Production method for iron carbide

Abstract

Provided is a technique for obtaining semi-reduced iron that has low metallization by a direct-reduction process that uses a shaft furnace and then efficiently carbonizing the semi-reduced iron. This production method for iron carbide has a reduction step for bringing a reducing gas into contact with an iron oxide starting material to obtain semi-reduced iron and a carbonization step for bringing a carbonizing gas into contact with the semi-reduced iron to obtain iron carbide. The reduction step is performed in a shaft furnace. The metallization of the semi-reduced iron is no more than 90%. The carbonization step is performed at a reaction temperature of 700°C–850°C. The carbonizing gas of the carbonization step is at least 30 vol% but less than 100 vol% methane gas and more than 0 vol% but no more than 40 vol% hydrogen gas.

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 Reducing carbon dioxide emissions has been studied. 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 of the resulting steel. Carbonizing the reduced iron is also desirable from the viewpoint of preventing reoxidation of the reduced iron and improving its handleability. One method for carbonizing the reduced iron is to bring a reducing gas into contact with an oxidized iron raw material to obtain the reduced iron, and then to bring the reduced iron into contact with a hydrocarbon gas or the like to carburize it. For example, Patent Document 1 discloses a vertical reduction furnace having a reduction zone and a cooling zone, which is provided with a cooling gas loop for circulating a cooling gas for cooling the reduced iron, and which uses a gas with a hydrocarbon concentration of 50% or more as the cooling gas.

[0003] On the other hand, as disclosed in Non-Patent Documents 1 and 2, there is a method for improving the overall reduction efficiency in the blast furnace process by using reduced iron as an iron source. In this case, the metallization rate of the reduced iron is said to be 74.5 to 94.4%.

[0004] International Publication No. 2021 / 029114

[0005] Kazuya KUNITOMO, Yasushi TAKAMOTO, Masaaki NAITO, Jun-ichiro YAG, Journal of the Japan Institute of Energy, 2005, Volume 84, Issue 2, 126. Yutaka UJISAWA, Kaoru NAKANO, Yoshinori MATSUKURA, Kohei SUNAHARA, Shusaku KOMATSU, Takaiku YAMAMOTO, ISIJ International, 2005, Volume 45, Issue 10, 1379.

[0006] In the prior art, when semi-reduced iron with a low metallization rate is obtained by a direct reduction process using a shaft furnace, a method for carbonizing the semi-reduced iron has not been sufficiently studied.

[0007] 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 bringing a reducing gas into contact with an oxidized iron raw material to obtain partially reduced iron; and a carbonization step of bringing a carbonization gas into contact with the partially reduced iron to obtain iron carbide, wherein the reduction step is carried out in a shaft furnace, wherein a metallization rate of the partially reduced iron is 90% or less, the reaction temperature in the carbonization step is 700°C or more and 850°C or less, and the carbonization gas in the carbonization step comprises: 30% by volume or more and less than 100% by volume of methane gas and more than 0% by volume and 40% by volume or less of hydrogen gas. <Aspect 2> The method for producing iron carbide according to Aspect 1, wherein the carbonization step is carried out in the shaft furnace. <Aspect 3> The method for producing iron carbide according to Aspect 1, wherein the carbonization step is carried out outside the shaft furnace. <Aspect 4> The method for producing iron carbide according to any one of Aspects 1 to 3, wherein the reducing gas contains one or both of hydrogen gas and methane gas. <Aspect 5> The method for producing iron carbide according to any one of Aspects 1 to 4, wherein the volume V of the hydrogen gas is １ and the volume V of the methane gas ２ Relative to V １ / V ２ is greater than 0 and not greater than 0.75. <Aspect 6> The method for producing iron carbide according to any one of Aspects 1 to 5, wherein the carbonizing gas contains 15 volume % or more and 25 volume % or less of hydrogen gas.

[0008] According to the method for producing iron carbide of the present disclosure, semi-reduced iron with a low metallization rate can be appropriately carbonized.

[0009] Fig. 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. 2 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. 3 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. 4 shows the relationship between the degree of carbonization (the proportion of iron that has become cementite to the total iron in the iron carbide) and the hydrogen gas concentration of the carbonization gas in the carbonization process.

