Iron carbide manufacturing method

Abstract

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

Description

TITLE: METHOD FOR PRODUCING CARBURIZED IRON FIELD

[0001] The present application discloses a method for producing carburized iron. BACKGROUND

[0002] Reduction in CO2 emission by direct reduction using reducing gas is being studied in the steel industry as a substitute technique for the blast furnace method. In a direct reduction process, a reducing gas is contacted with iron oxide feedstock to obtain direct reduced iron (DRI). For reduced iron, the carbon content is often increased (that is, carburization is carried out), in order to lower the melting temperature in the subsequent melting and refining steps, and in order to ensure strength as steel. Carburization of reduced iron is also preferred from the viewpoint of preventing reoxidation of the reduced iron and improving the handleability. Methods for carburization of reduced iron include a method comprising a reducing step in which a reducing gas is contacted with an iron oxide feedstock to obtain reduced iron, and a carburizing step in which methane gas, for example, is contacted with the reduced iron to cool the reduced iron while carburizing the metallic iron in the reduced iron. The temperature of the reduced iron obtained by the reducing step is about 700°C to 1050°C. The carburization reaction of the metallic iron by methane gas (CH4 (g) + 3Fe (s) ^ 2H2 (g) + FesC (s)) is an endothermic reaction which proceeds in a high-temperature range of 700°C or higher.

[0003] In relation to the technology described above, PTL 1 discloses providing a cooling gas loop that circulates a cooling gas that cools reduced iron, in a vertical reduction furnace having a reduction zone and a cooling zone, using a gas with a hydrocarbon concentration of 50% or greater as the cooling gas. PTL 2 discloses a method using a hydrocarbon such as methane, pentane or hexane as a cooling gas. [CITATION LIST] [PATENT LITERATURE]

[0004] [PTL 1] International Patent Publication No. WO2021 / 029114 [PTL 2] Japanese Unexamined Patent Publication No. S61-073805 SUMMARY [TECHNICAL PROBLEM]

[0005] Research is still inadequate in the prior art as concerns the conditions for obtaining reduced iron by a direct reduction process using a shaft furnace and subsequently carburizing the reduced iron to obtain carburized iron, in such a manner that the degree of cementite formation for the carburized iron is at or above a target value. [SOLUTION TO PROBLEM]

[0006] The present application discloses the following aspects as means for solving this problem. <Aspect 1> A method for producing carburized iron, the method comprising: a reducing step in which an iron oxide feedstock is reduced by a reducing gas to obtain reduced iron, and a carburizing step in which the reduced iron is cooled and carburized by a carburizing gas to obtain carburized iron, wherein the reducing step is carried out inside a shaft furnace and in the carburizing step, the carburizing gas comprises methane gas at 70 vol% or higher, the reduced iron is carburized up to or above a target degree of cementite formation, and the following relationships (1) and (2) are satisfied: Y > 0.24X - 0.31  (1) X > 3   (2) Y: time elapsed while the temperature of the reduced iron falls from 800°C to 700°C (min) X: target degree of cementite formation (%). <Aspect 2> The method for producing carburized iron according to aspect 1, wherein the elapsed time Y is 30 minutes or less. <Aspect 3> The method for producing carburized iron according to aspect 1, wherein the elapsed time Y is 20 minutes or less. <Aspect 4> The method for producing carburized iron according to any one of aspects 1 to 3, wherein the carburizing step is carried out inside the shaft furnace. <Aspect 5> The method for producing carburized iron according to any one of aspects 1 to 3, wherein the carburizing step is carried out outside the shaft furnace. <Aspect 6> The method for producing carburized iron according to any one of aspects 1 to 5, wherein the reducing gas comprises either or both hydrogen gas and methane gas. [EFFECTS OF INVENTION]

[0007] In the method for producing carburized iron according to the disclosure, when reduced iron is obtained by a direct reduction process using a shaft furnace and the reduced iron is then carburized to obtain carburized iron, a degree of cementite formation at or above a target value can be achieved for the carburized iron. BRIEF DESCRIPTION OF DRAWINGS

[0008] Fig. 1 is a schematic diagram illustrating an example of a method and system for producing carburized iron. Fig. 2 is a schematic diagram illustrating an example of a method and system for producing carburized iron. Fig. 3 is a schematic diagram illustrating an example of a method and system for producing carburized iron. Fig. 4 is a schematic diagram illustrating an example of a method and system for producing carburized iron. Fig. 5A shows the relationship between elapsed time and carbon concentration, for a reaction temperature of 700°C. Fig. 5B shows the relationship between elapsed time and carbon concentration, for a reaction temperature of 800°C. Fig. 5C shows the relationship between elapsed time and carbon concentration, for a reaction temperature of 900°C. Fig. 6 shows the relationship between elapsed time from temperature decrease from 800°C to 700°C, and degree of cementite formation. DESCRIPTION OF EMBODIMENTS

[0009] One embodiment of the method and system for producing carburized iron according to the present disclosure will now be explained. However, the method and system for producing carburized iron 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 “reduced iron” refers to reduced iron obtained as the intermediate product after the reducing step. The term “carburized iron” refers to reduced iron having an increased degree of cementite formation via the carburizing step.

