Method for producing pellets

Abstract

Provided is a pellet production method capable of more effectively suppressing bursting of pellets produced by a granulation step in which water is added to a starting material and granulation is performed and a firing step in which the granulated material is fired. This pellet production method comprises: a granulation step in which water is added to a raw material obtained by blending a powdery material containing iron ore having an iron component content of 63 mass% or less and an auxiliary raw material, and granulation is performed; and a firing step for firing the obtained granulated material, including a drying / preheating zone and a subsequent firing zone, and before the firing step, measuring the specific surface area and true density of the raw material and the amount of water added in the firing step, the obtained measurement values are used to estimate appropriate values for the drying temperature and the gas flow rate in the drying / preheating region in the firing step, and the estimated appropriate values for the drying temperature and the gas flow rate are used to process the drying / preheating region in the firing step.

Description

TECHNICAL FIELD

[0001] The present application relates to a method for manufacturing a pellet used as a charging material in a shaft furnace or the like. BACKGROUND

[0002] As a direct reduction iron-making method, in the reduction in the currently mainstream shaft furnace method, lump ore, pellet are used as charging materials. Pellets are manufactured through various processes of pulverizing, mixing, granulating, and firing of ore. It is known that in the firing process, due to the evaporation of moisture in the iron ore pellets before firing (i.e., so-called green pellets), phenomena such as bursting of green pellets, powdering, and the like occur, and the yield of finished pellets decreases.

[0003] A method for manufacturing green pellets containing iron ore and auxiliary raw materials, and an organic binder is disclosed in Patent Literature 1. In this manufacturing method, by controlling the viscosity of water in which the organic binder is dissolved, with respect to the blending ratio of the raw material iron ore containing a large amount of crystal water, bursting when firing the green pellets can be suppressed.

[0004] PRIOR ART DOCUMENTS

[0005] PATENT LITERATURE

[0006] Patent Literature 1: Japanese Patent Application Publication No. 2020-180371 SUMMARY

[0007] PROBLEMS TO BE SOLVED BY THE INVENTION

[0008] Currently, high-grade ore with little crystal water is generally used as a raw material ore (iron ore) for pellets. However, in the future, as the demand for high-grade ore increases, it is predicted that it will become difficult to stably obtain high-grade ore. Therefore, the full use of low-grade ore with a total Fe content of less than 63 mass% produced in Australia, India, and the like is being explored. If the crystal water in the iron ore pellets (green pellets) increases, bursting in the pellets after firing is likely to occur as described above, and therefore, it is desirable to improve the manufacturing method of pellets aimed at suppressing bursting.

[0009] In this regard, the green pellet manufacturing method of Patent Literature 1 does not take into account the appropriate raw material particle size and the amount of water addition of the green pellets, and the appropriate drying conditions. Therefore, the suppression of bursting of the pellets after firing is insufficient, and a new manufacturing method of pellets that can more effectively suppress bursting is desired.

[0010] The present application was completed in view of the above circumstances, and aims to provide a manufacturing method of pellets that can more effectively suppress bursting, the pellets being manufactured by a granulation process of adding moisture to a raw material and a firing process of firing the granulated material.

[0011] Problem-solving method

[0012] The pellet production method of the present application has: a granulation step of adding moisture to a raw material prepared by mixing a powdered substance containing iron ore having an iron component content of 63 mass% or less and a secondary raw material, and performing granulation; and a firing step of performing firing on the resultant granulated product, including a drying / preheating zone and a subsequent firing zone, before the firing step, the specific surface area and the true density of the raw material, and the amount of moisture added in the granulation step are measured, the obtained measured values are used to estimate appropriate values of the drying temperature and the gas flow rate in the drying / preheating zone of the firing step, and the drying / preheating zone in the firing step is processed using the estimated appropriate values of the drying temperature and the gas flow rate.

