Iron carbide manufacturing method

Abstract

This disclosure discloses a technique for achieving a cementitious ratio in iron carbide equal to or greater than a target value when reduced iron is obtained by a direct reduction process using a shaft furnace and then carbonized to obtain iron carbide. The method for producing iron carbide disclosed herein includes a reduction step in which an iron oxide raw material is reduced with a reducing gas to obtain reduced iron, and a carbonization step in which the reduced iron is carbonized with a carbonizing gas while being cooled to obtain iron carbide. The reduction step is carried out in a shaft furnace. In the carbonization step, the carbonizing gas contains 70% by volume or more of methane gas, the reduced iron is carbonized to a target cementitious ratio or greater, and the relationships Y≧0.24X−0.31 and X≧3 (Y: elapsed time (minutes) for the reduced iron to cool from 800°C to 700°C, X: the target cementitious ratio (%)) are satisfied.

Description

[Technical Field]

[0001] This application discloses a method for producing iron carbide. [Background technology]

[0002] In the steel industry, the direct reduction process using reducing gas is being considered as an alternative technology to the blast furnace process to reduce CO2 emissions. In the direct reduction process, direct reduced iron (DRI) is obtained by contacting reducing gas with oxidized iron raw materials. The carbon content of DRI is sometimes increased (i.e., carbonized) to lower the melting temperature in the subsequent melting and refining processes and to ensure the strength of steel. Carbonizing DRI is also desirable from the perspective of preventing reoxidation of the DRI and improving its handleability. One method for carbonizing DRI includes a reduction step in which reducing gas is contacted with oxidized iron raw materials to obtain DRI, and a carbonization step in which the DRI is contacted with methane gas or the like to cool the DRI and carbonize the metallic iron contained in the DRI. The temperature of the DRI obtained in the reduction step is approximately 700°C to 1050°C. In addition, the carbonization reaction of metallic iron by methane gas (CH4(g) + 3Fe(s) ⇒ 2H2(g) + Fe3C(s)) is an endothermic reaction that proceeds in the high temperature range of 700°C or higher.

[0003] In relation to the above-mentioned technology, Patent Document 1 discloses a vertical reduction furnace having a reduction zone and a cooling zone, in which a cooling gas loop is provided for circulating a cooling gas for cooling reduced iron, and a gas having a hydrocarbon concentration of 50% or more is used as the cooling gas. Patent Document 2 also discloses a method in which a hydrocarbon such as methane, pentane, or hexane is used as the cooling gas. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2021 / 029114 [Patent Document 2] Japanese Patent Publication No. 61-073805 Summary of the Invention [Problem to be solved by the invention]

[0005] In the prior art, when reduced iron is obtained by a direct reduction process using a shaft furnace and then the reduced iron is carbonized to obtain iron carbide, sufficient consideration has not been given to conditions for making the cementite conversion rate in the iron carbide equal to or greater than a target value. [Means for solving the problem]

[0006] The present application discloses the following aspects as means for solving the above problems. <Aspect 1> A method for producing iron carbide, comprising: a reduction step of reducing the oxidized iron raw material with a reducing gas to obtain reduced iron; a carbonization step of carbonizing the reduced iron while cooling it with a carbonizing gas to obtain iron carbide; and The reduction step is carried out in a shaft furnace, In the carbonization step, The carbonized gas contains 70% by volume or more of methane gas, The reduced iron is carbonized until the cementitization rate reaches a target cementitization rate or more, and The following relationships (1) and (2): Y≧0.24X-0.31 (1) X≧3 (2) Y: Time (minutes) elapsed until the temperature of the reduced iron reaches 700°C from 800°C X: The target cementitization rate (%) is satisfied, Method for producing iron carbide. <Aspect 2> A method for producing iron carbide according to aspect 1, comprising: The elapsed time Y is 30 minutes or less. Method for producing iron carbide. <Aspect 3> A method for producing iron carbide according to aspect 1, comprising: The elapsed time Y is 20 minutes or less. Method for producing iron carbide. <Aspect 4> The method for producing iron carbide according to any one of aspects 1 to 3, The carbonization step is carried out in the shaft furnace. Method for producing iron carbide. <Aspect 5> The method for producing iron carbide according to any one of aspects 1 to 3, The carbonization step is carried out outside the shaft furnace. Method for producing iron carbide. <Aspect 6> The method for producing iron carbide according to any one of aspects 1 to 5, The reducing gas includes one or both of hydrogen gas and methane gas. Method for producing iron carbide. [Effects of the Invention]

[0007] According to the method for producing iron carbide disclosed herein, when reduced iron is obtained by a direct reduction process using a shaft furnace and then the reduced iron is carbonized to obtain iron carbide, the cementite conversion rate in the iron carbide can be made equal to or greater than a target value. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a schematic diagram for explaining an example of a method for producing iron carbide and an example of a system for producing iron carbide. FIG. [Figure 2] 1 is a schematic diagram for explaining an example of a method for producing iron carbide and an example of a system for producing iron carbide. FIG. [Figure 3] 1 is a schematic diagram for explaining an example of a method for producing iron carbide and an example of a system for producing iron carbide. FIG. [Figure 4] 1 is a schematic diagram for explaining an example of a method for producing iron carbide and an example of a system for producing iron carbide. FIG. [Figure 5A]The graph shows the relationship between the elapsed time and the carbon concentration when the reaction temperature is 700°C. [Figure 5B] The graph shows the relationship between the elapsed time and the carbon concentration when the reaction temperature is 800°C. [Figure 5C] The graph shows the relationship between the elapsed time and the carbon concentration when the reaction temperature is 900°C. [Figure 6] The graph shows the relationship between the time elapsed from 800°C to 700°C and the cementite conversion rate. DETAILED DESCRIPTION OF THE INVENTION

[0009] An embodiment of the iron carbide manufacturing method and iron carbide manufacturing system of the present disclosure will be described below. However, the iron carbide manufacturing method and iron carbide manufacturing system of the present disclosure are not limited to the following embodiment. In this application, "iron oxide raw material" refers to a raw material containing iron oxide before the reduction step. "Reduced iron" refers to reduced iron as an intermediate product after the reduction step. "Iron carbide" refers to reduced iron whose cementitization rate has been increased through the carbonization step.

