Method for manufacturing sponge iron

Abstract

The present invention addresses the problem of manufacturing sponge iron with higher efficiency by increasing the generation rate of carbon monoxide required for reducing iron oxide powder as compared with a conventional technique. According to the present invention, when sponge iron is manufactured by mixing a carbonaceous reduction powder and a CO gasification catalyst powder to obtain a mixed powder, charging iron oxide powder and the mixed powder into a container and heating to thereby reduce the iron oxide powder and obtain sponge iron, the CO gasification catalyst powder is one or more selected from the group consisting of alkali metal compounds and alkaline earth metal compounds, the lower of the thermal decomposition temperature and the melting point of the CO gasification catalyst powder is less than 1000°C, and the content of the CO gasification catalyst in the mixed powder is 0.10-30.00 parts by mass per 100 parts by mass of the carbonaceous reduction powder.

Description

Method for manufacturing sponge iron

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

[0002] Sponge iron is porous iron obtained by reducing iron oxide powder. The reduction of iron oxide powder is carried out by heating the iron oxide powder and the reducing agent arranged in a container to a temperature lower than the melting temperature of iron. Usually, a carbonaceous reducing powder is used as the reducing agent. By pulverizing the sponge iron obtained by reducing the iron oxide powder, reduced iron powder can be produced. The reduced iron powder is mainly used as a raw material powder for sintered parts such as mechanical parts and magnetic materials.

[0003] FIGS. 1 and 2 are cross-sectional views schematically showing an example of the arrangement of raw material powders in a container used for manufacturing sponge iron. FIG. 1 is a vertical cross-sectional view seen from the side of the cross-section at the position of B - B' in FIG. 2. FIG. 2 is a horizontal cross-sectional view seen from directly above the cross-section at the position of A - A' in FIG. 1. In the example of the arrangement of the raw material powders shown in FIGS. 1 and 2, iron oxide powder 2 is arranged in a cylindrical shape and aggregated in a bottomed cylindrical container 1. The carbonaceous reducing powder 3 is filled without gaps around the iron oxide powder 2.

[0004] When the container 1 shown in FIGS. 1 and 2 is heated from the outside, a redox reaction accompanied by gas movement between the carbonaceous reducing powder 3 and the iron oxide powder 2 proceeds. As a result, the oxygen contained in the iron oxide combines with carbon monoxide and chemically changes to carbon dioxide and separates, and particulate iron is generated. The particulate iron placed at a temperature lower than the melting temperature forms a sintering neck at the contact points with other adjacent particles and binds three-dimensionally. In the final stage of the redox reaction, the particulate iron becomes porous sponge iron sintered in the same cylindrical shape as the arrangement of the iron oxide powder 2 shown in FIGS. 1 and 2.

[0005] In the aforementioned manufacturing method, the rate at which sponge iron is produced is limited by an oxidation-reduction reaction involving gas movement. For this reason, methods to increase the rate of the oxidation-reduction reaction have been considered. For example, Patent Document 1 describes an invention for a method of producing sponge iron, in which one or more compounds selected from alkali metal compounds are added to carbonaceous reduction powder in an amount of 2% to 50% by weight, in order to promote the CO gasification of the carbonaceous reduction powder. According to the manufacturing method described in Patent Document 1, numerous fine cracks are generated in the carbonaceous reduction powder due to the action of active alkali metal oxides produced by the thermal decomposition of the alkali metal compounds. As a result, the CO gasification of the carbonaceous reduction powder is promoted at around 1000°C.

[0006] Japanese Patent Application Publication No. 60-29408

[0007] The method for producing sponge iron described in Patent Document 1 is an excellent method because it can promote the CO gasification of carbonaceous reduced powder. However, the conventional technology has room for improvement in the selection of compounds that promote the CO gasification of carbonaceous reduced powder, and further improvement in the rate of the redox reaction was desired.

[0008] This invention has been made in view of the above 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 compound can be mixed with a carbonaceous reducing agent and generate or regenerate carbon monoxide with higher efficiency than the conventional technology. As a result of the investigation, they found that by using one or more compounds selected from the group consisting of alkali metal compounds and alkaline earth metal compounds, and having a thermal decomposition temperature or melting point, whichever is lower, below 1000°C, as a CO gasification catalyst powder to be mixed with carbonaceous reducing powder, the rate of carbon monoxide generation is significantly increased, and as a result, the production efficiency of sponge iron is dramatically improved.

