Method for manufacturing iron ore pellets

Abstract

Provided is an iron ore pellet production method, whereby high-strength iron ore pellets can be obtained, and it is possible to contribute to carbon neutrality. This iron ore pellet production method comprises: a mixing step for mixing iron ore having a total Fe content of 63 mass% or less, solid carbon, a binder, and auxiliary raw materials to obtain a mixture; a granulation step for granulating the mixture to obtain green pellets; and a firing step for firing the green pellets by heating the green pellets from the inside by burning the solid carbon while heating the green pellets from the outside by burning CH4 gas, to obtain iron ore pellets, wherein the solid carbon includes carbon generated from at least one selected from the group consisting of CO2 gas, CO gas, and CH4 gas.

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 form suitable for charging into a blast furnace or a solid reduction furnace (e.g., size, strength, reducibility, etc.). Iron ore pellets are generally produced by a process of mixing iron ore powder, a binder, and an optional auxiliary raw material to obtain a mixture, a process of granulating the mixture to obtain green pellets, and a firing process 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". Here, as described in Non-Patent Document 1, it is known to add a carbonaceous material such as anthracite to the mixture. In the firing process, natural gas mainly composed of CH4 gas is burned to heat the green pellets, but the heat generated by this combustion is transmitted from the surface to the inside of the green pellets. Therefore, the heating inside the pellets becomes insufficient, which may lead to a decrease in strength. Therefore, for heat compensation inside the pellets, a carbonaceous material such as anthracite is added to the green pellets, and by burning the carbonaceous material in the firing process, the green pellets are heated from the inside as well.

Prior Art Documents

Non-Patent Documents

[0003]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Ensuring the strength of green pellets, and consequently the strength of iron ore pellets, is crucial to prevent the green pellets from pulverizing during handling before being placed in the firing furnace and the resulting powder adhering to the inside of the furnace. Therefore, continuous improvement is required.

[0005] Furthermore, in response to recent public opinion regarding the reduction of CO2 emissions, carbon-neutral steelmaking methods are desired. In the production of iron ore pellets, the combustion of CH4 gas and carbon materials during the calcination process is the main source of CO2 emissions. As a method of producing iron ore pellets without CO2 emissions, it has been proposed to use organic and inorganic binders to produce uncalcined green pellets as iron ore pellets. However, uncalcined green pellets have the following quality issues. When organic binders are heated to high temperatures, carbonization or a gasification reaction with CO2 accompanying carbonization (C + CO2 → 2CO) occurs, making it impossible for them to exist as binders, and it is anticipated that it will be difficult to bring out the binder properties when organic binders are used at high temperatures. Inorganic binders include silicates (sodium silicate, calcium silicate), but silica itself needs to be separated as slag from pig iron or molten steel in either the pig ironmaking or steelmaking process, and excess heat is required to melt the pig iron. Furthermore, the inorganic binder is discharged as slag, i.e., excess material, which presents another challenge. In addition, the sodium in sodium silicate is solid at low temperatures but vaporizes below 1000°C, making it undesirable as a substance that accumulates in the blast furnace. There are currently no known carbon-neutral technologies for manufacturing iron ore pellets through a calcination process.

[0006] Therefore, in view of the above problems, the present invention aims to provide a method for producing iron ore pellets that can obtain high-strength iron ore pellets and contribute to carbon neutrality. [Means for solving the problem]

[0007] To achieve this objective, the inventors focused on improving the carbon material added to green pellets. Specifically, they conceived of using solid carbon containing carbon produced from one or more gases selected from the group consisting of CO2 gas, CO gas, and CH4 gas as the carbon material added to green pellets. In this case, although CO2 is generated by the combustion of the carbon material, the above greenhouse gases are consumed as raw materials for the carbon material, thus contributing to carbon neutrality. Furthermore, the inventors' research revealed that using solid carbon (carbon material) containing carbon produced from one or more gases selected from the group consisting of CO2 gas, CO gas, and CH4 gas unexpectedly increases the strength of iron ore pellets.