[0010] 1. Iron carbide manufacturing method 1 to 4, a method for producing iron carbide according to one embodiment includes a reduction step S1 in which an oxidized iron raw material 10 is reduced with a reducing gas to obtain reduced iron 20, and a carbonization step S2 in which the reduced iron 20 is carbonized with a carbonizing gas while being cooled to obtain iron carbide 30. The reduction step S1 is performed in a shaft furnace 100. In the carbonization step S2, the carbonizing gas contains 70% by volume or more of methane gas. In the carbonization step S2, the reduced iron 20 is carbonized until it reaches a target cementitization rate or higher. In the carbonization step S2, the following relationships (1) and (2) are satisfied: Y≧0.24X-0.31 (1) X≧3 (2) Y: Time (minutes) elapsed until the temperature of the reduced iron 20 reaches 700°C from 800°C X: The target cementitization rate (%) is satisfied.

[0011] 1.1 Reduction process In the reduction step S1, a reducing gas is brought into contact with the oxidized iron raw material 10. As a result, a portion of the iron oxide contained in the oxidized iron raw material 10 is reduced to obtain reduced iron 20. As shown in FIGS. 1 to 4, the reduction step S1 is performed in a shaft furnace 100. Note that, according to the technology of the present disclosure, it is believed that the desired effects can be obtained even when the reduction step S1 is performed in a kiln instead of the shaft furnace 100.

[0012] 1.1.1 Iron oxide raw materials The oxidized iron raw material 10 contains at least iron oxide. The oxidized iron raw material 10 may be, for example, at least one selected from iron ore pellets, iron ore, and sintered ore. The oxidized iron raw material 10 may contain, in addition to iron oxide, for example, one or both of silicon dioxide and aluminum oxide. The oxidized iron raw material 10 may contain impurities. The oxidized iron raw material 10 may have a particle size distribution or a uniform particle size. The average particle size of the oxidized iron raw material 10 may be, for example, 5.0 mm or more and 30.0 mm or less, or 10.0 mm or more and 15.0 mm or less. The "particle size" of the oxidized iron raw material refers to the sieve size of the raw material. The "average particle size" of the oxidized iron raw material refers to the weighted average value of the particle sizes of the raw material. The average particle size of the oxidized iron raw material is measured as follows. That is, the average particle size of the oxidized iron raw material can be measured by obtaining a mass-based particle size distribution by a dry sieving test described in JIS Z 8815: 1995, and then calculating the mass-weighted average of the maximum and minimum particle sizes of each sieve as a representative particle size. The oxidized iron raw material 10 may be formed into pellets or the like, may be in the form of a lump, or may have any other shape.

[0013] In the reduction step S1, the oxidized iron raw material 10 may be packed into the shaft furnace 100 to form a packed bed. The packing rate of the packed bed is not particularly limited and may be the same as that in a conventional reduced iron manufacturing method using a shaft furnace. The packed bed moves downward inside the shaft furnace 100. That is, the oxidized iron raw material 10 is substantially filled inside the shaft furnace 100 and gradually moves downward by dropping or the like. When focusing on a single raw material particle in the packed bed, the raw material particle may move downward continuously at a constant speed or may move intermittently by repeatedly dropping and stopping. When focusing on a single raw material particle in the packed bed, the average downward movement speed of the raw material particle is not particularly limited. For example, the average movement speed may be adjusted depending on the supply amount (feed rate) of the raw material. When moving the packed bed downward, a burden feeder or the like may be used to prevent hanging.

[0014] 1.1.2 Reducing gas The type of reducing gas is not particularly limited as long as it can reduce the oxidized iron raw material 10. The reducing gas may contain, for example, one or both of hydrogen gas and methane gas. The reducing gas may contain gases other than hydrogen gas and methane gas. Examples of other gases include CO gas, inert gas, CO gas, and water vapor. Examples of inert gases include nitrogen gas and argon gas. When the reducing gas contains hydrogen gas, the hydrogen concentration of the reducing gas may be, for example, 40% by volume to 100% by volume, 60% by volume to 100% by volume, or 80% by volume to 100% by volume. The supply temperature of the reducing gas (the temperature immediately before contact with the oxidized iron raw material 10) may be any temperature at which a reduction reaction with the iron oxide occurs, and may be, for example, 700°C or higher. The temperature of the reducing gas is preferably 800°C to 1100°C.

[0015] In the reduction step S1, the reducing gas may be supplied from the side wall of the shaft furnace 100 to the inside of the furnace. The method of supplying the reducing gas is not particularly limited. For example, a pipe or the like may be connected to a reducing gas supply port provided on the side wall of the shaft furnace 100, and the reducing gas may be supplied from the outside to the inside of the furnace through the pipe or the like.

[0016] 1.1.3 Reduced iron In the reduction step S1, at least a portion of the iron oxide contained in the oxidized iron raw material 10 is reduced to obtain reduced iron 20 containing metallic iron. The reduced iron 20 may contain, in addition to metallic iron, iron oxide that remains unreduced, and may further contain silicon dioxide, aluminum oxide, and the like. The temperature of the reduced iron 20 immediately after reduction (the temperature of the reduced iron 20 immediately before the carbonization step S2 described below) may be, for example, above 700°C. There is no particular upper limit to the temperature of the reduced iron 20 immediately after reduction, as long as it is a temperature at which the carbonization step S2 described below can be carried out. The temperature of the reduced iron 20 immediately after reduction may be, for example, 1100°C or lower. The temperature of the reduced iron 20 immediately after reduction may be higher than 700°C and lower than 1050°C, 750°C or higher and 1000°C or lower, or 800°C or higher and 900°C or lower.

