Iron ore pellet production method

Abstract

Provided is a pellet production method by which the occurrence of bursting can be suppressed. An iron ore pellet production method according to the present disclosure comprises: (a) a step for crushing iron ore; (b) a step for using the crushed iron ore to obtain a pellet raw material; (c) a step for granulating or molding the pellet raw material to obtain green pellets; and (d) a step for firing the green pellets, wherein the product XYZ (mass%∙mm∙ºC / min) of the average crystal water content X (mass%) of the iron ore constituting the pellet raw material, the average particle diameter Y (mm) of the green pellets before the firing, and a temperature rising rate Z (ºC / min) from 250ºC to 700ºC during the firing is adjusted so as to be equal to or less than a predetermined allowable value A.

Description

TITLE OF INVENTION: IRON ORE PELLET PRODUCTION METHOD TECHNICAL FIELD

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

[0002] Iron ore pellets (hereinafter also simply referred to as pellets) are a fired product obtained by agglomerating fine iron ore of 100 pm or less into particles with a size of about ten and several millimeters, and are used as a raw material for a blast furnace process and a direct reduction process.

[0003] Common methods for producing pellets include granulation and firing steps. In the granulation step, fine iron ore raw material whose particle size and water content are controlled is granulated into a spherical shape having a diameter of about ten and several millimeters by using a rolling granulator or the like, so that green pellets are produced. In the firing step, the green pellets are heated to the maximum temperature of about 1300°C and fired to form pellets.

[0004] Conventionally, high-quality hematite and magnetite ores with minimal crystalline water content have been used as raw materials for green pellets. However, with the depletion of high-quality iron ores in recent years, it has become necessary to use low-quality iron ores containing a large amount of crystalline water as the raw material for pellets. However, crystalline water thermally decomposes during the firing process to generate water vapor, which increases the internal pressure of the green pellets. As a result, green pellets produced from iron ores containing a large amount of crystalline water have a problem of being susceptible to bursting.

[0005] When such bursting occurs, the green pellet becomes pulverized, and the yield decreases. In addition, when the green pellets are pulverized in the firing process, the gaps in the green pellet packed bed are clogged by the generated powder, the permeability of the green pellet packed bed is inhibited, having an adverse effect on the production process. Therefore, it is necessary to suppress the occurrence of bursting in producing pellets.

[0006] 2024427813   25 Jun 2026 PTL 1 discloses a method for producing pellets with stable quality by controlling a preheating chamber temperature or / and a preheating time according to the strength that is reduced because the preheated pellets contain crystalline water.

[0007] PTL 2 discloses a method for suppressing the occurrence of bursting by temperature control focusing on the temperature difference between a dehydration chamber outlet grate temperature and a preheating chamber inlet temperature.

[0008] PTL 3 discloses a method for suppressing the occurrence of bursting by setting a heating rate from room temperature to 280°C to 70°C / min or less and a heating rate from more than 280°C to 1200°C to 200°C / min or less under the condition that the crystalline water content of iron ore powder is 3 to 7 mass%. CITATION LIST PATENT LITERATURE

[0009] PTL 1: Japanese Patent Application Publication No. JP 2000-87150 PTL 2: Japanese Patent Application Publication No. JP 2010-24477 PTL 3: Japanese Patent Application Publication No. JP 2023-33734 SUMMARY OF INVENTION [0009a] Any discussion of the prior art throughout the specification should not be considered as an express or implied admission that such prior art is widely known or forms part of the common general knowledge in the field. [0009b] It is an object of the present invention to overcome or ameliorate one or more of the disadvantages of the prior art, or at least to provide a useful alternative. [0009c] According to a first aspect, the present disclosure provides a method for producing iron ore pellets, the method comprising the steps of: (a) grinding iron ore; (b) obtaining a pellet raw material using the ground iron ore; (c) performing granulation or molding of the pellet raw material to obtain green pellets; and (d) firing the green pellets, wherein a product XYZ (mass%^mm^°C / min) of an average crystalline water content X (mass%) of the iron ore 2024427813   25 Jun 2026 constituting the pellet raw material, an average particle size Y (mm) of the green pellets before firing, and a heating rate Z (°C / min) from 250°C to 700°C during firing is controlled to be less than or equal to a predetermined allowable value A, the average particle size Y (mm) is a weightbased average value based on the short diameter of the green pellets, which is determined according to their shapes.

[0010] The present disclosure also provides a method for producing iron ore pellets capable of suppressing the occurrence of bursting.

