. 4-1 (Ex. 2-1)50:500.002120.776.236.9Ex. 4-250:500.253020.976.437.5Ex. 4-350:500.254021.076.337.6Ex. 4-450:500.503021.576.638.5Ex. 4-550:500.504021.976.439.2Ex. 4-650:501.003022.376.339.9Ex. 4-750:501.003522.676.140.3Ex. 4-850:501.004022.876.040.6Ex. 4-950:501.005022.975.840.7Ex. 4-1050:501.503522.876.040.6Ex. 4-1150:502.003022.876.540.9Ex. 4-1250:502.004023.276.141.3Ex. 4-1350:502.005022.875.740.5Ex. 4-1450:503.003023.176.741.4Ex. 4-1550:504.003023.276.141.3Comp. 2-1100:00.002119.476.035.1Test Results

[0190] The right columns of Table 9 show the test results (sintering speed, product yield, and production rate) for each test case. The cases in which the production rate was 36.0 t / (Dm2) or more in the test results are Examples of Example 4 (Test 1). The sintering speed, product yield, and production rate were determined in the same manner as in Example 1.Sintering Speed

[0191] Effect of oxygen concentration: The sintering speed increased until the oxygen concentration reached 40 vol. % (volume %), but tended to plateau at an oxygen concentration of 50 vol. % (Examples 4-9 and 4-13).

[0192] Effect of oxygen enrichment time: When the oxygen concentration was 30 vol. %, the effect was enhanced up to 2.0 minutes (Example 4-11), but the effect plateaued even when the time was extended to 3.0 minutes (Example 4-14) or 4.0 minutes (Example 4-15).Product Yield

[0193] The reignition method without oxygen enrichment (Example 4-1) yielded 76.2 mass %, while the reignition method with oxygen enrichment yielded 76.2±0.5 mass % for all test cases. Therefore, no effect on product yield was observed.Production Rate

[0194] FIG. 15 is a graph illustrating the test results, illustrating a relationship between the oxygen concentration (vol. %) and the production rate (t / (Dm2)). As illustrated in FIG. 15, the results are as follows.

[0195] Effect of oxygen concentration: The production rate increased until the oxygen concentration reached 40 vol. % (volume %), but tended to plateau at an oxygen concentration of 50 vol. % (Examples 4-9 and 4-13).

[0196] Effect of oxygen enrichment time: When the oxygen concentration was 30 vol. %, the effect was enhanced up to 2.0 minutes (Example 4-11), but the effect plateaued even when the time was extended to 3.0 minutes (Example 4-14) or 4.0 minutes (Example 4-15).

[0197] The results of calculating an oxygen utilization ratio and an oxygen consumption speed for Example 4-1 and Example 4-14 are shown in Table 10. Table 10 shows the test conditions and results for Example 4-14 (oxygen concentration 30 vol. %, oxygen enrichment time 3.0 minutes) that had the greatest effect on improving the production rate, and the test conditions and results for Comparatives 4-1 and 4-2 using single-stage ignition that were conducted for the comparison and evaluation for Example 4-14. In Comparative 4-2, oxygen enrichment was performed for 3.0 minutes immediately after the completion of ignition. The time period during which the oxygen utilization ratio and the oxygen consumption speed were evaluated was 3 minutes immediately after the completion of ignition in Comparative 4-1 and Comparative 4-2 (single-stage ignition sintering). In Example 4-1 and Example 4-14 (reignition sintering method), that time period was set to 3 minutes immediately after the completion of reignition. That is, for Comparative 4-2 and Example 4-14, the oxygen utilization ratio and oxygen consumption speed during the oxygen enrichment time period are shown. The oxygen release speed in exhaust gas (b) shown in Table was calculated from the results of exhaust gas analysis using a magnetic oxygen analyzer. The oxygen consumption speed (a) was calculated by subtracting the oxygen release speed in exhaust gas (b) from an oxygen flow speed sucked into the sintered layer. The oxygen flow speed sucked into the sintered layer can be 10 calculated by multiplying an inlet-side gas flow speed by the oxygen concentration.TABLE 10Comp.Ex.Comp.Ex.4-14-14-24-14Single-stage ignition sintering / Single-Reig-Single-Reig-Reignition sinteringstagenitionstagenitionOxygen enrichmentNoNoYes(*)Yes(*)((*) . . . Oxygen concentration30 vol. %, Oxygen enrichmenttime 3 minutes)Oxygen consumptionNm3 / min0.170.230.240.31speed (a)Oxygen release speedNm3 / min0.170.110.230.15in exhaust gas (b)Oxygen utilization%51685168ratio (a / (a + b))

