Method for producing cementite

Abstract

[Problem] To provide a method for producing cementite, capable of suppressing carbon precipitation and stably producing cementite over a long period of time. [Solution] Hematite (Fe2O3) is put into a reactor as an iron oxide serving as an iron source, and a carbonized gas formed from a mixed gas of carbon monoxide and carbon dioxide is supplied to the reactor. Cementite (Fe3C) can be efficiently produced for a long period of time without precipitating carbon by adjusting the carbon activity so that ac = 1 to 10 and subjecting the carbonized gas and hematite to a reduction reaction.

Description

Manufacturing method for cementite

[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.

[0002] Traditionally, steel production in Japan has developed mainly using the blast furnace-converter method. However, the current Japanese steel industry has been ２ We are being forced to address the issues of emissions, energy, and resources as quickly as possible. 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 its technological development on the production of high-quality steel using the blast furnace-converter method. As a result, Japan does not have accumulated technological expertise related to electric furnaces, particularly the technology for manufacturing high-quality 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 using 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. Therefore, it is difficult to produce high-quality steel such as steel sheets for automobiles using electric furnaces. For the production of high-quality steel using electric furnaces, direct reduced iron (DRI) and cementite (Fe ３ It is necessary to use an iron source with few impurities, such as iron source 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 storage and transportation costs are low. 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] Traditionally, natural gas, primarily methane, has been used as a raw material in cementite pilot plant operations. However, since natural gas is not produced domestically in Japan and the country is almost entirely dependent on imports, using natural gas as a reducing raw material for the industrial production of cementite would be extremely cost-inefficient.

[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 contact with each other in the presence of a reducing gas made of water vapor, an oxygen-containing gas (air or oxygen gas), carbon dioxide gas, or a mixture thereof, and heated under predetermined temperature conditions to reduce part of the iron oxide, thereby producing cementite.

[0008] JP 2012-57202 A

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

[0010] When a reducing gas with a high carbon activity is used, cementite is formed early, but carbon precipitation also occurs early. Once carbon precipitation occurs, cementite formation is interrupted and the cementite that has already formed 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 made in view of the above points, and has as its 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.

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

[0013] That is, the Fe—C—O system stability diagram at 527° C. is as shown in FIG. ３ To generate C, the carbon activity must be greater than 1. In other words, Fe ３ C cannot be produced, and Fe ３ To produce C, the carbon activity must be greater than 1.

[0014] Here, Fe ３ During the C generation process, carbon is converted to Fe ３ When precipitated on the C surface, Fe ３ Since the carbon activity in C is greater than 1, Fe ３ A carbon concentration gradient occurs between the C surface and the deposited carbon, and Fe ３ The carbon in C diffuses to the surface, and Fe ３ Decomposition of C occurs, and Fe ３ The production of C stops. This is known as metal dusting, and is a phenomenon in which the surface becomes powdery.

[0015] Therefore, metal dusting is a major problem in chemical plants that use highly reducing gases. As metal dusting progresses, the pipe wall thickness becomes thinner, which can lead to pipe rupture, and this must be avoided.

[0016] Specifically, copper-lined pipes are used in the presence of highly reducing gases that may cause metal dusting, because CO decomposition does not occur on the copper surface, and therefore carbon is not supplied.

[0017] That is, Fe is continuously added for a long period of time. ３ For industrial production of carbon, it is crucial to "prevent carbon deposition."

[0018] And, Fe due to carbon deposition ３ The decomposition behavior of C has been investigated in detail by Dr. H.J. Grabke, and its mechanism has been clarified (Fe ３ The decomposition mechanism of C is shown in Figure 10).

[0019] First, as shown in FIG. 10( a ), since the carbon activity in the gas is greater than 1, carbon is supplied to the surface from the carbonizing gas, and the carbon diffuses into the Fe.

[0020] Next, as shown in FIG. 10(b), when the carbon concentration in Fe reaches a critical point, the Fe—C alloy with a high carbon concentration becomes Fe ３ The phase changes to C.