[0011] The content of the present disclosure includes the following aspects. <Aspect 1> A method for producing iron ore pellets, the method comprising the steps of: (a) grinding iron ore; (b) obtaining a pellet raw material using the ground iron ore; (c) performing granulation or molding of the pellet raw material to obtain green pellets; and (d) firing the green pellets, wherein a product XYZ (mass%^mm-°C / min) of an average crystalline water content X (mass%) of the iron ore constituting the pellet raw material, an average particle size Y (mm) of the green pellets before firing, and a heating rate Z (°C / min) from 250°C to 700°C during firing is controlled to be less than or equal to a predetermined allowable value A. <Aspect 2> The method for producing iron ore pellets according to aspect 1, wherein before performing the steps (a) to (d), (e) with respect to the average crystalline water content X (mass%) of the iron ore constituting the pellet raw material, a plurality of production conditions under which bursting of iron ore pellets does not occur are identified; (f) for each of the plurality of production conditions identified in (e), a product XY1Z1 (mass%^mm^°C / min) of an average crystalline water content X (mass%) of the iron ore constituting the pellet raw material, an average particle size Y1 (mm) of green pellets before firing, and a heating rate Z1 (°C / min) from 250°C to 700°C during firing is identified; (g) a maximum value among the plurality of products XY1Z1 identified in (f) is identified; and (h) the maximum value identified in (g) is determined as the allowable value A. <Aspect 3> The method for producing iron ore pellets according to aspect 1, wherein the allowable value A is determined by Expression (1): A = 417X + 7190. <Aspect 4> The method for producing iron ore pellets according to any one of aspects 1 to 3, comprising one or more of: (i) changing a blending ratio 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 heating rate from 250°C to 700°C during firing. ADVANTAGEOUS EFFECTS OF INVENTION

[0012] According to the present disclosure, a method for producing iron ore pellets capable of suppressing the occurrence of bursting can be provided. As a result, improvement of productivity can be expected. BRIEF DESCRIPTION OF DRAWINGS

[0013] [Fig. 1] Fig. 1 is a graph showing a thermal decomposition behavior of Australian high-crystalline-water ore and a-FeOOH reagent by thermogravimetric analysis. [Fig. 2] Fig. 2 is a graph showing a relationship between a product (mass%^°C / min) of a crystalline water content of iron ore and a heating rate, and an amount of thermal decomposition of crystalline water per unit time (mass% / min). [Fig. 3] Fig. 3 is a schematic view of a test firing furnace used in examples. [Fig. 4] Fig. 4 is a graph in which a maximum value of a product XY1Z1 identified for each average crystalline water content X (mass%) in examples is plotted against the average crystalline water content X (mass%). [Fig. 5] Fig. 5 shows the results of examining the average particle size Y of MAC ore (average crystalline water content X: 6.44 mass%), the heating rate Z from 250°C to 700°C during firing, and the presence or absence of bursting. The curve indicates YZ = A / X = 9660 / 6.44. [Fig. 6] Fig. 6 shows the results of examining the average particle size Y of Pilbara blend ore (average crystalline water content X: 4.60 mass%), the heating rate Z from 250°C to 700°C during firing, and the presence or absence of bursting. The curve indicates YZ = A / X = 9200 / 4.60. [Fig. 7] Fig. 7 shows the results of examining the average particle size Y of Robe River ore (average crystalline water content X: 8.75 mass%), the heating rate Z from 250°C to 700°C during firing, and the presence or absence of bursting. The curve indicates YZ = A / X = 10938 / 8.75. [Fig. 8] Fig. 8 shows the results of examining the average particle size Y of Carajas ore (average crystalline water content X: 2.79 mass%), the heating rate Z from 250°C to 700°C during firing, and the presence or absence of bursting. The curve indicates YZ = A / X = 8370 / 2.79. DESCRIPTION OF EMBODIMENTS

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

[0015] In the present specification, “to” indicating a numerical range is used in the meaning including the numerical values described before and after as the lower limit value and the upper limit value, unless otherwise specified.

[0016] In the present specification, “crystalline water content” means the weight ratio of water contained in the ore other than hygroscopic water. In one embodiment, the term “crystalline water content” means a weight ratio of water generated while heating ore from 105°C to 950°C after hygroscopic water is removed by heating the ore to 105°C.

[0017] It is believed that bursting of green pellets occurs as follows. 1. During the firing process, water vapor is generated inside a green pellet by evaporation of moisture or thermal decomposition of crystalline water (mainly 2FeOOH ^ Fe2O3 + H2O). 2. As water vapor is generated, the pressure inside the green pellet increases, and when the stress generated by the internal pressure exceeds the matrix strength of the green pellet, bursting occurs.

[0018] Fig. 1 shows the thermal decomposition behavior of Australian high-crystalline-water ore with a crystalline water content of 4.70 mass% and an a-FeOOH reagent. As shown in Fig. 1, the weight loss of the Australian high-crystalline-water ore at around 300°C is considered to be mainly due to thermal decomposition of FeOOH contained in the ore (2FeOOH ^ Fe2O3 + H2O). That is, the thermal decomposition of the crystalline water described above in 1. occurs in a range of approximately 250°C to 350°C, as shown in Fig. 1. Therefore, it is considered that reducing the heating rate within this temperature range can suppress the rapid generation of water vapor. However, in the actual green pellet firing process, bursting may occur even in the region in which the firing temperature (the surface temperature of the green pellet or the temperature of the atmosphere near the surface) is higher than 350°C. It is considered that the reason for this is that the green pellet is heated from the surroundings by heat transfer from the atmospheric temperature, so that the temperature rise inside of the green pellet is delayed with respect to the rise in the firing temperature. Specifically, it is considered that the temperature inside the green pellet does not reach 350°C at the time when the firing temperature reaches 350°C, and the thermal decomposition of the crystalline water in the green pellet is not completed. As a result, it is not possible to suppress the occurrence of bursting simply by controlling the heating rate of the firing temperature from 250°C to 350°C. To suppress the occurrence of bursting, for example, it is necessary to properly control the heating rate even in the firing temperature range of higher than 350°C. Since the temperature inside the green pellet is usually difficult to measure, it is important to control the firing temperature in consideration of the delay in the temperature rise inside the green pellet.

