Method for producing sponge iron

Abstract

Provided is a method for producing sponge iron at a higher efficiency by increasing the speed of generation of carbon monoxide required for reduction of iron oxide powder as compared with the prior art. A method for producing sponge iron, characterized in that when producing sponge iron by charging an iron oxide powder and a carbonaceous reducing powder into a vessel and applying heat to reduce the iron oxide powder, the size L of crystallites obtained by the Scherrer equation (L = Kλ / (Bcosθ)) on the basis of a diffraction peak corresponding to the (002) diffraction plane of a graphite crystal from a diffraction pattern obtained by performing powder X-ray diffraction by using CuKα rays on the carbonaceous reducing powder is 0.01-25.0 nm. In the formula, K is the Scherrer constant 0.9, λ is the wavelength 0.15418 (nm) of the CuKα rays, B is the half width (rad) of the diffraction peak, and θ is the Bragg angle (degree) corresponding to the (002) diffraction plane of a graphite crystal.

Description

Method for producing sponge iron

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

[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 reducing agent, placed in a container, to a temperature lower than the melting point of iron. Carbonaceous reducing powder is typically used as the reducing agent. Reduced iron powder can be produced by grinding the sponge iron obtained by the reduction of 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] Figures 1 and 2 are schematic cross-sectional views illustrating an example of the arrangement of raw material powders within a container used in the production of sponge iron. Figure 1 is a vertical cross-sectional view taken from the side of the cross-section at position B-B' in Figure 2. Figure 2 is a horizontal cross-sectional view taken from above of the cross-section at position A-A' in Figure 1. In the example of raw material powder arrangement shown in Figures 1 and 2, iron oxide powder 2 is arranged in a cylindrical shape within a bottomed cylindrical container 1. 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 involving gas transfer proceeds between the carbonaceous reduction powder 3 and the iron oxide powder 2. As a result, the oxygen contained in the iron oxide combines with carbon monoxide to chemically change into carbon dioxide and is released, generating particulate iron. The particulate iron, placed at a temperature lower than its melting point, forms sintered necks at contact points with adjacent particles, bonding three-dimensionally. In the final stage of the redox reaction, the particulate iron becomes porous sponge iron sintered into the same cylindrical shape as the arrangement of the iron oxide powder 2 shown in Figures 1 and 2.

[0005] In the aforementioned manufacturing method, the rate at which sponge iron is produced is limited by a redox reaction involving gas movement. For this reason, methods to increase the rate of the redox reaction have been considered. For example, Patent Document 1 describes an invention for a method of producing sponge iron, characterized by filling a container with iron oxide powder and carbonaceous reduction powder alternately and in a helical pattern. With this arrangement of raw material powders in the manufacturing method, the distance between the iron oxide powder and carbonaceous reduction powder in the container becomes shorter compared to the arrangement of raw material powders shown in Figures 1 and 2. As a result, the time required for carbon monoxide produced in the carbonaceous reduction powder to move to the iron oxide powder, and the time required for carbon dioxide produced by the reduction of iron oxide to move to the carbonaceous reduction powder are both shortened, thus increasing the rate of the redox reaction.

[0006] Japanese Patent Publication No. 2004-250789

[0007] The method for producing sponge iron described in Patent Document 1 is excellent in that it can shorten the time it takes for the gas to move by diffusion between the iron oxide powder and the carbonaceous reduction powder. However, the conventional technology has room for improvement regarding the rate of carbon monoxide production necessary for the reduction of iron oxide powder, and further improvement in the rate of the redox reaction was needed.

[0008] This invention has been made in view of the above-mentioned problems, and aims to produce sponge iron with higher efficiency by increasing the rate of carbon monoxide generation necessary for the reduction of iron oxide powder compared to conventional technology.

[0009] To solve the above problems, the inventors investigated from various perspectives what kind of carbonaceous reduction powder can generate or regenerate carbon monoxide with higher efficiency than conventional technology. As a result of their investigations, they found that by adjusting the size of the crystallites contained in the carbonaceous reduction powder to a predetermined range, the rate of carbon monoxide generation increased significantly, and as a result, the production efficiency of sponge iron improved dramatically.

