Cementite manufacturing method

Abstract

The present invention provides a method for producing cementite that suppresses carbon precipitation and enables stable cementite formation over a long period of time. [Solution] Hematite (Fe2O3) is introduced into the rotary kiln as iron oxide, which is the iron source, and a carbonization gas consisting of a mixture of carbon monoxide and carbon dioxide is supplied to the rotary kiln. By adjusting the carbon activity ac = 1 to 100 and observing the rotation conditions such that the mixing behavior of the iron source in the rotary kiln is rolling or cascading, the carbonization gas and hematite are reduced, allowing cementite to be efficiently produced over a long period of time without carbon precipitation.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing cementite. Specifically, it relates to a method for manufacturing cementite that can suppress carbon precipitation and stably produce cementite over a long period of time.

Background Art

[0002] Conventionally, steel production in Japan has developed mainly based on the blast furnace-converter method. However, the current Japanese steel industry is being pressured to quickly address recent CO2 emission problems, energy problems, and resource problems, and there is a demand for conversion to electric furnaces with low CO2 emissions and low energy consumption.

[0003] However, different from Europe and the United States where steel production is centered around electric furnaces, in Japan, technology development has been mainly focused on the production of high-grade steel by the blast furnace-converter method. Therefore, there is no technical accumulation regarding electric furnaces, especially the technology for manufacturing high-grade steel using large electric furnaces, and many issues need to be solved for the transition from the blast furnace-converter method to electric furnaces.

[0004] The main iron raw material for the steelmaking method using an electric furnace is scrap. However, a major issue in steel production using this scrap is the accumulation of impurities such as Cu due to the repeated use of scrap. Therefore, it is difficult to manufacture high-grade steel such as steel sheets for automobiles using an electric furnace, and it is necessary to use iron sources with few impurities such as direct reduced iron (DRI) and cementite (Fe3C) in combination to reduce the impurity concentration.

[0005] Among the aforementioned iron sources, cementite not only has few impurities but also low energy consumption, and has the advantage of not incurring costs during storage and transportation due to its non-flammability. Therefore, cementite is considered an ideal iron source in steel production using an electric furnace, and research and development for the industrial production of cementite are underway.

[0006] Incidentally, since cementite is a thermodynamically unstable metastable substance, the carbon activity (ac) of the reducing gas used to produce cementite must be set to a value greater than 1. This inevitably indicates that carbon precipitation is always possible during the cementite manufacturing process. However, in conventional cementite manufacturing methods (for example, the manufacturing method disclosed in Patent Document 1), reducing gases with high carbon activity (for example, ac = 2000 or more) have been used to accelerate the cementite production rate.

[0007] Using a reducing gas with high carbon activity in this way leads to the rapid formation of cementite, but also accelerates carbon precipitation. Once carbon precipitates, cementite formation is interrupted, and the already formed cementite also decomposes, making subsequent operations difficult. This presents a challenge in that it is not possible to stably produce cementite over a long period of time.

[0008] Therefore, the inventors of the present invention have proposed a method that enables the stable production of cementite over a long period of time by using a reducing gas with low carbon activity instead of a reducing gas with high carbon activity that has been used conventionally (Patent Document 2). [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Japanese Patent Publication No. 2012-57202 [Patent Document 2] Patent No. 7640157 specification [Overview of the project] [Problems that the invention aims to solve]

[0010] While the cementite manufacturing method described in Patent Document 2 above can stably produce cementite, the range of carbon activity is limited (specifically, the carbon activity is limited to greater than 1 and 10 or less), and there was a need to relax the restriction on carbon activity.

[0011] This invention was conceived in view of the above points, and aims to provide a method for producing cementite that can stably generate cementite over a long period of time over a wider range of carbon activity. [Means for solving the problem]

[0012] As a result of various studies conducted by the inventors of this invention, the following findings were reached.

[0013] In other words, when a mixed gas of carbon monoxide (CO) and carbon dioxide (CO2) is supplied to a metal surface (Fe surface), if the ratio of the partial pressure of carbon monoxide (pCO) to the partial pressure of carbon dioxide (pCO2), pCO / pCO2, is greater than a predetermined value (this value is determined thermodynamically once the temperature is determined), carbon will precipitate on the Fe surface by the Boudouard reaction shown in [1] below. Note that (g) indicates the gas. 2CO(g) → C + CO2(g)···[1]

[0014] In reality, the reaction equation in [1] consists of the reactions shown in [2] and [3] below. (ad) indicates the adsorbed atom. CO(g)→C(ad)+O(ad)···[2] O(ad) + CO(g) → CO2(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 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, reaching near equilibrium as shown in [4] below. CO(g) = C(ad) + O(ad)···[4]

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

[0018] However, since the adsorbed oxygen atoms O(ad) decrease due to the reaction shown in [3], the adsorbed carbon atoms C(ad) removed from the Fe surface in the reaction shown in [5] decrease, and as a result, the adsorbed carbon atoms C(ad) on the Fe surface will gradually increase.

[0019] By the way, in the CO-CO2 mixed gas, when the CO2 concentration is high, as shown in [6] below, since the concentration of the adsorbed oxygen atoms O(ad) generated by the decomposition reaction of CO2 is high, most of the adsorbed carbon atoms C(ad) are removed from the Fe surface by the reaction shown in [5], so the concentration of the adsorbed carbon atoms C(ad) becomes almost zero and carbon precipitation does not occur. CO2(g) → O(ad) + CO(g) ··· [6]

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

[0021] Therefore, if the diffusion rate of the adsorbed carbon atoms C(ad) into the Fe is high, the diffusion amount of the adsorbed carbon atoms C(ad) into the Fe is larger 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 becomes almost zero and carbon precipitation does not occur.

[0022] Since the diffusion rate of the adsorbed carbon atoms C(ad) to Fe is proportional to the concentration gradient, as 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. Since the concentration was low until now, the adsorbed carbon atoms C(ad), which did not interact with each other, will combine with each other and carbon precipitation will occur.

[0023] In addition, as reactors for promoting gas-solid reactions for generating cementite from particles (iron raw materials such as iron oxide), various types such as fixed-bed reactors, moving-bed reactors, and fluidized-bed reactors can be mentioned, but the reactors that can be used for generating cementite are limited.

[0024] That is, in the case of a fixed-bed reactor or a moving-bed reactor, the pressure loss when the particles are filled becomes large, and the gas cannot flow uniformly in the packed bed, resulting in so-called channeling where the gas flows through a specific path, and thus they cannot be used.

