Method for producing cementite

Abstract

Provided is a method for producing cementite, which can suppress carbon precipitation and stably produce cementite for a long period of time. Hematite (Fe2O3) is used as an iron source and is supplied to a reaction furnace, and a carbonization gas composed of a mixed gas of carbon monoxide and carbon dioxide is supplied to the reaction furnace. Furthermore, the carbonization gas is adjusted so that the carbon activity becomes ac = 1 to 10, and a reduction reaction is performed between the carbonization gas and the hematite, whereby cementite (Fe3C) can be efficiently produced for a long period of time without carbon precipitation.

Description

Technical Field

[0001] This invention relates to a method for manufacturing cementite. More specifically, it relates to a method for manufacturing cementite capable of suppressing carbon precipitation and achieving long-term stable cementite formation. Background Technology

[0002] In the past, Japan's steel production has mainly relied on the blast furnace-converter process. However, Japan's steel industry is now facing urgent challenges in addressing recent CO2 emissions, energy, and resource issues, and urgently needs to transition to electric arc furnaces, which have lower CO2 emissions and lower energy consumption.

[0003] However, unlike Europe and the United States, which focus on steel production using electric furnaces, Japan has always focused on developing technology for producing high-grade steel using the blast furnace-converter method. As a result, it lacks technological accumulation regarding electric furnaces, especially the technology for producing high-grade steel using large electric furnaces. The transition from the blast furnace-converter method to electric furnaces requires solving many problems.

[0004] The main iron raw material for electric arc furnace steelmaking is scrap steel. However, a major problem in steel production using scrap steel is the accumulation of impurities such as Cu due to the repeated use of scrap steel. Therefore, it is difficult to manufacture high-grade steels such as automotive steel sheets using electric arc furnaces. When manufacturing high-grade steels using electric arc furnaces, it is necessary to use iron sources with fewer impurities, such as direct reduced iron (DRI) and cementite (Fe3C), in order to reduce the impurity concentration.

[0005] Among the aforementioned iron sources, cementite not only has fewer impurities but also lower energy consumption. Furthermore, its non-flammability eliminates the advantage of not incurring additional costs during storage and transportation. Therefore, cementite is considered an ideal iron source for steel production using electric arc furnaces, and research and development for its industrial production are underway.

[0006] Previously, in pilot plant operations for cementite production, natural gas, primarily composed of methane, was used as a raw material. However, Japan does not produce natural gas domestically and relies heavily on imports. Therefore, using natural gas as a reducing agent for the industrial production of cementite is very costly.

[0007] Regarding this, for example, Patent Document 1 discloses a manufacturing method in which an iron oxide-containing substance is brought into solid contact with a carbonaceous reducing agent in the presence of a reducing gas composed of water vapor, oxygen-containing gas (air, oxygen), carbon dioxide gas, or a mixture of these gases, and heated under specified temperature conditions, thereby generating cementite by reducing a portion of the iron oxide. Existing technical documents Patent documents

[0008] Patent Document 1: Japanese Patent Application Publication No. 2012-57202 Summary of the Invention The technical problem that the invention aims to solve

[0009] However, since cementite is a thermodynamically unstable metastable material, the carbon activity (ac) of the reducing gas used for cementite generation needs to be set to be greater than 1. This indicates that there is always a possibility of carbon precipitation during the cementite manufacturing process, but in the cementite manufacturing process to date, including the manufacturing method disclosed in Patent Document 1, reducing gases with high carbon activity (e.g., ac = 2000 or higher) have been used to accelerate the cementite generation rate.

[0010] If a reducing gas with high carbon activity is used in this way, cementite will form early, but on the other hand, carbon precipitation will also occur prematurely. Moreover, once carbon precipitates, the formation of cementite will be interrupted, and the cementite that has already formed will decompose, making subsequent operations difficult and creating a problem of not being able to form cementite stably in the long term.

[0011] The present invention was made in view of the above points, and its object is to provide a method for manufacturing cementite, wherein the method for manufacturing cementite uses a reducing gas with low carbon activity instead of the high carbon activity reducing gas used in the past, thereby enabling the long-term and stable generation of cementite. means of solving technical problems

[0012] Through various studies conducted by the inventors of this application, the following insights have been derived.

[0013] That is, the stable phase diagram of the Fe-CO system at 527℃ is as follows: Figure 9 As shown, the carbon activity needs to be greater than 1 to generate Fe3C. In other words, Fe3C cannot be generated using solid carbon, and even at other temperatures (temperatures other than 527℃), the carbon activity needs to be greater than 1 to generate Fe3C.