[0021] As shown in FIG. 10(c), the carbon supply from the gas continues, and Fe ３ Carbon diffuses through the C phase, but Fe ３ As the C phase gradually thickens, Fe ３ The concentration gradient of carbon in the C phase becomes smaller, the diffusion rate decreases, and a part of the carbon supplied to the surface remains on the surface (carbon deposition).

[0022] Next, as shown in FIG. 10(d), the carbon activity of the carbon precipitated on the surface is 1, but the carbon activity of the Fe ３ The carbon activity of the C phase is greater than 1, and a difference in carbon concentration occurs at the interface. ３ The carbon in the C begins to diffuse towards the surface. ３ The carbon concentration of C decreases and Fe ３ The decomposition of C begins.

[0023] In other words, once carbon begins to precipitate, Fe ３ The decomposition of C progresses, and Fe ３ C will no longer be produced.

[0024] In this way, (1) Fe was produced using solid oxygen (carbon activity = 1). ３ C cannot be produced, (2) Fe ３ When solid carbon precipitates on the C surface, Fe ３ C decomposes into Fe ３ This means that C generation will no longer be possible.

[0025] By the way, carbon monoxide (CO) and carbon dioxide (CO ２ When a mixed gas of carbon monoxide (pCO) and carbon dioxide (pCO) is supplied to a metal surface (Fe surface), the partial pressure of carbon monoxide (pCO) and carbon dioxide (pCO) ２ ) pCO / pCO ２When is greater than a predetermined value (which is determined thermodynamically when the temperature is determined), carbon precipitates on the Fe surface by the Boudouard reaction shown in [1] below. Note that (g) represents gas. 2CO(g) → C + CO ２ (g) ... [1]

[0026] In fact, the reaction formula [1] is composed of the following reactions [2] and [3]. Note that (ad) indicates an adatom. CO(g) → C(ad) + O(ad) ... [2] O(ad) + CO(g) → CO ２ (g) ... [3]

[0027] Here, [2] indicates 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] indicates the reaction in which adsorbed oxygen atoms O(ad) present on Fe react with CO gas and are removed from the Fe surface.

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

[0029] In other words, as shown in the following [5], 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]

[0030] However, since the adsorbed oxygen atoms O(ad) decrease due to the reaction [3], the amount of adsorbed carbon atoms C(ad) removed from the Fe surface in the reaction [5] decreases, and as a result, the amount of adsorbed carbon atoms C(ad) on the Fe surface increases little by little.

[0031] By the way, CO-CO ２ In the mixed gas, CO ２ When the concentration is large, CO ２Since the concentration of adsorbed oxygen atoms O(ad) generated by the decomposition reaction of CO 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. ２ (g)→O(ad)+CO(g)...[6]

[0032] Furthermore, the adsorbed carbon atoms C(ad) on the Fe surface diffuse into the Fe, forming an Fe-C alloy as shown in [7] below. Note that C represents carbon dissolved in Fe. C(ad) → C... [7]

[0033] Therefore, if the diffusion rate of the adsorbed carbon atoms C(ad) onto Fe is high, the amount of the adsorbed carbon atoms C(ad) diffused 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 almost zero, and no carbon deposition will occur.

[0034] Since 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, and the adsorbed carbon atoms C(ad) gradually increase, and the adsorbed carbon atoms C(ad) that had not interacted with each other because their concentration was so low until then bond to each other, resulting in carbon deposition.

[0035] 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 carburizing gas having a carbon activity greater than 1 and not greater than 10 into a reactor containing iron oxide; and heating the iron oxide under heating conditions such that the temperature falls within a 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, this does not include the line on the curve for carbon activity of 1 or the boundary line of the carbon precipitation region) in a phase stability diagram for an Fe-C-O 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 carburizing gases, thereby suppressing carbon precipitation and producing cementite.

[0036] 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 carbonizing gas having a carbon activity greater than 1 and not greater than 10 into a reactor containing iron oxide; and heating the iron oxide under heating conditions such that the temperature falls within a region surrounded by a line at 800°C, a curve with carbon activities of 1 and 10, and a phase boundary line between Fe and FeO, excluding the carbon precipitation region, 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 the carbonizing gas (however, this does not include the line on the curve with a carbon activity of 1 or the boundary line of the carbon precipitation region).

