Method for producing iron ore pellets

Abstract

Provided is a method for producing iron ore pellets with which it is possible to obtain high-strength green pellets capable of suppressing bursting. The method for producing iron ore pellets according to the present invention is characterized by comprising a step for mixing iron ore having a total Fe content of 63 mass% or less with a binder to obtain a mixture, a step for granulating the mixture to obtain green pellets, and a step for calcining the green pellets to obtain iron ore pellets, the iron ore has a core ore (10) and a fine ore (12), the core ore (10) having a particle size of more than 1 mm, and the fine ore (12) having a particle size of 1 mm or less.

Description

Technical Field

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

[0002] Iron ore pellets refer to pellets made by granulating iron ore powder into a shape suitable for being put into a blast furnace or a solid reduction furnace (e.g., size, strength, reducibility, etc.). For example, as described in Patent Document 1, iron ore pellets are generally manufactured by the following steps: a step of crushing iron ore raw materials to obtain powdered ore, a step of mixing the powdered 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 calcining the green pellets to obtain iron ore pellets. In this specification, the pellets in the granulated state before calcination are referred to as "green pellets".

[0003] In the manufacture of iron ore pellets, ensuring the strength of the green pellets is important in order to suppress the pulverization of the green pellets during the treatment before being placed in the calcining furnace and the adhesion of powder to the calcining furnace. In addition, suppressing the bursting of the green pellets is also important in order to ensure the strength of the iron ore pellets after calcination. Explosion refers to the phenomenon that the green pellets break due to the pressure of the steam generated from the inside when the green pellets are dried and dehydrated. If the bursting occurs, cracks will be generated in the green pellets, so the strength of the iron ore pellets after calcination will be significantly reduced. In particular, if the particle size of the powdered ore after crushing is reduced in order to ensure the strength of the green pellets, the green pellets will be highly dense and prone to bursting. That is, ensuring the strength of the green pellets and suppressing the bursting are difficult issues to take into account at the same time.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Publication No. 2022-34210 Summary of the invention

[0007] In order to ensure the strength of green pellets or to suppress the explosion of green pellets, bentonite or various organic and inorganic binders are added as binders. However, from the perspective of ensuring the strength of green pellets and suppressing explosion, there is room for improvement.

[0008] Therefore, the present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a method for producing iron ore pellets which can obtain green pellets having high strength and capable of suppressing explosion.

[0009] In order to achieve this purpose, the inventors of the present invention conducted in-depth research and obtained the following insights: in addition to iron ore with a particle size of less than 1 mm obtained by crushing iron ore raw materials, iron ore with a particle size of more than 1 mm obtained by not crushing the iron ore raw materials is also used, and they are mixed and granulated to produce green pellets, thereby ensuring the drop strength of the green pellets and suppressing explosion.

[0010] The gist of the present invention completed based on the above findings is as follows.

[0011] [1] A method for producing iron ore pellets, characterized by comprising:

[0012] A step of mixing iron ore having a total Fe content of 63% by mass or less with a binder to obtain a mixture,

[0013] a step of granulating the mixture to obtain green pellets, and

[0014] A step of calcining the green pellets to obtain iron ore pellets;

[0015] The iron ore includes core ore and powder ore. The iron ore has a particle size exceeding 1 mm, and the powder ore has a particle size of 1 mm or less.

[0016] [2] The method for producing iron ore pellets according to [1], wherein the mass ratio of the core ore to the iron ore is 15 mass % or more.

[0017] [3] The method for producing iron ore pellets according to [2], wherein the mass ratio of particles having a particle size exceeding 2.8 mm in the core ore is 15 mass % or more relative to the iron ore.

[0018] [4] The method for producing iron ore pellets according to [2], wherein the mass ratio of particles having a particle size exceeding 2.8 mm in the core ore is 30% by mass or more relative to the iron ore.

