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

Iron ore pellet manufacturing method

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

[0002] Iron ore pellets (hereinafter simply referred to as pellets) are fired products obtained 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 iron ore powder, the particle size and moisture of which have been adjusted, is granulated into spheres with a diameter of about 10 mm or so using a tumbling granulator or the like to produce green pellets. In the firing process, the green 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 is thermally decomposed 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 a 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 of stable quality by adjusting the temperature of a preheating chamber and / or the preheating time in accordance with 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 rate of temperature increase from room temperature to 280°C to 70°C / min or less and the rate of temperature increase 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%.

[0009] JP 2000-87150 A JP 2010-24477 A JP 2023-33734 A

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

[0011] The present disclosure includes the following aspects: <Aspect 1> A method for producing iron ore pellets, comprising: (a) a step of crushing iron ore, (b) a step of obtaining a pellet raw material using the crushed iron ore, (c) a step of granulating or molding the pellet raw material to obtain green pellets, and (d) a step of firing the green pellets, wherein a product XYZ (mass% mm °C / min) of an average crystal 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 adjusted to be equal to or less than a predetermined allowable value A. <Aspect 2> Before carrying out the steps (a) to (d), (e) a plurality of production conditions are identified under which no bursting of iron ore pellets occurs for the average crystal water content X (mass%) of the iron ore constituting the pellet raw material, and (f) for each of the plurality of production conditions identified 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, and (g) the multiple products XY specified in (f) are specified. １ Z １(h) specifying the maximum value among (a) and (b) defining the maximum value specified in (g) as the allowable value A. <Aspect 3> The method for producing iron ore pellets according to Aspect 1 above, wherein the allowable value A is defined by equation (1): A = 417X + 7190. <Aspect 4> The method for producing iron ore pellets according to any one of Aspects 1 to 3 above, including one or more of: (i) changing the blend of iron ores 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.

[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.

[0013] 1 is a graph showing the thermal decomposition behavior of an Australian ore with high water content and an α-FeOOH reagent by thermogravimetric analysis. 2 is a graph showing the relationship between the product of the water of crystallization content of iron ore and the heating rate (mass% ° C / min) and the amount of water of crystallization thermally decomposed per unit time (mass% / min). 3 is a schematic diagram of a test firing furnace used in the examples. 4 is a graph showing the relationship between the average water of crystallization content X (mass%) and the product XY specified for each average water of crystallization content X (mass%) in the examples. １ Z １1 is a graph plotting the maximum value of . 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 results of confirming whether or not explosion occurred. The curve shows the average particle size Y of Pilbara Blend ore (average crystal water content X: 4.60% by mass), which shows YZ = A / X = 9660 / 6.44, the heating rate Z from 250°C to 700°C during firing, and the results of confirming whether or not explosion occurred. The curve shows YZ = A / X = 9200 / 4.60. 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 from 250°C to 700°C during firing, and the results of confirming whether or not explosion occurred. The curve shows YZ = A / X = 10938 / 8.75. The graph shows the results of checking 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 whether or not explosion occurred. The curve shows YZ=A / X=8370 / 2.79.

[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 term "to" indicating a range of values ​​means that the values ​​before and after it are included as the lower and upper limits.

