Method for producing hot direct reduced iron

Abstract

Disclosed is a technique by which hot direct reduced iron having a heightened carbon concentration is produced through a direct reduction process.　This method for producing hot direct reduced iron comprises: a reduction step in which a reducing gas is brought into contact with an iron oxide feedstock to obtain iron metal; and a cooling step in which the iron metal is carbonized while being cooled, thereby obtaining hot direct reduced iron. The cooling step comprises a first step, in which after the reduction step, methane gas is brought into contact with the iron metal, and a second step, in which after the first step, CO gas is brought into contact with the iron metal. In the second step, the iron metal in contact with the CO gas has a temperature of 400-700°C. In the second step, the proportion of the volume of a discharge gas that is discharged from the system to the volume of the gas that is supplied is 40% or higher.

Description

TITLE: METHOD FOR PRODUCING HOT DIRECT REDUCED IRON FIELD

[0001] The present application discloses a method for producing hot direct reduced iron (HDRI). BACKGROUND

[0002] Reduction in CO2 emission by a direct reduction process using reducing gas is being studied in the steel industry as a substitute technique for the blast furnace method. A process using a shaft furnace, for example, has been investigated as a direct reduction process (PTL 1). In a direct reduction process, a reducing gas is contacted with iron oxide feedstock to obtain direct reduced iron (DRI). DRI includes two types: cold DRI (CDRI) and HDRI. Both CDRI and HDRI may have their carbon content increased in order to lower the melting temperature in the subsequent melting and refining steps, and to ensure strength as steel. [CITATION LIST] [PATENT LITERATURE]

[0003] [PTL 1] International Patent Publication No. WO2021 / 195160 SUMMARY [TECHNICAL PROBLEM]

[0004] Production of hot direct reduced iron by a direct reduction process in the prior art has been in need of improvement in terms of increasing the carbon content of the hot direct reduced iron while maintaining a high degree of reduction. [SOLUTION TO PROBLEM]

[0005] The present application discloses the following aspects as means for solving this problem. <Aspect 1> A method for producing hot direct reduced iron, the method comprising: a reducing step in which a reducing gas is contacted with an iron oxide feedstock to obtain metallic iron, and a cooling step in which the metallic iron is cooled while being carburized to obtain hot direct reduced iron, wherein the cooling step comprises a first step in which methane gas is contacted with the metallic iron after the reducing step, and a second step in which, after the first step, CO gas is contacted with the metallic iron, the temperature of the metallic iron in contact with the CO gas in the second step is 400°C or higher and 700°C or lower, and the proportion of the volume of the gas discharged outside the system with respect to the volume of the gas that is supplied in the second step is 40% or greater. <Aspect 2> The method for producing hot direct reduced iron according to aspect 1, wherein the proportion of the volume of gas discharged out of the system with respect to the volume of gas supplied in the second step is 70% or greater. <Aspect 3> The method for producing hot direct reduced iron according to aspect 1 or 2, wherein the temperature of the metallic iron in contact with CO gas in the second step is 550°C or higher and 700°C or lower. <Aspect 4> The method for producing hot direct reduced iron according to any one of aspects 1 to 3, wherein the discharged gas from the first step is used as the reducing gas in the reducing step. <Aspect 5> The method for producing hot direct reduced iron according to any one of aspects 1 to 4, wherein the reducing step and the cooling step are carried out inside a shaft furnace. <Aspect 6> The method for producing hot direct reduced iron according to any one of aspects 1 to 4, wherein the reducing step is carried out inside a shaft furnace, and the cooling step is carried out outside the shaft furnace. <Aspect 7> The method for producing hot direct reduced iron according to any one of aspects 1 to 4, wherein the reducing step and the first step are carried out inside a shaft furnace, and the second step is carried out outside the shaft furnace. <Aspect 8> The method for producing hot direct reduced iron according to any one of aspects 1 to 7, wherein the reducing gas comprises hydrogen gas. <Aspect 9> The method for producing hot direct reduced iron according to aspect 8, the method comprising: a dehydrating step in which the discharged gas of the reducing step is dehydrated to obtain a circulating gas, and a temperature-raising step in which the circulating gas and hydrogen gas are increased in temperature to obtain a reducing gas comprising the circulating gas and hydrogen gas. <Aspect 10> The method for producing hot direct reduced iron according to any one of aspects 1 to 9, wherein the temperature of the metallic iron in contact with the methane gas in the first step is 700°C or higher. [EFFECTS OF INVENTION]

[0006] According to the method for producing hot direct reduced iron of the disclosure it is possible to produce hot direct reduced iron with an increased carbon concentration while maintaining a high degree of reduction. BRIEF DESCRIPTION OF DRAWINGS

[0007] Fig. 1 is a schematic diagram illustrating an example of a method and system for producing HDRI. Fig. 2 is a schematic diagram illustrating an example of a method and system for producing HDRI. Fig. 3 is a schematic diagram illustrating an example of a method and system for producing HDRI. Fig. 4 is a schematic diagram illustrating an example of a method and system for producing HDRI. Fig. 5 shows simulation conditions for Cases 1 to 3. Fig. 6 shows the relationship between the outlet temperature of the transition zone (first step) (the temperature of the metallic iron in contact with CO gas in the cooling zone (second step)) and the carbon content and degree of reduction of the ultimately obtained HDRI, for Case 3. Fig. 7 shows the relationship between the outlet temperature of the transition zone (first step) (the temperature of the metallic iron in contact with CO gas in the cooling zone (second step)) and the temperature of the ultimately obtained HDRI, for Case 3. DESCRIPTION OF EMBODIMENTS

[0008] An embodiment of a production process and production system for hot direct reduced iron (hereunder, “HDRI”) according to the disclosure will now be described. However, the method and system for producing HDRI according to the disclosure are not limited to the following embodiment. For the present purpose, the term “iron oxide feedstock” refers to a feedstock that comprises iron oxide prior to the reducing step. The term “metallic iron” refers to the intermediate product obtained from the reducing step up until completion of the cooling step, and for convenience, the product having increased carbon concentration due to carburizing will also be referred to as “metallic iron”. The term “HDRI” refers to the HDRI product which is obtained after the cooling step.

[0009] 1. Method for producing HDRI As shown in Figs. 1 to 4, the method for producing HDRI according to one embodiment comprises: a reducing step S1 in which a reducing gas is contacted with an iron oxide feedstock 10 to obtain metallic iron 20, and a cooling step S2 in which the metallic iron 20 is carburized while cooling, to obtain HDRI 30. The cooling step S2 comprises: a first step S21 in which methane gas is contacted with the metallic iron 20 after the reducing step S1, and a second step S22 in which CO gas is contacted with the metallic iron 20 after the first step S21. The temperature of the metallic iron 20 in contact with the CO gas in the second step S22 is 400°C or higher and 700°C or lower. The proportion of the volume of gas discharged out of the system with respect to the volume of gas supplied in the second step S22 is 40% or greater.