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

[0009] [1] A mixing step to obtain a mixture by mixing iron ore with a total Fe content of 63% by mass or less, solid carbon, binder, and auxiliary raw materials, A granulation step is performed to granulate the aforementioned mixture to obtain green pellets, A firing process to obtain iron ore pellets involves heating the green pellets from the outside by burning CH4 gas and heating the green pellets from the inside by burning solid carbon, thereby firing the green pellets. It has, A method for producing iron ore pellets, characterized in that the solid carbon includes carbon produced from one or more gases selected from the group consisting of CO2 gas, CO gas, and CH4 gas.

[0010] [2] The method for producing iron ore pellets according to [1], wherein the solid carbon comprises carbon produced from one or both of CO2 gas and CO gas.

[0011] [3] The method for producing iron ore pellets according to [1], wherein the CH4 gas used in the production of carbon includes CH4 gas produced from one or both of CO2 gas and CO gas.

[0012] [4] A method for producing iron ore pellets according to any one of [1] to [3], wherein the carbon comprises carbon produced using iron as a catalyst.

[0013] [5] A method for producing iron ore pellets according to any one of [1] to [4], wherein the proportion of carbon in the solid carbon is 10% by mass or more.

[0014] [6] The method for producing iron ore pellets according to any one of [1] to [5], wherein the CH4 gas used in the calcination step includes CH4 gas generated from one or both of CO2 gas and CO gas. [Effects of the Invention]

[0015] The present invention provides a method for producing iron ore pellets that yields high-strength iron ore pellets and contributes to carbon neutrality. [Brief explanation of the drawing]

[0016] [Figure 1] A schematic diagram of the vertical reactor used in the carbon production test using CO gas as a raw material is shown. [Figure 2] The images show (a) sintered ore and (b) solid carbon after the carbon formation test. [Figure 3] (a) and (b) show schematic diagrams of the apparatus used in the carbon production test using CH4 gas as a raw material. [Figure 4] (a) to (c) show the TEM observation results of the solid carbon obtained in each carbon production experiment. [Modes for carrying out the invention]

[0017] The following describes embodiments of the method for producing iron ore pellets according to the present invention. Note that the embodiments described below are examples that embody the present invention, and these specific examples do not limit the configuration of the present invention.

[0018] The method for manufacturing iron ore pellets according to an embodiment of the present invention includes a mixing step of mixing iron ore, solid carbon (carbon material), a binder, and auxiliary raw materials to obtain a mixture, a granulation step of granulating the mixture to obtain green pellets, and a firing step of firing the green pellets to obtain iron ore pellets by burning CH4 gas to externally heat the green pellets and burning the solid carbon to internally heat the green pellets. And the solid carbon is characterized by containing carbon produced from one or more selected from the group consisting of CO2 gas, CO gas, and CH4 gas.

[0019] The mixture for manufacturing iron ore pellets consists of iron ore, solid carbon, a binder, and auxiliary raw materials. In this embodiment, the iron ore is assumed to include those with a total Fe content (T.Fe) of 63% by mass or less. Iron ore with a T.Fe of 63% by mass or less is inexpensive and suitable for production.

[0020] The blending amount of solid carbon in the iron ore pellets is preferably contained at 0.80% by mass or more based on the amount of iron ore in the iron ore pellets in order to sufficiently obtain the effect of improving the strength of the green pellets. On the other hand, if solid carbon is contained in excess, heat excess will occur in the firing step and melting of the iron ore pellets will occur. Therefore, the blending amount of solid carbon in the iron ore pellets is preferably contained at 2.00% by mass or less based on the amount of iron ore in the iron ore pellets.

[0021] The solid carbon contains carbon produced from one or more selected from the group consisting of CO2 gas, CO gas, and CH4 gas. That is, as the carbon material (solid carbon) added to the green pellets, carbon produced from one or more selected from the group consisting of CO2 gas, CO gas, and CH4 gas is used. In this case, although CO2 is generated by the combustion of the carbon material, the greenhouse gas is consumed as the raw material of the carbon material, which contributes to carbon neutrality. In addition, by adding carbon produced from one or more selected from the group consisting of CO2 gas, CO gas, and CH4 gas to the green pellets, the effect of increasing the strength of the green pellets and iron ore pellets can be obtained.

