Sponge iron manufacturing method

Abstract

The present invention provides a method for producing sponge iron with higher efficiency by increasing the rate of carbon monoxide production required for the reduction of iron oxide powder compared to the prior art. The method involves charging iron oxide powder and carbonaceous reducing powder into a vessel, heating the vessel, and reducing the iron oxide powder to produce sponge iron. α A method for producing sponge iron, characterized in that powder X-ray diffraction is performed using X-rays, and the crystallite size L calculated by Scherrer's formula (L = Kλ / (B cosθ)) based on the diffraction peak corresponding to the (002) diffraction plane of graphite crystals in the obtained diffraction pattern is 0.01 nm or more and 25.0 nm or less, where K is the Scherrer constant 0.9 and λ is CuK α where B is the half-width (rad) of the diffraction peak, and θ is the Bragg angle (degrees) corresponding to the (002) diffraction plane of the graphite crystal.

Description

[Technical Field]

[0001] The present invention relates to a method for producing sponge iron by reducing iron oxide powder with a carbonaceous reducing powder. [Background technology]

[0002] Sponge iron is a porous iron obtained by reducing iron oxide powder. The reduction of iron oxide powder is carried out by heating the iron oxide powder and a reducing agent placed in a container to a temperature lower than the melting point of iron. Carbonaceous reducing powder is usually used as the reducing agent. Reduced iron powder can be produced by crushing the sponge iron obtained by reducing iron oxide powder. Reduced iron powder is mainly used as a raw material powder for sintered parts such as machine parts and magnetic materials.

[0003] 1 and 2 are cross-sectional views schematically showing an example of the arrangement of raw material powder in a vessel used for producing sponge iron. FIG. 1 is a vertical cross-sectional view taken from the side of the cross-section taken at position BB' in FIG. 2. FIG. 2 is a horizontal cross-sectional view taken from directly above the cross-section taken at position AA' in FIG. 1. In the example of the arrangement of raw material powder shown in FIGS. 1 and 2, iron oxide powder 2 is arranged in a cylindrically-shaped container 1 with a bottom. Carbonaceous reduced powder 3 is packed tightly around the iron oxide powder 2.

[0004] When the container 1 shown in Figures 1 and 2 is heated from the outside, a redox reaction occurs, accompanied by the transfer of gas between the carbonaceous reducing powder 3 and the iron oxide powder 2. As a result, the oxygen contained in the iron oxide combines with carbon monoxide, chemically converting to carbon dioxide and then escaping, producing iron particles. When the iron particles are kept at a temperature lower than the melting point, they form sintered necks at the contact points with neighboring particles, bonding them three-dimensionally. At the final stage of the redox reaction, the iron particles become porous sponge iron, sintered into a cylindrical shape similar to the arrangement of the iron oxide powder 2 shown in Figures 1 and 2.

[0005] In the above-mentioned production method, the rate at which sponge iron is produced is determined by the oxidation-reduction reaction accompanying gas transfer. For this reason, methods for increasing the rate of the oxidation-reduction reaction have been investigated. For example, Patent Document 1 describes an invention for a method for producing sponge iron, characterized in that iron oxide powder and carbonaceous reduced powder are alternately packed into a container so that they are helically deposited. The raw material powder arrangement in this production method reduces the distance between the iron oxide powder and the carbonaceous reduced powder in the container compared to the raw material powder arrangement shown in Figures 1 and 2. This shortens both the time required for carbon monoxide generated in the carbonaceous reduced powder to travel to the iron oxide powder and the time required for carbon dioxide generated by the reduction of iron oxide to travel to the carbonaceous reduced powder, thereby increasing the rate of the oxidation-reduction reaction. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-250789 Summary of the Invention [Problem to be solved by the invention]

[0007] The method for producing sponge iron described in Patent Document 1 is an excellent method in that it can shorten the time required for gas to move by diffusion between the iron oxide powder and the carbonaceous reducing powder. However, the conventional technology still has room for improvement in the rate of carbon monoxide production required for the reduction of the iron oxide powder, and there is a need for further improvement in the rate of the oxidation-reduction reaction.

