Hot direct reduced iron, method for producing hot direct reduced iron, and method for producing hot briquetted iron

Abstract

Disclosed are a hot direct reduced iron containing carbon and having excellent hot formability, and a method for producing the same. The hot direct reduced iron according to the present disclosure has: a carbon content of at least 1.5 mass% but less than 5.0 mass% and a cementite ratio η of 80% or less; or a carbon content of at least 5.0 mass% but less than 6.0 mass% and a cementite ratio η of 75% or less; or a carbon content of at least 6.0 mass% but less than 10.0 mass% and a cementite ratio η of 70% or less. The method for producing hot direct reduced iron according to the present disclosure includes bringing a carburizing gas containing methane gas into contact with reduced iron to subject the reduced iron to a carburization treatment. Here, the temperature T of the carburization treatment is adjusted such that the cementite ratio η of the reduced iron after the carburization treatment reaches a target value, and the methane gas concentration CCH4 of the carburizing gas and the time t of the carburization treatment are adjusted such that the carbon content in the reduced iron after the carburization treatment reaches a target value.

Description

Hot-reduced iron, method for producing hot-reduced iron, and method for producing hot-formed reduced iron

[0001] This application discloses hot-direct reduced iron (HDRI), a method for producing hot-direct reduced iron, and a method for producing hot-briquetted reduced iron (HBI).

[0002] In the steel industry, CO2 reduction is achieved through a direct reduction process using reducing gas as an alternative technology to the blast furnace method. ２ Reducing emissions is being considered (for example, Patent Documents 1 and 2). In the direct reduction process, reduced iron (DRI) is obtained by contacting an iron oxide raw material with a reducing gas. This reduced iron is then processed into hot-formed reduced iron for purposes such as preventing re-oxidation during transport.

[0003] International Publication No. 2021 / 195160, Japanese Patent Publication No. Sho 61-073805

[0004] Toru Takayama, Reiko Murao, Masao Kimura, Quantitative Analysis of Mineral Phases in Iron-ore Sinter by the Rietveld Method of X-ray Diffraction Patterns, ISIJ International, 2018, Volume 58, Issue 6, Pages 1069-1078

[0005] Hot-formed reduced iron can be obtained, for example, by forming hot reduced iron. In this respect, hot reduced iron is required to have excellent hot-formability. On the other hand, hot reduced iron and hot-formed reduced iron are required to contain carbon from the viewpoint of lowering the melting temperature in the melting and refining processes. In view of the above, this application discloses hot reduced iron containing carbon and having excellent hot-formability, a method for producing the same, and a method for producing hot-formed reduced iron using said hot reduced iron.

[0006] This application discloses the following multiple embodiments as means for solving the above problems. <Embodiment 1> Hot-reduced iron having a carbon content of 1.5% by mass or more and less than 5.0% by mass and a cementitiousness η of 80% or less, or a carbon content of 5.0% by mass or more and less than 6.0% by mass and a cementitiousness η of 75% or less, or a carbon content of 6.0% by mass or more and less than 10.0% by mass and a cementitiousness η of 70% or less. <Embodiment 2> Hot-reduced iron according to Embodiment 1, having a carbon content of 1.5% by mass or more and 6.0% by mass or less and a cementitiousness η of 70% or less. <Embodiment 3> Hot-reduced iron according to Embodiment 1, having a carbon content of 3.0% by mass or more and 5.0% by mass or less and a cementitiousness η of 50% or less. <Aspect 4> A method for producing hot reduced iron according to any of aspects 1 to 3, comprising: bringing reduced iron into contact with a carbonizing gas containing methane gas to perform a carbonization treatment on the reduced iron, wherein the temperature T of the carbonization treatment is adjusted so that the cementitiousness η of the reduced iron after the carbonization treatment reaches a target value, and the methane gas concentration C of the carbonizing gas is adjusted so that the carbon content of the reduced iron after the carbonization treatment reaches a target value. ＣＨ４A method for producing hot reduced iron, wherein the time t of the carbonization treatment is adjusted. <Aspect 5> A method for producing hot reduced iron according to Aspect 4, wherein the target value of the cementitiousness η and the temperature T of the carbonization treatment are determined according to the following formula (1): lnη = 5281.7 / T - 1.206 …(1) T: temperature of the carbonization treatment (K) <Aspect 6> A method for producing hot reduced iron according to Aspect 4 or 5, comprising obtaining the reduced iron by performing a reduction treatment on an iron oxide raw material, wherein the reduction treatment and the carbonization treatment are performed in a shaft furnace. <Aspect 7> A method for producing hot-formed reduced iron, comprising obtaining a molded product by hot-forming the hot reduced iron according to any of Aspects 1 to 3. <Aspect 8> A method for producing hot-formed reduced iron according to aspect 7, comprising holding the formed product in an inert gas atmosphere at 700°C to 800°C for 10 minutes or more.

[0007] The technology disclosed herein provides hot-reduced iron containing carbon and exhibiting excellent hot-formability, a method for producing the same, and a method for producing hot-formed reduced iron using the hot-reduced iron.

[0008] This is a schematic diagram illustrating an example of a method for producing hot-reduced iron and a method for producing hot-formed reduced iron. It shows a comparison between the cementitization rate η and temperature. It also shows a comparison between actual and calculated values ​​of the cementitization rate η. Finally, it shows a comparison between actual and calculated values ​​of the carbon content.

[0009] The following description will refer to the drawings and explain hot-reduced iron and its manufacturing method according to the embodiments, as well as the manufacturing method for hot-formed reduced iron. However, the technology of this disclosure is not limited to the following embodiments.

[0010] As shown in Figure 1, hot reduced iron 10 is produced, for example, by reducing and carbonizing iron oxide raw material 1 in a reduction apparatus 100. Hot formed reduced iron 20 is produced by forming the hot reduced iron 10 in a hot forming apparatus 200.