[0019] [5] The method for producing iron ore pellets according to [1], wherein the mass ratio of particles having a particle size exceeding 4.8 mm in the core ore is 10 mass % or more relative to the iron ore.

[0020] [6] The method for producing iron ore pellets according to [5], wherein the mass ratio of particles having a particle size exceeding 4.8 mm in the core ore is 25 mass % or more relative to the iron ore.

[0021] [7] The 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 so that the number of the core ore contained in each of the iron ore pellets is 0.9 to 1.0 on average.

[0022] [8] The 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 size of 1 mm, the oversize material is used as the core ore, and the undersize material is crushed and used as the powder ore.

[0023] [9] The method for producing iron ore pellets according to any one of [1] to [8], wherein in the mixture, the mass of the powder ore is set to W1, the mass of the binder is set to W2, and W2 / W1×100 is greater than 1.0.

[0024]

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

[0025] According to the method for producing iron ore pellets of the present invention, green pellets having high strength and capable of suppressing explosion can be obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a diagram schematically showing a cross section of an iron ore pellet obtained according to an embodiment of the present invention.

[0027] Figure 2 It is a diagram schematically showing the growth process in the granulation step of green pellets in the embodiment of the present invention and the comparative example.

[0028] Figure 3 This is a diagram schematically showing the outline of an electric furnace used for measuring the burst temperature. DETAILED DESCRIPTION

[0029] Hereinafter, an embodiment of the method for producing iron ore pellets of the present invention will be described. It should be noted that the embodiment described below is an example of specific embodiment of the present invention, and the configuration of the present invention is not limited to this specific example.

[0030] The method for producing iron ore pellets according to one embodiment of the present invention is characterized by comprising: a step of mixing an iron ore having a total Fe content of 63% or less with a binder to obtain a mixture, a step of granulating the mixture to obtain green pellets, and a step of calcining the green pellets to obtain iron ore pellets. The iron ore used to obtain the mixture includes core ore and powder ore, the core ore having a particle size of more than 1 mm, and the powder ore having a particle size of 1 mm or less.

[0031] In this specification, iron ore particles having a particle size exceeding 1 mm obtained without pulverizing the 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 the iron ore raw material are referred to as "powder ore".

[0032] In the present embodiment, as the iron ore constituting the green pellets, core ore is used in addition to powder ore. Since the green pellets contain high-strength core ore, the strength of the green pellets can be ensured. In addition, compared with the prior art in which the iron ore constituting the green pellets is composed only of powder ore, in the present embodiment, since the green pellets contain core ore with a large particle size, the speed of drying and de-crystallization water can be reduced. By including the core ore, the porosity of the green pellets becomes low and dense, so it is difficult to carry moisture. In addition, since there are few gas passages, it is not easy to generate steam from the inside of the green pellets. As a result, explosion can be fully suppressed.

[0033] In the present invention, the "particle size" of iron ore corresponds to the nominal mesh size of the sieve according to JIS Z 8801: 2019. That is, iron ore having a particle size exceeding X mm refers to iron ore remaining on the sieve when sieved with a sieve having a nominal mesh size of X mm. Iron ore having a particle size of Y mm or less refers to iron ore that passes through the sieve and becomes undersize when sieved with a sieve having a nominal mesh size of Y mm.

[0034] The type and characteristics of the core ore are not particularly limited as long as it is an iron ore with a total Fe content of 63% or less and a particle size exceeding 1 mm. In addition, the core ore preferably has a particle size of 9.5 mm or less. If the particle size is 9.5 mm or less, the size of the finished iron ore pellets is appropriate, and subsequent reduction treatment can be uniformly performed.

[0035] The mass ratio of the core ore is preferably 15% by mass or more relative to the total iron ore. This is because the more core ore that is larger in volume than the normally crushed iron ore powder, the more effective the binder such as bentonite can be, and the strength improvement effect and the crack suppression effect can be appropriately obtained.