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

[0011] [1] A method for producing sponge iron, comprising mixing carbonaceous reduction powder and CO gasification catalyst powder to form a mixed powder, and then heating the mixture of iron oxide powder and the mixed powder in a container to reduce the iron oxide powder, wherein the CO gasification catalyst powder is one or more selected from the group consisting of alkali metal compounds and alkaline earth metal compounds, the lower of the thermal decomposition temperature and melting point of the CO gasification catalyst powder is less than 1000°C, and the content of the CO gasification catalyst powder in the mixed powder is 0.10 parts by mass or more and 30.00 parts by mass or less per 100 parts by mass of carbonaceous reduction powder.

[0012] [2] The method for producing sponge iron according to [1], wherein the content of the CO gasification catalyst powder in the mixed powder is 0.50 parts by mass or more and 25.00 parts by mass or less per 100 parts by mass of the carbonaceous reduction powder.

[0013] [3] The method for producing sponge iron according to [1] or [2] above, wherein the lower of the thermal decomposition temperature and melting point of the CO gasification catalyst powder is 400°C or higher.

[0014] [4] The method for producing sponge iron according to any one of [1] to [3] above, wherein the CO gasification catalyst powder is one or more selected from the group consisting of alkaline earth metal compounds.

[0015] [5] The alkaline earth metal is calcium, and the CO gasification catalyst powder is CuK α A method for producing sponge iron according to [4] above, wherein powder X-ray diffraction is performed using a line, and the full width at half maximum of the peak with the maximum diffraction intensity among the obtained diffraction patterns is 0.05 degrees or more and 5.50 degrees or less.

[0016] [6] The method for producing sponge iron according to [5] above, wherein the full width at half maximum is 0.10 degrees or more and 5.00 degrees or less.

[0017] According to the present invention, sponge iron can be manufactured with higher efficiency compared to conventional techniques.

[0018] This is a schematic vertical cross-sectional view showing an example of the arrangement of raw material powders in a container. This is a schematic horizontal cross-sectional view showing an example of the arrangement of raw material powders in a container. This is a graph showing the relationship between the content (parts by mass) of CO gasification catalyst powder per 100 parts by mass of carbonaceous reduction powder and the reduction rate of reduced iron powder. This is a graph showing the relationship between the full width at half maximum of the X-ray diffraction peak of the CO gasification catalyst powder and the reduction rate of reduced iron powder.

[0019] 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.

[0020] 1. A method for producing sponge iron In one embodiment of the present invention, the present invention is a method for producing sponge iron by mixing carbonaceous reduction powder and CO gasification catalyst powder to form a mixed powder, and then charging iron oxide powder and the mixed powder into a container and heating it to reduce iron oxide powder, wherein the CO gasification catalyst powder is one or more selected from the group consisting of alkali metal compounds and alkaline earth metal compounds, the lower of the thermal decomposition temperature and melting point of the CO gasification catalyst powder is less than 1000°C, and the content of CO gasification catalyst powder in the mixed powder is 0.10 parts by mass or more and 30.00 parts by mass or less per 100 parts by mass of carbonaceous reduction powder.

[0021] [Iron Oxide Powder] The iron oxide powder used in the method for producing sponge iron according to the present invention may be any iron oxide powder as long as it contains iron oxide. The iron oxide powder, together with the carbonaceous reduction powder and CO gasification catalyst powder described later, constitutes the raw material powder that is the raw material for sponge iron produced by the method according to the present invention.

[0022] 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(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.

[0023] Specific raw material powders that can be used as iron oxide powder in the present invention include, but are not limited to, powders obtained by crushing natural iron ore, natural iron sand, commercially available iron oxide powders as reagents or industrial raw materials, mill scale and other iron oxide powders generated on 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 in the iron oxide powder is preferably 50% or more, more preferably 65% ​​or more, and even more preferably 80% or more.

[0024] "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.

[0025] 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.

[0026] 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.

[0027] [Carbonaceous Reduction Powder] The carbonaceous reduction powder used in the method according to the present invention may be any powder containing carbon. 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.

[0028] 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.

[0029]

[0030] 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).

[0031]

[0032] 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).

[0033]

[0034] 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).

[0035]

[0036] 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.

