Iron ore pellet manufacturing method

Abstract

The present disclosure provides a method for producing pellets that can suppress the occurrence of bursting. The method for producing iron ore pellets of the present disclosure includes the steps of (a) crushing iron ore, (b) obtaining pellet raw material using the crushed iron ore, (c) granulating or molding the pellet raw material to obtain green pellets, and (d) firing the green pellets, in which the product XYZ (mass%·mm·°C / min) of the average crystal water content X (mass%) of the iron ore constituting the pellet raw material, the average particle size Y (mm) of the green pellets before firing, and the heating rate Z (°C / min) from 250°C to 700°C during firing is adjusted to be equal to or less than a predetermined allowable value A.

Description

[Technical Field]

[0001] The present disclosure relates to a method for producing iron ore pellets. [Background technology]

[0002] Iron ore pellets (hereinafter simply referred to as pellets) are sintered products made by agglomerating fine iron ore of 100 μm or less into a size of about 10 mm or so, and are used as raw materials in the blast furnace process and direct reduction process.

[0003] A typical pellet manufacturing method includes a granulation process and a firing process. In the granulation process, raw pellets are produced by granulating fine iron ore raw material, the particle size and moisture of which have been adjusted, into spheres with a diameter of about 10 mm using a tumbling granulator or the like. In the firing process, the raw pellets are heated to a maximum temperature of about 1,300°C and fired to produce pellets.

[0004] Conventionally, high-quality hematite ore and magnetite ore containing almost no water of crystallization have been used as raw materials for green pellets. However, in recent years, with the depletion of high-quality iron ore, there has been a demand for using inferior iron ore containing a large amount of water of crystallization as raw materials for pellets. However, since the water of crystallization thermally decomposes during the firing process to generate steam, which increases the internal pressure of the green pellets, green pellets produced from iron ore containing a large amount of water of crystallization have the problem of being prone to bursting (exploding).

[0005] When such bursting occurs, the green pellets are pulverized, resulting in a decrease in yield. In addition, when the green pellets are pulverized during the firing process, the resulting powder clogs the gaps in the green pellet packed bed, inhibiting the air permeability of the green pellet packed bed and adversely affecting the manufacturing process. Therefore, there is a need to suppress the occurrence of bursting in pellet manufacturing.

[0006] Patent Document 1 discloses a method for producing pellets with stable quality by adjusting the temperature in the preheating chamber and / or the preheating time according to the strength reduction caused by the inclusion of water of crystallization in the preheated pellets.

[0007] Patent Document 2 discloses a method for suppressing the occurrence of bursting by temperature control that focuses on the temperature difference between the grate temperature at the exit of the water separation chamber and the temperature at the entrance of the preheating chamber.

[0008] Patent Document 3 discloses a method for suppressing the occurrence of bursting by setting the heating rate from room temperature to 280°C to 70°C / min or less and the heating rate from over 280°C to 1200°C to 200°C / min or less under the condition that the content of water of crystallization in the iron ore powder is 3 to 7 mass%. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-87150 [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-24477 [Patent Document 3] Japanese Patent Application Publication No. 2023-33734 Summary of the Invention [Problem to be solved by the invention]

[0010] The present disclosure provides a method for producing iron ore pellets that can suppress the occurrence of bursting. [Means for solving the problem]

[0011] The present disclosure includes the following aspects. <Aspect 1> A method for producing iron ore pellets, comprising: (a) crushing iron ore; (b) obtaining pellet raw material using the crushed iron ore; (c) granulating or molding the pellet raw material to obtain green pellets; (d) firing the green pellets; Including, The product XYZ (mass% mm °C / min) of the average crystal water content X (mass%) of the iron ore constituting the pellet raw material, the average particle size Y (mm) of the raw pellets before firing, and the heating rate Z (°C / min) from 250°C to 700°C during firing is adjusted to be equal to or less than a predetermined allowable value A. Iron ore pellet manufacturing method. <Aspect 2> Before carrying out the steps (a) to (d), (e) A plurality of production conditions under which bursting of iron ore pellets does not occur are identified for the average crystal water content X (mass%) of the iron ore constituting the pellet raw material; (f) for each of the plurality of manufacturing conditions identified in (e), a product XY1Z1 (mass% mm °C / min) is identified, where X is the average crystal water content of the iron ore constituting the pellet raw material, Y1 is the average particle size of the raw pellets before firing (mm), and Z1 is the heating rate (°C / min) from 250°C to 700°C during firing; (g) identifying the maximum value of the multiple products XY1Z1 identified in (f); (h) the maximum value specified in (g) is defined as the tolerance A; A method for producing iron ore pellets according to the first aspect. <Aspect 3> The tolerance A is Formula (1): A=417X+7190 It is determined by A method for producing iron ore pellets according to the first aspect. <Aspect 4> (i) changing the composition of the iron ore constituting the pellet raw material; (ii) changing the average particle size of the green pellets before firing; and (iii) Changing the rate of temperature rise from 250°C to 700°C during firing; including one or more of the following: A method for producing iron ore pellets according to any one of the above first to third aspects. [Effects of the Invention]