[0036] Similarly, from the viewpoint of more fully obtaining the effect of the present invention, it is more preferred that the mass ratio of particles having a particle size of more than 2.8 mm in the nuclear ore is 15% by mass or more relative to the total iron ore, and further preferably 30% by mass or more. At this time, the mass ratio of the nuclear ore having a particle size of more than 1 mm and less than 2.8 mm is not particularly limited and may be 0% by mass.

[0037] Similarly, from the viewpoint of more fully obtaining the effect of the present invention, it is preferred that the mass ratio of particles having a particle size of more than 4.8 mm in the core ore is 10% by mass or more relative to the total iron ore, and more preferably 25% by mass or more. At this time, the mass ratio of the core ore having a particle size of more than 1 mm and less than 4.8 mm is not particularly limited and may be 0% by mass.

[0038] The mass ratio of the core ore is preferably 99 mass % or less, more preferably 75 mass % or less, relative to the total iron ore. In general iron ore raw materials, the mass ratio of particles having a particle size exceeding 1 mm is about 75 mass % at most.

[0039] Powdered ore can be obtained by pulverizing iron ore using a general ball mill or the like. The average particle size of the powdered ore is preferably about several tens of μm. The Blaine index of the powdered ore is preferably 2000 to 4000 cm 2 / g, more preferably 2500 to 3500 cm 2 / g. If the Blaine index is 2000cm 2 / g or more, the powder making efficiency is more appropriate. If the Blaine index is 4000cm 2 / g or less, the shrinkage caused by sintering during calcination is suppressed, and the strength is more suitable. It should be noted that the Blaine index is a value measured using the Blaine air permeation device specified in JIS R 5201: 2015, which represents the specific surface area of ​​the powder. In the pelletizing process, the Blaine index is used as a management index for ore particle size, and the higher the value, the more fine powder it becomes.

[0040] The production method of the core ore and the powder ore is not particularly specified, but it is preferred that the iron ore raw material is sieved with a sieve having a mesh size of 1 mm, the oversize is used as the core ore, and the undersize is crushed and used as the powder ore. Alternatively, each may be prepared separately from the iron ore raw material.

[0041] like Figure 1 As shown, each iron ore pellet obtained in this embodiment preferably contains one core ore 10, and the powder ore 12 is attached to the surface of the core ore 10. It is also preferable to do the same at the stage of green pellets. When the green pellets and the iron ore pellets contain one core ore, appropriate strength can be obtained.

[0042] The inventors of the present invention consider the following reason why high strength can be obtained when the core ore contained in the green pellets and the iron ore pellets is one. Figure 2A schematic diagram (cross-sectional view) showing the growth process in the granulation step of green pellets. As shown in the example of the present invention, it is believed that when the number of core ores contained in one pellet is one, layered granulation occurs in which powder ore 12 and a binder are attached to the surface of the core ore 10 in layers. It is believed that when the green pellet does not contain a core ore, a core 14 of a powder agglomerate based on the powder ore is formed instead of the core ore, and the powder ore covers the core 14 to perform granulation, but since the binder is dispersed in the green pellet, the strength is reduced. In addition, when there are more than one core ores contained in the green pellet, a layer of powder ore is required to wrap a plurality of core ores at a time during the growth process, and the particles grow rapidly, so the time for the coated layer to become dense is insufficient, and the strength is lowered compared to the case where there is only one core ore.

[0043] Therefore, in the present embodiment, it is preferred to set the particle size and mass ratio of the core ore in such a way that the number of core ores contained in each iron ore pellet is 0.9 to 1.0 on average. The weight ratio G of the core ore can be calculated from the sizes of the core ore and the green pellet as follows. It is considered that the weight ratio G of the core ore in the green pellet is equal to the effective volume ratio of the core ore in the green pellet (the volume ratio obtained by removing the pore part of the agglomerate of the powder ore). Let the volume ratio of the core ore to the volume of the green pellet (the "pile volume" of the pellet including the pores of the agglomerate part of the powder ore) be T, let the porosity of the agglomerate of the powder ore be K, and the weight ratio G can be expressed as the following formula (1).

