Manufacturing method of cementite

Abstract

Provided is a method for producing cementite, which can suppress carbon precipitation and produce cementite stably over a long period of time. [Solution] Hematite (Fe2O3) is charged into a reactor as iron oxide, which is an iron source, and a carbonization gas consisting of a mixed gas of carbon monoxide and carbon dioxide is supplied to the reactor. Then, the carbon activity is adjusted to ac = 1 to 10, and the carbonization gas and hematite are subjected to a reduction reaction, thereby efficiently producing cementite (Fe3C) for a long period of time without carbon precipitation.

Description

[Technical field]

[0001] The present invention relates to a method for producing cementite, and more particularly to a method for producing cementite that can suppress carbon precipitation and produce cementite stably over a long period of time. [Background technology]

[0002] Traditionally, steel production in Japan has developed mainly using the blast furnace-converter method. However, the current Japanese steel industry has been affected by the recent CO 2 There is an urgent need to address the issues of emissions, energy, and resources. 2 There is a demand for conversion to electric furnaces, which have lower emissions and consume less energy.

[0003] However, unlike Europe and the United States, where steel production is centered on electric furnaces, Japan has focused on technological development for the production of high-grade steel using the blast furnace-converter method. As a result, there is no accumulated technology related to electric furnaces, especially the technology for producing high-grade steel using large electric furnaces. Therefore, many challenges must be overcome in order to transition from the blast furnace-converter method to electric furnaces.

[0004] The main iron raw material for steelmaking in electric furnaces is scrap, but a major issue with using this scrap to make steel is the accumulation of impurities such as Cu due to repeated use of the scrap. For this reason, it is difficult to produce high-grade steel such as steel sheets for automobiles in electric furnaces, and direct reduced iron (DRI) and cementite (Fe 3 It is necessary to use an iron source with fewer impurities, such as C), in combination to reduce the impurity concentration.

[0005] Among the iron sources mentioned above, cementite has the advantages of not only containing few impurities, but also consuming little energy and being non-flammable, so that there are no costs involved in storage and transportation. Therefore, cementite is considered an ideal iron source for steel production using electric furnaces, and research and development into the industrial production of cementite is underway.

[0006] Conventionally, natural gas, mainly consisting of methane, has been used as the raw material in cementite pilot plant operations. However, since natural gas is not produced in Japan and the country is almost entirely dependent on imports, using natural gas as a reduction raw material for the industrial production of cementite is extremely disadvantageous in terms of cost.

[0007] In this regard, for example, Patent Document 1 discloses a production method in which an iron oxide-containing material and a carbonaceous reducing agent are brought into solid-state contact in the presence of a reducing gas consisting of water vapor, an oxygen-containing gas (air or oxygen gas), carbon dioxide gas, or a mixture of these gases, and heated under prescribed temperature conditions to reduce a portion of the iron oxide, thereby producing cementite. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2012-57202 A Summary of the Invention [Problem to be solved by the invention]

[0009] Incidentally, since cementite is a thermodynamically unstable metastable substance, the carbon activity (ac) of the reducing gas used to generate cementite needs to be set to be greater than 1. This means that there is inevitably a possibility of carbon being precipitated in the process of producing cementite, but in the production method disclosed in Patent Document 1 and other conventional methods of producing cementite, a reducing gas with a high carbon activity (e.g., ac = 2000 or more) has been used to increase the rate at which cementite is generated.

[0010] When a reducing gas with a high carbon activity is used in this way, cementite is formed early, but carbon precipitation also occurs early. Once carbon precipitates, the formation of cementite is interrupted and the cementite that has already been formed also decomposes, making subsequent operation difficult and resulting in the problem that cementite cannot be formed stably for a long period of time.

[0011] The present invention has been devised in view of the above points, and has an object to provide a method for producing cementite that can stably produce cementite over a long period of time by using a reducing gas with a low carbon activity instead of a reducing gas with a high carbon activity that has been conventionally used. [Means for solving the problem]

[0012] As a result of various studies conducted by the inventors of the present application, the following findings have been reached.