[0022] The proportion of the above-mentioned carbon in the solid carbon added to the green pellets is preferably 10% by mass or more, more preferably 50% by mass or more, and most preferably 100% by mass. The higher the amount of the above-mentioned carbon, the greater the effect of increasing strength and the greater the contribution to carbon neutrality. Furthermore, if the proportion of the above-mentioned carbon in the solid carbon added to the green pellets is less than 100% by mass, the remainder of the solid carbon may be, for example, anthracite.

[0023] The solid carbon preferably includes carbon produced from either or both CO2 gas and CO gas. Furthermore, if the solid carbon includes carbon produced from CH4 gas, the CH4 gas used in its production preferably includes CH4 gas produced from either or both CO2 gas and CO gas. There are no particular limitations on the reaction used to produce solid carbon from CO2 gas, CO gas, or CH4 gas, but examples include the following reactions.

[0024] [Reaction to produce solid carbon from CO gas] Solid carbon may also be produced from CO gas by the Boudouer reaction shown in reaction equation (1). The Boudouer reaction can produce solid carbon from CO gas at temperatures below approximately 700°C. 2CO = C + CO2 ... (1)

[0025] Solid carbon may also be produced from CO gas by the reverse water gasification reaction shown in reaction equation (2). The reverse water gasification reaction can produce solid carbon from CO gas at temperatures of approximately 650°C or below. CO + H2 = C + H2O ... (2)

[0026] The methanation reaction shown in reaction equation (3) may produce CH4 gas from CO gas, and the thermal decomposition reaction shown in reaction equation (4) may produce solid carbon from CH4 gas. The methanation reaction can produce CH4 gas from CO gas at temperatures below approximately 650°C, and the thermal decomposition reaction can produce solid carbon from CH4 gas in air at temperatures above approximately 500°C. CO + 3H2 = CH4 + H2O ... (3) CH4=C+2H2···(4)

[0027] Solid carbon may be produced from CO gas by the decomposition reaction shown in reaction equation (5). The decomposition reaction allows for the production of solid carbon from CO gas under low oxygen partial pressure. 2CO = 2C + O2 ... (5)

[0028] [Reaction to produce solid carbon from CO2 gas] Solid carbon may also be produced from CO2 gas by the reverse water gasification reaction shown in reaction equation (6). The reverse water gasification reaction can produce solid carbon from CO2 gas at temperatures of approximately 650°C or below. CO2 + 2H2 = C + 2H2O ... (6)

[0029] CO gas may be produced from CO2 gas by the reverse water-gas shift reaction shown in reaction equation (7), and solid carbon may be produced from CO gas by the reverse water-gasification reaction shown in reaction equation (2). The reverse water-gas shift reaction can produce CO gas from CO2 gas at temperatures of approximately 850°C or higher. CO2 + H2 = CO + H2O ... (7) CO + H2 = C + H2O ... (2)

[0030] The methanation reaction shown in reaction equation (8) may produce CH4 gas from CO2 gas, and the thermal decomposition reaction shown in reaction equation (4) may produce solid carbon from CH4 gas. The methanation reaction can produce CH4 gas from CO2 gas at temperatures below approximately 600°C. CO2 + 4H2 = CH4 + 2H2O ... (8) CH4=C+2H2···(4)

[0031] Solid carbon may also be produced from CO2 gas by the decomposition reaction shown in reaction equation (9). The decomposition reaction allows for the production of solid carbon from CO2 gas under low oxygen partial pressure. CO2 = C + O2 ... (9)

[0032] [Reaction to produce solid carbon from CH4 gas] Solid carbon may be produced from CH4 gas by the thermal decomposition reaction shown in reaction equation (4). CH4=C+2H2···(4)

[0033] The gas used in the carbon production reaction may be CO2 gas, CO gas, or CH4 gas individually, or a mixture of two or more of the three gases, or a mixed gas of one or more of the three gases with H2 gas, N2 gas, etc. For example, a mixed gas of CO:31%, H2:19%, and N2:50% by volume may be used.