[0044] G=T / {(1-T)*(1-K)+T}…(1)

[0045] Here, "(1-T)*(1-K)" is the volume ratio obtained by removing the pores of the aggregate of the powder ore of the green pellet. By adjusting the weight ratio of the core ore and the particle size of the core ore in a manner that satisfies formula (1), it is possible to make the green pellet contain an average of one core ore. It should be noted that the porosity K of the green pellet is generally determined by the crushing particle size of the powder ore, and for example, 0.33 can be used.

[0046] As the binder used in granulation, bentonite is preferred, but known or arbitrary binders such as organic and inorganic binders that can achieve the same effect can also be used. In addition, in the mixing process, in addition to iron ore and binders, limestone or dolomite can also be mixed as auxiliary raw materials. In addition, as auxiliary raw materials, various reducing agents, additives, etc. can be added according to the type of furnace used in the reduction treatment after calcination. Specifically, as a reducing agent, there can be carbon materials such as coal and coke.

[0047] As the amount of the binder, in the mixture, the mass of the powder ore is denoted as W1, and the mass of the binder is denoted as W2, and the amount satisfying W2 / W1×100≥1.0 is preferred.

[0048] If it is within the above range, the binder amount is appropriate for the powder ore, the binder effect can be appropriately obtained, and the strength can be ensured. The above range is more preferably 1.4 or more, and more preferably 1.6 or more. The more the amount of binder, the easier it is to ensure the strength of the pellets, but since the purity of the reduced iron decreases, it is preferably within the range of 3.0 or less.

[0049] Iron ore pellets are produced by general crushing, mixing, granulation, and calcination steps. The crushing step can be performed using a general crusher such as a ball mill, and is performed only on iron ore used as powder ore. The mixing step can be performed using a general concrete mixer, etc. The granulation step can be performed using a general granulator or drum mixer, etc. The calcination step can be performed using a general vertical furnace or rotary kiln, etc.

[0050] The size of the granulated green pellets is preferably about 9.5 to 12 mm. If the size of the green pellets is less than 9.5 mm, the air permeability when filling the blast furnace as calcined pellets is poor. If the size of the green pellets exceeds 12 mm, the reducibility is reduced.

[0051] Example

[0052] After the iron ore is dried at 105°C for 24 hours, it is screened using a Ro-tap sieve with mesh sizes of 1.0 mm, 2.8 mm, 4.8 mm, 6.7 mm, 8.0 mm, and 9.5 mm, and the iron ore on the sieve is used as the core ore. Table 1 shows the composition of the iron ore used as the raw material. It should be noted that T.Fe is the total iron content in the iron ore, and LOI (Loss on Ignition) is the strong heat loss during the measurement. After the iron ore with the same composition as above is dried at 105°C for 24 hours, the iron ore powder obtained by crushing it with a ball mill is used as powder ore. The total amount of powder ore passes through a sieve with a mesh size of 1.0 mm, that is, it has a particle size of less than 1 mm. The Blaine index of the powder ore is 2560 cm 2 / g.

[0053] [Table 1]

[0054] Table 1 (mass %)

[0055] T.Fe <![CDATA[SiO2]]> <![CDATA[Al2O3]]> CaO MgO LOI 61.5 3.7 2.2 0.07 0.09 5.4

[0056] The core ore and powder ore were prepared in a mass ratio shown in Table 2 in a total of 5000 g, and mixed with a predetermined ratio of bentonite using a concrete mixer at 20 rpm for 3 minutes. The amount of bentonite added to the total iron ore (the total of the core ore and powder ore) (mass %) is shown in the column "bentonite ratio" of Table 2. Next, the mixed raw materials were placed in A granulator is used to perform granulation while adding water. Pellets of 9.5 to 12 mm are collected and further rotated in a granulator for 10 minutes to obtain green pellets. It should be noted that in the invention example, the amount of core ore and powder ore is adjusted in such a way that the number of core ore contained in one green pellet is 1.0 on average. The volume ratio of core ore to green pellets and the number of core ore per green pellet are shown in Table 2. It should be noted that the porosity is 33%.