[0013] That is, carbon monoxide (CO) and carbon dioxide (CO 2 When a mixed gas of carbon monoxide (pCO) and carbon dioxide (pCO 2 ) is the ratio of pCO / pCO 2 When is greater than a certain value (the value here is determined thermodynamically once the temperature is determined), carbon precipitates on the Fe surface due to the Boudouard reaction shown in [1] below. Note that (g) denotes a gas. 2CO(g) → C+CO 2 (g) [1]

[0014] In fact, the reaction scheme [1] is composed of the following reactions [2][3], where (ad) stands for adatoms. CO(g) → C(ad) + O(ad) [2] O(ad)+CO(g)→CO 2 (g) [3]

[0015] Here, [2] shows the reaction in which CO gas is adsorbed onto the Fe surface and separated into adsorbed carbon atoms C(ad) and adsorbed oxygen atoms O(ad), and [3] shows the reaction in which the adsorbed oxygen atoms O(ad) present on Fe react with CO gas and are removed from the Fe surface.

[0016] And [2] is a very fast reaction, which is almost in equilibrium, as shown in [4] below. CO(g) = C(ad) + O(ad) [4]

[0017] In other words, as shown in [5] below, the reverse reaction of [2] also always occurs, and the same molar amounts of adsorbed carbon atoms C(ad) and adsorbed oxygen electrons O(ad) are removed from the Fe surface. C(ad)+O(ad)→CO(g) [5]

[0018] However, since the amount of adsorbed oxygen atoms O(ad) decreases in reaction [3], the amount of adsorbed carbon atoms C(ad) removed from the Fe surface in reaction [5] decreases, and as a result, the amount of adsorbed carbon atoms C(ad) on the Fe surface increases little by little.

[0019] By the way, CO-CO 2 In mixed gas, CO 2 When the concentration is large, CO 2 Since the concentration of adsorbed oxygen atoms O(ad) produced by the decomposition reaction of is large, most of the adsorbed carbon atoms C(ad) are removed from the Fe surface by the reaction shown in [5], so the concentration of adsorbed carbon atoms C(ad) becomes almost zero and no carbon deposition occurs. CO 2 (g) → O(ad) + CO(g) [6]

[0020] In addition, the adsorbed carbon atoms C(ad) on the Fe surface diffuse into the Fe, forming an Fe-C alloy as shown in [7] below. C represents carbon dissolved in Fe. C(ad) → C [7]

[0021] Therefore, if the diffusion rate of the adsorbed carbon atoms C(ad) onto Fe is large, the amount of diffusion of the adsorbed carbon atoms C(ad) onto Fe will be greater than the increase in the adsorbed carbon atoms C(ad) due to the decrease in the adsorbed oxygen atoms O(ad) on the Fe surface, and the concentration of the adsorbed carbon atoms C(ad) on the Fe surface will be nearly zero, and no carbon deposition will occur.

[0022] In addition, because the diffusion rate of the adsorbed carbon atoms C(ad) into Fe is proportional to the concentration gradient, when the adsorbed carbon atoms C(ad) diffuse into Fe and the concentration gradient gradually decreases, the diffusion rate of the adsorbed carbon atoms C(ad) from the Fe surface slows down. In this case, it becomes impossible to diffuse all of the adsorbed carbon atoms C(ad) present on the Fe surface into Fe, so the adsorbed carbon atoms C(ad) gradually increase, and the adsorbed carbon atoms C(ad) that did not interact with each other because their concentration was small until now bond with each other, resulting in carbon precipitation.

[0023] Based on this knowledge, in order to achieve the above object, the method for producing cementite of the present invention includes the steps of: supplying a mixed gas containing at least carbon monoxide and carbon dioxide as a carbonization gas having a carbon activity greater than 1 and not greater than 10 into a reactor containing iron oxide; and heating the iron oxide to produce cementite under heating conditions such that the temperature falls within a region surrounded by the line at 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 (however, excluding the curve with a carbon activity of 1 and the boundary line of the carbon precipitation region) in a phase stability diagram of an Fe-CO system that shows a carbon precipitation region, which is a region in which carbon precipitation occurs when carbon monoxide and carbon dioxide are used as carbonization gases.