[0025] Actually, apart from experiments in the laboratory (experiments where the sample is on the order of grams), in the production of cementite on the bench scale or pilot scale that has been carried out so far, all fluidized-bed reactors have been used, and there are no examples of using fixed-bed reactors or moving-bed reactors for cementite production.

[0026] And in the case of a fluidized-bed reactor, it is possible to control the residence time of the particles, the temperature and the concentration of the carbonizing gas are almost uniform throughout the reactor, and the reaction rate of all the particles in the reactor is almost constant. Therefore, it is possible to remove the particles that have reached a predetermined reaction amount (for example, the reaction amount is 95%) (in other words, it is possible to stop the carbon supply to the particles before carbon precipitation starts).

[0027] In other words, the cementite formation process is a competitive reaction in which the cementite formation reaction and the carbon precipitation reaction occur simultaneously. To produce only cementite, it is necessary to suppress the carbon precipitation reaction. This requires precise control of the reaction process, and in a fluidized bed reactor, by setting operating conditions that suppress the carbon precipitation reaction, these conditions can be achieved throughout the entire reactor, enabling stable cementite formation. Therefore, as mentioned above, fluidized bed reactors have been used for cementite production until now.

[0028] On the other hand, a rotary kiln is a system that uses a horizontal rotary furnace to impart rotational energy to the particles inside the furnace, causing the particles to mix and move within the furnace while the reaction proceeds.

[0029] Rotary kilns are widely used in cement production, and it seems that this is based on the rotary kiln used in cement production. The general understanding is that the temperature distribution and particle reaction rates within the reactor vary greatly from place to place, resulting in a non-uniform structure. Therefore, there are no reported cases of cementite being produced using a rotary kiln.

[0030] However, there is no theoretical basis to suggest that cementite cannot be produced using a rotary kiln.

[0031] Furthermore, because rotary kilns mix particles using mechanical energy, the gas flow rate can be arbitrarily set regardless of the weight or total volume of particles. Compared to fluidized bed reactors, this allows for a wider range of gas flow rate selection and enables significant changes in the ratio of "particle weight" to "gas flow rate."

[0032] Furthermore, a problem specific to rotary kilns is "ring formation" (a phenomenon in which particles partially melt and adhere to the furnace wall in the high-temperature zone, forming a circumferential mass (dam ring)). However, ring formation is caused by the partial melting of particles during high-temperature operation and is not considered to occur in cementite production.

[0033] Now, in a rotary kiln, as shown in Figure 1, the mixing behavior of the particles changes significantly depending on the rotation speed of the rotary furnace. Specifically, this can result in "slipping," "slumping," "rolling," "cascading," "cataracting," and "centrifuging."

[0034] Furthermore, in a rotary kiln, as gas passes through the space, particles on the surface of the particle layer react, and these reacted particles immediately move into the interior of the particle layer. Although some of the gas diffuses into the particle layer, it is only a small amount, and the main reactions in a rotary kiln occur on the surface of the particle layer.

[0035] This means that if the rotation speed of the rotary kiln is increased to a certain extent, the particles present on the surface are replaced in a short amount of time, so the time average of the reaction rate of each particle can be considered to be almost constant. In other words, in a rotary kiln, it can be assumed that the overall reaction rate of particles in the particle layer is approximately constant, which is equivalent to the situation in a fluidized bed reactor where the time average of the reaction rate of each particle is approximately constant.

[0036] Considering these points, when manufacturing cementite, it is important to create a state in which the particles remain on the surface for a predetermined short time. The rotation speed required for this must be such that the "rolling" and "cascading" phenomena shown in Figure 1 occur.

[0037] The conditions under which these "rolling" and "cascading" occur have been experimentally determined by numerous studies to date as functions of the particle size being reacted, the particle packing density, the radius of the rotary kiln, and the rotational speed of the rotary kiln, but the Fr number has the greatest influence. The Fr number is expressed as follows [8], where ω is the rotational speed of the rotary kiln, R is the radius of the rotary kiln, and g is the acceleration due to gravity. Fr=ω 2 R / g···[8]

[0038] Using this Fr number, the range in which rolling and other issues occur in a rotary kiln is classified as shown in Table 1 below. Note that Φ represents the angle of repose ("angle of repose" means "the maximum angle between a slope and a horizontal plane when a deposit of particles is stable and does not collapse"), and Table 1 shows the case where Φ = 35°. However, it should be understood that the values ​​in the classification in Table 1 will vary depending on the particles used, etc., and therefore only indicate a tendency.

[0039] [Table 1] JPEG0007896947000002.jpg54170

[0040] Furthermore, Figure 2 shows the results when variables such as the filling rate are added in addition to the Fr number. From Figure 2, it can be seen that the range in which rolling and other issues occur is determined by the operating conditions. However, the results shown in Figure 2 are not absolute values ​​that can be clearly determined, and it is reasonable to consider them as showing a general trend rather than something that changes drastically. In fact, experimental results like those shown in Figure 3 have also been reported, and results have been reported in which a transition region appears between "Rolling" and "Slumping".

[0041] Thus, it is important to recognize that the "Rolling" and "Cascading" domains are not fixed as absolute values, but rather fluctuate to some extent depending on operating conditions.

[0042] Furthermore, as shown in Figure 4, the particles inside the rotary kiln can be divided into two regions: the "active region" and the "passive region." The majority of the particles reside in the passive region. In this passive region, there is no relative movement between particles or between particles and the walls; in other words, each particle in the passive region is on a fixed orbit, rotating at the same angular velocity as the cylinder.

[0043] As the rotary kiln rotates, the particles enter the active region (indicated by symbols AB in Figure 4) in the upper half of the particle bed. Furthermore, when the particles pass the midpoint of the surface (indicated by a point OB in Figure 4), they begin to enter the inactive region along the line indicated by symbol CB in Figure 4.

[0044] Thus, the distances between the entry and exit points of particles into the inactive region as the rotary kiln rotates are not equidistant. In other words, the motion of particles in the active region is quite random, and therefore, almost all mixing is achieved in the active region.

[0045] When manufacturing cementite using a rotary kiln, it is necessary to achieve uniform mixing of particles in order to ensure that the carbonization reaction of each particle within the rotary kiln proceeds uniformly. To achieve this, it is important to operate the kiln in a state of "rolling" or "cascading" that creates an active region.

[0046] As mentioned above, it is difficult to pinpoint the conditions under which "rolling" or "cascading" occurs using precise numerical values. However, as shown in Figure 2, if the Fr number and bed depth during rotary kiln operation are determined, the region where "rolling" or "cascading" occurs can be roughly determined. Therefore, by performing preliminary operations with varying Fr numbers and bed depths to determine the regions where these phenomena occur, and then proceeding with operations under those conditions, it becomes possible to create an active region, thereby activating the mixing and stirring of particles within the rotary kiln and allowing the reaction to proceed uniformly.