[0008] The present invention has been made in view of the above-mentioned problems, and aims to produce sponge iron more efficiently by increasing the rate of carbon monoxide production required for the reduction of iron oxide powder compared to the prior art. [Means for solving the problem]

[0009] In order to solve the above problems, the present inventors have investigated from various perspectives what kind of carbonaceous reduced powder can produce or regenerate carbon monoxide more efficiently than conventional techniques. As a result of their investigations, they have found that by adjusting the crystallite size contained in the carbonaceous reduced powder to fall within a predetermined range, the rate of carbon monoxide production can be significantly increased, and as a result, the production efficiency of sponge iron can be dramatically improved.

[0010] The present invention has been completed based on the above findings, and has the following gist and configuration.

[0011] [1] A method for producing sponge iron by charging iron oxide powder and carbonaceous reduced powder into a container and heating the iron oxide powder to reduce the iron oxide powder, Regarding the carbonaceous reduced powder, CuK α A method for producing sponge iron, characterized in that powder X-ray diffraction is performed using X-rays, and the crystallite size L calculated by the following mathematical formula (1) based on the diffraction peak corresponding to the (002) diffraction plane of graphite crystals in the obtained diffraction pattern is 0.01 nm or more and 25.0 nm or less.

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[0012] [2] The method for producing sponge iron according to [1] above, wherein the crystallite size L is 0.05 nm or more and 20.0 nm or less. [Effects of the Invention]

[0013] According to the present invention, sponge iron can be produced more efficiently than in the prior art by increasing the rate of carbon monoxide production required for the reduction of iron oxide powder. [Brief explanation of the drawings]

[0014] [Figure 1] 3 is a vertical cross-sectional view schematically showing an example of the arrangement of raw material powder in a container. FIG. [Figure 2] 3 is a horizontal cross-sectional view schematically showing an example of the arrangement of raw material powder in a container. FIG. [Figure 3] 1 is a graph showing the relationship between the crystallite size of the carbonaceous reduced powder and the reduction rate of the reduced iron powder. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, embodiments of the present invention will be described in detail. In the following description, unless otherwise specified, "%" refers to mass percentage.

[0016] In one embodiment, the present invention provides a method for producing sponge iron by charging iron oxide powder and carbonaceous reducing powder into a container and heating the container to reduce the iron oxide powder, the method comprising: α The invention relates to a method for producing sponge iron, characterized in that powder X-ray diffraction is performed using X-rays, and the crystallite size L calculated by the following mathematical formula (1) based on the diffraction peak corresponding to the (002) diffraction plane of graphite crystals in the obtained diffraction pattern is 0.01 nm or more and 25.0 nm or less.

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[0017] [Iron oxide powder] The iron oxide powder used in the method for producing sponge iron according to this embodiment may be any powder containing iron oxide. The iron oxide powder, together with the carbonaceous reduced powder described below, constitutes the raw material powder that is the raw material for sponge iron.

[0018] The type of iron oxide contained in the iron oxide powder is not particularly limited. The iron oxide powder may contain one or more iron oxides selected from iron oxide (II), iron oxide (III), and iron oxide (II, III) in any proportion. The proportion of impurities other than iron oxide contained in the iron oxide powder is preferably as low as possible.

[0019] Specific raw material powders that can be used as the iron oxide powder in the sponge iron manufacturing method according to this embodiment include, but are not limited to, powder obtained by crushing natural iron ore, natural iron sand, iron oxide powders commercially available as reagents or industrial raw materials, mill scale and other iron oxide powders generated on steelworks production lines, and iron powders contained in disposable hand warmers and oxygen absorbers that have oxidized during use. The proportion of iron oxide contained in the iron oxide powder is preferably 50% or more, more preferably 65% ​​or more, and even more preferably 80% or more. There is no particular upper limit to the proportion of iron oxide contained in the iron oxide powder, and it may be 100%.