[0010] 1.1. Reducing step In the reducing step S1, a reducing gas is contacted with the iron oxide feedstock 10. Reduction reaction thereby takes place, yielding metallic iron 20. As shown in Figs. 1 to 4, the reducing step S1 may be carried out inside a shaft furnace 100, for example. Alternatively, the reducing step S1 may be carried out in an apparatus other than the shaft furnace 100 (such as a fluidized bed). A particularly notable effect can be expected if the reducing step S1 is carried out inside the shaft furnace 100.

[0011] 1.1.1. Iron oxide feedstock The iron oxide feedstock 10 comprises at least iron oxide. The iron oxide feedstock 10 may be one or more materials selected from among iron ore pellets, iron ore, and sintered ore, for example. The iron oxide feedstock 10 may also comprise either or both silicon dioxide and aluminum oxide in addition to iron oxide. The iron oxide feedstock 10 may be one having a particle size distribution, or one having a uniform particle size. The mean particle size of the iron oxide feedstock 10 may be 5.0 mm to 30.0 mm or 10.0 mm to 15.0 mm, for example. The “particle size” of the iron oxide feedstock 10 is the sieve diameter of the feedstock. The “mean particle size” of the iron oxide feedstock 10 is the weighted average value for the particle size of the feedstock. The mean particle size of the iron oxide feedstock is measured in the following manner. Specifically, the mass-based particle size distribution can be obtained by a dry sieve screening test according to JIS Z 8815:1995, and the weighted average by mass calculated using the averages for the maximum particle size and minimum particle size of each sieve as the representative particle size may be used for measurement of the mean particle size of the iron oxide feedstock. The iron oxide feedstock 10 may be formed into pellets, or it may be in a powder state or lump shape, or in any other shape.

[0012] When the reducing step S1 is carried out in the shaft furnace 100, the iron oxide feedstock 10 is filled into the shaft furnace 100, forming a packed bed. The packing rate of the packed bed is not particularly restricted and may be similar to the packing rate in the conventional method for producing reduced iron using a shaft furnace. The packed bed moves downward inside the shaft furnace 100. That is, inside the shaft furnace 100, the iron oxide feedstock 10 gradually moves downward by dropping while in a substantially packed state. From the viewpoint of a single feedstock particle in the packed bed, the feedstock particle may continuously move downward at a fixed speed, or it may move intermittently with repeated dropping and stopping. From the viewpoint of a single feedstock particle in the packed bed, the average downward traveling speed of the feedstock particle is not particularly restricted. The average traveling speed may be adjusted depending on the amount of feedstock that is fed (the feed rate), for example. A burden feeder, for example, may be provided to prevent bridging while the packed bed is being caused to move downward. A burden feeder is a rectifying apparatus with a mechanism such as a rotating mechanism.

[0013] 1.1.2. Reducing gas The type of reducing gas may be any one that can reduce the iron oxide feedstock 10, and it is not particularly restricted. If the reducing gas comprises hydrogen gas, then an even more notable effect can be expected when using the technology of the disclosure. The reducing gas may also comprise a gas other than hydrogen gas, in addition to hydrogen gas. Gases other than hydrogen include CO gas, inert gases, CO2 gas, and steam. Inert gases include nitrogen gas and argon gas. When the reducing gas comprises hydrogen gas, the hydrogen concentration of the reducing gas may be 80 vol% to 100 vol%, for example. The supply temperature of the reducing gas (the temperature right before contacting the iron oxide feedstock 10) may be any temperature at which reduction reaction takes place with iron oxide, and it may be 700°C or higher, for example. The temperature of the reducing gas may be 700°C or higher and 1100°C or lower, and is preferably 800°C or higher and 1100°C or lower.

[0014] When the reducing step S1 is carried out inside the shaft furnace 100, the reducing gas can be supplied in from the side wall of the shaft furnace 100. The system for supplying the reducing gas is not particularly restricted. For example, piping may be connected to a reducing gas supply port provided in the side wall of the shaft furnace 100, and reducing gas may be supplied through the piping from the outside to the inside of the furnace.

[0015] 1.1.3. Metallic iron If at least part of the iron oxide in the iron oxide feedstock 10 is reduced in the reducing step S1, a solid reaction product comprising metallic iron 20 is obtained. In addition to metallic iron 20, the solid reactant may also comprise iron oxide, silicon dioxide and aluminum oxide that remain without being reduced. The temperature of the metallic iron 20 immediately after reduction may be 700°C or higher, for example. The upper limit for the temperature of the metallic iron 20 immediately after reduction is not particularly restricted, and it may be a temperature that allows the subsequent cooling step S2 to be carried out. The temperature of the metallic iron 20 immediately after reduction may also be 1100°C or lower, for example. The temperature of the metallic iron 20 immediately after reduction may be 700°C or higher and 1100°C or lower, 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.

[0016] If the reducing step S1 is carried out inside the shaft furnace 100, then the solid reaction product comprising the metallic iron 20 can be recovered from the lower part of the shaft furnace 100 (below the position where the reducing gas is supplied).

[0017] 1.1.4. Shaft furnace When the reducing step S1 is carried out inside the shaft furnace 100, the shape of the body of the shaft furnace 100 may be similar to the shape of the body of a known shaft furnace. For example, the body of the shaft furnace 100 may have a furnace top part, a furnace bottom part, and a cylindrical part forming a side wall between the furnace top part and furnace bottom part. In this case, the cylindrical part may have a barrel part and a constricted part provided lower than the barrel part, with the inner diameter of the furnace being reduced from the top toward the bottom at the constricted part. The shaft furnace 100 may be provided with a burden feeder to prevent bridging of the packed bed of the feedstock when the packed bed is moved downward inside. The shaft furnace 100 may also have a cooling gas supply port for supply of cooling gas, or a cooling gas discharge port for discharge of cooling gas, provided below the reducing gas supply port. The cooling gas supply port may be provided in the side wall of the furnace, or it may be provided further inward from the side wall of the furnace. The cooling gas discharge port may also be provided in the side wall of the furnace. The manner of supply and discharge of each gas will now be described.

[0018] 1.2. Cooling step The temperature of the metallic iron 20 immediately after carrying out the reducing step S1 is 700°C or higher, for example. By contacting gas with the high-temperature metallic iron 20 during the cooling step S2, the metallic iron 20 is cooled while the metallic iron 20 is carburized. In other words, carbon-containing HDRI 30 is obtained. By thus carburizing the metallic iron 20 to obtain carbon-containing HDRI 30, the melting temperature falls during the subsequent step of melting and refining, further ensuring the strength of the steel.

