Iron carbide manufacturing method

Abstract

Disclosed is a technique in which the cementite conversion rate in iron carbide is greater than or equal to a target value when reduced iron is first obtained through a direct reduction process using a shaft furnace, and then the iron carbide is obtained by carburizing the reduced iron. The iron carbide manufacturing method according to the present disclosure has a reduction step in which an iron oxide raw material is reduced with a reducing gas to obtain reduced iron, and a carburization step in which the reduced iron is carburized with a carburizing gas while being cooled to obtain iron carbide. The reduction step is performed in a shaft furnace. In the carburization step, the carburizing gas includes 70 vol% or more of methane gas, the reduced iron is carburized until reaching the target cementite conversion rate or higher, and the relationship in which Y≥0.24X-0.31 and X≥3 (where "Y" is the elapsed time (minutes) for the reduced iron to reach 700°C from 800°C, and "X" is the target cementite conversion rate (%)) is satisfied.

Description

Iron carbide manufacturing method

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

[0002] In the steel industry, as an alternative to the blast furnace method, a direct reduction process using reducing gas is used to reduce CO ２ In the direct reduction process, direct reduced iron (DRI) is obtained by bringing a reducing gas into contact with an oxidized iron raw material. The carbon content of the reduced iron may be increased (i.e., carbonized) to lower the melting temperature in the subsequent melting and refining processes and to ensure the strength required for steel. Carbonizing the reduced iron is also desirable from the viewpoint of preventing reoxidation of the reduced iron and improving its handleability. Examples of methods for carbonizing the reduced iron include a reduction step in which a reducing gas is brought into contact with an oxidized iron raw material to obtain the reduced iron, and a carbonization step in which the reduced iron is brought into contact with methane gas or the like to cool the reduced iron and carbonize the metallic iron contained in the reduced iron. Here, the temperature of the reduced iron obtained in the reduction step is approximately 700°C to 1050°C. In addition, the carbonization reaction of metallic iron with methane gas (CH ４ (g)+3Fe(s)⇒2H ２ (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 techniques, 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 discloses a method in which a hydrocarbon such as methane, pentane, or hexane is used as the cooling gas.

[0004] International Publication No. 2021 / 029114 Japanese Patent Application Laid-Open No. 61-073805

[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.

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

[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.

[0008] FIG. 1 is a schematic diagram for explaining an example of a method for producing iron carbide and an iron carbide production system. FIG. 2 is a schematic diagram for explaining an example of a method for producing iron carbide and an iron carbide production system. FIG. 3 is a schematic diagram for explaining an example of a method for producing iron carbide and an iron carbide production system. FIG. 4 is a schematic diagram for explaining an example of a method for producing iron carbide and an iron carbide production system. FIG. 5 shows the relationship between elapsed time and carbon concentration when the reaction temperature is 700°C. FIG. 6 shows the relationship between elapsed time and carbon concentration when the reaction temperature is 800°C. FIG. 7 shows the relationship between elapsed time and carbon concentration when the reaction temperature is 900°C. FIG. 8 shows the relationship between elapsed time and carbon concentration when the reaction temperature is from 800°C to 700°C.

[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 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 volume % or more of methane gas. In the carbonization step S2, the reduced iron 20 is carbonized until the reduced iron 20 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), where Y is the elapsed time (minutes) required for the reduced iron 20 to cool from 800°C to 700°C, and X is the target cementitization rate (%).

[0011] 1.1 Reduction Step 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 Figures 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 effect can be obtained even when the reduction step S1 is performed in a kiln instead of the shaft furnace 100.

[0012] 1.1.1 Oxidized Iron Raw Material 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 may have a uniform particle diameter. The average particle diameter 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 diameter" of the oxidized iron raw material refers to the sieve diameter of the raw material. The "average particle diameter" of the oxidized iron raw material refers to the weighted average value of the particle diameters of the raw material. The average particle diameter 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 calculating a 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, inside the shaft furnace 100, the oxidized iron raw material 10 is substantially filled and gradually moves downward by falling or the like. When focusing on a single raw material particle in the packed bed, the raw material particle may move continuously downward at a constant speed or may move intermittently by repeatedly falling 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. In addition to hydrogen gas and methane gas, the reducing gas may also contain other gases. Examples of other gases include CO gas, inert gas, and CO ２ Examples of the reducing gas include hydrogen gas, 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 or more and 100% by volume or less, 60% by volume or more and 100% by volume or less, or 80% by volume or more and 100% by volume or less. 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 more. The temperature of the reducing gas is preferably 800°C or more and 1100°C or less.

[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 via 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 not only metallic iron but also remaining unreduced iron oxide, 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, greater than 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 greater than 700°C and less than 1050°C, greater than 750°C and less than 1000°C, or greater than 800°C and less than 900°C.