[0198] As shown in Table 10, when comparing the test cases without oxygen enrichment (Example 4-1 and Comparative 4-1), the oxygen consumption speed and oxygen utilization ratio were higher in the three minutes immediately after the completion of reignition than in the three minutes immediately after the completion of ignition in single-stage ignition. Comparing the test cases in which reignition was performed (Example 4-1 and Example 4-14), in Example 4-14 where oxygen enrichment was performed for 3 minutes immediately after the completion of reignition, the oxygen utilization ratio was maintained, and the oxygen consumption speed increased in proportion to the increase in oxygen concentration.Test 2

[0199] In this test, only the point of time when oxygen enrichment was started was changed, with reference to Example 4-10 in Test 1, in 10-second increments from the point of time when reignition was completed to examine the effects. In other words, the test conditions for Examples 4-16 to 4-21 were the same as those for Example 4-10, except for the point of time when oxygen enrichment was started. Table 11 shows the test conditions and test results for Example 4-10 and Examples 4-16 to 4-21. The oxygen enrichment start time in Table 11 indicates an elapsed time (in seconds) from the point of time when reignition was completed to the point of time when reignition was started. In Example 4-10 where oxygen enrichment was started immediately after the completion of reignition, “0 seconds” indicates a time more than 0 seconds and 2 seconds or less, including an operation time from the completion of reignition to the start of oxygen enrichment. In addition, “10 seconds” in Example 4-16 indicates a time of more than 10 seconds and 12 seconds or less, including an operation time from the completion of reignition to the start of oxygen enrichment. The number of seconds for the oxygen enrichment start time in each of Examples 4-17 to 4-21 was defined similarly to Example 4-10 and Example 4-16.TABLE 11Common conditions①Mass ratio of carbonOxygen enrichmentcontent of start timebonding agents(Based on whenOxygen(mass %) = 50:50reignition wasenrichmentOxygenSinteringProductProduction②Separation time:completed)timeconcentrationspeedyieldrate1.0(min)(sec)(min)(vol. %)(mm / min)(mass %)(t / (Dm2))Ex. 4-1001.53522.876.040.6Ex. 4-16101.53522.977.241.5Ex. 4-17201.53522.476.440.1Ex. 4-18301.53522.576.640.4Ex. 4-19401.53521.678.339.6Ex. 4-20501.53521.875.338.4Ex. 4-21601.53521.776.438.7Test Results

[0200] The right columns of Table 11 show the test results (sintering speed, product yield, and production rate) for each test case. The cases in which the production rate was 36.0 t / (Dm2) or more in the test results are Examples of Example 4 (Test 2). The sintering speed, product yield, and production rate were determined in the same manner as in Example 1. FIG. 16 is a graph illustrating the test results in the right columns of Table 4, illustrating a relationship between the oxygen enrichment time and the production rate. FIG. 16 and the test results (sintering speed and production rate) in the right columns of Table 4 show that oxygen enrichment is preferably started within 10 seconds after the point of time when reignition was completed, but a significant effect can be obtained even if oxygen enrichment is started within 30 seconds after the point of time when reignition was completed. Even after 60 seconds, the effect is greater than those of Examples 4-1 to 4-3 shown in Table 9.Example 5Test Cases

[0201] In Example 5, the same sinter pot test as in Example 1 was used to verify the applicability of the stand-support sintering technique to the invention. A description will be made below about the test conditions and test results of test cases of Example 5 (Comparatives 5-1 to 5-2 and Examples 5-1 to 5-6) using Table 12 described later. In the following description, duplicated descriptions of the conditions, test methods, and the like that are same as those in Example 1 will be omitted as appropriate.Blending of Materials, Etc.