[0037] Specific raw material powders that can be used as carbonaceous reduced powder in the present invention 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 in the carbonaceous reduced powder used in the method of the present invention is preferably 50% or more, more preferably 70% or more, and even more preferably 90% or more.

[0038] The particle size of the carbonaceous reducing powder used in the method according to the present invention is not particularly limited because it has little effect on the reduction rate of sponge iron. 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. The D50 of the carbonaceous reducing powder can be measured by the same method as the measurement method of the D50 of the iron oxide powder described above.

[0039] [CO gasification catalyst powder] In the method for producing sponge iron according to the present invention, reduction of iron oxide powder is carried out using a mixed powder obtained by mixing carbonaceous reducing powder and CO gasification catalyst powder. The CO gasification catalyst powder promotes the reduction of iron oxide represented by the above chemical formula (4) by promoting the CO gasification of the carbonaceous reducing powder represented by the above chemical formula (2) or chemical formula (3).

[0040] The CO gasification catalyst powder is at least one selected from the group consisting of compounds of alkali metals and compounds of alkaline earth metals. The alkali metals refer to those excluding hydrogen among the elements belonging to Group 1 in the periodic table. The alkali metals include lithium, sodium, potassium, rubidium, cesium, and francium. In the present invention, since they are generally easily available, it is preferable to use a compound of lithium, sodium, or potassium. Further, as the compound of an alkali metal, it is preferable to use a carbonate, bicarbonate, sulfate, nitrate, acetate, sulfide, hydroxide, oxide, or halide.

[0041] Alkaline earth metals are elements belonging to Group 2 of the periodic table. Alkaline earth metals include beryllium, magnesium, calcium, strontium, barium, and radium. In this invention, it is preferable to use compounds of calcium, strontium, or barium because they are generally readily available and easy to handle. Also, as with alkali metals, it is preferable to use carbonates, bicarbonates, sulfates, nitrates, acetates, sulfides, hydroxides, oxides, or halides as compounds of alkaline earth metals.

[0042] The CO gasification catalyst powder in this invention may be used as a single compound or as a mixture of two or more different compounds. When using a mixture of two or more compounds, alkali metal compounds and alkaline earth metal compounds may be mixed together.

[0043] If the lower of the thermal decomposition temperature and melting point of the CO gasification catalyst powder is too high, the amount of CO gas generated during the heating process until the iron oxide powder reaches the temperature required for reduction will be low, leading to a decrease in the reduction rate and reduction rate of the iron oxide powder. Therefore, the lower of the thermal decomposition temperature and melting point of the CO gasification catalyst powder should be less than 1000°C, preferably 850°C or lower. On the other hand, there is no particular limit on the lower of the thermal decomposition temperature and melting point of the CO gasification catalyst powder. However, if the lower of the thermal decomposition temperature and melting point of the CO gasification catalyst powder is too low, thermal decomposition or melting of the CO gasification catalyst powder will occur before the iron oxide powder reaches the temperature required for reduction. In that case, the amount of CO gas generated when the iron oxide powder reaches the temperature required for reduction will be low, which may lead to a decrease in the reduction rate and reduction rate of the iron oxide powder. Therefore, the lower of the thermal decomposition temperature and melting point of the CO gasification catalyst powder is preferably 400°C or higher, and more preferably 550°C or higher.

[0044] The thermal decomposition temperature and melting point of the CO gasification catalyst powder can be known from descriptions in literature regarding the compounds used in the CO gasification catalyst powder, safety data sheets (SDS) issued by the manufacturers of commercially available compounds, and the like. For a certain compound, when one of the values of the thermal decomposition temperature and melting point is not described in the literature or SDS, or even if it is described but the value is shown as a numerical range and not fixed at a single temperature, the temperature described as one of the thermal decomposition temperature and melting point is regarded as "the lower temperature of either the thermal decomposition temperature or the melting point". Further, when multiple types of compounds are used for the CO gasification catalyst powder, it is sufficient that the lowest temperature among the thermal decomposition temperatures and melting points of these multiple types of compounds is less than 1000°C.

[0045] In the method for producing sponge iron according to the present invention, when a mixed powder obtained by mixing a carbonaceous reducing powder and a specific CO gasification catalyst powder is used, although the reason why the reduction of iron oxide powder is promoted is not well understood, the inventors consider it as follows.