[0019] In the cooling step S2, (1) carburizing and cooling (and reduction) of the metallic iron 20 by methane gas is followed by (2) carburizing (and reduction) of the metallic iron 20 by CO gas. The present inventors have found that carburizing of the metallic iron 20 by methane gas tends to proceed in a high-temperature range of 700°C or higher, and is also an endothermic reaction. However, carburizing of the metallic iron 20 by CO gas proceeds most readily at 400°C or higher and 600°C or lower, and is an exothermic reaction. The term “carburizing” of the metallic iron 20 by methane gas or CO gas, for the present purpose, means one or more of the following: the metallic iron 20 is converted into a carbide produced by carbon extracted from methane or CO, the carbon extracted from methane or CO is in solid solution in the metallic iron 20, and / or the carbon extracted from methane or CO adheres to or is deposited on the surface of the metallic iron 20in elemental form. In the cooling step S2, first methane gas is contacted with the high-temperature metallic iron 20 obtained by the reducing step S1, to appropriately and efficiently promote the endothermic reaction. The endothermic reaction during carburizing, in addition to the physical heat absorption resulting from contact with methane gas which has a large specific heat, causes the temperature of the metallic iron 20 to easily fall to a temperature suitable for carburizing by CO gas. Contacting CO gas with the metallic iron 20 having such a lower temperature allows the exothermic reaction to proceed appropriately and efficiently, making it possible to greatly increase the carbon concentration in the ultimately obtained HDRI 30. As explained above, the combined steps of (1) carburization with methane gas and (2) carburization with CO gas in that order in the cooling step S2 causes the carbon concentration in the HDRI 30 to increase more than when each of these gases is contacted individually. Moreover, conducting (1) endothermic reaction with methane gas and (2) exothermic reaction with CO gas in that order can more easily yield HDRI 30 exhibiting at least a fixed temperature.

[0020] When the reducing step S1 is carried out inside the shaft furnace, the cooling step S2 may be carried out either inside the shaft furnace or outside the shaft furnace. In other words, as shown in Fig. 1, the reducing step S1 and cooling step S2 may both be carried out inside the shaft furnace 100. Alternatively, as shown in Fig. 2, the reducing step S1 may be carried out inside the shaft furnace 100 and the cooling step S2 may be carried out outside the shaft furnace 100. Also alternatively, as shown in Fig. 3, the reducing step S1 and the first step S21 of the cooling step S2 (described below) may be carried out in the shaft furnace 100, and the second step S22 of the cooling step S2 (described below) may be carried out outside the shaft furnace 100. When at least part of the cooling step S2 is carried out outside the shaft furnace 100, a cooling tower 200 may be provided downstream from the shaft furnace 100 as shown in Figs. 2 and 3, for example, with at least part of the cooling step S2 being carried out in the cooling tower 200. The term “downstream” as used herein means downstream from the process for producing HDRI. Specifically, for production of carbon-containing HDRI 30 obtained from the iron oxide feedstock 10 via the metallic iron 20, the upstream end is the iron oxide feedstock 10 end, while the downstream end is the carbon-containing HDRI 30 end.

[0021] The cooling step S2 comprises a first step S21 and a second step S22. For this embodiment, the temperature of the metallic iron 20 may be lowered during at least part of the cooling step S2, or the temperature of the metallic iron 20 may be increased during the cooling step S2. For example, as mentioned above, exothermic reaction may take place between the metallic iron 20 and the CO gas in the second step S22, raising the temperature of the metallic iron 20.

[0022] 1.2.1. First step The first step S21 corresponds to step (1) above. That is, in the first step S21, methane gas is contacted with the metallic iron 20 after the reducing step S1. This results in carburization of the metallic iron 20. The metallic iron 20 is partially carburized. When the reducing step S1 is carried out inside the shaft furnace, the first step S21 may be carried out either inside the shaft furnace or outside the shaft furnace (for example, in the cooling tower provided downstream from the shaft furnace). There are no particular restrictions on the system for contacting the methane gas with the metallic iron 20. For example, by connecting piping to a gas supply port provided in the side wall of the shaft furnace 100 or cooling tower 200, and supplying methane gas from the exterior to the interior through the piping, it is possible to contact the methane gas with the metallic iron 20. The temperature of the metallic iron 20 that contacts with the methane gas in the first step S21 may be any temperature that allows carburizing by the methane gas. If the temperature of the metallic iron 20 in contact with the methane gas in the first step S21 is 700°C or higher, for example, carburizing of the metallic iron 20 by the methane gas can proceed even more suitably. There is no particular restriction on the upper limit for the temperature of the metallic iron 20 in contact with the methane gas in the first step S21. The temperature of the metallic iron 20 in contact with the methane gas may be 1100°C or lower, for example. From the viewpoint of allowing carburizing reaction with methane gas to proceed more markedly, the temperature of the metallic iron 20 in contact with methane gas in the first step S21 may be 710°C or higher and 1100°C or lower, 730°C or higher and 1100°C or lower, 750°C or higher and 1100°C or lower, 770°C or higher and 1100°C or lower, 790°C or higher and 1100°C or lower, 810°C or higher and 1100°C or lower, 700°C or higher and 1070°C or lower, 700°C or higher and 1040°C or lower, 700°C or higher and 1000°C or lower, 700°C or higher and 970°C or lower, 700°C or higher and 940°C or lower, 700°C or higher and 900°C or lower, 710°C or higher and 1070°C or lower, 730°C or higher and 1040°C or lower, 750°C or higher and 1000°C or lower, 770°C or higher and 970°C or lower, 790°C or higher and 940°C or lower, or 810°C or higher and 900°C or lower. The temperature of the metallic iron 20 in the first step S21 (the temperature of the metallic iron 20 after the reducing step S1) can be controlled by the temperature of the iron oxide feedstock 10, the feed rate of the iron oxide feedstock 10, the temperature of the reducing gas and the feed rate of the reducing gas, in the reducing step S1. Incidentally, the “temperature of the metallic iron 20” in the first step S21 is the mean temperature in the radial direction of the shaft furnace or cooling tower. When the reducing step S1 and the first step S21 are carried out in the same device (such as the shaft furnace), the position (height position) P1 when measuring the “temperature of the metallic iron 20” in the first step S21 is a position downstream (downward) from the position (height position) P2 where the reducing gas is injected in during the reducing step S1, being a position (height position) within 1 m from position P2. Alternatively, when the reducing step S1 and the first step S21 are carried out in separate apparatuses (for example, the reducing step S1 is carried out in the shaft furnace while the first step S21 is carried out in the cooling tower provided downstream from the shaft furnace), the position P1 when measuring the “temperature of the metallic iron 20” during the first step S21 is a position within 1 m downstream (downward) from the metallic iron supply port of the device in which the first step S21 is carried out. The mean temperature of the metallic iron 20 in the radial direction may be determined, for example, by arranging rod-shaped members in the radial direction of the shaft furnace or cooling tower, providing a plurality of thermocouples in the members, and taking multiple temperature measurements in the radial direction. That is, when the first step S21 is carried out inside the shaft furnace, the plurality of thermocouples arranged in the radial direction of the shaft furnace may be used to determine the mean temperature of the metallic iron 20 in the radial direction. When the first step S21 is carried out inside the cooling tower, the mean temperature of the metallic iron 20 in the radial direction may be determined by the plurality of thermocouples arranged in the radial direction of the cooling tower. Assuming that the temperatures between measured points are linearly distributed in the radial direction, for example, the temperature distribution T (r) in the radial direction can be expressed by a combination of linear functions of “r”. The mean temperature Tave is defined by the following formula where, when measuring the temperature at N points, the radius of a measured point “i” (i = 1 to N) is ri. The values for r0 and rN+1 are for the positions of the furnace center (r0 = 0) and the furnace wall (rN+1 = R), with the temperatures at those points being determined by extrapolation. With this method, it is possible to determine the mean temperature in the radial direction as the average of multiple measured temperatures, even if a temperature distribution forms in the radial direction. The number of the thermocouples is not particularly restricted, but preferably five or more are arranged, for example. The temperature of the methane gas that contacts with the metallic iron 20 in the first step S21 is not particularly restricted. The temperature of the methane gas may be 25°C or higher and 600°C or lower, for example. [Mathematical Formula 1] 2nZi=o{ri+1rT(r)dr rp __ ave ~ nR2