[0012] According to the present disclosure, it is possible to provide a method for producing iron ore pellets that can suppress the occurrence of bursting, which is expected to result in improved productivity. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a graph showing the thermal decomposition behavior of an Australian high water of crystallization ore and an α-FeOOH reagent by thermogravimetric analysis. [Figure 2] 1 is a graph showing the relationship between the product of the content of water of crystallization in iron ore and the heating rate (mass %·° C. / min) and the amount of water of crystallization thermally decomposed per unit time (mass % / min). [Figure 3] FIG. 1 is a schematic diagram of a test firing furnace used in the examples. [Figure 4] 1 is a graph plotting the maximum value of the product XY1Z1 determined for each average crystal water content X (mass%) in the examples against the average crystal water content X (mass%). [Figure 5] The graph shows the average particle size Y of MAC ore (average crystal water content X: 6.44% by mass), the heating rate Z from 250°C to 700°C during firing, and the presence or absence of explosion. The curve shows YZ=A / X=9660 / 6.44. [Figure 6] The graph shows the average particle size Y of Pilbara blend ore (average crystal water content X: 4.60% by mass), the heating rate Z from 250°C to 700°C during firing, and the presence or absence of explosion. The curve shows YZ=A / X=9200 / 4.60. [Figure 7] The graph shows the average particle size Y of Robe River ore (average crystal water content X: 8.75% by mass), the heating rate Z during firing from 250°C to 700°C, and the presence or absence of explosion. The curve shows YZ=A / X=10938 / 8.75. [Figure 8]The graph shows the average particle size Y of Carajas ore (average crystal water content X: 2.79% by mass), the heating rate Z during firing from 250°C to 700°C, and the presence or absence of explosion. The curve shows YZ=A / X=8370 / 2.79. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, an embodiment of the method for producing iron ore pellets according to the present disclosure will be described in detail, although the method for producing iron ore pellets according to the present disclosure is not limited to the following embodiment.

[0015] In this specification, unless otherwise specified, the use of "to" to indicate a range of values ​​means that the values ​​before and after it are included as the lower limit and upper limit.

[0016] In this specification, the "crystal water content" refers to the weight ratio of water contained in the ore other than absorbed water. In one embodiment, the "crystal water content" refers to the weight ratio of water generated during heating from 105°C to 950°C after the ore is heated to 105°C to remove absorbed water.

[0017] Bursting of raw pellets is thought to occur as follows. 1. During the firing process, water vapor is generated inside the raw pellets due to evaporation of water or thermal decomposition of water of crystallization (mainly 2FeOOH → Fe2O3 + H2O). 2. As steam is generated, pressure rises inside the raw pellets, and bursting occurs when the stress caused by the internal pressure exceeds the matrix strength of the raw pellets.

[0018] Figure 1 shows the thermal decomposition behavior of an Australian high-water-of-crystallization ore with a crystallization water content of 4.70% by mass and α-FeOOH reagent. From Figure 1, the weight loss of the Australian high-water-of-crystallization ore at around 300°C is believed to be primarily due to the thermal decomposition of FeOOH contained in the ore (2FeOOH → Fe2O3 + HO). Specifically, the thermal decomposition of the crystallization water (1) occurs in the range of approximately 250 to 350°C, as shown in Figure 1. Therefore, rapid water vapor generation can be suppressed by suppressing the rate of temperature rise in this temperature range. However, in actual green pellet firing processes, bursting can occur even when the firing temperature (green pellet surface temperature or ambient temperature near the surface) exceeds 350°C. This is thought to be because the green pellets are heated from the surroundings by heat transfer from the ambient temperature, and the temperature rise inside the green pellets is delayed relative to the rise in firing temperature. Specifically, this is thought to be because the temperature inside the green pellets has not yet reached 350°C when the firing temperature reaches 350°C, and the thermal decomposition of the water of crystallization inside the green pellets has not yet been completed. Therefore, simply controlling the rate at which the firing temperature rises from 250°C to 350°C is insufficient to prevent bursting. To prevent bursting, it is necessary to appropriately control the rate at which the temperature rises even in the firing temperature range above 350°C, for example. Since it is usually difficult to measure the temperature inside the green pellets, it is important to control the firing temperature while taking into account the delay in temperature rise inside the green pellets.

[0019] On the other hand, a decrease in the heating rate is undesirable because it increases the firing time and ultimately reduces productivity. Therefore, the inventors focused on controlling the particle size of green pellets as a method for suppressing bursting while maintaining a relatively high heating rate. Reducing the particle size of green pellets shortens the distance that water vapor generated inside the pellets must travel to escape, thereby reducing the pressure inside the pellets and suppressing bursting. Bursting of green pellets is thought to occur at the shortest point from the center to the outside of the green pellets. Therefore, the inventors came up with the idea that it is important to focus on and control the minor axis of the green pellet particle size. In this specification, the "particle size" of green pellets refers to the "minor axis" of the green pellets. Focusing on the minor axis allows for more accurate identification of conditions under which bursting does not occur. On the other hand, reducing the particle size increases the specific surface area, facilitating heat transfer from the ambient temperature and facilitating thermal decomposition of the water of crystallization. Therefore, attention must be paid to controlling the heating rate in addition to particle size control.

