Method for manufacturing iron ore pellets

Abstract

Provided is a method for producing an iron ore pellet, whereby it is possible to obtain a high-strength green pellet in which bursting can be suppressed. This method for producing an iron ore pellet is characterized by having a step for mixing a binder and iron ore having a total Fe content of 63% by mass to obtain a mixture, a step for granulating the mixture to obtain a green pellet, and a step for firing the green pellet to obtain an iron ore pellet, the iron ore having a core ore 10 having a grain size of more than 1 mm, and a fine ore 12 having a grain size of 1 mm or less.

Description

Technical Field

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

Background Art

[0002] Iron ore pellets are those obtained by granulating iron ore powder into a shape suitable for charging into a blast furnace or a solid reduction furnace (such as size, strength, reducibility, etc.). For example, as described in Patent Document 1, iron ore pellets generally include a step of pulverizing an iron ore raw material to obtain pulverized ore, a step of mixing the pulverized ore, a binder, and any auxiliary raw materials to obtain a mixture, a step of granulating the mixture to obtain green pellets, and a step of firing the green pellets to obtain iron ore pellets. In this specification, pellets before firing while still in the granulated state are referred to as "green pellets".

[0003] In the production of iron ore pellets, ensuring the strength of green pellets is important for suppressing the pulverization of green pellets during handling before charging into the firing furnace and the adhesion of powder to the inside of the firing furnace. Also, suppressing the bursting of green pellets is important for ensuring the strength of iron ore pellets after firing. Bursting is a phenomenon in which green pellets burst due to the pressure of steam generated inside when green pellets dry and lose crystal water. When bursting occurs, cracks form in the green pellets, resulting in a significant decrease in the strength of iron ore pellets after firing. In particular, in order to ensure the strength of green pellets, when the particle size of the pulverized ore is reduced, the green pellets become denser and bursting is likely to occur. That is, ensuring the strength of green pellets and suppressing bursting have been difficult problems to reconcile.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

[0005] To ensure the strength of green pellets or to suppress bursting, bentonite or various organic and inorganic binders are sometimes added as binders. However, there was room for improvement in terms of ensuring the strength of green pellets and suppressing bursting.

[0006] Therefore, in view of the above problems, the present invention aims to provide a method for producing iron ore pellets that are high in strength and can suppress bursting, thereby yielding green pellets. [Means for solving the problem]

[0007] To achieve this objective, the inventors conducted diligent research and found that by using iron ore with a particle size of 1 mm or less obtained by crushing iron ore raw materials, as well as iron ore with a particle size of more than 1 mm obtained without crushing iron ore raw materials, and mixing and granulating these to produce green pellets, the drop strength of the green pellets can be ensured and bursting can be suppressed.

[0008] Based on the above findings, the gist of the present invention is as follows.

[0009] [1] A step of mixing iron ore with a binder to obtain a mixture, with a total Fe content of 63% by mass or less. The process of granulating the above mixture to obtain green pellets, The process involves firing the aforementioned green pellets to obtain iron ore pellets, It has, A method for producing iron ore pellets, characterized in that the iron ore comprises a core ore having a particle size greater than 1 mm and a powder ore having a particle size of 1 mm or less.

[0010] [2] The method for producing iron ore pellets according to [1], wherein the mass ratio of the core ore is 15% by mass or more relative to the iron ore.

[0011] [3] The method for producing iron ore pellets according to [2], wherein the mass ratio of particles having a particle size of 2.8 mm or more among the core ore is 15% by mass or more with respect to the iron ore.

[0012] [4] The method for producing iron ore pellets according to [2], wherein the mass ratio of particles having a particle size of 2.8 mm or more among the core ore is 30% by mass or more with respect to the iron ore.

[0013] [5] The method for producing iron ore pellets according to [1], wherein the mass ratio of particles having a particle size of 4.8 mm or more among the core ore is 10% by mass or more with respect to the iron ore.

[0014] [6] The method for producing iron ore pellets according to [5], wherein the mass ratio of particles having a particle size of 4.8 mm or more among the core ore is 25% by mass or more with respect to the iron ore.