[0047] Based on these findings, in order to achieve the above objective, the present invention provides a method for producing cementite, comprising the steps 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 100 or less into a rotary kiln into which a predetermined iron raw material has been introduced; and heating the iron raw material under heating conditions such that it falls within the region enclosed by the straight line at a temperature of 800°C, the curves for carbon activities of 1 and 100, the phase boundary line between Fe and FeO, and the phase boundary line between Fe and Fe3O4 in the Fe-CO phase stability diagram, and rotating the rotary kiln under rotation conditions such that the mixing behavior of the iron raw material in the rotary kiln is rolling or cascading, thereby suppressing carbon deposition and producing cementite.

[0048] Furthermore, the present invention's method for producing cementite comprises the steps of: supplying a mixed gas containing at least carbon monoxide and carbon dioxide as a first carbonization gas with a carbon activity greater than 1 and 100 or less into a rotary kiln into which iron oxide has been introduced; heating the iron oxide under heating conditions such that it falls within the region enclosed by the straight line at 800°C, the curves for carbon activities of 1 and 100, the phase boundary line between Fe and FeO, and the phase boundary line between Fe and Fe3O4 in the Fe-CO phase stability diagram; and rotating the rotary kiln under rotation conditions such that the mixing behavior of the iron oxide in the rotary kiln is rolling or cascading, thereby reducing the iron oxide to iron The process includes: generating a mixture of at least carbon monoxide and carbon dioxide as a second carbonization gas having a carbon activity greater than 1 and less than or equal to 100; and heating the iron under heating conditions such that it falls within the region enclosed by the straight line at 800°C, the curves for carbon activities of 1 and 100, the phase boundary line between Fe and FeO, and the phase boundary line between Fe and Fe3O4 in the Fe-CO phase stability diagram, and rotating the rotary kiln under rotation conditions such that the mixing behavior of the iron in the rotary kiln is rolling or cascading, thereby suppressing carbon deposition and generating cementite.

[0049] Here, by supplying carbonization gas (a mixed gas containing at least carbon monoxide and carbon dioxide) into the rotary kiln into which the iron raw material is fed, the carbonization gas reacts with the iron raw material as a reducing gas, and cementite is produced.

[0050] Similarly, by supplying a second carbonization gas (a mixed gas containing at least carbon monoxide and carbon dioxide) into the rotary kiln where iron is produced, the second carbonization gas reacts with the iron as a reducing gas to produce cementite.

[0051] Furthermore, by supplying a carbonization gas (a mixed gas containing at least carbon monoxide and carbon dioxide) with a carbon activity greater than 1 and less than or equal to 100, carbon precipitation during the cementite formation process can be suppressed.

[0052] Similarly, by supplying a second carbonization gas (a mixed gas containing at least carbon monoxide and carbon dioxide) with a carbon activity greater than 1 and less than or equal to 100, carbon precipitation during the cementite formation process can be suppressed.

[0053] Furthermore, by heating the "iron raw material" and "produced iron" under heating conditions that fall within the region enclosed by the straight line at 800°C, the curves for carbon activities of 1 and 100, the phase boundary between Fe and FeO, and the phase boundary between Fe and Fe3O4 in the Fe-CO phase stability diagram, and by rotating the rotary kiln under rotation conditions that result in rolling or cascading mixing behavior of the "iron raw material" and "produced iron" within the rotary kiln, carbon deposition is suppressed and cementite is produced, thereby enabling the stable acquisition of cementite over a long period of time.

[0054] Note that the region (the region indicating heating conditions) here includes "the straight line at a temperature of 800°C," "the curve with a carbon activity of 100," "the phase boundary between Fe and FeO," and "the phase boundary between Fe and Fe3O4," but does not include "the curve with a carbon activity of 1."

[0055] Furthermore, the heating conditions here are defined as a region enclosed by a straight line at 800°C, and including the area on the straight line at 800°C; in other words, a region where the temperature is 800°C or lower. This is because if the ambient temperature of the carbonized gas exceeds 800°C, there is a risk that the decomposition of cementite will proceed.

[0056] Furthermore, the heating conditions here are the region enclosed by the "curve where carbon activity is 1," and not included on the curve where carbon activity is 1; in other words, the region where carbon activity is greater than 1. This is because, if the carbon activity is less than 1, the reaction "2CO(g) → C + CO2(g)" in [1] above proceeds to the left (in other words, the reaction "CO(g) → C(ad) + O(ad)" in [2] above proceeds to the left), which reduces the number of adsorbed carbon atoms C(ad) that diffuse into Fe, making it difficult to form cementite.

[0057] Similarly, if the carbon activity is 1, it means that the reaction "2CO(g) → C + CO2(g)" in [1] above reaches equilibrium (in other words, it means that the reaction "CO(g) → C(ad) + O(ad)" in [2] above reaches equilibrium), and the amount of adsorbed carbon atoms C(ad) that diffuse into Fe does not increase, making it difficult to form cementite.

[0058] Furthermore, the heating conditions here are the region enclosed by the "curve for carbon activity of 100" and including the area on the curve for carbon activity of 100, in other words, the region where carbon activity is 100 or less. This is because if the carbon activity exceeds 100, the reaction "2CO(g) → C + CO2(g)" in [1] above proceeds rapidly to the right (in other words, the reaction "CO(g) → C(ad) + O(ad)" in [2] above proceeds rapidly to the right), the amount of adsorbed carbon atoms C(ad) on the Fe surface increases, making it impossible to diffuse the adsorbed carbon atoms C(ad) present on the Fe surface into the Fe, and causing the adsorbed carbon atoms C(ad) to bond with each other and precipitate as carbon. Furthermore, if carbon precipitation occurs, the formation of cementite will be interrupted, and there is a risk that even the cementite that has been formed will decompose.

[0059] Incidentally, while the heating conditions in Patent Document 2 are described as "a region where the carbon activity is 10 or less" (see paragraph number

[0033] of Patent Document 2), the heating conditions of the present invention are described as "a region where the carbon activity is 100 or less," thus expanding the upper limit of the carbon activity. This is because, under rotational conditions where the mixing behavior of the "iron raw material" and "produced iron" in the rotary kiln is rolling or cascading, even if carbon deposition occurs on the particle surface, the deposited carbon is detached from the particle surface due to friction and impact between particles, and is then transported by the high-speed airflow in the rotary kiln and discharged outside the system (see Figure 5).