[0020] The inventors considered the factors governing bursting to be as follows. According to the Kozeny-Carman equation, the pressure ΔP generated when a fluid passes through a powder bed is proportional to the length L of the powder bed and the superficial velocity u of the fluid. Applying this to the bursting phenomenon of raw pellets, the pressure ΔP generated inside the raw pellets due to the thermal decomposition of water of crystallization (steam generation) is thought to be proportional to the distance the water vapor passes and the superficial velocity of the water vapor. The distance the water vapor passes is proportional to the particle size of the raw pellets. Assuming that the cross-sectional area of ​​the flow path through which the water vapor passes is constant, the superficial velocity of the water vapor is proportional to the amount of water of crystallization thermally decomposed per unit time (amount of water vapor generated). Thermogravimetric analysis revealed that the amount of water of crystallization thermally decomposed per unit time (mass% / min) is approximately proportional to the product of the water of crystallization content of the iron ore and the heating rate (mass% °C / min), as shown in Figure 2. Therefore, it is considered that the pressure ΔP generated inside the green pellets due to the thermal decomposition of the water of crystallization is proportional to the product of the particle size of the green pellets, the content of water of crystallization in the iron ore, and the rate of temperature rise.

[0021] Based on this finding, the present inventors have considered that the pressure ΔP generated inside the green pellets due to the thermal decomposition of the water of crystallization can be controlled by the product of the particle size of the green pellets, the content of water of crystallization in the iron ore, and the rate of temperature rise, and have arrived at the present invention.

[0022] One embodiment of a method for producing iron ore pellets includes: (a) crushing iron ore; (b) obtaining raw pellet material using the crushed iron ore; (c) granulating or molding the raw pellet material to obtain green pellets; and (d) firing the green pellets. The product XYZ (mass%·mm·°C / min) of the average water of crystallization content X (mass%) of the iron ore constituting the raw pellet material, the average particle size Y (mm) of the raw pellets before firing, and the heating rate Z (°C / min) from 250°C to 700°C during firing is adjusted to be equal to or less than a predetermined allowable value A. By adjusting the product XYZ of the average water of crystallization content X (mass%), the average particle size Y (mm), and the heating rate Z (°C / min) to be equal to or less than the allowable value A, explosion of the raw pellets in the preheating zone can be suppressed. As a result, smooth operation of the firing process and improved pellet product yield can be expected.

[0023] The method for producing iron ore pellets may include one or more of (i) changing the composition of the iron ore constituting the pellet raw material, (ii) changing the average particle size of the raw pellets before firing, and (iii) changing the rate of temperature increase from 250°C to 700°C during firing. This allows the product XYZ to be changed during production.

[0024] <(a) Process> Step (a) is a step of pulverizing iron ore. The method for pulverizing iron ore is not particularly limited, and any known pulverizing method can be used. A preferred example is a pulverization method using a ball mill.

[0025] The size of the iron ore after pulverization is not particularly limited, but as an example, it is preferable to pulverize the iron ore so that the ratio of raw material particles having a diameter of 100 μm or less is 90 mass % or more. The remainder is iron ore having a diameter of more than 100 μm. The size of the iron ore after pulverization can be adjusted, for example, by the pulverization time. The iron ore after pulverization may be classified so that it has a desired particle size distribution.

[0026] The iron ore may include iron ore having a crystal water content of 3 mass% or more (hereinafter, sometimes referred to as "first iron ore"), and may further include iron ore having a crystal water content of less than 3 mass% (hereinafter, sometimes referred to as "second iron ore").

[0027] (Iron Ore No. 1) The first iron ore has a crystal water content of 3% by mass or more. The crystal water content of the first iron ore may be 4% by mass or more, 5% by mass or more, or 6% by mass or more, and may be 10% by mass or less, 9% by mass or less, 8% by mass or less, or 7% by mass or less. These upper and lower limit values ​​can be combined arbitrarily.

[0028] Specific examples of primary iron ore include Pilbara blend ore, MAC ore, Yandi ore, and Robe River ore.

[0029] The first iron ore may be used alone or in combination of two or more kinds in any ratio.

[0030] (Second iron ore) The second iron ore has a crystal water content of less than 3% by mass. The crystal water content of the second iron ore may be 0.1% by mass or more, 0.5% by mass or more, or 1.0% by mass or more, and may be 2.9% by mass or less, 2.5% by mass or less, or 2.0% by mass or less. These upper and lower limit values ​​can be combined arbitrarily.