[0010] 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 process. "Semi-reduced iron" refers to an intermediate product that has been produced after the reduction process and before the final product has been produced through the carbonization process, and has a metallization rate of 90% or less. "Iron carbide" refers to reduced iron that has been produced through the carbonization process and has an increased carbon concentration.

[0011] 1 to 4 , a method for producing iron carbide according to one embodiment includes a reduction step S1 in which a reducing gas is brought into contact with an oxidized iron raw material 10 to obtain partially reduced iron 20, and a carbonization step S2 in which a carbonization gas is brought into contact with the partially reduced iron 20 to obtain iron carbide. Here, the reduction step S1 is performed in a shaft furnace 100. The partially reduced iron 20 has a metallization rate of 90% or less. The reaction temperature in the carbonization step S2 is 700°C or higher and 850°C or lower. The carbonization gas in the carbonization step S2 contains 30% by volume or higher and lower than 100% by volume of methane gas and more than 0% by volume and 40% by volume or lower of hydrogen gas.

[0012] 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 semi-reduced iron 20. As shown in Figures 1 and 2, 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.

[0013] 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.

[0014] 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.

[0015] 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.

[0016] 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.

[0017] 1.1.3 Semi-reduced iron In the reduction step S1, a portion of the iron oxide contained in the oxidized iron raw material 10 is reduced to obtain semi-reduced iron 20 containing metallic iron. The semi-reduced iron 20 contains 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 semi-reduced iron 20 immediately after reduction may be, for example, 700°C or higher. There is no particular upper limit to the temperature of the semi-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 semi-reduced iron 20 immediately after reduction may be, for example, 1100°C or lower. The temperature of the semi-reduced iron 20 immediately after reduction may be 700°C or higher and 1050°C or lower, 700°C or higher and 1000°C or lower, or 700°C or higher and 900°C or lower.

[0018] In the present embodiment, the metallization rate of the semi-reduced iron 20 obtained in the reduction step S1 ([mass of metallic iron in the semi-reduced iron 20] / [mass of all iron in the semi-reduced iron 20] × 100) is 90% or less. Carbonization of semi-reduced iron 20 with a low metallization rate is considered to proceed in two stages: (1) reduction of iron oxide, and (2) carbonization of reduced metallic iron. Therefore, the overall carbonization rate of the semi-reduced iron 20 is considered to be affected by the reduction reaction rate of the previous stage. In the present embodiment, in the carbonization step S2 described below, the carbonization of the semi-reduced iron 20 is promoted by using not only methane gas but also hydrogen gas, which has the effect of promoting reduction. The lower limit of the metallization rate of the semi-reduced iron 20 is not particularly limited. The metallization rate of the semi-reduced iron 20 may be, for example, 50% to 90%, 60% to 90%, 70% to 90%, or 80% to 90%.

[0019] The measurement method for the "metallization rate of semi-reduced iron" may be performed in accordance with ISO 5416:2006. In either case where both the reduction step and the carbonization step described below are performed inside a shaft furnace, or where the reduction step is performed inside a shaft furnace and the carbonization step is performed outside the shaft furnace, the metallization rate of semi-reduced iron obtained in the reduction step may be determined as an estimated value based on mass balance from the amount and composition of the gas supplied to the shaft furnace, the amount and composition of the gas discharged from the shaft furnace, the amount and composition of the raw materials supplied to the shaft furnace, and the amount and composition of the reduced iron discharged from the shaft furnace. In addition, when the semi-reduced iron obtained in the reduction step can be sampled, the metallization rate of the semi-reduced iron may be determined by directly analyzing the sampled semi-reduced iron, and the metallization rate may be used as a reference for the estimated value obtained by the mass balance.

[0020] 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.

[0021] 1.2 Carbonization Step The temperature of the partially reduced iron 20 immediately after the reduction step S1 is, for example, 700°C or higher. In the carbonization step S2, the partially reduced iron 20 at such a high temperature is brought into contact with a carbonizing gas containing methane gas and hydrogen gas. This causes an endothermic reaction, a carbon deposition reaction by the methane gas, and an endothermic reaction, a reduction reaction by the hydrogen gas, to proceed, and the partially reduced iron 20 is carbonized and cooled. Through the carbonization step S2, iron carbide 30 (reduced iron with an increased carbon concentration) is obtained. Carbonizing the partially reduced iron 20 to obtain iron carbide 30 in this way lowers the melting temperature in the subsequent melting and refining steps and ensures the strength required for steel. Furthermore, reoxidation of the reduced iron is prevented, improving handling, such as transportation and storage.

