How iron ore pellets are made

Abstract

Provided is a method for producing a pellet, the method being capable of suppressing green pellet bursting. This method for producing an iron ore pellet comprises: a crushing step for obtaining ore powder by crushing iron ore having an iron content of 63 mass% or less. The volatile content of the iron ore is 3.3 mass% or greater. The iron ore powder has a cumulative 90% diameter in the grain size distribution by volume of 150 μm or less and a grain size distribution index based on the harmonic mean diameter of 14,700 to 510,000.

Description

[Technical field]

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

[0002] In reduction using the shaft furnace method, which is currently the mainstream direct reduction ironmaking method, lump ore or pellets are used as the charging raw materials. Pellets are manufactured through the processes of crushing, mixing, granulating, and firing the ore. In this manufacturing process, iron ore pellets before firing (so-called green pellets) are required to have a strength sufficient to prevent them from crumbling or pulverizing during handling, such as transportation from the granulation process to the firing process. For this reason, efforts are being made to improve the strength of green pellets.

[0003] Japanese Patent Laid-Open Publication No. 2000-273552 (Patent Document 1) discloses a method for producing reducing agent-mixed pellets. In this pellet production method, at least one of carboxymethyl cellulose and polyvinyl alcohol or tar is added to powdered iron ore and coal as a reducing agent, and moisture is added in a granulator to form the pellets into pellets to produce reducing agent-mixed pellets. The reducing agent-mixed pellets are further dried in a dryer at an atmospheric temperature of 150°C to 170°C. It is said that the drop strength of the dried reducing agent-mixed pellets can be significantly improved by using a hydrocarbon binder such as carboxymethyl cellulose, polyvinyl alcohol, or tar.

[0004] JP 2002-206120 A (Patent Document 2) discloses pellets for reduction furnaces, a manufacturing method thereof, and a method for reducing metal oxide. In this pellet manufacturing method, raw materials discharged from a raw material storage tank are kneaded in a kneading device, and pellets are manufactured in a pan-type granulator using fine powder containing metal oxide and carbon as raw materials. It is said that this pellet manufacturing method can prevent the raw material pellets from collapsing when they are classified in a pellet sieve device, dried in a pellet dryer, and fired and reduced in a rotary hearth type reduction furnace.

[0005] JP 2009-161791 A (Patent Document 3) discloses a method for producing carbon-containing non-calcined pellets for blast furnaces. The method for producing the pellets involves adding a hydraulic binder to fine iron-containing raw materials and fine carbonaceous materials, adjusting the blending ratio of the fine carbonaceous materials so that the carbon content in the total raw materials is 15 to 25 mass%, mixing and granulating while adjusting the moisture, and then curing the pellets to obtain a pellet having a cold crushing strength of 85 kg / cm. 2 This is a method for producing the above-mentioned carbon-containing non-calcined pellets for blast furnaces. In this pellet production method, the curing treatment is performed by leaving the pellets after granulation in the air for 12 to 48 hours in a primary curing yard, then loading the pellets into a shaft furnace for secondary curing, and subjecting the pellets to steam injection treatment at a temperature of 60 to 90°C for a treatment time of 5 hours or more in the shaft furnace, followed by continuous drying treatment, with the total treatment time for the steam injection treatment and drying treatment being within 8 hours. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2000-273552 A [Patent Document 2] JP 2002-206120 A [Patent Document 3] JP 2009-161791 A Summary of the Invention [Problem to be solved by the invention]

[0007] As described above, iron ore pellets (green pellets) are produced using ore powder obtained by crushing iron ore into powder form. However, if the particle size (particle diameter) of the ore powder is reduced to increase the strength of the green pellets, the green pellets become denser and are more likely to burst during the drying process or firing process. Bursting here refers to a phenomenon in which the green pellets burst, explode, or break into powder due to the pressure of the steam of the volatile matter generated inside the green pellets when the volatile matter of the iron ore in the green pellets vaporizes. The volatile matter of the iron ore in the green pellets is, for example, the water of crystallization of the iron ore in the green pellets. If bursting occurs, cracks will occur in the pellets, and the strength after firing will be significantly reduced.