[0010] 1. Method for producing carburized iron As shown in Figs. 1 to 4, the method for producing carburized iron according to one embodiment comprises a reducing step S1 in which the iron oxide feedstock 10 is reduced by a reducing gas to obtain reduced iron 20, and a carburizing step S2 in which the reduced iron 20 is cooled and carburized by carburizing gas to obtain carburized iron 30. The reducing step S1 is carried out inside the shaft furnace 100. In the carburizing step S2, the carburizing gas comprises methane gas at 70 vol% or greater. In the carburizing step S2, the reduced iron 20 is carburized up to or above the target degree of cementite formation. In the carburizing step S2, the following relationships (1) and (2) are satisfied: Y > 0.24X - 0.31   (1) X > 3   (2) Y: time elapsed while the temperature of the reduced iron 20 falls from 800°C to 700°C (min) X: target degree of cementite formation (%).

[0011] 1.1. Reducing step In the reducing step S1, a reducing gas is contacted with the iron oxide feedstock 10. A portion of the iron oxide in the iron oxide feedstock 10 is thus reduced to obtain reduced iron 20. As shown in Figs. 1 to 4, the reducing step S1 is carried out inside the shaft furnace 100. It is likely that the technology of the disclosure will produce the desired effect even if the reducing step S1 is carried out in a kiln instead of a shaft furnace 100.

[0012] 1.1.1. Iron oxide feedstock The iron oxide feedstock 10 comprises at least iron oxide. The iron oxide feedstock 10 may be at least one material 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 also contain impurities. 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 is the sieve diameter of the feedstock. The “mean particle size” of the iron oxide feedstock 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 massbased 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 lumpy state, or in any other shape.

[0013] In the reducing step S1, the iron oxide feedstock 10 may be filled into the shaft furnace 100 to form a packed bed. The packing rate of the packed bed is not particularly restricted and may be similar to the packing rate used in a 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. For example, the average traveling speed may be adjusted depending on the amount of feedstock that is fed (the feed rate). A burden feeder, for example, may be provided to prevent bridging while the packed bed is being caused to move downward.

[0014] 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. The reducing gas may comprise either or both hydrogen gas and methane gas, for example. The reducing gas may also comprise other gases in addition to hydrogen gas or methane gas. Other gases include CO gas, inert gases, CO2 gas and water vapor. Inert gases include nitrogen gas and argon gas. When the reducing gas comprises hydrogen gas, the hydrogen concentration of the reducing gas may be 40 vol% to 100 vol%, 60 vol% to 100 vol% or 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 is preferably 800°C to 1100°C.

[0015] In the reducing step S1, the reducing gas may be supplied from the side wall of the shaft furnace 100 to the interior of the furnace. 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.

[0016] 1.1.3 Reduced iron If at least part of the iron oxide in the iron oxide feedstock 10 is reduced in the reducing step S1, reduced iron 20 comprising metallic iron is obtained. The reduced iron 20 may also comprise, in addition to metallic iron, also iron oxide that remains without being reduced, and may also comprise silicon dioxide or aluminum oxide. The temperature of the reduced iron 20 immediately after reduction (the temperature of the reduced iron 20 immediately before the subsequent carburizing step S2) may be higher than 700°C, for example. The upper limit for the temperature of the reduced iron 20 immediately after reduction is not particularly restricted, and it may be a temperature that allows the subsequent carburizing step S2 to be carried out. The temperature of the reduced iron 20 immediately after reduction may be 1100°C or lower, for example. The temperature of the reduced iron 20 immediately after reduction may be in the range of higher than 700°C and 1050°C or lower, 750°C to 1000°C or 800°C to 900°C.

[0017] For this embodiment, there is no particular restriction on the degree of metallization of the reduced iron 20 obtained in the reducing step S1 ([mass of metallic iron in reduced iron 20] / mass of total iron in reduced iron 20) x 100). According to one embodiment, the degree of metallization of the reduced iron 20 obtained in the reducing step S1 may be 50% to 100%, 60% to 100%, 70% to 100%, 80% to 100% or 90% to 100%.

[0018] 1.1.4 Shaft furnace The shape of the furnace body of the shaft furnace 100 may be the same as the shape of the furnace body of a publicly 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 (carburizing gas for cooling and carburization), 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.

[0019] 1.2 Carburizing step The temperature of the reduced iron 20 immediately after carrying out the reducing step S1 is higher than 700°C, for example. In the carburizing step S2, a carburizing gas comprising methane gas is contacted with the high-temperature reduced iron 20, whereby the reduced iron 20 is carburized while being cooled. The carburizing step S2 yields carburized iron (reduced iron with an increased carbon concentration) 30. By thus carburizing the reduced iron 20 to obtain carburized iron 30, the melting temperature is lower in the subsequent melting and refining steps, and the strength as steel is also ensured. Reoxidation of the reduced iron is also prevented, and handleability is improved, including transport and storage.