[0014] In the process of Fe3C formation, if carbon precipitates on the Fe3C surface, a carbon concentration gradient will be generated between the Fe3C surface and the precipitated carbon because the carbon activity in Fe3C is greater than 1. This causes carbon within the Fe3C to diffuse to the surface, resulting in the decomposition of Fe3C and thus stopping the formation of Fe3C. This is called metal dusting, a phenomenon in which the surface becomes pulverized.

[0015] Therefore, metal pulverization is a significant problem in chemical plants that use highly reducing gases. As metal pulverization progresses, the pipe walls become thinner, which can lead to pipe rupture. This phenomenon must be avoided.

[0016] Specifically, when highly reducing gases that could potentially pulverize metals are present, copper-lined pipes are used. This is because CO decomposition does not occur on the copper surface, thus preventing the supply of carbon.

[0017] In other words, "preventing carbon precipitation" is crucial for the long-term, continuous industrial production of Fe3C.

[0018] Furthermore, Dr. HJ Grabke has conducted detailed research on the decomposition behavior of Fe3C accompanying carbon precipitation and elucidated its mechanism (in... Figure 10 The decomposition mechanism of Fe3C is shown in the figure.

[0019] First, such as Figure 10 As shown in (a), carbon is supplied to the surface from the carbonized gas because the activity of carbon in the gas is greater than 1, and the carbon diffuses in the Fe.

[0020] Next, as Figure 10 As shown in (b), when the carbon concentration in Fe reaches the critical stage, the Fe-C alloy with high carbon concentration transforms into Fe3C.

[0021] It should be noted that, as Figure 10 As shown in (c), the carbon supply from the gas continues, and the carbon diffuses through the Fe3C phase. However, as the Fe3C phase gradually thickens, the carbon concentration gradient within the Fe3C phase decreases, and the diffusion rate slows down, resulting in some of the carbon supplied to the surface remaining on the surface (carbon precipitation).

[0022] Next, as Figure 10 As shown in (d), the carbon activity of the carbon precipitated on the surface is 1, but the carbon activity of the Fe3C phase in contact with it is greater than 1. Therefore, a carbon concentration gradient is generated at the interface, and carbon in Fe3C begins to diffuse toward the surface. As a result, the carbon concentration of Fe3C decreases, and Fe3C begins to decompose.

[0023] In other words, once carbon begins to precipitate, the decomposition of Fe3C will proceed, and Fe3C will no longer be generated.

[0024] Therefore, (1) Fe3C cannot be generated using solid carbon (carbon activity = 1), and (2) Fe3C will decompose when solid carbon precipitates on the surface of Fe3C, thus Fe3C cannot be generated.

[0025] However, when a mixture of carbon monoxide (CO) and carbon dioxide (CO2) is supplied to the metal surface (Fe surface), and the ratio of the partial pressure of carbon monoxide (pCO) to the partial pressure of carbon dioxide (pCO2), pCO / pCO2, is greater than a specified value (which is thermodynamically determined once the temperature is set), carbon will precipitate on the Fe surface via the Boudouard reaction shown below [1]. It should be noted that (g) represents a gas. 2CO(g)→C+CO2(g)···[1]

[0026] In addition, reaction formula [1] is actually composed of the reactions shown in [2][3] below. It should be noted that (ad) represents the adsorbed atom. CO(g)→C(ad)+O(ad)···[2] O(ad)+CO(g)→CO2(g)···[3]

[0027] Here, [2] means "the reaction in which CO gas is adsorbed on the Fe surface and dissociates into adsorbed carbon atoms C (ad) and adsorbed oxygen atoms O (ad)," and [3] means "the reaction in which adsorbed oxygen atoms O (ad) present on Fe react with CO gas and are thus removed from the Fe surface."

[0028] Moreover, [2] is a very fast reaction, as shown in [4] below, and is almost in equilibrium. CO(g)=C(ad)+O(ad)···[4]

[0029] In other words, as shown below [5], the reverse reaction of [2] also always occurs, and the same amount of adsorbed carbon atoms C (ad) and adsorbed oxygen atoms O (ad) are removed from the Fe surface. C(ad)+O(ad)→CO(g)···[5]

[0030] However, since the reaction shown in [3] reduces the number of adsorbed oxygen atoms O (ad), the number of adsorbed carbon atoms C (ad) removed from the Fe surface in the reaction shown in [5] will also decrease, resulting in a gradual accumulation and increase of adsorbed carbon atoms C (ad) on the Fe surface.