[0034] Furthermore, as solid carbon to be added to the green pellets, one of the following may be used alone: ​​carbon produced from CO2 gas, carbon produced from CO gas, and carbon produced from CH4 gas; or two or more may be used in mixture form.

[0035] It is preferable that the solid carbon is produced using iron as a catalyst. The Boudouar reaction shown in reaction equation (1) and the CH4 thermal decomposition reaction shown in reaction equation (4) are known to be able to use iron as a catalyst. In addition, the reverse water-gas shift reaction shown in reaction equation (7) is known to be able to use iron oxide as a catalyst. Therefore, when producing solid carbon, sintered ore or directly reduced iron may be charged into the furnace, and the iron or iron oxide contained in the sintered ore or directly reduced iron may be used as a catalyst. Furthermore, in high-temperature reactions such as the CH4 thermal decomposition reaction shown in reaction equation (4), alumina may be charged into the furnace to maintain the furnace temperature.

[0036] The solid carbon is preferably in a fibrous form, and preferably has an aspect ratio (length / diameter) of 10 or more. When the solid carbon is fibrous, the strength of the iron ore pellets can be suitably obtained. The solid carbon may also be in a spherical form. When the solid carbon is spherical, a binder effect of fine particles can be obtained, and the strength of the iron ore pellets can be suitably obtained. Furthermore, when the solid carbon is spherical, its cumulative particle size D90 is preferably about 10 to 50 μm. The solid carbon (carbon material) produced from one or more gases selected from the group consisting of CO2 gas, CO gas, and CH4 gas preferably has a carbon content of 50% by mass or more, and the remainder may contain Fe, FeO, etc.

[0037] Bentonite is preferred as the binder for iron ore pellets, but any known or arbitrary binder, such as organic or inorganic binders that provide similar effects, may be used. To fully obtain its effects, the binder is preferably contained in an amount of 0.1% by mass or more relative to the amount of iron ore in the iron ore pellets. On the other hand, if the binder is included in excess, the manufacturing cost increases, and the effect diminishes as the content increases. Therefore, it is preferable that the binder is contained in an amount of 4.0% by mass or less relative to the amount of iron ore in the iron ore pellets.

[0038] Iron ore pellets may be mixed with auxiliary materials such as quicklime, limestone (CaCO3), and dolomite (CaMg(CO3)2). The basicity of the iron ore pellets is adjusted by the addition of these auxiliary materials. The basicity of the iron ore pellets is calculated based on the weight ratio of CaO / SiO2 contained in the iron ore pellets. The basicity of the iron ore pellets is preferably between 0.01 and 1.5.

[0039] Iron ore pellets are manufactured through general crushing, mixing, granulation, and calcination processes. The crushing process may use general crushing machines such as ball mills. The mixing process may use general concrete mixers or high-speed agitators. The granulation process may use general pelletizers or drum mixers.

[0040] The Blaine index of crushed iron ore is 2000-4000 cm³. 2 A value of approximately / g is preferable. The Blaine index is measured using a Blaine air permeation device as 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.

[0041] The granulated green pellets should preferably have a particle size of approximately 9.5 to 12 mm. If the particle size of the green pellets is less than 9.5 mm, the permeability will deteriorate when they are filled into a blast furnace as calcined pellets. If the particle size of the green pellets exceeds 12 mm, the reducibility will decrease.

[0042] The firing process may use a general rotary kiln or electric furnace. In the firing process, the green pellets are heated from the outside by burning CH4 gas, while the green pellets are heated from the inside by burning solid carbon. Preferably, the firing conditions are a furnace temperature of 1200 to 1350°C and a holding time of 5 to 30 minutes.