[0008] As described above, since the presence of crystallization water can cause bursting, currently, high-grade ore with little crystallization water is generally used as the raw ore (iron ore) for pellets. However, it is predicted that it will become difficult to secure a stable supply of high-grade ore in the future as demand for high-grade ore increases. For this reason, the use of low-grade ore with an iron content (total Fe content) of 63 mass% or less, which is produced in Australia, India, etc., is being considered. However, if the amount of crystallization water in iron ore pellets (green pellets) increases, bursting becomes more likely as described above. Therefore, it is desired to provide a pellet manufacturing method that can suppress the bursting of green pellets.

[0009] The present disclosure has been made in view of the above circumstances, and an object of the present disclosure is to provide a method for producing pellets that can suppress bursting of green pellets. [Means for solving the problem]

[0010] In order to achieve the above object, the method for producing iron ore pellets according to the present disclosure is as follows.

[0011] [1] A milling process for milling iron ore having an iron content of 63% by mass or less to obtain iron ore powder, The volatile content of the iron ore is 3.3% by mass or more, The ore powder is The cumulative 90% diameter in the volume-based particle size distribution is 150 μm or less, and A method for producing iron ore pellets having a particle size distribution index defined by the following formulas (1) to (4) of 14,700 or more and 510,000 or less.

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[0012] The method for producing iron ore pellets according to the present disclosure may further be as follows.

[0013] [2] The method for producing iron ore pellets according to [1] above, wherein the iron ore has a volatile content of 12.0 mass% or less.

[0014] [3] The method for producing iron ore pellets according to the above [1] or [2], wherein the ore powder has a cumulative 50% diameter in a volume-based particle size distribution of less than 61 μm.

[0015] [4] The method for producing iron ore pellets according to any one of [1] to [3] above, wherein the ore powder has a cumulative 10% diameter in a volume-based particle size distribution of less than 7.1 μm.

[0016] [5] The method for producing iron ore pellets according to any one of [1] to [4] above, wherein the ore powder has a harmonic mean diameter of less than 13 μm. Effect of the Invention

[0017] According to the method for producing iron ore pellets of the present disclosure, a method for producing pellets that can suppress bursting of green pellets can be provided. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] A method for producing iron ore pellets according to an embodiment of the present disclosure will be described.

[0019] First, an overview of the manufacturing method of iron ore pellets according to the present embodiment will be described. The iron ore pellets according to the present embodiment are so-called green pellets (pellets before sintering). The manufacturing method of iron ore pellets according to the present embodiment includes a crushing step of crushing iron ore having an iron content of 63 mass% or less (so-called low-grade ore) to obtain ore powder. In the following description, iron ore having an iron content of 63 mass% or less will be simply referred to as iron ore.

[0020] In the method for producing iron ore pellets according to this embodiment, the volatile content of the iron ore used to obtain the ore powder is 3.3 mass % or more.

[0021] The ore powder has a cumulative 90% diameter of 150 μm or less in the volume-based particle size distribution. The particle size distribution index of the ore powder defined by the above formulas (1) to (4) is 14,700 or more and 510,000 or less. Note that the particle ratio wi in formulas (1), (3), and (4) is the particle ratio in the representative particle size range di when the particle size distribution is divided into predetermined particle size ranges for evaluation.

[0022] According to the method for producing iron ore pellets according to the present embodiment, it is possible to provide a method for producing pellets that can suppress bursting of green pellets.

[0023] In this embodiment, bursting refers to a phenomenon in which, when the volatile matter of the iron ore in the green pellets vaporizes, the green pellets burst, explode, or break into powder due to the pressure of the vapor of the volatile matter generated inside the green pellets.