[0202] The materials blended were the same as those in Example 1 (Table 4), except for the bonding agents. The blending of the bonding agents (carbonaceous materials) in the blended material and the particle size of the bonding agents (the average particle size of the low combustible carbonaceous material and the ratio of the highly combustible carbonaceous material with a particle size of 2.8 mm or more) were the same as those in Comparative 1-2 or Example 1-2 of Example 1. In Comparatives 5-1 and 5-2, as with the former (Comparative 1-2), only 4.5 mass % (not included in total mass) of coke breeze (low combustible carbonaceous material) was used relative to the new materials (carbon content mass ratio of bonding agents: 0 mass % for semi-coke and 100 mass % for coke breeze), and the average particle size of coke breeze was 1.0 mm. In Examples 5-1 to 5-6, as with the latter (Example 1-2), the carbon content mass ratio was 50 mass % coke breeze (low combustible carbonaceous material) and 50 mass % semi-coke (highly combustible carbonaceous material), the average particle size of coke breeze was 1.00 mm, and the ratio of the highly combustible carbonaceous material with a particle size of 2.8 mm or more was 30 mass %. In all test cases of Example 5, the granulation method was bulk granulation and the charging method was normal charging.Firing Conditions

[0203] In all test cases of Example 5, the ignition time for ignition (corresponding to ignition by the ignition furnace 3 (initial ignition)) and the ignition time for reignition (corresponding to reignition by the reignition furnace 4) were each 1 minute (heat amount: 25 MJ / ton of blended material). As shown in the column for separation time in Table 12, the separation time was set to either 0.5 minutes, 1 minute, 2.5 minutes, or 3.5 minutes. As with Example 1, the suction pressure was adjusted by changing the valve opening on the suction side of the blower so that the measured value under the pot was constant at 1,300 mmAq (12.75 kPa) in all test cases of Example 5.

[0204] Table 12 shows the test conditions and test results for each test case in Example 5. As shown in Table 12, for cases where semi-coke (highly combustible carbonaceous material) was blend at 50 mass %, the effect of the stand-support sintering technique was evaluated at three levels of separation time (0.5 minutes, 2.5 minutes, and 3.5 minutes) (Examples 5-1 to 5-6). For comparison, the test cases in which semi-coke was blended at 0 mass % were evaluated with a separation time of 0.5 minutes (Comparatives 5-1 and 5-2). For reference, Comparative 1-2 and Example 1-2 of Example 1 are also shown.TABLE 12OxygenMass ratio of carbon content ofconcentrationbonding agentsbetween(mass %)ignition(Low combustible carbonaceous Reignition-furnace andSinteringmaterial:HighlySeparationreignitionspeedProductProductioncombustible carbonaceoustimefurnace(BTS)yieldratematerial)Stand-support(min)(vol. %)(mm / min)(mass %)(t / (Dm2))Comp. 5-1100:0—0.52115.073.526.5Comp. 5-2100:0Yes0.52116.971.728.5Comp. 1-2100:0—1.02116.073.527.6Ex. 1-250:50—1.02117.873.830.5Ex. 5-150:50—0.52117.873.730.5Ex. 5-250:50—2.52117.773.230.0Ex. 5-350:50—3.52117.573.529.8Ex. 5-450:50Yes0.52119.873.934.7Ex. 5-550:50Yes2.52119.573.633.9Ex. 5-650:50Yes3.52118.973.032.4Test Results

[0205] Table 12 shows the test results (sintering speed, product yield, and production rate) for each test case. The cases in which the production rate was 29.5 t / (Dm2) or more in the test results are Examples of Example 5. The sintering speed, product yield, and production rate were determined in the same manner as in Example 1. FIG. 17 (semi-coke blend: 50 mass %) and FIG. 18 (semi-coke blend: 0 mass %) are each a graph illustrating the test results of Table 12, illustrating a relationship between the separation time (min) and the production rate (t / (Dm2)).

[0206] As shown in Table 12 and FIG. 17, in the test cases where semi-coke was blended at 50 mass %, when the stand-support sintering technique was applied with a separation time of 0.5 or 2.5 minutes, the production rate had a high value, and the sintering speed, product yield, and production rate were all significantly improved.

[0207] On the other hand, as shown in Table 12 and FIG. 18, when the semi-coke was blended at 0 mass %, the effect of the stand-support sintering technique was reduced in all of the sintering speed, product yield, and production rate. Thus, the synergistic effect beyond expectation between the blending of the highly combustible carbonaceous material (semi-coke) and the stand-support sintering technique was confirmed in the reignition sintering method.

[0208] In the test cases (Examples 1 to 5) of the invention, the ignition time and reignition time were each 1 minute (heat amount: as sensible heat of the suction gas, 25 MJ / ton of blended material), but the exemplary embodiments of the invention are not limited thereto. This is because the ignition time in the test cases was set taking into account the heat loss in the pot test. In an actual machine (commercial sintering machine), if the ignition time is, for example, 30 seconds, it is not necessary to set the ignition time to 1 minute. The reignition sintering method can be performed by maintaining the ignition time of the actual operation. Similarly, the reignition time does not have to be 1 minute on the actual machine.