[0037] 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 reacts with the iron oxide as a reducing gas to produce cementite.

[0038] Furthermore, by supplying a carbonizing gas (a mixed gas containing at least carbon monoxide and carbon dioxide) having a carbon activity greater than 1 and not greater than 10, carbon precipitation during the cementite production process can be suppressed.

[0039] Furthermore, when cementite is produced while suppressing carbon precipitation by heating iron oxide under heating conditions that fall within a 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 in a phase stability diagram of an Fe-C-O system that shows the carbon precipitation region (the region in which carbon precipitation occurs) when carbon monoxide and carbon dioxide are used as carbonizing gases (however, the curve for carbon activity of 1 and the boundary line of the carbon precipitation region are not included), carbon precipitation is suppressed in the cementite production process, and therefore cementite can be produced stably over a long period of time.

[0040] Similarly, in a phase stability diagram for an Fe-C-O system showing the carbon precipitation region (region in which carbon precipitation occurs) when carbon monoxide and carbon dioxide are used as carbonizing gases, when carbon precipitation is suppressed and cementite is produced by heating iron oxide under heating conditions that fall within a region surrounded by the line at a temperature of 800°C, the curves for carbon activities of 1 and 10, and the phase boundary between Fe and FeO, but excluding the carbon precipitation region (however, not including the curve for carbon activity of 1 or the boundary line of the carbon precipitation region), carbon precipitation is suppressed in the cementite production process, and therefore cementite can be produced stably over a long period of time.

[0041] The region here (region indicating the heating conditions) includes "across the line where the temperature is 800°C," "across the curve where the carbon activity is 10," and "across the phase boundary between Fe and FeO," but does not include "across the curve where the carbon activity is 1" or "across the boundary of the carbon precipitation region."

[0042] The heating conditions here are a region surrounded by the "straight line of 800°C temperature" and including the line of 800°C temperature, in other words, a region where the temperature is 800°C or less. This is because if the ambient temperature of the carbonizing gas exceeds 800°C, decomposition of cementite may proceed.

[0043] Furthermore, the heating conditions here are the region surrounded by the "curve of carbon activity 1" but not included on the curve of carbon activity 1, in other words, the region where the carbon activity is greater than 1. This is because, if the carbon activity is less than 1, the reaction of "2CO(g) → C + CO" in [1] above will not occur. ２This is because the reaction of "CO(g) → C(ad) + O(ad)" in [2] above proceeds to the left side (in other words, the reaction of "CO(g) → C(ad) + O(ad)" in [2] above proceeds to the left side), and the number of adsorbed carbon atoms C(ad) diffusing into Fe decreases, making it difficult to form cementite.

[0044] Similarly, if the carbon activity is 1, then the above [1] "2CO (g) → C + CO ２ This is because the reaction of "CO(g) → C(ad) + O(ad)" in [2] above reaches equilibrium (in other words, the reaction of "CO(g) → C(ad) + O(ad)" in [2] above reaches equilibrium), and the number of adsorbed carbon atoms C(ad) diffusing into Fe does not increase, making it difficult to produce cementite.

[0045] Furthermore, the heating conditions here are the region surrounded by the "curve of carbon activity 10" and including the curve of carbon activity 10, in other words, the region where the carbon activity is 10 or less. This is because when the carbon activity exceeds 10, the reaction of "2CO(g) → C + CO ２ This means that the reaction "CO(g) → C(ad) + O(ad)" in [2] above rapidly progresses to the right (in other words, the reaction "CO(g) → C(ad) + O(ad)" in [2] above rapidly progresses to the right), and the adsorbed carbon atoms C(ad) on the Fe surface increase, making it impossible for the adsorbed carbon atoms C(ad) present on the Fe surface to diffuse into the Fe, resulting in the adsorbed carbon atoms C(ad) bonding together and carbon precipitation. Note that when carbon precipitation occurs, the formation of cementite is interrupted and there is a risk that even the formed cementite may decompose.

