Method for producing iron ore pellets

Abstract

The present invention can provide a pellet production method capable of suppressing cracking of green pellets. This method for producing iron ore pellets comprises a crushing step for crushing iron ore having an iron component content of 63 mass% or less to obtain ore powder, the iron ore having a volatile component content of 3.3 mass% or more, the cumulative 90% diameter in the volume-based particle size distribution of the ore powder being 150 [mu] m or less, and the particle size distribution index based on the blended average diameter being 14700-510000.

Description

Technical Field

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

[0002] In the reduction of the shaft furnace method, which is the mainstream of the direct reduction ironmaking method, lump ore and pellets are used as the raw materials for the furnace. The pellets are manufactured through the processes of crushing, mixing, granulating and firing the ore. In this manufacturing process, the iron ore pellets before firing (so-called green pellets) need to have a certain degree of strength to suppress the disintegration and pulverization caused by the transportation from the granulation process to the firing process. Therefore, efforts are being made to improve the strength of the green pellets.

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

[0004] Japanese Patent Application Laid-Open No. 2002-206120 (Patent Document 2) discloses a pellet for a reduction furnace, a method for producing the same, and a method for reducing oxidized metals. In the method for producing the pellets, raw materials output from a raw material storage tank are kneaded in a kneading device, and pellets are produced using fine powder containing oxidized metals and carbon particles as raw materials in a pan granulating device. It is claimed that the pellets can prevent the raw material pellets from disintegrating when they are dried in a pellet drying device after being classified in a pellet sieving device and calcined and reduced in a rotary hearth type reduction furnace.

[0005] Japanese Patent Application Laid-Open No. 2009-161791 (Patent Document 3) discloses a method for producing carbon-containing non-fired pellets for blast furnaces. The method for producing the pellets is to add a hydraulic binder to a fine powder iron-containing raw material and a fine powder carbon material, and adjust the proportion of the fine powder carbon material so that the carbon content in the total raw material is 15 to 25% by mass, and then mix and granulate while adjusting the water content, and then perform curing, thereby producing a pellet with a cold crushing strength of 85 kg / cm 2The method for producing carbon-containing non-fired pellets for blast furnaces is as follows. In the method for producing pellets, the pellets after granulation are placed in the air in a primary curing area for 12 to 48 hours, and then the pellets are charged into a secondary curing shaft furnace, steam is blown into the shaft furnace at a temperature of 60 to 90° C. for a treatment time of more than 5 hours, and then drying is continuously performed, and the total treatment time of the steam blowing treatment and the drying treatment is within 8 hours.

[0006] Prior art literature

[0007] Patent Literature

[0008] Patent Document 1: Japanese Patent Application Publication No. 2000-273552

[0009] Patent Document 2: Japanese Patent Application Publication No. 2002-206120

[0010] Patent Document 3: Japanese Patent Application Publication No. 2009-161791 Summary of the invention

[0011] As described above, in the manufacture of iron ore pellets (green pellets), ore powder obtained by crushing iron ore into powder form is used. However, in order to improve the strength of green pellets, if the particle size (particle size) of the ore powder is reduced, the green pellets become high-density and are prone to collapse during the drying process or the firing process. Here, collapse refers to the phenomenon that when the volatile components of the iron ore in the green pellets are gasified, the green pellets are caused to break, explode or pulverize due to the pressure of the vapor of the volatile components generated inside the green pellets. The volatile components of the iron ore in the green pellets refer to, for example, the crystal water of the iron ore in the green pellets. If collapse occurs, cracks will occur in the pellets, and thus the strength after firing will be significantly reduced.

[0012] As mentioned above, the presence of crystal water may be the cause of collapse, so now the raw material ore (iron ore) of the pellet generally uses high-grade ore with less crystal water. However, with the increase in demand for high-grade ore in the future, it is expected that the stability of high-grade ore will become difficult to ensure. Therefore, it is considered to use low-grade ores with an iron content (total Fe content) of 63% by mass or less produced in Australia, India, etc. However, if the crystal water in the iron ore pellets (green pellets) increases, collapse is likely to occur as described above. Therefore, it is desirable to provide a method for manufacturing pellets that can suppress the collapse of green pellets.