[0017] In this embodiment, there is no particular limitation on the metallization rate of the reduced iron 20 obtained in the reduction step S1 ([mass of metallic iron in the reduced iron 20] / [mass of all iron in the reduced iron 20]×100). In one embodiment, the metallization rate of the reduced iron 20 obtained in the reduction step S1 may be 50% or more and 100% or less, 60% or more and 100% or less, 70% or more and 100% or less, 80% or more and 100% or less, or 90% or more and 100% or less.

[0018] 1.1.4 Shaft furnace The shape of the shaft furnace 100 may be the same as that of a known shaft furnace. For example, the shaft furnace 100 may have a furnace top, a furnace bottom, and a cylindrical portion (cylindrical portion) forming a sidewall between the furnace top and the furnace bottom. In this case, the cylindrical portion may have a barrel portion and a tapered portion provided below the barrel portion, and the inner diameter of the furnace may decrease from top to bottom at the tapered portion. The shaft furnace 100 may be equipped with a burden feeder or the like to prevent the packed bed of raw materials from hanging when moving the packed bed downward inside the shaft furnace 100. The shaft furnace 100 may also be equipped with a cooling gas supply port for supplying a cooling gas (carbonization gas for cooling and carbonization) and a cooling gas outlet for discharging the cooling gas below the reducing gas supply port. The cooling gas supply port may be provided in the sidewall of the furnace or may be provided inside the sidewall of the furnace. The cooling gas outlet port may be provided in the sidewall of the furnace.

[0019] 1.2 Carbonization process The temperature of the reduced iron 20 immediately after the reduction step S1 is, for example, above 700°C. In the carbonization step S2, the reduced iron 20 at such a high temperature is brought into contact with a carbonizing gas containing methane gas, thereby carbonizing the reduced iron 20 and cooling it. Through the carbonization step S2, iron carbide 30 (reduced iron with an increased carbon concentration) is obtained. By carbonizing the reduced iron 20 to obtain iron carbide 30 in this way, the melting temperature in the subsequent melting and refining step is lowered and the strength required for steel is ensured. Furthermore, reoxidation of the reduced iron is prevented, improving handling properties during transportation and storage.

[0020] The carbonization step S2 may be performed inside or outside the shaft furnace. That is, as shown in FIG. 1, the reduction step S1 and the carbonization step S2 may be performed inside the shaft furnace 100. Alternatively, as shown in FIG. 2, the reduction step S1 may be performed inside the shaft furnace 100, and the carbonization step S2 may be performed outside the shaft furnace 100. When the carbonization step S2 is performed outside the shaft furnace 100, for example, as shown in FIG. 2, a cooling tower 200 may be provided downstream of the shaft furnace 100, and the carbonization step S2 may be performed in the cooling tower 200. In addition, in the present application, the "downstream side" refers to the downstream side in the iron carbide production process. That is, when iron carbide 30 is produced from the iron oxidized raw material 10 via reduced iron 20, the iron oxidized raw material 10 side is the upstream side, and the iron carbide 30 side is the downstream side.

[0021] In the carbonization step S2, the reduced iron 20 is cooled and carbonized by being brought into contact with the carbonizing gas. The reduced iron 20 is partially carbonized. There are no particular limitations on the method for bringing the carbonizing gas into contact with the reduced iron 20. For example, the carbonizing gas can be brought into contact with the reduced iron 20 by connecting a pipe or the like to a gas supply port provided on a side wall of the shaft furnace 100 or the cooling tower 200 and supplying the carbonizing gas from the outside to the inside through the pipe or the like.

[0022] In the carbonization step S2, contacting the reduced iron 20 with the carbonization gas can cause a cementite production reaction represented by the following formula (A) and a cementite decomposition reaction represented by the following formula (B). According to the findings of the present inventors, the cementitization rate can change depending on the reaction temperature and reaction time (elapsed time) in the carbonization step S2. C(s)+3Fe(s)⇒Fe3C(s) ···(A) Fe3C(s)⇒C(s)+3Fe(s) ···(B)

[0023] 1.2.1 Reaction temperature The reaction temperature in the carbonization step S2 (the surface temperature of the reduced iron 20 in contact with the carbonizing gas) is the temperature at which the carbonization of metallic iron by methane gas contained in the carbonizing gas progresses. According to the findings of the present inventors, at temperatures significantly exceeding 800°C, carbon can be precipitated in the reduced iron 20 to increase the carbon concentration, but the amount of cementite produced decreases. Furthermore, according to the findings of the present inventors, cementite is particularly likely to be produced when the reduced iron 20 is heated from 800°C to 700°C. One feature of the present embodiment is that, when the reduced iron 20 is cooled and carbonized by a predetermined carbonizing gas in the carbonization step S2, the elapsed time Y for the reduced iron 20 to heat from 800°C to 700°C is set to a predetermined time or more, thereby making the cementitization rate of the reduced iron 20 equal to or greater than a target value X (obtaining iron carbide 30 having a cementitization rate equal to or greater than the target value X).

[0024] The surface temperature of the reduced iron 20 in the carbonization step S2 (the surface temperature of the reduced iron 20 after the reduction step S1) can be controlled by the temperature of the oxidized iron raw material 10 in the reduction step S1, the supply amount of the oxidized iron raw material 10, the temperature of the reducing gas, the supply amount of the reducing gas, etc. Note that the "surface temperature of the reduced iron 20 in contact with the carbonization gas" in the carbonization step S2 is the average temperature in the radial direction of the shaft furnace or the cooling tower. The average temperature of the reduced iron 20 in the radial direction can be determined, for example, by installing a rod-shaped member in the radial direction of the shaft furnace or the cooling tower, providing multiple thermocouples on the member, and measuring multiple temperatures in the radial direction. For example, assuming that the temperature between the measurement points is distributed linearly in the radial direction, the temperature distribution T(r) in the radial direction can be expressed as a combination of linear functions of r. In this case, the average temperature T ave If the temperature is measured at N points, the radius of measurement point i (i=1~N) is r i is defined by the following formula: N+1 corresponds to the center of the furnace (r0=0) and the furnace wall (r N+1=R), and the temperature at that point is determined by extrapolation. With this method, even if there is a temperature distribution in the radial direction, the average temperature in the radial direction can be determined by averaging multiple measured temperatures. The number of thermocouples is not particularly limited, but it is preferable that five or more thermocouples are disposed, for example. Alternatively, the surface temperature of the reduced iron 20 that comes into contact with the carbonizing gas may be a temperature estimated by simulation. In the carbonization step S2, the temperature of the carbonizing gas that comes into contact with the reduced iron 20 is not particularly limited. The temperature of the carbonizing gas may be, for example, 25°C or higher and 600°C or lower.