[0043] From the viewpoint of carbon neutrality, the CH4 gas used in the above calcination process preferably includes CH4 gas generated from either or both CO2 gas and CO gas. There are no particular restrictions on the method of generating CH4 gas, but it may be generated by, for example, the methanation reaction shown in reaction formulas (3) and (8). CO2 + 4H2 = CH4 + 2H2O ... (8) CO + 3H2 = CH4 + H2O ... (3) [Examples]

[0044] Iron ore, binder, and auxiliary materials were prepared as raw materials for iron ore pellets. Table 1 shows the composition of the iron ore used. The iron ore was dried at 105°C for 24 hours and then crushed, resulting in a Blaine index of 2560 cm³. 2 Iron ore powder with a volume-average diameter of 95 μm was obtained. Bentonite was used as a binder and limestone as a secondary material.

[0045] [Table 1]

[0046] Next, we conducted tests to generate solid carbon from CO gas or CH4 gas. Details are described below. In addition, we prepared anthracite as solid carbon that was not generated from any of the CO, CO2, or CH4 gases. The anthracite was prepared in particulate form with a particle size of 1 mm or less and a carbon content of 85% by mass.

[0047] [Test of carbon production from CO gas] Carbon was produced from CO gas using a vertical reactor. Figure 1 shows a schematic diagram of the vertical reactor. An alumina support base 12, alumina balls 14, and sintered ore 16 were charged into a reactor core tube 10 with an inner diameter of φ80 mm in the order described above. A gas mixture (hereinafter referred to as CO mixed gas) with volume % CO:31%, H2:19%, and N2:50% was flowed through a gas introduction pipe 18 at 550°C or 800°C (heated with heater 20 and measured with thermocouple 22) for 3 hours at a gas flow rate of 17 L / min to produce solid carbon. The sintered ore was used as a catalyst for carbon production due to the iron content it contained.

[0048] The reduced sample was sieved through a 0.125 mm sieve to separate it into sintered ore and solid carbon. Solid carbon comprised 1.5 wt% of the reduced sample. Figure 2 shows photographs of (a) sintered ore and (b) solid carbon after a carbon generation test from a CO-mixed gas at 800°C. The solid carbon was a fine powder, and cumulative particle sizes were measured as follows: D10: 2.1 μm, D50: 6.61 μm, D90: 14.8 μm. That is, more than 90% of the cumulative powder particles had a particle size of 14.8 μm or less. Table 2 shows the results of the component analysis of the solid carbon.

[0049] [Table 2]

[0050] [Test of carbon production from CH4 gas] Carbon was produced from CH4 gas using the apparatus shown in Figures 3(a) and 3(b). First, 30g (approximately 20-30 pellets) of directly reduced iron (DRI) 32, obtained by reducing φ10mm iron ore pellets, was charged into the electric furnace 30 shown in Figure 3(a) as a catalyst. 100% CH4 gas was then introduced through the gas inlet 34 at 900°C for 1 hour with a gas flow rate of 1.0 L / min to produce solid carbon. Next, 500g of φ6mm alumina balls 38 were charged into the φ80mm furnace core tube (alumina tube) 36 shown in Figure 3(b) to form a homogenized zone approximately 50mm high. 100% CH4 gas was then introduced through the gas inlet 40 at a gas temperature of 1400 (±10)°C (heated by heater 42) for 1 hour with a gas flow rate of 1.0 L / min to produce solid carbon. The reduced samples obtained from each test were sieved using a 0.125 mm sieve to separate them into DRI or alumina balls and solid carbon, respectively.

[0051] Table 3 shows the form and carbon content of solid carbon and anthracite produced under each condition. Figure 4 shows the TEM observation results of (a) solid carbon produced from a CO mixed gas at 550°C, (b) solid carbon produced from CH4 gas at 900°C, and (c) solid carbon produced from CH4 gas at 1400°C. The solid carbon obtained from reactions below 900°C was fibrous with an aspect ratio of 10 or more. The solid carbon obtained from the reaction at 1400°C was spherical with a particle size of approximately 0.2 to 2.0 μm. This is thought to be because, at 1400°C, the high temperature leads to high reactivity and the main reaction is carbon formation in the gas phase, while below 900°C, the thermal decomposition reaction of CH4 gas does not proceed easily, and the main reaction is a formation reaction catalyzed by iron.