[0046] 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 becomes a thermodynamically apparent stable phase. As a result of the inventors' investigation, it was found that in a region where the carbon activity is greater than 1, carbon or cementite becomes a stable phase, but cementite cannot be produced without passing through metallic Fe. Therefore, the heating conditions are set to the "region where Fe becomes a thermodynamically apparent stable phase" and the heating conditions are set to FeO or Fe ３ O ４ This excludes the region where the phase appears thermodynamically stable.

[0047] The heating conditions here are a region surrounded by the "curve for carbon activity 10" and the "phase boundary between Fe and FeO," and considering that the "curve for carbon activity 10" and the "phase boundary between Fe and FeO" intersect at approximately 600°C, the region is a temperature region of approximately 600°C or higher. This is because if the ambient temperature of the carbonizing gas is below approximately 600°C, the reaction does not proceed and it takes a long time to produce cementite.

[0048] 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, if carbon precipitation occurs, the formation of cementite is interrupted and even the formed cementite may be decomposed.

[0049] The heating conditions here are, as described above, "a region in which the carbon activity is greater than 1" and "a region in which the carbon activity is 10 or less."

[0050] Then, the partial pressure of carbon monoxide (pCO) and the partial pressure of carbon dioxide (pCO ２ ) with CO-CO ２ The carbon activity of the mixed gas is calculated by the above [1] "2CO (g) → C + CO ２ (g)" can be determined thermodynamically based on the reaction. That is, if the equilibrium constant of the reaction [1] above is K, the equilibrium state of the reaction [1] above can be expressed as [7]. Note that ac represents the carbon activity of the mixed gas. K = (pCO) ２ / (ac·pCO ２ ) [7]

[0051] Then, by rewriting the above [7], it can be expressed as [8]: ac = (pCO) ２ / K(pCO ２ ) ... [8]

[0052] In other words, CO-CO ２ The carbon activity of a gas mixture, a, is calculated from the partial pressure of carbon monoxide and the partial pressure of carbon dioxide (pCO). ２This can be determined by knowing the / (pCO) ratio and the equilibrium constant K. The value of the equilibrium constant K at each temperature can be calculated thermodynamically, and is given by [9], where T is the absolute temperature. log K = 8460 / T - 8.85...[9]

[0053] Therefore, at a given temperature, the partial pressure of carbon monoxide (pCO ) and the partial pressure of carbon dioxide (pCO ) can be calculated from the above equations [8] and [9]. ２ ) the carbon activity ac can be calculated.

[0054] Here, CO and CO ２ The explanation is given for a binary gas mixture of CH ４ Mixed gases containing CO and CH ４ Even for multi-component mixed gases containing these elements simultaneously, the carbon activity can be obtained if the "gas composition" and the "equilibrium constant K of each reaction occurring between the various gases that make up the mixed gas" are known.

[0055] Furthermore, when carbonized gas is generated by adding carbon monoxide gas to blast furnace exhaust gas discharged from a blast furnace or by removing carbon dioxide from blast furnace exhaust gas discharged from a 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 in steelworks, and carbonized gas can be obtained at extremely low cost.

[0056] If the particle size of the iron oxide is less than 1 mm, the reaction with the carbonizing 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 carbonizing gas takes time, and the reduction reaction may not proceed sufficiently.

[0057] Furthermore, when the iron oxide is hematite, the reduction reaction with the carbonizing gas is promoted, and cementite can be produced more efficiently.

[0058] Furthermore, when magnetite or wustite is contained as the iron oxide, the reduction reaction with the carbonizing gas can be promoted by oxidizing the magnetite or wustite to form hematite. That is, since magnetite or wustite as iron oxide requires time for the reduction reaction with the carbonizing gas, if the magnetite or wustite is subjected to the reduction reaction as it is, the reduction reaction may take a long time. However, by first forming hematite, the problem of the reduction reaction taking a long time can be solved.

[0059] 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.