[0010] This invention was completed based on the above findings, and its gist is as follows.

[0011] [1] A method for producing sponge iron by reducing iron oxide powder and carbonaceous reduction powder by heating the iron oxide powder in a container, wherein the carbonaceous reduction powder is CuK α A method for producing sponge iron, characterized in that powder X-ray diffraction is performed using a line, and the crystallite size L, which is determined by the following formula (1) based on the diffraction peak corresponding to the (002) diffraction plane of the graphite crystal in the obtained diffraction pattern, is between 0.01 nm and 25.0 nm. However, K is Scherrer's constant 0.9, and λ is CuK. α The wavelength of the line is 0.15418 nm, B is the full width at half maximum (rad) of the diffraction peak, and θ is the Bragg angle (degrees) corresponding to the (002) diffraction plane of the graphite crystal.

[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.

[0013] According to the present invention, sponge iron can be produced with higher efficiency by increasing the rate of carbon monoxide generation required for the reduction of iron oxide powder compared to conventional techniques.

[0014] This is a schematic vertical cross-sectional view showing an example of the arrangement of raw material powders within a container. This is a schematic horizontal cross-sectional view showing an example of the arrangement of raw material powders within a container. This is a graph showing the relationship between the crystallite size of carbonaceous reduction powder and the reduction rate of reduced iron powder.

[0015] The embodiments for carrying out the present invention will be described in detail below. Unless otherwise specified in the following description, "%" refers to mass percentage.

[0016] In one embodiment, the present invention relates to a method for producing sponge iron by reducing iron oxide powder by charging iron oxide powder and carbonaceous reduction powder into a container and heating, wherein the carbonaceous reduction powder is CuK αThis invention relates to a method for producing sponge iron, characterized in that powder X-ray diffraction is performed using a line, and the crystallite size L, which is determined by the following formula (1) based on the diffraction peak corresponding to the (002) diffraction plane of the graphite crystal in the obtained diffraction pattern, is between 0.01 nm and 25.0 nm. However, K is Scherrer's constant 0.9, and λ is CuK. α The wavelength of the line is 0.15418 nm, B is the full width at half maximum (rad) of the diffraction peak, and θ is the Bragg angle (degrees) corresponding to the (002) diffraction plane of the graphite crystal.

[0017] [Iron Oxide Powder] The iron oxide powder used in the method for producing sponge iron according to this embodiment can be any iron oxide powder, as long as it contains iron oxide. The iron oxide powder, together with the carbonaceous reduced powder described later, constitutes the raw material powder that serves as 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 types of iron oxide selected from iron(II) oxide, iron(III) oxide, and iron(II,III) oxide in any proportion. It is preferable that the proportion of impurities other than iron oxide contained in the iron oxide powder be as low as possible.

[0019] Specific raw material powders that can be used as iron oxide powder in the method for producing sponge iron according to this embodiment include, but are not limited to, powder obtained by crushing natural iron ore, natural iron sand, commercially available iron oxide powder as a reagent or industrial raw material, mill scale and other iron oxide powder generated in the production lines of steel mills, and iron powder contained in disposable hand warmers and oxygen absorbers that has been oxidized by 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 the oxide film formed on the surface of hot-rolled steel sheets when the steel material is oxidized by oxygen in the atmosphere during the manufacturing process. Mill scale is usually removed from the surface of hot-rolled steel sheets by methods such as pickling or shot blasting. The mill scale removed and recovered from the surface of hot-rolled steel sheets is a flaky powder composed of iron(II) oxide, iron(III) oxide, and a complex oxide containing iron(II,III) oxide. Mill scale has a lower impurity content compared to natural iron ore. Therefore, when aiming 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 necessary for the oxidation-reduction reaction to pass through the portion consisting of the iron oxide powder, or it may be difficult to pulverize the sponge iron. For this reason, it is preferable that the particle size of the iron oxide powder be 10 μm or larger at D50, which is the median value in the volume 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 oxide powder to the center, or the reduced iron powder obtained from the sponge iron may have a particle size that is too large and difficult to handle. For this reason, it is preferable that the particle size of the iron oxide powder be 10 mm or less at D50.