[0013] Note that, in the pellet production method of the present application configured as described above, a more preferable problem-solving method is:

[0014] (1) the drying temperature and the gas flow rate are determined in such a manner that the burst index X calculated from the following formula (1) is 0.100 or more and 3.000 or less, using the measured values of the specific surface area and the true density of the raw material, and the amount of moisture added in the granulation step, and the estimation of the appropriate values of the drying temperature and the gas flow rate in the drying / preheating zone of the firing step is performed thereby,

[0015]

[0016] In the formula,

[0017] S m : the specific surface area of the raw material (cm 2 / g),

[0018] p: the true density of the raw material (g / cm 3 ),

[0019] M: the amount of moisture added in the granulation step (mass%),

[0020] T: the drying temperature (°C),

[0021] V: the gas flow rate (m / s).

[0022] Effects of the invention

[0023] According to the pellet production method of the present application, the burst of the pellets produced by the granulation step of adding moisture to the raw material and the firing step of firing the granulated product can be more effectively suppressed. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1is a flowchart for explaining a procedure of one embodiment of the pellet production method of the present application. DETAILED DESCRIPTION

[0025] Hereinafter, one embodiment of the present application will be specifically described. Note that the following embodiment illustrates an apparatus, a method, and the like for embodying the technical idea of the present application, and does not limit the present application to the following content. That is, the technical idea of the present application can be applied to various changes within the technical scope of the present application.

[0026] Figure 1 is a flowchart for explaining a procedure of one embodiment of the pellet production method of the present application. Hereinafter, one embodiment of the pellet production method of the present application will be described with reference to Figure 1 One embodiment of the pellet production method of the present application will be described. The pellet production method of the present embodiment includes a granulation process and a firing process. In the granulation process, water is added to a raw material which is a mixture of a fine substance of an iron ore (so-called low-grade ore) having an iron component content of 63 mass% or less and a secondary raw material, and granulation is performed, whereby a granulated product is obtained. The firing process includes a drying / preheating zone and a subsequent firing zone, and the obtained granulated product is fired, whereby a (fired) pellet is obtained. Note that, in the present application, the object is limited to an iron ore (so-called low-grade ore) having an iron component content of 63 mass% or less, because the effect of bursting is not significant for an iron ore (so-called high-grade ore) having an iron component content exceeding 63 mass%.

[0027] The pellet production method of the present embodiment is characterized by the following aspects. First, before the firing process, the specific surface area and the true density of the raw material and the amount of water added in the granulation process are measured. Then, using the obtained measurement values, appropriate values of the drying temperature and the gas flow rate in the drying / preheating zone of the firing process are estimated. Then, the drying / preheating zone of the firing process is processed using the estimated appropriate values of the drying temperature and the gas flow rate. Note that, in the following description, an iron ore having an iron component content of 63 mass% or less is simply referred to as an iron ore.

[0028] According to the pellet production method of the present embodiment, bursting can be suppressed. Note that bursting refers to a phenomenon in which an iron ore pellet (green pellet) bursts and is pulverized due to evaporation of water in the green pellet when the green pellet is heated.

[0029] Hereinafter, the specific surface area and the true density in the pellet production method of the present embodiment, the water addition amount, the drying temperature, the gas flow rate, and the shatter index X are described in detail. Note that in the following description, in order to distinguish between the pellets before and after firing, the iron ore pellets (pellets before firing) are sometimes referred to as green pellets, and the pellets after firing are referred to as fired pellets. The green pellets and the fired pellets can contain a sub raw material other than the iron ore (for example, bentonite). In addition, the kind of the iron ore as a raw material of the green pellets, and the blending (ore blending) are not particularly limited. The raw material of the green pellets can be composed of a single iron ore, or a plurality of ores can be mixed in an arbitrary blending.

[0030] <Regarding the specific surface area and the true density>

[0031] The water evaporated from the green pellets increases the internal pressure, and thus causes shattering, and therefore, for the green pellets as the powder packing layer, it can be considered that if the resistance to the penetration of steam is high, shattering is likely to occur. For the specific surface area, since the method of measuring it can reflect the resistance to the fluid penetrating the powder packing layer, it is an important factor that affects the shatterability.