[0020] The carburizing step S2 may be carried out inside the shaft furnace or outside the shaft furnace. In other words, the reducing step S1 and carburizing step S2 may both be carried out inside the shaft furnace 100, as shown in Fig. 1. Alternatively, as shown in Fig. 2, the reducing step S1 may be carried out inside the shaft furnace 100 and the carburizing step S2 may be carried out outside the shaft furnace 100. When the carburizing step S2 is to be carried out outside the shaft furnace 100, a cooling tower 200 may be provided downstream from the shaft furnace 100 as shown in Fig. 2, for example, with the carburizing step S2 being carried out in the cooling tower 200. The term “downstream” as used herein means downstream in the process for producing carburized iron. That is, when carburized iron 30 is produced from iron oxide feedstock 10 via reduced iron 20, the iron oxide feedstock 10 end is the upstream end while the carburized iron 30 end is the downstream end.

[0021] In the carburizing step S2, the reduced iron 20 is cooled and carburized by contacting the reduced iron 20 with a carburizing gas. The reduced iron 20 is partially carburized. There are no particular restrictions on the system used for contacting the carburizing gas with the reduced iron 20. For example, the carburizing gas can be contacted with the reduced iron 20 by connecting piping to a gas supply port provided in the side wall of the shaft furnace 100 or cooling tower 200, and supplying carburizing gas from the exterior to the interior through the piping.

[0022] In the carburizing step S2, contact of the reduced iron 20 with the carburizing gas can result in reaction that produces cementite represented by the following formula (A), and decomposition reaction of cementite, represented by formula (B). The present inventors have found that the reaction temperature and reaction time (elapsed time) in the carburizing step S2 can affect the degree of cementite formation. C (s) + 3Fe (s) ^ Fe3C (s) (A) Fe3C (s) ^ C (s) + 3Fe (s) (B)

[0023] 1.2.1 Reaction temperature The reaction temperature in the carburizing step S2 (the surface temperature of the reduced iron 20 in contact with the carburizing gas) is the temperature at which carburization of the metallic iron proceeds by the methane gas in the carburizing gas. The present inventors have found that at temperatures significantly above 800°C, it is possible to increase the carbon concentration by deposition of carbon in the reduced iron 20, while the amount of cementite produced is low. The present inventors have further found that production of cementite tends to occur most notably during the period while the temperature of the reduced iron 20 falls from 800°C to 700°C. One feature of this embodiment is that during cooling and carburization of the reduced iron 20 with the predetermined carburizing gas in the carburizing step S2, the degree of cementite formation of the reduced iron 20 is specified to be at or above a target value X (carburized iron 30 is obtained having a degree of cementite formation of the target value X or higher), by setting at least a predetermined value for the elapsed time Y during which the temperature of the reduced iron 20 falls from 800°C to 700°C.

[0024] The surface temperature of the reduced iron 20 in the carburizing step S2 (the surface temperature of the reduced 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. The “surface temperature of the reduced iron 20 that contacts with the carburizing gas” in the carburizing step S2 is the mean temperature in the radial direction of the shaft furnace or cooling tower. The mean temperature of the reduced 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. 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. Alternatively, the surface temperature of the reduced iron 20 in contact with the carburizing gas may be the temperature estimated in a simulation. The temperature of the carburizing gas contacting the reduced iron 20 in the carburizing step S2 is not particularly restricted. The temperature of the carburizing gas may be 25°C to 600°C, for example. [Mathematical Formula 1] f^'rT(r)dr

[0025] 1.2.2 Carburizing gas The carburizing gas contacted with the reduced iron 20 in the carburizing step S2 comprises methane gas at 70 vol% or greater. This will allow carburizing reaction of the reduced iron 20 to proceed efficiently. The proportion of methane gas in the carburizing gas in the carburizing step S2 may be 70 vol% to 100 vol%, 75 vol% to 100 vol%, 80 vol% to 100 vol%, 85 vol% to 100 vol%, 90 vol% to 100 vol% or 95 vol% to 100 vol%, for example. The composition of the carburizing gas is based on the concentration of the gas before (immediately before) it is introduced into the shaft furnace 100, and it can be controlled by the amount of methane gas and other gas supplied.