[0016] In this specification, the "crystallization water content" refers to the weight ratio of water contained in the ore other than absorbed water. In one embodiment, the "crystallization 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 green pellets is thought to occur as follows: 1. Evaporation of water or thermal decomposition of water of crystallization during the firing process (mainly 2FeOOH → Fe ２ O ３ +H ２2. As steam is generated, pressure rises inside the green pellets, and bursting occurs when the stress caused by the internal pressure exceeds the matrix strength of the green 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 mass% and the α-FeOOH reagent. Figure 1 shows that the weight loss of the Australian high water of crystallization ore at around 300°C is mainly due to the thermal decomposition of FeOOH contained in the ore (2FeOOH → Fe ２ O ３ +H ２ This is thought to be due to the thermal decomposition of the water of crystallization (1) above. As shown in FIG. 1, the thermal decomposition of the water of crystallization occurs in the range of approximately 250 to 350°C. Therefore, it is believed that rapid water vapor generation can be suppressed by suppressing the rate of temperature rise in this temperature range. However, in an actual green pellet firing process, bursting can occur even in a region where the firing temperature (surface temperature of the green pellet 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 lags behind the rise in the firing temperature. Specifically, it is thought that when the firing temperature reaches 350°C, the temperature inside the green pellets has not yet reached 350°C, and the thermal decomposition of the water of crystallization inside the green pellets has not yet been completed. Therefore, the occurrence of bursting cannot be suppressed simply by controlling the rate of temperature rise of the firing temperature from 250°C to 350°C. To suppress the occurrence of bursting, it is necessary to appropriately control the rate of temperature rise even in a region where the firing temperature exceeds 350°C, for example. Usually, it is often difficult to measure the temperature inside the green pellets, so it is important to control the firing temperature while taking into consideration 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 flow 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 considered to be proportional to the distance the water vapor passes through and the superficial flow velocity of the water vapor. The distance the water vapor passes through 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 flow 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 (mass% ° C / min) of the water of crystallization content of iron ore and the heating rate, 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 knowledge, 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 the method for producing iron ore pellets includes: (a) crushing iron ore; (b) obtaining pellet raw material using the crushed iron ore; (c) granulating or molding the pellet raw material to obtain raw pellets; and (d) firing the raw pellets. The product XYZ (mass% mm °C / min) of the average water of crystallization 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. 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 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 products X, Y, and Z to be changed during production.

[0024] <Step (a)> Step (a) is a step of pulverizing iron ore. The method for pulverizing iron ore is not particularly limited, and any known pulverization 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] (First iron ore) 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] <Step (b)> 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, as necessary, a step of pulverizing other optional components prior to 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 Components) 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 kinds 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 that adjusts the product XYZ 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 arbitrarily.

[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] <Step (c)> 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 typically 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 typically 10 mm to 20 mm, and when the green pellets are briquette-shaped, their major diameter is typically 10 mm to 40 mm, and their minor diameter is typically 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] When the green pellets are spherical, the "minor diameter" of the green pellets is the diameter of the sphere. When the green pellets are cylindrical, the "minor diameter" of the green pellets is the smaller of the diameter of the circle and the height of the cylinder. When the green pellets are in a shape other than spherical or cylindrical, the "minor diameter" of the green pellets is the shortest distance between two parallel planes when the green pellets are sandwiched between them. The "average particle size Y" of the green pellets is a weight-based average value based on the minor diameter determined according to the shape of the green 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 may be measured with a vernier caliper or the like, and the weight-based average value of the values ​​obtained may 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 width of the particle size distribution of the green pellets, the better. Specifically, the maximum value of the 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 value of the 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 (diameter / minor axis) is preferably 1.10 to 2.00. The diameter of the green pellets is calculated from the following formula: In the formula, d is the equivalent sphere diameter of the green pellet, and V is the volume of the green pellet. When the green pellet has a spherical shape, the equivalent sphere diameter and the minor axis of the green pellet are equal. When the green pellet has a shape other than a sphere, the equivalent sphere diameter of the green pellet is larger than the minor axis. From the viewpoint of suppressing bursting, the larger the ratio of the equivalent sphere diameter to the minor axis of the green pellet (equivalent sphere diameter / minor axis), the better. On the other hand, from the viewpoint of ensuring the strength of the green pellet, the ratio of the equivalent sphere diameter to the minor axis of the green pellet (equivalent sphere diameter / minor axis) is preferably 2.00 or less. From the viewpoint of suppressing bursting, the lower limit of the ratio of the equivalent sphere diameter to the minor axis of the green pellet is preferably 1.11 or more, more preferably 1.12 or more, and even more preferably 1.13 or more. From the viewpoint of ensuring the strength of the green pellet, the upper limit of the ratio of the equivalent sphere diameter to the minor axis of the green pellet is preferably 1.80 or less, more preferably 1.60 or less, and even more preferably 1.40 or less.