[0032] In addition, the true density is basically determined by the kind of the iron ore. The average particle diameter of the packing layer can be estimated using the following equation (2) from the specific surface area and the true density. If the average particle diameter is small, a dense packing structure is likely to be formed, and thus the shattering is affected. Note that the specific surface area can be measured based on JIS R 5021, and the true density can be measured based on JIS M 8717.

[0033] [Mathematical expression 1]

[0034]

[0035] <Regarding the water addition amount>

[0036] The water addition amount of the green pellets is an important factor that affects the shatterability. As described above, since the water evaporated from the green pellets increases the internal pressure and causes shattering, the amount of water as a source of steam affects the shatterability. The water addition amount can be calculated by measuring the weight change when the green pellets are dried at 105°C.

[0037] <Regarding the drying temperature>

[0038] The firing process can be generally divided into a drying / preheating zone and a firing zone. When the green pellets sent to the firing process are heated in the drying / preheating zone, the internal moisture evaporates and causes cracking. Therefore, the drying temperature can affect the cracking. In the case of too high drying temperature, cracking can easily occur due to intense evaporation, on the other hand, in the case of too low, it cannot be sufficiently dried, which can result in a decrease in the strength of fired pellets, a decrease in productivity, and thus drying needs to be performed at an appropriate temperature. Generally, the drying temperature in the drying / preheating zone is in the range of 100 to 500°C.

[0039] <Regarding the gas flow rate>

[0040] In the drying / preheating zone in the firing process, heating is performed by blowing hot air to the green pellets. If the gas flow rate of the hot air becomes fast, the heat transfer rate increases, the temperature easily rises, and cracking can easily occur due to intense evaporation. In the case of slow gas flow rate, it cannot be sufficiently dried, which can result in a decrease in the strength of fired pellets, a decrease in productivity, and thus drying needs to be performed at an appropriate gas flow rate. Generally, the gas flow rate in the drying / preheating zone is in the range of 0.1 to 3.0 m / s.

[0041] <Regarding the cracking index X>

[0042] In the present application, the cracking index X is expressed by Equation (1), which is a mathematical equation obtained by modeling the degree of influence of the above-mentioned factors "specific surface area", "true density", "moisture addition amount", "drying temperature", "gas flow rate" that affect cracking based on actual implementation data.

[0043]

[0044] In the equation,

[0045] S m : specific surface area of raw material (cm 2 / g),

[0046] p: true density of raw material (g / cm 3 ),

[0047] M: moisture addition amount in the granulation process (mass%),

[0048] T: drying temperature (°C),

[0049] V: gas flow rate (m / s).

[0050] The burst index X represented by Formula (1) indicates the easiness of burst, and the larger the value, the more easily the burst occurs. In the case where the value of the burst index X is 3.000 or less, the fired pellet can be obtained while suppressing the burst. The value of the burst index X is preferably 2.000 or less, and further preferably 1.000. On the other hand, if the value of the burst index X is too small, drying is insufficient, and the strength is not sufficiently increased in the firing process. In the case where the value of the burst index X is 0.100 or more, the strength after firing can be ensured. Although the strength after firing is not particularly limited, it is preferably 250 kg / pellet or more, which is generally required in the technical field.

[0051] Next, the pelletizing process and the sintering process in the method for producing the pellet of the present application will be described in detail.

[0052] The pelletizing process is a process of pelletizing the ore powder to obtain the iron ore pellet (green pellet). The pelletizing method in the pelletizing process is not limited. In the pelletizing process, for example, the ore powder can be pelletized using a pelletizer. As the pelletizer, for example, a pan-type pelletizer (so-called pan pelletizer) can be used. In the firing process, the green pellet is supplied to a firing zone after passing through a drying / preheating zone to be fired, and a fired pellet is produced. The firing temperature in the firing zone is, for example, 1150°C to 1350°C.