[0026] In the carburizing step S2, another gas may also be contacted with the reduced iron 20, together with the methane gas. The carburizing gas in the carburizing step S2 may also comprise nitrogen gas, hydrogen gas, CO gas, CO2 gas or water vapor, in addition to methane gas. The proportion of gas other than methane gas in the carburizing gas may be 0 vol% to 30 vol%, 0 vol% to 25 vol%, 0 vol% or higher and lower than 20 vol%, 0 vol% to 15 vol%, 0 vol% to 10 vol% or 0 vol% to 5 vol%, for example. The proportion of nitrogen gas in the carburizing gas may be 0 vol% to 30 vol%, 0 vol% to 25 vol%, 0 vol% or higher and lower than 20 vol%, 0 vol% to 15 vol%, 0 vol% to 10 vol% or 0 vol% to 5 vol%, for example. The proportion of hydrogen gas in the carburizing gas may be 0 vol% to 30 vol%, 0 vol% to 25 vol%, 0 vol% or higher and lower than 20 vol%, 0 vol% to 15 vol%, 0 vol% to 10 vol% or 0 vol% to 5 vol%, for example. The proportion of CO gas in the carburizing gas is preferably low. The proportion of CO gas in the carburizing gas may be 0 vol% to 10 vol% or 0 vol% to 5 vol%. The proportion of CO2 gas in the carburizing gas may be 0 vol% to 30 vol%, 0 vol% to 25 vol%, 0 vol% or higher and lower than 20 vol%, 0 vol% to 15 vol%, 0 vol% to 10 vol% or 0 vol% to 5 vol%, for example. The proportion of water vapor in the carburizing gas may be 0 vol% to 30 vol%, 0 vol% to 25 vol%, 0 vol% or higher and lower than 20 vol%, 0 vol% to 15 vol%, 0 vol% to 10 vol% or 0 vol% to 5 vol%, for example. The carburizing gas in the carburizing step S2 may also comprise natural gas, for example. The temperature of the carburizing gas in the carburizing step S2 is not particularly restricted so long as it allows carburization reaction of the metallic iron to proceed.

[0027] The discharge gas in the carburizing step S2 comprises methane gas or hydrogen gas, for example. The discharge gas may be discharged out of the system and utilized as fuel, or it may be utilized as the above-mentioned reducing gas. If the discharge gas from the carburizing step S2 is utilized as the reducing gas for the reducing step S1, in particular, more efficient operation can be accomplished while reducing the amount of reducing gas used. When the carburizing step S2 is carried out inside the shaft furnace 100, the gas obtained by carburization in the carburizing step S2 may pass through the transition zone inside the shaft furnace 100 and further rise inside the furnace, to be used as part of the reducing gas in the reduction zone.

[0028] 1.2.3 Elapsed time Y In the carburizing step S2, contact of methane gas with the reduced iron 20 causes carburization of the metallic iron in the reduced iron 20 to proceed, as mentioned above. The carburization reaction with methane gas is an endothermic reaction, and therefore the reduced iron 20 is cooled while being carburized in the carburizing step S2. As mentioned above, carburization of the reduced iron 20 tends to take place at 700°C to 800°C. The present inventors have found that if the elapsed time Y during which the temperature of the reduced iron 20 falls from 800°C to 700°C in the carburizing step S2 is at least a predetermined value, it is possible to carburize the reduced iron 20 up to or above a target degree of cementite formation X. Specifically, it is important for the following relationships (1) and (2) to be satisfied in the carburizing step S2.

[0029] Y > 0.24X - 0.31  (1) X > 3   (2) Y: Time elapsed while temperature of reduced iron 20 falls from 800°C to 700°C (min). X: Target degree of cementite formation (%)

[0030] The lower limit for the elapsed time Y is determined based on the formula shown above, depending on the target degree X of cementite formation. The upper limit for the elapsed time Y is not particularly restricted, however, and it may be appropriately determined in consideration of productivity. From the viewpoint of increasing productivity, the elapsed time Y may be 30 minutes or less, 25 minutes or less or 20 minutes or less. The elapsed time Y can be controlled based on the temperature of the iron oxide feedstock 10 in the reducing step S1, the temperature of the reducing gas that contacts with the iron oxide feedstock 10 in the reducing step S1, the temperature of the reduced iron 20 reaching the carburizing step S2, the supply rate (feed rate) of the reduced iron 20, the discharge speed of the carburized iron 30 in the carburizing step S2, the discharge temperature of the carburized iron 30, the temperature of the carburizing gas, the supply position of the carburizing gas, the supply rate of the carburizing gas, the discharge position of the discharge gas in the carburizing step S2, the discharge amount of the discharge gas, and the shape and volume of the reactor (shaft furnace or cooling tower) in which the carburizing step S2 is carried out.

[0031] 1.2.4 Target degree X of cementite formation In the carburizing step S2, as mentioned above, the reduced iron 20 is carburized up to or above a target degree X of cementite formation to obtain carburized iron 30. The target degree X of cementite formation may be appropriately set according to the purpose of use of the carburized iron 30. The target degree X of cementite formation is 3% or higher, and for example, it may be 5% to 100% or 10% to 30%.