[0019] On the other hand, lowering of the heating rate is not desirable because it increases the firing time and thus lowers the productivity. As such, the present inventors have focused on controlling the particle size of green pellets as a method for suppressing the occurrence of bursting while maintaining a relatively high heating rate. It is considered that when the particle size of the green pellets is reduced, the distance for water vapor generated inside the green pellets to be released to the outside becomes shorter, so that the pressure generated inside the green pellets decreases, thereby suppressing the occurrence of bursting. Bursting of a green pellet is considered to occur at the location where the distance from the center of the green pellet to the outside is shortest. Therefore, the idea has been obtained that it is important to focus on and control the short diameter as the particle size of the green pellets. As used herein, the “particle size” of a green pellet is the “short diameter” of the green pellet. By focusing on the short diameter, the conditions under which bursting does not occur can be more accurately identified. On the other hand, reducing the particle size increases the specific surface area and promotes heat transfer from the atmospheric temperature, so that the thermal decomposition of the crystalline water easily progresses. Thus, it is necessary to pay attention not only to the control of the particle size, but also to the control of the heating rate.

[0020] The present inventors have considered the factors governing bursting as follows. According to the Kozeny-Carman equation, a pressure AP 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. When this is applied to the bursting phenomenon of a green pellet, it is considered that a pressure AP generated inside the green pellet due to thermal decomposition of crystalline water (generation of water vapor) is proportional to the distance through which the water vapor passes and the superficial velocity of the water vapor. The distance through which the water vapor passes is proportional to the particle size of the green pellet. The superficial velocity of the water vapor is considered to be proportional to the amount of thermal decomposition of crystalline water per unit time (amount of water vapor generated), assuming that the cross-sectional area of the flow path through which the water vapor passes is constant. As a result of thermogravimetric analysis, it has been found that the amount of thermal decomposition of crystalline water per unit time (mass% / min) is approximately proportional to the product (mass%^°C / min) of the crystalline water content of iron ore and the heating rate, as shown in Fig. 2. It is therefore considered that the pressure AP generated inside the green pellet due to thermal decomposition of crystalline water is proportional to the product of the particle size of the green pellet, the crystalline water content of the iron ore, and the heating rate.

[0021] Based on such findings, the present inventors considered that the pressure AP generated inside green pellets due to thermal decomposition of crystalline water can be controlled by the product of the particle size of the green pellets, the crystalline water content of the iron ore, and the heating rate, and conceived the present invention.

[0022] A method for producing iron ore pellets of one embodiment comprises the steps of : (a) grinding iron ore; (b) obtaining a pellet raw material using the ground iron ore; (c) performing granulation or molding of the pellet raw material to obtain green pellets; and (d) firing the green pellets, wherein a product XYZ (mass%^mm^°C / min) of an average crystalline water content X (mass%) of the iron ore constituting the pellet raw material, an average particle size Y (mm) of the green pellets before firing, and a heating rate Z (°C / min) from 250°C to 700°C during firing is controlled to be less than or equal to a predetermined allowable value A. By controlling the product XYZ of the average crystalline water content X (mass%), the average particle size Y (mm), and the heating rate Z (°C / min) to be less than or equal to the allowable value A, the bursting of green pellets in a preheating zone can be suppressed. As a result, a smooth operation of the firing process and an improvement in the yield of pellet products can be expected.

[0023] The method for producing iron ore pellets may include one or more of: (i) changing a blending ratio 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 heating rate from 250°C to 700°C during firing. As a result, the product XYZ can be modified during production.

[0024] <Step (a)> Step (a) is a step of grinding iron ore. The grinding method of iron ore is not particularly limited, and known grinding methods can be used. Preferable examples thereof include a grinding method using a ball mill.

[0025] The size of the iron ore after grinding is not particularly limited, although it is preferable, for example, to grind the iron ore such that the ratio of raw material particles with a diameter of 100 pm or less is 90 mass% or more. In this case, the remainder is iron ore with a diameter greater than 100 pm. The size of the iron ore after grinding can be controlled by, for example, grinding time. Classification may be performed so that the iron ore after grinding has a desired grain size distribution.

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

[0027] (First Iron Ore) The crystalline water content of the first iron ore is 3 mass% or more. The crystalline water content of the first iron ore may be 4 mass% or more, 5 mass% or more, or 6 mass% or more, and may be 10 mass% or less, 9 mass% or less, 8 mass% or less, or 7 mass% or less. The upper limit value and the lower limit value may be arbitrarily combined.