[0060] Furthermore, when the reactor is of the fluidized bed type, the atmospheric temperature and the concentration of the carbonizing gas inside 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.

[0061] In a fluidized bed reactor, if the particle size of the iron oxide is less than 10 μm, the iron oxide will be blown away when the carbonizing gas is supplied into the reactor, and some of the iron oxide will adhere to the inner wall of the reactor and may not be used in the reduction reaction. Therefore, when a fluidized bed reactor is used, the particle size of the iron oxide is preferably 10 μm or more.

[0062] In the method for producing cementite according to the present invention, carbon precipitation can be suppressed and cementite can be produced stably over a long period of time.

[0063] 3(a) is a process diagram relating to a method for producing cementite according to a first embodiment of the present invention. FIG. 3(b) is a time series graph of cementite generation when a carbonizing gas having a high carbon activity is used. FIG. 3(c) is a graph for explaining production conditions in a method for producing cementite according to a first embodiment of the present invention. FIG. 3(d) is a partial enlarged view of the graph shown in FIG. 3(a). FIG. 3(c) is a process diagram relating to a method for producing cementite according to a second embodiment of the present invention. FIG. 3(d) is a schematic diagram of an experimental apparatus used in the examples. FIG. 3(d) is a graph showing the results of analysis by XRD in Example 1. FIG. 3(c) is a graph showing the results of analysis by XRD in Example 2. FIG. 3(d) is a graph showing the results of analysis by XRD in Example 3. ３ FIG. 1 is a schematic diagram for explaining the decomposition mechanism of C.

[0064] 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.

[0065] [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 charged into a reactor (Step 1). Further, a carbonization gas is supplied to the reactor (Step 2), and the hematite and the carbonization gas are heated under predetermined heating conditions (Step 3). ３ C).

[0066] The iron oxide used as the iron source in step 1 is not limited to hematite. However, as a result of investigations by the inventors, it has been found that in the cementite production step, hematite most effectively promotes a reduction reaction with the carbonizing gas, and therefore cementite can be produced more efficiently.

[0067] The reactor used for producing cementite can be appropriately selected from fixed bed, moving bed, fluidized bed, etc. Among these, the fluidized bed type is suitable for making the atmospheric temperature and the carbonizing gas concentration uniform in the reactor, and therefore can efficiently produce high-quality cementite. Therefore, it is preferable to use the fluidized bed type as the reactor.

[0068] Here, from the viewpoint of promoting the reduction reaction with the carbonization gas, it is preferable that the hematite be in the form of a powder with a small particle size. However, as mentioned above, when a fluidized bed 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, preventing them from being used in the reduction reaction. Therefore, in order to achieve both floating in the fluid in the reactor and promoting the reaction, the particle size of the hematite is preferably about 10 μm to 1 mm.

[0069] 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, and may adhere to the inner wall of the reactor, preventing it from being used in the reduction reaction, as described above. On the other hand, if the particle size of the hematite is more than 1 mm, it may take a long time for the hematite to react with the carbonization gas, and the reduction reaction may not proceed sufficiently.

[0070] In step 2, the carbonizing gas supplied into the reactor is carbon monoxide (CO) and carbon dioxide (CO ２ ) and CO mixed gas is used. ２ is a carbonizing gas with low carbon activity (ac = 1 to 10), which enables stable production of cementite without carbon precipitation during the reduction reaction with iron oxide.

[0071] Regarding carbon activity, when heating is performed in a gas stream containing a predetermined concentration of CO, the reaction of CO occurs as defined by the following formula (1): 2CO → C + CO ２ ...(1)

[0072] Carbon activity is a value that indicates the tendency of the reaction represented by formula (1) to proceed to the right. For example, when carbon activity ac > 1, the reaction of formula (1) proceeds to the right, resulting in carbon deposition. On the other hand, when carbon activity ac < 1, the reaction of formula (1) proceeds to the left, resulting in reduced carbon deposition. When ac = 1, the reaction of formula (1) reaches equilibrium, and the amount of carbon remains constant.