[0022] 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 semi-reduced iron 20, the oxidized iron raw material 10 side is the upstream side, and the iron carbide 30 side is the downstream side.

[0023] In the carbonization step S2, the half-reduced iron 20 is brought into contact with the carbonizing gas, thereby reducing and carbonizing the half-reduced iron 20. The half-reduced iron 20 is partially carbonized. There are no particular limitations on the method for bringing the carbonizing gas into contact with the half-reduced iron 20. For example, the carbonizing gas can be brought into contact with the half-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.

[0024] The reaction temperature in the carbonization step S2 (the surface temperature of the semi-reduced iron 20 in contact with the carbonizing gas) is the temperature at which reduction by the hydrogen gas contained in the carbonizing gas and carbonization by the methane gas contained in the carbonizing gas proceed. According to the findings of the present inventors, when the reaction temperature in the carbonization step S2 is 700°C or higher and 850°C or lower, the reduction reaction by the hydrogen gas and the carbon deposition reaction by the methane gas can proceed more appropriately, and the carbon concentration in the finally obtained iron carbide 30 becomes appropriate. The reaction temperature in the carbonization step S2 may be 730°C or higher or 750°C or higher, or 830°C or lower or 800°C or lower.

[0025] The surface temperature of the semi-reduced iron 20 in the carbonization step S2 (the surface temperature of the semi-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 semi-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 semi-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 be placed, for example. In the carbonization step S2, the temperature of the carbonization gas that comes into contact with the semi-reduced iron 20 is not particularly limited. The temperature of the carbonization gas may be, for example, 25°C or higher and 600°C or lower.

[0026] In the carbonization step S2, it is important to bring the carbonization gas containing 30 vol % or more and less than 100 vol % methane gas and more than 0 vol % and 40 vol % or less hydrogen gas into contact with the half-reduced iron 20 at the above-mentioned reaction temperature. This allows the carbonization reaction to proceed efficiently for the half-reduced iron 20 with a low metallization rate. If the carbonization gas does not contain hydrogen gas, the reduction efficiency of the iron oxide contained in the half-reduced iron 20 decreases, and as a result, the carbonization degree of the finally obtained iron carbide 30 also decreases. On the other hand, if the carbonization gas contains an excessive amount of hydrogen gas, the proportion of methane gas in the carbonization gas decreases relatively, the carbonization efficiency of metallic iron decreases, and as a result, the carbonization degree of the finally obtained iron carbide 30 also decreases. The proportion of methane gas in the carbonization gas in the carbonization step S2 may be, for example, 33 vol% or more, 35 vol% or more, 37 vol% or more, 40 vol% or more, 43 vol% or more, 45 vol% or more, 47 vol% or more, 50 vol% or more, 53 vol% or more, 55 vol% or more, 57 vol% or more, 60 vol% or more, 63 vol% or more, 65 vol% or more, 67 vol% or more, 70 vol% or more, 73 vol% or more, 75 vol% or more, or 77 vol% or more, or may be 99 vol% or less, 97 vol% or less, 95 vol% or less, 93 vol% or less, 91 vol% or less, 89 vol% or less, 87 vol% or less, 85 vol% or less, 83 vol% or less, 81 vol% or less, or 80 vol% or less. In one embodiment, the proportion of methane gas in the carbonization gas may be 30 vol% or more and 95 vol% or less, 30 vol% or more and 90 vol% or less, 30 vol% or more and 85 vol% or less, 30 vol% or more and 80 vol% or less, 30 vol% or more and 75 vol% or less, 30 vol% or more and 70 vol% or less, or 30 vol% or more and less than 65 vol%; alternatively, 60 vol% or more and less than 100 vol%, 65 vol% or more and 99 vol% or less, 67 vol% or more and 97 vol% or less, 70 vol% or more and 93 vol% or less, 70 vol% or more and 90 vol% or less, 70 vol% or more and 85 vol% or less, 70 vol% or more and 80 vol% or less, 75 vol% or more and 85 vol% or less, or 75 vol% or more and 80 vol% or less.The proportion of hydrogen gas in the carbonization gas may be, for example, 1 vol% or more, 3 vol% or more, 5 vol% or more, 9 vol% or more, 11 vol% or more, 13 vol% or more, 15 vol% or more, 17 vol% or more, 19 vol% or more, or 20 vol% or more, or 35 vol% or less, 33 vol% or less, 30 vol% or less, or 27 vol% or less. In one embodiment, the proportion of hydrogen gas in the carbonization gas may be 1 vol% or more and 35 vol% or less, 3 vol% or more and 33 vol% or less, 5 vol% or more and 30 vol% or less, 7 vol% or more and 30 vol% or less, 10 vol% or more and 30 vol% or less, 15 vol% or more and 30 vol% or less, 20 vol% or more and 30 vol% or less, 15 vol% or more and 25 vol% or less, or 20 vol% or more and 25 vol% or less. The volume V of hydrogen gas in the carbonization gas is １ and the volume of methane gas V ２ Relative to V １ / V ２ may be, for example, greater than 0 and not greater than 1.33, greater than 0 and not greater than 1.20, greater than 0 and not greater than 1.10, greater than 0 and not greater than 1.00, greater than 0 and not greater than 0.90, greater than 0 and not greater than 0.80, greater than 0 and not greater than 0.75, 0.10 or greater and not greater than 0.75, 0.20 or greater and not greater than 0.70, 0.30 or greater and not greater than 0.65, or 0.40 or greater and not greater than 0.60. 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 hydrogen gas, etc.