[0032] The “degree of cementite formation” is the mass ratio of iron composing the cementite among the total iron in the reduced iron 20 (carburized iron 30) after the carburizing step S2 ([mass of iron composing cementite] / mass of total iron] x 100). The degree of cementite formation can be determined by pulverizing the reduced iron 20 (carburized iron 30) after the carburizing step S2 to form a powder, and then obtaining an X-ray diffraction pattern by powder X-ray diffraction and further carrying out Rietveld analysis. Regarding the measuring conditions for powder X-ray diffraction, CoKa radiation and a focusing optic system are used as the ray source, with a tube voltage of 40 kV and a tube current of 36 mA, and the measurement is conducted at a scanning speed of 0.02° / step, 2.0° / min, using a scanning angle of 20 = 5 to 120°. Rietveld analysis is well known, as described in NPL 1, for example. For Rietveld analysis, background correction is performed using the B-spline method, and the peak profile function used is the split pseudo-Voigt function. The selective orientation is corrected using the March-Dollase function. In this analysis, the parameters for background function, the lattice constant for each mineral phase, the profile function and the crystal structure factor are refined. With the analysis conditions described above, Rietveld analysis is carried out for the crystal phases identified by qualitative analysis, and each crystal phase is identified. The crystal phases analyzed are hematite (ICDD:01-080-2377), magnetite (ICDD:01-089-0688), wustite (ICDD:01-089-0686), a-Fe (ICDD:00-006-0696), Y-Fe (ICDD:01-089-04185), cementite (ICDD:04-014-3159), graphite (ICDD:01-071-3739), other iron carbides (Hagg carbide (x—Fe5C2) (ICDD:01-089-2544) and Hexagonal carbide (s-Fe2C) (ICDD:01-089-2544)), and gangue-derived compounds (quartz (ICDD:00-046-1045), wollastonite (ICDD:04-016-5334), larnite (ICDD:01-083-0465) and gehlenite (ICDD:04-016-0209)). The target phase for analysis is selected each time depending on the raw material conditions and operating conditions. For example, when the iron oxide feedstock 10 used comprises alumina at a high concentration, the reduced iron 20 (carburized iron 30) obtained after the carburizing step S2 presumably contains gehlenite, and therefore this is included as a phase to be analyzed. If the iron oxide feedstock 10 used has added CaO and a large amount of free SiO2, then 2CaO^SiO2, such as larnite, is also included as a phase to be analyzed. NPL 1: Takayuki Harano, Yu Nemoto, Reiko Murao, Masao Kimura, ISIJ International, 2020, Volume 60, Issue 12, 2851.

[0033] The “target degree of cementite formation” is the target value for the degree of cementite formation in the carburized iron 30 obtained from the carburizing step S2. In the carburizing step S2, the reduced iron 20 is carburized up to or above the target degree of cementite formation. When an additional carburizing step S3 is carried out after the carburizing step S2, as explained below, it is sufficient if the reduced iron 20 is carburized up to or above the target degree of cementite formation after the carburizing step S2 and before the carburizing step S3.

[0034] 1.3 Additional steps For this embodiment, other steps may also be carried out in addition to the reducing step S1 and carburizing step S2 described above. For example, according to one embodiment, as shown in Fig. 3, carburization with a carburizing gas other than methane gas (carburizing step S3) may also be carried out, separately from the carburizing step S2 described above. According to another embodiment, cooling with a cooling gas other than methane gas (cooling step S4) may also be carried out, separately from the carburizing step S2. For this embodiment, it is sufficient for the temperature of the reduced iron 20 to be lowered in the carburizing step S2, and the temperature of the reduced iron 20 may be increased in a different step separate from the carburizing step S2 (for example, a step after the carburizing step S2). For example, when the carburizing step S3 is carried out with CO gas after the carburizing step S2, exothermic reaction takes place between the reduced iron 20 and CO gas, potentially increasing the temperature of the reduced iron 20. 1.3.1 Additional carburizing step with carburizing gas other than methane gas According to one embodiment, an additional carburizing step S3 may also be carried out, by further contacting the reduced iron 20 with CO gas after the carburization by contact with carburizing gas in the carburizing step S2. This will help to further increase the degree of carburization of the final obtained carburized iron 30. The additional carburizing step S3 with CO gas may be carried out either inside the shaft furnace or outside the shaft furnace. There are no particular restrictions on the system used for contacting CO gas with the reduced iron 20 after carburization with methane gas. 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 reduced iron 20 that has been carburized by methane gas. In addition to CO gas, another gas may also be contacted with the reduced iron 20 that has been carburized with methane gas. Gases other than CO gas include nitrogen gas, methane gas, hydrogen gas and CO2 gas. CO-containing gas may be Linz-Donawitz gas (LDG), for example.

[0036] When CO gas is contacted after the reduced iron 20 has been contacted with carburizing gas for carburization in the carburizing step S2, discharge gas is generated from the CO gas. The discharge gas comprises mainly CO gas and CO2 gas, for example. The discharge gas may also be discharged out of the system and utilized as fuel.

[0037] 1.3.2 Additional cooling step with cooling gas According to one embodiment, after the carburizing step S2 has been carried out by contacting the reduced iron 20 with carburizing gas in the carburizing step S2, or after the additional carburizing step S3 with CO gas, the cooling gas may be contacted with the carburized reduced iron 20, further cooling the carburized reduced iron 20. The cooling step S4 may be carried out inside the shaft furnace or outside the shaft furnace. In the additional cooling step S4 it is sufficient if cooling of the carburized reduced iron 20 proceeds, and multiple different types of cooling gases may also be contacted with the carburized reduced iron 20. The cooling gas may comprise an inert gas such as methane gas or nitrogen gas, or hydrogen gas or water vapor, for example. When the gas contacting with the carburized reduced iron 20 in the further cooling step S4 is composed of a plurality of different gases, the volume ratio of methane gas or the volume ratio of inert gas may be highest among the volume ratios of each of the gases.