[0015] [7] A method for producing iron ore pellets according to any one of [1] to [6], wherein the particle size and mass ratio of the core ore are set such that the number of core ore contained in each iron ore pellet is an average of 0.9 to 1.0.

[0016] [8] A method for producing iron ore pellets according to any one of [1] to [7], wherein the iron ore raw material is sieved with a sieve having a mesh opening of 1 mm, the portion on the sieve is used as the core ore, and the portion below the sieve is crushed to be used as the powdered ore.

[0017] [9] A method for producing iron ore pellets according to any one of [1] to [8], wherein the mass of the powdered ore in the mixture is W1, the mass of the binder is W2, and W2 / W1 × 100 is 1.0 or more.

[0018] The method for producing an iron ore pellet according to any one of [1] to [9], wherein the binder is bentonite.

Advantages of the Invention

[0019] According to the method for producing an iron ore pellet of the present invention, it is possible to obtain a green pellet having high strength and capable of suppressing bursting.

Brief Description of the Drawings

[0020] [Figure 1] It is a diagram schematically showing a cross section of an iron ore pellet obtained according to an embodiment of the present invention. [Figure 2] It is a diagram schematically showing the growth process in the granulation process of a green pellet in an embodiment of the present invention and a comparative example. [Figure 3] It is a diagram schematically showing an outline of an electric furnace used for measuring the bursting temperature.

Modes for Carrying Out the Invention

[0021] Hereinafter, embodiments of the method for producing an iron ore pellet according to the present invention will be described. The embodiments described below are an example of embodying the present invention, and do not limit the configuration of the present invention with the specific examples.

[0022] The method for producing an iron ore pellet according to an embodiment of the present invention includes a step of mixing an iron ore having a total Fe content of 63% by mass or less and a binder to obtain a mixture, a step of granulating the mixture to obtain a green pellet, and a step of firing the green pellet to obtain an iron ore pellet. The iron ore for obtaining the mixture is characterized by having core ore having a particle size exceeding 1 mm and fine ore having a particle size of 1 mm or less.

[0023] In the present specification, iron ore particles having a particle size exceeding 1 mm obtained without pulverizing an iron ore raw material are referred to as "core ore", and iron ore particles having a particle size of 1 mm or less obtained by pulverizing an iron ore raw material are referred to as "fine ore".

[0024] In this embodiment, core ore is used in addition to powdered iron ore as the iron ore constituting the green pellets. By including high-strength core ore in the green pellets, the strength of the green pellets can be ensured. Furthermore, compared to conventional technology in which the iron ore constituting the green pellets consists only of powdered iron ore, in this embodiment, the green pellets contain larger-grained core ore, which can reduce the rate of drying and decrystallization water. The inclusion of core ore reduces the porosity of the green pellets, making them denser and less likely to hold moisture. In addition, because there are fewer gas passages, steam is less likely to be generated from inside the green pellets. As a result, bursting can be sufficiently suppressed.

[0025] In this invention, the "grain size" of iron ore corresponds to the nominal mesh size of a sieve in accordance with JIS Z 8801:2019. That is, iron ore with a grain size greater than X mm is the iron ore that remains on the sieve when sieved with a sieve with a nominal mesh size of X mm. Iron ore with a grain size of Y mm or less is the iron ore that passes through the sieve and remains below the sieve when sieved with a sieve with a nominal mesh size of Y mm.

[0026] The core ore is not particularly limited in type or characteristics, as long as it is iron ore with a particle size greater than 1 mm and a total Fe content of 63% by mass or less. Furthermore, it is preferable that the core ore has a particle size of 9.5 mm or less. A particle size of 9.5 mm or less results in a suitable size for the finished iron ore pellets, allowing for uniform subsequent reduction treatments.

[0027] The mass ratio of core ore is preferably 15% by mass or more relative to the total iron ore. This is because the larger the volume of core ore compared to ordinary crushed iron ore powder, the more effective the binder such as bentonite can be even with a small amount of binder, resulting in improved strength and suppression of bursting.