[0013] The present disclosure has been made in view of such actual circumstances, and an object of the present disclosure is to provide a method for producing pellets that can suppress the collapse of green pellets.

[0014] The method for producing iron ore pellets of the present disclosure for achieving the above-mentioned object is as follows.

[0015] [1] A method for producing iron ore pellets, comprising a pulverizing step of pulverizing iron ore having an iron content of 63% by mass or less to obtain ore powder,

[0016] The volatile content of the iron ore is 3.3% by mass or more.

[0017] The ore powder has a cumulative 90% diameter in a volume-based particle size distribution of 150 μm or less, and a particle size distribution index defined by the following formulas (1) to (4) of 14,700 to 510,000.

[0018] D p =1 / Σ(w i / d i ) (1)

[0019]

[0020] I P =(1 / D p ) 2 ∑w i (d i -Dp) 2 (4)

[0021] Wherein, Dp is the harmonic mean diameter (μm), Isp is the particle size distribution index, wi is the particle ratio, and di (μm) is the representative particle size.

[0022] The method for producing the iron ore pellets according to the present disclosure may also be as follows.

[0023] [2] The method for producing iron ore pellets according to [1] above, wherein the volatile matter content of the iron ore is 12.0 mass % or less.

[0024] [3] The method for producing iron ore pellets according to [1] or [2] above, wherein the cumulative 50% diameter in the volume-based particle size distribution of the ore powder is less than 61 μm.

[0025] [4] The method for producing iron ore pellets according to any one of [1] to [3] above, wherein the cumulative 10% diameter in the volume-based particle size distribution of the ore powder is less than 7.1 μm.

[0026] [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.

[0027] According to the method for producing iron ore pellets of the present disclosure, it is possible to provide a method for producing pellets that can suppress the collapse of green pellets. DETAILED DESCRIPTION

[0028] A method for producing iron ore pellets according to an embodiment of the present disclosure will be described.

[0029] First, the outline of the method for producing iron ore pellets according to the present embodiment is described. The iron ore pellets according to the present embodiment are so-called green pellets (pellets before firing). The method for producing iron ore pellets according to the present embodiment includes a pulverizing step of pulverizing iron ore (so-called low-grade ore) having an iron content of 63% by mass or less to obtain ore powder. In the following description, the iron ore having an iron content of 63% by mass or less is referred to as iron ore.

[0030] In the method for producing iron ore pellets according to the present embodiment, the volatile matter content of the iron ore for obtaining the ore powder is 3.3% by mass or more.

[0031] The cumulative 90% diameter in the volume-based particle size distribution of the ore powder is 150 μm or less. In addition, the particle size distribution index defined by the above formulas (1) to (4) in the ore powder is 14700 to 510000. It should be noted that the particle ratio wi in the formulas (1), (3) and (4) is the particle ratio in the interval of the representative particle size di when the particle size distribution is divided into intervals of a predetermined particle size range for evaluation.

[0032] 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 the collapse of green pellets.

[0033] In the present embodiment, the collapse refers to a phenomenon in which the green pellets are broken, exploded, or pulverized due to the pressure of the vapor of the volatile components generated inside the green pellets when the volatile components of the iron ore in the green pellets are gasified.

[0034] Hereinafter, the method for producing iron ore pellets according to the present embodiment will be described in detail.

[0035] The iron ore pellets in this embodiment refer to pellets before firing, so-called green pellets. In the following description, in order to distinguish the pellets before and after firing, the iron ore pellets involved in this embodiment (pellets before firing) are sometimes referred to as green pellets, and the pellets after firing are sometimes referred to as fired pellets. In this embodiment, the green pellets and fired pellets may contain auxiliary raw materials (such as bentonite) other than iron ore.

[0036] The method for producing iron ore pellets according to the present embodiment may further include a granulation step of granulating ore powder to obtain green pellets in addition to the above-mentioned pulverization step. The green pellets are then calcined in a calcination step to obtain calcined pellets.