[0060] Furthermore, the heating conditions here refer to a region enclosed by the "Fe-FeO phase boundary" and the "Fe-Fe3O4 phase boundary," and including the phase boundary between Fe and FeO and the phase boundary between Fe and Fe3O4. In other words, it is the region where Fe is the thermodynamically apparent stable phase. The inventors' investigations revealed that in regions where the carbon activity is greater than 1, carbon or cementite becomes the stable phase, but cementite cannot be formed without going through metallic Fe. Therefore, the heating conditions are defined as "the region where Fe is the thermodynamically apparent stable phase," excluding regions where FeO or Fe3O4 are the thermodynamically apparent stable phases.

[0061] Furthermore, the heating conditions here are in the region enclosed by the "curve with a carbon activity of 100" and the "phase boundary between Fe and Fe3O4," and considering that the intersection of the "curve with a carbon activity of 100" and the "phase boundary between Fe and Fe3O4" is approximately 507°C, the temperature is in the region of approximately 507°C or higher. This is because if the ambient temperature of the carbonization gas falls below approximately 507°C, the reaction will not proceed, and it will take a long time for cementite to form.

[0062] Furthermore, the heating conditions described here do not exclude the "carbon precipitation region, which is the region where carbon precipitation occurs when carbon monoxide and carbon dioxide are used as the carbonizing gas" (in Patent Document 2, the heating conditions are defined as the region enclosed by the "boundary line of the carbon precipitation region," or the region excluding the "carbon precipitation region" and not including the area on the boundary line of the carbon precipitation region). This is the same reason as the aforementioned "reason for the expansion of the upper limit of carbon activity," because under rotational conditions where the mixing behavior of the "iron raw material" and "produced iron" in the rotary kiln is rolling or cascading, even if carbon deposition occurs on the particle surface, the deposited carbon is detached from the particle surface due to friction and impact between particles, and is transported out of the system by the high-speed airflow flowing through the rotary kiln (see Figure 5).

[0063] Furthermore, the rotational condition here is that the mixing behavior of the "iron raw material" and "produced iron" in the rotary kiln is "rolling" or "cascading". This activates the mixing and stirring of particles in the rotary kiln, allowing the reaction to proceed uniformly, and even if the upper limit of carbon activity is 100, carbon precipitation can be suppressed, and cementite can be stably produced over a long period of time.

[0064] Figure 6(a) shows the XRD (X-ray diffraction) analysis results of the product obtained by placing 25 g of Fe2O3 with a particle size of 250-500 μm into a rotary kiln, raising the temperature to 700°C, then introducing a CO-CO2 mixed gas with a carbon activity of 50 at a rate of 2000 ml / min and holding for 120 minutes. The rotary kiln rotation conditions were such that the mixing behavior of Fe2O3 was rolling.

[0065] Figure 6(b) shows the XRD analysis results of the product obtained by adding 25 g of Fe2O3 with a particle size of 250-500 μm to a fluidized bed reactor, raising the temperature to 700°C, then introducing a CO-CO2 mixed gas with a carbon activity of 50 at a rate of 2000 ml / min and holding for 120 minutes.

[0066] As shown in Figure 6(a), carbon precipitation could not be observed when a rotary kiln was used. On the other hand, as shown in Figure 6(b), carbon precipitation could be observed when a fluidized bed reactor was used.

[0067] Here, if the heating conditions are defined as a region enclosed by the straight lines between temperatures of 550°C and 800°C, and which includes the straight line at 550°C, in other words, a region where the temperature is 550°C or higher, cementite can be efficiently produced. This is because the reaction rate decreases significantly below 550°C.

[0068] Furthermore, if the iron raw material is iron oxide, and the heating conditions are enclosed by a straight line at 650°C, and the temperature range includes a region on the straight line at 650°C, in other words, if the temperature range is 650°C or lower, cementite can be stably produced.

[0069] Furthermore, if the raw material is iron, and the heating conditions are such that the region is enclosed by the "curve where the carbon activity is 10" and also includes the region where the carbon activity is 10, in other words, if the region is such that the carbon activity is 10 or greater, then high-purity cementite with almost no FeO content can be synthesized.

[0070] Furthermore, when iron oxide is used as the starting material, and iron oxide is reduced to produce iron, followed by the production of cementite (two-stage reduction method), high-purity cementite can be produced stably.

[0071] Furthermore, if the heating conditions in the process of producing iron using the two-stage reduction method are set to a region enclosed by a straight line between temperatures of 750°C and 800°C, and also by a curve with a carbon activity of 1.5, in other words, if the temperature is 750°C or higher and the carbon activity is 1.5 or higher, iron can be produced with high efficiency.

[0072] Furthermore, if the heating conditions in the process of producing cementite using the two-stage reduction method are set to the region enclosed by the "curve for carbon activity of 10," in other words, if the carbon activity is set to the region of 10 or more, then higher purity cementite can be produced stably. [Effects of the Invention]

[0073] The cementite production method according to the present invention enables the stable production of cementite over a long period of time within a wider range of carbon activity. [Brief explanation of the drawing]

[0074] [Figure 1] This is a schematic diagram illustrating the mixing behavior of particles inside a rotary kiln. [Figure 2] This is a schematic diagram illustrating the relationship between operating conditions and mixed behavior. [Figure 3] This is an example of experimental results regarding the relationship between operating conditions and mixing behavior. [Figure 4] This is a schematic diagram illustrating the active and inactive regions of particles within a rotary kiln. [Figure 5] This is a schematic diagram illustrating how precipitated carbon is expelled from the system. [Figure 6] These graphs show the results of XRD analysis using a rotary kiln and the results of XRD analysis using a fluidized bed reactor. [Figure 7] This is a process diagram relating to the method for producing cementite according to the first embodiment of the present invention. [Figure 8] This is a time-series graph of cementite formation when using a high-carbon activity carbonization gas. [Figure 9] This graph illustrates the heating conditions in the method for producing cementite according to the first embodiment of the present invention. [Figure 10] This is a process diagram relating to a method for producing cementite according to a second embodiment of the present invention. [Figure 11]This is a schematic diagram illustrating the rotary kiln experimental apparatus used in the embodiment of the present invention. [Figure 12] This graph shows the results of the XRD analysis in Example 1. [Figure 13] This graph shows the results of the XRD analysis in Example 2. [Figure 14] This graph shows the results of the XRD analysis in Example 3. [Figure 15] This graph shows the results of the XRD analysis in Example 4. [Figure 16] This graph shows the results of the XRD analysis in Example 5. [Figure 17] Graph (1) shows the results of the XRD analysis in Example 6. [Figure 18] Graph (2) shows the results of the XRD analysis in Example 6. [Figure 19] This graph shows the results of the XRD analysis in Example 7. [Modes for carrying out the invention]

[0075] The following describes embodiments for carrying out the invention (hereinafter referred to as "embodiments") with reference to the drawings, in order to facilitate understanding of the present invention.