[0022] The D50 of iron oxide powder shall be measured by the method described below. First, the raw material powder to be measured is placed in a solvent. For example, ethanol can be used as the solvent. Next, the iron oxide powder is dispersed in the solvent by applying ultrasonic vibration for 30 seconds or more to the solvent containing the raw material powder. Then, the volume-based particle size frequency distribution of the iron oxide powder dispersed in the solvent is measured using a laser diffraction particle size 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 Reduction Powder] The carbonaceous reduction powder used in the method for producing sponge iron according to this embodiment may be any powder that contains carbon and has a graphite crystallite size within a certain range. In the method for producing sponge iron according to the present invention, the carbonaceous reduction 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. Carbon monoxide is used to reduce the iron oxide powder. These series of chemical reactions will be described in detail below.

[0024] When the container 1 shown in Figures 1 and 2 is heated from the outside, the carbonaceous reduced powder 3 first burns due to the oxygen contained in the air present in the gaps between the carbonaceous reduced powder 3, and carbon dioxide is produced by the chemical reaction shown in chemical formula (1). In all the chemical formulas shown below, the symbol (s) indicates that the substance represented by the molecular formula to which this symbol is attached is in a solid phase state, and the symbol (g) indicates that the substance represented by the molecular formula to which this symbol is attached is in a gaseous phase state.

[0025]

[0026] The generated carbon dioxide is reduced by the carbon contained in carbonaceous reduction powder 3 and chemically converted to carbon monoxide through the chemical reaction shown in chemical formula (2).

[0027]

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

[0029]

[0030] The carbon monoxide produced by the chemical reaction shown in chemical formula (2) or chemical formula (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). The reduction of iron oxide produces solid-state iron and carbon dioxide. Here, chemical formula (4) is the chemical formula that shows 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-state iron and carbon dioxide are produced by a chemical reaction similar to the reduction reaction shown in chemical formula (4).

[0031]

[0032] The carbon dioxide generated at the location of iron oxide powder 2 reaches the location of carbonaceous reduction powder 3 by diffusion and is reduced to carbon monoxide by the chemical reaction shown in chemical formula (2). The generated carbon monoxide reaches the location of iron oxide powder 2 and reduces the iron oxide by the chemical reaction shown in chemical formula (4). Through this series of oxidation-reduction reactions involving gas movement, the iron oxide powder is reduced and porous sponge iron is produced.

[0033] Specific raw material powders that can be used as 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 dry distillation of coal powder to increase the carbon content, and biomass powder obtained by dry distillation of plant-derived biomass to increase the carbon content. The carbon content in 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 in the carbonaceous reduced powder, and it may be 100%.

[0034] In the method for producing sponge iron according to this embodiment, carbonaceous reduced powder with a crystallite size L of 0.01 nm to 25.0 nm is used. "Crystallite" refers to the smallest unit that contributes to X-ray diffraction and is the region in the carbonaceous reduced powder that can be considered as a single crystal of graphite. "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 sponge iron is higher compared to 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] While the exact reason why the crystallite size L of carbonaceous reduction powder affects the reduction rate of sponge iron is unclear, the inventors believe the following: When the crystallite size L is between 0.01 nm and 25.0 nm, carbonaceous reduction powder with such crystallite sizes contains many grain boundaries. It also has low crystallinity and contains regions where the arrangement of carbon atoms is irregular. Therefore, at low temperatures, the bonds between carbon atoms are easily broken, and the carbon atoms combine with the surrounding oxygen to easily produce carbon dioxide through the chemical reaction shown in chemical formula (1). The produced carbon dioxide easily combines with the easily broken carbon atoms and is chemically converted to carbon monoxide through the chemical reaction shown in chemical formula (2). Furthermore, if oxygen is insufficient, carbon monoxide is easily produced directly through the chemical reaction shown in chemical formula (3). As a result of these combined effects, it is thought that the reduction rate of iron oxide is improved.