[0024] The method for producing cementite of the present invention includes the steps of: supplying a mixed gas containing at least carbon monoxide and carbon dioxide as a carbonization gas having a carbon activity greater than 1 and not greater than 10 into a reactor containing iron oxide; and heating the iron oxide to produce cementite under heating conditions that are within a region surrounded by a line indicating a temperature of 800°C, curves indicating carbon activities of 1 and 10, and a phase boundary between Fe and FeO, excluding the carbon precipitation region, in a phase stability diagram of an Fe-CO system showing a carbon precipitation region in which carbon precipitation occurs when carbon monoxide and carbon dioxide are used as carbonization gases (however, this does not include the curve indicating a carbon activity of 1 and the boundary line of the carbon precipitation region).

[0025] Here, by supplying a carbonizing gas (a mixed gas containing at least carbon monoxide and carbon dioxide) into the reactor containing the iron oxide, the carbonizing gas acts as a reducing gas and reacts with the iron oxide to produce cementite.

[0026] Furthermore, by supplying a carburizing gas (a mixed gas containing at least carbon monoxide and carbon dioxide) whose carbon activity is greater than 1 and equal to or less than 10, carbon precipitation in the process of producing cementite can be suppressed.

[0027] Furthermore, when cementite is produced by heating iron oxide under heating conditions that fall within the region surrounded by the line at 800°C, the curves for carbon activities of 1 and 10, the phase boundary between Fe and FeO, and the boundary line of the carbon precipitation region (however, excluding the curve for carbon activity of 1 and the boundary line of the carbon precipitation region) in a phase stability diagram for the Fe-CO system, which shows the carbon precipitation region (the region in which carbon precipitation occurs) when carbon monoxide and carbon dioxide are used as carbonizing gases, carbon precipitation is suppressed in the cementite production process, so that cementite can be produced stably over a long period of time.

[0028] Similarly, when cementite is produced by heating iron oxide under heating conditions that are within the region surrounded by the line indicating the temperature of 800°C, the curves indicating the carbon activity of 1 and 10, and the phase boundary between Fe and FeO, excluding the carbon precipitation region (however, not including the curve indicating the carbon activity of 1 and the boundary line of the carbon precipitation region) in a phase stability diagram of the Fe-CO system, which shows the carbon precipitation region (region in which carbon precipitation occurs) when carbon monoxide and carbon dioxide are used as carbonizing gases, carbon precipitation is suppressed in the cementite production process, so cementite can be produced stably over a long period of time.

[0029] The region here (region showing the heating conditions) includes "the line where the temperature is 800°C," "the curve where the carbon activity is 10," and "the phase boundary between Fe and FeO," but does not include "the curve where the carbon activity is 1" or "the boundary of the carbon precipitation region."

[0030] The heating conditions here are a region surrounded by the "straight line of 800°C temperature" and including the straight line of 800°C temperature, in other words, a region of 800°C or lower temperature. This is because if the temperature of the carbonizing gas atmosphere exceeds 800°C, decomposition of cementite may proceed.

[0031] Furthermore, the heating conditions here are in a region surrounded by the "curve with carbon activity of 1" but not including the curve with carbon activity of 1; in other words, a region where the carbon activity is greater than 1. This is because when the carbon activity is less than 1, the reaction [1] above, "2CO(g) → C + CO 2 This means that the reaction "CO(g) → C(ad) + O(ad)" in [2] above proceeds to the left, and the number of adsorbed carbon atoms C(ad) diffusing into the Fe decreases, making it difficult to form cementite.

[0032] Similarly, if the carbon activity is 1, then the above [1] "2CO(g) → C + CO 2 This means that the reaction "CO(g) → C(ad) + O(ad)" in [2] above will be in equilibrium, and the number of adsorbed carbon atoms C(ad) diffusing into the Fe will not increase, making it difficult to form cementite.