[0037] The type and composition (ore composition) of the iron ore used as the raw material of green pellets are not particularly limited. The raw material of green pellets may be a raw material composed of a single iron ore or a raw material obtained by mixing a plurality of ores in an arbitrary composition.

[0038] In the present embodiment, the volatile components of the iron ore as the raw material of the green pellets refer to the moisture adsorbed on the iron ore and the crystal water contained in the crystals in the iron ore. That is, the volatile components of the iron ore in the green pellets refer to, for example, the moisture adsorbed on the iron ore in the green pellets and the crystal water of the iron ore in the green pellets. It should be noted that the content of the volatile components can be measured as the weight loss (LOI: Loss On Ignition) of the iron ore when the iron ore is kept at 1000°C for 30 minutes.

[0039] In this embodiment, iron ore having a volatile content of 3.3% by mass or more is used as a raw material for green pellets. Thus, low-cost or easily available iron ore raw materials can be used. The volatile content of the iron ore is preferably 12.0% by mass or less.

[0040] The pulverizing step is a step of pulverizing iron ore to obtain ore powder (iron ore powder) that makes the iron ore powder. In the present embodiment, the pulverizing method of the iron ore in the pulverizing step and the pulverizing time required for pulverizing are not particularly limited. The pulverization of the iron ore can be carried out, for example, using a ball mill, a hammer mill, a roller mill, etc.

[0041] 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, all pass through a 250 μm sieve, so-called 250 μm pass). If the ore powder contains coarse particles with a particle size greater than 250 μm, the strength of the green pellets may be reduced.

[0042] In the present embodiment, the particle size distribution of the ore powder is a value measured using a laser diffraction particle size analyzer (Mastersizer 3000 manufactured by Malvern). Water is used as a dispersion medium in the measurement of the particle size distribution.

[0043] In the particle size distribution of the ore powder, the cumulative 90% diameter in the volume-based particle size distribution is 150 μm or less. In addition, in the present embodiment, as described above, the particle size distribution index defined by the above formulas (1) to (4) in the particle size distribution of the ore powder, that is, the particle size distribution index based on the harmonic mean diameter is 14700 to 510000. This can suppress the collapse of the pellets.

[0044] As described above, the collapse is a phenomenon in which the green pellets burst or pulverize due to the evaporation of the volatile components of the iron ore in the green pellets when the green pellets are heated. Specifically, the collapse is a phenomenon in which the green pellets are broken, burst or pulverized due to the pressure of the steam generated inside the green pellets when the green pellets are dried or the crystal water in the green pellets is separated from the iron ore crystals.

[0045] The harmonic mean diameter is often used, for example, when evaluating the air permeability of a particle layer or a filling layer. Therefore, it can be considered suitable for evaluating the ease with which steam escapes from the inside of a green pellet to the outside. In addition, generally in a powder filling layer, for example, the wider the particle size distribution, the larger the particle size ratio of the largest particle to the smallest particle, and the relatively small particles enter the space between the relatively large particles, so the porosity of the powder filling layer decreases. Therefore, it can be considered that the particle size distribution index, which represents the width of the particle size distribution, is suitable for evaluating the ease with which steam escapes from the inside of a green pellet to the outside.

[0046] In the particle size distribution of the ore powder, the cumulative 50% diameter in the volume-based particle size distribution is preferably less than 61 μm. This can better suppress the collapse of the pellets. In addition, in the particle size distribution of the ore powder, the cumulative 10% diameter in the volume-based particle size distribution is preferably less than 7.1 μm. This can also better suppress the collapse of the pellets.

[0047] In the particle size distribution of the ore powder, the harmonic mean diameter is preferably less than 13 μm.

[0048] The granulation step is a step of granulating the ore powder to obtain iron ore pellets (green pellets). The granulation method in the granulation step is not limited. In the granulation step, for example, the ore powder can be granulated using a granulator. As a granulator, for example, a disk-type granulator (so-called disk granulator) can be used.