[0036] However, when the crystallite size L is too small, crystallization is insufficient, and many of the carbon atoms contained in the carbonaceous reducing powder are in an amorphous state with weak bonds. When such a carbonaceous reducing powder and iron oxide powder are charged into a container and heated, a large amount of carbon monoxide is temporarily generated from the carbonaceous reducing powder at the initial stage of the temperature increase process by the chemical reactions shown in Chemical Formulas (1) to (3). However, at the initial stage of the temperature increase process, the temperature of the iron oxide powder has not been sufficiently increased. In this case, the carbon monoxide is released outside the container without being used for the reduction of the iron oxide powder shown in Chemical Formula (4), and the reduction rate of sponge iron decreases. 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 still more preferably 1.00 nm or more.

[0037] On the other hand, when the crystallite size L is too large, the carbonaceous reducing powder has advanced graphitization during the grain growth process and has become a single crystal of highly crystalline graphite. In this case, the arrangement of carbon atoms is regular, and the bonds between carbon atoms become strong. Therefore, the generation of carbon dioxide shown in Chemical Formula (1) and the generation of carbon monoxide shown in Chemical Formulas (2) and (3) are suppressed, and the reduction rate of sponge iron decreases. 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 still more preferably 3.00 nm or less.

[0038] To adjust the crystallite size L of the carbonaceous reducing powder to the range of 0.01 nm or more and 25.0 nm or less, for example, when producing the carbonaceous reducing powder by carbonizing coal or biomass, the carbonization conditions may be controlled. The longer the carbonization time at a constant temperature or the higher the carbonization temperature at a constant time, the larger the crystallite size L will be.

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

[0040] [Method for obtaining the crystallite size L of the carbonaceous reducing powder] In the method for producing sponge iron according to this embodiment, powder X-ray diffraction is performed on the carbonaceous reducing powder using CuK α line, and based on the diffraction peak corresponding to the (002) diffraction plane of the graphite crystal in the obtained diffraction pattern, the crystallite size L is obtained by the following formula (1). However, K is the Scherrer constant 0.9, λ is the wavelength 0.15418 (nm) of the CuK α line, B is the full width at half maximum (rad) of the diffraction peak, and θ is the Bragg angle (degree) corresponding to the (002) diffraction plane of the graphite crystal. The unit of the crystallite size L calculated by formula (1) is the same as the unit of λ, which is nanometer.

[0041] The above formula (1) is known as the Scherrer equation showing the relationship between the full width at half maximum of the diffraction peak and the crystallite size. As shown in formula (1), when K, λ, and θ are constant, the magnitude of the full width at half maximum B of the diffraction peak is inversely proportional to the crystallite size L. That is, the larger L is, the sharper the diffraction peak becomes and the smaller the full width at half maximum B is. Conversely, the smaller L is, the broader the diffraction peak becomes and the larger the full width at half maximum B is.

[0042] The reason why the relationship between crystallite size L and full width at half maximum B, as shown in Scherrer's equation, holds true can be considered as follows. According to Bragg's law, when 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 planes of the graphite crystal, and θ is the Bragg angle, if the following equation (2) holds true, then the path difference of X-rays scattered by different (002) atomic planes is always an integer multiple of λ. In such cases, the diffraction intensity of X-rays by the (002) diffraction planes is increased.

[0043]

[0044] (002) When the incident angle of X-rays on the 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 cases, if the crystallite size is sufficiently large, (002) diffraction planes where the path difference of the X-rays is exactly half of λ exist within the crystallite. Since these scattered X-rays have opposite phases, they cancel each other out, reducing the diffraction intensity. For this reason, the diffraction intensity is increased when the diffraction angle 2θ is exactly twice the Bragg angle, and at other diffraction angles the diffraction intensity becomes zero, resulting in a sharp diffraction peak. However, when the crystallite size is very small, (002) diffraction planes where the path difference of the X-rays is exactly half of λ no longer exist within the same crystallite, so complete cancellation of scattered X-rays does not occur. For this reason, the diffraction peak becomes broad.

[0045] In this invention, the crystallite size L is calculated based on the full width at half maximum (FWHM) of the diffraction peak corresponding to the (002) diffraction plane of the graphite crystal, using the relationship shown in formula (1) of carbonaceous reduced powder. The crystallite size L obtained by this method precisely corresponds to the thickness of the crystallite as viewed in the direction of the normal to the (002) diffraction plane of the graphite crystal.

