METHOD FOR PRODUCING IRON ORE PELLET

Abstract

A pellet production method that can suppress bursting of green pellets can be provided. The method for producing an iron ore pellet includes grinding iron ore with iron content of 63 mass % or less to obtain ore powder, in which a volatile matter of the iron ore is 3.3 mass % or more, the ore powder has a cumulative 90% diameter of 150 m or less in a volume-based particle size distribution and a particle size distribution index based on the harmonic mean diameter of 14,700 or more and 510,000 or less.

Claims

1. A method for producing an iron ore pellet, comprising grinding iron ore with iron content of 63 mass % or less to obtain ore powder, wherein a volatile matter of the iron ore is 3.3 mass % or more, a cumulative 90% diameter of the ore powder is 150 m or less in a volume-based particle size distribution, and a particle size distribution index of the ore powder defined by Equations (1) to (4) below is 14,700 or more and 510,000 or less, $\left[\mathrm{Math}.1\right]$ $\begin{array}{cc}{D}_p=1/.Math.\left(w_i/d_i\right)& \left(1\right)\end{array}$ $\left[\mathrm{Math}.2\right]$ $\begin{array}{cc}{I}_{\mathrm{sp}}=100\sqrt{I_S{I}_P}& \left(2\right)\end{array}$ $\left[\mathrm{Math}.3\right]$ $\begin{array}{cc}{I}_S=D_P^2-.Math.{w_i\left(1/d_i-1/D_p\right)}^2& \left(3\right)\end{array}$ $\left[\mathrm{Math}.4\right]$ $\begin{array}{cc}{I}_P={\left(1/D_p\right)}^2.Math.{w_i\left(d_i-D_p\right)}^2& \left(4\right)\end{array}$ where, Dp is a harmonic mean diameter (m), Isp is a particle size distribution index, wi is a particle ratio, and di (m) is a representative particle size.

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

3. The method for producing an iron ore pellet according to claim 1, wherein the ore powder has a cumulative 50% diameter of less than 61 m in the volume-based particle size distribution.

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

5. The method for producing an iron ore pellet according to claim 1, wherein the ore powder has a harmonic mean diameter of less than 13 m.

6. The method for producing an iron ore pellet according to claim 2, wherein the ore powder has a cumulative 50% diameter of less than 61 m in the volume-based particle size distribution.

7. The method for producing an iron ore pellet according to claim 2, wherein the ore powder has a cumulative 10% diameter of less than 7.1 m in the volume-based particle size distribution.

8. The method for producing an iron ore pellet according to claim 3, wherein the ore powder has a cumulative 10% diameter of less than 7.1 m in the volume-based particle size distribution.

9. The method for producing an iron ore pellet according to claim 6, wherein the ore powder has a cumulative 10% diameter of less than 7.1 m in the volume-based particle size distribution.

10. The method for producing an iron ore pellet according to claim 2, wherein the ore powder has a harmonic mean diameter of less than 13 m.

11. The method for producing an iron ore pellet according to claim 3, wherein the ore powder has a harmonic mean diameter of less than 13 m.

12. The method for producing an iron ore pellet according to claim 4, wherein the ore powder has a harmonic mean diameter of less than 13 m.

13. The method for producing an iron ore pellet according to claim 6, wherein the ore powder has a harmonic mean diameter of less than 13 m.

14. The method for producing an iron ore pellet according to claim 7, wherein the ore powder has a harmonic mean diameter of less than 13 m.

15. The method for producing an iron ore pellet according to claim 8, wherein the ore powder has a harmonic mean diameter of less than 13 m.

16. The method for producing an iron ore pellet according to claim 9, wherein the ore powder has a harmonic mean diameter of less than 13 m.

Description

DETAILED DESCRIPTION

[0024] This section describes a method for producing an iron ore pellet according to an embodiment of this disclosure.

[0025] First, an overview of the method for producing an iron ore pellet according to the present embodiment is described. The iron ore pellet according to the present embodiment is a so-called green pellet (pellet before baking). The method for producing an iron ore pellet according to the present embodiment includes the grind process in which iron ore with iron content of 63 mass % or less (so-called low-grade ore) is grinded to obtain ore powder. In the following description, iron ore with iron content of 63 mass % or less is simply referred to as iron ore.