[0031] Specific examples of secondary iron ores include Carajas ore, hematite ore, Brazilian blend ore, and magnetite ore.

[0032] The second iron ore may be used alone or in combination of two or more kinds in any ratio.

[0033] <(b) Process> Step (b) is a step of obtaining a pellet raw material using the pulverized iron ore obtained in step (a). The pellet raw material contains one or more types of iron ore, and thus has a predetermined average crystal water content X. The pellet raw material may contain other optional components in addition to the iron ore.

[0034] The method for producing iron ore pellets may include a step of pulverizing other optional components, if necessary, before step (b). The pulverizing method for the other optional components is not particularly limited, and a known pulverizing method can be used. A preferred example is a pulverizing method using a ball mill.

[0035] The size of the other optional components after pulverization is not particularly limited, but as an example, it is preferable to pulverize so that the proportion of particles with a diameter of 100 μm or less is 90 mass % or more. The remainder at this time is particles with a diameter of more than 100 μm. The size of the other optional components after pulverization can be adjusted, for example, by the pulverization time. Classification may be performed so that the other optional components after pulverization have a desired particle size distribution.

[0036] The amount of the first iron ore contained in the pellet raw material is not particularly limited. For example, the content of the first iron ore in the pellet raw material may be 10% by mass or more, 20% by mass or more, or 30% by mass or more, and may be 100% by mass or less, 90% by mass or less, or 80% by mass or less. These upper and lower limit values ​​can be combined arbitrarily.

[0037] The amount of the second iron ore contained in the pellet raw material is not particularly limited. The content of the second iron ore can be adjusted depending on the average crystal water content X described below. The mass of the second iron ore relative to the total mass of the first iron ore and the second iron ore may be, for example, 0% by mass or more, 5% by mass or more, or 10% by mass or more, and may be 50% by mass or less, 40% by mass or less, or 30% by mass or less. These upper and lower limit values ​​can be combined arbitrarily.

[0038] (Other optional ingredients) Specific examples of other optional components include carbonaceous materials, limestone, dolomite, magnesite, olivine, and bentonite. Examples of carbonaceous materials include coke, anthracite, and charcoal.

[0039] The upper limit of the total content of other optional components in the pellet raw material is preferably 9% by mass or less to avoid a decrease in the iron content. It is more preferably 6% by mass or less, and even more preferably 3% by mass or less. The content of carbonaceous material in the pellet raw material is preferably 2% by mass or less, and more preferably 1% by mass or less. The total content of limestone, dolomite, magnesite, and olivine in the pellet raw material is preferably 6% by mass or less, and more preferably 3% by mass or less. The content of bentonite in the pellet raw material is preferably 1% by mass or less, and more preferably 0.5% by mass or less. The lower limit of the total content of other optional components in the pellet raw material is not particularly limited, but may be, for example, 0% by mass or more, or 0.1% by mass or more.

[0040] The other optional components may be used alone or in combination of two or more in any ratio.

[0041] (Average crystal water content X) The average crystal water content X of the iron ore constituting the pellet raw material (also simply referred to as the average crystal water content X) may be 1% by mass or more, 2% by mass or more, 3% by mass or more, 4% by mass or more, 5% by mass or more, or 6% by mass or more, and may be 10% by mass or less, 9% by mass or less, 8% by mass or less, or 7% by mass or less. The method for producing iron ore pellets in which the product XYZ is adjusted is particularly effective in suppressing the occurrence of bursting when the average crystal water content X is high. From this perspective, the average crystal water content X is preferably 2.79% by mass or more, more preferably more than 3% by mass. From the perspective of reducing the occurrence of cracks in the pellets after firing, it is preferable that the upper limit of the average crystal water content X be 7% by mass or less. These upper and lower limits can be combined as desired.

[0042] The average crystal water content X can be determined by measuring the crystal water content of the iron ore contained in the pellet raw material. Specifically, when the pellet raw material contains one type of iron ore, the crystal water content of that iron ore is determined as the average crystal water content X. When the pellet raw material contains two or more types of iron ore, the weighted average of the crystal water contents of each type or the crystal water content measured for the mixture of iron ores constituting the pellet raw material is determined as the average crystal water content X. The crystal water content of iron ore can be measured, for example, by JIS M8211:1995 "Iron ore - Method for determining water of combination."

[0043] The average crystal water content X can be adjusted, for example, by selecting the iron ore used as a raw material, and by selecting the blending ratio when two or more types of iron ore are used. Specifically, when iron ore with a relatively high crystal water content is used as the raw material, the average crystal water content X is relatively high, and when iron ore with a relatively low crystal water content is used as the raw material, the average crystal water content X is relatively low. When two or more types of iron ore are used, the average crystal water content X increases by increasing the blending amount of iron ore with a relatively high crystal water content, and decreases by increasing the blending amount of iron ore with a relatively low crystal water content.

[0044] <(c) Process> Step (c) is a step of granulating or molding the pellet raw material obtained in step (b) to obtain green pellets. The pellet raw material is preferably humidified before granulating or molding. After granulating or molding, the green pellets may be dried.