[0028] Similarly, from the viewpoint of obtaining the effects of the present invention to the fullest extent, it is more preferable that the mass ratio of particles having a particle size greater than 2.8 mm in the core ore is 15% by mass or more, and even more preferable that it is 30% by mass or more, relative to the total iron ore. In this case, there are no particular restrictions on the mass ratio of core ore with a particle size greater than 1 mm and less than or equal to 2.8 mm, and it may be 0% by mass.

[0029] Similarly, from the viewpoint of obtaining the effects of the present invention to the fullest extent, it is preferable that the mass ratio of particles having a particle size greater than 4.8 mm in the core ore is 10% by mass or more, and more preferably 25% by mass or more, relative to the total iron ore. In this case, there are no particular restrictions on the mass ratio of core ore particles with a particle size greater than 1 mm and less than or equal to 4.8 mm, and it may be 0% by mass.

[0030] Furthermore, the mass ratio of core ore is preferably 99% by mass or less, and more preferably 75% by mass or less, relative to the total iron ore. In general iron ore raw materials, the mass ratio of particles with a particle size exceeding 1 mm is at most about 75% by mass.

[0031] Iron ore powder is obtained by crushing iron ore using a general-purpose ball mill or similar device. The average particle size of the iron ore powder is preferably around several tens of micrometers. The Blaine index of the iron ore powder is 2000-4000 cm². 2 It is preferable that the amount is around / g, and the range is 2500-3500 cm. 2 A value of approximately / g is preferable. A Brain index of 2000 cm 2 If the amount is greater than or equal to / g, the efficiency of powder milling is more favorable. A Blaine index of 4000 cm² is desirable. 2 When the value is less than / g, shrinkage during sintering is suppressed, resulting in more favorable strength. The Blaine index is measured using a Blaine air permeation device specified in JIS R 5201:2015 and represents the specific surface area of ​​the powder. In the pellet manufacturing process, the Blaine index is used as a control index for ore particle size, and a higher value means that the powder is finer.

[0032] There are no specific requirements for the manufacturing method of core ore and powdered ore, but it is preferable to sieve the iron ore raw material with a sieve having a mesh size of 1 mm, use the top of the sieve as core ore, and crush the bottom of the sieve to use as powdered ore. Alternatively, each may be prepared separately from the iron ore raw material.

[0033] As shown in Figure 1, it is preferable that each iron ore pellet obtained in this embodiment contains one core ore 10, with powdered ore 12 adhering to the surface of the core ore 10. The same is preferable for the green pellet stage. When both the green pellet and the iron ore pellet contain one core ore, suitable strength can be obtained.

[0034] The inventors believe that the reason why high strength can be obtained when the core ore in green pellets and iron ore pellets is reduced to one is as follows. Figure 2 shows a schematic diagram (cross-sectional view) of the growth process in the granulation process of green pellets. As in the present invention example, when the number of core ore in one pellet is reduced to one, it is thought that layering granulation occurs in which powdered ore 12 and binders are attached in layers to the surface of the core ore 10. When green pellets do not contain core ore, it is thought that a nucleus 14 of powdered ore aggregates is formed instead of core ore, and that granulation proceeds as the powdered ore covers it, but the binder is dispersed within the green pellet, resulting in a decrease in strength. Furthermore, when there is more than one core ore in the green pellet, the powdered ore layer needs to enclose multiple core ore at once during the growth process, and the particles grow rapidly, so there is insufficient time for the covering layer to become dense, resulting in lower strength than when there is one core ore.

[0035] Therefore, in this embodiment, it is preferable to set the particle size and mass ratio of the core ore so that the average number of core ore particles contained in one iron ore pellet is 0.9 to 1.0. The weight percentage G of the core ore can be calculated from the size of the core ore and the green pellet as follows. The weight percentage G of the core ore in the green pellet is considered to be equal to the effective volume percentage of the core ore in the green pellet (volume ratio excluding the pore portion in the aggregate of powdered ore). Let T be the volume ratio of the core ore to the volume of the green pellet (the "bulk volume" of the pellet, including the pores in the aggregate portion of the powdered ore), and K be the porosity in the aggregate of powdered ore. The weight percentage G can be expressed as shown in the following formula (1). G = T / {(1-T)*(1-K)+T} ... (1) Here, "(1-T)*(1-K)" is the volume ratio of the aggregates of powdered ore in the green pellet, excluding the porosity. By adjusting the weight ratio of core ore and the particle size of the core ore to satisfy equation (1), it is possible to ensure that each green pellet contains an average of one core ore particle. The porosity K of the green pellet is generally determined by the particle size of the powdered ore, and for example, 0.33 can be used.