[0076] [First Embodiment] Figure 7 shows a process diagram of a cementite manufacturing method according to the first embodiment of the present invention. The cementite manufacturing method according to this embodiment mainly consists of a series of steps, in which first, hematite (Fe2O3) is prepared as iron oxide, which is the iron source, and put into a rotary kiln (step 1). Furthermore, carbonization gas is supplied to the rotary kiln (step 2), the hematite and carbonization gas are heated under predetermined heating conditions (step 3), and the rotary kiln is rotated under predetermined rotation conditions (step 3) to manufacture cementite (Fe3C).

[0077] The iron oxide used as the iron source in step 1 is not limited to hematite, but the inventors' research showed that in the cementite manufacturing process, hematite is most readily available for reduction with carbon gas, making it possible to produce cementite more efficiently.

[0078] In this regard, from the viewpoint of promoting the reduction reaction with carbonized gas, it is preferable that the hematite be in the form of a small particle size powder.

[0079] In step 2, the carbonization gas supplied into the rotary kiln is a mixed gas of carbon monoxide (CO) and carbon dioxide (CO2) in this embodiment of the present invention. CO and CO2 are carbonization gases with low carbon activity (ac=1~100), which allows for the stable formation of cementite without carbon deposition during the reduction reaction with iron oxide.

[0080] To elaborate on carbon activity, for example, when heating is performed under a gas stream of CO at a predetermined concentration, the reaction defined by equation (1) below occurs. 2CO → C + CO2···(1)

[0081] Carbon activity is a value that indicates the tendency for the reaction shown in equation (1) to proceed to the right. For example, if carbon activity ac > 1, the reaction in equation (1) proceeds to the right and carbon precipitates. On the other hand, if carbon activity ac < 1, the reaction in equation (1) proceeds to the left and carbon precipitation decreases. And when carbon activity ac = 1, the reaction in equation (1) reaches equilibrium, and the amount of carbon becomes constant.

[0082] Except for the technology described in Patent Document 2, in conventional cementite production, a carbonization gas with high carbon activity (ac=2000 or more) is often used as a reducing gas in order to accelerate the cementite formation rate. As previously mentioned, in such cases, carbon precipitation inevitably occurs during operation according to equation (1).

[0083] Here, regarding carbon activity and carbon precipitation, the results of a test conducted by the US steel manufacturer Nucor in 1996 are relevant (Figure 8). Figure 8 is a time-series graph showing the change in cementite formation over time in the test plant. Cementite formation reaches almost 100% in about 10 hours from the start of formation, and then the formation rate gradually decreases. On the other hand, the formation rate of iron increases, which is thought to be because the cementite decomposes into iron and carbon.

[0084] On the other hand, the amount of metallic iron gradually increases. Since it is difficult to completely separate the precipitated carbon from 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 through the reaction shown in equation (2) below. Fe3C→Fe+3C···(2)

[0085] As described above, using a carbonized gas with high carbon activity as a reducing gas promotes cementite formation, but carbon precipitation inevitably occurs within a short time from the start of operation, which not only interrupts cementite formation but also risks decomposing the cementite that has already been formed. In contrast, in the embodiment of the present invention, a carbonized gas with low carbon activity is used as a reducing gas, thereby suppressing carbon precipitation and enabling stable cementite formation over a long period of time.

[0086] Here, high-purity CO and CO2 gases may be used as the carbonization gas, but blast furnace exhaust gas containing CO-CO2 may also be used. Table 2 shows an example of the composition of blast furnace exhaust gas. CO gas may be added to this blast furnace exhaust gas, or CO2 may be removed from the blast furnace exhaust gas and used as carbonized gas. Various general methods can be used to selectively separate CO2 from blast furnace exhaust gas, including chemical absorption (amine absorption), physical adsorption (adsorption on zeolite or activated carbon), and membrane separation.

[0087] [Table 2] JPEG0007896947000003.jpg44170

[0088] Next, the heating conditions for step 3 will be explained based on Figure 9. Figure 9(a) is a phase stability diagram of the Fe-CO system showing region B where carbon precipitation occurs when a CO-CO2 mixed gas is used as the carbonization gas. The horizontal axis represents temperature (°C), and the vertical axis represents the percentage of CO in the CO-CO2 mixed gas (%). Figure 9(b) is a magnified view of a portion of Figure 9(a).

[0089] As shown in Figures 9(a) and 9(b), the heating conditions are set so that they fall within the region enclosed by the straight line at 800°C, the curves for carbon activity ac=1.0 and ac=100, the phase boundary between Fe and FeO, and the phase boundary between Fe and Fe3O4 (region A, indicated by the shaded area in Figures 9(a) and 9(b)). More specifically, the CO-CO2 mixing ratio and heating temperature are set such that the carbon activity ac is greater than 1 and in the range of 100 or less, the ambient temperature (reaction temperature) of the carbonized gas is 800°C or less, and it is on the Fe side of the Fe-FeO phase boundary and on the Fe side of the Fe-Fe3O4 phase boundary.

[0090] Furthermore, the shaded region A in Figures 9(a) and 9(b) does not include the line of the "curve for carbon activity ac=1," but it does include the line of the "800°C straight line," the "curve for carbon activity ac=100," the "phase boundary line between Fe and FeO," and the "phase boundary line between Fe and Fe3O4."

[0091] Here, if the ambient temperature of the carbonized gas is below 507°C (as is clear from Figures 9(a) and 9(b), the "curve for carbon activity ac=100" and the "phase boundary line between Fe and Fe3O4" intersect at approximately 507°C. That is, the shaded region A in Figures 9(a) and 9(b) is approximately 507°C or higher), the reduction reaction may not proceed, and therefore, cementite may not be sufficiently produced. Furthermore, if the ambient temperature of the carbonizing gas exceeds 800°C, carbon deposition is more likely to occur, and there is a risk that cementite will decompose into iron and carbon.

[0092] Furthermore, the operating conditions in the embodiments of the present invention include rotation conditions that result in "rolling" and "cascading." These rotation conditions can be achieved by setting predetermined Fr numbers and bed depth conditions. Specifically, as shown in Table 1, the value of Fr is 0.5 × 10 -3 from 0.8 × 10 -1 While keeping the results within a reasonable range, preliminary operations are conducted by varying the Fr number and bed depth to determine the areas where "rolling" or "cascading" occurs, and then operations proceed under those conditions.

[0093] Although the cementite produced by steps 1 to 3 may contain unreacted iron oxide, it is estimated that even if the amount of iron oxide is small (approximately 20% by weight or less), the cementite will still be able to function properly.