[0073] In conventional cementite production, a carbonizing gas having a high carbon activity (ac=2000 or more) is generally used as a reducing gas in many cases for the purpose of accelerating the rate of cementite formation. In such cases, as described above, carbon deposition inevitably occurs during operation according to formula (1).

[0074] Regarding carbon activity and carbon precipitation, the results of a test 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. The cementite formation rate reaches nearly 100% about 10 hours after the start of formation, and then gradually decreases. Meanwhile, 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. However, 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)

[0076] As described above, when a carbide 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 carbide gas with a low carbon activity is used as a reducing gas, which suppresses carbon precipitation and enables cementite to be formed stably for a long period of time.

[0077] Here, the carbonizing gas is high-purity gas CO, CO ２ You can also use CO-CO ２ An example of the composition of blast furnace exhaust gas is shown in Table 1. CO gas may be added to such blast furnace exhaust gas or CO may be extracted 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 There are various general-purpose methods for selectively separating the above, such as chemical absorption (amine absorption), physical adsorption (adsorption on zeolite or activated carbon), and membrane separation.

[0078] [Table 1]

[0079] Next, the heating conditions in step 3 will be explained based on Fig. 3. Fig. 3(a) shows the heating conditions in step 3. ２ This is a phase stability diagram for the Fe—C—O system showing region B where carbon deposition occurs when using a mixed gas. The horizontal axis is temperature (°C) and the vertical axis is CO—CO ２ 3(b) shows the ratio (%) of CO in the mixed gas. Note that FIG. 3(b) is a partially enlarged view of FIG. 3(a).

[0080] As shown in Figures 3(a) and 3(b), the heating conditions are set so that the carbon activity ac falls within the region surrounded by the straight line at 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 deposition region B (region A indicated by diagonal lines in Figures 3(a) and 3(b)). More specifically, the carbon activity ac is set so that the carbon activity ac is greater than 1 and not more than 10, the ambient temperature (reaction temperature) of the carbonizing gas is not more than 800°C, the temperature is closer to Fe than the phase boundary between FeO and Fe, and is higher than the boundary of carbon deposition region B. ２ The mixing ratio of the two is set, and the heating temperature is also set.

[0081] Furthermore, the region A indicated by diagonal lines in FIGS. 3( a) and 3(b) does not include the "curve of carbon activity ac = 1" and the "boundary line of carbon deposition region B," but does include the "800°C line," the "curve of carbon activity ac = 10," and the "phase boundary line of Fe and FeO."

[0082] Here, when the temperature of the carbonizing gas atmosphere is less than 600°C (as is clear from Figures 3(a) and 3(b) , the "curve for carbon activity ac = 10" and the "phase boundary line between Fe and FeO" intersect at approximately 600°C. In other words, the area A indicated by diagonal lines in Figures 3(a) and 3(b) is approximately 600°C or higher), the reduction reaction does not proceed, and there is a risk that cementite will not be sufficiently produced. Furthermore, when 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.

[0083] The cementite produced by 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) can adequately function as cementite.

[0084] By carrying out the above steps 1 to 3, carbon precipitation can be suppressed and stable cementite production can be achieved for a long period of time.

[0085] Second Embodiment Next, a second embodiment will be described. In the second embodiment, instead of hematite used in the first embodiment, wustite (FeO) or magnetite (Fe ３ O ４ ) is used as the iron source iron oxide. In the following, explanations of parts common to the first embodiment will be omitted.

[0086] As a result of the inventors' investigations, when wüstite or magnetite is used as the iron source for producing cementite, cementite tends to be more difficult to produce than hematite. This is thought to be largely influenced by the stability of the chemical structure, hardness, particle size, iron content, etc.

[0087] 4, in the second embodiment in which wüstite or magnetite is used as the iron source, the iron source is first oxidized to produce hematite (Step 1). The subsequent steps are the same as those in the first embodiment, and the hematite produced in Step 1 is introduced into a reactor (Step 2), a carbonizing gas is further introduced into the reactor (Step 3), and the hematite and the carbonizing gas are heated under predetermined heating conditions (Step 4), thereby producing cementite.