[0031] However, in a CO-CO2 mixed gas, when the CO2 concentration is high, as shown below [6], since the concentration of adsorbed oxygen atoms O (ad) generated by the CO2 decomposition reaction is high, most of the adsorbed carbon atoms C (ad) will be removed from the Fe surface through the reaction shown in [5], so the concentration of adsorbed carbon atoms C (ad) is almost zero and no carbon precipitation will occur. CO2(g)→O(ad)+CO(g)···[6]

[0032] In addition, adsorbed carbon atoms C(ad) on the Fe surface diffuse into the Fe, as shown below [7], forming an Fe-C alloy. It should be noted that, C This represents carbon dissolved in Fe. C(ad) → C ...[7]

[0033] Therefore, if the diffusion rate of adsorbed carbon atoms C(ad) into Fe is high, the amount of adsorbed carbon atoms C(ad) into Fe is greater than the increase in adsorbed carbon atoms C(ad) caused by the decrease in adsorbed oxygen atoms O(ad) on the Fe surface. As a result, the concentration of adsorbed carbon atoms C(ad) on the Fe surface is almost zero, and carbon precipitation will not occur.

[0034] It should be noted that since the diffusion rate of adsorbed carbon atoms C(ad) into Fe is proportional to the concentration gradient, as the concentration gradient gradually decreases, the diffusion rate of adsorbed carbon atoms C(ad) from the Fe surface slows down. In this situation, not all adsorbed carbon atoms C(ad) present on the Fe surface can diffuse into the Fe. The number of adsorbed carbon atoms C(ad) gradually increases, and because their concentration was previously low, the previously non-interacting adsorbed carbon atoms C(ad) combine with each other, leading to carbon precipitation.

[0035] Based on the above insights, in order to achieve the above objectives, the method for manufacturing cementite according to the present invention comprises: a step of supplying a mixed gas containing at least carbon monoxide and carbon dioxide as a carburizing gas with a carbon activity greater than 1 and less to a reactor containing iron oxide; and a step of heating the iron oxide under heating conditions that fall within the region enclosed by a straight line at a temperature of 800°C, curves with carbon activities of 1 and 10, the phase boundary line between Fe and FeO, and the boundary line of the carbon precipitation region (excluding points on the curve with a carbon activity of 1 and points on the boundary line of the carbon precipitation region), thereby suppressing carbon precipitation and generating cementite.

[0036] Furthermore, the method for manufacturing cementite according to the present invention includes: a step of supplying a mixed gas containing at least carbon monoxide and carbon dioxide as a carburizing gas with a carbon activity greater than 1 and less to a reactor containing iron oxide; and a step of heating the iron oxide under heating conditions that fall within the region surrounded by a straight line at a temperature of 800°C, curves with carbon activities of 1 and 10, and the phase boundary between Fe and FeO, and excluding the region where carbon precipitation occurs (excluding points on the curve with a carbon activity of 1 and points on the boundary line of the carbon precipitation region), thereby suppressing carbon precipitation and generating cementite.

[0037] Here, by supplying a carburizing gas (a mixture of at least carbon monoxide and carbon dioxide) into a reactor containing iron oxide, the carburizing gas reacts with the iron oxide as a reducing gas to generate cementite.

[0038] In addition, by supplying a carbonizing gas (a mixture of at least carbon monoxide and carbon dioxide) with a carbon activity greater than 1 and less than 10, carbon precipitation during cementite formation can be suppressed.

[0039] Furthermore, in the Fe-CO system stable phase diagram representing the carbon precipitation region (the region where carbon precipitation occurs) when carbon monoxide and carbon dioxide are used as carbonizing gases, when iron oxide is heated under heating conditions that fall within the region enclosed by the straight line at a temperature of 800°C, the curves with carbon activities of 1 and 10, the phase boundary between Fe and FeO, and the boundary line of the carbon precipitation region (excluding the points on the curve with a carbon activity of 1 and the points on the boundary line of the carbon precipitation region), thereby suppressing carbon precipitation and generating cementite, cementite can be generated stably for a long period of time because carbon precipitation is suppressed during the cementite formation process.

[0040] Similarly, in the Fe-CO system stable phase diagram representing the carbon precipitation region (the region where carbon precipitation occurs) when carbon monoxide and carbon dioxide are used as carbonizing gases, if iron oxide is heated under heating conditions that fall within the region enclosed by the straight line at a temperature of 800°C, the curves with carbon activities of 1 and 10, and the phase boundary between Fe and FeO, and exclude the region where carbon precipitation occurs (excluding the points on the curve with a carbon activity of 1 and the points on the boundary line of the carbon precipitation region), thereby suppressing carbon precipitation and generating cementite, cementite can be generated stably for a long time because carbon precipitation is suppressed during the cementite formation process.

[0041] It should be noted that the region here (representing the heating conditions) includes "points on the straight line with a temperature of 800℃", "points on the curve with a carbon activity of 10", and "points on the phase boundary between Fe and FeO", but does not include "points on the curve with a carbon activity of 1" and "points on the boundary line of the carbon precipitation region".