[0094] By following the above steps 1 to 3, carbon precipitation can be suppressed, enabling the long-term stable formation of cementite.

[0095] [Second Embodiment] Next, a second embodiment will be described. The second embodiment is one in which wustite (FeO) or magnetite (Fe3O4) is used as the iron oxide source instead of hematite used in the first embodiment. Note that the following description will omit the explanation of parts that are common to the first embodiment.

[0096] The inventors' research revealed that when using wustite or magnetite as iron sources for cementite formation, cementite tends to be less easily formed compared to hematite. This is thought to be largely influenced by factors such as chemical structural stability, hardness, grain size, and the iron content.

[0097] Therefore, as shown in the process diagram of Figure 10, in the second embodiment, which uses wustite or magnetite as an iron source, first these iron sources are oxidized to produce hematite (step 1). The subsequent steps are the same as in the first embodiment, where the hematite produced in step 1 is put into a rotary kiln (step 2), carbonization gas is further put into the rotary kiln (step 3), the hematite and carbonization gas are heated under predetermined heating conditions (step 4), and the rotary kiln is rotated under predetermined rotation conditions (step 4), thereby producing cementite.

[0098] As described above, in the second embodiment, even when using wustite or magnetite as the iron source, carbon precipitation can be suppressed and stable cementite production over a long period of time can be achieved.

[0099] Next, embodiments of the present invention will be described.

[0100] Figure 11 is a schematic diagram illustrating the rotary kiln experimental apparatus used in the embodiment. In the example, 320g of Fe2O particles smaller than 45μm were placed in a rotary kiln and rotated at 0, 50, and 100 rpm while N2 gas was circulated. Subsequently, the temperature was raised to a predetermined temperature (650-740°C) using a tubular furnace. After reaching the predetermined temperature, a CO-CO2 mixed gas, controlled by a mass flow meter to have a predetermined carbon activity (ac=3.0-50.0), was introduced at a rate of 1 L / min, and a heating reduction treatment was performed for 120 minutes. After processing, the sample was cooled to 200°C by air cooling, and then cooled to room temperature under an N2 gas atmosphere. After cooling to room temperature, the sample was removed and the product was analyzed by X-ray diffraction (XRD).

[0101] [Example 1]: Effect of rotation In Example 1, the effect of rotation speed on the reduction behavior was investigated under three conditions: "0 rpm with no stirring at all," "50 rpm resulting in a rolling state," and "100 rpm resulting in stronger stirring and a cascading state." Other conditions were carbon activity ac=20, holding temperature 650°C, and holding time 120 minutes.

[0102] Figure 12 shows the results of the XRD analysis of the product obtained by the reduction reaction.

[0103] As shown in Figure 12, although cementite formation was observed at 0 rpm, the peaks for FeO and metallic Fe remained prominently, indicating that the reduction reaction was insufficient.

[0104] On the other hand, at 50 rpm (rolling state) and 100 rpm (cascading state), the peak states of FeO and metallic Fe were significantly reduced compared to 0 rpm, confirming that a more uniform reduction reaction was proceeding.

[0105] Under these conditions, the rotation facilitated mixing of the sample, improving the renewal of the reaction interface and enhancing heat and mass transfer, which likely allowed the reduction reaction to proceed efficiently throughout the entire sample. Furthermore, while weight loss was similar at 50 rpm and 100 rpm compared to 0 rpm, XRD analysis revealed clear differences in the rate and uniformity of the reaction.

[0106] Furthermore, comparing 50 rpm and 100 rpm, no significant difference was observed in the product composition despite the stronger stirring at 100 rpm. This suggests that, under these experimental conditions, a rotation speed of around 50 rpm already provides sufficient stirring to mix the sample and promote the reaction. Therefore, excessive increases in rotation speed are not necessarily effective in improving the reduction behavior.

[0107] Based on the results above, in the experiments from Example 2 onward, the rotation speed was fixed at 50 rpm, and cementite formation was investigated by changing other conditions.

[0108] [Example 2]: Effect of temperature Based on the results of the study in Example 1 (i.e., the results of the rotation speed study), in Example 2, the rotation speed was fixed at 50 rpm, and the holding temperature was set to 650°C, 680°C, 700°C, 720°C, and 740°C to investigate the effect of the holding temperature on the reduction behavior. Other conditions were carbon activity AC = 20 and holding time 120 minutes.

[0109] At 650°C, 680°C, and 700°C, some sample sintering was observed after the reaction, but overall, a good state of agitation was maintained throughout the holding time. On the other hand, at 720°C and 740°C, although good stirring was observed in the initial stages of heating, significant sintering of the sample occurred approximately 60 minutes after the start of heating, and sufficient mixing became impossible due to the agglomeration of the sample. This progression of sintering behavior is considered to be a factor that affects the uniformity of the reaction and the resulting phase.

[0110] Figure 13 shows the results of the XRD analysis of the product obtained by the reduction reaction.

[0111] As shown in Figure 13, at 650°C, a product containing a mixture of FeO, cementite, and metallic Fe was obtained. Furthermore, as the holding temperature increased to 680°C, 700°C, 720°C, and 740°C, a tendency was observed for the peak intensities of FeO and cementite to decrease, while the peak intensity of metallic Fe to increase.

[0112] This suggests that the reduction reaction progresses with increasing temperature, and the resulting phase shifts to one primarily composed of Fe. These results are thought to be due to the thermodynamic instability of cementite in the high-temperature range (approximately 750°C to 850°C), and its tendency to decompose into Fe and C as carbon precipitates. In addition, it is possible that the effective reaction surface area decreases due to the progression of sintering of the sample under high-temperature conditions, suppressing the cementite formation reaction.

[0113] Based on these results, we determined that 650°C is the optimal holding temperature for stably and efficiently generating cementite under these experimental conditions, as it minimizes the effects of particle sintering and allows cementite to exist in a relatively stable state.

[0114] [Example 3]: Effect of activity Based on the results of the study in Example 1 (i.e., the results of the study on rotational speed) and the results of the study in Example 2 (i.e., the results of the study on holding temperature), we investigated the reduction behavior and the effect on the product phase when the rotational speed was fixed at 50 rpm and the holding temperature at 650°C, and the carbon activity AC was changed to 3, 5, 10, 20, and 50.

[0115] Figure 14 shows the results of the XRD analysis of the product obtained by the reduction reaction.