[0088] 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 cementite production can be achieved for a long period of time.

[0089] Next, examples 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, and one side of the test tube is a CO-CO ２ A gas inlet is open for introducing the mixed gas. The other end of the test tube is sealed with an end cap, and a quartz tube that connects the inside and outside of the test tube and a thermocouple for measuring the temperature inside the test tube are supported through the end caps. The mixed gas introduced from the gas inlet passes through a porous filter and is supplied into the test tube, and then is exhausted to the outside through the quartz tube.

[0090] [Example 1] In Example 1, the effect of carbon activity on the formation of cementite was investigated. As a preliminary preparation, 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, and the atmospheric temperature in the test tube was kept at 700°C, and the powdered hematite was circulated in the test tube for about 60 minutes. ２ The reduction reaction was carried out in each of the patterns of ac=1.5, ac=5, and ac=10 while adjusting the mixing ratio of the mixed gas.

[0091] The results of XRD (X-ray diffraction) analysis of the reaction product obtained by the reduction reaction are shown in Figure 6. As shown in Figure 6, at ac = 1.5, a small amount of iron oxide and iron was detected, but at ac = 5 and ac = 10, no iron oxide remained, confirming that cementite could be produced over a wide range of low carbon activities.

[0092] [Example 2] Next, in Example 2, in order to investigate the influence 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 relatively little carbon deposition occurred in Example 1, with holding temperatures of 550°C, 650°C, 750°C, and 850°C. The underlying experimental conditions were the same as those in Example 1.

[0093] The results of XRD analysis of the reaction product obtained in Example 2 are shown in Figure 7. When the holding temperature was 550°C, only magnetite was reduced and cementite was not produced. Furthermore, when the holding temperature was 650°C, peaks of iron oxide and iron were detected, indicating insufficient reduction, but it was also found that a cementite peak was detected.

[0094] When the holding temperature reaches 750°C, it can be seen that only cementite is formed, although some carbon peaks can be confirmed. At 850°C, carbon precipitation is prominent, and it can be seen that iron is formed preferentially. From the above, it can be seen that cementite is difficult to form at holding temperatures below 550°C or above 850°C.

[0095] [Example 3] As shown in Example 1, 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 as the carbon activity increases, in Example 3, a reaction experiment was carried out with ac = 1.5 and the holding temperature raised to 750°C. The experimental conditions underlying the experiment were the same as those in Examples 1 and 2.

[0096] The results of XRD analysis of the reaction product obtained in Example 3 are shown in Figure 8. As shown in Figure 8, under the conditions of a holding temperature of 750°C and ac = 1.5, no peaks of wüstite or carbon were confirmed, and high-purity cementite was obtained.

[0097] As mentioned above, CO-CO ２ It has become clear that cementite can be produced at ac = 1.5 by adjusting the composition of the mixed gas. This indicates that cementite can be produced using blast furnace gas as a reducing raw material for industrial cementite production, which is a great advantage for Japan, which does not produce natural gas.

[0098] 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. A method for producing cementite, comprising: a step 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 a step of heating the iron oxide under heating conditions such that the temperature falls within a 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, the line on the curve for carbon activity of 1 and the boundary line of the carbon precipitation region are not included) in a phase stability diagram of the Fe-C-O system, which shows the carbon precipitation region, which is the region where carbon precipitation occurs when carbon monoxide and carbon dioxide are used as the carbonization gas, thereby suppressing carbon precipitation and producing cementite.

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

3. The method for producing cementite according to claim 1, further comprising a step of adding carbon monoxide gas to blast furnace exhaust gas discharged from a blast furnace, or removing carbon dioxide from blast furnace exhaust gas discharged from a blast furnace, to generate the carbonized gas.

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

5. The method for producing cementite according to claim 1, wherein the iron oxide is hematite.

6. The method for producing cementite according to claim 1, comprising a step of containing magnetite or wustite as the iron oxide and oxidizing the magnetite or wustite to produce hematite.

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

8. The method for producing cementite according to claim 1, wherein the reactor is of a fluidized bed type.

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

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