[0042] Furthermore, the heating conditions here are defined by a straight line at 800°C, encompassing the region containing points on that line—in other words, the region with temperatures below 800°C. This is because when the atmosphere temperature of the carbonizing gas exceeds 800°C, it may lead to the decomposition of the cementite.

[0043] Furthermore, the heating conditions here are the region enclosed by the "curve with carbon activity of 1" and do not include the points on the curve with carbon activity of 1; in other words, the region with carbon activity greater than 1. This is because when the carbon activity is less than 1, it means that the reaction “2CO(g)→C+CO2(g)” in the above [1] proceeds to the left (in other words, it means that the reaction “CO(g)→C(ad)+O(ad)” in the above [2] proceeds to the left), which leads to a reduction in the adsorbed carbon atoms C(ad) diffused into Fe, making it difficult to form cementite.

[0044] Similarly, this is because when the carbon activity is 1, it means that the reaction “2CO(g) → C + CO2(g)” in the above [1] reaches equilibrium (in other words, it means that the reaction “CO(g) → C(ad) + O(ad)” in the above [2] reaches equilibrium, which leads to the adsorbed carbon atoms C(ad) diffused into Fe not increasing, making it difficult to form cementite).

[0045] Furthermore, the heating conditions here are enclosed by the "curve with carbon activity of 10" and include the region containing the points on the curve with carbon activity of 10, in other words, the region with carbon activity below 10. This is because when the carbon activity exceeds 10, it means that the reaction "2CO(g) → C + CO2(g)" in the above [1] proceeds sharply to the right (in other words, the reaction "CO(g) → C(ad) + O(ad)" in the above [2] proceeds sharply to the right), thereby increasing the number of adsorbed carbon atoms C(ad) on the Fe surface, making it impossible for the adsorbed carbon atoms C(ad) on the Fe surface to diffuse into the Fe, and the adsorbed carbon atoms C(ad) combine with each other, leading to carbon precipitation. It should be noted that when carbon precipitation occurs, the formation of cementite will be interrupted, and even the cementite that has already been formed may decompose.

[0046] Furthermore, the heating condition here is defined by the "Fe-FeO phase boundary," encompassing the region containing points on that boundary—in other words, the region where Fe is thermodynamically apparent as a stable phase. This is because the inventors' research showed that in regions with carbon activity greater than 1, although carbon or cementite becomes a stable phase, cementite will not form without the passage of metallic Fe. Therefore, by using the "region where Fe is thermodynamically apparent as a stable phase" as the heating condition, the regions where FeO and Fe3O4 are thermodynamically apparent as stable phases are excluded.

[0047] Furthermore, the heating conditions here are defined by the region enclosed by the curve representing carbon activity of 10 and the phase boundary between Fe and FeO. Considering that the curve representing carbon activity of 10 intersects with the phase boundary between Fe and FeO at approximately 600°C, this region represents a temperature above approximately 600°C. This is because when the atmosphere temperature of the carbonizing gas is below approximately 600°C, the reaction does not proceed, resulting in a longer time required for cementite formation.

[0048] Furthermore, the heating conditions here are either surrounded by or exclude the "carbon precipitation region" but do not include points on the boundary line of the carbon precipitation region. This is because when carbon precipitation occurs, the formation of cementite is interrupted, and even the already formed cementite may decompose.

[0049] In addition, as mentioned above, the heating conditions here are "regions with carbon activity greater than 1" and "regions with carbon activity less than 10".

[0050] Moreover, the carbon activity of a CO-CO2 mixture with partial pressures of carbon monoxide (pCO) and carbon dioxide (pCO2) can be thermodynamically determined based on the reaction “2CO(g) → C + CO2(g)” described above [1]. That is, when the equilibrium constant of the above [1] reaction is set to K, the equilibrium state of the above [1] reaction can be represented by [7]. It should be noted that ac represents the carbon activity of the mixed gas. K = (pCO) 2 / (ac·pCO2)···[7]

[0051] Moreover, if the above [7] is rewritten, it can be represented by [8]. ac = (pCO) 2 / K(pCO2)···[8]

[0052] In other words, the carbon activity (ac) of a CO-CO2 mixture can be obtained by knowing the partial pressures of carbon monoxide and carbon dioxide (pCO). 2 The pCO ratio and the equilibrium constant K can then be determined. It should be noted that the value of the equilibrium constant K at each temperature can be obtained through thermodynamics. When the absolute temperature is set to T, it becomes [9]. logK=8460 / T-8.85···[9]

[0053] Therefore, at a specified temperature, the carbon activity ac can be determined by the partial pressure of carbon monoxide (pCO) and the partial pressure of carbon dioxide (pCO2) according to the above [8][9].