[0017] In the present 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 total 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 similar to that of known shaft furnaces. 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 located 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 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 discharge port for discharging the cooling gas below the reducing gas supply port. The cooling gas supply port may be provided on the sidewall of the furnace or inside the sidewall of the furnace. The cooling gas outlet may be provided in the side wall of the furnace.

[0019] 1.2 Carbonization Step 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 the iron carbide 30 in this way, the melting temperature in the subsequent melting and refining steps 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. Note that in this application, the "downstream side" refers to the downstream side in the iron carbide production process. That is, when iron carbide 30 is produced from oxidized iron raw material 10 via reduced iron 20, the oxidized iron 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 brought into contact with the carbonizing gas, thereby cooling and carbonizing the reduced iron 20. 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, the reduced iron 20 is brought into contact with the carbonization gas, whereby a cementite production reaction represented by the following formula (A) and a cementite decomposition reaction represented by the following formula (B) can occur. According to the knowledge 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) ⇒ Fe ３ C(s) ...(A) Fe ３ C(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 longer, 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 measured 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 the measurement point i (i = 1 to N) is r ｉ is defined by the following formula: ０ , r Ｎ＋１ corresponds to the center of the furnace (r ０ = 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 in contact with the carbonizing gas may be a temperature estimated by simulation. In the carbonization step S2, the temperature of the carbonizing gas in 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.

[0025] 1.2.2 Carbonization Gas The carbonization gas brought into contact with the reduced iron 20 in the carbonization step S2 contains 70 vol% 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 vol% to 100 vol%, 75 vol% to 100 vol%, 80 vol% to 100 vol%, 85 vol% to 100 vol%, 90 vol% to 100 vol%, or 95 vol% to 100 vol%. The composition of the carbonization gas is based on the concentration of the gas before (just before) it is 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 the methane gas. The carbonization gas in the carbonization step S2 may be nitrogen gas, hydrogen gas, CO gas, CO ２The carbonization gas may contain methane gas, water vapor, etc. The proportion of gases other than methane gas in the carbonization gas may be, for example, 0 vol% to 30 vol%, 0 vol% to 25 vol%, 0 vol% to less than 20 vol%, 0 vol% to 15 vol%, 0 vol% to 10 vol%, or 0 vol% to 5 vol%. The proportion of nitrogen gas in the carbonization gas may be 0 vol% to 30 vol%, 0 vol% to 25 vol%, 0 vol% to less than 20 vol%, 0 vol% to 15 vol%, 0 vol% to 10 vol%, or 0 vol% to 5 vol%. The proportion of hydrogen gas in the carbonization gas may be 0 vol% to 30 vol%, 0 vol% to 25 vol%, 0 vol% to less than 20 vol%, 0 vol% to 15 vol%, 0 vol% to 10 vol%, or 0 vol% to 5 vol%. The proportion of CO gas contained in the carbonization gas is preferably small. 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. ２ The proportion of the gas may be 0 vol% or more and 30 vol% or less, 0 vol% or more and 25 vol% or less, 0 vol% or more and less than 20 vol%, 0 vol% or more and 15 vol% or less, 0 vol% or more and 10 vol% or less, or 0 vol% or more and 5 vol% or less. The proportion of water vapor in the carbonization gas may be 0 vol% or more and 30 vol% or less, 0 vol% or more and 25 vol% or less, 0 vol% or more and less than 20 vol%, 0 vol% or more and 15 vol% or less, 0 vol% or more and 10 vol% or less, or 0 vol% or more and 5 vol% 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 above-mentioned reducing gas. 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 carbonization of metallic iron contained in the reduced iron 20 progresses by bringing methane gas into contact with the reduced iron 20. Furthermore, 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, by setting the elapsed time Y during which the reduced iron 20 is cooled from 800°C to 700°C to a predetermined value or longer in the carbonization step S2, the reduced iron 20 can be carbonized to a target cementite conversion rate X 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 (minutes) required for the reduced iron 20 to reach 700° C. from 800° C. X: target cementite conversion rate (%)

[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 cementitious ratio X or more to obtain iron carbide 30. The target cementitious ratio X may be appropriately determined depending on the application of the iron carbide 30. The target cementitious ratio X is 3% or more, and may be, for example, 5% or more and 100% or less, or 10% or more and 30% or less.