[0036] Bentonite is preferred as the binder used during granulation, but any known or arbitrary binder, such as organic or inorganic binders that produce similar effects, may be used. In the mixing process, in addition to iron ore and binder, limestone, dolomite, etc., may be mixed in as auxiliary materials. Furthermore, various reducing agents and additives may be added as auxiliary materials, depending on the type of furnace used for the reduction treatment after calcination. Specifically, carbon materials such as coal and coke may be used as reducing agents.

[0037] As for the amount of binder, in the mixture, let W1 be the mass of the powdered ore and W2 be the mass of the binder. W2 / W1 × 100 ≥ 1.0 It is preferable that the amount satisfies the above conditions. Within the above range, the amount of binder is suitable for the powdered ore, the effect of the binder is suitably obtained, and strength can be ensured. The above range is more preferably 1.4 or higher, and even more preferably 1.6 or higher. The more binder there is, the easier it is to ensure pellet strength, but the purity of reduced iron decreases, so it is preferable that the amount be in the range of 3.0 or lower.

[0038] Iron ore pellets are manufactured through general crushing, mixing, granulation, and calcination processes. The crushing process may be carried out using a general crushing machine such as a ball mill, and is performed only on iron ore intended for use as powder. The mixing process may be carried out using a general concrete mixer, etc. The granulation process may be carried out using a general pelletizer or drum mixer, etc. The calcination process may be carried out using a general shaft furnace or rotary kiln, etc.

[0039] The granulated green pellets are preferably about 9.5 to 12 mm in size. If the green pellets are smaller than 9.5 mm, the permeability will be poor when they are filled into the blast furnace as calcined pellets. If the green pellets are larger than 12 mm, their reducibility will decrease. [Examples]

[0040] Iron ore was dried at 105°C for 24 hours, then sieved using a rotap with mesh sizes of 1.0 mm, 2.8 mm, 4.8 mm, 6.7 mm, 8.0 mm, and 9.5 mm. The iron ore on these sieves was used as the core ore. Table 1 shows the composition of the iron ore used as raw material. T.Fe represents the total iron content in the iron ore, and LOI (Loss on Ignition) is the loss on ignition during measurement. Iron ore of the same composition as above was similarly dried at 105°C for 24 hours, and then crushed in a ball mill to obtain the iron ore powder. The iron ore powder passed completely through a sieve with a mesh size of 1.0 mm, meaning it had a particle size of 1 mm or less. The Blaine index of the iron ore powder was 2560 cm². 2 It was / g.

[0041] [Table 1]

[0042] The core ore and powdered ore were prepared in the mass ratios shown in Table 2, totaling 5000g. This mixture was then combined with a predetermined proportion of bentonite and mixed using a concrete mixer at 20rpm for 3 minutes. The amount of bentonite added (mass%) relative to the total iron ore (sum of core ore and powdered ore) is shown in the "Bentonite Ratio" column of Table 2. Next, the mixed raw materials were placed in a 1.2mφ pelletizer and granulation was carried out while adding water. Pellet particles of 9.5-12mm were collected and further tumbled in the pelletizer for 10 minutes to obtain green pellets. In this invention example, the proportions of core ore and powdered ore were adjusted so that the average number of core ore particles in each green pellet was 1.0. The volume ratio of core ore to green pellets and the number of core ore particles per green pellet are shown in Table 2. The porosity was set to 33%.