[0046] In powder X-ray diffraction, it is known that the full width at half maximum of the diffraction peak increases when there is non-uniform strain in the crystal lattice. Also, K α１ Line and K α２ In some cases, slight differences in the wavelength of the lines can cause the diffraction peak to split into two. These phenomena become more pronounced as the diffraction angle 2θ increases. However, CuKα In powder X-ray diffraction using a line, since the diffraction angle 2θ corresponding to the (002) diffraction plane of the graphite crystal is as small as approximately 26.3 degrees, the influence of lattice strain can be ignored. Also, at this diffraction angle, almost no separation of diffraction peaks is observed. Therefore, for the value of λ in formula (1), the weighted average value of the wavelengths of CuK α１ line and CuK α２ line, which is 0.15418 (nm), is used.

[0047] When obtaining the half-value width B of the diffraction peak corresponding to the (002) diffraction plane of the graphite crystal, it is preferable to perform background processing to subtract the diffraction intensity of the baseline at diffraction angles that are not diffraction peaks from the diffraction intensity of the diffraction peak. Also, when there are many up-and-down movements due to noise in the measurement signal in the diffraction pattern near the diffraction peak, it is preferable to perform smoothing processing on the diffraction intensity. By performing these data processes on the diffraction intensity, a measured value closer to the true value can be obtained for the half-value width B of the diffraction peak. It should be noted that in the calculation of the crystallite size L based on formula (1), while the unit of the Bragg angle θ is an angle (degree) using the degree method, the unit of the half-value width B is an angle (rad) using the radian method.

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

[0049] When the value of the full width at half maximum (FWHM) 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, indicating the presence of amorphous carbon, appears. In such cases, it becomes impossible to accurately measure the FWHM B, and therefore the crystallite size L cannot be evaluated.

[0050] Furthermore, attempts have been 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 Constants and Crystallite Size of Carbon Materials," established June 20, 2007) describes a standardized measurement method for the crystallite size of carbon materials (hereinafter referred to as the "JIS method"). According to the JIS method, errors caused by the equipment used for measurement are reduced, and highly reproducible measurements of the crystallite size L can be obtained. Therefore, in the method for producing sponge iron according to the present invention, it is preferable to determine the crystallite size L by 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, sponge iron is produced by reducing the iron oxide powder by charging the iron oxide powder and carbonaceous reduction powder into a container and heating it. The arrangement of the iron oxide powder and carbonaceous reduction powder in the container can be any arrangement that is convenient for the carbon monoxide generated from the carbonaceous reduction powder to come into contact with the iron oxide powder.

[0052] In the method for producing sponge iron according to this embodiment, it is preferable to charge the raw material powders, iron oxide powder and carbonaceous reduced powder, into a container without mixing them, so that the portion consisting of iron oxide powder and the portion consisting of carbonaceous reduced powder are separated from each other. This makes it easier for the carbon dioxide produced by the chemical reaction of chemical formula (1) in the portion consisting of carbonaceous reduced powder to immediately change into carbon monoxide by the chemical reaction of chemical formula (2). Alternatively, oxygen may be deficient in the portion consisting of carbonaceous reduced powder, making it easier for carbon monoxide to be directly produced by chemical formula (3).

[0053] For example, in the raw material powder arrangement shown in Figures 1 and 2, as described above, iron oxide powder 2 is arranged in a cylindrical shape inside a bottomed cylindrical container 1. Carbonaceous reduced powder 3 is packed around the iron oxide powder 2. In such an arrangement, carbon monoxide generated in the portion consisting of carbonaceous reduced powder 3 diffuses toward the portion consisting of iron oxide powder 2, driven by the carbon monoxide concentration gradient. Similarly, in the raw material powder arrangement described in Patent Document 1, carbon monoxide diffuses toward the portion consisting of iron oxide powder, which is packed in an alternating spiral pattern within the container.