[0026] In the method for producing an iron ore pellet according to the present embodiment, the volatile matter of the iron ore used to obtain the ore powder is 3.3 mass % or more.

[0027] The ore powder has a cumulative 90% diameter of 150 m or less in the volume-based particle size distribution. The ore powder has a particle size distribution index of 14,700 or more and 510,000 or less as defined by Equations (1) to (4) above. The particle ratio wi in Equations (1), (3), and (4) is the particle ratio in the section of representative particle size di when the particle size distribution is divided into sections of a specified particle size range and evaluated.

[0028] The method for producing an iron ore pellet according to the present embodiment can provide a method for producing a pellet that can suppress bursting of green pellets.

[0029] In the present embodiment, bursting is a phenomenon in which, when the volatile matter of iron ore in the green pellet vaporizes, the green pellet bursts, explodes, or powders due to the pressure of vapor of the volatile matter generated inside the green pellet.

[0030] The following is a detailed description of the method for producing an iron ore pellet according to the present embodiment.

[0031] The iron ore pellet according to the present embodiment is a pellet before baking, so-called green pellet. In the following description, to distinguish between pellets before and after baking, the iron ore pellet (pellet before baking) according to the present embodiment may be referred to as green pellet and the pellet after baking as baked pellet. In the present embodiment, the green pellet and baked pellet can contain an auxiliary law material (e.g., bentonite) other than iron ore.

[0032] In addition to the grind process described above, the method for producing an iron ore pellet according to the present embodiment may further include a granulation process in which the ore powder is granulated to obtain green pellets. The green pellets may then be baked in a baking process to become baked pellets.

[0033] The type and mix proportion (ore mix proportion) of iron ore used as raw material for green pellets are not limited. The raw material for green pellets may consist of a single iron ore or a mixture of multiple ores in any mix proportion.

[0034] In the present embodiment, the volatile matter of the iron ore used as the raw material for green pellets refers to the water adsorbed on the iron ore and water of crystallization contained in the crystals in the iron ore. In detail, the volatile matter of the iron ore in the green pellets is, for example, water adsorbed on the iron ore in the green pellets and water of crystallization of the iron ore in the green pellets. The volatile matter content can be measured as the amount of weight loss of iron ore when the iron ore is held at 1000 C. for 30 minutes (LOI: Loss On Ignition).

[0035] In the present embodiment, iron ore with a volatile matter content of 3.3 mass % or more is used as raw material for green pellets. This allows the use of inexpensive or readily available iron ore raw materials. The volatile matter of iron ore is preferably 12.0 mass % or less.

[0036] The grind process is the process of grinding iron ore to obtain ore powder (iron ore powder), in which the iron ore is in powder form. In the present embodiment, the grind method of iron ore and the grind time required for grind in the grind process are not limited. Iron ore can be grinded, for example, in a ball mill, hammer mill, and roll grinder.

[0037] The shape of the ore powder is not limited. The particle size (particle diameter) of the ore powder is preferably, for example, 250 m or less (e.g., passing completely through a 250 m sieve, so called, 250 m pass). If the ore powder contains coarse grains with a particle size exceeding 250 m, the strength of the green pellets may be reduced.

[0038] The values of the particle size distribution of the ore powder in the present embodiment are measured using laser diffraction particle size measurement (Mastersizer 3000 manufactured by Malvern). Water is used as the dispersant when measuring the particle size distribution.

[0039] The particle size distribution of ore powder has a cumulative 90% diameter of 150 m or less in the volume-based particle size distribution. In the present embodiment, the particle size distribution of ore powder has a particle size distribution index defined by Equations (1) to (4) above, i.e., a particle size distribution index based on harmonic mean diameter of 14,700 or more and 510,000 or less, as described above. These can suppress bursting pellets.

[0040] As mentioned above, bursting is a phenomenon in which the volatile matter of iron ore in the green pellet vaporizes when the green pellet is heated, causing the green pellet to explode or powder. In detail, bursting is a phenomenon in which the green pellet bursts, explodes, or powders due to the pressure of vapor generated inside the green pellet when the green pellet is dried or when water of crystallization is desorbed from the iron ore crystals in the green pellet.