[0046] As an example of the compound used for the CO gasification catalyst powder, the carbonate of an alkaline earth metal will be described. First, when the carbonate of an alkaline earth metal is heated and the temperature reaches the thermal decomposition temperature, the carbonate of the alkaline earth metal is thermally decomposed, and an oxide of the alkaline earth metal and carbon dioxide are generated by the chemical reaction shown in Chemical Formula (5). Here, M represents an alkaline earth metal.

[0047] [[ID=ll]]

[0048] The generated oxide of the alkaline earth metal is reduced by the carbon contained in the carbonaceous reducing powder, and an alkaline earth metal and carbon monoxide are generated by the chemical reaction shown in Chemical Formula (6).

[0049] <00OO108>

[0050] The generated alkaline earth metal reacts with, for example, the carbon dioxide generated by the chemical reaction shown in Chemical Formula (5), and an oxide of the alkaline earth metal and carbon monoxide are generated by the chemical reaction shown in Chemical Formula (7).

[0051]

[0052] When attempting to reduce iron oxide powder using only carbonaceous reduction powder without mixing it with CO gasification catalyst powder, the chemical reactions that produce carbon monoxide, as shown in chemical formulas (2) and (3), hardly proceed unless the temperature of the container containing the oxide powder and mixed powder reaches a high temperature of 1000°C or higher. In contrast, in the method for producing sponge iron according to the present invention, the lower of the thermal decomposition temperature or melting point of the CO gasification catalyst powder contained in the mixed powder is less than 1000°C. When the thermal decomposition temperature of the CO gasification catalyst powder, which is made of alkaline earth metal carbonate, is less than 1000°C, carbon monoxide is produced in the temperature range below 1000°C by the chemical reactions shown in chemical formulas (5) to (7). In particular, the lower the temperature at which the reaction of chemical formula (5) begins, the faster the chemical reactions shown in chemical formulas (6) and (7) proceed, and carbon monoxide is produced. The produced carbon monoxide comes into contact with the iron oxide powder, and the reduction of the iron oxide powder shown in chemical formula (4) proceeds.

[0053] After the temperature of the container holding the oxide powder and mixed powder reaches a high temperature of 1000°C or higher, the iron oxide powder is reduced mainly by carbon monoxide produced by the chemical reactions shown in chemical formulas (1) to (3).

[0054] Furthermore, if the CO gasification catalyst powder is an alkaline earth metal hydroxide, an alkali metal carbonate, or an alkali metal hydroxide compound with a thermal decomposition temperature of less than 1000°C, it is considered that carbon monoxide will be generated in the temperature range below 1000°C, similar to the case of the alkaline earth metal carbonate mentioned above, and the reduction of iron oxide powder will be promoted.

[0055] On the other hand, even if the compounds constituting the CO gasification catalyst powder are thermally stable and do not decompose at temperatures below their melting point, or if they do not produce carbon monoxide even if they do decompose, it is thought that the CO gasification of the carbonaceous reduction powder is promoted by a mechanism different from the chemical reaction described above. If the compounds constituting the CO gasification catalyst powder are compounds that do not decompose at temperatures below their melting point, they become liquid when the heating temperature reaches the melting point, and come into contact with the carbonaceous reduction powder more easily than in the powder state. As a result, an electron transfer mechanism acts between the alkali metal or alkaline earth metal contained in the compound and carbon. The electron transfer mechanism is a mechanism in which electrons are transferred between carbon and the catalyst, and the carbon is activated by the resulting change in the π-electron structure of the carbon. The electron transfer mechanism makes it easier to break the C-C bond in the carbonaceous reduction powder, and the CO gasification of the carbonaceous reduction powder is promoted.

[0056] Furthermore, if the compounds constituting the CO gasification catalyst powder are compounds that do not produce carbon monoxide even when thermally decomposed, the powder particles disintegrate due to thermal decomposition, making them more readily in contact with the carbonaceous reduction powder compared to the powder state. As a result, it is thought that an electron transfer mechanism acts between the alkali metal or alkaline earth metal contained in the decomposition product and carbon, facilitating the cleavage of C-C bonds in the carbonaceous reduction powder and promoting the CO gasification of the carbonaceous reduction powder.