[0045] The pellet raw material can be granulated using, for example, a rolling granulator such as a pan pelletizer or a drum pelletizer, without any particular limitation. The pellet raw material can be molded using, for example, a molding machine such as a compression molding machine or an extrusion molding machine, without any particular limitation. The pellet raw material is preferably molded using a compression molding machine or an extrusion molding machine.

[0046] The shape of the green pellets is not particularly limited, and specific examples thereof include spherical, cylindrical, and briquette shapes (for example, pillow-shaped and almond-shaped).

[0047] When the green pellets are spherical, their diameter is usually 10 mm to 20 mm. If the diameter is 10 mm or more, a flow path for the reducing gas is ensured in the subsequent reduction process in a furnace such as a blast furnace or a direct reduction shaft furnace, thereby increasing productivity. If the diameter is 20 mm or less, reduction to the inside of the green pellets is easy. For the same reason, when the green pellets are cylindrical, their bottom diameter and height are usually 10 mm to 20 mm, and when the green pellets are briquette-shaped, their major diameter is usually 10 mm to 40 mm, and their minor diameter is usually 10 mm to 20 mm. The "major diameter" of green pellets refers to the maximum diameter of the green pellets. The "minor diameter" will be described later.

[0048] The average particle size Y (mm) of the green pellets before firing (also simply referred to as average particle size Y) can be adjusted, for example, as follows. When a tumbling granulator is used, examples of methods for adjusting the average particle size Y include adjusting the granulation residence time and adjusting the mesh size or roller screen width when classifying the green pellets. When a molding machine such as a compression molding machine or an extrusion molding machine is used, examples of methods for adjusting the average particle size Y include adjusting the die diameter.

[0049] If the raw pellets are spherical, the "minor diameter" of the raw pellets is the diameter of the sphere. If the raw pellets are cylindrical, the "minor diameter" of the raw pellets is the smaller of the diameter of the circle and the height of the cylinder. If the raw pellets are shaped other than spherical or cylindrical, the "minor diameter" of the raw pellets is the shortest distance between two parallel planes when the raw pellets are sandwiched between them. The "average particle size Y" of the raw pellets is a weight-based average value based on the minor diameter determined according to the shape of the raw pellets.

[0050] There is no particular limitation on the method for determining the average particle size Y. For example, a certain number or more of green pellets can be measured with a vernier caliper or the like, and the weight-based average value of the values ​​obtained can be determined as the particle size Y. When green pellets are produced using a mold, the average particle size Y may be determined from the size of the mold.

[0051] The average particle size Y may be 10 mm or more, or 12 mm or more, and may be 20 mm or less, or 18 mm or less. These upper and lower limits can be combined arbitrarily. The narrower the particle size distribution of the green pellets, the better. Specifically, the maximum minor axis of the green pellets is preferably the average particle size Y+1.0 mm or less, and more preferably the average particle size Y+0.5 mm or less. The minimum minor axis of the green pellets is preferably the average particle size Y-1.0 mm or more, and more preferably the average particle size Y-0.5 mm or more.

[0052] The ratio of the diameter to the minor axis of the green pellets equivalent to an equivalent volume sphere (diameter / minor axis) is preferably 1.10 to 2.00. The diameter of the green pellets equivalent to an equivalent volume sphere is calculated by the following formula.

number

[0053] The carbon content of the green pellets is preferably 1% by mass or less, more preferably 0.5% by mass or less, on a dry basis. The lower limit of the carbon content of the green pellets is not particularly limited, but may be, for example, 0.1% by mass or more, or 0.2% by mass or more.

[0054] <(d) Process> Step (d) is a step of firing the green pellets obtained in step (c). The method of firing the green pellets is not particularly limited, and can be performed using a known firing furnace. Examples of the type of firing furnace include a traveling grate type, a grate kiln type, and a pot type.

[0055] The heating rate Z (°C / min) from 250°C to 700°C during firing (also simply referred to as heating rate Z) can be calculated, for example, by measuring the temperature using a thermocouple fixed within the green pellet layer or near the green pellet surface (for example, within 10 mm from the surface of the green pellet) so that the relative positional relationship between the green pellets and the thermocouple is constant, and measuring the time it takes for the temperature to rise from 250°C to 700°C.

[0056] In traveling grate or grate kiln-type kilns, it is difficult to install thermocouples as described above. In such cases, the temperature rise rate Z may be calculated from the ambient temperature measurements made by multiple fixed thermocouples installed directly above the pellet layer or directly below the pallet cart in the furnace. Specifically, the temperature rise rate Z is calculated by determining two positions 1 and 2 within the temperature rise process from 250°C to 700°C, and then using the following formula to calculate the temperature at those positions and the time elapsed between those two positions: Z = (T2 - T1) × (V / D) Here, T1 is the temperature measured at position 1, T2 is the temperature measured at position 2, D is the distance between positions 1 and 2 in the direction of travel of the pallet cart, and V is the travel speed of the pallet cart.