[0116] As shown in Figure 14, at a carbon activity of ac=3, mainly FeO was produced, with only a small amount of metallic Fe being detected. At a carbon activity of ac=5, the amount of metallic Fe produced increased, but cementite formation was not observed. On the other hand, at a carbon activity of ac=10, a product containing a mixture of FeO, Fe, and cementite was obtained, confirming that the carburizing reaction proceeded simultaneously with the reduction reaction. Furthermore, at carbon activities of ac=20 and ac=50, products containing FeO, Fe, and cementite were also obtained, but no significant difference in the composition of the resulting phases was observed. In addition, no carbon precipitation was observed at a carbon activity of ac=50.

[0117] [Example 4]: Effect of activity when iron is used as the starting material In Examples 1 to 3, Fe2O3 was used as the starting material, but in all cases, small amounts of Fe and FeO were present. Therefore, in order to investigate the possibility of producing cementite without these elements, we investigated cementite formation using metallic iron powder instead of Fe2O3 as the starting material.

[0118] In reality, even if small amounts of Fe or FeO are present, when added to molten steel, the Fe dissolves as is, and the FeO reacts with the carbon produced by cementite decomposition to form CO microbubbles, which are then removed from the molten steel, so it does not pose a problem.

[0119] In Example 4, 20 g of electrolytic iron powder classified to 40 μm or less was used as the raw material. Under conditions of a rotation speed of 50 rpm, a holding temperature of 650°C, and a holding time of 120 minutes, the effect of carbon activity ac on the resulting phase was investigated, with Fe as the starting material.

[0120] Figure 15 shows the results of the XRD analysis of the product obtained by the reduction reaction.

[0121] As shown in Figure 15, at a carbon activity of ac=3, a small amount of FeO and cementite formation was observed, but a large amount of metallic Fe remained, indicating that the carburizing reaction did not proceed sufficiently. On the other hand, at carbon activities of ac=10 and ac=20, almost single-phase cementite was obtained. Furthermore, under these conditions, no peaks attributable to carbon precipitation were observed, indicating that stable cementite formation without carbon precipitation was achieved.

[0122] Based on these results, when Fe is used as the starting material, cementite was produced under the condition of carbon activity ac > 10. This means that, unlike when Fe2O3 is used as the starting material, it is possible to synthesize high-purity cementite that contains almost no FeO.

[0123] [Example 5]: Effect of sample weight In Example 4, 20 g of sample was used. In Example 5, the case where the sample amount was increased to 50 g was investigated. The experimental conditions were the same as in the 20 g case, with the carbon activity ac = 20.

[0124] The weight of a 50g sample due to carbonization is 52.3737g. Based on the stoichiometric composition of cementite, the theoretical weight if all 50g of Fe were converted into cementite would be approximately 53.6g. Compared to this theoretical value, the weight obtained in Example 5 was slightly smaller, but the measured value was close to the theoretical value, suggesting that the majority of the sample was converted into cementite.

[0125] Figure 16 shows the results of the XRD analysis of the product obtained by the reduction reaction.

[0126] As shown in Figure 16, although slight peaks of FeO and metallic Fe were observed, similar to the case of the 20g sample, the main phase of the product was cementite, and no peaks attributable to carbon precipitation were observed. This result is in good agreement with the degree of carburization reaction estimated from the weight change, indicating that stable cementite formation is possible even when the sample amount is increased to 50g, without excessive carburization or carbon precipitation.

[0127] [Example 6] The results of Example 5 confirmed that using Fe as the starting material facilitates cementite formation, but also revealed some challenges. Multiple experiments were conducted to confirm reproducibility, and it was frequently observed that the sample sintered and adhered along the inner wall of the reaction tube. This adhesion prevented sufficient stirring, resulting in cases where cementite containing almost no FeO was not formed.

[0128] Furthermore, from an industrial perspective, using Fe2O3 as the starting material is more cost-effective than using Fe as the starting material. Therefore, cementite was produced using a two-stage reduction method in which Fe2O3 is first reduced entirely to Fe, and then converted to cementite.

[0129] In the first reduction stage, 320g of Fe2O particles smaller than 45μm were placed in the reaction tube and rotated at 50rpm. The temperature was raised to a predetermined temperature (700°C to 750°C) using a tubular furnace while flowing N2 gas. After reaching the predetermined temperature, a CO-CO2 mixed gas, controlled by a mass flow meter to achieve carbon activity ac = 1.5, 3, and 5, was introduced at 1 L / min. Under each condition, the mixture was heated for 120 minutes, then air-cooled, and the resulting product was analyzed by XRD.

[0130] At a holding temperature of 700°C, even with a high carbon activity, cementite was formed before FeO was completely reduced, resulting in a mixture of FeO and cementite. Therefore, with the aim of completely reducing FeO to Fe, the experiment was conducted with the holding temperature set to 750°C.

[0131] The weight change was from 20.05g to 14.360g at a carbon activity of AC=1, and from 20.3g to 14.6142g at a carbon activity of AC=1.5, confirming a weight decrease.

[0132] Figure 17 shows the results of the XRD analysis of the product obtained by the reduction reaction.

[0133] As shown in Figure 17, at 750°C and a carbon activity of ac=1, a small amount of FeO remained. However, at a carbon activity of ac=1.5, all FeO was reduced to Fe, and no cementite formation was observed.

[0134] Based on these results, the optimal conditions for the first stage of Fe production were determined to be a holding temperature of 750°C and a carbon activity of ac = 1.5.

[0135] Based on the results of the first stage described above, it was shown that 320g of Fe2O can be completely reduced to Fe under the conditions of a holding temperature of 750°C, a carbon activity ac = 1.5, and a holding time of 120 minutes. Furthermore, based on the cementite formation experiment results using iron powder (Fe) as the starting material shown in Example 4, the conditions for the carbon activity ac were investigated with a holding temperature of 650°C in the second stage.

[0136] As described above, 320g of Fe2O particles smaller than 45μm were placed in a reaction tube, and N2 gas was circulated while rotating at 50 rpm, raising the temperature to 750°C. After reaching 750°C, a CO-CO2 mixed gas with a controlled carbon activity ac=1.5 was introduced at 1 L / min and held for 120 minutes to convert all of the 320g of Fe2O into metallic iron.

[0137] Subsequently, the gas was switched back to N2 and the temperature was lowered to 650°C. After reaching 650°C, a CO-CO2 mixed gas, controlled to have carbon activities ac=3, 5, 10, and 20, was introduced at a rate of 1 L / min while rotating at 50 rpm, and the mixture was heated for 120 minutes. After the treatment, the mixture was air-cooled to 200°C, then switched back to an N2 gas atmosphere and cooled to room temperature, and the product was analyzed by XRD.

[0138] Figure 18 shows the results of the XRD analysis of the product obtained by the reduction reaction.