number

[0025] 1.2.2 Carbonizing gas The carbonization gas brought into contact with the reduced iron 20 in the carbonization step S2 contains 70% by volume or more of methane gas. This allows the carbonization reaction in the reduced iron 20 to proceed efficiently. The proportion of methane gas in the carbonization gas in the carbonization step S2 may be, for example, 70% by volume or more and 100% by volume or less, 75% by volume or more and 100% by volume or less, 80% by volume or more and 100% by volume or less, 85% by volume or more and 100% by volume or less, 90% by volume or more and 100% by volume or less, or 95% by volume or more and 100% by volume or less. The composition of the carbonization gas is based on the concentration of the gas before (just before) being introduced into the shaft furnace 100 and can be controlled by the supply amounts of methane gas and other gases, etc.

[0026] In the carbonization step S2, other gases may be brought into contact with the reduced iron 20 in addition to methane gas. The carbonization gas in the carbonization step S2 may contain nitrogen gas, hydrogen gas, CO gas, CO2 gas, water vapor, etc. in addition to methane gas. The proportion of gases other than methane gas in the carbonization gas may be, for example, 0% by volume or more and 30% by volume or less, 0% by volume or more and 25% by volume or less, 0% by volume or more and less than 20% by volume, 0% by volume or more and 15% by volume or less, 0% by volume or more and 10% by volume or less, or 0% by volume or more and 5% by volume or less. The proportion of nitrogen gas in the carbonization gas may be 0% by volume or more and 30% by volume or less, 0% by volume or more and 25% by volume or less, 0% by volume or more and less than 20% by volume, 0% by volume or more and 15% by volume or less, 0% by volume or more and 10% by volume or less, or 0% by volume or more and 5% by volume or less. The proportion of hydrogen gas in the carbonization gas may be 0% by volume or more and 30% by volume or less, 0% by volume or more and 25% by volume or less, 0% by volume or more and less than 20% by volume, 0% by volume or more and 15% by volume or less, 0% by volume or more and 10% by volume or less, or 0% by volume or more and 5% by volume or less. A lower proportion of CO gas contained in the carbonization gas is preferable. The proportion of CO gas in the carbonization gas may be 0% by volume or more and 10% by volume or less, or 0% by volume or more and 5% by volume or less. The proportion of CO gas in the carbonization gas may be 0% by volume or more and 30% by volume or less, 0% by volume or more and 25% by volume or less, 0% by volume or more and less than 20% by volume, 0% by volume or more and 15% by volume or less, 0% by volume or more and 10% by volume or less, or 0% by volume or more and 5% by volume or less. The proportion of water vapor in the carbonization gas may be 0% by volume or more and 30% by volume or less, 0% by volume or more and 25% by volume or less, 0% by volume or more and less than 20% by volume, 0% by volume or more and 15% by volume or less, 0% by volume or more and 10% by volume or less, or 0% by volume or more and 5% by volume or less. The carbonization gas in the carbonization step S2 may contain, for example, natural gas. The temperature of the carbonization gas in the carbonization step S2 is not particularly limited as long as it can cause the carbonization reaction of metallic iron to proceed.

[0027] The exhaust gas from the carbonization step S2 includes, for example, methane gas and hydrogen gas. The exhaust gas may be discharged to the outside of the system and used as fuel, or may be used as the reducing gas described above. In particular, by using the exhaust gas from the carbonization step S2 as the reducing gas in the reduction step S1, efficient operation is possible while reducing the amount of reducing gas used. When the carbonization step S2 is performed in the shaft furnace 100, the gas after carbonization in the carbonization step S2 may pass through a transition zone in the shaft furnace 100, further rise within the furnace, and be used as part of the reducing gas in the reduction zone.

[0028] 1.2.3 Elapsed Time Y In the carbonization step S2, as described above, the reduced iron 20 is brought into contact with methane gas, thereby carbonizing the metallic iron contained in the reduced iron 20. The carbonization reaction by methane gas is an endothermic reaction, and the reduced iron 20 is carbonized and cooled in the carbonization step S2. As described above, the carbonization of the reduced iron 20 is likely to occur at a temperature of 700°C or higher and 800°C or lower. According to the findings of the present inventors, in the carbonization step S2, by setting the elapsed time Y for the reduced iron 20 to cool from 800°C to 700°C to a predetermined time or longer, the reduced iron 20 can be carbonized until the cementite conversion rate X is reached or higher. Specifically, in the carbonization step S2, it is important that the following relationships (1) and (2) are satisfied.

[0029] Y≧0.24X-0.31 (1) X≧3 (2) Y: Time (min) elapsed until reduced iron 20 reaches 700°C from 800°C X: Target cementitious ratio (%)

[0030] The lower limit of the elapsed time Y is determined based on the above formula in accordance with the target cementitization rate X. On the other hand, the upper limit of the elapsed time Y is not particularly limited and may be determined appropriately taking into consideration productivity and the like. From the viewpoint of increasing productivity, the elapsed time Y may be 30 minutes or less, 25 minutes or less, or 20 minutes or less. Note that the elapsed time Y can be controlled based on the temperature of the oxidized iron raw material 10 in the reduction step S1, the temperature of the reducing gas contacting the oxidized iron raw material 10 in the reduction step S1, the temperature of the reduced iron 20 leading to the carbonization step S2, the supply amount (supply rate) of the reduced iron 20, the discharge rate of the iron carbide 30 in the carbonization step S2, the discharge temperature of the iron carbide 30, the temperature of the carbonization gas, the supply position of the carbonization gas, the supply amount of the carbonization gas, the discharge position of the exhaust gas from the carbonization step S2, the discharge amount of the exhaust gas, and the shape and volume of the reactor (shaft furnace or cooling tower) in which the carbonization step S2 is performed.