[0057] The temperature rise rate Z can be adjusted, for example, as follows. When a traveling grate or grate kiln calciner is used, methods for adjusting the temperature rise rate Z include, for example, adjusting the burner firing rate (amount of heat supplied) during the temperature rise process from 250°C to 700°C, adjusting the movement speed of the pallet cart, and adding a substance that generates oxidation heat, such as magnetite ore, to the raw materials. Specifically, the temperature rise rate Z increases by increasing the amount of heat supplied, and decreases by decreasing the movement speed of the pallet cart. When a pot-type calciner is used, methods for adjusting the temperature rise rate Z include, for example, adjusting the movement speed of the pot that contains the raw pellets, and adding a substance that generates oxidation heat, such as magnetite ore, to the raw materials.

[0058] The heating rate Z may be 50°C / min or more, greater than 70°C / min, 75°C / min or more, 80°C / min or more, 100°C / min or more, or 120°C / min or more, and may be 250°C / min or less, 200°C / min or less, 180°C / min or less, or 160°C / min or less. These upper and lower limits can be arbitrarily combined. From the viewpoint of productivity, it is preferable that the heating rate Z is as high as possible under conditions that do not cause bursting. According to the method of the present disclosure, the heating rate Z can be determined according to the average crystal water content X and the average particle size Y, thereby achieving both suppression of bursting and production efficiency. Furthermore, the process from 250°C to 700°C can be operated with consistent heating control, which is advantageous from the viewpoint of manufacturing efficiency. The smaller the fluctuation in the heating rate Z from 250°C to 700°C during firing, the more preferable it is. Specifically, the difference between the heating rate from 250°C to 280°C during firing and the heating rate from above 280°C to 700°C during firing is preferably 40°C / min or less, more preferably 20°C / min or less. There is no need to set a lower limit for the difference between the heating rate from 250°C to 280°C during firing and the heating rate from above 280°C to 700°C during firing, but it is 0°C / min or more. The difference between the heating rate from 250°C to 350°C during firing and the heating rate from above 350°C to 700°C during firing is preferably 40°C / min or less, more preferably 20°C / min or less. There is no need to set a lower limit for the difference between the heating rate from 250°C to 350°C during firing and the heating rate from above 350°C to 700°C during firing, but it is 0°C / min or more.

[0059] <How to determine tolerance A> The tolerance value A is a threshold value determined based on whether or not bursting occurs. The method for determining the tolerance value A is not particularly limited. The tolerance value A may be determined, for example, as follows. That is, before performing the above steps (a) to (d), (e) identify a plurality of production conditions under which bursting of the iron ore pellets does not occur for the average crystal water content X (mass%) of the iron ore constituting the pellet raw material, (f) identify, for each of the plurality of production conditions identified in (e), the product XY1Z1 (mass%·mm·°C / min) of the average crystal water content X (mass%) of the iron ore constituting the pellet raw material, the average particle size Y1 (mm) of the raw pellets before firing, and the heating rate Z1 (°C / min) from 250°C to 700°C during firing, (g) identify the maximum value of the plurality of XY1Z1 values ​​identified in (f), and (h) determine the maximum value identified in (g) as the tolerance value A.

[0060] Specifically, for a given average crystal water content X, one or more of the average particle size Y1 (also simply referred to as average particle size Y1) (mm) of the raw pellets before firing and the heating rate Z1 (also simply referred to as heating rate Z1) (°C / min) from 250°C to 700°C during firing are changed, and a finite number of trials are conducted to check for the presence or absence of explosion.The maximum value of the product XY1Z1 in the trials in which explosion did not occur can be determined as the allowable value A for the average crystal water content X.

[0061] The presence or absence of bursting can be evaluated by visually observing the sample after firing, and if the sample breaks into two or more particles, it can be determined that bursting has occurred. The fired sample can also be sieved through a specified sieve size, and the presence or absence of bursting can be evaluated based on the amount of powder generated after firing. As an example, the fired sample can be sieved through a sieve with a 6.3 mm mesh size, and if the proportion of sample that falls under the sieve is 5% by mass or more, it can be determined as a failure (bursting has occurred). The sieve size and the proportion of sample that falls under the sieve, which serve as the evaluation criteria, can be set appropriately depending on the size of the raw pellets used and the acceptable rate of bursting.