[0023] It is sufficient if carburizing of the metallic iron 20 by the methane gas proceeds in the first step S21, and another gas may also be contacted with the metallic iron 20 along with the methane gas. In other words, it is sufficient if the gas that contacts with the metallic iron 20 in the first step S21 (first contact gas) comprises methane gas. The first contact gas that contacts with the metallic iron 20 in the first step S21 may also comprise hydrogen gas, nitrogen gas, CO gas, CO2 gas or water vapor in addition to the methane gas. The first contact gas contacting the metallic iron 20 in the first step S21 may also be natural gas, for example. As mentioned above, in the first step S21, the temperature of the metallic iron 20 decreases due to carburizing reaction of the metallic iron 20 by the methane gas (endothermic reaction). So long as such endothermic reaction by methane gas proceeds predominantly in the first step S21, a portion of the first contact gas that contacts the metallic iron 20 may comprise a gas that is accompanied by exothermic reaction (such as CO gas). However, when the first contact gas contacting the metallic iron 20 during the first step S21 comprises CO gas, the volume ratio of the CO gas will be lower than the volume ratio of the methane gas. If the first contact gas that contacts with the metallic iron 20 during the first step S21 comprises multiple different types of gas, then the volume ratio of the methane gas is highest from among the volume ratios of each of the other gases. The first contact gas that contacts with the metallic iron 20 in the first step S21 comprises methane gas at 50 vol% or greater, 60 vol% or greater or 70 vol% or greater, for example.

[0024] The gas resulting from reaction between the metallic iron 20 and methane gas in the first step S21 may comprise methane gas and hydrogen gas. The reaction gas may be discharged out of the system as discharged gas and utilized as fuel, or it may be utilized as the above-mentioned reducing gas. If the gas from reaction between the metallic iron 20 and methane gas in the first step S21 is utilized as the reducing gas in the reducing step S1, this will allow efficient operation while reducing the amount of reducing gas used.

[0025] 1.2.2. Second step The second step S22 corresponds to step (2) above. Specifically, in the second step S22, CO gas is contacted with the metallic iron 20 after the first step S21. This results in further carburization of the metallic iron 20. The metallic iron 20 is partially carburized. When the reducing step S1 is carried out inside the shaft furnace, the second step S22 may be carried out either inside the shaft furnace or outside the shaft furnace (for example, in the cooling tower provided downstream from the shaft furnace). When the first step S21 is carried out outside the shaft furnace, the second step S22 is also necessarily carried out outside the shaft furnace. There are no particular restrictions on the system for contacting the CO gas with the metallic iron 20. For example, by connecting piping to a gas supply port provided in the side wall of the shaft furnace 100 or cooling tower 200, and supplying CO gas from the exterior to the interior through the piping, it is possible to contact the CO gas with the metallic iron 20. If the temperature of the metallic iron 20 in contact with CO gas is 400°C or higher and 700°C or lower in the second step S22, it will be possible to properly promote carburizing reaction with the CO gas while obtaining HDRI 30 at a suitable temperature after the second step. From the viewpoint of markedly promoting the carburizing reaction by CO gas, the temperature of the metallic iron 20 in contact with the CO gas in the second step S22 may be 410°C or higher and 700°C or lower, 420°C or higher and 700°C or lower, 430°C or higher and 700°C or lower, 440°C or higher and 700°C or lower, 450°C or higher and 700°C or lower, 460°C or higher and 700°C or lower, 470°C or higher and 700°C or lower, 480°C or higher and 700°C or lower, 490°C or higher and 700°C or lower, 500°C or higher and 700°C or lower, 510°C or higher and 700°C or lower, 520°C or higher and 700°C or lower, 530°C or higher and 700°C or lower, 540°C or higher and 700°C or lower, 550°C or higher and 700°C or lower, 560°C or higher and 700°C or lower, 570°C or higher and 700°C or lower, or 580°C or higher and 700°C or lower. Particularly when the temperature of the metallic iron 20 in contact with CO gas in the second step S22 is 550°C or higher and 700°C or lower, the temperature of the HDRI 30 obtained after the second step S22 is higher, tending to result in a temperature that is suitable for briquetting of the HDRI 30 (for example, 650°C or higher). That is, the HDRI 30 may be directly briquetted to obtain HBI (Hot Briquetted Iron). The temperature of the metallic iron 20 in the second step S22 (the temperature of the metallic iron 20 after the first step S21) can be controlled by the temperature of the metallic iron 20, the supply rate of the metallic iron 20, the temperature of the methane gas and the supply rate of the methane gas in the first step S21. Incidentally, the “temperature of the metallic iron 20” in the second step S22 is also the mean temperature in the radial direction of the shaft furnace or the cooling tower, similar to the first step S21. That is, when the second step S22 is carried out inside the shaft furnace, the plurality of thermocouples arranged in the radial direction of the shaft furnace may be used to determine the mean temperature of the metallic iron 20 in the radial direction. When the second step S22 is carried out inside the cooling tower, the mean temperature of the metallic iron 20 in the radial direction may be determined by the plurality of thermocouples arranged in the radial direction of the cooling tower. When the first step S21 and the second step S22 are carried out in the same device (such as the shaft furnace), the position (height position) P3 when measuring the “temperature of the metallic iron 20” in the second step S22 is a position downstream (downward) from the position (height position) P4 where the methane gas is injected in during the first step S21, being a position (height position) within 1 m from position P4. Alternatively, when the first step S21 and the second step S22 are carried out in separate apparatuses (for example, the first step S21 is carried out in the shaft furnace while the second step S22 is carried out in a cooling tower provided downstream from the shaft furnace), the position P3 when measuring the “temperature of the metallic iron 20” during the second step S22 is a position within 1 m downstream (downward) from the metallic iron supply port of the device in which the second step 22 is carried out. The temperature of the CO gas contacting the metallic iron 20 in the second step S22 is not particularly restricted. The temperature of the CO gas may be 25°C or higher and 700°C or lower, for example.