[0054] The container can be heated by placing it in a furnace where the temperature is kept constant. If the oxygen concentration in the furnace is too high when heating the container, the generation of carbon monoxide through the reduction of carbon dioxide will be less likely to occur. Therefore, it is preferable that the atmosphere inside the furnace has an oxygen concentration of 15 vol% or less. Also, if the temperature inside the furnace is too low, the reduction of iron oxide powder will be insufficient. Therefore, it is preferable that the temperature inside the furnace be 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 inside the furnace is too high, the iron produced will sinter firmly, making it difficult to crush. Therefore, it is preferable that the temperature inside the furnace be 1300°C or lower, and more preferably 1200°C or lower. The heating time for the container may be determined appropriately according to the size of the container, the amount of raw material powder to be charged, etc.

[0055] Next, the present invention will be described in more detail based on the following examples. However, the present invention is not limited in any way by the following examples. The present invention can be modified as appropriate within the scope of its spirit, and any such modified embodiment falls within the technical scope of the present invention.

[0056] As iron oxide powder, mill scale with an oxygen content of 24% was ground into a powder. The D50 of this iron oxide powder, measured by the method described above, was 0.2 mm. As carbonaceous reduced powder, wood powder was prepared by dry distillation. Dry distillation of the wood powder was carried out in a nitrogen gas atmosphere at a temperature of 1200°C, with the dry distillation time varied in 18 ways from 3 minutes to 270 minutes, as shown in Table 1. The D50 of the obtained carbonaceous reduced powder was measured by the method described above.

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

[0058] Next, 200 grams of one of the 18 types of carbonaceous reduction powders shown 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 heating rate of 4°C per minute under a nitrogen atmosphere, held at 1000°C for 9 hours, and then cooled. Next, the sintered, lump-like sponge iron was removed and crushed using a pulverizer until the particle size was 150 μm or less to obtain reduced iron powder. A hammer mill manufactured by Hosokawa Micron Corporation was used as the pulverizer. Next, the oxygen content of the obtained reduced iron powder was measured, and the reduction rate was calculated using the following formula (3). These procedures were repeated for all 18 types of carbonaceous reduction powders shown in Table 1.

[0059]

[0060] Table 1 shows the calculated reduction rates. Figure 3 shows the relationship between the crystallite size L of the carbonaceous reduction powder and the reduction rate of the reduced iron powder, shown on a logarithmic scale.

[0061]

[0062] As shown in Table 1 and Figure 3, the reduced iron powders in Invention Examples 1 to 14, which were reduced using carbonaceous reduced powder with a crystallite size L of 0.01 nm to 25.0 nm as defined in the method for producing sponge iron according to the present invention, showed a high reduction rate of 50% to 95%.

[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 the graphite had not crystallized due to the short carbonization time, a large amount of carbon monoxide was generated at once in the initial stages of the heating 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 powder with a crystallite size L exceeding 25.0 nm, were also less than 50%. This is thought to be because the crystallization of graphite in the carbonaceous reduced powder was progressing, making it difficult to generate carbon monoxide, which resulted in a decrease in the reduction rate.

[0065] Furthermore, in the reduced iron powders of Invention Examples 3 to 12, in which reduction was performed using carbonaceous reduced powder having a crystallite size L in a preferred range of 0.05 nm to 20.0 nm, the reduction rate was 60% to 95%, indicating a higher reduction rate compared to the other Invention Examples.

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

Claims

1. A method for producing sponge iron by reducing iron oxide powder by heating iron oxide powder and carbonaceous reduction powder in a container, wherein the carbonaceous reduction powder contains CuK α A method for producing sponge iron, characterized in that powder X-ray diffraction using a line is performed, and the crystallite size L, which is determined by the following formula (1) based on the diffraction peak corresponding to the (002) diffraction plane of the graphite crystal in the obtained diffraction pattern, is 0.01 nm or more and 25.0 nm or less. However, K is Scherrer's constant 0.9, and λ is CuK. α The wavelength of the line is 0.15418 nm, B is the full width at half maximum (rad) of the diffraction peak, and θ is the Bragg angle (degrees) corresponding to the (002) diffraction plane of the graphite crystal.

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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