[0011] 1. Hot Reduced Iron (HDRI) The hot reduced iron 10 according to this embodiment has a carbon content of 1.5% by mass or more and less than 5.0% by mass and a cementitiousness η of 80% or less, or a carbon content of 5.0% by mass or more and less than 6.0% by mass and a cementitiousness η of 75% or less, or a carbon content of 6.0% by mass or more and less than 10.0% by mass and a cementitiousness η of 70% or less. The carbon content of the hot reduced iron 10 is 1.5% by mass or more, which allows the melting temperature in the melting and refining processes to be sufficiently reduced, and also suppresses the re-oxidation of iron. Furthermore, the cementitiousness η of the hot reduced iron 10 is below a certain level, which reduces the Young's modulus of the hot reduced iron 10 and improves the hot formability of the hot reduced iron 10. For example, if the carbon content of the hot-reduced iron 10 is 1.5% by mass or more and less than 5.0% by mass, the hot-formability of the hot-reduced iron 10 will be sufficiently excellent if the cementitiousness η of the hot-reduced iron 10 is 80% or less. Also, if the carbon content of the hot-reduced iron 10 is 5.0% by mass or more and less than 6.0% by mass, the hot-formability of the hot-reduced iron 10 will be sufficiently excellent if the cementitiousness η of the hot-reduced iron 10 is 75% or less. Furthermore, if the carbon content of the hot-reduced iron 10 is 6.0% by mass or more and less than 10.0% by mass, the hot-formability of the hot-reduced iron 10 will be sufficiently excellent if the cementitiousness η of the hot-reduced iron 10 is 70% or less. In particular, when the hot-reduced iron 10 has a carbon content of 1.5% by mass or more and 6.0% by mass or less and a cementitiousness η of 70% or less, and especially when it has a carbon content of 3.0% by mass or more and 5.0% by mass or less and a cementitiousness η of 50% or less, the balance between solubility in the melting process and hot formability in the refining process becomes even better.

[0012] 1.1 Carbon Content As described above, the carbon content of hot-reduced iron 10 is 1.5% by mass or more. If the carbon content of hot-reduced iron 10 is 1.5% by mass or more, the liquid phase formation start temperature of the hot-formed reduced iron 10 will be 1160°C or lower, which is particularly advantageous in subsequent processes such as the melting and refining processes. In particular, if the carbon content of hot-reduced iron 10 is 2.0% by mass or more, and especially 3.0% by mass or more, the melting temperature in the melting and refining processes can be further reduced, and the re-oxidation of iron can be further suppressed. Furthermore, the carbon content of hot-reduced iron 10 is less than 10.0% by mass. If the carbon content of hot-reduced iron 10 is less than 10.0% by mass, hot-formability is ensured, the shape can be maintained after hot-forming, and the decarburization load in the subsequent refining process can be sufficiently reduced. In particular, when the carbon content of the hot-reduced iron 10 is 6.0% by mass or less, especially 5.0% by mass or less, it can be expected that excessive carbon addition and the resulting burden on the decarburization process can be further suppressed in the steelmaking process. That is, a higher effect can be expected when the carbon content of the hot-reduced iron 10 is 1.5% by mass or more and 6.0% by mass or less, especially when it is 2.0% by mass or more and 6.0% by mass or less, and especially when it is 3.0% by mass or more and 5.0% by mass or less. In this application, the carbon content (by mass) of the hot-reduced iron will be determined by quantifying the amount of carbon contained in the hot-reduced iron in accordance with JIS G1211-1:2011. Here, the "carbon content of the hot-reduced iron" will be determined by pulverizing the entire hot-reduced iron and then quantifying the amount of carbon contained in the entire hot-reduced iron.

[0013] 1.2 Cementitization Rate η As described above, in this embodiment, the upper limit of the cementitization rate η of the hot-reduced iron 10 is determined according to the carbon content of the hot-reduced iron 10. However, when the cementitization rate η of the hot-reduced iron 10 is 70% or less, and especially 50% or less, the hot formability of the hot-reduced iron 10 can be significantly improved regardless of the carbon content of the hot-reduced iron 10. The lower limit of the cementitization rate η is not particularly limited. The cementitization rate η may be, for example, 5% or more, or 10% or more. In this application, "cementitization rate η" refers to the ratio of cementitious iron to the total iron contained in the hot-reduced iron (η (%) = 100 × [amount of cementitious iron contained in the hot-reduced iron (g)] / [total amount of iron contained in the hot-reduced iron (g)]). In this application, hot-reduced iron is pulverized into powder, an X-ray diffraction pattern is obtained by powder X-ray diffraction, and then Rietveld analysis is performed to determine the cementitization rate of hot-reduced iron. For the measurement conditions of powder X-ray diffraction, a CoKα 1-ray source and a focusing optical system are used, with a tube voltage of 40 kV and a tube current of 36 mA, and measurements are performed at a scanning angle 2θ = 5 to 120° with a scanning speed of 0.02° / step and 2.0° / min. Rietveld analysis is well known, for example, as described in Non-Patent Document 1 below. In the Rietveld analysis, background correction is performed using the B-spline method, and the peak profile function is the divided pseudo-Voigt function. In addition, selective orientation is corrected using the March-Dollase function. In this analysis, the background function, lattice constants of each mineral phase, profile function, and crystal structure factor are targeted for refinement. Under the above analytical conditions, Rietveld analysis is performed on the crystalline phases identified by qualitative analysis to identify each crystalline phase.The crystalline phases to be analyzed include hematite (ICDD: 01-080-2377), magnetite (ICDD: 01-089-0688), wustite (ICDD: 01-089-0686), α-Fe (ICDD: 00-006-0696), γ-Fe (ICDD: 01-089-04185), cementite (ICDD: 04-014-3159), graphite (ICDD: 01-071-3739), and other iron carbides (Hagg carbide (χ-Fe). ５ C ２ ) (ICDD:01-089-2544), Hexagonal carbide (ε-Fe ２ C) (ICDD: 01-089-2544)), compounds derived from gangue (quartz (ICDD: 00-046-1045), urastonite (ICDD: 04-016-5334), lanite (ICDD: 01-083-0465), ghelenite (ICDD: 04-016-0209)) are used. Note that "ICDD" is an abbreviation for International Centre for Diffraction Data. The phase to be analyzed is selected each time according to the raw material conditions and operating conditions. For example, if a high concentration of alumina is used as the iron oxide raw material 1 described below, it is considered that ghelenite is included in the hot reduced iron 10, and therefore it is included as the phase to be analyzed. In addition, if CaO is added to the iron oxide raw material 1, and single SiO ２ If a material with a high concentration is used, such as lanite, 2CaO·SiO ２ The phase to be analyzed shall be included. Non-patent document 1: Toru Takayama, Reiko Murao, Masao Kimura, Quantitative Analysis of Mineral Phases in Iron-ore Sinter by the Rietveld Method of X-ray Diffraction Patterns, ISIJ International, 2018, Volume 58, Issue 6, Pages 1069-1078

[0014] 1.3 Iron Content The iron content of hot-reduced iron 10 may be, for example, 75% by mass or more and 98% by mass or less, 80% by mass or more and 97% by mass or less, 85% by mass or more and 95% by mass or less, or 88% by mass or more and 94% by mass or less. Hot-reduced iron 10 having such an iron content is suitably used, for example, as a raw material for steel materials. In this application, the iron content (by mass) of hot-reduced iron shall be determined by performing a chemical analysis of the hot-reduced iron in accordance with the method of JIS M8212:2022 and quantifying the total amount of iron contained in the hot-reduced iron.