[0041] The harmonic mean diameter is often used, for example, to evaluate the gas permeability of a particle or filling layer. Therefore, it can be considered suitable for evaluating the ease of vapor escape from the inside of the green pellet to the outside. In general, the void ratio of the powder-filled layer decreases, because, for example, as the particle size distribution widens, the particle size ratio of the largest to the smallest particles increases, and relatively small particles enter the spaces between relatively large particles. Therefore, the particle size distribution index, which represents the spread of the particle size distribution, can be considered suitable for evaluating the ease of vapor escape from the inside to the outside of the green pellets.

[0042] The particle size distribution of ore powder preferably has a cumulative 50% diameter of less than 61 m in the volume-based particle size distribution. This can suppress pellet bursting better. Further, the particle size distribution of ore powder preferably has a cumulative 10% diameter of less than 7.1 m in the volume-based particle size distribution. This also can suppress pellet bursting better.

[0043] The particle size distribution of the ore powder preferably has a harmonic mean diameter of less than 13 m.

[0044] The granulation process is the process of granulating 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, a pelletizer may be used to granulate the ore powder. For example, a pan-type granulator (so-called pan pelletizer) may be used as a pelletizer.

[0045] The shape and size of the iron ore pellets (e.g., particle size or average particle size) are not limited in the present embodiment and may be of any value. For the size of the green pellets, for example, the major and minor axis diameters are measured using a caliper, and their average value may be evaluated as particle size of the green pellets. The particle size of the green pellets is preferably 9 mm or more, which is the usual size used in this technical field. The particle size of the green pellets is preferably 16 mm or less, which is the usual size used in this technical field.

[0046] The green pellets are then subjected to the baking process and baked to become baked pellets. The baking temperature in the baking process is, for example, from 1200 C. to 1350 C.

[0047] The ease of bursting, or bursting property, can be evaluated based on the temperature at which bursting occurs, for example, when the green pellets are heated gradually to increase the temperature of the green pellets. In detail, when the temperature at which bursting occurs is low, bursting is more likely to occur (bursting property is high) and can be evaluated as not good from the viewpoint of suppressing bursting. Conversely, when the temperature at which bursting occurs is high, bursting is less likely to occur (bursting property is low) and can be evaluated as good from the perspective of suppressing bursting.

EXAMPLES

[0048] Green pellets were produced using the following procedures and evaluated for pellet strength and bursting property.

[0049] As the iron ore used as the raw material (raw material ore) for the green pellets, ores (A, B) having the chemical compositions listed in Table 1 were used. The LOI in Table 1 is represented as the content (in mass %) of volatile matter such as water of crystallization, as described above. In Table 1, T.Fe represents the mass percent of iron content (total Fe content) in the iron ore. The T.Fe is the value determined based on the method for determination of total iron content for iron ores specified in JIS M 8212:2022. As listed in Table 1, Ore A and Ore B, the raw materials for the green pellets, are both iron ore with T.Fe of 63 mass % or less (so-called low-grade ore). In the following examples, the case where only Ore A or Ore B is a raw material for green pellets is exemplified, but if Ore A and Ore B are mixed in any proportion, the volatile matter of the iron ores as raw materials for the green pellets can be changed or adjusted in the range from 3.3 mass % to 12.0 mass %.

TABLE-US-00001 TABLE 1 Chemical Composition (mass %) Ore T.Fe SiO.sub.2 CaO Al.sub.2O.sub.3 MgO LOI A 62.62 3.68 0.1 2.17 0.43 3.3 B 55.59 5.68 0.1 3.09 0.08 12

[0050] Ore A or Ore B listed in Table 1 was grinded in a ball mill to obtain ore powders of Production No. 1 to No. 13 with the particle size distributions listed in Table 2. In grinding of Ore A or Ore B, the particle size distributions of the ore powders were made to differ by adjusting the grind time and sieving.