[0038] When methane gas is used in the cooling step S4, discharge gas derived from the methane gas or other gas is generated in the cooling step S4. The discharge gas comprises methane gas and hydrogen gas, for example. The discharge gas may be discharged out of the system and used as fuel, or it may be used as a portion of the reducing gas, or it may be used as a portion of the carburizing gas in the carburizing step S2 with methane gas and hydrogen gas, or it may be used as a portion of the carburizing gas in the carburizing step S3 with CO gas. When an inert gas is used in the cooling step S4, discharge gas comprising the inert gas is generated in the cooling step S4. The discharge gas may be discharged out of the system or may be reutilized as inert gas in the cooling step S4.

[0039] 1.3.3. Dehydrating step and temperature-raising step As shown in Fig. 4, the method for producing reduced iron according to the embodiment may have a dehydrating step S5 in which the discharge gas from the reducing step S1 is dehydrated to obtain a circulating gas, and a temperature-raising step S6 in which the circulating gas and hydrogen gas are raised in temperature to obtain a reducing gas comprising the circulating gas and hydrogen gas. In the dehydrating step S5, the discharge 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 is 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 discharge gas from the reducing step S1. Dehydration of this discharge gas produces a circulating gas comprising the reducing gas. The circulating gas obtained from the dehydrating step S5 comprises reducing gas, but the amount is not sufficient. Since the circulating gas obtained from the dehydrating step S5 is also at low temperature, using it directly after the reducing step S1 is poorly efficient. In the temperature-raising step S6, 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 S6, 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 hydrogen gas may be obtained by electrolysis of water vapor in the discharge gas from the reducing step S1. The temperature-raising step S3 may be carried out using a known heating apparatus.

[0040] As explained above, in the method for producing carburized iron according to one embodiment, the reducing step S1 and carburizing step S2 may both be carried out inside the shaft furnace 100. In this case, the discharge gas from the carburizing step S2 may be raised in temperature while in the furnace and added to the reducing gas in the reducing step S1, and used for reduction. As shown in Fig. 3, the discharge gas from the optional carburizing step S3 with CO gas may be discharged out of the system at the downstream end from the carburizing step S2. Such a configuration causes the CO gas or CO2 gas in the discharge gas from the carburizing step S3 to be eliminated from the reducing step S1. In other words, the CO gas or CO2 gas no longer contaminates the discharge gas from the reducing step S1, so that the discharge gas from the reducing step S1 consists entirely of the reducing gas (such as hydrogen gas) and water vapor. Carrying out the dehydrating step S5 for the discharge gas in this manner allows reuse of the reducing gas. Although methane gas remains in the discharge gas from the carburizing step S2, the methane gas is degraded in the reduction zone. The methane gas is also merged and diluted with other gases in the reduction zone. That is, virtually no methane gas is present in the discharge 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.

[0041] The method of the disclosure may also combine the modes illustrated in Figs. 1 to 4. For example, the dehydrating step S4 or temperature-raising step S5 shown in Fig. 4 may be carried out in the production methods illustrated in Figs. 1 to 3. The additional carburizing step S3 and cooling step S4 shown in Fig. 3 may also be carried out in the production method illustrated in Fig. 2.

[0042] 1.4 Carburized iron The reducing step S1 and carburizing step S2 are carried out to produce carburized iron 30 (reduced iron with an increased carbon concentration). In addition to carbon and iron, the carburized iron 30 may also comprise iron oxide, silicon dioxide and aluminum oxide that remain without being reduced. The degree of cementite formation in the carburized iron 30 is equal to or greater than the target degree of cementite formation. The “carbon” in the carburized iron 30 includes carbon present as cementite, as well as free carbon. The carbon concentration of the carburized iron 30 may be greater than 0 mass% and 5 mass% or lower, or 0.5 mass% to 5 mass%, for example. The temperature of the carburized iron 30 produced by the method of this embodiment (the surface temperature of the carburized iron 30 on the outer side of the shaft furnace 100 or cooling tower 200) may be 150°C or lower or 80°C or lower, for example. That is, the carburized iron 30 may be CDRI with an increased carbon concentration. Alternatively, the temperature of the carburized iron 30 may be 100°C or higher, 150°C or higher or 200°C or higher, for example. That is, the carburized iron 30 may be HDRI with an increased carbon concentration.

[0043] 2. System for producing carburized iron The technology of the present disclosure also has an aspect as a system for producing carburized iron. Specifically, as shown in Figs. 1 to 4, the system for producing carburized iron according to one embodiment comprises a reducing unit 110 in which the iron oxide feedstock 10 is reduced by a reducing gas to obtain reduced iron 20, and a carburizing unit 120 in which the reduced iron 20 is cooled and carburized by carburizing gas to obtain carburized iron 30. The reducing unit 110 is provided in the shaft furnace 100. The carburizing gas supplied to the carburizing unit 120 comprises methane gas at 70 vol% or greater. In the carburizing unit 120, the reduced iron is carburized up to or above the target degree of cementite formation. The carburizing unit 120 is constructed so as to satisfy the following relationship (1): Y > 0.24X - 0.31   (1) Y: time elapsed while temperature of reduced iron falls from 800°C to 700°C (min) X: target degree of cementite formation (%).