[0054] It should be noted that although this explanation uses a binary gas mixture of CO and CO2, carbon activity can be obtained even for gas mixtures containing CH4 or multi-component gas mixtures containing both CO and CH4, as long as the "gas composition" and "equilibrium constant K of the reactions between the various gases constituting the gas mixture" are known.

[0055] In addition, by adding carbon monoxide gas to the blast furnace exhaust gas or removing carbon dioxide from the blast furnace exhaust gas to generate carbonized gas, only minor adjustments to the composition of the large amount of by-product blast furnace exhaust gas in the ironmaking plant are needed to obtain carbonized gas at a very low cost.

[0056] Furthermore, when the particle size of iron oxide is less than 1 mm, its reaction with the carbide gas in the reactor can be promoted. It should be noted that when the particle size of iron oxide exceeds 1 mm, the reduction reaction may not proceed fully because the reaction between iron oxide and carbide gas requires time.

[0057] In addition, when the iron oxide is hematite, it can promote the reduction reaction with the carbide gas, thereby enabling the formation of cementite more efficiently.

[0058] In addition, as an oxidized iron oxide, in the case of magnetite or aragonite, hematite is generated by oxidizing magnetite or aragonite, thereby promoting the reduction reaction with carbonized gas. That is, the reduction reaction of magnetite and aragonite with iron oxide and carbide gas requires time. Therefore, if these magnetite and aragonite are directly supplied to the reduction reaction, it may cause the reduction reaction to take a long time. However, if hematite is generated in advance, the problem of long reduction reaction time can be eliminated.

[0059] Furthermore, as a result of the reduction reaction with the carbonizing gas, the cementite can fully perform its function as a cementite when it contains 0% to 20% iron oxide by weight.

[0060] In addition, when the reactor is a fluidized bed type, the uniform temperature and concentration of the carbonizing gas in the reactor result in all iron oxide powders having almost the same reduction rate, thus enabling the generation of high-quality cementite.

[0061] It should be noted that in a fluidized bed reactor, when the iron oxide particle size is less than 10 μm, the iron oxide will be dispersed when the carburizing gas is supplied into the reactor, and some of the iron oxide will adhere to the inner wall of the reactor, which may prevent it from being used for the reduction reaction. Therefore, when using a fluidized bed reactor, the iron oxide particle size is preferably 10 μm or larger. The effects of the invention

[0062] In the cementite manufacturing method of the present invention, carbon precipitation can be suppressed, thereby enabling the long-term stable generation of cementite. Attached Figure Description

[0063] Figure 1 This is a process diagram involving the method for manufacturing cementite according to the first embodiment of the present invention. Figure 2 It is a time-series curve of cementite formation when using carbonizing gas with high carbon activity. Figure 3(a) is a graph illustrating the manufacturing conditions in the method for manufacturing cementite according to the first embodiment of the present invention. Figure 3(b) is a partially enlarged view of the curve shown in Figure 3(a). Figure 4 This is a process diagram related to the method for manufacturing cementite according to the second embodiment of the present invention. Figure 5 This is a schematic diagram of the experimental setup used in the embodiments. Figure 6 This is a graph representing the XRD analysis results of Example 1. Figure 7 This is a graph representing the XRD analysis results of Example 2. Figure 8 This is a graph representing the XRD analysis results of Example 3. Figure 9 This is the stable phase diagram of the Fe-CO system at 527℃. Figure 10 This is a simulation diagram used to illustrate the decomposition mechanism of Fe3C. Detailed Implementation

[0064] Hereinafter, with reference to the accompanying drawings, a mode for carrying out the invention (hereinafter referred to as "implementation mode") will be described to provide an understanding of the invention.

[0065] [First Implementation] Figure 1 This diagram illustrates the process of manufacturing cementite according to the first embodiment of the present invention. The method for manufacturing cementite according to this embodiment mainly consists of a series of steps. First, hematite (Fe2O3) is prepared as the iron source iron oxide and fed into a reactor (step 1). Next, carburizing gas is supplied to the reactor (step 2), and the hematite and carburizing gas are heated under specified heating conditions (step 3), thereby manufacturing cementite (Fe3C).

[0066] Regarding the iron oxide used as the iron source in step 1, it is not limited to hematite. However, the inventors' research shows that in the cementite manufacturing process, the reduction reaction between hematite and carbide gas is most easily promoted, thus enabling the more efficient generation of cementite.