[0027] In the carbonization step S2, it is sufficient that the reduction of iron oxide proceeds through a reduction reaction with hydrogen gas and the carbonization of metallic iron proceeds through a carbon deposition reaction with methane gas, and other gases may be brought into contact with the semi-reduced iron 20 in addition to methane gas and hydrogen gas. The carbonization gas in the carbonization step S2 may be nitrogen gas, CO gas, CO ２The carbonization gas may contain methane gas, water vapor, etc. The proportion of gases other than methane gas and hydrogen gas in the carbonization gas may be, for example, 0 vol% or more and less than 70 vol%, 0 vol% or more and less than 60 vol%, 0 vol% or more and less than 50 vol%, 0 vol% or more and less than 40 vol%, 0 vol% or more and less than 30 vol%, 0 vol% or more and less than 20 vol%, or 0 vol% or more and less than 10 vol%. The proportion of nitrogen gas in the carbonization gas may be 0 vol% or more and less than 70 vol%, 0 vol% or more and less than 60 vol%, 0 vol% or more and less than 50 vol%, 0 vol% or more and less than 40 vol%, or 0 vol% or more and less than 30 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 vol% or more and less than 10 vol%, or 0 vol% or more and less than 5 vol%. ２ The proportion of the gas may be 0 vol% or more and less than 70 vol%, 0 vol% or more and less than 60 vol%, 0 vol% or more and less than 50 vol%, 0 vol% or more and less than 40 vol%, or 0 vol% or more and less than 30 vol%. The proportion of water vapor in the carbonization gas may be 0 vol% or more and less than 70 vol%, 0 vol% or more and less than 60 vol%, 0 vol% or more and less than 50 vol%, 0 vol% or more and less than 40 vol%, or 0 vol% or more and less than 30 vol%. The carbonization gas in the carbonization step S2 may contain, for example, natural gas.

[0028] 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.

[0029] 1.3 Other Steps In the present 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 and hydrogen gas (cooling step S4) may be performed in addition to the carbonization step S2. Note that in the present embodiment, it is sufficient that the temperature of the half-reduced iron 20 is reduced in the carbonization step S2. The temperature of the half-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 half-reduced iron 20 and the CO gas may occur, causing the temperature of the half-reduced iron 20 to increase.

[0030] 1.3.1 Further Carbonization Step Using Carbonizing Gas Other than Methane Gas In one embodiment, after the semi-reduced iron 20 is carbonized by contacting it with a carbonizing gas in the carbonization step S2, a further carbonization step S3 may be performed by contacting it with CO gas. 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 semi-reduced iron 20 after carbonization with methane gas and hydrogen gas with CO gas is not particularly limited. For example, by connecting a pipe or the like to a gas supply port provided on the side wall of the shaft furnace 100 or the cooling tower 200 and supplying CO gas from the outside to the inside through the pipe or the like, the semi-reduced iron 20 after carbonization with methane gas and hydrogen gas can be brought into contact with CO gas. Note that other gases may be brought into contact with the semi-reduced iron 20 after carbonization with methane gas and hydrogen gas, along with CO gas. Examples of gases other than CO gas include nitrogen gas, methane gas, hydrogen gas, and CO ２ The CO-containing gas may be, for example, converter gas (LDG).