[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] <Step (d)> 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 the firing 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 temperature rise rate Z (°C / min) from 250°C to 700°C during firing (also simply referred to as the temperature rise rate Z) can be calculated, for example, by measuring the temperature using a thermocouple fixed in 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 a traveling grate or grate kiln type firing furnace, it is difficult to install thermocouples as described above. In such cases, the temperature rise rate Z may be calculated from the results of measuring the ambient temperature using 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 using the temperature at those positions and the time elapsed between those two points using the following formula: Z=(T ２ -T １ ) × (V / D) where T １ is the temperature measured at position 1, T ２ 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-type calcination furnace is used, methods for adjusting the temperature rise rate Z include, for example, adjusting the burner firing amount (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 calcination furnace 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 in which bursting does not occur. 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 excellent 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. Specifically, the difference between the temperature rise rate from 250°C to 280°C during firing and the temperature rise 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 temperature rise rate from 250°C to 280°C during firing and the temperature rise rate from above 280°C to 700°C during firing, but it is 0°C / min or more. There is no need to set a lower limit for the difference between the temperature rise rate from 250°C to 350°C during firing and the temperature rise rate from above 350°C to 700°C during firing, but it is 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 temperature rise rate from 250°C to 350°C during firing and the temperature rise rate from above 350°C to 700°C during firing, but it is 0°C / min or more.

[0059] <Method of determining tolerance value A> The tolerance value A is a threshold value determined based on the occurrence or non-occurrence of bursting. The method of determining the tolerance value A is not particularly limited. For example, the tolerance value A may be determined as follows. That is, before performing the above steps (a) to (d), (e) identify a plurality of production conditions under which bursting of iron ore pellets does not occur for the average crystal water content X (mass%) of the iron ore constituting the pellet raw material, and (f) for each of the plurality of production conditions identified in (e), determine 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. 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 (g) specifying the XY (mass % mm °C / min) specified in (f) above; １ Z １ (h) the maximum value identified in (g) above can be set as the allowable value A.

[0060] Specifically, for a certain average crystal water content X, the average particle size Y of the raw pellets before firing is １ (simply average particle size Y １ ) (mm), and the temperature rise rate Z from 250°C to 700°C during firing １ (Simply heating rate Z １ ) (℃ / min), and a finite number of trials are conducted to check whether or not an explosion occurs. １ Z １ The maximum value among these can be determined as the allowable value A for the average crystal water content X.

[0061] The presence or absence of explosion can be evaluated by visual observation of the sample after firing, and if the sample breaks and separates into two or more particles, it can be determined that bursting has occurred. The fired sample may be sieved using a predetermined sieve size, and the presence or absence of bursting may be evaluated based on the amount of powder generated after firing. As an example, the fired sample may be sieved using a sieve with a sieve size of 6.3 mm, and if the proportion of samples that fall under the sieve is 5% by mass or more, it may be determined to be unacceptable (bursting has occurred). The sieve size and the proportion of samples that fall under the sieve, which serve as the evaluation criteria, can be appropriately set depending on the size of the raw pellets used and the acceptable rate of bursting occurrence.

[0062] As shown in FIG. 4, the average crystal water content X and the product XY in trials in which no explosion occurred １ Z １ The relationship between the maximum value of the product XY １ Z １ It has been found that the maximum value of the tolerance A can be approximated by the following formula (1):

[0063] The crystallization water content of iron ore is generally constant for each type. For example, Carajas ore has a crystallization water content of 2 to 3 mass %, Pilbara blend ore has a crystallization water content of 4 to 5 mass %, MAC ore has a crystallization water content of 6 to 7 mass %, and Robe River ore has a crystallization water content of 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.