[0052] [Table 3]

[0053] 1000g of iron ore powder was prepared, and various solid carbons in the proportions shown in Table 4, as well as 1% by mass of bentonite relative to the amount of iron ore, were added. Furthermore, limestone in an amount set to achieve a basicity of 0.1 in the iron ore powder was added, and the mixture was mixed for 3 minutes at 20 rpm using a concrete mixer. Note that the proportion of solid carbon in Nos. 2 to 7 was set to be the same as the amount of carbon contained in anthracite in No. 1. The percentage of carbon in anthracite that was replaced by carbon contained in solid carbons 1 to 4 is shown in the "Carbon Replacement Rate" column of Table 4. Next, the mixed raw materials were placed in a 1.2 mφ pelletizer and granulation was carried out while adding water. Pellet particles with a particle size of 9.5 to 12 mm were collected and tumbled in the pelletizer for another 10 minutes to obtain green pellets.

[0054] [Table 4]

[0055] [Measuring the drop strength of green pellets] 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 3.

[0056] Green pellets not used in the above test were placed in an electric furnace and fired. Under an atmospheric environment, the temperature was increased at 10°C / min, held at 1300°C for 10 minutes, and then cooled at 10°C / min. After cooling, the sample was removed to obtain iron ore pellets. In the actual firing process, natural gas mainly composed of CH4 gas is burned to heat the green pellets, but in this test, an electric furnace was used to simulate that heating process.

[0057] [Measuring the crushing strength of iron ore pellets] In each example and comparative example, the crushing strength (kgf) of 10 iron ore pellets was measured using an autograph. The displacement rate was set to 2 mm / min, and the average value for 10 pellets is shown in Table 3.

[0058] Table 4 shows that the inventive example is superior to the comparative example in both the drop strength of the green pellets and the crushing strength of the iron ore pellets. In addition to the above, by replacing the solid carbon with carbon produced from one or more gases selected from the group consisting of CO2 gas, CO gas, and CH4 gas, the reduction of CO2 emissions can also be contributed to, and the effects of the present invention are clear. Furthermore, it is clear that carbon neutrality can be achieved in the production of iron ore pellets by using CH4 gas produced from one or both CO2 gas and CO gas in the carbon production and calcination process, with a solid carbon replacement rate of 100% by mass. [Industrial applicability]

[0059] According to the present invention, it is possible to obtain high-strength iron ore pellets and to provide a method for producing iron ore pellets that contributes to carbon neutrality. [Explanation of Symbols]

[0060] 10 core tubes 12 Alumina support base 14 Alumina balls 16 Sintered Ore 18 Gas inlet pipe 20 Heater 22 Thermocouples 30 Electric Furnaces 32. DRI obtained by reducing iron ore pellets 34 Gas inlet 36 Furnace tube 38 Alumina balls 40 Gas inlet 42 Heater

Claims

1. A mixing step to obtain a mixture by mixing iron ore with a total Fe content of 63% by mass or less, solid carbon, binder, and auxiliary raw materials, A granulation step is performed to granulate the aforementioned mixture to obtain green pellets, CH ４ A firing process to obtain iron ore pellets by firing the green pellets by burning gas to heat them from the outside and burning solid carbon to heat them from the inside, It has, The aforementioned solid carbon is CO ２ Gas, CO gas, and CH ４ A method for producing iron ore pellets, characterized by containing carbon produced using iron as a catalyst from one or more gases selected from the group.

2. The aforementioned solid carbon is CO ２ A method for producing iron ore pellets according to claim 1, comprising carbon generated from one or both of gas and CO gas.

3. The CH used in the generation of the carbon ４ Gas, CO ２ CH4 produced from one or both of gas and CO gas ４ A method for producing iron ore pellets according to claim 1, comprising gas.

4. A method for producing iron ore pellets according to any one of claims 1 to 3, wherein the proportion of carbon in the solid carbon is 10% by mass or more.

5. The CH used in the firing process ４ gas is CH generated from one or both of CO ２ gas and CO gas ４ The method for producing an iron ore pellet according to any one of claims 1 to 3, comprising the CH gas

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