[0024] The method for producing iron ore pellets according to this embodiment will be described in detail below.

[0025] The iron ore pellets in this embodiment are pellets before firing, so-called green pellets. In the following description, in order to distinguish between pellets before and after firing, the iron ore pellets according to this embodiment (pellets before firing) may be referred to as green pellets, and the pellets after firing may be referred to as fired pellets. In this embodiment, the green pellets and the fired pellets may contain auxiliary materials other than iron ore (e.g., bentonite).

[0026] In addition to the above-mentioned crushing step, the method for producing iron ore pellets according to the present embodiment may further include a granulation step of granulating the ore powder to obtain green pellets. The green pellets may then be fired in a firing step to obtain fired pellets.

[0027] The type and composition (ore composition) of the iron ore used as the raw material for the green pellets are not particularly limited. The raw material for the green pellets may consist of a single iron ore or may be a mixture of multiple ores in any composition.

[0028] In this embodiment, the volatile content of the iron ore used as the raw material of the green pellets refers to the moisture adsorbed by the iron ore and the crystal water contained in the crystals in the iron ore. That is, the volatile content of the iron ore in the green pellets refers to, for example, the moisture adsorbed by the iron ore in the green pellets and the crystal water of the iron ore in the green pellets. The content of the volatile content can be measured as the weight loss of the iron ore (LOI: Loss On Ignition) when the iron ore is held at 1000°C for 30 minutes.

[0029] In this embodiment, iron ore with a volatile content of 3.3% by mass or more is used as the raw material for green pellets. This makes it possible to use cheap or easily available iron ore raw material. The volatile content of the iron ore is preferably 12.0% by mass or less.

[0030] The crushing process is a process in which iron ore is crushed to obtain ore powder (iron ore powder) in which the iron ore is in a powder form. In this embodiment, the crushing method of the iron ore in the crushing process and the crushing time required for crushing are not particularly limited. The crushing of the iron ore can be performed, for example, by a ball mill, a hammer mill, a roll crusher, or the like.

[0031] The shape of the ore powder is not particularly limited. The particle size (particle diameter) of the ore powder is preferably, for example, 250 μm or less (for example, passing through a 250 μm sieve, so-called 250 μm pass). If the ore powder contains coarse particles with a particle size exceeding 250 μm, the strength of the green pellet may decrease.

[0032] In this embodiment, the particle size distribution of the ore powder is a value measured using a laser diffraction particle size measurement (Malvern, Mastersizer 3000). Water is used as a dispersion medium when measuring the particle size distribution.

[0033] The particle size distribution of the ore powder is such that the cumulative 90% diameter in the volumetric particle size distribution is 150 μm or less. Also, in this embodiment, as described above, the particle size distribution index of the ore powder is defined by the above formulas (1) to (4), i.e., the particle size distribution index based on the harmonic mean diameter, is 14,700 or more and 510,000 or less. This makes it possible to suppress the bursting of the pellets.

[0034] As described above, bursting is a phenomenon in which the green pellets explode or break into powder when the volatile content of the iron ore in the green pellets evaporates when the green pellets are heated. More specifically, bursting is a phenomenon in which the green pellets burst, explode, or break into powder due to the pressure of steam generated inside the green pellets when the green pellets are dried or when water of crystallization is released from the iron ore crystals in the green pellets.

[0035] The harmonic mean diameter is often used, for example, when evaluating the air permeability of a particle layer or a packed bed. Therefore, it is considered to be suitable for evaluating the ease of steam escape from the inside to the outside of green pellets. In addition, in general, in a powder packed bed, for example, the broader the particle size distribution, the larger the particle size ratio between the largest particle and the smallest particle becomes, and the relatively small particles enter the space between the relatively large particles, so the void ratio of the powder packed bed decreases. Therefore, the particle size distribution index, which indicates the broadness of the particle size distribution, is considered to be suitable for evaluating the ease of steam escape from the inside to the outside of green pellets.