[0020] "Mill scale" refers to an oxide film formed on the surface of a steel material by oxidation with oxygen in the air during the production of hot-rolled steel sheet. Mill scale is usually removed from the surface of hot-rolled steel sheet by means of pickling, shot blasting, or the like. The mill scale removed and recovered from the surface of the hot-rolled steel sheet is a flaky powder composed of iron (II) oxide, iron (III) oxide, and a composite oxide containing iron (II, III) oxide. Mill scale contains fewer impurities than natural iron ore. For this reason, when attempting to produce high-purity sponge iron, it is preferable to use mill scale as iron oxide powder.

[0021] The particle size of the iron oxide powder is not particularly limited. However, if the particle size of the iron oxide powder is too small, it may be difficult for the gas required for the oxidation-reduction reaction to pass through the iron oxide powder, or it may be difficult to pulverize the sponge iron. Therefore, it is preferable that the particle size of the iron oxide powder be 10 μm or more in terms of D50, which is the median value in the volumetric particle size distribution of the powder. On the other hand, if the particle size of the iron oxide powder is too large, it may take a long time to reduce the iron oxide powder to its center, or the particle size of the reduced iron powder obtained from the sponge iron may be too large and difficult to handle. Therefore, it is preferable that the particle size of the iron oxide powder be 10 mm or less in terms of D50.

[0022] The D50 of iron oxide powder is measured by the following method. First, the raw material powder to be measured is placed in a solvent. Ethanol, for example, can be used as the solvent. Next, ultrasonic vibrations are applied to the solvent containing the raw material powder for 30 seconds or more to disperse the iron oxide powder in the solvent. Next, the volumetric particle size frequency distribution of the iron oxide powder dispersed in the solvent is measured using a laser diffraction particle size distribution analyzer. Finally, the particle size corresponding to 50 vol% of the total volume of all particles in the cumulative distribution calculated from the obtained particle size frequency distribution is taken as the measured value of D50.

[0023] [Carbonaceous reduced powder] The carbonaceous reducing powder used in the sponge iron manufacturing method according to this embodiment may be any powder containing carbon and having graphite crystallites with a certain range of sizes. In the sponge iron manufacturing method according to the present invention, the carbonaceous reducing powder functions as a carbon source that combines with oxygen in the atmosphere to produce carbon dioxide, and as a reducing agent that reduces the produced carbon dioxide to produce carbon monoxide. The carbon monoxide is used to reduce the iron oxide powder. This series of chemical reactions will be described in detail below.

[0024] 1 and 2 is heated from the outside, the carbonaceous reducing powder 3 is first combusted by the oxygen contained in the air present in the gaps between the carbonaceous reducing powder 3, and carbon dioxide is produced by the chemical reaction shown in chemical formula (1). In all chemical formulas shown below, the symbol (s) indicates that the substance represented by the molecular formula with this symbol is in a solid phase, and the symbol (g) indicates that the substance represented by the molecular formula with this symbol is in a gas phase.

[0025] [ka]

[0026] The produced carbon dioxide is reduced by the carbon contained in the carbonaceous reducing powder 3 and chemically converted into carbon monoxide by the chemical reaction shown in chemical formula (2).

[0027] [ka]

[0028] When oxygen is insufficient or the temperature is high, carbon monoxide is produced directly from the carbonaceous reduced powder through the chemical reaction shown in chemical formula (3).