[0031] 1.2.4 Target cementitious ratio X In the carbonization step S2, as described above, the reduced iron 20 is carbonized to a target cementite conversion rate X or higher to obtain iron carbide 30. The target cementite conversion rate X may be determined appropriately depending on the application of the iron carbide 30. The target cementite conversion rate X is 3% or higher, and may be, for example, 5% or higher and 100% or lower, or 10% or higher and 30% or lower.

[0032] The "cementitization rate" refers to the mass ratio of iron constituting cementite to the total iron contained in the reduced iron 20 (iron carbide 30) after the carbonization step S2 ([mass of iron constituting cementite] / [total mass of iron] × 100). The cementitization rate can be determined by pulverizing the reduced iron 20 (iron carbide 30) after the carbonization step S2 to obtain a powder, obtaining an X-ray diffraction pattern by powder X-ray diffraction, and then performing Rietveld analysis. The powder X-ray diffraction measurement conditions are as follows: CoKα radiation as the radiation source, a focused optical system, a tube voltage of 40 kV, a tube current of 36 mA, a scan angle 2θ of 5 to 120°, a scan rate of 0.02° / step, and a scan rate of 2.0° / min. Rietveld analysis is well known, as described in, for example, Non-Patent Document 1 below. In the Rietveld analysis, background correction is performed using the B-spline method, and a divided pseudo-Voigt function is used as the peak profile function. In addition, preferred orientation is corrected using the March-Dollase function. In this analysis, the background function, lattice constants of each mineral phase, profile function, and crystal structure factors are refined. Under the above analysis conditions, Rietveld analysis is performed on the crystalline phases identified in the qualitative analysis to identify each crystalline phase. The crystalline phases analyzed include hematite (ICDD:01-080-2377), magnetite (ICDD:01-089-0688), wustite (ICDD:01-089-0686), α-Fe (ICDD:00-006-0696), γ-Fe (ICDD:01-089-04185), cementite (ICDD:04-014-3159), graphite (ICDD:01-071-3739), other iron carbides (Hagg carbide (χ-Fe5C2) (ICDD:01-089-2544), hexagonal carbide (ε-FeC) (ICDD: 01-089-2544)), and gangue-derived compounds (quartz (ICDD: 00-046-1045), wollastonite (ICDD: 04-016-5334), larnite (ICDD: 01-083-0465), gehlenite (ICDD: 04-016-0209)). The phase to be analyzed is selected each time depending on the raw material conditions and operating conditions.For example, if a high concentration of alumina is used as the oxidized iron raw material 10, the reduced iron 20 (iron carbide 30) after the carbonization step S2 is likely to contain gehlenite, and this is included as the phase to be analyzed. Also, if a material containing CaO and a large amount of simple SiO2 is used as the oxidized iron raw material 10, 2CaO SiO2 such as larnite is included as the phase to be analyzed. Non-patent literature 1: Takayuki Harano, Yu Nemoto, Reiko Murao, Masao Kimura, ISIJ International, 2020, Volume 60, Issue 12, 2851.

[0033] The "target cementitization rate" refers to a target value of the cementitization rate for iron carbide 30 obtained by the carbonization step S2. In the carbonization step S2, the reduced iron 20 is carbonized until the cementitization rate reaches or exceeds the target cementitization rate. When a further carbonization step S3 is performed after the carbonization step S2 as described below, it is sufficient that the reduced iron 20 is carbonized to or exceeds the target cementitization rate after the carbonization step S2 and before the carbonization step S3.

[0034] 1.3 Other processes In this embodiment, other processes may be performed in addition to the reduction process S1 and the carbonization process S2. For example, as shown in FIG. 3, in one embodiment, carbonization using a carbonizing gas other than methane gas (carbonization process S3) may be performed in addition to the carbonization process S2. Also, in one embodiment, cooling using a cooling gas other than methane gas (cooling process S4) may be performed in addition to the carbonization process S2. Note that in this embodiment, it is sufficient that the temperature of the reduced iron 20 is reduced in the carbonization process S2, and the temperature of the reduced iron 20 may be increased in a process other than the carbonization process S2 (for example, a process subsequent to the carbonization process S2). For example, when the carbonization process S3 using CO gas is performed subsequent to the carbonization process S2, an exothermic reaction between the reduced iron 20 and the CO gas may occur, causing the temperature of the reduced iron 20 to increase.

[0035] 1.3.1 Further carbonization process using carbonization gases other than methane gas In one embodiment, after the reduced iron 20 is carbonized by contacting the carbonization gas in the carbonization step S2, a further carbonization step S3 may be performed by further contacting the reduced iron 20 with CO gas. This tends to further increase the carbonization degree of the finally obtained iron carbide 30. The further carbonization step S3 using CO gas may be performed inside or outside the shaft furnace. The method for contacting the reduced iron 20 after carbonization using methane gas with CO gas is not particularly limited. For example, a pipe or the like may be connected to a gas supply port provided on a side wall of the shaft furnace 100 or the cooling tower 200, and CO gas may be supplied from the outside to the inside through the pipe or the like, thereby bringing the CO gas into contact with the reduced iron 20 after carbonization using methane gas. Note that other gases may be contacted with the reduced iron 20 after carbonization using methane gas together with the CO gas. Examples of gases other than CO gas include nitrogen gas, methane gas, hydrogen gas, and CO2 gas. The CO-containing gas may be, for example, converter gas (LDG).

[0036] When CO gas or the like is further brought into contact with the reduced iron 20 after the carbonization step S2 in which the reduced iron 20 is brought into contact with the carbonization gas for carbonization, an exhaust gas derived from the CO gas or the like is generated. The exhaust gas mainly contains, for example, CO gas and CO gas. The exhaust gas may be discharged to the outside of the system and used as fuel.