[0033] Furthermore, the heating conditions here are the region surrounded by the "curve with carbon activity of 10" and including the curve with carbon activity of 10, in other words, the region with carbon activity of 10 or less. This is because when the carbon activity exceeds 10, the above [1] "2CO(g) → C + CO 2 This means that the reaction "CO(g) → C(ad) + O(ad)" in [2] above progresses rapidly to the right, causing an increase in the number of adsorbed carbon atoms C(ad) on the Fe surface, making it impossible for the adsorbed carbon atoms C(ad) present on the Fe surface to diffuse into the Fe, and the adsorbed carbon atoms C(ad) bond together to form carbon precipitates. If carbon precipitation occurs, the formation of cementite is interrupted and even the formed cementite may decompose.

[0034] The heating conditions here are a region surrounded by the "phase boundary between Fe and FeO" and including the phase boundary between Fe and FeO, in other words, a region where Fe is a thermodynamically apparent stable phase. The inventors' investigation revealed that in a region where the carbon activity is greater than 1, carbon or cementite is a stable phase, but cementite cannot be produced without passing through metallic Fe. For this reason, the heating conditions are set to "a region where Fe is a thermodynamically apparent stable phase", and FeO or Fe 3 O 4 This excludes the region in which the phase appears thermodynamically stable.

[0035] The heating conditions here are in the region surrounded by the "curve with carbon activity of 10" and the "phase boundary between Fe and FeO", and considering that the "curve with carbon activity of 10" and the "phase boundary between Fe and FeO" intersect at approximately 600°C, the temperature is approximately 600°C or higher. This is because if the ambient temperature of the carbonizing gas falls below approximately 600°C, the reaction does not proceed and it takes a long time to generate cementite.

[0036] Furthermore, the heating conditions here are a region surrounded by the "boundary line of the carbon precipitation region" or excluding the "carbon precipitation region" and not including the boundary line of the carbon precipitation region. This is because, when carbon precipitation occurs, the formation of cementite is interrupted and even the formed cementite may decompose.

[0037] In addition, when carbonization gas is generated by adding carbon monoxide gas to the blast furnace exhaust gas discharged from the blast furnace, or by removing carbon dioxide from the blast furnace exhaust gas discharged from the blast furnace, it is only necessary to make slight composition adjustments to the blast furnace exhaust gas that is produced in large quantities as a by-product within the steelworks, and carbonization gas can be obtained at extremely low cost.

[0038] In addition, if the particle size of the iron oxide is less than 1 mm, the reaction with the carbonization gas in the reactor is accelerated, whereas if the particle size of the iron oxide is more than 1 mm, the reaction between the iron oxide and the carbonization gas takes time, and there is a risk that the reduction reaction will not proceed sufficiently.

[0039] Furthermore, when the iron oxide is hematite, the reduction reaction with the carbonization gas is promoted, and cementite can be produced more efficiently.

[0040] Furthermore, when magnetite or wustite is contained as the iron oxide, the reduction reaction with the carbonization gas can be promoted by oxidizing the magnetite or wustite to form hematite. That is, since magnetite and wustite as iron oxides require time to undergo a reduction reaction with carbonization gas, if these magnetite and wustite are subjected to a reduction reaction as they are, the reduction reaction may take a long time. However, by first generating hematite, the problem of the reduction reaction taking a long time can be eliminated.

[0041] Furthermore, when the cementite contains 0% to 20% by weight of iron oxide as a result of the reduction reaction with the carbonizing gas, the cementite can fully exhibit its function.

[0042] Furthermore, when the reactor is of the fluidized bed type, the atmospheric temperature and the concentration of the carbonizing gas in the reactor are uniform, so that all of the iron oxide powder has approximately the same reduction rate, making it possible to produce high-quality cementite.