[0015] 1.4 Metallization Rate The metallization rate of iron contained in the hot-reduced iron 10 may be, for example, 92% or more, 94% or more, or 96% or more. The upper limit of the metallization rate is not particularly limited and may be, for example, 98% or less. The metallization rate refers to the proportion of iron that constitutes metallic iron out of the total iron contained in the hot-reduced iron 10 (metallization rate (%) = 100 × [amount of metallic iron contained in the hot-reduced iron (mass%)] / [amount of all iron contained in the hot-reduced iron (mass%)]). In this application, the metallization rate of iron in the hot-reduced iron is determined by determining the proportion of metallic iron (M.Fe) contained in the hot-reduced iron according to JIS M 8213:1995 (Iron ore - Method for determining acid-soluble iron(II)) and determining the proportion of all iron (T.Fe) contained in the hot-reduced iron according to JIS M 8212:2022 (Titanium(III) chloride-reduced potassium dichromate titration method).

[0016] 1.5 Other Elements As described above, hot-reduced iron 10 contains iron and carbon. In hot-reduced iron 10, some iron exists as metallic iron, and some iron exists as compounds such as cementite and iron oxide. Also, in hot-reduced iron 10, some carbon exists as elemental carbon, and some carbon exists as compounds such as cementite. Furthermore, hot-reduced iron 10 may contain elements other than iron and carbon as impurities. For example, hot-reduced iron 10 may contain other elements such as silicon, aluminum, calcium, magnesium, or compounds of these elements. The content of other elements in hot-reduced iron 10 may be, for example, 0% by mass or more and 20% by mass or less, 0% by mass or more and 15% by mass or less, 0% by mass or more and 10% by mass or less, or 0% by mass or more and 5% by mass or less.

[0017] 1.6 Shape The shape of the hot-reduced iron 10 is not particularly limited. When hot-reduced iron 10 is obtained by reducing iron oxide raw material 1, the hot-reduced iron 10 may have a shape corresponding to the iron oxide raw material 1. The iron oxide raw material 1 will be described later.

[0018] 1.7 Temperature In this application, "hot reduced iron (HDRI)" means reduced iron (DRI) having a surface temperature of 100°C or higher. The surface temperature of the hot reduced iron 10 may be 650°C or higher and 950°C or lower, 650°C or higher and 850°C or lower, or 650°C or higher and 750°C or lower.

[0019] 2. Method for producing hot reduced iron (HDRI) One embodiment of the method for producing hot reduced iron 10 includes bringing the reduced iron into contact with a carbonization gas containing methane gas to perform a carbonization treatment on the reduced iron. Here, the temperature T of the carbonization treatment is adjusted so that the cementitiousness η of the reduced iron after the carbonization treatment reaches a target value, and the methane gas concentration C of the carbonization gas is adjusted so that the carbon content of the reduced iron after the carbonization treatment reaches a target value. ＣＨ４ The carbonization treatment time t is adjusted.

[0020] 2.1 Reduction treatment As shown in FIG. 1, in the reduction apparatus 100, reduced iron is obtained by performing a reduction treatment on the iron oxide raw material 1. In FIG. 1, an example is shown in which the reduction treatment is performed in the reduction zone 101 on the upstream side of the shaft furnace as the reduction apparatus 100, and the carbonization treatment is performed in the carbonization zone 103 on the downstream side. That is, the method for producing the hot-reduced iron 10 according to one embodiment may include obtaining reduced iron by performing a reduction treatment on the iron oxide raw material 1. In this case, the reduction treatment and the carbonization treatment may be performed in a shaft furnace. Thus, by performing both the reduction treatment and the carbonization treatment in the shaft furnace, the hot-reduced iron 10 can be more appropriately produced. The reduction treatment using a shaft furnace may be an existing shaft-type direct reduction treatment such as the Midrex method or the HyL / ENERGIRON method.

[0021] 2.1.1 Iron oxide raw material The iron oxide raw material 1 contains iron oxide. The iron oxide raw material 1 may be, for example, one or more selected from iron ore pellets, lump ore, and sintered ore. In addition to iron oxide, the iron oxide raw material 1 may contain, for example, one or both of silicon dioxide and aluminum oxide. The iron oxide raw material 1 may have a particle size distribution or may have a uniform particle size. When performing the reduction treatment in a shaft furnace, the average particle size of the iron oxide raw material 1 may be, for example, 5.0 mm or more and 20.0 mm or less, or may be 10.0 mm or more and 15.0 mm or less.

[0022] In the present application, the "particle size" of the iron oxide raw material is measured by the diameter of the sieve (sieve opening) when the iron oxide raw material is sieved. When measuring the particle size of the iron oxide raw material, a plurality of sieves with different openings are prepared. Specifically, a total of seven types of sieves with openings of 8 mm, 10 mm, 12 mm, 14 mm, 16 mm, 18 mm, and 20 mm are prepared, and the iron oxide raw material is sieved according to the dry sieving test described in JIS Z 8815:1994. At this time, the particle size D of the iron oxide raw material that did not pass through the sieve with the largest opening of 20 mm １It is assumed that the particle size is 20 mm. Furthermore, the particle size D of the iron oxide raw material that passed through the 20 mm sieve but did not pass through the 18 mm sieve is also considered. ２ The particle size D of the iron oxide raw material that passed through the sieve with an 18 mm mesh size but not through the sieve with an 16 mm mesh size is considered to be the average of 20 mm and 18 mm. ３ Assuming that the particle size is (18 + 16) / 2 mm, the particle size D of the iron oxide raw material that passed through a sieve with a mesh size of 16 mm but did not pass through a sieve with a mesh size of 14 mm is calculated. ４ Assuming that the particle size is (16 + 14) / 2 mm, the particle size D of the iron oxide raw material that passed through a sieve with a mesh size of 14 mm but did not pass through a sieve with a mesh size of 12 mm is calculated. ５ Assuming that the particle size is (14 + 12) / 2 mm, the particle size D of the iron oxide raw material that passed through a sieve with a mesh size of 12 mm but did not pass through a sieve with a mesh size of 10 mm is calculated. ６ Assuming that the particle size is (12 + 10) / 2 mm, the particle size D of the iron oxide raw material that passed through a sieve with a mesh size of 10 mm but did not pass through a sieve with a mesh size of 8 mm is calculated. ７ It is assumed that the particle size is (10 + 8) / 2 mm. Finally, the particle size D of the iron oxide raw material that passed through the smallest sieve with a mesh size of 8 mm. ８ It is assumed to be 8 mm.