[0044] For this embodiment, the reducing step S1 is carried out at the reducing unit 110 and the carburizing step S2 is carried out at the carburizing unit 120. It is sufficient if the reducing unit 110 and carburizing unit 120 are configured so as to allow the reducing step S1 and the carburizing step S2 to be carried out, respectively. The reducing unit 110 and carburizing unit 120 may be provided in the shaft furnace 100, for example, as shown in Fig. 1. In this case, the reduction zone inside the shaft furnace 100 corresponds to the reducing unit 110, while the cooling zone corresponds to the carburizing unit 120. Alternatively, as shown in Fig. 2, the reducing unit 110 may be provided in the shaft furnace 100, with the carburizing unit 120 being provided separately from the shaft furnace 100. When the carburizing unit 120 is provided separately from the shaft furnace 100, the carburizing unit 120 may be installed in the cooling tower 200 provided downstream from the shaft furnace 100, for example.

[0045] 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 either or both hydrogen gas and methane gas, for example.

[0046] A carburizing gas comprising methane gas is supplied to the carburizing unit 120. The composition and manner of supply of the carburizing gas are as described above. The carburizing unit 120 is also constructed so as to satisfy relationship (1) above. The upper limit for the elapsed time Y in the carburizing unit 120 is as explained above. That is, from the viewpoint of increasing productivity, the carburizing unit 120 may be constructed so that the elapsed time Y is 30 minutes or less, 25 minutes or less or 20 minutes or less.

[0047] As shown in Fig. 3, the system for producing carburized iron according to one embodiment may have a second carburizing unit 130 to which carburizing gas other than methane gas is supplied, and a cooling unit 140 to which cooling gas is supplied. The configurations of the second carburizing unit 130 and cooling unit 140 are sufficient if they allow the additional carburizing step S3 and cooling step S4 to be carried out, respectively. The second carburizing unit 130 and cooling unit 140 may be provided in the shaft furnace 100, or they may be provided separately from the shaft furnace 100.

[0048] As shown in Fig. 4, the system for producing carburized iron according to one embodiment may have a dehydration apparatus 150 in which the discharge gas from the reducing unit 110 is dehydrated to obtain a circulating gas, and a temperature-raising apparatus 160 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 dehydration apparatus 150 and temperature-raising apparatus 160 are apparatuses in which the dehydrating step S5 and temperature-raising step S6, respectively, are carried out. The details are as explained above.

[0049] The temperatures of the line for discharge gas from the reducing unit 110 and carburizing unit 120, and of the reduced iron 20 in the reducing unit 110 and carburizing unit 120, respectively, are also as described above.

[0050] 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.

[0051] When the cooling tower 200 is used as the carburizing 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.

[0052] 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 dehydration apparatus 150 or temperature-raising apparatus 160 shown in Fig. 4 may be combined. The second carburizing unit 130 and cooling unit 140 shown in Fig. 3 may also be combined in the production system shown in Figs. 2 and 4.

[0053] 3. Effect As explained above, in the method and system for producing carburized iron according to this embodiment, after reduced iron 20 has been obtained by a direct reduction process using a shaft furnace 100 and the reduced iron 20 is then carburized to obtain carburized iron 30, it is possible for the carburized iron 30 to have a degree of cementite formation that is at or above a target value. Various effects can be obtained by controlling the degree of cementite formation of the carburized iron 30. For example, when a refining step is carried out by melting of the carburized iron 30, or when a step of conversion to HBI is carried out by hot forming of the carburized iron 30, it is sometimes preferred for the carburized iron 30 to comprise more graphite (free carbon), and sometimes preferred for it to comprise more cementite. For example, in a step of HBI conversion of the carburized iron 30 by hot forming, it is sometimes preferred for the carburized iron 30 to comprise more graphite (free carbon). However, thermal efficiency is more satisfactory if a large amount of cementite is present in the melting step for carburized iron 30. By using this embodiment it is possible to suitably control the degree of cementite formation of the carburized iron 30, and to produce carburized iron 30 having a suitable degree of cementite formation for the subsequent steps. Moreover, by increasing the degree of cementite formation of the carburized iron 30 to at least a target value, it is possible to facilitate melting of the carburized iron 30 in the subsequent melting step or refining step, for example, and to reduce the amount of carbon material added during the melting step or refining step. EXAMPLES

[0054] 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.

[0055] 1. Experiment method Iron oxide pellets having the chemical composition and particle sizes listed in Table 1 below were reduced with hydrogen to obtain reduced iron, and then a carburization test was conducted using a carburizing gas comprising methane gas at 70 vol% and nitrogen gas at 30 vol%, under 3 conditions with a reaction temperature of 700°C, 800°C or 900°C, and 4 conditions with a reaction time of 5 minutes, 10 minutes, 20 minutes or 30 minutes (a total of 12 different conditions). The iron oxide pellets had sizes (10 to 15 mm) suited for a direct reduction process using a shaft furnace. The sample after the carburization test was supplied for X-ray diffraction, and the cementite (Fe3C) content of the sample was quantitatively analyzed by Rietveld analysis of the obtained diffraction pattern.