[0043] In a fluidized bed reactor, if the particle size of the iron oxide is less than 10 μm, the iron oxide is blown away when the carbonization gas is supplied into the reactor, and some of the iron oxide adheres to the inner wall of the reactor and may not be used for the reduction reaction. Therefore, when using a fluidized bed reactor, the particle size of the iron oxide is preferably 10 μm or more. Effect of the Invention

[0044] According to the method for producing cementite of the present invention, carbon precipitation is suppressed and cementite can be produced stably for a long period of time. [Brief description of the drawings]

[0045] [Figure 1] FIG. 2 is a process diagram relating to the method for producing cementite according to the first embodiment of the present invention. [Diagram 2] 1 is a time series graph of cementite formation when a carbonizing gas having a high carbon activity is used. [Figure 3(a)] 2 is a graph for explaining production conditions in the method for producing cementite according to the first embodiment of the present invention. [Figure 3(b)] FIG. 4 is a partially enlarged view of the graph shown in FIG. [Figure 4] FIG. 4 is a process diagram relating to a method for producing cementite according to a second embodiment of the present invention. [Diagram 5] FIG. 1 is a schematic diagram of an experimental apparatus used in the examples. [Figure 6] 1 is a graph showing the analysis results of Example 1 by XRD. [Figure 7] 1 is a graph showing the analysis results of Example 2 by XRD. [Figure 8] 1 is a graph showing the analysis results of Example 3 by XRD. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0046] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, modes for carrying out the invention (hereinafter referred to as "embodiments") will be described with reference to the drawings to facilitate understanding of the present invention.

[0047] [First embodiment] FIG. 1 shows a process diagram of a method for producing cementite according to a first embodiment of the present invention. In the method for producing cementite according to this embodiment, first, hematite (Fe 2 O 3) is prepared and put into a reactor (step 1). Furthermore, carbonization gas is supplied to the reactor (step 2), and the hematite and carbonization gas are heated under specified heating conditions (step 3), to produce cementite (Fe 3 C)

[0048] The iron oxide used as the iron source in step 1 is not limited to hematite. However, as a result of the investigations conducted by the inventors, in the cementite production process, hematite is most likely to undergo a reduction reaction with carbonization gas, making it possible to produce cementite more efficiently.

[0049] The reactor used for producing cementite can be appropriately selected from fixed bed type, moving bed type, fluidized bed type, etc. Among these, the fluidized bed type is suitable for uniformizing the atmospheric temperature and the concentration of the carbonizing gas in the reactor, and therefore can efficiently produce high-quality cementite. Therefore, it is preferable to use the fluidized bed type as the reactor.

[0050] Here, from the viewpoint of promoting the reduction reaction with the carbonization gas, it is preferable that the hematite is in the form of a powder with a small particle size. However, as described above, when a fluidized bed type reactor is used, when a fluid (carbonization gas) is ejected upward from the bottom of the reactor, the solid particles may be blown away and adhere to the inner wall of the reactor, and may not be used for the reduction reaction. Therefore, in order to float in the fluid in the reactor and promote the reaction at the same time, the particle size of the hematite is preferably about 10 μm to 1 mm.

[0051] If the particle size of the hematite is less than 10 μm, it may be blown away when the carbonization gas is supplied into the reactor as described above, and may adhere to the inner wall of the reactor and not be used for the reduction reaction. On the other hand, if the particle size of the hematite is more than 1 mm, it may take time for the hematite to react with the carbonization gas, and the reduction reaction may not proceed sufficiently.

[0052] In step 2, the carbonization gas supplied to the reactor is carbon monoxide (CO) and carbon dioxide (CO 2 ) mixed gas is used. 2 is a carbonization gas with low carbon activity (ac = 1 to 10), which enables stable formation of cementite without carbon precipitation during the reduction reaction with iron oxide.