[0037] 1.3.2 Further cooling process with cooling gas In one embodiment, after the carbonization step S2 in which the reduced iron 20 is brought into contact with a carbonization gas to perform the carbonization step S2, or after the further carbonization step S3 using CO gas is performed, the carbonized reduced iron 20 may be brought into contact with a cooling gas to further cool the carbonized reduced iron 20. The further cooling step S4 may be performed inside or outside the shaft furnace. In the further cooling step S4, it is only necessary that the cooling of the carbonized reduced iron 20 proceeds, and multiple types of cooling gases may be brought into contact with the carbonized reduced iron 20. The cooling gas may contain an inert gas such as methane gas or nitrogen gas, hydrogen gas, water vapor, or the like. When the gas that contacts the carbonized reduced iron 20 in the further cooling step S4 is made up of multiple types of gases, for example, the volume proportion of methane gas or the volume proportion of the inert gas may be the largest among the volume proportions of the respective gases.

[0038] When methane gas is used in the further cooling step S4, exhaust gas derived from methane gas or the like is generated from the cooling step S4. The exhaust gas includes, for example, methane gas and hydrogen gas. The exhaust gas may be discharged to the outside of the system and used as fuel, or may be used as part of the reducing gas described above, or may be used as part of the carbonization gas in the carbonization step S2 using methane gas and hydrogen gas described above, or may be used as part of the carbonization gas in the carbonization step S3 using CO gas or the like described above. When an inert gas is used in the further cooling step S4, exhaust gas containing the inert gas is generated from the cooling step S4. The exhaust gas may be discharged to the outside of the system, or may be reused as the inert gas in the cooling step S4.

[0039] 1.3.3 Dehydration process and heating process As shown in FIG. 4 , a method for producing reduced iron according to one embodiment may include a dehydration step S5 in which the exhaust gas from the reduction step S1 is dehydrated to obtain a circulation gas, and a temperature increase step S6 in which the circulation gas and hydrogen gas are heated to obtain a reducing gas containing the circulation gas and hydrogen gas. In the dehydration step S5, the exhaust gas from the reduction step S1 is dehydrated to obtain a circulation gas. The dehydration may be performed using a known dehydration device. In the reduction step S1, water is produced by the reaction between the reducing gas and the oxidized iron raw material 10. However, in the reduction step S1, not all of the reducing gas is necessarily used. That is, the reducing gas remains in the exhaust gas from the reduction step S1 along with water. By dehydrating such exhaust gas, a circulation gas containing the reducing gas is obtained. Although the circulation gas obtained in the dehydration step S5 contains the reducing gas, the amount thereof is insufficient. Furthermore, since the temperature of the circulation gas obtained in the dehydration step S5 is low, it is inefficient to use the circulation gas directly in the reduction step S1. Therefore, in the temperature-raising step S6, the circulation gas and hydrogen gas are heated to obtain a reducing gas containing the circulation gas and hydrogen gas. In other words, the reducing gas is obtained by heating and mixing hydrogen gas as a make-up gas together with the circulation gas. In the temperature-raising step S6, the circulation gas and hydrogen gas may be mixed after heating, or the circulation gas and hydrogen gas may be mixed and then heated. The hydrogen gas may be obtained by electrolysis of water or membrane separation from synthesis gas (gas obtained by steam reforming or partial combustion of coal or biomass). The hydrogen gas may also be obtained by electrolyzing water vapor contained in the exhaust gas from the reduction step S1. The temperature-raising step S3 may be performed using a known heating device.

[0040] As described above, in one embodiment of the method for producing iron carbide, both the reduction step S1 and the carbonization step S2 may be performed in the shaft furnace 100. In this case, the exhaust gas from the carbonization step S2 rises directly within the furnace and is added to the reducing gas in the reduction step S1 and used for reduction. Furthermore, as shown in FIG. 3, the exhaust gas from the optional carbonization step S3 using CO gas may be discharged downstream of the carbonization step S2. This configuration eliminates CO gas and CO2 gas in the exhaust gas from the carbonization step S3 from the reduction step S1. This eliminates the inclusion of CO gas and CO2 gas in the exhaust gas from the reduction step S1, and the exhaust gas from the reduction step S1 consists only of reducing gas (e.g., hydrogen gas) and water vapor. By subjecting this exhaust gas to the dehydration step S5 described above, it can be reused as a reducing gas. Note that methane gas remains in the exhaust gas from the carbonization step S2, but this methane gas is decomposed in the reduction zone. Furthermore, the methane gas is diluted by being mixed with other gases in the reduction zone. That is, the exhaust gas from the reduction step S1 contains almost no methane gas, and even if it does contain methane, it is only about 1% by volume. Therefore, there is almost no effect of concentration of carbon-containing gases due to circulation. In other words, it can be handled by only a normal partial discharge process of the reduction gas outside the system, and no special CO2 removal process is required when circulating the exhaust gas.

[0041] In the manufacturing method of the present disclosure, the embodiments shown in Figures 1 to 4 may be combined. For example, in the manufacturing method shown in Figures 1 to 3, a dehydration step S4 and a temperature-raising step S5 as shown in Figure 4 may be performed. In addition, in the manufacturing method shown in Figure 2, a carbonization step S3 and a cooling step S4 as shown in Figure 3 may be further performed.

[0042] 1.4 Iron carbide Through the reduction step S1 and the carbonization step S2, iron carbide 30 (reduced iron with an increased carbon concentration) is produced. The iron carbide 30 may contain, in addition to carbon and iron, iron oxide, silicon dioxide, aluminum oxide, and the like that remain unreduced. The cementitization rate of the iron carbide 30 is equal to or higher than the target cementitization rate described above. The carbon in the iron carbide 30 may exist as elemental carbon in addition to cementite. The carbon concentration of the iron carbide 30 may be, for example, more than 0% by mass and 5% by mass or less, or 0.5% by mass or more and 5% by mass or less. The temperature of the iron carbide 30 produced by the method according to this embodiment (the surface temperature of the iron carbide 30 at the outlet of the shaft furnace 100 or the cooling tower 200) may be, for example, 150°C or less, or 80°C or less. That is, the iron carbide 30 may be CDRI with an increased carbon concentration. Alternatively, the temperature of the iron carbide 30 may be, for example, 100° C. or higher, 150° C. or higher, or 200° C. or higher. That is, the iron carbide 30 may be HDRI with an increased carbon concentration.