[0051] In Table 2, D10 indicates the cumulative 10% diameter on a volume basis, D50 indicates the cumulative 50% diameter (median size) on a volume basis, and D90 indicates the cumulative 90% diameter on a volume basis. In Table 2, Dp indicates the harmonic mean diameter and Isp indicates the particle size distribution index based on the harmonic mean diameter (see Equation 2). The particle size distributions listed in Table 2 were derived based on the particle sizes of the ore powders measured using laser diffraction particle size measurement (Mastersizer 3000 manufactured by Malvern). The dispersant used in the measurement of particle size distribution is water. The measurement range of the particle size distribution was 0.01 m or more and 3500 m or less, and this range was divided into 101 sections for measurement. As listed in Table 2, the ore powders from Production No. 1 to No. 13 have a cumulative 90% diameter of 150 m or less in the volume-based particle size distribution.

TABLE-US-00002 TABLE 2 No. Ore D10 D50 D90 Dp Isp 1 A 7.1 16.4 29.2 13.0 14524 Comp. Ex. 1 2 A 6.5 15.7 27.4 11.1 14949 Ex. 1 3 A 0.6 5.2 40.1 1.7 148198 Ex. 2 4 A 2.1 44.6 126.7 6.6 227586 Ex. 3 5 A 1.1 35.3 125.4 4.2 304790 Ex. 4 6 A 1.2 49.8 140.1 4.2 526592 Comp. Ex. 2 7 B 7.2 17.0 30.1 24.7 13868 Comp. Ex. 3 8 B 7.1 17.2 30.3 12.2 14909 Ex. 5 9 B 2.9 33.4 96.1 8.1 125044 Ex. 6 10 B 1.5 30.3 123.7 5.0 248661 Ex. 7 11 B 1.1 57.5 147.2 4.5 391882 Ex. 8 12 B 1.1 60.1 143.9 4.3 502482 Ex. 9 13 B 1.2 53.4 141.2 4.4 523476 Comp. Ex. 4

[0052] Next, the ore powders from Production No. 1 to No. 13 were granulated in a pan-type granulator (pelletizer with pan diameter of 1.2 m) to obtain green pellets (granulation process). Granulation was performed by gradually adding water while rolling each ore powder on a rotating pan (rotation speed: 10 times/s). The rolled ore powder was gradually granulated with the addition of water and rolling and grew into pellets ranging from 9.5 mm to 12 mm in diameter, and then the pellets were collected. The pellets that grew from 9.5 mm to 12 mm were rolled on the pan for another 10 minutes to produce generally spherical green pellets. The amount of water added during the above granulation is about 10% of the weight of the green pellets.

[0053] Table 3 lists the evaluation results of the strength (drop strength in Table 3) and bursting property (bursting temperature in Table 3) of the green pellets from Production No. 1 to No. 13. Table 3 reiterates the same Isp as listed in Table 2 for viewing convenience.

TABLE-US-00003 TABLE 3 Bursting Drop Strength Temperature No. Ore Isp (number) ( C.) 1 A 14524 4.6 360 Comp. Ex. 1 2 A 14949 5.1 360 Ex. 1 3 A 148198 6.6 340 Ex. 2 4 A 227586 7.7 320 Ex. 3 5 A 304790 8.5 300 Ex. 4 6 A 526592 15.4 240 Comp. Ex. 2 7 B 13868 4.4 360 Comp. Ex. 3 8 B 14909 5.1 360 Ex. 5 9 B 125044 6.2 340 Ex. 6 10 B 248661 8.1 320 Ex. 7 11 B 391882 12.3 280 Ex. 8 12 B 502482 14.3 260 Ex. 9 13 B 523476 14.7 240 Comp. Ex. 4

[0054] In Table 3, drop strength indicates the strength evaluation results based on the drop test of the green pellets. In this example, the drop test was conducted by dropping the green pellets of each Production No. from a height of 50 cm and measuring the number of times the green pellets cracked or were broken (number of drops), and the greater or lesser number of drops was adopted as the drop strength. In detail, as the value of drop strength is larger, the strength of green pellets is higher. As the value of drop strength is smaller, the strength of green pellets is lower. The number of drops adopted as the drop strength is not counted when the green pellets crack or the green pellets are broken. In other words, drop strength is the number of times the green pellets can be dropped without cracking or breaking. In this example, the average (additive average) of 10 green pellets was used for the drop strength.

[0055] In Table 3, bursting temperature indicates evaluation results pertaining to the bursting property. The bursting temperature indicates the temperature at which bursting occurs when the green pellets are heated gradually to increase the temperature of the green pellets. In detail, when the bursting temperature is low, the bursting property is high. Further, when the bursting temperature is high, the bursting property is low.