[0036] The particle size distribution of the ore powder is preferably such that the cumulative 50% diameter in the volumetric particle size distribution is less than 61 μm. This can better suppress the bursting of the pellets. Also, the particle size distribution of the ore powder is preferably such that the cumulative 10% diameter in the volumetric particle size distribution is less than 7.1 μm. This can also better suppress the bursting of the pellets.

[0037] The particle size distribution of the ore powder preferably has a harmonic mean diameter of less than 13 μm.

[0038] The granulation process is a process of granulating the ore powder to obtain iron ore pellets (green pellets). The granulation method in the granulation process is not limited. In the granulation process, for example, the ore powder may be granulated using a pelletizer. As the pelletizer, for example, a pan-type granulator (so-called pan pelletizer) may be used.

[0039] In this embodiment, the shape and size (e.g., particle size and average particle size) of the iron ore pellets are not particularly limited and may be any value. The size of the green pellets may be evaluated, for example, by measuring the major axis diameter and minor axis diameter using a caliper and averaging the measurements as the particle size of the green pellets. The particle size of the green pellets is preferably 9 mm to 16 mm, which is commonly used in this technical field.

[0040] The green pellets are then subjected to a firing process to be fired into fired pellets. The firing temperature in the firing process is, for example, 1200°C to 1350°C.

[0041] The likelihood of bursting, i.e., bursting property, can be evaluated based on the temperature at which bursting occurs when the temperature of the green pellets is increased by gradually heating the green pellets. That is, when the temperature at which bursting occurs is low, bursting is likely to occur (high bursting property), and it can be evaluated as not being good from the viewpoint of suppressing bursting. Conversely, when the temperature at which bursting occurs is high, bursting is unlikely to occur (low bursting property), and it can be evaluated as good from the viewpoint of suppressing bursting. EXAMPLES

[0042] Green pellets were produced according to the following procedure, and the pellet strength and bursting properties were evaluated.

[0043] As the iron ore used as the raw material (raw ore) of the green pellets, ores (A, B) having the chemical composition shown in Table 1 were used. As described above, "LOI" in Table 1 indicates the content (mass%) of volatile matter such as crystal water. In addition, in Table 1, "T.Fe" indicates the mass% of the iron content (total Fe content) in the iron ore. In addition, "T.Fe" is a value quantified based on the total iron quantification method of iron ore specified in JIS M 8212:2022. As shown in Table 1, Ore A and Ore B used as raw materials of the green pellets are both iron ores having a T.Fe of 63 mass% or less (so-called low-grade ores). In the following examples, only the case where the raw material of the green pellets is Ore A or only the case where the raw material of the green pellets is Ore B is exemplified, but if Ore A and Ore B are mixed in any combination, the volatile content of the iron ore used as the raw material of the green pellets can be changed or adjusted in the range of 3.3 mass% to 12.0 mass%.

[0044] [Table 1]

[0045] Ore A or Ore B shown in Table 1 was pulverized in a ball mill to obtain ore powders No. 1 to No. 13 with particle size distributions shown in Table 2. In pulverizing Ore A or Ore B, the particle size distribution of the ore powder was differentiated by adjusting the pulverization time and sieving.

[0046] In Table 2, "D10" indicates the cumulative 10% diameter based on volume, "D50" indicates the cumulative 50% diameter (median diameter) based on volume, and "D90" indicates the cumulative 90% diameter based on volume. In addition, in Table 2, "Dp" indicates the harmonic mean diameter, and "Isp" indicates the particle size distribution index (see formula 2) based on the harmonic mean diameter. The particle size distribution shown in Table 2 was derived based on the particle size of the ore powder measured using a laser diffraction particle size measurement (Malvern, Mastersizer 3000). The dispersion medium used in measuring the particle size distribution was water. The measurement range of the particle size distribution was 0.01 μm or more and 3500 μm or less, and this range was divided into 101 sections for measurement. As shown in Table 2, the ore powders No. 1 to No. 13 have a cumulative 90% diameter of 150 μm or less in the particle size distribution based on volume.