[0029] [ka]

[0030] Carbon monoxide produced by the chemical reaction shown in chemical formula (2) or (3) reaches the position of iron oxide powder 2 by diffusion and reduces the iron oxide by the chemical reaction shown in chemical formula (4). Reduction of iron oxide produces solid-phase iron and carbon dioxide. Here, chemical formula (4) is a chemical formula showing the reduction reaction when the iron oxide is iron (II, III) oxide. In the reduction reaction when the iron oxide is iron (II) oxide or iron (III) oxide, solid-phase iron and carbon dioxide are produced by a chemical reaction similar to the reduction reaction shown in chemical formula (4).

[0031] [ka]

[0032] Carbon dioxide generated at the position of iron oxide powder 2 diffuses to the position of carbonaceous reducing powder 3, where it is reduced to carbon monoxide through the chemical reaction shown in chemical formula (2). The generated carbon monoxide then reaches the position of iron oxide powder 2 and reduces iron oxide through the chemical reaction shown in chemical formula (4). Through this series of oxidation-reduction reactions involving gas movement, the iron oxide powder is reduced to produce porous sponge iron.

[0033] Specific raw material powders that can be used as the carbonaceous reduced powder in the method for producing sponge iron according to this embodiment include, but are not limited to, coal powder, coke powder obtained by carbonizing coal powder to increase the carbon content, and biomass powder obtained by carbonizing plant-derived biomass to increase the carbon content. The carbon content of the carbonaceous reduced powder used in the method according to the present invention is preferably 50% or more, more preferably 70% or more, and even more preferably 90% or more. There is no particular upper limit to the carbon content of the carbonaceous reduced powder, and it may be 100%.

[0034] In the sponge iron manufacturing method according to this embodiment, a carbonaceous reduced powder having a crystallite size L of 0.01 nm or more and 25.0 nm or less is used. A "crystallite" is the smallest unit that contributes to X-ray diffraction, and refers to a region in the carbonaceous reduced powder that can be regarded as a single crystal of graphite. Furthermore, "crystallite size" refers to the crystallite size determined by powder X-ray diffraction. When the crystallite size L of the carbonaceous reduced powder is within the above range, the reduction rate of the sponge iron is higher than when it is outside the above range. The method for determining the crystallite size L of the carbonaceous reduced powder will be described later.

[0035] Although the details of why the crystallite size L of the carbonaceous reduced powder affects the reduction rate of sponge iron are unclear, the inventors believe the following. When the crystallite size L is 0.01 nm or more and 25.0 nm or less, the carbonaceous reduced powder with such a crystallite size contains many grain boundaries. Furthermore, the crystallinity is low and the carbon atoms contain irregularities. Therefore, at low temperatures, the bonds between carbon atoms are easily broken, and the carbon atoms easily combine with surrounding oxygen to generate carbon dioxide through the chemical reaction shown in chemical formula (1). The generated carbon dioxide easily combines with easily broken carbon atoms and is chemically converted to carbon monoxide through the chemical reaction shown in chemical formula (2). Furthermore, when oxygen is insufficient, carbon monoxide is easily generated directly through the chemical reaction shown in chemical formula (3). These combined effects are thought to improve the reduction rate of iron oxide.

[0036] However, if the crystallite size L is too small, crystallization is insufficient, and many of the carbon atoms in the carbonaceous reducing powder are in an amorphous state with weak bonds. When such carbonaceous reducing powder and iron oxide powder are placed in a container and heated, a large amount of carbon monoxide is generated from the carbonaceous reducing powder at the beginning of the heating process due to the chemical reactions shown in chemical formulas (1) to (3). However, at the beginning of the heating process, the temperature of the iron oxide powder has not yet risen sufficiently. In this case, the carbon monoxide is released outside the container without being used in the reduction of the iron oxide powder shown in chemical formula (4), resulting in a decrease in the reduction rate of the sponge iron. Therefore, the crystallite size L is set to 0.01 nm or more, preferably 0.05 nm or more, more preferably 0.30 nm or more, and even more preferably 1.00 nm or more.