[0043] [Table 2]

[0044] [Drop Strength Measurement] In each example and comparative example, the drop strength of 10 green pellets was measured, simulating the transport and loading processes in actual operation. The green pellets were repeatedly dropped from a height of 50 cm, and the process was terminated when cracks or breakage were observed in the green pellets. The drop strength was defined as the number of attempts before termination (i.e., the attempt in which cracks or breakage were observed), and the average drop strength of the 10 pellets is shown in Table 2.

[0045] [Bursing temperature measurement] In each example and comparative example, the temperature at which the green pellets burst in the furnace (hereinafter referred to as the bursting temperature) was measured. Figure 3 schematically shows the electric furnace used in this example. A green pellet filling basket 32 ​​filled with 200g of green pellets was placed in the electric furnace 30, and hot air (air) at 200°C (measured with a thermocouple 34) was flowed from a heating gas source 36 at a flow rate of 1.2 m / sec and held for 10 minutes. After holding, the sample was removed and checked for bursting. If no bursting was observed, the temperature of the hot air was increased in 40°C increments, and a new sample was placed in the furnace and the same test was repeated. The temperature at which the sample bursting was observed was defined as the bursting temperature, and the results are shown in Table 2.

[0046] Referring to Table 2, a comparison of the inventive example and the comparative example with the same bentonite ratio shows that the inventive example, which contains core ore, has higher drop strength and a higher bursting temperature, clearly demonstrating the effectiveness of the invention. Furthermore, it was confirmed that increasing the proportion of high-grain core ore or increasing the bentonite ratio improves strength and bursting temperature, indicating that more favorable manufacturing conditions can be achieved. [Industrial applicability]

[0047] According to the present invention, it is possible to provide a method for producing iron ore pellets that are high in strength and can suppress bursting, resulting in green pellets. [Explanation of symbols]

[0048] Core ore with a particle size exceeding 10 mm 12. Powdered ore with a particle size of 1 mm or less 14. Core of aggregates made of powdered ore with a particle size of 1 mm or less 30 Electric Furnaces 32 Green Pellet Filling Baskets 34 Thermocouples 36. Heating gas source

Claims

1. A process of mixing iron ore with a binder to obtain a mixture, wherein the total Fe content is 63% by mass or less. The process involves granulating the aforementioned mixture to obtain green pellets that are within the size range of 9.5 to 12 mm, A process of firing the aforementioned green pellets to convert them directly into iron ore pellets, It has, A method for producing iron ore pellets, characterized in that the iron ore comprises a core ore having a particle size of more than 1 mm and a powder ore having a particle size of 1 mm or less.

2. The method for producing iron ore pellets according to claim 1, wherein the mass ratio of the core ore is 15% by mass or more relative to the iron ore.

3. The method for producing iron ore pellets according to claim 2, wherein the mass ratio of particles having a particle size of 2.8 mm or more among the core ore is 15% by mass or more relative to the iron ore.

4. The method for producing iron ore pellets according to claim 2, wherein the mass ratio of particles having a particle size of 2.8 mm or more among the core ore is 30% by mass or more relative to the iron ore.

5. The method for producing iron ore pellets according to claim 1, wherein the mass ratio of particles having a particle size greater than 4.8 mm among the core ore is 10% by mass or more relative to the iron ore.

6. The method for producing iron ore pellets according to claim 5, wherein the mass ratio of particles having a particle size of 4.8 mm or more among the core ore is 25% by mass or more relative to the iron ore.

7. A method for producing iron ore pellets according to any one of claims 1 to 6, wherein the particle size and mass ratio of the core ore are set such that the number of core ore particles contained in each iron ore pellet is, on average, 0.9 to 1.

0.

8. A method for producing iron ore pellets according to any one of claims 1 to 6, comprising sieving iron ore raw material with a sieve having a mesh opening of 1 mm, using the sieved portion as the core ore, and crushing the sieved portion to use as the powdered ore.

9. A method for producing iron ore pellets according to any one of claims 1 to 6, wherein in the mixture, the mass of the powdered ore is W1, the mass of the binder is W2, and W2 / W1 × 100 is 1.0 or more.

10. A method for producing iron ore pellets according to any one of claims 1 to 6, wherein the binder is bentonite.

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