[0026] It is sufficient for carburization of the metallic iron 20 to proceed by the CO gas in the second step S22, and other gases in addition to CO gas may also be contacted with the metallic iron 20. In other words, it is sufficient if the gas that contacts with the metallic iron 20 in the second step S22 (second contact gas) comprises CO gas. The second contact gas that contacts with the metallic iron 20 in the second step S22 may also comprise nitrogen gas, hydrogen gas and CO2 gas, in addition to CO gas. The second contact gas contacting the metallic iron 20 in the second step S22 may be Linz-Donawitz gas (LDG), for example. When the second contact gas that contacts with the metallic iron 20 in the second step S22 comprises methane gas, the volume ratio of the methane gas is lower than the volume ratio of the CO gas. If the second contact gas in contact with the metallic iron 20 in the second step S22 comprises a plurality of types of gases, the volume ratio of the CO gas is largest from among the volume ratios of each of the other gases. The second contact gas that contacts with the metallic iron 20 in the second step S22 comprises CO gas at 50 vol% or greater, 60 vol% or greater or 70 vol% or greater, for example.

[0027] The gas supplied in the second step S22 comprises CO gas, as mentioned above, and optionally comprises another gas. The gas resulting from reaction between the metallic iron 20 and CO gas in the second step S22 may comprise CO gas and CO2 gas. The reaction gas may be discharged out of the system and utilized as fuel, or it may also be utilized as part of the cooling gas during the first step S21 described above. In the production method of the disclosure, it is important for the proportion of the volume of gas discharged out of the system with respect to the volume of gas that is supplied in the second step S22 ([(Volume of discharged gas) / (volume of supply gas)] x 100) to be at least 40%. This allows reduction reaction in the reducing step S1 and carburizing reaction with methane gas in the first step S21 to proceed more suitably, and can increase the degree of reduction and carbon content of the HDRI 30. According to one embodiment, the proportion of the volume of gas discharged out of the system with respect to the volume of gas that is supplied in the second step S22 may be 50% or greater, 60% or greater, 70% or greater, 80% or greater, 90% or greater or 100%. If the proportion of the volume of gas discharged out of the system with respect to the volume of gas that is supplied in the second step S22 is 70% or greater, in particular, the carbon content and degree of reduction of the final produced HDRI will tend to be even more notably increased. For example, the gas in the first step S21 may be injected into the second step S22 within the system, resulting in a greater volume of gas discharged out of the system than the volume of gas supplied in the second step S22. When the volume of the gas discharged out of the system is greater than the volume of the gas supplied in the second step S22, the proportion of the volume of gas discharged out of the system with respect to the volume of gas that is supplied in the second step S22 is considered to be “100%”. The method for discharging the discharged gas from the second step S22 out of the system is not particularly restricted. For example, the discharged gas may be discharged out of the system through a gas discharge port provided on the side wall of the shaft furnace 100 or cooling tower 200. The gas discharge port may also be provided further downstream than the reducing step S1, or further downstream than the first step S21. By controlling the flow rate of the discharged gas through the gas discharge port, it is possible to control the proportion of gas discharged out of the system with respect to the discharged gas in the second step S22.

[0028] 1.2.3 Hot direct reduced iron (HDRI) HDRI 30 is produced as a result of the reducing step S1 and cooling step S2 described above. As used herein, “HDRI” refers to DRI having a surface temperature of 100°C or higher.

[0029] In addition to carbon and iron, the HDRI 30 may also comprise iron oxide, silicon dioxide and aluminum oxide that remain without being reduced. The carbon content of the HDRI 30 may be 2.5 mass% to 5 mass%, for example.

[0030] 1.3.Dehydrating Step and Temperature-Raising Step The method for producing HDRI of the present embodiment may also comprise other steps in addition to the reducing step S1 and cooling step S2. For example, the method for producing HDRI of the embodiment may also have a dehydrating step S3 for dehydration of the gas discharged from the reducing step S1 to obtain a circulating gas. The method for producing HDRI of the embodiment may also have a temperature-raising step S4 for raising the temperature of the discharged gas from the reducing step S1 or raising the temperature of the circulating gas obtained by dehydrating the discharged gas. The temperature-raising step S4 may also be a step of raising the temperature of the discharged gas from the reducing step S1 or the circulating gas obtained by dehydrating that discharged gas, and also the hydrogen gas, and obtaining a reducing gas comprising either or both the discharged gas and circulating gas, and the hydrogen gas. The dehydrating step S3 and temperature-raising step S4 may also be combined. For example, as shown in Fig. 4, the method for producing HDRI according to the embodiment may have a dehydrating step S3 in which the discharged gas from the reducing step S1 is dehydrated to obtain a circulating gas, and a temperature-raising step S4 in which the circulating gas and hydrogen gas are raised in temperature to obtain a reducing gas comprising the circulating gas and hydrogen gas.

[0031] 1.3.1. Dehydrating step In the dehydrating step S3, the discharged gas from the reducing step S1 is dehydrated and a circulating gas is obtained. The dehydration may be carried out using a known dehydration apparatus. In the reducing step S1, water can be produced by reaction between the reducing gas and the iron oxide feedstock 10. However, it is not necessary for 100% of the reducing gas to be used in the reducing step S1. In other words, reducing gas may remain with water in the discharged gas from the reducing step S1. Dehydration of this discharged gas produces a circulating gas comprising the reducing gas.

[0032] 1.3.2. Temperature-raising step The circulating gas obtained from the dehydrating step S3 comprises reducing gas, but the amount is not sufficient. Since the circulating gas obtained from the dehydrating step S3 is also at low temperature, directly reusing it in the reducing step S1 is inefficient. In the temperatureraising step S4, therefore, the circulating gas and hydrogen gas are raised in temperature to obtain a reducing gas comprising the circulating gas and hydrogen gas. In other words, the reducing gas is obtained by raising the temperature of and mixing the circulating gas with the hydrogen gas used as makeup gas. In the temperature-raising step S4, the circulating gas and hydrogen gas may be mixed after raising the temperature, or the temperature may be raised after the circulating gas and hydrogen gas have been mixed. The hydrogen gas may be obtained by electrolysis of water or membrane separation from synthetic gas (gas obtained by steam reforming or partial combustion of coal or biomass). The temperature-raising step S4 may be carried out using a known heating apparatus.