[0053] Further, as described above, the burst refers to a phenomenon in which the green pellet is burst and pulverized due to evaporation of water in the green pellet when the green pellet is heated. The easiness of burst, i.e., the burst property, can be evaluated based on, for example, the proportion of the pellet of 5.6 mm or less produced after the green pellet is subjected to a heat treatment at a given temperature (for example, 300°C) or after firing of the green pellet.

[0054] Specifically, in the case where the burst occurs when the green pellet is subjected to a heat treatment at a given temperature or in the firing process (hereinafter referred to as the firing process or the like), the green pellet is burst or pulverized to produce fine (for example, 5.6 mm or less) pellet fragments. Therefore, if the amount of the pellet fragments produced in the firing process or the like is large, the burst easily occurs (the burst property is high), and it can be evaluated as not good from the viewpoint of suppressing the burst. On the contrary, if the amount of the pellet fragments produced in the firing process or the like is small, the burst does not easily occur (the burst property is low), and it can be evaluated as good from the viewpoint of suppressing the burst.

[0055] If the amount of the pellet fragments in the firing process or the like is 3.0 mass% or less, preferably 1.5 mass% or less, and further preferably 1.0 mass% or less, the reduction in the yield of the fired pellets can be suppressed. Thus, if the amount of the pellet fragments in the firing process or the like is 3.0 mass% or less, the high-quality pellets in which the bursting is suppressed can be evaluated.

[0056] Examples

[0057] The green pellets were manufactured by the following steps, and the bursting property was evaluated.

[0058] As the raw material of the green pellets, the ores (Ore A, Ore B, and Ore C) having the chemical compositions shown in Table 1 below were used. Note that “LOI” in Table 1 indicates the content of the crystallization water (mass%), which is the weight change when the ore is kept at 1000°C for 30 minutes. In addition, in Table 1, “T.Fe” indicates the mass% of the total Fe component in the iron ore. Note that “T.Fe” is a value obtained by quantification based on the total iron quantification method for iron ore prescribed in JIS M 8212:2022. As shown in Table 1, both Ore A and Ore B, and Ore C, which are the raw materials of the green pellets, are low-grade ores in which T.Fe is less than 63 mass%.

[0059]

[0060] The information (ore properties) of the ores used for the green pellets, the conditions of the pelletizing process and the firing process, or the evaluation results are shown in Table 2 below. Hereinafter, from the ores having the properties shown in Table 2, the fired pellets of Experimental Nos. 1 to 17 were obtained by the pelletizing process and the firing process described later.

[0061] First, the raw material ores shown in Table 2 were pulverized (batch processing) with a ball mill to obtain ore powder. Then, the specific surface area and the true density of the ore powder were measured. In addition, the particle diameter calculated from the specific surface area and the true density is also described in Table 2.

[0062] Next, bentonite was added to the ore powder at 1.0 mass% to obtain a mixed powder. Note that although bentonite is added as a binder at the time of pelletizing, it is not essential in the present embodiment. The mixed powder was pelletized by a disc-type pelletizer (pelletizer) to obtain green pellets (pelletizing step). At the time of pelletizing, water was added to the mixed powder and made to rotate in such a manner that the pelletizing moisture (amount of water added) reached each water addition level. Note that the pelletizing moisture can be determined by measuring the change in weight of the green pellets before and after being kept at 105°C for 24 hours. For example, in the case where the change in weight of the green pellets after being kept at 105°C for 24 hours was -10 mass%, the amount of water in the pelletizing step was 10 mass%.

[0063] For the particle diameter of the green pellets, the major axis diameter and the minor axis diameter were measured using a vernier caliper, and the average thereof (arithmetic mean) was used. Note that for each green pellet of each experimental No., the particle diameter of 10 pellets was measured, and the average thereof (arithmetic mean) was used as the particle diameter of the green pellet of the experimental No.