[0023] Furthermore, in this application, the "average particle size" of the iron oxide raw material means the weighted average value of the particle size of the iron oxide raw material. Specifically, the average particle size of the iron oxide raw material can be measured by obtaining a mass-based particle size distribution by the dry sieving test described in JIS Z 8815:1994, and using the average of the maximum and minimum diameters of each sieve as the representative diameter, and then weighting the average by mass. More specifically, as described above, a total of seven types of sieves with mesh openings of 8 mm, 10 mm, 12 mm, 14 mm, 16 mm, 18 mm, and 20 mm are prepared, and the iron oxide raw material is sieved according to the dry sieving test described in JIS Z 8815:1994. At this time, the iron oxide raw material that did not pass through the sieve with the largest mesh opening of 20 mm (particle size D １ The mass of 20 mm is X １(g) is the iron oxide raw material (particle size D) that passed through a sieve with a mesh size of 20 mm and did not pass through a sieve with a mesh size of 18 mm. ２ The mass of (20 + 18) / 2 mm is X ２ (g) is the iron oxide raw material (particle size D) that passed through a sieve with an 18 mm mesh size and did not pass through a sieve with an 16 mm mesh size. ３ The mass of (18 + 16) / 2 mm is X ３ (g) is the iron oxide raw material (particle size D) that passed through a sieve with a mesh size of 16 mm and did not pass through a sieve with a mesh size of 14 mm. ４ The mass of (16 + 14) / 2 mm is X ４ (g) is the iron oxide raw material (particle size D) that passed through a sieve with a mesh size of 14 mm and did not pass through a sieve with a mesh size of 12 mm. ５ The mass of (14 + 12) / 2 mm is X ５ (g) is the iron oxide raw material (particle size D) that passed through a sieve with a mesh size of 12 mm and did not pass through a sieve with a mesh size of 10 mm. ６ The mass of (12 + 10) / 2 mm is X ６ (g) is the iron oxide raw material (particle size D) that passed through a sieve with a mesh size of 10 mm and did not pass through a sieve with a mesh size of 8 mm. ７ The mass of (10 + 8) / 2 mm is X ７ (g) represents the iron oxide raw material that passed through the smallest sieve with a mesh size of 8 mm (particle size D ８ The mass of 8 mm is X ８ If (g), the average particle size of the iron oxide raw material (weighted average value of the particle size of the iron oxide raw material) D ａｖｅ D is calculated as follows: ａｖｅ = [X １ ×D １ +X ２ ×D ２ +X ３ ×D ３ +X ４ ×D ４ +X ５ ×D ５ +X ６ ×D ６ +X ７ ×D ７ +X ８ ×D ８ ] / [X １ +X ２+X ３ +X ４ +X ５ +X ６ +X ７ +X ８ ]

[0024] The iron oxide raw material 1 may be formed into pellets or the like, in powder form, in lump form, or in any other form.

[0025] The amount of iron oxide raw material 1 supplied to the reduction device 100 should be selected to be optimal depending on the scale and operating conditions of the reduction device 100. When a shaft furnace is used as the reduction device 100, the iron oxide raw material 1 is supplied from the top of the shaft furnace into the interior. In this case, the supply position of the iron oxide raw material 1 should be above the supply position of the reducing gas. The iron oxide raw material 1 may be supplied, for example, through a raw material supply port provided at the top of the shaft furnace. The method of supplying the iron oxide raw material 1 is not particularly limited, and it may be supplied, for example, by a hopper or chute. The iron oxide raw material 1 may also be supplied by free fall. As the iron oxide raw material 1 is supplied from the top of the shaft furnace into the interior, a packed bed is formed inside the shaft furnace. The packing density of the packed bed is not particularly limited. The iron oxide raw material 1 moves downward inside the shaft furnace while forming a packed bed. That is, inside the shaft furnace, the iron oxide raw material 1 moves gradually downward by falling or the like in a substantially filled state. The average downward movement speed of the iron oxide raw material 1 is not particularly limited. In the packed bed inside the shaft furnace, the iron oxide raw material 1 may have a particle size distribution and temperature distribution from the top to the bottom of the shaft furnace and / or in the radial direction of the shaft furnace. The packed bed may have a regular particle size distribution or an irregular particle size distribution. Furthermore, the temperature distribution of the packed bed is not particularly limited.

[0026] 2.1.2 In the reducing gas reduction device 100, the iron oxide raw material 1 contained in the packed bed becomes reduced iron via the reduction zone 101. For example, as shown in Figure 1, in the reduction zone 101 of the reduction device 100, the iron oxide raw material 1 is brought into contact with the iron oxide raw material 1 contained in the packed bed by a reducing gas, thereby reducing the iron oxide raw material 1 to reduced iron. The reducing gas is a gas capable of reducing the iron oxide contained in the iron oxide raw material 1. The reducing gas includes, for example, hydrogen gas. By reducing the iron oxide raw material 1 with hydrogen gas, reduced iron with a low carbon content can be obtained. In this embodiment, the subsequent carbonization treatment can increase the carbon content of the reduced iron to above a certain level while keeping the cementitization rate η below a certain level. In one embodiment, the reducing gas may contain hydrogen gas in an amount of 40% to 100% by volume. In addition to hydrogen gas, the reducing gas may also contain gases other than hydrogen gas. Examples of gases other than hydrogen gas include CO gas, hydrocarbon gases, and inert gases. Examples of inert gases include nitrogen gas, argon gas and other noble gases, and CO ２ The reducing gas may be one or more selected from gases, water vapor, etc. The proportion of hydrogen gas in the reducing gas may be 40% or more by volume, 50% or more by volume, 55% or more by volume, 60% or more by volume, 65% or more by volume, 70% or more by volume, 75% or more by volume, 80% or more by volume, 85% or more by volume, 90% or more by volume, or 95% or more by volume. The reducing gas may optionally contain CO gas along with hydrogen gas. In the reducing gas, the proportion of the volume of hydrogen gas to the total volume of hydrogen gas and the volume of CO gas may be 65% or more, 70% or more, 75% or more, 80% or more, 85% or more, 90% or more, or 95% or more.