[0033] 1.3.3 Other aspects As explained above, in the method for producing HDRI according to one embodiment, the reducing step S1 and cooling step S2 may both be carried out inside the shaft furnace 100. For example, the first step S21 may be carried out at a lower part inside the shaft furnace 100 than the reducing step S1. In this case, as shown in Fig. 4, the discharged gas from the first step S21 may rise through the interior of the furnace and be added to the reducing gas used in the reducing step S1, and utilized for reduction. As shown in Figs. 1 to 4, the discharged gas from the second step S22 may be discharged out of the system at a point downstream from the first step S21. Such a configuration causes the CO gas or CO2 gas in the discharged gas from the second step S22 to be eliminated from the reducing step S1. In other words, the CO gas or CO2 gas no longer contaminates the discharged gas from the reducing step S1, so that the discharged gas from the reducing step S1 consists entirely of the reducing gas (such as hydrogen gas) and steam. Carrying out the dehydrating step S3 for the discharged gas in this manner allows reuse of the reducing gas. Although methane gas remains in the discharged gas from the first step S21, the methane gas is decomposed in the reduction zone. The methane gas is merged and diluted with other gases in the reduction zone. That is, virtually no methane gas is present in the discharged gas from the reducing step S1, or even if present it is only in an amount of about 1 vol%. Therefore, the concentration of the carbon-containing gas during circulation has very little impact. In other words, the usual step of partial discharge of the reducing gas out of the system can be used, without requiring a special step for removal of CO2 during circulated reuse of the discharged gas.

[0034] The method of the disclosure may also combine the modes illustrated in Figs. 1 to 4. For example, the dehydrating step S3 or temperature-raising step S4 shown in Fig. 4 may be carried out in the production methods illustrated in Figs. 1 to 3.

[0035] 2. HDRI production system The technology of the disclosure also encompasses a system for producing HDRI, in addition to the method for producing HDRI described above. Specifically, as shown in Figs. 1 to 4, the system for producing HDRI according to one embodiment comprises: a reducing unit 110 in which a reducing gas is contacted with an iron oxide feedstock 10 to obtain metallic iron 20, and a cooling unit 120 in which the metallic iron 20 is carburized while cooling, to obtain HDRI 30. The cooling unit 120 has: a first section 121 where methane gas is contacted with the metallic iron 20, further downstream than the reducing unit 110, and a second section 122 where CO gas is contacted with the metallic iron 20, further downstream than the first section 121. The system for producing HDRI according to one embodiment is configured so that: the temperature of the metallic iron 20 in contact with CO gas in the second section 122 is 400°C or higher and 700°C or lower, and the proportion of the volume of gas discharged out of the system with respect to the volume of gas that is supplied in the second section 122 is 40% or greater.

[0036] For this embodiment, the reducing step S1 is carried out in the reducing unit 110 and the cooling step S2 is carried out in the cooling unit 120. It is sufficient if the reducing unit 110 and cooling unit 120 are configured so as to allow the reducing step S1 and the cooling step S2 to be carried out. For example, the reducing unit 110 and cooling unit 120 may be provided in the shaft furnace 100, as shown in Fig. 1. Alternatively, the reducing unit 110 may be provided in the shaft furnace 100 while the cooling unit 120 may be provided separately from the shaft furnace 100, as shown in Fig. 2.The reducing unit 110 and the first section 121 of the cooling unit 120 may also be provided in the shaft furnace 100, while the second section 122 of the cooling unit 120 may be provided separately from the shaft furnace 100, as shown in Fig. 3. When at least part of the cooling unit 120 is provided separately from the shaft furnace 100, the cooling unit 120 may be a cooling tower 200 provided downstream from the shaft furnace 100, for example.

[0037] A reducing gas is supplied to the reducing unit 110. The type and manner of supply of the reducing gas are as described above. The reducing gas may comprise hydrogen gas, for example.

[0038] As shown in Fig. 4, the system of producing HDRI according to one embodiment may comprise: a dehydrating apparatus 130 in which the discharged gas from the reducing unit 110 is dehydrated to obtain a circulating gas, and a temperature-raising apparatus 140 in which the circulating gas and hydrogen gas are increased in temperature to obtain a reducing gas comprising the circulating gas and hydrogen gas. The dehydrating apparatus 130 and temperature-raising apparatus 140 are apparatuses in which the dehydrating step S3 and temperature-raising step S4, respectively, are carried out. The details are as explained above.

[0039] The system for discharge of gas from the reducing unit 110 and the cooling unit 120 is also as described above. Specifically, the production system according to one embodiment is configured so that the proportion of the volume of the gas discharged outside the system with respect to the volume of the gas supplied in the second section 122 (corresponding to the discharged gas which is discharged out of the system in the second step S22) is 40% or greater. If the system is configured so that the proportion of the volume of gas discharged out of the system with respect to the volume of gas that is supplied in the second section 122 is 70% or greater, in particular, the carbon content and degree of reduction of the final produced HDRI will tend to be even more notably increased. The production system according to one embodiment may also be configured so that the discharged gas from the first section 121 (corresponding to the discharged gas from the first step S21) is utilized as reducing gas for the reducing unit 110.

[0040] The temperature of the metallic iron 20 at both the reducing unit 110 and the cooling unit 120 is as explained above. Specifically, the production system according to one embodiment is configured so that the temperature of the metallic iron 20 in contact with CO gas in the second section 122 is 400°C or higher and 700°C or lower. As mentioned above, the configuration is most preferably such that the temperature of the metallic iron 20 in contact with the CO gas in the second section 122 is 550°C or higher and 700°C or lower. The production system according to one embodiment may also be configured so that the temperature of the metallic iron 20 in contact with the methane gas in the first section 121 is 700°C or higher. The temperature of the metallic iron 20 in the first section 121 can be controlled by the temperature of the iron oxide feedstock 10, the feed rate of the iron oxide feedstock 10, the temperature of the reducing gas and the feed rate of the reducing gas, in the reducing unit 110. The temperature of the metallic iron 20 in the second section 122 can be controlled by the temperature of the metallic iron 20, the supply rate of the metallic iron 20, the temperature of the methane gas or the supply rate of the methane gas, in the first section 121.

[0041] When the shaft furnace 100 is used as the reducing unit 110 in the production system of the disclosure, the pressure at the top of the shaft furnace is not particularly restricted but may be in the range of 0 MPa to 0.8 MPa, as gauge pressure. The pressure can be measured using a pressure gauge provided at the top of the shaft furnace, for example.

[0042] When the cooling tower 200 is used as the cooling unit 120 in the production system of the disclosure, the pressure at the top of the cooling tower is not particularly restricted but may be in the range of 0 MPa to 0.8 MPa, as gauge pressure. The pressure can similarly be measured using a pressure gauge provided at the top of the cooling tower, for example.

[0043] The production system of the disclosure may also combine the modes illustrated in Figs. 1 to 4. For example, in the production systems illustrated in Figs. 1 to 3, the dehydrating apparatus 130 or temperature-raising apparatus 140 shown in Fig. 4 may be combined.