[0209] Although the preferred exemplary embodiments and Examples of the invention are described in detail above with reference to the accompanying drawings, the invention is not limited thereto. It is clear that a person with ordinary knowledge in the technical field to which the invention pertains can reach various modified or altered examples within the scope of the technical ideas recited in Claims, and it is understood that those also naturally fall within the technical scope of the invention.EXPLANATION OF CODES

[0210] 1 . . . sinter material bins, 2 . . . drum mixer, 3 . . . ignition furnace, 31 . . . igniter, 32 . . . hood of ignition furnace, 32a . . . partition wall (downstream side), 4 . . . reignition furnace, 41 . . . reigniter, 42 . . . hood of reignition furnace, 42a . . . partition wall (upstream side), 42b . . . partition wall (downstream side), 5 . . . pallet, 5x . . . pallet traveling direction, 51 . . . grate bar, 52 . . . main frame, 53 . . . pallet side wall, 6 . . . downward suction device, 6x . . . downward suction, 61 . . . wind box, 62 . . . wind leg, 63 . . . damper, 64 . . . duct, 65 . . . blower, 7 . . . atmosphere suction area, 7x . . . oxygen enrichment gas suction area, 8 . . . gradient plate chute-type charging device, 81 . . . sinter mixture surge hopper, 82 . . . gradient plate chute, 9 . . . oxygen enrichment gas supplying unit, 91 . . . hood of oxygen enrichment gas supplying unit, 92 . . . gas pipe, 10 . . . sinter material packed bed, 10x . . . slope, 10A . . . combustion zone, 10A1 . . . initial ignition combustion zone, 10A2 . . . reignition combustion zone, 10B . . . sinter cake, 16 . . . stand, 16a . . . sinter cake supporting surface, 101,103,104 . . . . DL-type sintering machine, S . . . section.

Examples

first exemplary embodiment

[0057]First, a description will be made on a Dwight-Lloyd (DL) type sintering machine used in a reignition sintering method and a method for producing sintered ore using the DL-type sintering machine. The DL-type sintering machine used in the reignition sintering method includes a reigniter that performs a second ignition. The reigniter is located at a predetermined interval (corresponding to a “separation distance” described later) downstream in a pallet traveling direction of an igniter that performs a first ignition. The reigniter is a flame heater that heats with flame an upper surface (surface) of a sinter material packed bed after the first ignition is completed.

[0058]FIG. 1 schematically illustrates an exemplary Dwight-Lloyd (DL) type sintering machine used in a reignition sintering method. In the following description, an ore supply side (left side in FIG. 1) is defined as an upstream side, and an ore discharge side (right side in FIG. 1) is defined as a downstream side, bas...

second exemplary embodiment

[0102]A preferred example in which an air volume control technique is used in the invention will be described below as a second exemplary embodiment. As described in the first exemplary embodiment, the production rate is improved by using a predetermined amount (30 mass % or more and 80 mass % or less) of the coarse highly combustible carbonaceous material in the invention. However, since the blended material contains the highly combustible carbonaceous material with a particle size of less than 2.8 mm, the highly combustible carbonaceous material (especially fine particles) is present in the upper part of the sinter material packed bed. Since the highly combustible carbonaceous material has a fast combustion speed, the temperature of the upper part of the sintered layer (up to a depth of about 100 mm from the surface layer) rapidly drops in the section from the first ignition to the start of reignition, reducing the yield and decreasing the effect of improving the production rate. ...

third exemplary embodiment

[0115]A preferred example in which an oxygen enrichment technique is used in the invention will be described below as a third exemplary embodiment. In the invention using the reignition sintering method, the production rate may be further improved by extending a high-temperature retention time of the sintered layer. In that case, if an attempt is made to extend the separation time, which is a time from the completion of initial ignition until reignition is performed (a time required for the pallet to move through the section between the ignition furnace and the reignition furnace), the temperature of the upper part of the sintered layer would drop too much before reignition is performed, and so there has been a limit to how long the separation time can be extended. In particular, when using the highly combustible carbonaceous material, even if the particles thereof are coarse (with a particle size of 2.8 mm or more being 30 mass % to 80 mass %), the combustion time is short and the ...