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

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

[0030] (Second Iron Ore) The crystalline water content of the second iron ore is less than 3 mass%. The crystalline water content of the second iron ore may be 0.1 mass% or more, 0.5 mass% or more, or 1.0 mass% or more, and may be 2.9 mass% or less, 2.5 mass% or less, or 2.0 mass% or less. The upper limit value and the lower limit value may be arbitrarily combined.

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

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

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

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

[0035] The size of the other optional components after grinding is not particularly limited, and it is preferable, for example, to grind the particles such that the ratio of particles with a diameter of 100 gm or less is 90 mass% or more. In this case, the remainder is particles with a diameter greater than 100 gm. The size of other optional components after grinding can be controlled by, for example, the grinding time. Classification may be performed so that other optional components after grinding have a desired grain size distribution.

[0036] The amount of 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 mass% or more, 20 mass% or more, or 30 mass% or more, and may be 100 mass% or less, 90 mass% or less, or 80 mass% or less. The upper limit value and the lower limit value may be arbitrarily combined.

[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 controlled according to the average crystalline water content X described below. The mass of the second iron ore with respect to the total mass of the first iron ore and the second iron ore may be, for example, 0 mass% or more, 5 mass% or more, or 10 mass% or more, and may be 50 mass% or less, 40 mass% or less, or 30 mass% or less. The upper limit value and the lower limit value may be arbitrarily combined.

[0038] (Other Optional Components) Specific examples of other optional components include carbonaceous material, limestone, dolomite, magnesite, olivine, and bentonite. Examples of the carbonaceous material include coke, anthracite, charcoal, and the like.

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

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

[0041] (Average Crystalline Water Content X) The average crystalline water content X of the iron ore constituting the pellet raw material (also simply referred to as the average crystalline water content X) may be 1 mass% or more, 2 mass% or more, 3 mass% or more, 4 mass% or more, 5 mass% or more, or 6 mass% or more, and may be 10 mass% or less, 9 mass% or less, 8 mass% or less, or 7 mass% or less. The iron ore pellet production method in which the product XYZ is controlled has a particularly high effect of suppressing the occurrence of bursting when the average crystalline water content X is high. In this respect, the average crystalline water content X is preferably 2.79 mass% or more, and more preferably more than 3 mass%. From the viewpoint of reducing the occurrence of cracks in the pellets after firing, it is preferable that the upper limit of the average crystalline water content X be 7 mass% or less. The upper limit value and the lower limit value may be arbitrarily combined.

[0042] The average crystalline water content X can be determined by measuring the crystalline water content of iron ore contained in the pellet raw material. Specifically, when the pellet raw material contains one type of iron ore, the crystalline water content of the iron ore is determined as the average crystalline water content X. When the pellet raw material includes two or more types of iron ore, a weighted average value of the respective crystalline water contents, or a crystalline water content measured for a mixture of the iron ores constituting the pellet raw material, is determined as an average crystalline water content X. The crystalline water content of iron ore can be measured, for example, by JIS M8211: 1995 “Iron ores - Method for determination of combined water content.”

[0043] The average crystalline water content X can be controlled, for example, by selecting iron ore to be used as the raw material, and by selecting the blending ratio when two or more types of iron ore are used. Specifically, when an iron ore having a relatively high crystalline water content is used as the iron ore used as the raw material, the average crystalline water content X is relatively high, and when an iron ore having a relatively low crystalline water content is used as the iron ore used as the raw material, the average crystalline water content X is relatively low. When two or more types of iron ore are used, the average crystalline water content X is increased by increasing the blending amount of the iron ore having a relatively high crystalline water content, and the average crystalline water content X is reduced by increasing the blending amount of the iron ore having a relatively low crystalline water content.

[0044] <Step (c)> Step (c) is the step of performing granulation or molding of the pellet raw material obtained in the above step (b) to obtain green pellets. The pellet raw material is preferably subjected to humidity conditioning before it is granulated or molded. After granulation or molding, the green pellets may be dried.

[0045] The granulation of the pellet raw material is not particularly limited, and may be performed using, for example, a rolling granulator such as a pan pelletizer or a drum pelletizer. The molding of the pellet raw material is not particularly limited, and can be performed, for example, using a molding machine such as a compression molding machine, an extrusion molding machine, and the like. The molding of the pellet raw material is preferably performed using a compression molding machine or an extrusion molding machine.

[0046] The shape of the green pellet is not particularly limited, and specific examples thereof include a spherical shape, a cylindrical shape, a briquette shape (e.g., a pillow shape and an almond shape), and the like.

[0047] When a green pellet has a spherical shape, its diameter is usually 10 mm to 20 mm. When it is 10 mm or more, the flow path of the reducing gas in the reducing process in the later furnace such as a blast furnace or a direct reduction shaft furnace, for example, is ensured, thereby enhancing productivity. When it is 20 mm or less, it is easy to reduce to the inside of the green pellet. For a similar reason, when the green pellet has a cylindrical shape, its bottom surface diameter and height are usually 10 mm to 20 mm, and when the green pellet has a briquette shape, the long diameter is usually 10 mm to 40 mm, and the short diameter is usually 10 mm to 20 mm. The “long diameter” of a green pellet refers to the maximum diameter of the green pellet. The “short diameter” will be described below.