[0037] On the other hand, if the crystallite size L is too large, the carbonaceous reduced powder undergoes graphitization during grain growth, resulting in a highly crystalline graphite single crystal. In this case, the carbon atoms are regularly arranged and the bonds between the carbon atoms are strong, suppressing the generation of carbon dioxide (represented by chemical formula (1)) and carbon monoxide (represented by chemical formulas (2) and (3)), thereby reducing the reduction rate of the sponge iron. Therefore, the crystallite size L is set to 25.0 nm or less, preferably 20.0 nm or less, more preferably 10.0 nm or less, and even more preferably 3.00 nm or less.

[0038] The crystallite size L of the carbonaceous reduced powder can be adjusted to a range of 0.01 nm to 25.0 nm by, for example, controlling the carbonization conditions when carbonizing coal or biomass to produce the carbonaceous reduced powder. The crystallite size L increases as the carbonization time at a constant temperature increases or as the carbonization temperature increases for a constant time.

[0039] The particle size of the carbonaceous reduced powder used in the method for producing sponge iron according to this embodiment is not particularly limited because, unlike the crystallite size L, it has little effect on the reduction rate of the sponge iron. However, if the particle size of the carbonaceous reduced powder is too small, it may be difficult for carbon monoxide and carbon dioxide to pass through the portion consisting of the carbonaceous reduced powder. Therefore, the particle size of the carbonaceous reduced powder is preferably 5.0 μm or more in D50. On the other hand, if the particle size of the carbonaceous reduced powder is too large, it may be difficult to handle the carbonaceous reduced powder. Therefore, the particle size of the carbonaceous reduced powder is preferably 5.0 mm or less in D50, and more preferably 3.0 mm or less.

[0040] [How to determine the crystallite size L of reduced carbonaceous powder] In the method for producing sponge iron according to this embodiment, the carbonaceous reduced powder is CuK α Powder X-ray diffraction is performed using X-rays, and the crystallite size L is calculated using the following mathematical formula (1) based on the diffraction peak corresponding to the (002) diffraction plane of the graphite crystal in the obtained diffraction pattern.

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[0041] The above equation (1) is known as the Scherrer equation, which shows the relationship between the half-width of a diffraction peak and the size of the crystallite. As shown in equation (1), when K, λ, and θ are constant, the half-width B of the diffraction peak is inversely proportional to the crystallite size L. In other words, the larger L, the sharper the diffraction peak and the smaller the half-width B. Conversely, the smaller L, the broader the diffraction peak and the larger the half-width B.

[0042] The reason why the relationship between the crystallite size L and the half-width B shown in the Scherrer formula holds is thought to be as follows. According to Bragg's law, when the following formula (2) holds, where n is an integer greater than or equal to 1, λ is the wavelength of the X-ray, d is the interplanar spacing of the (002) diffraction plane of a graphite crystal, and θ is the Bragg angle, the path difference of the X-rays scattered by different (002) atomic planes is always an integer multiple of λ. In such cases, the diffraction intensity of the X-rays by the (002) diffraction plane is enhanced.

[0043]

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[0044] When the angle of incidence of X-rays on the (002) diffraction plane deviates from the Bragg angle θ, the path difference of the X-rays is no longer an integer multiple of λ. In such a case, if the crystallite size is large enough, a (002) diffraction plane with an X-ray path difference of exactly half λ exists within the crystallite. Because these scattered X-rays are out of phase with each other, they cancel each other out, resulting in a reduced diffraction intensity. Therefore, the diffraction intensity is enhanced when the diffraction angle 2θ is exactly twice the Bragg angle, and is zero at other diffraction angles, resulting in a sharp diffraction peak. However, if the crystallite size is very small, a (002) diffraction plane with an X-ray path difference of exactly half λ no longer exists within the same crystallite, and the scattered X-rays do not completely cancel each other out. This results in a broad diffraction peak.