[0044] 3. Effect As explained above, in the production method and production system of this embodiment, it is possible to efficiently produce HDRI 30 with increased carbon concentration while maintaining a high degree of reduction, by (1) obtaining metallic iron 20 by reduction of iron oxide feedstock 10, and (2) carburizing and cooling the metallic iron 20 with methane gas, followed by (3) carburizing the metallic iron 20 by CO gas. EXAMPLES

[0045] The present invention will now be further described using Examples, with the understanding that the Examples are not intended to be limitative on the invention. The invention may be applied under a variety of different conditions that do not fall outside of its gist and that allow its object to be achieved. In the following Examples, conditions for increasing the degree of reduction and carburization amount of HDRI were examined using numerical simulations. The numerical simulations in the Examples were carried out by adding the reactions described in (7), (9) and (10) of NPL 2, to a shaft furnace mathematical model developed by applying the blast furnace mathematical model described in NPL 1. NPL 1: Nishioka et al. “Development of Mathematical Models for Blast Furnaces”, NIPPON STEEL & SUMITOMO METAL TECHNICAL REPORT No. 120(2018) NPL 2:Hamzeh Hamadeh et al., “Detailed Modeling of the Direct Reduction of Iron Ore in a Shaft Furnace”, Materials 2018,11(10), 1865 (https: / / doi.org / 10.3390 / ma11101865)

[0046] 1. Simulation conditions Each numerical simulation was carried out assuming operation in the shaft furnace. First, H2 at 950°C was injected in against iron oxide pellets charged into the shaft furnace, causing reduction reaction to proceed. The temperature of the metallic iron immediately after completion of the reduction reaction was 861°C. The following Cases 1 to 3 for the cooling step were then carried out for the metallic iron at 861°C. The pressure at the top of the shaft furnace was 0.04 MPa or 0.7 MPa (gauge pressure). Case 1 (corresponding to Comparative Example): CH4 (25°C) was injected into lower part of the cooling zone of the shaft furnace, and the metallic iron was simultaneously cooled during carburizing. Case 2 (corresponding to Comparative Example): Mixture of CH4 and CO (400°C) was injected in the transition zone of the shaft furnace, and the metallic iron was simultaneously cooled during carburizing. Case 3 (corresponding to Example): CH4 (25°C) was injected into the transition zone of the shaft furnace, the metallic iron was cooled simultaneously while carburizing, and then CO (25°C or 400°C) was injected in at the bottom of the cooling zone of the shaft furnace for further carburizing.

[0047] Fig. 5 shows the types of gas introduced into the transition zone and cooling zone of the shaft furnace, and the locations of introduction, for each of Cases 1 to 3. The structures shown in Fig. 5 (the black colored portions in Fig. 5) are the left half of each structure in the furnace, when divided into a right half and a left half around the central axis of the shaft furnace, based on a cross-section running through the central axis of the shaft furnace and running along the central axis. For Case 3, the reduction zone in Fig. 5 corresponds to the reducing step S1, the transition zone corresponds to the first step S21 in the cooling step S2, and the bottom of the cooling zone corresponds to the second step S22 in the cooling step S2. For some of the calculation conditions, extraction of gas out of the system was from the top of the cooling zone in a specified extraction amount, with the amount of gas extracted from the top of the cooling zone being zero for some of the calculation conditions.

[0048] 2. Calculation results 1 Tables 1 and 2 show the shaft furnace top pressure, the manner of gas injection (Cases 1 to 3), the results of calculating the temperature of the metallic iron in contact with gas in the transition zone (first step), the gas type, supply rate and temperature of the gas in the transition zone (first step), the results of calculating the temperature of the metallic iron in contact with gas in the cooling zone (second step), the type, supply rate and temperature of the gas in the cooling zone (second step), the amount of discharged gas and gas extracted out of the system in the cooling zone (second step), the proportion of discharged gas discharged out of the system in the cooling zone (second step), and the temperature, carbon content and degree of reduction of HDRI discharged from the bottom end of the shaft furnace.

[0049] For the Examples, an HDRI carbon content of 2.5% or higher and a degree of reduction of 91.0% or higher was judged to represent HDRI with both a high carbon content and a high degree of reduction.

[0050] Comparative Example Example 1-1 1-2 1-3 1-4 1-5 1-6 1-1 1-2 1-3 1-4 1-5 Cooling step calculation conditions Shaft furnace top pressure (MPa) (gauge pressure) 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.7 Gas injection method Case 1 Case 2 Case 3 Case 3 Case 3 Case 3 Case 3 Case 3 Case 3 Case 3 Case 3 First step (transition zone) Metallic iron temperature (°C) 861 861 861 861 861 861 861 861 861 861 861 CH4 (Nm3 / min) 0 175 250 280 200 500 200 250 250 500 250 CO (Nm3 / min) 0 525 0 0 0 0 0 0 0 0 0 Injection temperature (°C) - 400 25 25 25 25 25 25 25 25 25 Second step (cooling zone) Metallic iron temperature (°C) 861 716 644 572 720 392 608 588 581 411 580 CH4 (Nm3 / min) 100 0 0 0 0 0 0 0 0 0 0 CO (Nm3 / min) 0 0 350 350 300 500 300 350 350 350 350 Injection temperature (°C) 25 - 400 500 25 400 25 400 400 400 400 Amount extracted from system (Nm3 / min) 100 - 0 100 500 350 200 350 150 350 350 Percentage volume of gas discharged from system with respect to volume of supply gas (%) 100 - 0 29 100 70 67 100 43 100 100 HDRI quality Temperature (°C) 675 677 720 698 693 600 673 673 697 582 725 Carbon content (mass%) 1.1 2.0 2.2 2.6 2.0 3.8 2.6 3.0 2.6 3.8 3.5 Degree of reduction (mass%) 91.9 92.8 88.8 90.0 91.4 90.5 93.3 93.9 91.3 92.4 94.5 Comp. Example Example 2-1 2-1 2-2 2-3 2-4 Cooling step calculation conditions Shaft furnace top pressure (MPa) (gauge pressure) 0.04 0.04 0.04 0.04 0.04 Gas injection method Case 3 Case 3 Case 3 Case 3 Case 3 First step (transition zone) Metallic iron temperature (°C) 861 861 861 861 861 CH4 (Nm3 / min) 250 250 250 250 250 CO (Nm3 / min) 0 0 0 0 0 Injection temperature (°C) 25 25 25 25 25 Second step (cooling zone) Metallic iron temperature (°C) 644 581 564 555 588 CH4 (Nm3 / min) 0 0 0 0 0 CO (Nm3 / min) 350 350 350 350 350 Injection temperature (°C) 400 400 400 400 400 Amount extracted from system (Nm3 / min) 0 150 200 250 350 Percentage volume of gas discharged from system with respect to volume of supply gas (%) 0 43 57 71 100 HDRI quality Temperature (°C) 720 697 687 676 673 Carbon content (mass%) 2.2 2.6 2.8 3.0 3.0 Degree of reduction (mass%) 88.8 91.3 92.3 93.4 93.9

[0051] The following conclusions may be drawn from the results in Tables 1 and 2. The carbon 5     content of the final produced HDRI was increased in Case 3, where cooling was performed separately using CH4 and CO in that order (Examples 1-1 to 1-5 and 2-1 to 2-4), compared to Case 1 where cooling was with CH4 alone (Comparative Example 1-1) or Case 2 where cooling was with a mixture of CH4 and CO (Comparative Example 1-2). When the amount of discharged gas extracted out of the system in the second step was insufficient with Case 3 (Comparative 10     Examples 1-3, 1-4 and 2-1), the carbon content and degree of reduction of the ultimately obtained HDRI were reduced. Based on the results shown in Tables 1 and 2, it may be concluded that in order to maintain a high degree of reduction for the ultimately obtained HDRI, it is necessary for the proportion of the volume of gas discharged out of the system with respect to the volume of gas supplied in the second step to be 40% or greater. If the proportion of the 15     volume of gas discharged out of the system with respect to the volume of gas supplied in the second step is 70% or greater, in particular, the carbon content and degree of reduction of the ultimately obtained HDRI will tend to be even more notably increased. When using a high temperature for the metallic iron in contact with CO gas in the second step (Comparative Example 1-5), it was not possible to sufficiently increase the carbon content of the ultimately obtained HDRI. When using a low temperature for the metallic iron in contact with CO gas in the second step (Comparative Example 1-6), it was not possible to sufficiently increase the degree of reduction of the ultimately obtained HDRI.