[0062] As shown in Figure 4, it was found that the relationship between the average crystal water content X and the maximum value of the product XY1Z1 in trials in which no explosion occurred can be approximated by the maximum value of the product XY1Z1 = 417X + 7190. Therefore, the allowable value A may be determined by the following formula (1). Formula (1): Tolerance A = 417X + 7190

[0063] The water of crystallization content of iron ore is roughly constant for each type. For example, the water of crystallization content of Carajas ore is 2 to 3 mass %, the water of crystallization content of Pilbara blend ore is 4 to 5 mass %, the water of crystallization content of MAC ore is 6 to 7 mass %, and the water of crystallization content of Robe River ore is 8 to 9 mass %. When the average crystal water content X is 2 to 3 mass%, the allowable value A may be 8370 mm·mass%·°C / min; when the average crystal water content X is 3 to 4 mass%, the allowable value A may be 8733 mm·mass%·°C / min; when the average crystal water content X is 4 to 5 mass%, the allowable value A may be 9200 mm·mass%·°C / min; when the average crystal water content X is 6 to 7 mass%, the allowable value A may be 9660 mm·mass%·°C / min; and when the average crystal water content X is 8 to 9 mass%, the allowable value A may be 10938 mm·mass%·°C / min. The allowable value A varies depending on the raw pellets used, but is, for example, 8000 to 11000 mm·mass%·°C / min. [Example]

[0064] The present invention will be specifically described below based on examples and comparative examples, but the present invention is not limited to these examples.

[0065] Table 1 shows whether or not bursting occurred when pellets were fired under several conditions of heating rate Z and average particle size Y using pellet raw materials composed of MAC ore with a crystal water content of 6.44 mass% as iron ore and bentonite.

[0066] (Test Method) The content of water of crystallization in iron ore was measured according to JIS M8211:1995 "Iron ore - Method for determining water of crystallization."

[0067] (Production of raw pellets) Iron ore was crushed to 100 μm or less using a ball mill. Then, 0.5% by mass of bentonite was added to the crushed iron ore as a binder to obtain pellet raw material. After adding 8% by mass of water to the blended raw material, the mixture was compressed using dies with different inner diameters to produce cylindrical green pellets with different particle sizes. The height of the cylindrical green pellets was adjusted to be the same as their diameter. That is, the ratio of the diameter to the minor axis of the green pellets (equivalent volume sphere diameter / minor axis) was 1.14. The inner diameter of the die corresponding to the diameter of the resulting green pellets was determined as the average particle size Y. Because the green pellets were produced using a die of a specified size, there was little variation in particle size, and the maximum and minimum minor axis values ​​of the green pellets were consistent with Y (average particle size Y within ±0.5 mm).

[0068] (Roasted pellet firing) The prepared green pellets were fired using a test firing furnace capable of controlling the heating rate. Figure 3 shows a schematic diagram of the test firing furnace used in the examples. The test firing furnace was equipped with a sample cage 13 installed in an alumina tube 11 with an inner diameter of 42 mm, and the sample cage 13 was capable of containing green pellets 14. During firing, the alumina tube 11 was heated to 1,360°C by a heater 12. The temperature of the green pellets 14 contained in the sample cage 13 was measured using a thermocouple 15 installed directly above the green pellets 14. The temperature of the green pellets during firing could be controlled by moving the sample cage 13 up and down within the alumina tube 11. The heating rate from room temperature to 250°C was 50°C / min. The heating rate Z from 250°C to 700°C was as shown in Table 1. The difference between the heating rate from 250°C to 280°C and the heating rate from above 280°C to 700°C, and the difference between the heating rate from 250°C to 350°C and the heating rate from above 350°C to 700°C were all 10°C / min or less. The heating rate from 700°C to 1300°C was 200°C / min, and after reaching 1300°C, the temperature was maintained for 10 minutes and then air-cooled to obtain pellets.

[0069] (Method for assessing the presence or absence of bursting) The presence or absence of bursting was judged by visual observation of the fired pellets. If the fired pellets maintained their shape without cracking, it was judged as no bursting, and if the pellets cracked and separated into two or more particles, it was judged as bursting.

[0070] [Table 1]

[0071] Among the conditions under which bursting did not occur, the conditions under which the product XYZ was greatest were Test Nos. 3 and 5. The product XYZ at this time can be defined as the allowable value A. In other words, bursting during firing can be suppressed by adjusting the product XYZ of the average crystal water content X, the average particle size Y, and the heating rate Z to 9660 (mm·mass%·°C / min) or less.

[0072] The presence or absence of bursting was evaluated in the same manner as for MAC ores, except that Pilbara blend ore (crystallization water content: 4.60 mass%), Robe River ore (crystallization water content: 8.75 mass%), or Carajas ore (crystallization water content: 2.79 mass%) was used as the iron ore. The results are shown in Tables 2 to 4.

[0073] [Table 2]

[0074] [Table 3]

[0075] [Table 4]

[0076] For each pellet raw material, the maximum value of the product XYZ under conditions where bursting did not occur was determined as the allowable value A. The results are summarized in Table 5 together with the results for MAC ore.