[0049] In the present embodiment, the shape and size (e.g., particle size, average particle size) of the iron ore pellets are not particularly limited and can be any value. The size of the green pellets can be measured, for example, using a vernier caliper to measure the major axis diameter and the minor axis diameter, and the average value thereof is used as the particle size of the green pellets for evaluation. In the present technical field, it is preferred that the particle size of the green pellets be the commonly used 9 mm to 16 mm.

[0050] Then, the green pellets are subjected to a firing step and fired to become fired pellets. The firing temperature in the firing step is, for example, 1200°C to 1350°C.

[0051] The ease with which the cracking occurs, i.e., the cracking property, can be evaluated, for example, based on the temperature at which the cracking occurs when the green pellets are gradually heated to increase the temperature of the green pellets. That is, when the temperature at which the cracking occurs is low, the cracking easily occurs (the cracking property is high), and from the viewpoint of suppressing the cracking, it can be evaluated as poor. On the contrary, when the temperature at which the cracking occurs is high, the cracking does not easily occur (the cracking property is low), and from the viewpoint of suppressing the cracking, it can be evaluated as good.

[0052] Example

[0053] Green pellets were produced according to the following procedure, and the strength and collapse resistance of the pellets were evaluated.

[0054] As the iron ore of the raw material (raw material ore) of green pellets, ores (A, B) of the chemical composition shown in Table 1 were used. It should be noted that, as described above, "LOI" in Table 1 represents the content (mass %) of volatile components such as crystal water. In addition, in Table 1, "T.Fe" represents the mass % of the iron content (total Fe content) in the iron ore. It should be noted that "T.Fe" is a value quantified according to the total iron quantification method of iron ore specified in JIS M8212: 2022. As shown in Table 1, Ore A and Ore B, which are raw materials for green pellets, are both iron ores (so-called low-grade ores) having a T.Fe of 63% by mass or less. In the following examples, only the case where the raw material of green pellets is ore A or ore B is illustrated, but ore A and ore B can be mixed in any combination to change or adjust the volatile components of the iron ore as the raw material of green pellets within the range of 3.3% by mass to 12.0% by mass.

[0055] Table 1

[0056]

[0057] Ore A or Ore B shown in Table 1 was pulverized using a ball mill to obtain ore powders of Production No. 1 to No. 13 having the particle size distribution shown in Table 2. In the pulverization of Ore A or Ore B, the particle size distribution of the ore powder was differentiated by adjusting the pulverization time and sieving.

[0058] In Table 2, "D10" represents the cumulative 10% diameter on a volume basis, "D50" represents the cumulative 50% diameter (median diameter) on a volume basis, and "D90" represents the cumulative 90% diameter on a volume basis. In addition, in Table 2, "Dp" represents the harmonic mean diameter, and "Isp" represents the particle size distribution index based on the harmonic mean diameter (reference formula 2). It should be noted that the particle size distribution shown in Table 2 is derived from the particle size of the ore powder measured using a laser diffraction particle size measurement (Mastersizer 3000 manufactured by Malvern). It should be noted that the dispersion medium used in the measurement of the particle size distribution is water. In addition, the measurement range of the particle size distribution is set to 0.01 μm to 3500 μm, and the range is divided into 101 intervals for measurement. As shown in Table 2, the cumulative 90% diameter in the volume-based particle size distribution of the ore powders produced No. 1 to No. 13 is less than 150 μm.