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

[0049] When the green pellet has a spherical shape, the “short diameter” of the green pellet is the diameter of the sphere. When the green pellet has a cylindrical shape, the “short diameter” of the green pellet is the smaller of the diameter of the circle and the height of the cylinder. When the green pellet has a shape other than a spherical shape or a cylindrical shape, the “short diameter” of the green pellet is the shortest distance between two parallel planes that sandwich the green pellet. The “average particle size Y” of green pellets is a weight-based average value based on the short diameter of the green pellets, which is determined according to their shapes.

[0050] The method for determining the average particle size Y is not particularly limited. For example, a weight-based average value of values obtained by measuring a certain number or more of green pellets with calipers or the like 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 can be 10 mm or more, or 12 mm or more, and can be 20 mm or less, or 18 mm or less. The upper limit value and the lower limit value may be arbitrarily combined. It is preferable that the grain size distribution of the green pellets be narrower. Specifically, the maximum value of short diameter of the green pellets is preferably less than or equal to the average particle size Y + 1.0 mm, and more preferably less than or equal to the average particle size Y + 0.5mm. The minimum value of the short diameter of the green pellets is preferably equal to or greater than the average particle size Y - 1.0 mm, and more preferably equal to or greater than the average particle size Y - 0.5mm.

[0052] It is preferable that the ratio of the volume-equivalent spherical diameter to the short diameter of the green pellets (volume-equivalent spherical diameter / short diameter) be 1.10 to 2.00. The volume-equivalent spherical diameter of a green pellet is calculated by the following expression. [Math. 1] 1 / 6V\3 d - — \ n / In the expression, d is the volume-equivalent spherical diameter of the green pellet, and V is the volume of the green pellet. When the shape of the green pellet is a sphere, the volumeequivalent spherical diameter and the short diameter of the green pellet are equal. When the shape of the green pellet is other than a sphere, the volume-equivalent spherical diameter of the green pellet is larger than the short diameter thereof. From the viewpoint of suppressing bursting, the larger the ratio of the volume-equivalent spherical diameter to the short diameter of the green pellet (volume-equivalent spherical diameter / short diameter), the more preferable it is. On the other hand, from the viewpoint of securing the strength of the green pellet, the ratio of the volume-equivalent spherical diameter to the short diameter of the green pellet (volumeequivalent spherical diameter / short diameter) is preferably 2.00 or less. From the viewpoint of suppressing bursting, the lower limit value of the ratio of the volume-equivalent spherical diameter to the short diameter of the green pellet is preferably 1.11 or more, more preferably 1.12 or more, and still more preferably 1.13 or more. From the viewpoint of securing the strength of the green pellet, the upper limit value of the ratio of the volume-equivalent spherical diameter to the short diameter of the green pellet is preferably 1.80 or less, more preferably 1.60 or less, and still more preferably 1.40 or less.

[0053] The carbon content of the green pellets is preferably 1 mass% or less, and more preferably 0.5 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 mass% or more, or 0.2 mass% or more.

[0054] <Step (d)> Step (d) is the step of firing the green pellets obtained in the above step (c). The method of firing the green pellets is not particularly limited, and may 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 determined, for example, by measuring the temperature with a thermocouple fixed within the green pellet layer or near the surface of the green pellets (for example, within 10 mm from the surface of the green pellets) so that the relative position between the green pellets and the thermocouple is constant, and measuring the time required for the temperature to increase from 250°C to 700°C.

[0056] The above-mentioned thermocouple is difficult to be installed in the traveling grate type or grate kiln type firing furnace. In that case, the heating rate Z may be determined from the measurement results of the atmospheric temperature with a plurality of stationary thermocouples installed directly above the pellet layer in the furnace or directly below the pallet truck. Specifically, the heating rate Z is determined by determining two positions 1 and 2 within a heating process from 250°C to 700°C, and calculating the heating rate Z according to the following expression based on the temperatures at the two positions and the elapsed time between the two positions. Z = (T2 - T1) x (V / D) wherein, T1 is the temperature measured at position 1, T2 is the temperature measured at position 2, D is a distance in a traveling direction of a pallet truck between position 1 and position 2, and V is a traveling speed of the pallet truck.

[0057] The heating rate Z can be controlled, for example, as follows. When a traveling grate type firing furnace or a grate kiln type firing furnace is used, examples of a method for controlling the heating rate Z include controlling a firing amount (supplied heat amount) of burners in the heating process from 250°C to 700°C, controlling the traveling speed of the pallet truck, and blending a material that generates oxidation heat, such as magnetite ore, into the raw material. Specifically, the heating rate Z is increased by increasing the supplied heat amount, and the heating rate Z is reduced by reducing the traveling speed of the pallet truck. When a pot type firing furnace is used, examples of a method for controlling the heating rate Z include controlling a traveling speed of a pot that contains green pellets, and blending a material that generates oxidation heat, such as magnetite ore, into the raw material.