[0064] The explosiveness was evaluated by the amount of pellet fragments produced after the firing step. Specifically, the explosiveness was evaluated as described below. First, 500 g of green pellets were loaded into a vertical furnace (cylindrical shape with a diameter of 60 mm). Air heated to each temperature was ventilated from the lower side of the layer of green pellets toward the upper side at each flow rate for 10 minutes. Next, the green pellets after being exposed to the air at each temperature were heated to 1300°C in another electric furnace different from the above-described furnace, and fired at that temperature for 25 minutes (firing step). Then, the green pellets were taken out of the furnace, sieved with a sieve having a pore diameter of 5.6 mm, and the passing ratio (mass%) of the pellet fragments that passed through the sieve was measured as an evaluation of the explosiveness. In Table 2, the passing ratio of the pellet fragments is indicated as the "-5.6 mm ratio". In addition, after the explosiveness was evaluated, the strength of the pellet that did not explode was measured. The strength measurement was performed by Autograph set to a displacement speed of 1.0 mm / min, and the average of 10 was used. In Table 2, it is indicated as the "fired pellet strength".

[0065]

[0066] From the results of Table 2, the following can be known. First, for the fired pellets of Experimental Nos. 1 to 14 in which the value of the bursting index X represented by Formula (1) is 3.000 or less, the passing ratio of the pellet fragments (-5.6 mm ratio) as an index of the bursting property is 3.0 mass% or less, specifically 1.5 mass% or less. That is, in these Experimental Nos. 1 to 14 of the present embodiment, it is possible to obtain the fired pellets while suppressing the bursting. On the other hand, in Experimental Nos. 16 and 17 in which the value of the bursting index X represented by Formula (1) is not 3.000 or less, the passing ratio of the pellet fragments (-5.6 mm ratio) exceeds 3.0 mass%. That is, the bursting cannot be suppressed in Experimental Nos. 16 and 17. On the other hand, it is known that in the case of Experimental No. 15 in which the value of the bursting index X represented by Formula (1) is less than 0.100, the fired strength is 250 kg / pellet or less, and the required strength is not satisfied.

[0067] From the above results, the following can be known. In the drying / preheating zone of the firing process, the passing ratio of the pellet fragments (-5.6 mm ratio) is 3.0 mass% or less for the inventive examples processed under the conditions of the drying temperature and the gas flow rate estimated in such a manner that the bursting index X represented by Formula (1) is 0.100 to 3.00. Therefore, it is known that the bursting property is good in the pellets after firing. In addition, it is known that in the above inventive examples, the fired pellet strength exceeds 250 kg / pellet, and the required strength is satisfied.

[0068] Industrial applicability

[0069] In the method of manufacturing the pellets of the present invention, for the pellets manufactured by the pelletizing process of granulating by adding moisture to the raw material and the firing process of firing the pelletized product, the bursting can be more effectively suppressed, and it is industrially useful.

Claims

1. A method for manufacturing ore pellets, the method comprising: The granulation process involves adding water to a raw material composed of iron ore with an iron content of less than 63 mass% and powdered by-products, and then granulating it. The firing process includes a drying / preheating zone and a subsequent firing zone to fire the resulting granules. Before the firing process, the specific surface area and true density of the raw material and the amount of moisture added in the granulation process are measured. The measured values ​​are used to estimate appropriate values ​​for the drying temperature and gas flow rate in the drying / preheating zone of the firing process, and the estimated appropriate values ​​for the drying temperature and gas flow rate are used to treat the drying / preheating zone of the firing process.

2. A method for manufacturing ore pellets, wherein, Using the specific surface area and true density of the raw material, and the measured values ​​of the amount of moisture added during the granulation process, the drying temperature and gas flow rate are determined such that the bursting index X, calculated by the following formula (1), is 0.100 or higher and 3.000 or lower. Based on this, appropriate values ​​for the drying temperature and gas flow rate in the drying / preheating zone of the firing process are estimated. In the formula, S m Specific surface area of ​​raw materials (cm²) 2 / g), ρ: True density of the raw material (g / cm³) 3 ), M: Moisture addition amount (mass%) during the granulation process. T: Drying temperature (°C) VV: Gas velocity (m / s).

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