[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, and measurement at a scan angle 2θ = 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, the preferred orientation is corrected using the March-Dollase function. In this analysis, the background function, the lattice constant of each mineral phase, the profile function, and the crystal structure factor are the targets of refinement. 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 to be 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), and other iron carbides (χ-Fe ５ C ２ ) (ICDD:01-089-2544), Hexagonal carbide (ε-Fe ２C) (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 operation conditions. For example, when a material containing a high concentration of alumina is used as the oxidized iron raw material 10, it is considered that gehlenite is included in the reduced iron 20 (iron carbide 30) after the carbonization step S2, and therefore gehlenite is included as the phase to be analyzed. Furthermore, when CaO is added as the oxidized iron raw material 10 and the sole SiO ２ When a material with a high content of 2CaO.SiO, such as Larnite, is used, ２ The phases to be analyzed are to be included. Non-Patent Document 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 the 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 Steps In this embodiment, other steps may be performed in addition to the reduction step S1 and the carbonization step S2. For example, as shown in FIG. 3 , in one embodiment, carbonization using a carbonizing gas other than methane gas (carbonization step S3) may be performed in addition to the carbonization step S2. In addition to the carbonization step S2, cooling using a cooling gas other than methane gas (cooling step S4) may be performed in addition to the carbonization step S2. Note that in this embodiment, it is sufficient that the temperature of the reduced iron 20 is reduced in the carbonization step S2. The temperature of the reduced iron 20 may be increased in a step other than the carbonization step S2 (e.g., a step subsequent to the carbonization step S2). For example, when the carbonization step S3 using CO gas is performed subsequent to the carbonization step 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 Step Using Carbonizing Gas Other Than Methane Gas In one embodiment, after the reduced iron 20 is carbonized by contacting the carbonizing gas with the reduced iron 20 in the carbonization step S2, the reduced iron 20 may be further carbonized with CO gas in a further carbonization step S3. This tends to further increase the carbonization degree of the iron carbide 30 finally obtained. 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 with 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 the 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 reduced iron 20 after carbonization with methane gas into contact with CO gas. Note that other gases may be brought into contact with the reduced iron 20 after carbonization with methane gas together with CO gas. Examples of gases other than CO gas include nitrogen gas, methane gas, hydrogen gas, CO ２ The CO-containing gas may be, for example, converter gas (LDG).

[0036] In the carbonization step S2, after the reduced iron 20 is carbonized by bringing the carbonization gas into contact with the reduced iron 20, if the reduced iron 20 is further brought into contact with CO gas or the like, exhaust gas derived from the CO gas or the like is generated. The exhaust gas is, for example, mainly CO gas and CO２ The exhaust gas may be discharged to the outside of the system and used as fuel.

[0037] 1.3.2 Further Cooling Step 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 in the above-described carbonization step S2, or after the further carbonization step S3 using CO gas is further carried out, 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. In the further cooling step S4, when the gas that comes into contact with the carbonized reduced iron 20 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 Step and Temperature-Raising Step As shown in FIG. 4 , a method for producing reduced iron according to an 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-raising 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 an 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 of the reducing gas is insufficient. Furthermore, since the circulating gas obtained in the dehydration step S5 has a low temperature, it is inefficient to use it directly in the reduction step S1. Therefore, in the temperature increase step S6, the circulating gas and hydrogen gas are increased in temperature to obtain a reducing gas containing the circulating gas and hydrogen gas. In other words, the reducing gas is obtained by increasing the temperature and mixing hydrogen gas as a make-up gas together with the circulating gas. In the temperature increase step S6, the circulating gas and hydrogen gas may be mixed after being increased in temperature, or the circulating gas and hydrogen gas may be mixed and then increased in temperature. 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 increase step S3 may be performed using a known heating device.

[0040] As described above, in the method for producing iron carbide according to one embodiment, 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 is used for reduction. Also, as shown in FIG. 3, the exhaust gas from the carbonization step S3 using CO gas, which is an optional step, may be discharged outside the system downstream of the carbonization step S2. With these configurations, the CO gas and CO ２ Gas is removed from the reduction step S1. That is, CO gas and CO are removed from the exhaust gas of the reduction step S1. ２ There is no gas contamination, and the exhaust gas from the reduction step S1 consists only of reducing gas (e.g., hydrogen gas) and water vapor. By subjecting such exhaust gas to the above-mentioned dehydration step S5, it can be reused as 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, this methane gas is combined with other gases in the reduction zone and diluted. In other words, 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, this can be achieved by only a normal partial out-of-system discharge step of the reducing gas, and no special CO 2 removal is required when circulating the exhaust gas. ２ No process is required.