Claims

1. A method for producing sintered ore using a Dwight-Lloyd type sintering machine, the machine comprising an ignition furnace for initial ignition and a reignition furnace for reignition arranged at a predetermined distance downstream of the ignition furnace, and being configured to advance sintering by downward suction,the method comprising using, as bonding agents for a blended material, a low combustible carbonaceous material with a combustion start temperature exceeding 550 degrees C. and a highly combustible carbonaceous material with a combustion start temperature of 550 degrees C. or less, whereina ratio of the highly combustible carbonaceous material with a particle size of 2.8 mm or more is 30 mass % or more and 80 mass % or less.

2. The method for producing sintered ore according to claim 1, wherein a crushed product obtained by crushing a compression-molded product, which is provided by compressing and molding an aggregate of carbonized wood, is used as the highly combustible carbonaceous material.

3. The method for producing sintered ore according to claim 2, wherein the crushed product of the compression-molded product provided by compressing and molding the aggregate of the carbonized wood is produced by:carbonizing a wood material to produce the carbonized wood;crushing the produced carbonized wood as necessary to produce carbonized wood particles, and kneading the carbonized wood particles alone or with a binder to produce the aggregate of the carbonized wood;compressing and molding the aggregate to produce the compression-molded product; andcrushing the compression-molded product.

4. The method for producing sintered ore according to claim 1, wherein a mass ratio of a carbon content of the highly combustible carbonaceous material to a carbon content of the bonding agents is 25 mass % or more and 75 mass % or less.

5. The method for producing sintered ore according to claim 4, wherein an average particle size of the low combustible carbonaceous material is in a range from 0.8 mm to 1.2 mm.

6. The method for producing sintered ore according to claim 5, wherein a segregation-strengthened type charging device is used as a charging device for the blended material.

7. The method for producing sintered ore according to claim 5, wherein, of the bonding agents, only the low combustible carbonaceous material is added in a latter half of a granulation process.

8. The method for producing sintered ore according to claim 1, wherein a downward suction air volume is reduced only in a section up to an outlet of the reignition furnace on a sinter strand upstream side.

9. The method for producing sintered ore according to claim 8, wherein an average superficial air volume of the atmosphere sucked in the section up to the outlet of the reignition furnace on the sinter strand upstream side is set to 60% or more and 80% or less of an average superficial air volume of the atmosphere sucked in a section downstream of the outlet of the reignition furnace.

10. The method for producing sintered ore according to claim 8, wherein an average negative pressure in a wind box or a wind leg in the section up to the outlet of the reignition furnace on the sinter strand upstream side is set to 40% or more and 70% or less of an average negative pressure in a wind box or a wind leg in the section downstream of the outlet of the reignition furnace.

11. The method for producing sintered ore according to claim 8, wherein a separation time, which is a time required for a pallet to pass through a section between the ignition furnace and the reignition furnace, is set to 30 seconds or more and 2 minutes or less.

12. The method for producing sintered ore according to claim 1, wherein a separation time, which is a time required for a pallet to pass through a section between the ignition furnace and the reignition furnace, is set to 1 minute or more, andan oxygen concentration of a suction gas sucked downward from a surface layer side of a sintered layer in the section is set to 30 volume % or more.

13. The method for producing sintered ore according to claim 12, wherein the separation time is set to 5 minutes or less and the oxygen concentration of the suction gas is set to 40 volume % or less.

14. The method for producing sintered ore according to claim 1, whereinoxygen enrichment of a suction gas sucked downward from a surface layer side of a sintered layer is started after the reignition is completed,an oxygen enrichment time from the start of the oxygen enrichment to a completion of the oxygen enrichment is 30 seconds or more, andan oxygen concentration of the suction gas sucked downward during the oxygen enrichment time is set to 30 volume % or more.

15. The method for producing sintered ore according to claim 14, wherein the oxygen enrichment time is set to 2 minutes or less and the oxygen concentration of the suction gas is set to 40 volume % or less.

16. The method for producing sintered ore according to claim 14, wherein the oxygen enrichment is started at a timing more than 0 seconds and within 30 seconds after a point of time when the reignition is completed.

17. The method for producing sintered ore according to claim 16, wherein the oxygen enrichment is started at a timing more than 0 seconds and within 10 seconds after the point of time when the reignition is completed.

18. The method for producing sintered ore according to claim 1, wherein a pallet, into which the blended material is to be charged, is provided with a support member having a sinter cake supporting surface, the support member being erected on a grate bar to be embedded in a sinter material packed bed.

Images