[0047] [Table 2]

[0048] Next, the ore powder from production No. 1 to No. 13 was granulated in a pan-type granulator (pelletizer, pan diameter 1.2 m) to obtain green pellets (granulation process). Granulation was performed by gradually adding water while rolling the ore powder on a rotating pan (rotation speed: 10 times / s). The rolling ore powder was gradually granulated with the addition of water and rolling, and when it grew into pellets with a diameter of 9.5 mm to 12 mm, these were collected. The pellets that grew from 9.5 mm to 12 mm were rolled on the pan for another 10 minutes to become green pellets with a roughly spherical shape. The amount of water added during the above granulation was about 10% of the weight of the green pellets.

[0049] Table 3 shows the evaluation results of the strength ("Drop strength" in Table 3) and bursting property ("Burst temperature" in Table 3) of green pellets produced No. 1 to No. 13. For ease of reference, Table 3 lists the same "Isp" as shown in Table 2.

[0050] [Table 3]

[0051] In Table 3, "drop strength" indicates the evaluation result of the strength based on the drop test of the green pellets. In this embodiment, the drop test was performed by dropping each green pellet of each production number from a height of 50 cm, and measuring the number of times (number of drops) until the green pellet cracked or was broken, and the number of drops was used as the drop strength. That is, the higher the value of the drop strength, the higher the strength of the green pellet. Also, the lower the value of the drop strength, the lower the strength of the green pellet. Note that the number of drops used as the drop strength does not include the number of drops when the green pellet cracked or was broken. In other words, the drop strength is the number of times the green pellet could be dropped without cracking or being broken. In this embodiment, the average value (arithmetic mean) of 10 green pellets was used as the drop strength.

[0052] In Table 3, "bursting temperature" indicates the evaluation result regarding bursting property. The bursting temperature indicates the temperature at which bursting occurs when the temperature of the green pellet is increased by gradually heating the green pellet. In other words, when the bursting temperature is low, the bursting property is high. On the other hand, when the bursting temperature is high, the bursting property is low.

[0053] In this embodiment, the "bursting temperature" is a temperature measured as follows. First, 200 g of green pellets are charged into a vertical cylindrical furnace (diameter 50 mm). Then, air heated to 200°C is passed from the bottom to the top of the layer of green pellets at a wind speed of 1.0 m / sec (however, converted to 0°C / 1 atmospheric pressure) for 10 minutes. Then, the presence or absence of bursting of the green pellets is confirmed. If the green pellets burst or break into powder during the period in which the air heated to 200°C is passed through the layer of green pellets or at the end of the period, the bursting temperature of the green pellets is determined to be 200°C. If the bursting of the green pellets is not confirmed, the temperature of the hot air is further increased by 20°C and passed for 10 minutes, and the presence or absence of bursting of the green pellets is confirmed. Thereafter, the temperature is increased by 20°C in a similar manner (i.e., the green pellets are gradually heated) to identify the temperature at which the green pellets burst. After repeated heating of the hot air, the temperature at which the green pellets burst is identified as the bursting temperature of the green pellets, as described above.

[0054] In this embodiment, from the perspective of whether the green pellets have sufficient strength to prevent disintegration or powdering due to handling such as transportation until they are made into fired pellets (from the granulation process to the firing process), a drop strength of 5 or more is judged to be good, and a drop strength of less than 5 times is judged to be poor.

[0055] In addition, whether or not bursting was suppressed was judged based on whether the bursting temperature was 250° C. or higher or lower than 250° C. In other words, if the bursting temperature was 250° C. or higher, the bursting property was low and good, and if the bursting temperature was lower than 250° C., the bursting property was high and poor.