[0057] Alkali metals and alkaline earth metals are elements with high catalytic activity. Therefore, the electron transfer mechanism between alkali metals or alkaline earth metals and carbon, as described above, is thought to be at work even when the compounds constituting the CO gasification catalyst powder undergo thermal decomposition to produce carbon monoxide. In this invention, we focus on the catalytic activity universally possessed by alkali metals or alkaline earth metals contained in the powder mixed with the carbonaceous reduction powder, and refer to this powder as "CO gasification catalyst powder".

[0058] In order for the CO gasification catalyst powder to fully exhibit its function, it is preferable that the CO gasification catalyst powder is sufficiently dispersed within the carbonaceous reduction powder during mixing, rather than being unevenly distributed. The mixing of the carbonaceous reduction powder and the CO gasification catalyst powder can be carried out using a known mixer or the like used for mixing powders.

[0059] In the method for producing sponge iron according to the present invention, the content of CO gasification catalyst powder in the mixed powder of carbonaceous reduction powder and CO gasification catalyst powder is 0.10 parts by mass or more and 30.00 parts by mass or less per 100 parts by mass of carbonaceous reduction powder. If the content of CO gasification catalyst powder per 100 parts by mass of carbonaceous reduction powder is too low, the effect of promoting CO gasification of the carbonaceous reduction powder is small, and the effect of increasing the reduction rate of iron oxide powder is poor. For this reason, the content of CO gasification catalyst powder per 100 parts by mass of carbonaceous reduction powder is set to 0.10 parts by mass or more, preferably 0.50 parts by mass or more, and more preferably 5.00 parts by mass or more. On the other hand, if the content of CO gasification catalyst powder per 100 parts by mass of carbonaceous reduction powder is too high, CO gasification of the carbonaceous reduction powder is promoted, but the total amount of CO gas generated decreases because the weight of carbonaceous reduction powder charged in the container decreases, and the reduction rate of iron oxide powder decreases. Therefore, the content of CO gasification catalyst powder per 100 parts by mass of carbonaceous reduction powder shall be 30.00 parts by mass or less, preferably 25.00 parts by mass or less. When two or more different types of compounds are mixed and used as CO gasification catalyst powder, the total content of all CO gasification catalyst powders per 100 parts by mass of carbonaceous reduction powder shall be 0.10 parts by mass or more and 30.00 parts by mass or less.

[0060] In preferred embodiments, the compounds are one or more selected from the group consisting of alkaline earth metal compounds. Alkaline earth metal compounds pose a lower risk of damaging the refractory material used for the inner wall of the furnace used to heat the container compared to alkali metal compounds. Therefore, it is preferable that the compounds constituting the CO gasification catalyst powder are one or more selected from the group consisting of alkaline earth metal compounds.

[0061] In a more preferred embodiment, the alkaline earth metal is calcium. That is, in this embodiment, the CO gasification catalyst powder is one or more selected from the group consisting of calcium compounds, and CuK α Powder X-ray diffraction is performed using a line, and the full width at half maximum of the peak with the maximum diffraction intensity in the obtained diffraction pattern is between 0.05 degrees and 5.50 degrees. Here, "degrees" is the unit of angle in the degree system.

[0062] Among alkaline earth metal compounds, calcium compounds are preferred because they are readily available as inexpensive commercial products. Preferably, calcium carbonate, calcium bicarbonate, calcium sulfate, calcium nitrate, calcium acetate, calcium sulfide, calcium hydroxide, calcium oxide, or calcium halide are used as calcium compounds. From the viewpoint of generating carbon monoxide through thermal decomposition, calcium carbonate or calcium hydroxide is more preferable.

[0063] When a compound selected from the group consisting of calcium compounds has low crystallinity, its thermal decomposition temperature is lower than that of a normal thermal decomposition temperature. In other words, lower crystallinity of a compound is preferable because it makes it easier for the thermal decomposition of the compound shown in chemical formula (5) to occur at a lower temperature, and the chemical reactions shown in chemical formulas (6) and (7) proceed more quickly, making it easier to produce carbon monoxide.