[0058] The heating rate Z may be 50°C / min or more, more 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. The upper limit value and the lower limit value may be arbitrarily combined. From the viewpoint of productivity, the heating rate Z is preferably as high as possible under conditions where bursting does not occur. According to the method of the present disclosure, since the heating rate Z can be determined in accordance with the average crystalline water content X and the average particle size Y, it is possible to suppress bursting while maintaining production efficiency. In addition, since the process from 250°C to 700°C can be operated under consistent heating control, the method is excellent from the viewpoint of manufacturing efficiency. It is preferable that fluctuations in the heating rate Z from 250°C to 700°C during firing be smaller. Specifically, the difference between the heating rate from 250°C to 280°C during firing and the heating rate from more than 280°C to 700°C during firing is preferably 40°C / min or less, and more preferably 20°C / min or less. The lower limit value of the difference between the heating rate from 250°C to 280°C during firing and the heating rate from more than 280°C to 700°C during firing does not need to be set, and 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 more than 350°C to 700°C during firing is preferably 40°C / min or less, and more preferably 20°C / min or less. The lower limit value of the difference between the heating rate from 250°C to 350°C during firing and the heating rate from more than 350°C to 700°C during firing does not need to be set, and is 0°C / min or more.

[0059] <Method for Determining Allowable Value A> The allowable value A is a threshold determined based on the presence or absence of the occurrence of bursting. A method for determining the allowable value A is not particularly limited. The allowable value A may be determined, for example, as follows. That is, before performing the above steps (a) to (d), (e) with respect to the average crystalline water content X (mass%) of the iron ore constituting the pellet raw material, a plurality of production conditions under which bursting of iron ore pellets does not occur are identified; (f) for each of the plurality of production conditions identified in (e), a product XY1Z1 (mass%^mm^°C / min) of an average crystalline water content X (mass%) of the iron ore constituting the pellet raw material, an average particle size Y1 (mm) of green pellets before firing, and a heating rate Z1 (°C / min) from 250°C to 700°C during firing is identified; (g) a maximum value among the plurality of products XY1Z1 identified in (f) is identified; and (h) the maximum value identified in (g) is determined as the allowable value A.

[0060] Specifically, for a given average crystalline water content X, a finite number of trials are performed in which one or more of the average particle size Y1 (also simply referred to as average particle size Y1) (mm) of green pellets before firing and the heating rate Z1 from 250°C to 700°C during firing (also simply referred to as heating rate Z1) (°C / min) are varied, and the presence or absence of bursting is checked. The maximum value among products XY1Z1 obtained in trials in which bursting did not occur can be determined as the allowable value A for the average crystalline water content X.

[0061] The presence or absence of bursting can be evaluated by visual observation of the sample after firing, and when the sample is broken and divided into two or more particles, it can be determined that bursting has occurred. The sample after firing may be sieved with a predetermined sieve size, and the presence or absence of bursting may be evaluated from the amount of generated powder after firing. As one example, the sample after firing may be sieved using a sieve having a sieve size of 6.3 mm, and when a ratio of samples passing through the sieve is 5 mass% or more, the sample may be determined to be unacceptable (occurrence of bursting). The sieve size and the ratio of samples passing through the sieve used as a determination criterion can be appropriately set according to the size of green pellets used and the allowable occurrence rate of bursting.

[0062] As shown in Fig. 4, it was found that the relationship between the average crystalline water content X and the maximum value among the products XY1Z1 in trials in which bursting did not occur can be approximated by the maximum value of XY1Z1 = 417X + 7190. Accordingly, the allowable value A may be determined by the following expression (1). Expression (1): Allowable value A = 417X + 7190

[0063] The crystalline water content of iron ore is generally constant for each type. For example, the crystalline water content of Carajas ore is 2 to 3 mass%, the crystalline water content of Pilbara blend ore is 4 to 5 mass%, the crystalline water content of MAC ore is 6 to 7 mass%, and the crystalline water content of Robe River ore is 8 to 9 mass%. When the average crystalline water content X is 2 to 3 mass%, the allowable value A may be 8370 mm^mass%^°C / min, when the average crystalline water content X is 3 to 4 mass%, the allowable value A may be 8733 mm^mass%^°C / min, when the average crystalline water content X is 4 to 5 mass%, the allowable value A may be 9200 mm^mass%^°C / min, when the average crystalline water content X is 6 to 7 mass%, the allowable value A may be 9660 mm^mass%^°C / min, and when the average crystalline 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 green pellets used, and may be, for example, 8000 to 11000 mm^mass%^°C / min. EXAMPLES

[0064] The present invention will be described below in detail on the basis of examples and comparative examples, but the present invention is not limited to these examples.

[0065] Table 1 shows the presence or absence of bursting when pellets were fired under various heating rates Z and average particle sizes Y, using a pellet raw material composed of MAC ore with a crystalline water content of 6.44 mass%, as iron ore, and bentonite.

[0066] (Test Method) The crystalline water content of iron ore is measured in accordance with JIS M8211: 1995 “Iron ores - Method for determination of combined water content.”