[0067] The reactor used for producing cementite can be appropriately selected from fixed-bed, moving-bed, and fluidized-bed reactors. Among them, the fluidized-bed reactor is suitable for homogenizing the atmosphere temperature and carbonizing gas concentration within the reactor, thus enabling the efficient generation of high-quality cementite. Therefore, the fluidized-bed reactor is preferred as the reactor type.

[0068] From the viewpoint of promoting the reduction reaction with the carbonized gas, hematite is preferably in the form of a small-particle-size powder. However, as mentioned above, when using a fluidized bed reactor, when the fluid (carbonized gas) is sprayed upwards from the lower side of the reactor, solid particles may be blown away and adhere to the inner wall of the reactor, thus hindering the reduction reaction. Therefore, in order to balance suspension in the fluid within the reactor and promotion of the reaction, the particle size of the hematite is preferably around 10 μm to 1 mm.

[0069] It should be noted that when the hematite particle size is less than 10 μm, as mentioned above, it will be dispersed when the carbide gas is supplied into the reactor and will adhere to the inner wall of the reactor, potentially preventing it from being supplied to the reduction reaction. On the other hand, when the hematite particle size exceeds 1 mm, the reaction between the hematite and the carbide gas requires time, and the reduction reaction may not proceed sufficiently.

[0070] In step 2, a mixture of carbon monoxide (CO) and carbon dioxide (CO2) is used as the carbonizing gas supplied to the reactor in this embodiment of the invention. CO and CO2 are carbonizing gases with low carbon activity (ac=1 to 10), and can stably generate cementite without carbon precipitation during the reduction reaction with iron oxide.

[0071] Here, if the carbon activity is supplemented, for example when CO is heated under a gas stream of a specified concentration, CO will undergo a reaction defined by the following equation (1). 2CO→C+CO2···(1)

[0072] Furthermore, carbon activity is a value that indicates the tendency of the reaction shown in equation (1) to proceed to the right. For example, when the carbon activity ac > 1, the reaction in equation (1) proceeds to the right, and carbon is released. On the other hand, when the carbon activity ac < 1, the reaction in equation (1) proceeds to the left, and carbon release is reduced. Moreover, when ac = 1, the reaction in equation (1) reaches equilibrium, and the amount of carbon becomes a fixed amount.

[0073] In the past, in order to accelerate the formation of cementite, carbonizing gas with high carbon activity (ac=2000 or above) was usually used as the reducing gas. In this case, as mentioned above, carbon precipitation will inevitably occur during operation according to formula (1).

[0074] Regarding carbon activity and carbon deposition, the test results conducted by the American steel manufacturer Nucor in 1996 can be used as a reference. Figure 2 ). Figure 2 This is a time-series curve showing the change in cementite formation over time in the experimental equipment. The cementite formation rate reaches almost 100% after about 10 hours, after which the formation rate gradually decreases. On the other hand, the iron formation rate increases, which is thought to be due to the decomposition of cementite into iron and carbon.

[0075] On the other hand, the amount of metallic iron gradually increases. Since it is difficult to completely separate the precipitated carbon from the cementite and metallic iron, the amount of carbon is not shown, but the decrease in cementite and the corresponding increase in metallic iron are considered to be due to the decomposition of cementite into iron and carbon according to the reaction shown in the following formula (2). Fe3C→Fe+3C···(2)

[0076] As described above, when a high-carbon-activity carburizing gas is used as the reducing gas, although it promotes the formation of cementite, carbon precipitation inevitably occurs within a short period of time from the start of operation. This not only interrupts the formation of cementite but may even cause the already formed cementite to decompose. In contrast, in the embodiments of the present invention, since a low-carbon-activity carburizing gas is used as the reducing gas, carbon precipitation can be suppressed, and cementite can be formed stably over a long period of time.

[0077] Here, high-purity gases such as CO and CO2 can be used as carbonizing gases, or blast furnace exhaust gas containing CO and CO2 can be used. Table 1 shows an example of the composition of blast furnace exhaust gas. Mixed CO gas can be added to such blast furnace exhaust gas, or CO2 can be removed from the blast furnace exhaust gas and used as a carbonization gas. It should be noted that various common methods for selectively separating CO2 from blast furnace exhaust gas include chemical absorption (amine absorption), physical adsorption (adsorption to zeolite, activated carbon), and membrane separation.

[0078] [Table 1]

[0079] Next, the heating conditions for process 3 will be explained based on Figure 3. Figure 3(a) is a stable phase diagram of the Fe-CO system in region B, where carbon precipitation occurs when a CO-CO2 mixture is used as the carbonizing gas. The horizontal axis represents temperature (°C), and the vertical axis represents the proportion of CO in the CO-CO2 mixture (%). It should be noted that Figure 3(b) is a partially enlarged view of Figure 3(a).