[0064] The crystalline nature of calcium compounds is expressed as CuK for calcium compounds. αThe crystallinity can be evaluated by performing powder X-ray diffraction using a line and measuring the full width at half maximum (FWHM) of the peak with the highest diffraction intensity in the obtained diffraction pattern. A larger FWHM indicates lower crystallinity. Here, "FWHM" refers to the full width at half maximum (full width at half maximum). That is, the FWHM is defined as the difference in diffraction angles (2θ) at two points where the X-ray diffraction intensity is half of the maximum peak value. If the FWHM is too small, the crystallinity is excessively high, making it difficult for the CO gasification catalyst powder to decompose thermally, resulting in a low catalytic effect and difficulty in CO gasification of the carbonaceous reduction powder. Therefore, the FWHM is preferably 0.05 degrees or higher, more preferably 0.10 degrees or higher, and even more preferably 1.00 degrees or higher. On the other hand, if the FWHM is too large, the crystallinity is excessively low, making it easy for the CO gasification catalyst powder to decompose thermally, so CO gasification of the carbonaceous reduction powder occurs at a considerably lower temperature than the temperature at which reduction of iron oxide powder begins. If this occurs, a large amount of CO gas will be released outside the container before the reduction of the iron oxide powder begins, making it impossible to secure a sufficient amount of CO gas for the reduction of the iron oxide powder. For this reason, the full width at half maximum is preferably 5.50 degrees or less, more preferably 5.00 degrees or less, and even more preferably 2.00 degrees or less.

[0065] When determining the full width at half maximum (FWHM) of the peak with the maximum diffraction intensity for calcium compounds, it is preferable to perform background processing by subtracting the baseline diffraction intensity at non-diffraction angles from the diffraction intensity of the diffraction peak. Furthermore, if there are many vertical fluctuations in the diffraction pattern near the diffraction peak due to noise in the measurement signal, it is preferable to perform smoothing processing on the diffraction intensity. By applying these data processing steps to the diffraction intensity, it is possible to obtain a measurement value for the FWHM of the diffraction peak that is closer to the true value.

[0066] The method for adjusting the full width at half maximum (FWHM) of the X-ray diffraction peak obtained by the aforementioned method for calcium compounds to be between 0.05 degrees and 5.50 degrees is not particularly limited, and the most suitable method can be used depending on the type of compound. For example, when using calcium carbonate as the calcium compound, the crystallinity of calcium carbonate can be increased by heating it to a temperature lower than its thermal decomposition temperature of 825°C. By utilizing this property, the crystallinity of calcium carbonate can be adjusted by changing the heating temperature and time so that the FWHM value is between 0.05 degrees and 5.50 degrees.

[0067] The particle size of the CO gasification catalyst powder used in the method for producing sponge iron according to the present invention is not particularly limited, as long as there are no handling problems. However, if the particle size of the CO gasification catalyst powder is too small, handling may become difficult. For this reason, it is preferable that the particle size of the CO gasification catalyst powder be 0.5 μm or more in terms of 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 CO gasification catalyst powder is too large, the contact surface area with the carbonaceous reduction powder may be too small, and the catalyst may not be able to exert its effect sufficiently. For this reason, it is preferable that the particle size of the CO gasification catalyst powder be 300 μm or less in terms of D50, and more preferably 100 μm or less. The D50 of the CO gasification catalyst powder can be measured by the same method as the method for measuring the D50 of the iron oxide powder described above.

[0068] [Reduction of Iron Oxide Powder] As described above, in the method for producing sponge iron according to the present invention, carbonaceous reduction powder and CO gasification catalyst powder are mixed to form a mixed powder, and the iron oxide powder and the mixed powder are placed in a container and heated to reduce the iron oxide powder and produce sponge iron. The arrangement of the iron oxide powder and the mixed powder in the container can be any arrangement that is convenient for the carbon monoxide generated from the mixed powder to come into contact with the iron oxide powder.

[0069] In the method for producing sponge iron according to the present invention, it is preferable to charge the raw material powder, iron oxide powder, and the mixed powder into a container without mixing them, so that the portion consisting of iron oxide powder and the portion consisting of the mixed powder are separated from each other. This makes it easier for carbon dioxide generated in the portion consisting of the mixed powder by the chemical reaction shown in chemical formula (1) or (5) to immediately change into carbon monoxide by the chemical reaction shown in chemical formula (2) or (6). Alternatively, oxygen may be deficient in the portion consisting of the mixed powder, making it easier for carbon monoxide to be directly generated by chemical formula (3) or (7).