[0027] The reducing gas is supplied, for example, from the side wall of the shaft furnace, which serves as the reduction device 100, into the interior of the shaft furnace. The method of supplying the reducing gas from the side wall of the shaft furnace is not particularly limited. For example, the reducing gas may be supplied into the interior of the shaft furnace through an opening provided in the side wall of the shaft furnace. The opening may be provided, for example, at an appropriate position (below the reduction zone) depending on the position of the reduction zone 101 of the shaft furnace. Piping or the like may be connected to the opening. In this case, the tip of the piping may or may not protrude inward from the inner wall of the furnace inside the shaft furnace. The temperature of the reducing gas supplied to the reduction device 100 should be such that the reduction reaction of iron oxide can occur. For example, the supply temperature of the reducing gas may be between 900°C and 1150°C. However, if the temperature of the reducing gas is too high, the packed bed may become blocked by fusion, which may hinder the diffusion of the reducing gas in the packed bed.

[0028] 2.1.3 Exhaust Gas Treatment As shown in Figure 1, the gas remaining after the reaction with iron oxide in the reduction treatment is discharged from the top of the reduction device 100 to the outside of the system. Here, the exhaust gas from the reduction treatment is, for example, water and CO ２ and a reducing gas. In this embodiment, the exhaust gas from the reduction treatment is subjected to dust removal, dehydration and CO2 removal. ２ At least one of the following may be performed to obtain a circulating gas, the circulating gas may be mixed with a raw material gas (hydrogen gas, natural gas, etc.) to obtain a mixed gas, and the mixed gas may be optionally heated to obtain a reducing gas. Dust removal, dewatering and / or CO2 removal from the exhaust gas of the reduction treatment may also be performed. ２ These processes may be carried out by the first exhaust gas treatment device 110. Furthermore, the circulating gas and the raw material gas may be mixed in any ratio. The mixed gas may be heated by the first heating device 120. The heating method of the first heating device 120 is not particularly limited.

[0029] 2.2 Carbonization Treatment In this embodiment, the reduced iron obtained by the reduction treatment described above is subjected to carbonization treatment by contacting it with a carbonization gas containing methane gas. Specifically, the reduced iron obtained by the reduction treatment is carbonized in the carbonization zone 103 downstream of the transition zone 102, after passing through the transition zone 102 downstream of the reduction zone 101. That is, in the carbonization zone 103, the reduced iron contained in the packed bed is carbonized by supplying the carbonization gas to the packed bed, etc.

[0030] The carbonization gas is supplied, for example, from the side wall of the shaft furnace, which serves as the reduction device 100, into the interior of the shaft furnace. The method of supplying the carbonization gas from the side wall of the shaft furnace is not particularly limited. For example, the carbonization gas may be supplied into the interior of the shaft furnace through an opening provided in the side wall of the shaft furnace. The opening may be provided, for example, at an appropriate position (below the carbonization zone) depending on the position of the carbonization zone 103 of the shaft furnace. Piping or the like may be connected to the opening. In this case, the tip of the piping may or may not protrude inward from the inner wall of the furnace inside the shaft furnace.

[0031] According to the inventor's new findings, when reduced iron is subjected to carbonization treatment, the carbon content and cementitation rate η of the reduced iron (hot reduced iron 10) after carbonization treatment change depending on the methane gas concentration of the carbonization gas, the carbonization treatment temperature, and the carbonization treatment time. Specifically, the cementitation rate η becomes less affected by the carbonization treatment time as the amount of cementite produced saturates after a certain treatment time, and depends on the carbonization treatment temperature. On the other hand, the carbon content increases as the methane gas concentration increases, the carbonization treatment temperature increases, and the carbonization treatment time increases. In other words, in this embodiment, the carbonization treatment temperature T is adjusted so that the cementitation rate η of the reduced iron (hot reduced iron 10) after carbonization treatment reaches the target value, and the methane gas concentration C of the carbonization gas is adjusted so that the carbon content of the reduced iron (hot reduced iron 10) after carbonization treatment reaches the target value. ＣＨ４ The carbonization treatment time t is also adjusted.

[0032] 2.2.1 Temperature of Carbonization Treatment In this embodiment, the cementitization rate η of the hot reduced iron 10 finally obtained is controlled by controlling the temperature T of the carbonization treatment. In one embodiment, the temperature T of the carbonization treatment may be 650°C or more and 950°C or less, 700°C or more and 900°C or less, or 750°C or more and 850°C or less. In particular, by controlling the temperature T of the carbonization treatment within the range of 650°C or more and 850°C or less, the carbonization rate can be improved while controlling the cementitization rate η to a certain level or less, and the carbon content can be increased in a short time. Note that "temperature T of the carbonization treatment" refers to the temperature of the reduced iron that comes into contact with the carbonization gas during the carbonization treatment. When the carbonization treatment is carried out in a shaft furnace, the position (height position) P1 when measuring the "temperature of the reduced iron" is downstream (below) the position (height position) P2 into which the reducing gas is blown during the reduction treatment, and is within 1 m of the position (height position) P2. Alternatively, if the reduction treatment and carbonization treatment are performed in separate devices (for example, if the reduction treatment is performed in a shaft furnace and the carbonization treatment is performed in a carbonization furnace located downstream of the shaft furnace), the position P1 for measuring the "temperature of reduced iron" shall be within 1 m downstream (downward) from the reduced iron supply port of the device where the carbonization treatment is performed. The "temperature of reduced iron" in the carbonization treatment is the average temperature in the radial direction of the shaft furnace or carbonization furnace. The average temperature of reduced iron in the radial direction can be determined, for example, by installing a rod-shaped member in the radial direction of the shaft furnace or carbonization furnace, attaching multiple thermocouples to the member, and measuring multiple temperatures in the radial direction. That is, if the carbonization treatment is performed in a shaft furnace, the average temperature of reduced iron in the radial direction is determined by multiple thermocouples installed in the radial direction of the shaft furnace. Also, if the carbonization treatment is performed in a carbonization furnace separate from the shaft furnace, the average temperature of reduced iron in the radial direction is determined by multiple thermocouples installed in the radial direction of the carbonization furnace. Here, for example, assuming that the temperature between measured points is linearly distributed in the radial direction, the radial temperature distribution T(r) can be expressed as a combination of linear functions of r. In this case, the average temperature T ａｖｅ If we measure the temperature at N points, the radius of the measurement point i (i = 1 to N) is r. ｉ It is defined by the following formula. Note that r０ ,r Ｎ＋１ The corresponding point is the center of the furnace (r ０ = 0) and furnace wall (r Ｎ＋１ The location is R), and the temperature at that point is determined by extrapolation. With this method, even if a temperature distribution occurs in the radial direction, the average temperature in the radial direction can be determined by averaging multiple measured temperatures. The number of thermocouples is not particularly limited, but it is preferable to have five or more thermocouples, for example.