[0080] As shown in Figures 3(a) and 3(b), the heating conditions were set to fall within the region (region A represented by the diagonal line in Figures 3(a) and 3(b)) enclosed by the straight line of 800℃, the curve of carbon activity ac=1.0 and carbon activity ac=10.0, the phase boundary of Fe and FeO, and the boundary line of carbon precipitation region B. More specifically, the carbon activity ac is in the range of greater than 1 and less than 10. The mixing ratio of CO-CO2 is set so that the temperature of the atmosphere (reaction temperature) as the carbonization gas is less than 800°C, closer to the Fe side than the phase boundary between FeO and Fe, and closer to the high temperature side than the boundary of the carbon precipitation region B. At the same time, the heating temperature is set.

[0081] In addition, in Figures 3(a) and 3(b), the region A represented by the diagonal line does not include the points on the lines of the "curve with carbon activity ac=1" and the "boundary line of carbon precipitation region B", but it does include the points on the lines of the "straight line at 800℃", the "curve with carbon activity ac=10", and the "phase boundary line between Fe and FeO".

[0082] Here, when the atmosphere temperature of the carbonizing gas is less than 600°C (as can be clearly seen in Figures 3(a) and 3(b), the "curve with carbon activity ac=10" intersects with the "phase boundary between Fe and FeO" at approximately 600°C. That is, region A, represented by the diagonal line in Figures 3(a) and 3(b), is above approximately 600°C), the reduction reaction does not occur, so cementite may not be fully generated. In addition, when the temperature of the carbonized gas atmosphere exceeds 800°C, carbon is easily released, and the cementite may decompose into iron and carbon.

[0083] It should be noted that the cementite produced by processes 1 to 3 sometimes contains unreacted iron oxide, but it is speculated that, for example, if it is a small amount of iron oxide (less than 20% by weight), its function as cementite can still be fully utilized.

[0084] Through the above processes 1 to 3, carbon precipitation can be suppressed, thereby achieving the formation of long-term stable cementite.

[0085] [Second Implementation] Next, the second embodiment will be described. The second embodiment is an embodiment in which aragonite (FeO) or magnetite (Fe3O4) is used as the iron source to oxidize iron, instead of hematite used in the first embodiment. It should be noted that the description of the parts common to the first embodiment will be omitted below.

[0086] The inventors' research indicates that, when using aragonite or magnetite as iron sources for cementite formation, cementite tends to be less readily formed compared to hematite. This is believed to be significantly influenced by factors such as chemical structure stability, hardness, grain size, and iron content.

[0087] Therefore, as Figure 4 As shown in the process diagram, in the second embodiment using argumentite and magnetite as iron sources, these iron sources are first oxidized to generate hematite (step 1). Subsequent steps are the same as in the first embodiment: the hematite generated in step 1 is fed into a reactor (step 2); further, carburizing gas is introduced into the reactor (step 3); and the hematite and carburizing gas are heated under specified heating conditions (step 4) to generate cementite.

[0088] As described above, in the second embodiment, when using ferromagnetite or magnetite as the iron source, carbon precipitation can also be suppressed, thereby achieving long-term stable cementite formation.

[0089] The embodiments of the present invention will be described next. first, Figure 5 This is a schematic diagram of the experimental apparatus used in the embodiment. The experimental apparatus uses a cylindrical test tube with a narrowed top as a fluidized bed reactor. One side of the test tube has a gas inlet for introducing a CO-CO2 mixed gas. The other side of the test tube is sealed by an end cap. A quartz tube connecting the inside and outside of the test tube, as well as a thermocouple for measuring the temperature inside the test tube, are supported through the end cap. The apparatus is structured such that the mixed gas introduced from the gas inlet is supplied to the inside of the test tube through a porous filter and further discharged to the outside through the quartz tube.

[0090] [Example 1] In Example 1, the effect of carbon activity on cementite formation was investigated. As a preliminary step, 0.1 g of powdered hematite, pulverized to a particle size of less than 45 μm, was added to a test tube. Then, a CO-CO2 mixture was supplied vertically upward from the gas inlet, maintaining the atmosphere temperature inside the test tube at 700°C, allowing the powdered hematite to circulate within the test tube for approximately 60 minutes. At this point, while adjusting the mixing ratio of the CO-CO2 mixture, reduction reactions were carried out under conditions of ac=1.5, ac=5, and ac=10.

[0091] The XRD (X-ray diffraction) analysis results of the reactants obtained by the reduction reaction are shown in the figure. Figure 6 . like Figure 6 As shown, although a small amount of iron oxide and iron were detected at ac=1.5, no iron oxide residue was found at ac=5 and ac=10, confirming that cementite can be generated under a wide range of low carbon activity.