[0045] In the present invention, the crystallite size L is calculated based on the half-width B of the diffraction peak corresponding to the (002) diffraction plane of the graphite crystal in the diffraction pattern measured for the carbonaceous reduced powder, using the relationship shown in mathematical formula (1). The crystallite size L calculated by this method corresponds to the crystallite thickness as viewed in the direction normal to the (002) diffraction plane of the graphite crystal, to be precise.

[0046] In powder X-ray diffraction, it is known that when there is non-uniform distortion in the crystal lattice, the half-width of the diffraction peak becomes large. α1 Line and K α2 The diffraction peak may split into two due to a slight difference in the wavelength of the line. This phenomenon becomes more pronounced as the diffraction angle 2θ increases. However, CuK α In powder X-ray diffraction using X-rays, the diffraction angle 2θ corresponding to the (002) diffraction plane of graphite crystal is small, at approximately 26.3 degrees, so the effect of lattice distortion can be ignored. Furthermore, at this diffraction angle, the separation of the diffraction peaks is hardly observed. Therefore, the value of λ in formula (1) is determined by the CuK α1 wire and CuK α2 The weighted average value of the wavelength of the line, 0.15418 (nm), is used.

[0047] When determining the half-width B of the diffraction peak corresponding to the (002) diffraction plane of a graphite crystal, it is preferable to perform background processing, in which the baseline diffraction intensity at diffraction angles other than the diffraction peak is subtracted from the diffraction intensity of the diffraction peak. Furthermore, if the diffraction pattern near the diffraction peak exhibits significant fluctuations due to noise in the measurement signal, it is preferable to perform smoothing processing on the diffraction intensity. By performing these data processing operations on the diffraction intensity, a measurement value closer to the true value of the half-width B of the diffraction peak can be obtained. It should be noted that in calculating the crystallite size L based on Equation (1), the Bragg angle θ is expressed in degrees, while the half-width B is expressed in radian degrees.

[0048] Graphite crystals are hexagonal, with lattice constants c=0.67 nm and a=0.14 nm. In the present invention, when the crystallite size L calculated based on Equation (1) is less than 1.00 nm, the size is equal to or smaller than the size of the graphite crystal lattice. Therefore, in the numerical range of 0.01 nm or more and less than 1.00 nm, the relationship between the crystallite size L and the half-width B indicated by the Scherrer formula does not necessarily hold. However, as mentioned above, even in the numerical range of 0.01 nm or more and less than 1.00 nm, a clear correlation is observed between the value of L and the reduction rate of sponge iron. Therefore, in the present invention, the crystallite size L is used as a target value for controlling the production conditions, even in the numerical range of 0.01 nm or more and less than 1.00 nm. In the present invention, L calculated based on Equation (1) is referred to as the "crystallite size," regardless of whether the relationship between the crystallite size L and the half-width B indicated by the Scherrer formula holds.

[0049] When the value of the half-width B exceeds approximately 0.26 rad (15 degrees), which corresponds to L = 0.01 nm, the intensity of the diffraction peak corresponding to the (002) diffraction plane usually decreases, and a broad diffraction pattern called a halo pattern, which indicates the presence of amorphous carbon, appears. In such cases, it is no longer possible to accurately measure the half-width B, and therefore the value of the crystallite size L cannot be evaluated.

[0050] Attempts are being made to standardize the method for measuring the crystallite size of carbon materials. For example, Japanese Industrial Standard JIS R 7651:2007 ("Method for measuring lattice constant and crystallite size of carbon materials," established on June 20, 2007) describes a standardized method for measuring the crystallite size of carbon materials (hereinafter referred to as the "JIS method"). The JIS method reduces errors due to the equipment used for measurement, allowing highly reproducible measurements of the crystallite size L to be obtained. Therefore, in the method for producing sponge iron according to the present invention, it is preferable to determine the crystallite size L using a method conforming to the JIS method.