[0077] [Table 5]

[0078] Fig. 4 shows a graph plotting the relationship between the average crystal water content X and the maximum value of the product XYZ (i.e., the product XY1Z1) under conditions where no bursting occurred, determined as the allowable value A. From Fig. 4, it was found that the relationship between the average crystal water content X and the maximum value of the product XY1Z1 can be approximated by the following formula. Maximum product of XY1Z1 = 417X + 7190

[0079] It was observed that the larger the average water of crystallization content X, the larger the maximum value of the product XY1Z1. This is thought to be due to the fact that the iron ore with a higher water of crystallization content has a larger amount of porosity within the ore, and the greater heat absorption, which reduces the actual temperature rise rate inside the raw pellets. Taking this effect into consideration, by changing the allowable value A according to the average water of crystallization content X, it is possible to more precisely control explosion. As a result, it is possible to further increase productivity.

[0080] Figures 5-8 show the curves YZ = A / X for MAC ore (average crystallization water content X: 6.44 mass%, tolerance A: 9660 mm⋅mass%⋅°C / min), Pilbara blend ore (average crystallization water content X: 4.60 mass%, tolerance A: 9200 mm⋅mass%⋅°C / min), Robe River ore (average crystallization water content X: 8.75 mass%, tolerance A: 10938 mm⋅mass%⋅°C / min), and Carajas ore (average crystallization water content X: 2.79 mass%, tolerance A: 8370 mm⋅mass%⋅°C / min). Figures 5-8 also show the results of Tables 1-4. Figures 5-8 confirm that pellet bursting can be suppressed by adjusting the product XYZ to be less than the tolerance A, i.e., by adjusting YZ≦A / X.

[0081] Pilbara blend ore (crystallization water content: 4.60% by mass) was used as the first iron ore, and Carajas ore (crystallization water content: 2.79% by mass) was used as the second iron ore. They were mixed in a mass ratio of 1:1 to obtain iron ore with an average crystallization water content X of 3.70% by mass. The allowable value A was then determined to be 8733% by mass mm °C / min using equation (1). Green pellets were then prepared in the same manner as above, fired, and evaluated for the presence or absence of bursting. The average particle size Y and heating rate Z were as shown in Table 6. The results are shown in Table 6. As shown in Table 6, bursting could be suppressed under conditions where the product XYZ was adjusted to be equal to or less than the allowable value A.

[0082] [Table 6] [Explanation of symbols]

[0083] 11 Alumina tube 12 Heater 13 Sample basket 14 Raw pellets 15 Thermocouple

Claims

1. A method for producing iron ore pellets, comprising: (a) crushing iron ore; (b) obtaining pellet raw material using the crushed iron ore; (c) granulating or molding the pellet raw material to obtain green pellets; (d) firing the green pellets; Including, The product XYZ (mass% mm ° C / min) of the average crystal water content X (mass%) of the iron ore constituting the pellet raw material, the average particle size Y (mm) of the raw pellets before firing, and the heating rate Z (° C / min) from 250° C. to 700° C. during firing is adjusted to be equal to or less than a predetermined allowable value A, The average particle size Y (mm) is a weight-based average value based on the minor axis determined depending on the shape of the raw pellets. Iron ore pellet manufacturing method.

2. the ratio of the diameter to the minor axis of the green pellets (diameter / minor axis) of the green pellets to the equivalent diameter of a sphere of equal volume is 1.10 to 2.00; The method for producing iron ore pellets according to claim 1.

3. The average crystal water content X (mass%) of the iron ore constituting the pellet raw material is more than 3 mass%. The method for producing iron ore pellets according to claim 1.

4. The maximum minor axis of the green pellets is equal to or less than the average particle diameter Y (mm) of the green pellets before firing + 1.0 mm. The method for producing iron ore pellets according to claim 1.

5. The difference between the temperature rise rate from 250°C to 280°C during firing and the temperature rise rate from over 280°C to 700°C during firing is 40°C / min or less. The method for producing iron ore pellets according to claim 1.

6. In the step (c), the pellet raw material is molded using a compression molding machine or an extrusion molding machine. The method for producing iron ore pellets according to claim 1.

7. Before carrying out the steps (a) to (d), (e) A plurality of production conditions are identified under which bursting of the iron ore pellets does not occur with respect to the average crystal water content X (mass%) of the iron ore constituting the pellet raw material; (f) For each of the plurality of manufacturing conditions specified in (e), the average crystal water content X (mass%) of the iron ore constituting the pellet raw material and the average particle size Y of the raw pellets before firing are calculated. １ (mm), and the temperature rise rate Z from 250 ° C to 700 ° C during firing １ (°C / min) and product XY １ Z １ (mass% mm °C / min) is specified, (g) the plurality of products XY identified in (f) １ Z １ The maximum value of (h) the maximum value specified in (g) is defined as the tolerance A; The method for producing iron ore pellets according to claim 1.

8. The tolerance A is Formula (1): A=417X+7190 It is determined by The method for producing iron ore pellets according to claim 1.

9. (i) changing the composition of the iron ore constituting the pellet raw material; (ii) changing the average particle size of the green pellets before firing; and (iii) Changing the rate of temperature rise from 250°C to 700°C during firing; including one or more of the following: The method for producing iron ore pellets according to any one of claims 1 to 8.

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