[0067] (Preparation of Green Pellets) The iron ore was ground by a ball mill to 100 gm or less. Then, 0.5 mass% bentonite as a binder was added to the ground iron ore to obtain pellet raw materials. Furthermore, after adding an additional 8 mass% water to the blended raw material, compression molding was performed using dies with different inner diameters to prepare (cylindrical) green pellets having different particle sizes. The height of the cylindrical green pellet was controlled to be the same as the diameter thereof. That is, the ratio of the volume-equivalent spherical diameter to the short diameter of the green pellet (volume-equivalent spherical diameter / short diameter) was 1.14. The inner diameter of the die corresponding to the diameter of the resulting green pellet was determined as the average particle size Y. Since the green pellets were prepared using dies of a predetermined size, there was almost no variation in particle size, and the maximum and minimum values of the short diameters of the green pellets corresponded to Y (within the range of average particle size Y ± 0.5 mm).

[0068] (Firing of Green Pellets) The green pellets thus prepared were fired using a test firing furnace whose heating rate can be controlled. Fig. 3 shows a schematic view of the test firing furnace used in the example. The test firing furnace is provided with a sample cage 13 installed in an alumina tube 11 having an inner diameter of 42 mm^, and the sample cage 13 can contain green pellets 14. During firing, the alumina tube 11 is heated to 1360°C by a heater 12. The temperature of the green pellets 14 contained in the sample cage 13 can be measured by a thermocouple 15 installed directly above the green pellets 14. The temperature of the green pellets during firing can 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 set to 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 more than 280°C to 700°C, as well as the difference between the heating rate from 250°C to 350°C and the heating rate from more than 350°C to 700°C was 10°C / min or less in both cases. The heating rate from 700°C to 1300°C was 200°C / min, and after holding at 1300°C for 10minutes, pellets were obtained by air cooling.

[0069] (Method for Evaluating Presence or Absence of Bursting) The presence or absence of bursting was determined by visual observation of the pellet after firing. It was determined that bursting was absent when the pellet after firing had maintained its shape without cracking, and it was determined that bursting was present when the pellet was broken and divided into two or more particles.

[0070] [Table 1] Table 1 Test No. Average crystalline water content X (mass%) Average particle size Y (mm) Heating rate Z (°C / m i n) Product XYZ (mm ■ mass% ■ °C / min) Presence or absence of bursting 1 6.44 10.0 100 6440 Absent 2 6. 44 12.5 100 8050 Absent 3 6. 44 15.0 100 9660 Absent 4 6. 44 10. 0 125 8050 Absent 5 6.44 10. 0 150 9660 Absent 6 6.44 12.5 150 12075 Present 7 6. 44 15. 0 150 14490 Present 8 6. 44 10. 0 200 12880 Present 9 6.44 15.0 200 19320 Present

[0071] Among conditions under which bursting did not occur, the conditions under which the 5 product XYZ was maximum were test Nos. 3 and 5. The product XYZ under these conditions can be determined as the allowable value A. That is, by controlling the product XYZ of the average crystalline water content X, the average particle size Y, and the heating rate Z to be 9660 (mm'mass%°C / min) or less, it is possible to suppress the occurrence of bursting during firing.

[0072] 10         The presence or absence of bursting was evaluated in the same manner as in the case of MAC ore except that Pilbara blend ore (crystalline water content: 4.60 mass%), Robe River ore (crystalline water content: 8.75 mass%), or Carajas ore (crystalline water content: 2.79 mass%) was used as the iron ore. The results are shown in Tables 2 to 4.

[0073] 15 [Table 2] Table 2 Test No. Average crystalline water content X (mass%) Average particle size Y (mm) Heating rate Z (°C / min) Product XYZ (mm ■ mass% • °C / min) Presence or absence of bursting 1 4. 60 15. 0 100 6900 Absent 2 4.60 10.0 150 6900 Absent 3 4. 60 15. 0 200 1 3800 Present 4 4. 60 12. 5 225 12937. 5 Present 5 4.60 10.0 200 9200 Absent

[0074] [Table 3] Table 3 Test No. Average crystalline water content X (mass%) Average particle size Y (mm) Heating rate Z (°C / min) Product XYZ (mm • mass% - °C / min) Presence or absence of bursting 1 8. 75 15. 0 50 6562. 5 Absent 2 8.75 10. 0 100 8750 Absent 3 8. 75 15. 0 150 19687. 5 Present 4 8. 75 10. 0 200 17500 Present 5 8. 75 12.5 100 10938 Absent

[0075] [Table 4] Table 4 Test No. Average crystalline water content X (mass%) Average particle size Y (mm) Heating rate Z (°C / mi n) Product XYZ (mm ■ mass% • °C / min) Presence or absence of bursting 1 2. 79 12. 5 200 6975 Absent 2 2. 79 10. 0 250 6975 Absent 3 2. 79 15. 0 250 10462.5 Present 4 2. 79 15.0 200 8370 Absent

[0076] For each pellet raw material, the maximum value of the products XYZ was determined as the allowable value A among the conditions under which bursting did not occur. The results are shown in Table 5 together with the results for MAC ore. 10   

[0077] [Table 5] Table 5 Iron ore Average crystalline water content X (mass%) Allowable value A (mm ■ mass% • °C / mi n) Carajas ore 2. 79 8370 Pilbara blend ore 4. 60 9200 MAC ore 6. 44 9660 Robe river ore 8. 75 10938