[0031] When CO gas or the like is further brought into contact with the semi-reduced iron 20 after the carbonization step S2 in which the semi-reduced iron 20 is brought into contact with the carbonization gas for carbonization, exhaust gas derived from the CO gas or the like is generated. The exhaust gas is, for example, mainly CO gas and CO ２ The exhaust gas may be discharged to the outside of the system and used as fuel.

[0032] 1.3.2 Further Cooling Step with Cooling Gas In one embodiment, after the carbonization step S2 in which the carbonization gas is brought into contact with the half-reduced iron 20 in the above-described carbonization step S2, or after the further carbonization step S3 using CO gas is further carried out, the half-reduced iron 20 may be brought into contact with a cooling gas to further cool the half-reduced iron 20. The further cooling step S4 may be carried out inside or outside the shaft furnace. In the further cooling step S4, it is only necessary that the cooling of the half-reduced iron 20 by the cooling gas proceeds, and multiple types of cooling gases may be brought into contact with the half-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 comes into contact with the half-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.

[0033] 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.

[0034] 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.

[0035] 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, as shown in FIGS. 1 to 4, 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.

[0036] 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.

[0037] 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 carbon content of the iron carbide 30 may be, for example, more than 0% by mass and not more than 5% by mass, or 0.5% by mass or more and not more than 5% by mass. 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 more, 150°C or more, or 200°C or more. That is, the iron carbide 30 may be HDRI with an increased carbon concentration.

[0038] 2. Iron Carbide Production System The technology disclosed herein also has an aspect as an iron carbide production system. That is, as shown in FIGS. 1 to 4 , an iron carbide production system according to one embodiment includes a reduction unit 110 that brings a reducing gas into contact with an oxidized iron raw material 10 to obtain semi-reduced iron 20, and a carbonization unit 120 that brings a carbonization gas into contact with the semi-reduced iron 20 to obtain iron carbide 30. The reduction unit 110 is provided in a shaft furnace 100. The semi-reduced iron 20 obtained in the reduction unit 110 has a metallization rate of 90% or less. The reaction temperature in the carbonization unit 120 is 700°C or higher and 850°C or lower. The carbonization gas supplied to the carbonization unit 120 contains 30% by volume or higher and lower than 100% by volume of methane gas and more than 0% by volume and 40% by volume or lower of hydrogen gas.

[0039] 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.

[0040] 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.

[0041] A carbonizing gas containing methane gas and hydrogen gas is supplied to the carbonizing section 120. The composition and supply form of the carbonizing gas are as described above.

[0042] 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.

[0043] 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.

[0044] The exhaust gas system from the reduction section 110, the carbonization section 120, etc., and the temperatures of the semi-reduced iron 20 in each of the reduction section 110 and the carbonization section 120 are also as described above.

[0045] 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.

[0046] 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.

[0047] In the manufacturing system of the present disclosure, the configurations shown in Figures 1 to 4 may be combined. For example, the manufacturing system 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 system shown in Figure 2 may be combined with a second carbonization section 130 and a cooling section 140 as shown in Figure 3.

[0048] 3. Effects As described above, the iron carbide manufacturing method and iron carbide manufacturing system according to the present embodiment can appropriately carbonize partially reduced iron 20 having a low metallization rate to obtain iron carbide 30. In the iron carbide manufacturing method and iron carbide manufacturing system according to the present embodiment, it is believed that by bringing partially reduced iron having a low metallization rate into contact with a carbonization gas containing both methane gas and hydrogen gas, the hydrogen gas can reduce iron oxide to produce metallic iron, while the methane gas can appropriately carbonize the metallic iron. More specifically, when partially reduced iron having a low metallization rate (partially reduced iron containing iron oxide) is carbonized, intentionally including hydrogen gas, which does not contribute to carbonization, in the carbonization gas along with methane gas, causes the iron oxide in the partially reduced iron to be hydrogen-reduced to metallic iron in the carbonization step, increasing the number of carbonization reaction sites for methane gas. This is believed to more appropriately carbonize the metallic iron by methane gas, thereby increasing the carbonization rate of the iron carbide obtained after the carbonization step.