[0056] In this example, the bursting temperature is the temperature measured as follows First, 200 g of the green pellets are loaded in a vertical cylindrical furnace (with a diameter of 50 mm). Then, air heated to 200 C. is ventilated as hot blast from below to above the layer of the green pellets for 10 minutes at a wind speed of 1.0 m/sec (converted to 0 C./1 atm). The presence or absence of green pellet bursting is then checked. When bursting or powdering of the green pellets occurs during or at the end of the period when air heated to 200 C. is ventilated into the layer of the green pellets, then the bursting temperature of the green pellets is identified as 200 C. When bursting of green pellets is not observed, the temperature of the hot blast is increased by another 20 C. and ventilated for 10 minutes to check for green pellet bursting. Thereafter, the temperature is increased in the same manner, in 20 C. increments (i.e., the green pellets are heated gradually), and the temperature at which the green pellets burst is identified. After repeated hot blast heating, the temperature at which the green pellets burst is identified as the bursting temperature of the green pellets, as described above.

[0057] In this example, from the viewpoint of whether the green pellets have enough strength to suppress disintegration or powdering during handling such as transportation until the green pellets are made into baked pellets (from the granulation process to the baking process), the green pellets are judged to have good strength when the drop strength is 5 times or more, and poor strength when the drop strength is less than 5 times.

[0058] Whether or not bursting was suppressed was determined by whether the bursting temperature was 250 C. or higher or lower than 250 C. In detail, when the bursting temperature is 250 C. or higher, the bursting property is judged as low and good, and when the bursting temperature is lower than 250 C., the bursting property is judged as high and poor.

[0059] If the bursting property is good but the strength is poor, the comprehensive evaluation is judged as poor. This is because even if the bursting property is good, if the strength is poor, properly baking and use afterwards cannot be achieved. Similarly, if the strength is good but the bursting property is poor, the comprehensive evaluation is also judged as poor. This is because even if the strength is good, if the bursting property is poor, proper use after baking cannot be achieved.

[0060] When the green pellets of each Production No. are evaluated according to the above, the green pellets of No. 1 and 7 are poor in terms of strength. In addition, the green pellets of No. 6 and 13 are poor in terms of bursting property. The green pellets other than the above (Nos. 2-5 and 8-12) have good strength and bursting property. In Tables 2 and 3, in consideration of these evaluations, Nos. 1, 6, 7, and 13 are labeled as Comparative Examples 1 to 4 in this order. Nos. 2-5 and 8-12 are also labeled as Examples 1-9 in this order.

[0061] In the green pellets of Production No. 1 to No. 13, as the particle size distribution index is smaller (the particle size distribution is sharper), the bursting temperature is higher. In other words, for the green pellets of No. 1 to No. 13, as the particle size distribution index is smaller, the bursting property is better.

[0062] For the green pellets of Production No. 1 to No. 13, as the particle size distribution index is larger (the particle size distribution is broader), the drop strength is higher. In other words, for the green pellets of No. 1 to No. 13, as the particle size distribution index is larger, the strength is better.

[0063] Considering the particle size distribution indices of the green pellets of Production Nos. 2-5 and 8-12, which have good strength and bursting property, the particle size distribution index needs to be 14,700 or more and 510,000 or less in order to suppress bursting of green pellets, assuming that the green pellets have good strength.

[0064] With respect to the particle size distribution, if the cumulative 50% diameter is 5.0 m or more and 61.0 m or less, or at least 5.2 m or more and 60.1 m, it can be determined that both strength and bursting property can be achieved. If the cumulative 10% diameter is at least 0.6 m or more and 7.1 m or less, it can be determined that both strength and bursting property can be achieved.

[0065] With respect to the harmonic mean diameter, if the diameter is at least 1.7 m or more and less than 13.0 m, it can be determined that both strength and bursting property can be achieved.

[0066] The method for producing an iron ore pellet can be provided in the above manner.

[0067] The embodiments disclosed herein are examples, and the embodiments of this disclosure are not limited thereto and may be modified as appropriate to the extent not to depart from the scope of this disclosure.

INDUSTRIAL APPLICABILITY

[0068] This disclosure is applicable to a method for producing an iron ore pellet.