[0059] Table 2

[0060] No. ore D10 D50 D90 Dp Isp 1 A 7.1 16.4 29.2 13.0 14524 Comparative Example 1 2 A 6.5 15.7 27.4 11.1 14949 Example 1 3 A 0.6 5.2 40.1 1.7 148198 Example 2 4 A 2.1 44.6 126.7 6.6 227586 Example 3 5 A 1.1 35.3 125.4 4.2 304790 Example 4 6 A 1.2 49.8 140.1 4.2 526592 Comparative Example 2 7 B 7.2 17.0 30.1 24.7 13868 Comparative Example 3 8 B 7.1 17.2 30.3 12.2 14909 Example 5 9 B 2.9 33.4 96.1 8.1 125044 Example 6 10 B 1.5 30.3 123.7 5.0 248661 Example 7 11 B 1.1 57.5 147.2 4.5 391882 Example 8 12 B 1.1 60.1 143.9 4.3 502482 Example 9 13 B 1.2 53.4 141.2 4.4 523476 Comparative Example 4

[0061] Next, the ore powders produced in No. 1 to No. 13 are granulated using a disk-type granulator (granulator, disk diameter 1.2 m) to obtain raw pellets (granulation process). Granulation is carried out by rolling the ore powder on a rotating disk (rotation speed of 10 turns / s) and gradually adding water. As the rolling ore powder gradually forms particles and grows into pellets with a diameter of 9.5 mm to 12 mm with the addition of water and rolling, they are collected. The pellets grown into 9.5 mm to 12 mm are further rolled on the disk for 10 minutes to form roughly spherical raw pellets. It should be noted that the amount of water added in the above granulation is approximately 10% of the weight of the raw pellets.

[0062] Table 3 shows the evaluation results of the strength ("drop strength" in Table 3) and the disintegration property ("disintegration temperature" in Table 3) of the green pellets produced No. 1 to No. 13. For ease of reading, the same "Isp" as shown in Table 2 is shown again in Table 3.

[0063] Table 3

[0064]

[0065] In Table 3, "drop strength" indicates the evaluation result of the strength based on the drop test of the green pellets. In the present embodiment, the drop test is performed by dropping the green pellets of each manufacturing No. from a height of 50 cm, and measuring the number of times (number of drops) until cracks appear on the green pellets or the green pellets are destroyed, and the magnitude of the number of drops is used as the drop strength. That is, the larger the value of the drop strength, the higher the strength of the green pellets. In addition, the smaller the value of the drop strength, the lower the strength of the green pellets. It should be noted that the number of drops used as the drop strength does not include the number of drops when cracks appear on the green pellets or the green pellets are destroyed. In other words, the drop strength refers to the number of times that the green pellets can be dropped without cracks appearing in the green pellets or the green pellets being destroyed. In the present embodiment, the drop strength uses the average value (arithmetic mean) of 10 green pellets.

[0066] In Table 3, "collapse temperature" indicates the evaluation result related to collapsibility. Collapse temperature indicates the temperature at which green pellets are gradually heated to increase the temperature of the green pellets. That is, when the collapsibility temperature is low, the collapsibility is high. On the other hand, when the collapsibility temperature is high, the collapsibility is low.

[0067] In the present embodiment, the "cracking temperature" is a temperature measured as follows. First, 200 g of green pellets are placed in a vertical cylindrical furnace (diameter 50 mm). Then, air heated to 200°C is ventilated as hot air from the bottom of the green pellet layer to the top at a wind speed of 1.0 m / sec (converted to 0°C / 1 atmospheric pressure) for 10 minutes. Then, the presence or absence of cracking of the green pellets is confirmed. Assuming that the green pellets are cracked or pulverized during or at the end of the period when the air heated to 200°C is ventilated to the green pellet layer, the collapse temperature of the green pellets is determined to be 200°C. If the cracking of the green pellets is not confirmed, the temperature of the hot air is further increased by 20°C, and the air is ventilated for 10 minutes to confirm the presence or absence of cracking of the green pellets. Thereafter, the temperature is increased by 20°C each time (i.e., the green pellets are gradually heated) to confirm the temperature at which the green pellets are cracked. After repeatedly raising the temperature of the hot air, as described above, the temperature at which the green pellets are broken is determined as the collapse temperature of the green pellets.

[0068] In this embodiment, from the perspective of whether the strength is sufficient to suppress the degree of disintegration and pulverization caused by operations such as transportation until the green pellets become fired pellets (the green pellets are supplied from the granulation step to the firing step), the case where the drop strength is 5 times or more is judged to have good strength, and the case where the drop strength is less than 5 times is judged to have poor strength.