[0033] In the carbonization process, the temperature of the carbonizing gas in contact with the reduced iron is not particularly limited. The temperature of the carbonizing gas may be, for example, between 25°C and 600°C. The carbonizing gas may be heated as desired before being supplied to the carbonization zone 103. The heating of the carbonizing gas may be performed, for example, by a second heating device 140. The heating method by the second heating device 140 is not particularly limited.

[0034] 2.2.2 Target Value of Cementitization Rate η In this embodiment, as described above, the carbonization treatment temperature T is adjusted so that the cementitization rate η of the reduced iron (hot reduced iron 10) after carbonization treatment reaches the target value. The target value of the cementitization rate η can be appropriately determined according to the formability of the desired hot reduced iron 10. In this embodiment, the target value of the cementitization rate η and the carbonization treatment temperature T may be determined according to the following formula (1): lnη = 5281.7 / T - 1.206 …(1) T: carbonization treatment temperature (K) This makes it easier for the actual cementitization rate η of the hot reduced iron 10 finally obtained to match the target value.

[0035] In the above equations (1) and (2), the partial pressure of methane gas in the carbonized gas is P. ＣＨ４ This changes depending on the proportion of methane gas in the carbonized gas. As mentioned above, the proportion of methane gas in the carbonized gas may be, for example, 50% by volume or more and 100% by volume or less, that is, the partial pressure P ＣＨ４ For example, it may be between 0.5 and 1.0.

[0036] 2.2.3 Methane gas concentration of carbonized gas In this embodiment, the methane gas concentration of the carbonized gas is C ＣＨ４ By controlling this, the carbon content of the final hot-reduced iron 10 is controlled. In one embodiment, the carbonization gas may contain methane gas in an amount of 10% to 100% by volume. The proportion of methane gas in the carbonization gas may be 10% to 40% or 70% or more by volume, or 100% or less by volume, 70% or less by volume, or 40% or less by volume. A specific example of such a carbonization gas is natural gas. In addition to methane gas, the carbonization gas may contain gases other than methane gas. Examples of gases other than methane gas include hydrogen gas, CO gas, hydrocarbon gases other than methane gas, and inert gases. Examples of inert gases include nitrogen gas, noble gases such as argon gas, and CO ２ It may be one or more types selected from gases, water vapor, etc.

[0037] 2.2.4 Carbonization Treatment Time In this embodiment, the methane gas concentration of the carbonized gas is C ＣＨ４ Furthermore, by controlling the carbonization treatment time t, the carbon content of the final hot reduced iron 10 is controlled. In one embodiment, the carbonization treatment time t may be 5 minutes or more and 20 minutes or less, 7 minutes or more and 18 minutes or less, or 10 minutes or more and 15 minutes or less. Note that "carbonization treatment time t" refers to the time during which the reduced iron is in contact with the carbonization gas. When the carbonization treatment is carried out in a shaft furnace, the carbonization treatment time t refers to the time during which the reduced iron passes through the carbonization zone 103.

[0038] In this embodiment, the carbonization treatment temperature is set so that the cementitization rate reaches the target value, and the methane gas concentration C of the carbonized gas is set so that the carbon content reaches the target value. ＣＨ４ And the carbonization treatment time is determined. The following formula (3) is an example of a means for determining the carbonization treatment time. Here, C Ｔ，Ｃ (mol / m) ３ ) is the target value for carbon content (T.C), t(s) is the carbonization treatment time, and k １ (mol / m) ３ / s) is the reaction rate constant of T.C generation, and P ＣＨ４ (-) is the partial pressure of methane gas in the carbonized gas, and P Ｈ２ (-) is the partial pressure of hydrogen gas in the carbonized gas, and a Ｃ (-) is the C activity, and K １ (-) is the equilibrium constant, and C Ｍ．Ｆｅ (mol / m ３ ) is the amount of metallic iron, and C Ｔ．Ｆｅ (mol / m ３ ) is the total iron content (T.Fe), T(K) is the temperature of the carbonization treatment, and C ＦｅＯｘ (mol / m ３ ) is the iron oxide content, and C Ｆｅ３Ｃ (mol / m ３ ) is the amount of cementite.

[0039] 2.2.5 Exhaust Gas Treatment As shown in FIG. 1, the gas after reacting with reduced iron in the carbonization treatment is discharged out of the system, for example, on the downstream side of the transition zone 102 of the reduction device 100. The gas after reacting with reduced iron in the carbonization treatment contains, for example, methane gas and hydrogen gas. In this regard, in the present embodiment, hydrogen separation or the like may be performed on the exhaust gas of the carbonization treatment, and the obtained hydrogen gas may be reused as a reducing gas, and the methane gas may be reused as a carbonized gas. Hydrogen separation or the like for the exhaust gas of the carbonization treatment may be performed by the second exhaust gas treatment device 130. As shown in FIG. 1, the hydrogen gas separated by the second exhaust gas treatment device 130 may be supplied to the reduction zone 101 via the first heating device 120, and the methane gas separated by the second exhaust gas treatment device 130 may be supplied to the carbonization zone 103 via the second heating device 140.