[0056] [Table 1] (Table 1)_______________________________________________________________________________________ T.Fe mass% FeO mass% M.Fe mass% CaO mass% SiO2 mass% Al2O3 mass% Particle size mm Iron oxide pellets 66.03 0.26 0.16 2.04 2.29 0.47 10-15 mm

[0057] 2. Experiment results The experiment results are shown in Figs. 5A to C. In Figs. 5A to C, “C concentration” is the carbon concentration of the carburized iron after the carburization test, “time” is the reaction time mentioned above, “Total C” is the total carbon in the carburized iron after the carburization test, “C as free C” is the carbon present as free carbon among the carbon in the carburized iron after the carburization test, and “C as Fe3C” is the carbon present as the cementite among the carbon in the carburized iron after the carburization test. Fig. 5A shows the results for a reaction temperature of 700°C, Fig. 5B shows the results for a reaction temperature of 800°C, and Fig. 5C shows the results for a reaction temperature of 900°C. As shown in Figs. 5A to C, when the reaction temperature was 700 to 800°C, the degree of cementite formation increased as the reaction time progressed, with the degree of cementite formation becoming essentially saturated within 30 minutes. When the reaction temperature was 900°C, on the other hand, there was virtually no increase in the degree of cementite formation even after elapse of the carburization time, the cementite decomposing and being deposited as free carbon.

[0058] Table 2 shows a summary of elapsed times and degree of cementite formation at reaction temperatures of 700°C and 800°C, based on the results shown in Figs. 5A and B.

[0059] [Table 2] (Table 2) Time elapsed with reaction temperature of 700°C (min) Time elapsed with reaction temperature of 800°C (min) Average for 700°C to 800°C (min) Degree of cementite formation 10% 2.5 2.0 2.25 20% 4.5 4.0 4.25 40% 10.0 8.0 9.0 60% 13.0 15.0 14.0

[0060] Fig. 6 shows a graph summarizing the averages for 700 to 800°C in Table 2 as representative values, as the relationship between elapsed time Y during temperature decrease from 800°C to 700°C, and degree X of cementite formation. As shown in Fig. 6, the relationship between elapsed time Y and degree X of cementite formation is generally linear. Specifically, the relationship Y = 0.24X - 0.31 is satisfied. In other words, it may be concluded that carburization of reduced iron can be achieved at or above the target degree of cementite formation when the relationship Y > 0.24X - 0.31 is satisfied. This relationship can be applied when the target degree X of cementite formation is 3% or greater, for example.

[0061] 3. Summary Based on the results described above, it may be concluded that the method for producing carburized iron comprising the following constituent features (A) to (E), allows a degree of cementite formation at or above a target value to be achieved for carburized iron, when reduced iron is obtained by a direct reduction process using a shaft furnace and the reduced iron is then carburized to obtain carburized iron. (A) The method for producing carburized iron comprises a reducing step in which the iron oxide feedstock is reduced by a reducing gas to obtain reduced iron, and a carburizing step in which the reduced iron is cooled and carburized by carburizing gas to obtain carburized iron. (B) The reducing step is carried out inside the shaft furnace. (C) In the carburizing step, the carburizing gas comprises methane gas at 70 vol% to 100 vol%. (D) In the carburizing step, the reduced iron is carburized up to or above the target degree of cementite formation. (E) In the carburizing step, the following relationships (1) and (2) are satisfied: Y > 0.24X - 0.31   (1) X > 3   (2) Y: time elapsed while temperature of reduced iron falls from 800°C to 700°C (min) X: target degree of cementite formation (%). REFERENCE SIGNS LIST

[0062] 10 Iron oxide feedstock 5     20 Reduced iron 30 Carburized iron (reduced iron product) 100 Shaft furnace 110 Reducing unit 120 Carburizing unit 10     130 Second carburizing unit 140 Cooling unit 200 Cooling tower

Claims

1. A method for producing carburized iron, the method comprising:a reducing step in which an iron oxide feedstock is reduced by a reducing gas to obtain reduced iron, anda carburizing step in which the reduced iron is cooled and carburized by a carburizing gas to obtain carburized iron, whereinthe reducing step is carried out inside a shaft furnace andin the carburizing step,the carburizing gas comprises methane gas at 70 vol% or higher,the reduced iron is carburized up to or above a target degree of cementite formation, andthe following relationships (1) and (2) are satisfied:Y > 0.24X - 0.31  (1)X > 3   (2)Y: time elapsed while the temperature of the reduced iron falls from 800°C to 700°C (min)X: target degree of cementite formation (%).

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

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

4. The method for producing carburized iron according to any one of claims 1 to 3, whereinthe carburizing step is carried out inside the shaft furnace.

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

6. The method for producing carburized iron according to any one of claims 1 to 5, wherein the reducing gas comprises either or both hydrogen gas and methane gas.

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