[0051] [Reduction of iron oxide powder] As described above, in the method for producing sponge iron according to this embodiment, the iron oxide powder and the carbonaceous reduced powder are placed in a container and heated to reduce the iron oxide powder and produce sponge iron. The iron oxide powder and the carbonaceous reduced powder may be arranged in any manner in the container as long as the arrangement allows convenient contact of carbon monoxide produced from the carbonaceous reduced powder with the iron oxide powder.

[0052] In the method for producing sponge iron according to this embodiment, it is preferable that the raw material powders, ie, iron oxide powder and carbonaceous reduced powder, are not mixed, but are charged into a container in a separated state from each other. This makes it easier for carbon dioxide generated by the chemical reaction of chemical formula (1) in the carbonaceous reduced powder portion to be immediately converted to carbon monoxide by the chemical reaction of chemical formula (2). Alternatively, oxygen becomes insufficient in the carbonaceous reduced powder portion, making it easier for carbon monoxide to be directly generated by the chemical reaction of chemical formula (3).

[0053] For example, in the example of the raw material powder arrangement shown in Figures 1 and 2, as described above, iron oxide powder 2 is arranged in a cylindrically-shaped aggregate in a bottomed cylindrical container 1. Carbonaceous reducing powder 3 is packed around the iron oxide powder 2. In this arrangement, carbon monoxide generated in the portion consisting of carbonaceous reducing powder 3 diffuses and moves toward the portion consisting of iron oxide powder 2, driven by the carbon monoxide concentration gradient. Also in the raw material powder arrangement described in Patent Document 1, carbon monoxide diffuses and moves from the portion consisting of carbonaceous reducing powder packed so as to be alternately and spirally deposited in the container, toward the portion consisting of iron oxide powder.

[0054] The container can be heated by placing it in a furnace maintained at a constant temperature. If the oxygen concentration in the furnace is too high when heating the container, carbon monoxide generation by reduction of carbon dioxide becomes difficult. Therefore, the oxygen concentration of the atmosphere in the furnace is preferably 15 vol% or less. Furthermore, if the temperature in the furnace is too low, the reduction of the iron oxide powder becomes insufficient. Therefore, the temperature in the furnace is preferably 700°C or higher, more preferably 800°C or higher, even more preferably 900°C or higher, and even more preferably 1000°C or higher. On the other hand, if the temperature in the furnace is too high, the produced iron will sinter strongly, making it difficult to pulverize. Therefore, the temperature in the furnace is preferably 1300°C or lower, more preferably 1200°C or lower. The time for heating the container may be determined appropriately depending on the size of the container, the amount of raw material powder charged, etc. [Example]

[0055] Next, the present invention will be described in more detail based on examples. However, the present invention is not limited by the following examples. The present invention can be modified appropriately within the scope of its purpose, and all such modified embodiments are included in the technical scope of the present invention.

[0056] Iron oxide powder was prepared by crushing mill scale with an oxygen content of 24% into a powder. The D50 of this iron oxide powder, measured using the method described above, was 0.2 mm. Carbonaceous reduced powder was prepared by carbonizing wood powder. The wood powder was carbonized in a nitrogen gas atmosphere at a temperature of 1200°C for 18 different carbonization times, ranging from 3 minutes to 270 minutes, as shown in Table 1. The D50 of the resulting carbonaceous reduced powder was measured using the method described above.

[0057] Next, the obtained carbonaceous reduced powder was subjected to powder X-ray diffraction using an X-ray diffractometer. The X-ray diffractometer used was a SmartLab manufactured by Rigaku Corporation, equipped with a Cu target X-ray tube. The scanning speed of the diffraction angle 2θ of the counter fixed to the goniometer was 4 degrees per minute. The scanning range of the diffraction angle 2θ was 10 degrees or more and 60 degrees or less. Of the obtained diffraction patterns, background processing was performed on the diffraction peak corresponding to the (002) diffraction plane of the graphite crystal, and the half-width B of the diffraction peak was then determined. The crystallite size L was then calculated based on Equation (1). The D50 and L values ​​for the obtained carbonaceous reduced powder are shown in Table 1. However, for the carbonaceous reduced powder of Comparative Example 1, no diffraction peak corresponding to the (002) diffraction plane of the graphite crystal was observed, and the half-width B and crystallite size L could not be determined.