[0040] 3. Method for Manufacturing Hot-Formed Reduced Iron (HBI) As described above, the hot-formed reduced iron 10 of this disclosure has a carbon content above a certain level and a cementitiousness rate η below a certain level. The cementitiousness rate η of the hot-formed reduced iron 10 is below a certain level, resulting in a smaller Young's modulus for the hot-formed reduced iron 10, which exhibits excellent hot-formability. Such a hot-formable hot-formable hot-formed reduced iron 10 allows for the easy manufacture of hot-formed reduced iron 20. That is, a method for manufacturing hot-formed reduced iron 20 according to one embodiment includes hot-forming the hot-formed reduced iron 10 of this disclosure to obtain a molded product. By molding the hot-formed reduced iron 10 to obtain hot-formed reduced iron 20, a dense hot-formed reduced iron 20 is obtained, improving oxidation resistance. As a result, for example, sea transport becomes possible.

[0041] 3.1 Hot Forming In the method for producing hot-formed reduced iron 20 according to this embodiment, a molded product 15 is obtained by hot-forming the hot-formed reduced iron 10. The molded product 15 may be used as hot-formed reduced iron 20 as is, or it may be made into hot-formed reduced iron 20 after undergoing the heat retention treatment described later. Hot forming may be performed in a hot forming apparatus 200. A known forming apparatus (briquette machine) may be used as the hot forming apparatus 200. As shown in Figure 1, the hot forming apparatus 200 may obtain a molded product 15 by rotating a pair of rolls, supplying hot-formed reduced iron 10 between the rolls, and applying pressure to the hot-formed reduced iron 10 to form the hot-formed reduced iron 10 into a shape corresponding to the irregularities provided on the surface of the rolls. The temperature and pressure in hot forming may be adjusted as appropriate according to the target porosity and target density of the molded product 15. The temperature in hot forming may be, for example, 650°C or higher. By setting the temperature during hot forming to 650°C or higher, a molded product 15 having the desired porosity can be more easily obtained. The temperature during hot forming may be 650°C to 850°C, 650°C to 800°C, or 650°C to 750°C. Note that "temperature during hot forming" refers to the forming temperature in the hot forming apparatus 200, and the surface temperature of the molded product 15 immediately after forming by the hot forming apparatus 200. The pressure during hot forming may be adjusted as appropriate according to the desired density and porosity.

[0042] 3.2 Heat Retention Treatment A molded product 15 is obtained by the hot forming described above. As described above, the hot reduced iron 10 before hot forming has a cementitious rate η of a certain level or less. Therefore, the molded product 15 is also likely to have a cementitious rate η of a certain level or less. On the other hand, the oxidation resistance of the hot reduced iron 20 can be further improved by increasing the cementitious rate of the hot reduced iron 20. In this regard, in this embodiment, the cementitious rate of the molded product 15 may be increased by applying a heat retention treatment to the molded product 15. Specifically, the method for manufacturing the hot reduced iron 20 according to this embodiment may include holding the molded product 15 at 700°C to 800°C for 10 minutes or more in an inert gas atmosphere. The heat retention of the molded product 15 may be carried out in a heat retention furnace 300. Specifically, an inert gas is supplied into the heat retention furnace 300 to create an inert atmosphere inside the heat retention furnace 300. The inert gas is heated as needed by the third heating device 310 and then supplied into the heat-retaining furnace 300. The molded product 15 is then placed inside the heat-retaining furnace 300, which is now in an inert atmosphere, and the heat-retaining furnace 300 is maintained at a temperature of 600°C to 800°C for 10 minutes or more. After that, the molded product 15 is removed from the heat-retaining furnace 300, thereby obtaining hot-formed reduced iron 20 with a high cementitious content.

[0043] The effects of the technology of this disclosure will be explained in more detail below with reference to examples, but the technology of this disclosure is not limited to the following examples.

[0044] 1. Experimental Method Iron oxide pellets were reduced with hydrogen gas to obtain reduced iron (metallization rate: 92%, carbon content: 0 mass%). The obtained reduced iron was subjected to carbonization treatment by contacting it with one of the carbonization gases A to C shown in Table 1 below, and holding it at a treatment temperature of 700°C, 800°C, or 900°C for a treatment time of 5 minutes, 10 minutes, 20 minutes, or 30 minutes.

[0045] Chemical analysis was performed on the reduced iron after carbonization, and X-ray diffraction measurements and Rietveld analysis were also conducted to determine the carbon content and cementitation rate η.

[0046] 2. Results 2.1 Relationship between carbonization treatment conditions, carbon content, and cementitization rate The results of the above experiment showed that when reduced iron is subjected to carbonization treatment, the carbon content and cementitization rate η of the reduced iron (hot reduced iron) after carbonization treatment change depending on the methane gas concentration of the carbonization gas, the temperature of the carbonization treatment, and the carbonization treatment time. Specifically, it was found that the cementitization rate η becomes less affected by the carbonization treatment time once the amount of cementite produced saturates, and depends on the temperature of the carbonization treatment. It was also found that the cementitization rate does not depend on the methane concentration, and the time it takes for the amount of cementite produced to saturate depends on the temperature. On the other hand, it was found that the carbon content increases as the methane gas concentration increases, as the carbonization treatment temperature increases, and as the carbonization treatment time increases.

[0047] Based on the above results, the carbonization temperature T was adjusted so that the cementitization rate η in the reduced iron (hot reduced iron) after carbonization reached the target value, and the methane gas concentration C of the carbonization gas was adjusted so that the carbon content in the reduced iron (hot reduced iron) after carbonization reached the target value. ＣＨ４ Furthermore, it was found that by adjusting the carbonization treatment time t, hot reduced iron having a carbon content above a certain level and a cementitiousness η below a certain level can be obtained.

[0048] 2.2 Relationship between Carbonization Treatment Conditions and Cementitization Rate η As described above, after a certain carbonization treatment time, the change in the cementitization rate η is small. Therefore, representative data for carbonization treatment times of 20 minutes and 30 minutes were used to confirm the relationship between the carbonization treatment conditions (carbonization treatment temperature and methane gas concentration) and the cementitization rate η. Figure 2 shows the result of comparing the cementitization rate η and the carbonization treatment temperature. Table 2 below shows the carbonization treatment conditions, the actual values of the carbon content of the samples, and the actual values of the cementitization rate η in the samples. From the results shown in Table 2, it was推测 that the influence of the methane gas concentration on the cementitization rate is small. When organizing the cementitization rate η and the carbonization treatment temperature T from the conditions and actual values shown in Table 2 below, it was found that the cementitization rate η follows the following formula (1): lnη = 5281.7 / T - 1.206... (1) T: Carbonization treatment temperature (K) Table 2 below shows the calculated values of the cementitization rate η calculated from the above formula (1). Also, Figure 3 shows the comparison result between the actual values and the calculated values of the cementitization rate. As shown in Table 2 and Figure 3, it can be seen that the actual values and the calculated values are in good agreement. That is, it can be said that when adjusting the carbonization treatment temperature T so that the cementitization rate η in the reduced iron (hot-reduced iron) after carbonization treatment reaches the target value, the target value of the cementitization rate η may be determined according to the above formula (1).