[0092] [Example 2] Next, in Example 2, to investigate the effect of the atmosphere temperature (holding temperature) in the test tube during cementite formation, reduction reactions were carried out under the condition of ac=5, where carbon precipitation was relatively low, at holding temperatures of 550°C, 650°C, 750°C, and 850°C. It should be noted that the experimental conditions were the same as in Example 1.

[0093] The XRD analysis results of the reactants obtained in Example 2 are shown in Figure 7 At a temperature of 550℃, only magnetite was reduced, and no cementite was formed. Furthermore, it was observed that at a temperature of 650℃, although peaks of iron oxide and iron were detected and the reduction was not yet complete, peaks of cementite were also detected.

[0094] When the temperature is maintained at 750℃, although carbon peaks can be identified in some samples, it is confirmed that only cementite is formed. Moreover, at 850℃, carbon precipitation is significant, indicating that iron is preferentially formed. As can be seen from the above, cementite is difficult to form at temperatures below 550℃ or above 850℃.

[0095] [Example 3] As shown in Example 1, under the conditions of maintaining a temperature of 700°C and ac=1.5, although the reduction was incomplete, carbon precipitation was not confirmed. Since carbon precipitation tends to occur more easily with higher carbon activity, in Example 3, ac=1.5 was set and the holding temperature was increased to 750°C for the reaction experiment. It should be noted that the experimental conditions were the same as those in Examples 1 and 2.

[0096] The XRD analysis results of the reactants obtained in Example 3 are shown in Figure 8 .like Figure 8 As shown, under the conditions of maintaining a temperature of 750℃ and ac=1.5, no peaks of ferromagnetic ore or carbon were detected, thus enabling the production of high-purity cementite.

[0097] As described above, by precisely adjusting the composition of the CO-CO2 mixture, it has been clearly demonstrated that cementite can be manufactured at AC=1.5. This indicates that blast furnace gas can be used as a reducing agent for the industrial production of cementite, which is a significant advantage for Japan, which does not produce natural gas.

[0098] The method for manufacturing cementite according to the present invention can suppress carbon precipitation, thereby enabling the long-term and stable generation of cementite.

Claims

1. A method for manufacturing cementite, characterized in that, have: A process of supplying a mixed gas containing at least carbon monoxide and carbon dioxide as a carbonization gas with a carbon activity greater than 1 and less than 10 into a reactor containing iron oxide. In the Fe-CO stable phase diagram representing the carbon precipitation region when carbon monoxide and carbon dioxide are used as carbonizing gases, the process involves heating the iron oxide under heating conditions that fall within the region enclosed by a straight line at 800°C, curves with carbon activities of 1 and 10, the phase boundary between Fe and FeO, and the boundary line of the carbon precipitation region, but excluding points on the curve with a carbon activity of 1 and points on the boundary line of the carbon precipitation region, thereby suppressing carbon precipitation and generating cementite.

2. A method for manufacturing cementite, characterized in that, have: A process of supplying a mixed gas containing at least carbon monoxide and carbon dioxide as a carbonization gas with a carbon activity greater than 1 and less than 10 into a reactor containing iron oxide. In the Fe-CO stable phase diagram representing the carbon precipitation region when carbon monoxide and carbon dioxide are used as carbonizing gases, the process involves heating the iron oxide under heating conditions that suppress carbon precipitation and generate cementite, with the heating conditions falling within the region enclosed by a straight line at 800°C, curves with carbon activities of 1 and 10, and the phase boundary between Fe and FeO, excluding the carbon precipitation region but not including points on the curve with a carbon activity of 1 and points on the boundary line of the carbon precipitation region.

3. The method for manufacturing cementite according to claim 1, characterized in that, have: The process of adding carbon monoxide gas to the blast furnace exhaust gas discharged from the blast furnace, or removing carbon dioxide from the blast furnace exhaust gas discharged from the blast furnace to generate the carbonized gas.

4. The method for manufacturing cementite according to claim 1, characterized in that, The iron oxide particles have a diameter of less than 1 mm.

5. The method for manufacturing cementite according to claim 1, characterized in that, The iron oxide mentioned is hematite.

6. The method for manufacturing cementite according to claim 1, characterized in that, The iron oxide comprises magnetite or aragonite, and includes a step of oxidizing the magnetite or aragonite to generate hematite.

7. The method for manufacturing cementite according to claim 1, characterized in that, The cementite contains 0% to 20% by weight of iron oxide.

8. The method for manufacturing cementite according to claim 1, characterized in that, The reactor is a fluidized bed reactor.

9. The method for manufacturing cementite according to claim 8, characterized in that, The iron oxide has a particle size of 10 μm or larger.

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