[0053] Regarding carbon activity, for example, when heating is performed in a gas flow with a certain concentration of CO, the CO undergoes a reaction defined by the following formula (1). 2CO → C+CO 2 (1)

[0054] Carbon activity is a value that indicates the tendency of the reaction shown in formula (1) to proceed to the right. For example, when carbon activity ac>1, the reaction in formula (1) proceeds to the right and carbon precipitates. On the other hand, when carbon activity ac<1, the reaction in formula (1) proceeds to the left and carbon precipitates less. When ac=1, the reaction in formula (1) is in equilibrium and the amount of carbon remains constant.

[0055] In conventional cementite production, a carbonizing gas having a high carbon activity (ac=2000 or more) is generally used as a reducing gas in order to increase the rate at which cementite is produced. In such cases, as described above, carbon precipitation inevitably occurs during operation according to formula (1).

[0056] Regarding carbon activity and carbon precipitation, the test results conducted by Nucor, a US steel manufacturer, in 1996 are useful for reference (Figure 2). Figure 2 is a time series graph showing the change over time in cementite formation in a test plant, with the cementite formation rate reaching nearly 100% about 10 hours after formation begins, after which the formation rate gradually decreases. Meanwhile, the formation rate of iron increases, which is thought to be due to the decomposition of cementite into iron and carbon.

[0057] 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 thought to be due to the decomposition of cementite into iron and carbon by the reaction shown in the following formula (2). Fe 3 C → Fe + 3C (2)

[0058] As described above, when a carbonizing gas with a high carbon activity is used as a reducing gas, the formation of cementite is promoted, but carbon precipitation inevitably occurs within a short time after the start of operation, which not only interrupts the formation of cementite, but also may cause the formed cementite to decompose. In contrast, in the embodiment of the present invention, a carbonizing gas with a low carbon activity is used as a reducing gas, which suppresses carbon precipitation and enables the formation of cementite stably for a long period of time.

[0059] Here, the carbonizing gas is high-purity gas CO, CO 2 You can use CO-CO 2 Blast furnace exhaust gas containing these may also be used. An example of the composition of blast furnace exhaust gas is shown in Table 1. By adding CO gas to such blast furnace exhaust gas or extracting CO from the blast furnace exhaust gas, 2 It is also possible to remove CO from blast furnace exhaust gas and use it as carbonization gas. 2 Methods for selectively separating the above-mentioned compounds include various general-purpose methods such as chemical absorption (amine absorption), physical adsorption (adsorption on zeolite or activated carbon), and membrane separation.

[0060] [Table 1] JPEG0007640157000002.jpg34119

[0061] Next, the heating conditions in step 3 will be described with reference to FIG. Figure 3(a) shows the carbonization gas CO-CO 2 This is a phase stability diagram for the Fe-CO system showing region B where carbon deposition occurs when using mixed gases. The horizontal axis is temperature (°C) and the vertical axis is CO-CO2 The ratio (%) of CO in the mixed gas is shown. Note that Fig. 3(b) is a partially enlarged view of Fig. 3(a).

[0062] As shown in Fig. 3(a) and Fig. 3(b), the heating conditions are set so that the temperature falls within the region surrounded by the straight line of 800°C, the curves of carbon activity ac = 1.0 and carbon activity ac = 10.0, the phase boundary between Fe and FeO, and the boundary of carbon precipitation region B (region A shown by diagonal lines in Fig. 3(a) and Fig. 3(b)). More specifically, the carbon activity ac is in the range of more than 1 and not more than 10, the ambient temperature (reaction temperature) of the carbonizing gas is 800°C or less, the carbonization gas is on the Fe side of the FeO-Fe phase boundary, and the carbonization gas is on the higher temperature side of the boundary of the carbon precipitation region B. 2 The mixing ratio is set, and the heating temperature is also set.

[0063] In addition, the region A indicated by the diagonal lines in Figures 3(a) and 3(b) does not include the "curve for carbon activity ac = 1" and the "boundary line of carbon precipitation region B," but does include the "straight line at 800°C," the "curve for carbon activity ac = 10," and the "phase boundary line of Fe and FeO."