[0049] 2.3 Relationship between Carbonization Treatment Conditions and Carbon Content The carbon content was measured for each reduced iron after carbonization treatment. The results are shown in Tables 3-1 to 3-3 below.

[0050] The carbonization rate formula shown by the following formula (3) was derived from the actual values shown in Tables 3-1 to 3-3. Here, C Ｔ，Ｃ (mol / m ３ ) is the carbon content (T.C), t (s) is the carbonization treatment time, k １ (mol / m ３ / s) is the reaction rate constant for T.C generation, P ＣＨ４(-) represents the partial pressure of methane gas in the carbonized gas, and P Ｈ２ (-) represents the partial pressure of hydrogen gas in the carbonized gas, and a Ｃ (-) represents the C activity, and K １ (-) is the equilibrium constant, C Ｍ．Ｆｅ (mol / m) ３ ) is the amount of metallic iron, C Ｔ．Ｆｅ (mol / m) ３ ) is the total iron content (T.Fe), T(K) is the temperature of the carbonization treatment, and C ＦｅＯｘ (mol / m) ３ ) is the iron oxide content, C Ｆｅ３Ｃ (mol / m) ３ ) represents the amount of cementite.

[0051] Figure 4 shows a comparison of actual carbon content values ​​with calculated values ​​based on the above formula (3) for cases where the carbonization temperature is 800°C and the methane gas concentration is 70 vol% (methane gas partial pressure 0.7), 50 vol% (methane gas partial pressure 0.5), and 30 vol% (methane gas partial pressure 0.3). As shown in Figure 4, it can be seen that the actual values ​​and calculated values ​​are in good agreement. Therefore, the methane gas concentration C of the carbonization gas should be set so that the carbon content of the reduced iron (hot reduced iron) after carbonization reaches the target value. ＣＨ４ Furthermore, when the carbonization treatment time t is adjusted, the target value of the carbon content may be determined according to the above formula (3). Also, the carbon content and the cementitization rate η can be adjusted independently.

[0052] 3. What can be said from the above examples According to the method disclosed in the above examples, when obtaining hot reduced iron by subjecting reduced iron to a carbonization treatment, the carbon content and cementitization rate η of the reduced iron after carbonization can be controlled by controlling the conditions of the carbonization treatment (temperature, time, and methane gas concentration of the carbonization gas). For example, it is possible to produce hot reduced iron having a carbon content above a certain level and a cementitization rate η below a certain level. According to the method of this disclosure, for example, it is possible to produce hot reduced iron having: (1) a carbon content of 1.5% by mass or more and less than 5.0% by mass and a cementitization rate η of 80% or less; (2) a carbon content of 5.0% by mass or more and less than 6.0% by mass and a cementitization rate η of 75% or less; and (3) a carbon content of 6.0% by mass or more and less than 10.0% by mass and a cementitization rate η of 70% or less. Such hot-reduced iron has excellent solubility in the melting and refining processes because it contains a certain amount of carbon, and it has a low cementitiousness η, resulting in a low Young's modulus and excellent formability. Furthermore, according to the method of this disclosure, it is also possible to produce hot-reduced iron with an even better balance of solubility and formability, such as (4) hot-reduced iron having a carbon content of 1.5% by mass or more and 6.0% by mass or less and a cementitiousness η of 70% or less, and (5) hot-reduced iron having a carbon content of 3.0% by mass or more and 5.0% by mass or less and a cementitiousness η of 50% or less.

[0053] 1 Iron oxide raw material 10 Hot-reduced iron (HDRI) 15 Molded product 20 Hot-formed reduced iron (HBI) 100 Reduction device 110 First exhaust gas treatment device 120 First heating device 130 Second exhaust gas treatment device 140 Second heating device 200 Hot forming device 300 Heat retention furnace 310 Third heating device

Claims

1. Hot-reduced iron having a carbon content of 1.5% by mass or more and less than 5.0% by mass and a cementitiousness η of 80% or less, or a carbon content of 5.0% by mass or more and less than 6.0% by mass and a cementitiousness η of 75% or less, or a carbon content of 6.0% by mass or more and less than 10.0% by mass and a cementitiousness η of 70% or less.

2. Hot-reduced iron according to claim 1, having a carbon content of 1.5% by mass or more and 6.0% by mass or less, and a cementitization rate η of 70% or less.

3. Hot-reduced iron according to claim 1, having a carbon content of 3.0% by mass or more and 5.0% by mass or less, and a cementitization rate η of 50% or less.

4. A method for producing hot reduced iron according to any one of claims 1 to 3, comprising: bringing reduced iron into contact with a carbonizing gas containing methane gas to perform a carbonization treatment of the reduced iron, wherein the temperature T of the carbonization treatment is adjusted so that the cementitiousness η of the reduced iron after the carbonization treatment is a target value, and the methane gas concentration C of the carbonizing gas is adjusted so that the carbon content of the reduced iron after the carbonization treatment is a target value. ＣＨ４ A method for producing hot reduced iron, wherein the time t of the carbonization treatment is adjusted.

5. A method for producing hot reduced iron according to claim 4, wherein the target value of the cementitiousness η and the temperature T of the carbonization treatment are determined according to the following formula (1): lnη = 5281.7 / T - 1.206 …(1) T: temperature of the carbonization treatment (K) 6. A method for producing hot reduced iron according to claim 4 or 5, comprising: obtaining the reduced iron by performing a reduction treatment on an iron oxide raw material, wherein the reduction treatment and the carbonization treatment are performed in a shaft furnace.

7. A method for producing hot-formed reduced iron, comprising hot-forming the hot-formed reduced iron described in any one of claims 1 to 3 to obtain a molded product.

8. A method for producing hot-formed reduced iron according to claim 7, comprising holding the formed product in an inert gas atmosphere at 700°C to 800°C for 10 minutes or more.

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