[0043] 2. Iron carbide manufacturing system The technology of the present disclosure also has an aspect as a system for producing iron carbide. That is, as shown in Figs. 1 to 4, the system for producing iron carbide according to one embodiment includes a reduction section 110 that reduces an oxidized iron raw material 10 with a reducing gas to obtain reduced iron 20, and a carbonization section 120 that carbonizes the reduced iron 20 while cooling it with a carbonization gas to obtain iron carbide 30. Here, the reduction section 110 is provided in a shaft furnace 100. The carbonization gas supplied to the carbonization section 120 contains 70% by volume or more of methane gas. In the carbonization section 120, the reduced iron is carbonized until the target cementitization rate or higher is reached. The carbonization section 120 satisfies the following relationship (1): Y≧0.24X-0.31 (1) Y: Time (minutes) elapsed until the temperature of the reduced iron reaches 700°C from 800°C X: The target cementitization rate (%) It is configured to satisfy the following.

[0044] In this embodiment, the reduction step S1 is performed in the reduction section 110, and the carbonization step S2 is performed in the carbonization section 120. The reduction section 110 and the carbonization section 120 may be configured to be able to perform the reduction step S1 and the carbonization step S2, respectively. For example, as shown in FIG. 1, the reduction section 110 and the carbonization section 120 may be provided in a shaft furnace 100. In this case, the reduction zone in the shaft furnace 100 corresponds to the reduction section 110, and the cooling zone corresponds to the carbonization section 120. Alternatively, as shown in FIG. 2, the reduction section 110 may be provided in the shaft furnace 100, and the carbonization section 120 may be provided separately from the shaft furnace 100. When the carbonization section 120 is provided separately from the shaft furnace 100, the carbonization section 120 may be provided in, for example, a cooling tower 200 provided downstream of the shaft furnace 100.

[0045] A reducing gas is supplied to the reduction unit 110. The type and supply form of the reducing gas are as described above. The reducing gas may contain, for example, one or both of hydrogen gas and methane gas.

[0046] A carbonization gas containing methane gas is supplied to the carbonization section 120. The composition and supply form of the carbonization gas are as described above. The carbonization section 120 is configured to satisfy the above relationship (1). The upper limit of the elapsed time Y in the carbonization section 120 is as described above. That is, from the viewpoint of increasing productivity, the carbonization section 120 may be configured so that the elapsed time Y is 30 minutes or less, 25 minutes or less, or 20 minutes or less.

[0047] 3, the iron carbide manufacturing system according to one embodiment may include a second carbonization section 130 to which a carbonization gas other than methane gas is supplied, and a cooling section 140 to which a cooling gas is supplied. The second carbonization section 130 and the cooling section 140 may be configured to be able to perform the above-described further carbonization step S3 and cooling step S4, respectively. The second carbonization section 130 and the cooling section 140 may be provided in the shaft furnace 100, or may be provided separately from the shaft furnace 100.

[0048] As shown in Fig. 4, the iron carbide manufacturing system according to one embodiment may include a dehydration device 150 that dehydrates the exhaust gas from the reduction section 110 to obtain a circulation gas, and a temperature raising device 160 that raises the temperatures of the circulation gas and hydrogen gas to obtain a reduction gas containing the circulation gas and hydrogen gas. The dehydration device 150 and the temperature raising device 160 are used to perform the dehydration step S5 and the temperature raising step S6, respectively. Details are as described above.

[0049] The exhaust gas system from the reduction section 110, the carbonization section 120, etc., and the temperatures of the reduced iron 20 in each of the reduction section 110 and the carbonization section 120 are also as described above.

[0050] Furthermore, in the production system of the present disclosure, when a shaft furnace 100 is employed as the reduction section 110, the pressure at the top of the shaft is not particularly limited, but may be in the range of 0 MPa to 0.8 MPa in gauge pressure. The pressure can be measured, for example, using a pressure gauge provided at the top of the shaft.

[0051] Furthermore, in the manufacturing system of the present disclosure, when a cooling tower 200 is employed as the carbonization section 120, the pressure at the top of the cooling tower is not particularly limited, but may be in the range of 0 MPa to 0.8 MPa in gauge pressure. The pressure can be measured, for example, using a pressure gauge installed at the top of the cooling tower.

[0052] In the manufacturing system of the present disclosure, the configurations shown in Figures 1 to 4 may be combined. For example, the manufacturing systems shown in Figures 1 to 3 may be combined with a dehydration device 150 and a heating device 160 as shown in Figure 4. Furthermore, the manufacturing systems shown in Figures 2 and 4 may be combined with a second carbonization section 130 and a cooling section 140 as shown in Figure 3.

[0053] 3.Effects As described above, according to the iron carbide manufacturing method and iron carbide manufacturing system of the present embodiment, when reduced iron 20 is obtained by a direct reduction process using the shaft furnace 100 and then the reduced iron 20 is carbonized to obtain iron carbide 30, the cementitization rate of the iron carbide 30 can be set to a target value or higher. Controlling the cementitization rate of the iron carbide 30 provides various benefits. For example, when melting the iron carbide 30 for a refining process or hot-forming the iron carbide 30 to produce HBI, it is sometimes better for the iron carbide 30 to contain more graphite (elementary carbon) and sometimes better for the iron carbide 30 to contain more cementite. For example, in the process of hot-forming the iron carbide 30 to produce HBI, it is sometimes better for the iron carbide 30 to contain more graphite (elementary carbon). Furthermore, in the process of melting the iron carbide 30, it is better for the iron carbide 30 to contain more cementite. According to this embodiment, it is possible to appropriately control the cementitization rate in the iron carbide 30, and to manufacture iron carbide 30 having an appropriate cementitization rate according to the subsequent process. Furthermore, by increasing the cementitization rate of the iron carbide 30 to a target value or higher, the iron carbide 30 becomes more easily melted in the subsequent processes, such as the melting process and the refining process, and it is also possible to reduce the amount of carbonaceous material added in the melting process and the refining process. [Example]

[0054] The present invention will be further described below with reference to examples, but the present invention is not limited to the following examples. The present invention allows various conditions to be adopted as long as the object is achieved without departing from the gist of the present invention.