[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. Furthermore, 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. In addition to carbon and iron, the iron carbide 30 may contain unreduced iron oxide, silicon dioxide, aluminum oxide, and the like. 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 include elemental carbon as well as 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 disclosed herein also has an aspect as an iron carbide manufacturing system. That is, as shown in FIGS. 1 to 4 , an iron carbide manufacturing system according to one embodiment includes a reduction unit 110 that reduces an oxidized iron raw material 10 with a reducing gas to obtain reduced iron 20, and a carbonization unit 120 that carbonizes the reduced iron 20 while cooling it with a carbonization gas to obtain iron carbide 30. Here, the reduction unit 110 is provided in a shaft furnace 100. The carbonization gas supplied to the carbonization unit 120 contains 70% or more by volume of methane gas. In the carbonization unit 120, the reduced iron is carbonized until the reduced iron reaches a target cementitization rate or higher. The carbonization unit 120 is configured to satisfy the following relationship (1): Y≧0.24X−0.31 (1), where Y is the elapsed time (minutes) required for the reduced iron to cool from 800°C to 700°C, and X is the target cementitization rate (%).

[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 configurations of the reduction section 110 and the carbonization section 120 need only 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 circulating gas, and a temperature raising device 160 that raises the temperatures of the circulating gas and hydrogen gas to obtain a reducing gas containing the circulating 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 unit 110, the carbonization unit 120, etc., and the temperatures of the reduced iron 20 in each of the reduction unit 110 and the carbonization unit 120 are also as described above.

[0050] Furthermore, in the production system of the present disclosure, when the shaft furnace 100 is employed as the reduction section 110, the shaft furnace top pressure 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 shaft furnace top.

[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 in the iron carbide 30 can be set to a target value or higher. Controlling the cementitization rate of the iron carbide 30 provides various effects. For example, when melting the iron carbide 30 to perform a refining process or when 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 sometimes 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.

[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 having the chemical composition and particle size shown in Table 1 below were reduced with hydrogen to obtain reduced iron, and then a carburization test was carried out using a carburizing gas containing 70% by volume of methane gas and 30% by volume of nitrogen gas under a total of 12 conditions: three reaction temperatures of 700°C, 800°C, and 900°C, and four reaction times of 5 minutes, 10 minutes, 20 minutes, and 30 minutes. The iron oxide pellets used here had a size (10-15 mm) suitable for a direct reduction process using a shaft furnace. The samples after the carburization test were subjected to X-ray diffraction, and the resulting diffraction patterns were analyzed by Rietveld analysis to determine the cementite (Fe) content in the samples. ３C) The content was quantitatively analyzed.

[0056]

[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 described above, "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 reaction time and roughly saturated after 30 minutes. On the other hand, when the reaction temperature is 900°C, the cementitious ratio hardly increases even with the passage of carburizing time, and it is found 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 cementitization rate at a reaction temperature of 700° C. or 800° C. is summarized as shown in Table 2 below.

[0059]

[0060] The relationship between the time Y elapsed from 800°C to 700°C and the cementitization rate X is shown in FIG. 6, where the average of 700 to 800°C shown in Table 2 is used as a representative value. As shown in FIG. 6, the relationship between the time Y elapsed and the cementitization 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 cementitization rate or higher. This relationship is applicable, for example, when the target cementitization rate X is 3% or higher.

[0061] 3. Summary From the above results, it can be said that a method for producing iron carbide comprising the following configurations (A) to (E) can achieve a cementitization rate in the iron carbide equal to or higher than a target value 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. (A) The 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 performed in a shaft furnace. (C) In the carbonization step, the carbonizing gas contains 70 vol% to 100 vol% methane gas. (D) In ​​the carbonization step, the reduced iron is carbonized until the cementitization rate reaches or exceeds a target value. (E) In the carbonization step, the following relationships (1) and (2) are satisfied: Y≧0.24X−0.31 (1) X≧3 (2), where Y is the elapsed time (minutes) for the reduced iron to reach a temperature of 800° C. to 700° C., and X is the target cementite conversion rate (%).

[0062] REFERENCE SIGNS LIST 10 iron oxide raw material 20 reduced iron 30 iron carbide (product reduced iron) 100 shaft furnace 110 reduction section 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 an iron oxide raw material with a reducing gas to obtain reduced iron; and a carbonization step of carbonizing the reduced iron while cooling it with a carbonizing gas to obtain iron carbide, wherein the reduction step is carried out in a shaft furnace, and in the carbonization step, the carbonizing gas contains 70 volume % or more of methane gas, and the reduced iron is carbonized to a target cementitization rate or higher, and the following relationships (1) and (2) are satisfied: Y≧0.24X−0.31 (1), X≧3 (2), Y: elapsed time (minutes) for the reduced iron to cool from 800°C to 700°C, and X: target cementitization rate (%).

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

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

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

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

6. A method for producing iron carbide according to any one of claims 1 to 5, wherein the reducing gas contains one or both of hydrogen gas and methane gas.

Images