[0056] In addition, when the bursting property is good but the strength is poor, it is judged as poor as a comprehensive evaluation. This is because when the bursting property is good but the strength is poor, it is impossible to properly sinter and use thereafter. Similarly, when the strength is good but the bursting property is poor, it is judged as poor as a comprehensive evaluation. This is because when the strength is good but the bursting property is poor, it is impossible to properly use after sintering.

[0057] When the green pellets of each production number were evaluated according to the above, green pellets No. 1 and No. 7 were poor in terms of strength. Green pellets No. 6 and No. 13 were poor in terms of bursting properties. The other green pellets (No. 2-5 and No. 8-12) had good strength and bursting properties. In Tables 2 and 3, taking these evaluations into consideration, No. 1, 6, 7, and No. 13 are listed in that order as Comparative Examples 1 to 4. No. 2-5 and No. 8-12 are listed in that order as Examples 1 to 9.

[0058] In general, the smaller the particle size distribution index (the sharper the particle size distribution) is, the higher the bursting temperature is for the green pellets produced No. 1 to No. 13. In other words, the smaller the particle size distribution index is for the green pellets produced No. 1 to No. 13, the better the bursting properties are.

[0059] In addition, in the green pellets of Production No. 1 to No. 13, the larger the particle size distribution index (the broader the particle size distribution), the higher the drop strength. In other words, in the green pellets of Production No. 1 to No. 13, the larger the particle size distribution index, the better the strength.

[0060] Considering the particle size distribution index of the green pellets of Production Nos. 2-5 and 8-12, which have good strength and bursting resistance, in order to suppress bursting of the green pellets while maintaining good strength, the particle size distribution index needs to be 14,700 or more and 510,000 or less.

[0061] Regarding particle size distribution, if the cumulative 50% diameter is 5.0 μm or more and 61.0 μm or less, or at least 5.2 μm or more and 60.1 μm or less, it can be determined that both strength and bursting properties can be achieved. Also, if the cumulative 10% diameter is at least 0.6 μm or more and 7.1 μm or less, it can be determined that both strength and bursting properties can be achieved.

[0062] Regarding the harmonic mean diameter, it can be determined that both strength and bursting properties can be achieved if the harmonic mean diameter is at least 1.7 μm or more and less than 13.0 μm.

[0063] In this manner, a method for producing iron ore pellets can be provided.

[0064] Note that the embodiments disclosed in this specification are merely examples, and the embodiments of the present disclosure are not limited thereto, and may be modified as appropriate within the scope of the purpose of the present disclosure. [Industrial Applicability]

[0065] The present disclosure is applicable to methods for producing iron ore pellets.

Claims

1. The method includes a crushing step of crushing iron ore having an iron content of 63% by mass or less to obtain iron ore powder, The volatile content, which is the moisture adsorbed on the iron ore and the water of crystallization contained in the crystals in the iron ore, is 3.3 mass% or more, The ore powder is The cumulative 90% diameter in the volume-based particle size distribution is 150 μm or less, and A method for producing iron ore pellets having a particle size distribution index defined by the following formulas (1) to (4) of 14,700 or more and 510,000 or less: [0010] [0025] [0030] [0045] where Dp is the harmonic mean diameter (μm), Isp is the particle size distribution index, wi is the particle ratio in the i-th interval when the particle size distribution is measured in the range of 0.01 to 3500 μm and the range is divided into 101 intervals, and di (μm) is the representative particle size in the i-th interval.

2. The method for producing iron ore pellets according to claim 1, wherein the volatile content of the iron ore is 12.0 mass% or less.

3. The method for producing iron ore pellets according to claim 1 or 2, wherein the ore powder has a cumulative 50% diameter in a volumetric particle size distribution of less than 61 μm.

4. The method for producing iron ore pellets according to claim 1 or 2, wherein the ore powder has a cumulative 10% diameter in a volume-based particle size distribution of less than 7.1 μm.

5. The method for producing iron ore pellets according to claim 1 or 2, wherein the ore powder has a harmonic mean diameter of less than 13 μm.

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