[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 the occurrence of bursting when pellets were fired under conditions of multiple heating rates Z and average particle diameters Y using pellet raw materials composed of MAC ore as iron ore with a crystal water content of 6.44 mass% 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] (Preparation of Green Pellets) Iron ore was pulverized to 100 μm or less using a ball mill. Then, 0.5% by mass of bentonite was added to the pulverized iron ore as a binder to obtain a pellet raw material. Furthermore, 8% by mass of water was added to the blended raw material, and compression molding was performed using dies with different inner diameters to produce green pellets (cylindrical) 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 equivalent diameter to the minor axis of the green pellets (equivalent 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. Since the green pellets were prepared using a die of a predetermined size, there was little variation in particle size, and the maximum and minimum minor axis values ​​of the green pellets were consistent with Y (within ±0.5 mm of the average particle size Y).

[0068] (Firing of Green Pellets) The prepared green pellets were fired using a test firing furnace capable of controlling the heating rate. FIG. 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 evaluating the presence or absence of bursting) The presence or absence of bursting was judged by visual observation of the pellets after firing. When the pellets after firing maintained their shape without cracking, it was judged that no bursting occurred, and when the pellets cracked and separated into two or more particles, it was judged that bursting occurred.

[0070]

[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, 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, the occurrence of bursting during firing can be suppressed.

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

[0073]

[0074]

[0075]

[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 the MAC ore.

[0077]

[0078] The average crystal water content X and the product XYZ (i.e., product XY) of the conditions under which bursting did not occur, determined as the allowable value A, are １ Z １ 4 shows a graph plotting the relationship between the average crystal water content X and the product XY １ Z １ It has been found that the relationship between the maximum value of the product X and Y can be approximated by the following formula: １ Z １ Maximum value of = 417X + 7190

[0079] The larger the average crystal water content X, the greater the product XY １ Z １ The maximum value of also tended to increase. 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 in the ore, and the heat absorption becomes larger, 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 the explosion. As a result, it is possible to further increase productivity.

[0080] 5 to 8 show the curves YZ = A / X for MAC ore (average crystal water content X: 6.44 mass%, tolerance A: 9660 mm mass% ° C / min), Pilbara blend ore (average crystal water content X: 4.60 mass%, tolerance A: 9200 mm mass% ° C / min), Robe River ore (average crystal water content X: 8.75 mass%, tolerance A: 10938 mm mass% ° C / min), and Carajas ore (average crystal water content X: 2.79 mass%, tolerance A: 8370 mm mass% ° C / min). Figures 5 to 8 also show the results of Tables 1 to 4. From Figures 5 to 8, it was confirmed that pellet bursting can be suppressed by adjusting the product XYZ to be equal to or less than the tolerance A, that is, by adjusting so that 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 formula (1). Green pellets were then prepared and fired in the same manner as above, and the presence or absence of bursting was evaluated. 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]

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

Claims

1. A method for producing iron ore pellets, comprising: (a) a step of crushing iron ore; (b) a step of obtaining pellet raw material using the crushed iron ore; (c) a step of granulating or molding the pellet raw material to obtain green pellets; and (d) a step of firing the green pellets, wherein 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.

2. Before carrying out the steps (a) to (d), (e) a plurality of manufacturing conditions are identified under which bursting of iron ore pellets does not occur for the average crystal water content X (mass%) of the iron ore constituting the pellet raw material, and (f) for each of the plurality of manufacturing conditions identified 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 determined. 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, and (g) the multiple products XY specified in (f) are specified. １ Z １ The method for producing iron ore pellets according to claim 1, wherein a maximum value among the above is specified, and (h) the maximum value specified in (g) is set as the allowable value A.

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

4. A method for producing iron ore pellets according to any one of claims 1 to 3, comprising 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 green pellets before firing; and (iii) changing the rate of temperature increase from 250°C to 700°C during firing.

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