[0064] Here, when the atmospheric temperature of the carbonizing gas is less than 600°C (as is clear from Figs. 3(a) and 3(b), the "curve for carbon activity ac=10" and the "phase boundary line of Fe and FeO" intersect at approximately 600°C. In other words, the region A indicated by diagonal lines in Figs. 3(a) and 3(b) is approximately 600°C or higher), there is a risk that cementite will not be sufficiently generated because the reduction reaction does not proceed. Furthermore, if the temperature of the carbonizing gas atmosphere exceeds 800° C., carbon precipitation is likely to occur, and there is a risk that cementite will decompose into iron and carbon.

[0065] The cementite produced in steps 1 to 3 may contain unreacted iron oxide. However, it is estimated that a small amount of iron oxide (about 20% by weight or less) is sufficient to function as cementite.

[0066] By carrying out the above steps 1 to 3, carbon precipitation can be suppressed and stable production of cementite can be achieved for a long period of time.

[0067] [Second embodiment] Next, a second embodiment will be described. In the second embodiment, instead of the hematite used in the first embodiment, wustite (FeO) or magnetite (Fe 3 O 4 ) is used as the iron source, that is, iron oxide. In the following, the description of the parts common to the first embodiment will be omitted.

[0068] As a result of the inventors' investigation, when wustite or magnetite is used as the iron source for producing cementite, cementite tends to be more difficult to produce than hematite. This is believed to be largely influenced by the stability of the chemical structure, hardness, particle size, or the iron content.

[0069] 4, in the second embodiment in which wustite or magnetite is used as the iron source, first, these iron sources are oxidized and then hematite is produced (step 1). The subsequent steps are the same as those in the first embodiment, and the hematite produced in step 1 is put into a reactor (step 2), and further a carbonization gas is put into the reactor (step 3), and the hematite and the carbonization gas are heated under predetermined heating conditions (step 4), thereby producing cementite.

[0070] As described above, in the second embodiment, even when wustite or magnetite is used as the iron source, carbon precipitation can be suppressed and stable production of cementite can be achieved for a long period of time.

[0071] Next, an embodiment of the present invention will be described. First, Fig. 5 is a schematic diagram of the experimental apparatus used in the examples. The experimental apparatus uses a cylindrical test tube with a tapered end as a fluidized bed reactor. One side of the test tube is a CO2-CO 2A gas inlet for introducing the mixed gas is opened. The other side of the test tube is sealed with an end cap, and a quartz tube that communicates the inside and outside of the test tube and a thermocouple for measuring the temperature inside the test tube are supported through the end cap. The mixed gas introduced from the gas inlet is supplied to the inside of the test tube through a porous filter and is then exhausted to the outside through the quartz tube.

[0072] [Example 1] In Example 1, the effect of carbon activity on the formation of cementite was investigated. As a preliminary step, 0.1 g of powdered hematite pulverized to a particle size of 45 μm or less was placed in a test tube. Then, CO-CO 2 The mixed gas was supplied so that the atmospheric temperature inside the test tube was kept at 700°C, and the powdered hematite was circulated inside the test tube for approximately 60 minutes. At this time, CO-CO 2 The reduction reaction was carried out under the conditions of ac=1.5, ac=5, and ac=10 while adjusting the mixing ratio of the mixed gas.

[0073] FIG. 6 shows the results of XRD (X-ray diffraction) analysis of the reaction product obtained by the reduction reaction. As shown in Fig. 6, a small amount of iron oxide and iron were detected at ac = 1.5, but no iron oxide remained at ac = 5 and ac = 10, confirming that cementite could be formed over a wide range of low carbon activity.

[0074] [Example 2] Next, in Example 2, in order to investigate the effect of the atmospheric temperature (holding temperature) in the test tube on the formation of cementite, a reduction reaction was carried out at ac=5, where carbon precipitation was relatively small in Example 1, with holding temperatures of 550°C, 650°C, 750°C, and 850°C. The experimental conditions were the same as those in Example 1.