[0052] 3. Calculation results 2 The relationship between the outlet temperature of the transition zone (first step) (the temperature of the metallic iron in contact with CO gas in the cooling zone (second step)) and the carbon content and degree of reduction of the ultimately obtained HDRI, for Case 3, was examined in detail. The results are shown in Fig. 6. In Fig. 6, the plots for C concentration (carbon content) among the plots corresponding to Calculation results 1 are represented as square outlines, and the plots for degree of reduction are represented as circular outlines. Also in Fig. 6, the plots for C concentration (carbon content) among the plots corresponding to calculation results other than Calculation results 1 are represented as solid black squares, while the plots for degree of reduction are represented as solid black circles. For Calculation results 2, the gas flow rate and temperature in the cooling step were changed to vary the outlet temperature in the first step within the range of 250°C or higher and 750°C or lower.

[0053] As shown in Fig. 6, the carbon content of HDRI increased as the outlet temperature in the first step was decreased to an outlet temperature approaching a temperature conducive to carburization by CO. Excessive lowering of the outlet temperature in the first step, however, was found to reduce the degree of reduction of HDRI. Based on the results shown in Fig. 6, it may be concluded that in order to maintain the degree of reduction of HDRI while increasing the carbon content, it is effective for the temperature of the metallic iron in contact with CO gas in the second step to be 400°C or higher and 700°C or lower.

[0054] 4. Calculation results 3 The relationship between the outlet temperature of the transition zone (first step) (the temperature of the metallic iron in contact with CO gas in the cooling zone (second step)) and the temperature of the ultimately obtained HDRI, for Case 3, was examined in detail. The results are shown in Fig. 7. In Fig. 7, the plots corresponding to Calculation results 1 are represented as circular outlines, while the other plots are represented as solid black circles. For Calculation results 3, the gas flow rate and temperature in the cooling step were changed to vary the outlet temperature in the first step within the range of 250°C or higher and 750°C or lower.

[0055] As shown in Fig. 7, a lower temperature of the metallic iron after the first step also resulted in a lower HDRI temperature. When subsequent briquetting is to be carried out, the HDRI temperature is preferably 650°C or higher. This suggests that when HDRI is used to produce HBI, the temperature of the metallic iron in contact with CO gas in the second step is preferably 550°C or higher and 700°C or lower.

[0056] 5. Supplement The Examples are exemplary cases where hydrogen gas was used as the reducing gas, but the type of reducing gas is not limited to these cases. If a gas other than hydrogen gas (for example, a carbon-containing gas such as CO gas or CH4 gas) is used as the reducing gas, then carburization may result as the iron oxide is reduced in the reducing step, and the carbon content of the metallic iron after the reducing step may increase. By carrying out the cooling step of the disclosure with metallic iron having such a high carbon concentration, it is possible to further increase the carbon content of the ultimately obtained HDRI.

[0057] 5. Summary Based on these results, it may be concluded that the carbon content of HDRI can be increased while maintaining the degree of reduction, by carrying out the following reducing step and cooling step.

[0058] In the reducing step, a reducing gas is contacted with iron oxide feedstock to obtain metallic iron. In the cooling step, the metallic iron is cooled while being carburized is carried out, to obtain HDRI. The cooling step comprises a first step in which methane gas is contacted with the metallic iron after the reducing step, and a second step in which CO gas is contacted with the metallic iron after the first step. The temperature of the metallic iron that contacts with the CO gas in the second step is 400°C or higher and 700°C or lower. The proportion of the volume of gas discharged out of the system with respect to the volume of gas that is supplied in the second step is 40% or greater. REFERENCE SIGNS LIST

[0059] 10 Iron oxide feedstock 20 Metallic iron 30 HDRI 100 Shaft furnace 110 Reducing unit 120 Cooling unit 121 First section 5     122 Second section 200 Cooling tower

Claims

1. A method for producing hot direct reduced iron, the method comprising:a reducing step in which a reducing gas is contacted with an iron oxide feedstock to obtain metallic iron, anda cooling step in which the metallic iron is cooled while being carburized to obtain hot direct reduced iron, whereinthe cooling step comprisesa first step in which methane gas is contacted with the metallic iron after the reducing step, anda second step in which, after the first step, CO gas is contacted with the metallic iron,the temperature of the metallic iron in contact with the CO gas in the second step is400°C or higher and 700°C or lower, andthe proportion of the volume of the gas discharged outside the system with respect to the volume of the gas that is supplied in the second step is 40% or greater.

2. The method for producing hot direct reduced iron according to claim 1, whereinthe proportion of the volume of gas discharged out of the system with respect to the volume of gas that is supplied in the second step is 70% or greater.

3. The method for producing hot direct reduced iron according to claim 1 or 2, whereinthe temperature of the metallic iron in contact with CO gas in the second step is 550°C or higher and 700°C or lower.

4. The method for producing hot direct reduced iron according to any one of claims 1 to 3, whereinthe discharged gas from the first step is used as the reducing gas in the reducing step.

5. The method for producing hot direct reduced iron according to any one of claims 1 to 4, whereinthe reducing step and the cooling step are carried out inside a shaft furnace.

6. The method for producing hot direct reduced iron according to any one of claims 1 to 4, whereinthe reducing step is carried out inside a shaft furnace, andthe cooling step is carried out outside the shaft furnace.

7. The method for producing hot direct reduced iron according to any one of claims 1 to 4, whereinthe reducing step and the first step are carried out inside a shaft furnace, andthe second step is carried out outside the shaft furnace.

8. The method for producing hot direct reduced iron according to any one of claims 1 to 7, whereinthe reducing gas comprises hydrogen gas.

9. The method for producing hot direct reduced iron according to claim 8, the method comprising:a dehydrating step in which the discharged gas from the reducing step is dehydrated to obtain a circulating gas, anda temperature-raising step in which the circulating gas and hydrogen gas are increased in temperature to obtain a reducing gas comprising the circulating gas and hydrogen gas.

10. The method for producing hot direct reduced iron according to any one of claims 1 to 9, whereinthe temperature of the metallic iron in contact with the methane gas in the first step is 700°C or higher.

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