[0058] Next, 200 grams of one of the 18 types of carbonaceous reduced powders listed in Table 1 and 500 grams of iron oxide powder were placed in a container as shown in Figures 1 and 2. The container was then placed in an electric furnace and heated to 1000°C at a rate of 4°C per minute under a nitrogen atmosphere. The temperature was then maintained at 1000°C for 9 hours and then cooled. The sintered, agglomerated sponge iron was removed and crushed using a crusher to obtain reduced iron powder with a particle size of 150 μm or less. A hammer mill manufactured by Hosokawa Micron Corporation was used as the crusher. The oxygen content of the resulting reduced iron powder was measured, and the reduction ratio was calculated using the following formula (3). This procedure was repeated for all 18 types of carbonaceous reduced powders listed in Table 1.

[0059]

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[0060] The calculated reduction ratios are shown in Table 1. The graph in Figure 3 shows the relationship between the crystallite size L of the carbonaceous reduced powder, shown on a logarithmic scale, and the reduction ratio of the reduced iron powder.

[0061] [Table 1]

[0062] As shown in Table 1 and FIG. 3, the reduced iron powders of Examples 1 to 14, which were reduced using carbonaceous reduced powders having a crystallite size L of 0.01 nm or more and 25.0 nm or less, as specified in the method for producing sponge iron according to the present invention, showed high reduction rates of 50% or more and 95% or less.

[0063] On the other hand, the reduction rate of the reduced iron powder in Comparative Example 1 was 48%. This is thought to be because when reduction was performed using amorphous carbonaceous reduced powder in which graphite was not crystallized due to a short dry distillation time, a large amount of carbon monoxide was generated all at once in the early stage of the temperature rise process, and the carbon monoxide was released outside the container before the reduction of the iron oxide powder began.

[0064] Furthermore, the reduction rates of the reduced iron powders in Comparative Examples 2 to 4, which were reduced using carbonaceous reduced powders with a crystallite size L exceeding 25.0 nm, were also less than 50%. This is thought to be because the progress of graphite crystallization in the carbonaceous reduced powder made it difficult to generate carbon monoxide, resulting in a decrease in the reduction rate.

[0065] Furthermore, in the reduced iron powders of Examples 3 to 12, which were reduced using carbonaceous reduced powders having a crystallite size L in the preferred range of 0.05 nm to 20.0 nm, the reduction rates were 60% to 95%, which is higher than the reduction rates of the other Examples. [Explanation of symbols]

[0066] 1 container 2. Iron oxide powder 3. Carbonaceous reduced powder A, A' Cutting position of horizontal cross section of Figure 2 B, B´ Cutting position of vertical cross section of Figure 1

Claims

1. 1. A method for producing sponge iron by charging iron oxide powder and carbonaceous reduced powder into a vessel and heating the vessel to reduce the iron oxide powder, comprising: Regarding the carbonaceous reduced powder, CuK α A method for producing sponge iron, characterized in that powder X-ray diffraction is performed using X-rays, and the crystallite size L calculated by the following mathematical formula (1) based on the diffraction peak corresponding to the (002) diffraction plane of graphite crystals in the obtained diffraction pattern is 0.01 nm or more and 25.0 nm or less: [Equation 1] where K is the Scherrer constant 0.9, and λ is CuK α The wavelength of the line is 0.15418 (nm), B is the half-width (rad) of the diffraction peak, and θ is the Bragg angle (degrees) corresponding to the (002) diffraction plane of the graphite crystal.

2. 2. The method for producing sponge iron according to claim 1, wherein the crystallite size L is 0.05 nm or more and 20.0 nm or less.

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