[0075] The results of XRD analysis of the reaction product obtained in Example 2 are shown in Figure 7. At a holding temperature of 550°C, only magnetite was reduced and cementite was not produced. At a holding temperature of 650°C, peaks of iron oxide and iron were detected, indicating insufficient reduction, but it can be seen that a cementite peak was also detected.

[0076] At a holding temperature of 750°C, it can be seen that only cementite has formed, although some carbon peaks can be confirmed. At 850°C, carbon precipitation is prominent, and it can be seen that iron is formed first. From the above, it is found that cementite is difficult to form at a holding temperature of 550°C or lower or 850°C or higher.

[0077] [Example 3] As shown in Example 1, the reduction was insufficient under the conditions of a holding temperature of 700°C and ac=1.5, but carbon deposition was not confirmed. Since carbon deposition is more likely to occur with a higher carbon activity, in Example 3, a reaction experiment was performed with ac=1.5 and the holding temperature raised to 750°C. The experimental conditions were the same as those in Examples 1 and 2.

[0078] The results of XRD analysis of the reaction product obtained in Example 3 are shown in Fig. 8. As shown in Fig. 8, under the conditions of a holding temperature of 750°C and ac = 1.5, no peaks of wustite or carbon were confirmed, and high-purity cementite was obtained.

[0079] As mentioned above, CO-CO 2 It was revealed that cementite can be produced at ac = 1.5 by adjusting the composition of the mixed gas. This shows that cementite can be produced using blast furnace gas as a reducing raw material for industrial production of cementite, which is a great advantage for Japan, which does not produce natural gas.

[0080] As described above, the method for producing cementite according to the present invention can suppress carbon precipitation and produce cementite stably over a long period of time.

Claims

1. supplying a mixed gas containing at least carbon monoxide and carbon dioxide as a carbonizing gas having a carbon activity greater than 1 and less than or equal to 10 into a reactor containing iron oxide; and heating the iron oxide under heating conditions in a phase stability diagram of an Fe-C-O system showing a carbon precipitation region where carbon precipitation occurs when carbon monoxide and carbon dioxide are used as carbonizing gases, such that the temperature falls within a region surrounded by a straight line at 800°C, curves with carbon activities of 1 and 10, a phase boundary line between Fe and FeO, and the boundary line of the carbon precipitation region (however, the curve with carbon activity of 1 and the boundary line of the carbon precipitation region are not included), thereby suppressing carbon precipitation and generating cementite. A method for producing cementite.

2. supplying a mixed gas containing at least carbon monoxide and carbon dioxide as a carbonizing gas having a carbon activity greater than 1 and less than or equal to 10 into a reactor containing iron oxide; and heating the iron oxide under heating conditions that are within a region surrounded by a line at a temperature of 800° C., curves with carbon activities of 1 and 10, and a phase boundary line between Fe and FeO in a phase stability diagram of an Fe—C—O system showing a carbon precipitation region in which carbon precipitation occurs when carbon monoxide and carbon dioxide are used as carbonizing gases, and that are excluding the carbon precipitation region (however, the curve with a carbon activity of 1 and the boundary line of the carbon precipitation region are not included), thereby suppressing carbon precipitation and generating cementite. A method for producing cementite.

3. The carbonization gas is generated by adding carbon monoxide gas to the blast furnace exhaust gas discharged from the blast furnace or by removing carbon dioxide from the blast furnace exhaust gas discharged from the blast furnace. The method for producing cementite according to claim 1 or 2.

4. The particle size of the iron oxide is less than 1 mm. The method for producing cementite according to claim 1 or 2.

5. The iron oxide is hematite. The method for producing cementite according to claim 1 or 2.

6. The iron oxide includes magnetite or wustite, and the method includes a step of oxidizing the magnetite or wustite to generate hematite. The method for producing cementite according to claim 1 or 2.

7. The cementite contains 0% to 20% by weight of iron oxide. The method for producing cementite according to claim 1 or 2.

8. The reactor is of the fluidized bed type. The method for producing cementite according to claim 1 or 2.

9. The particle size of the iron oxide is 10 μm or more. The method for producing cementite according to claim 8.

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