[0057] [Table 2]

[0058]

[0059] [Drop strength measurement]

[0060] In each of the invention examples and comparative examples, the drop strength of 10 green pellets was measured assuming the actual operation of conveying, charging, etc. The operation of dropping the green pellets from a height of 50 cm was repeated, and the operation was considered to be completed after cracks or damage of the green pellets were confirmed. The number of times until the end (i.e., the number of times cracks or damage were confirmed) was taken as the drop strength, and the average drop strength of 10 pellets is shown in Table 2.

[0061] [Determination of burst temperature]

[0062] In each of the inventive examples and comparative examples, the temperature at which the green pellets exploded in the furnace (hereinafter referred to as the explosion temperature) was measured. Figure 3 The outline of the electric furnace used in this embodiment is schematically shown. A green pellet filling basket 32 ​​filled with 200 g of green pellets is placed in an electric furnace 30, and hot air (air) at 200°C (measured by a thermocouple 34) flows out from a heating gas source 36 at a flow rate of 1.2 m / sec and is maintained for 10 minutes. After maintaining, take out the sample and check whether it is cracked. If no crack is confirmed, increase the temperature of the hot air by 40°C increments, put a new sample into the furnace and repeat the same test. The temperature when the sample is confirmed to be cracked is taken as the burst temperature, and the results are shown in Table 2.

[0063] By comparing the invention examples and comparative examples at the same bentonite ratio with reference to Table 2, it can be confirmed that the drop strength of the invention example having the core ore becomes higher and the burst temperature also becomes higher, and the effect of the invention is obvious. In addition, it can be confirmed that by increasing the ratio of the core ore with a high particle size or increasing the bentonite ratio, the strength and burst temperature are improved, and it can be seen that more suitable manufacturing conditions are obtained.

[0064] Industrial Applicability

[0065] According to the present invention, there can be provided a method for producing iron ore pellets, which can obtain green pellets having high strength and capable of suppressing explosion.

[0066] Explanation of symbols

[0067] 10 Nuclear ores with a particle size exceeding 1 mm

[0068] 12 Powdered ore with a particle size of less than 1 mm

[0069] 14 Core based on agglomerates of powdered ore with a particle size of less than 1 mm

[0070] 30 Electric furnace

[0071] 32 green pellets fill the basket

[0072] 34 Thermocouple

[0073] 36 Heating gas source

Claims

1. A method for producing iron ore pellets, characterized in that: have: A step of mixing iron ore having a total Fe content of 63% by mass or less with a binder to obtain a mixture, a step of granulating the mixture to obtain green pellets, and A step of calcining the green pellets to obtain iron ore pellets; The iron ore includes core ore having a particle size exceeding 1 mm and 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 nuclear ore to the iron ore is 15 mass % or more.

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

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

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

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

7. The 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 so that the number of the core ore contained in one of the iron ore pellets is 0.9 to 1.0 on average.

8. The method for producing iron ore pellets according to any one of claims 1 to 7, wherein: The iron ore raw material was sieved with a sieve having a mesh size of 1 mm, the oversize was used as the core ore, and the undersize was crushed and used as the powder ore.

9. The method for producing iron ore pellets according to any one of claims 1 to 8, wherein: In the mixture, the mass of the powder ore is denoted as W1, the mass of the binder is denoted as W2, and W2 / W1×100 is set to be 1.0 or more.

10. The method for producing iron ore pellets according to any one of claims 1 to 9, wherein: The binder is bentonite.

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