[0055] 1. Experimental Method Iron oxide pellets with the chemical composition and particle size shown in Table 1 below were reduced with hydrogen to obtain reduced iron. Carburization tests were then conducted using a carbonization gas containing 70% by volume of methane gas and 30% by volume of nitrogen gas under 12 conditions: three reaction temperatures of 700°C, 800°C, and 900°C, and four reaction times of 5, 10, 20, and 30 minutes. The iron oxide pellets used were 10-15 mm in size, suitable for a direct reduction process using a shaft furnace. After the carburization tests, the samples were subjected to X-ray diffraction analysis, and the cementite (FeC) content in the samples was quantitatively analyzed using the Rietveld refinement method on the obtained diffraction patterns.

[0056] [Table 1]

[0057] 2. Experimental Results Figures 5A-C show the experimental results. In Figures 5A-C, "C concentration" refers to the carbon concentration in the iron carbide after the carburization test, "time" refers to the reaction time, "Total C" refers to the total carbon contained in the iron carbide after the carburization test, "C as free C" refers to the carbon present as elemental carbon in the iron carbide after the carburization test, and "C as Fe3C" refers to the carbon present as cementite in the iron carbide after the carburization test. Figure 5A shows the results when the reaction temperature was 700°C, Figure 5B shows the results when the reaction temperature was 800°C, and Figure 5C shows the results when the reaction temperature was 900°C. As shown in Figures 5A-C, when the reaction temperature was 700-800°C, the cementitization rate increased with the reaction time and was roughly saturated after 30 minutes. On the other hand, when the reaction temperature is 900°C, the cementite conversion rate hardly increases even with the passage of carburization time, and it is clear that cementite decomposes and precipitates as elemental carbon.

[0058] From the results shown in FIGS. 5A and 5B, the relationship between the elapsed time and the cementitious ratio at a reaction temperature of 700° C. or 800° C. is summarized as shown in Table 2 below.

[0059] [Table 2]

[0060] Using the average of 700 to 800°C shown in Table 2 as a representative value, the relationship between the time Y elapsed from 800°C to 700°C and the cementite conversion rate X is summarized in FIG. 6. As shown in FIG. 6, the relationship between the time Y and the cementite conversion rate X is approximately linear. Specifically, the relationship Y = 0.24X - 0.31 is satisfied. In other words, when the relationship Y ≥ 0.24X - 0.31 is satisfied, it can be said that reduced iron can be carbonized to a target cementite conversion rate or higher. This relationship is applicable, for example, when the target cementite conversion rate X is 3% or higher.

[0061] 3. Summary From the above results, it can be said that according to the method for producing iron carbide comprising the following configurations (A) to (E), when reduced iron is obtained by a direct reduction process using a shaft furnace and then the reduced iron is carbonized to obtain iron carbide, the cementite conversion rate in the iron carbide can be made equal to or higher than a target value. (A) A method for producing iron carbide includes a reduction step in which an iron oxide raw material is reduced with a reducing gas to obtain reduced iron, and a carbonization step in which the reduced iron is carbonized with a carbonizing gas while being cooled to obtain iron carbide. (B) The reduction step is carried out in a shaft furnace. (C) In the carbonization step, the carbonization gas contains methane gas in an amount of 70% by volume or more and 100% by volume or less. (D) In ​​the carbonization step, the reduced iron is carbonized until the cementitization rate reaches a target cementitization rate or higher. (E) In the carbonization step, the following relationships (1) and (2) are satisfied: Y≧0.24X-0.31 (1) X≧3 (2) Y: Time (minutes) elapsed until the temperature of the reduced iron reaches 700°C from 800°C X: The target cementitization rate (%) is satisfied. [Explanation of symbols]

[0062] 10 Iron oxide raw materials 20 Reduced iron 30 Iron carbide (product reduced iron) 100 shaft furnace 110 Reduction Department 120 Carbonization section 130 Second carbonization section 140 Cooling section 200 Cooling Tower

Claims

1. A method for producing iron carbide, comprising: a reduction step of reducing the oxidized iron raw material with a reducing gas to obtain reduced iron; a carbonization step of carbonizing the reduced iron while cooling it with a carbonizing gas to obtain iron carbide; and The reduction step is carried out in a shaft furnace, In the carbonization step, The carbonized gas contains 70% by volume or more of methane gas, The reduced iron is carbonized until the cementitization rate reaches a target cementitization rate or more, and The following relationships (1) and (2): Y≧0.24X-0.31...(1) X≧3 (2) Y: Time (minutes) elapsed until the temperature of the reduced iron reaches 700°C from 800°C X: the target cementitious ratio (%) is satisfied, Method for producing iron carbide.

2. The method for producing iron carbide according to claim 1, The elapsed time Y is 30 minutes or less. Method for producing iron carbide.

3. The method for producing iron carbide according to claim 1, The elapsed time Y is 20 minutes or less. Method for producing iron carbide.

4. The method for producing iron carbide according to any one of claims 1 to 3, The carbonization step is carried out in the shaft furnace. Method for producing iron carbide.

5. The method for producing iron carbide according to any one of claims 1 to 3, The carbonization step is carried out outside the shaft furnace. Method for producing iron carbide.

6. The method for producing iron carbide according to any one of claims 1 to 3, The reducing gas includes one or both of hydrogen gas and methane gas. Method for producing iron carbide.