[0049] 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.

[0050] 1. Experimental Method Iron oxide pellets (Nibrasco pellets: iron content 66% by mass) were semi-reduced with hydrogen and then carbonized using the carbonization gases shown in Table 1 below under a total of 16 conditions: reaction temperatures of 700°C, 750°C, 800°C, and 950°C, and reaction times of 30, 60, 90, and 120 minutes. The iron oxide pellets had a size (10-15 mm) suitable for a direct reduction process using a shaft furnace. The metallization rate after semi-reduction was 90% by mass. Cooling after carbonization was performed using N ２ The carburized sample was subjected to powder X-ray diffraction, and the Fe content in the sample was determined by Rietveld refinement of the obtained diffraction pattern. ３ The C content was measured.

[0051] 2. Experimental Results Table 1 below shows the average carbonization degree of 16 points obtained for each hydrogen concentration in the carbonization gas. Here, the carbonization degree is the ratio of Fe to the total iron content of the sample.３ 5 shows the relationship between the degree of carbonization (the proportion of iron that has become cementite to the total iron in iron carbide) and the hydrogen gas concentration of the carbonization gas in the carbonization process.

[0052]

[0053] From the results shown in Table 1 and FIG. 5, the regression equation expressing the effect of the hydrogen gas concentration x in the cooling gas on the carbonization degree y was obtained as the following equation (1): y=−0.0379x ２ +1.5277x+47.6...(1)

[0054] From the results shown in Figure 5 and the above formula (1), it can be said that when the hydrogen gas concentration is more than 0 vol% and not more than 40 vol%, the degree of carbonization is improved compared to when the hydrogen gas concentration is 0 vol%. In particular, when the hydrogen concentration is about 20 vol%, the degree of carbonization is maximized, reaching about 63%. Note that the above formula (1) is derived from the average value of the carbonization degrees obtained at 16 points for each hydrogen concentration in the carbonized gas, and it was confirmed that the carbonization degrees at each level exhibit similar behavior, although the values ​​of the carbonization degrees differ.

[0055] 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 efficiently carbonize partially reduced iron obtained through a direct reduction process using a shaft furnace, after which the partially reduced iron has a low metallization rate. (A) The method for producing iron carbide includes a reduction step in which a reducing gas is brought into contact with an iron oxide raw material to obtain partially reduced iron, and a carbonization step in which a carbonizing gas is brought into contact with the partially reduced iron to obtain iron carbide. (B) The reduction step is carried out in a shaft furnace. (C) The partially reduced iron has a metallization rate of 90% or less. (D) The reaction temperature in the carbonization step is 700°C or higher and 850°C or lower. (E) The carbonization gas in the carbonization step contains 30% by volume or higher and lower than 100% by volume of methane gas and more than 0% by volume and 40% by volume or lower of hydrogen gas.

[0056] REFERENCE SIGNS LIST 10 iron oxide raw material 20 semi-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 bringing a reducing gas into contact with an iron oxide raw material to obtain partially reduced iron; and a carbonization step of bringing a carbonization gas into contact with the partially reduced iron to obtain iron carbide, wherein the reduction step is carried out in a shaft furnace, the partially reduced iron has a metallization rate of 90% or less, the reaction temperature in the carbonization step is 700°C or more and 850°C or less, and the carbonization gas in the carbonization step comprises methane gas of 30% by volume or more and less than 100% by volume, and hydrogen gas of more than 0% by volume and 40% by volume or less.

2. A method for producing iron carbide according to claim 1, wherein the carbonization step is carried out in the shaft furnace.

3. A method for producing iron carbide according to claim 1, wherein the carbonization step is carried out outside the shaft furnace.

4. A method for producing iron carbide according to any one of claims 1 to 3, wherein the reducing gas contains one or both of hydrogen gas and methane gas.

5. The method for producing iron carbide according to any one of claims 1 to 4, wherein the volume V of the hydrogen gas is １ and the volume V of the methane gas ２ Relative to V １ / V ２ is greater than 0 and 0.75 or less.

6. A method for producing iron carbide according to any one of claims 1 to 5, wherein the carbonizing gas contains 15% by volume or more and 25% by volume or less of hydrogen gas.

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