[0139] As shown in Figure 18, at a carbon activity of ac=3, only a small amount of cementite was formed, and the majority of the product was metallic Fe, indicating an insufficient carburizing reaction. On the other hand, at carbon activities of ac=5, 10, and 20, only a small amount of FeO remained under all conditions, and the majority of the product consisted of cementite.

[0140] These results clearly indicate that higher purity cementite is obtained compared to when attempting to produce cementite in a single step using Fe2O3 as the starting material. Furthermore, the detection of trace amounts of Fe at carbon activity ac=20 is presumed to be due to carbon precipitation occurring under high carbon activity conditions, which in turn caused some of the cementite to decompose. The slightly higher weight compared to other carbon activity levels ac=5 and 10 is also thought to be due to carbon precipitation.

[0141] Based on these results, by employing a two-stage reduction method (two-step reduction process) in which Fe2O3 is completely reduced to Fe in the first stage and a carburizing reaction with controlled carbon activity is carried out in the second stage, weight changes and a phase composition mainly composed of cementite can be obtained that are close to the theoretical values, and high-purity cementite can be stably produced.

[0142] [Example 7]: Effect of retention time The results of Example 6 demonstrated that high-purity cementite can be produced by reducing Fe2O3 in two steps. Furthermore, although the second reduction step was performed for 120 minutes, considering the progress of the reaction, it is possible that similar products could be obtained in a shorter time. Under the same temperature and carbon activity conditions as in Example 6, the second-stage reduction treatment time was shortened to 60 minutes, and the effects of this were investigated.

[0143] Figure 19 shows the results of the XRD analysis of the product obtained by the reduction reaction.

[0144] As shown in Figure 19, cementite was formed at a carbon activity of ac=5, but small amounts of FeO and Fe remained. Comparing the results for 60 minutes and 120 minutes shown in Figure 18, the Fe peak was smaller at 120 minutes, indicating that higher purity cementite was obtained. Therefore, it was determined that 120 minutes is the optimal reduction treatment time for a carbon activity of ac=5.

[0145] On the other hand, at a carbon activity of ac=10, no Fe was detected even after 60 minutes, with only a small amount of FeO remaining. Since no significant difference was observed when compared with the results for 120 minutes at a carbon activity of ac=10 in Figure 18, it can be concluded that a reduction treatment time of 60 minutes is sufficient.

[0146] Furthermore, at a carbon activity of ac=20, the formation of Fe and carbon was suppressed at 60 minutes, yielding better results than at 120 minutes. Since the formation of Fe and C was confirmed at 120 minutes, it is thought that carbon precipitation progressed with longer treatment times, and cementite decomposed as a result.

Claims

1. A step 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 100 or less into a rotary kiln into which a predetermined iron raw material has been introduced, In the phase stability diagram of the Fe-C-O system, the straight line at a temperature of 800°C, the curves for carbon activity of 1 and 100, the phase boundary between Fe and FeO, and Fe and Fe ３ O ４ The process includes heating the iron raw material under heating conditions such that it falls within a region enclosed by the phase boundary line, and rotating the rotary kiln under rotation conditions such that the mixing behavior of the iron raw material in the rotary kiln is Rolling or Cascading, thereby suppressing carbon deposition and generating cementite. A method for producing cementite.

2. The process for producing the cementite is as follows: In the phase stability diagram of the Fe-C-O system, the straight lines at temperatures of 550°C and 800°C, the curves for carbon activity of 1 and 100, the phase boundary between Fe and FeO, and Fe and Fe ３ O ４ The iron raw material is heated under heating conditions such that it falls within the region enclosed by the phase boundary line. A method for producing cementite according to claim 1.

3. The aforementioned iron raw material is iron oxide, The process for producing the cementite is as follows: In the phase stability diagram of the Fe-C-O system, the straight line at a temperature of 650°C, the curves for carbon activity of 1 and 100, the phase boundary between Fe and FeO, and Fe and Fe ３ O ４ The iron oxide is heated under heating conditions such that it falls within the region enclosed by the phase boundary line. A method for producing cementite according to claim 1.

4. The aforementioned iron raw material is iron, The carbonized gas has a carbon activity of 10 or more. The process for producing the cementite is as follows: In the phase stability diagram of the Fe-C-O system, the straight line at a temperature of 800°C, the curves for carbon activity of 10 and 100, the phase boundary between Fe and FeO, and Fe and Fe ３ O ４ The iron is heated under heating conditions such that it falls within the region enclosed by the phase boundary line. A method for producing cementite according to claim 1.

5. A step of supplying a mixed gas containing at least carbon monoxide and carbon dioxide as a first carbonization gas having a carbon activity greater than 1 and 100 or less into a rotary kiln into which iron oxide has been introduced, In the Fe—C—O phase stability diagram, a straight line at a temperature of 800° C., a curve with a carbon activity of 1 and 100, a phase boundary line between Fe and FeO, and a phase boundary line between Fe and Fe ３ O ４ heating the iron oxide under heating conditions such that it falls within the region surrounded by the phase boundary line, and rotating the rotary kiln under rotational conditions such that the mixing behavior of the iron oxide in the rotary kiln becomes Rolling or Cascading, thereby reducing the iron oxide to produce iron; A step of supplying a mixed gas containing at least carbon monoxide and carbon dioxide as a second carbonization gas having a carbon activity greater than 1 and 100 or less into a rotary kiln where iron is produced, In the phase stability diagram of the Fe-C-O system, the straight line at a temperature of 800°C, the curves for carbon activity of 1 and 100, the phase boundary between Fe and FeO, and Fe and Fe ３ O ４ The process includes heating the iron under heating conditions such that it falls within a region enclosed by the phase boundary line, and rotating the rotary kiln under rotation conditions such that the mixing behavior of the iron in the rotary kiln is Rolling or Cascading, thereby suppressing carbon deposition and generating cementite. A method for producing cementite.

6. The first carbonized gas has a carbon activity greater than 1.

5. The process for producing the aforementioned iron is: The iron oxide is heated under heating conditions such that, in the phase stability diagram of the Fe-C-O system, the temperature falls within the region enclosed by the straight line between 750°C and 800°C, and the curves between carbon activities of 1.5 and 100. The method for producing cementite according to claim 5.

7. The second carbonized gas has a carbon activity greater than 5. The process for producing the cementite is as follows: In the phase stability diagram of the Fe-C-O system, the straight line at a temperature of 800°C, the curves for carbon activity values ​​of 5 and 100, the phase boundary between Fe and FeO, and Fe and Fe ３ O ４ The iron is heated under heating conditions such that it falls within the region enclosed by the phase boundary line. The method for producing cementite according to claim 5.

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