[0078] Fig. 4 shows a graph plotting the relationship between the average crystalline water 15 content X and the maximum value of the product XYZ (i.e., XY1Z1) among conditions under which bursting did not occur, which was determined as the allowable value A. From Fig. 4, it was found that the relationship between the average crystalline water content X and the maximum value of product XY1Z1 can be approximated by the following expression. Maximum value of product XY1Z1 = 417X + 7190

[0079] It was observed that the larger the average crystalline water content X, the larger the maximum value of product XY1Z1 tends to be. This is considered to be attributable to factors such as an increase in the amount of pores inherent in the ore as the crystalline water content of the iron ore increases, and a decrease in the actual heating rate inside the green pellet due to the larger endothermic effect as the crystalline water content of the iron ore increases. Taking this effect into account, it is possible to control the bursting more precisely by changing the allowable value A according to the average crystalline water content X. As a result, the productivity can be further enhanced.

[0080] Figs. 5 to 8 show the curve of YZ = A / X for each of MAC ore (average crystalline water content X: 6.44 mass%, allowable value A: 9660 mm^mass%^°C / min), Pilbara blend ore (average crystalline water content X: 4.60 mass%, allowable value A: 9200 mm^mass%^°C / min), Robe River ore (average crystalline water content X: 8.75 mass%, allowable value A: 10938 mm^mass%^°C / min), and Carajas ore (average crystalline water content X: 2.79 mass%, allowable value A: 8370 mm^mass%^°C / min). Figs. 5 to 8 also show the results of Tables 1 to 4. From Figs. 5 to 8, it was confirmed that controlling the product XYZ to be less than or equal to the allowable value A, that is, controlling to be YZ < A / X, can suppress bursting of pellets.

[0081] Pilbara blend ore (crystalline water content: 4.60 mass%) was used as the first iron ore, and Carajas ore (crystalline water content: 2.79 mass%) was used as the second iron ore. These were mixed at a mass ratio of 1:1 to obtain iron ore with an average crystalline water content X of 3.70 mass%. Then, the allowable value A was determined to 8733 mass%^mm^°C / min according to expression (1). Green pellets were then prepared, fired, and evaluated for the presence or absence of bursting in the same manner as described above. 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, under the conditions in which the product XYZ was controlled to be less than or equal to the allowable value A, it was possible to suppress the occurrence of bursting.

[0082] [Table 6] Table 6 Test No. Average crystalline water content X (mass%) Average particle size Y (mm) Heating rate Z CC / min) Product XYZ (mm ■ mass% ■ °C / min) Presence or absence of bursting 1 3. 70 10. 0 200 7400 Absent 2 3. 70 15. 0 150 8325 Absent 3 3. 70 12.5 250 11563 Present 4 3. 70 15. 0 200 11100 Present REFERENCE SINGS LIST 5    

[0083] 11   Alumina tube 12   Heater 13   Sample cage 14   Green pellet 10   15 Thermocouple

Claims

1. A method for producing iron ore pellets, the method comprising the steps of:(a) grinding iron ore;(b) obtaining a pellet raw material using the ground iron ore;(c) performing granulation or molding of the pellet raw material to obtain green pellets; and(d) firing the green pellets, whereina product XYZ (mass%^mm^°C / mm) of an average crystalline water content X (mass%) of the iron ore constituting the pellet raw material, an average particle size Y (mm) of the green pellets before firing, and a heating rate Z (°C / min) from 250°C to 700°C during firing is controlled to be less than or equal to a predetermined allowable value A, the average particle size Y (mm) is a weight-based average value based on the short diameter of the green pellets, which is determined according to their shapes.

2. The method for producing iron ore pellets according to claim 1, wherein the ratio of the volume-equivalent spherical diameter to the short diameter of the green pellets (volumeequivalent spherical diameter / short diameter) is 1.10 to 2.00.

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

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

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

6. The method for producing iron ore pellets according to claim 1, wherein, in the step (c), the molding of the pellet raw material is performed using a compression molding machine or an extrusion molding machine.2024427813  03 Jun 2026

7. The method for producing iron ore pellets according to claim 1, whereinbefore performing the steps (a) to (d),(e) with respect to the average crystalline water content X (mass%) of the iron ore constituting the pellet raw material, a plurality of production conditions under which bursting of iron ore pellets does not occur are identified;(f) for each of the plurality of production conditions identified in (e), a product XY1Z1 (mass%^mm^°C / min) of an average crystalline water content X (mass%) of the iron ore constituting the pellet raw material, an average particle size Y1 (mm) of green pellets before firing, and a heating rate Z1 (°C / min) from 250°C to 700°C during firing is identified;(g) a maximum value among the plurality of products XY1Z1 identified in (f) is identified; and(h) the maximum value identified in (g) is determined as the allowable value A.

8. The method for producing iron ore pellets according to claim 1, wherein the allowable value A is determined byExpression (1): A = 417X + 7190.

9. The method for producing iron ore pellets according to any one of claims 1 to 8, comprising one or more of:(i) changing a blending ratio 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 heating rate from 250°C to 700°C during firing.Nippon Steel CorporationPatent Attorneys for the Applicant / Nominated PersonSPRUSON & FERGUSON

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