[0069] Whether the chipping is suppressed is determined by whether the chipping temperature is 250° C. or higher or lower. That is, if the chipping temperature is 250° C. or higher, the chipping is low and is determined as good, and if the chipping temperature is lower than 250° C., the chipping is high and is determined as bad.

[0070] It should be noted that when the chipping property is good but the strength is poor, it is judged as poor as a comprehensive evaluation. This is because when the chipping property is good but the strength is poor, it is impossible to achieve proper firing and subsequent use. Similarly, when the strength is good but the chipping property is poor, it is judged as poor as a comprehensive evaluation. This is because when the strength is good but the chipping property is poor, it is impossible to achieve proper use after firing.

[0071] According to the above evaluation, the green pellets of each production No. are inferior from the viewpoint of strength. In addition, the green pellets of No. 6 and 13 are inferior from the viewpoint of disintegration. The strength and disintegration of the green pellets other than the above (No. 2-5 and 8-12) are good. In Tables 2 and 3, considering these evaluations, No. 1, 6, 7 and 13 are marked as Comparative Examples 1-4, respectively. In addition, No. 2-5 and 8-12 are marked as Examples 1-9, respectively.

[0072] In the green pellets of No. 1 to No. 13, basically, the smaller the particle size distribution index (the sharper the particle size distribution), the higher the disintegration temperature. In other words, in the green pellets of No. 1 to No. 13, the smaller the particle size distribution index, the better the disintegration property.

[0073] In addition, in the green pellets of No. 1 to No. 13, basically, the larger the particle size distribution index (the wider the particle size distribution), the greater the drop strength. In other words, in the green pellets of No. 1 to No. 13, the larger the particle size distribution index, the better the strength.

[0074] Considering the particle size distribution index of the green pellets of Production Nos. 2-5 and 8-12, which are excellent in strength and disintegration, it is necessary to set the particle size distribution index to 14700 to 510000 in order to suppress the disintegration of the green pellets on the premise of having good strength.

[0075] Regarding the particle size distribution, if the cumulative 50% diameter is 5.0 μm to 61.0 μm, at least 5.2 μm to 60.1 μm, it can be judged that both strength and disintegration properties are achieved. In addition, if the cumulative 10% diameter is at least 0.6 μm to 7.1 μm, it can be judged that both strength and disintegration properties are achieved.

[0076] When the harmonic mean diameter is at least 1.7 μm or more and less than 13.0 μm, it can be judged that both strength and chipping resistance can be achieved.

[0077] As described above, a method for producing iron ore pellets can be provided.

[0078] It should be noted that the embodiments disclosed in this specification are examples, and the embodiments of the present disclosure are not limited thereto, and can be appropriately modified within the scope not departing from the purpose of the present disclosure.

[0079] Industrial Applicability

[0080] The present disclosure is applicable to a method for producing iron ore pellets.

Claims

1. A method for producing iron ore pellets, comprising a pulverizing step of pulverizing iron ore having an iron content of 63% by mass or less to obtain ore powder, The volatile content of the iron ore is 3.3% by mass or more, The ore powder has a cumulative 90% diameter in a volume-based particle size distribution of 150 μm or less, and a particle size distribution index defined by the following formulas (1) to (4) is in the range of 14,700 to 510,000. D p =1 / ∑(w i / d i ) (1) in, Dp is the harmonic mean diameter, the unit is μm, Isp is the particle size distribution index, wi is the particle ratio, and di is the representative particle size, the unit is μm.

2. The method for producing iron ore pellets according to claim 1, wherein: The volatile matter 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 cumulative 50% diameter in the volume-based particle size distribution of the ore powder is less than 61 μm.

4. The method for producing iron ore pellets according to any one of claims 1 to 3, wherein: The cumulative 10% diameter in the volume-based particle size distribution of the ore powder is less than 7.1 μm.

5. The method for producing iron ore pellets according to any one of claims 1 to 4, wherein: The harmonic mean diameter of the ore powder is less than 13 μm.

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