[0070] For example, in the arrangement of raw material powders shown in Figures 1 and 2, as described above, the iron oxide powder 2 is arranged in a cylindrical shape inside a bottomed cylindrical container 1. The iron oxide powder 2 is surrounded by a mixed powder 3. In this arrangement, the carbon monoxide generated in the portion consisting of the mixed powder 3 diffuses toward the portion consisting of the iron oxide powder 2, driven by the carbon monoxide concentration gradient.

[0071] 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.

[0072] The ratio of the mixed powder to the iron oxide powder charged into the container is not particularly limited, and the amount of mixed powder necessary for the reduction of the iron oxide powder may be charged. However, if the ratio of the mixed powder to the iron oxide powder is too small, the reduction rate of the reduced iron powder described later will decrease. For this reason, it is preferable that the ratio of the mixed powder to 100 parts by mass of iron oxide powder be 20 parts by mass or more. On the other hand, if the ratio of the mixed powder to the iron oxide powder is too large, the carbonaceous reduction powder and the CO gasification catalyst powder will be wasted. For this reason, it is preferable that the ratio of the mixed powder to 100 parts by mass of iron oxide powder be 70 parts by mass or less.

[0073] 2. Method for Reducing Iron Oxide Powder In another embodiment, the present invention relates to a method for reducing iron oxide powder by mixing carbonaceous reduction powder and CO gasification catalyst powder to form a mixed powder, and then heating the iron oxide powder and the mixed powder in a container, wherein the CO gasification catalyst powder is one or more selected from the group consisting of alkali metal compounds and alkaline earth metal compounds, the lower of the thermal decomposition temperature and melting point of the CO gasification catalyst powder is less than 1000°C, and the content of the CO gasification catalyst powder in the mixed powder is 0.10 parts by mass or more and 30.00 parts by mass or less per 100 parts by mass of carbonaceous reduction powder. The method for reducing iron oxide powder according to the present invention can increase the reduction rate of iron oxide powder compared to the conventional technology.

[0074] 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.

[0075] [Example 1] 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, coke powder was prepared by carbonizing coal powder at 1200°C to obtain a carbon content of 88%. The D50 of this coke powder, measured by the method described above, was 3.0 mm.

[0076] Next, commercially available CO gasification catalyst powders of calcium carbonate, calcium hydroxide, sodium carbonate, barium chloride, and strontium carbonate were prepared. Table 1 shows the D50 measured by the method described above, the thermal decomposition temperature listed in the safety data sheet, and the melting point for these compounds. However, since the melting point of calcium hydroxide is unknown, the thermal decomposition temperature of 580°C is considered to be the lower of either the thermal decomposition temperature or the melting point for calcium hydroxide. Similarly, since the thermal decomposition temperature of barium chloride is also unknown, the melting point of 962°C is considered to be the lower of either the thermal decomposition temperature or the melting point for barium chloride. For four of these compounds, excluding strontium carbonate, the lower of either the thermal decomposition temperature or the melting point was all below 1000°C.

[0077] Next, 100 parts by mass of coke powder and the CO gasification catalyst powder in the amounts (parts by mass) shown in Table 1 were mixed thoroughly using a mixer to prepare mixed powders with different types and amounts of CO gasification catalyst powder. Next, 200 grams of one type of the obtained mixed powder and 500 grams of iron oxide powder were placed in a container as shown in Figures 1 and 2. Next, the container was charged into an electric furnace and heated to 1100°C at a heating rate of 4°C per minute under a nitrogen atmosphere, held at 1100°C for 9 hours, and then cooled. Next, the sintered sponge iron, which had become a lump, 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. As a reference example, reduced iron powder was also obtained by reducing iron oxide powder using only coke powder under the same conditions as above, without mixing in CO gasification catalyst powder.

[0078] Next, the oxygen content of the obtained reduced iron powder was measured, and the reduction rate of the reduced iron powder was calculated using the following formula (1). The measurement of the oxygen content in the reduced iron powder was performed in accordance with the method described in Japanese Industrial Standard JIS Z 2613 "General Rules for Determination of Oxygen in Metallic Materials". These procedures were repeated for all the test No. samples shown in Table 1. Table 1 shows the calculated reduction rates of the reduced iron powder.

[0079]

[0080]

[0081] Next, coke powder was mixed with the same calcium hydroxide and sodium carbonate as shown in Table 1, and thoroughly mixed using a mixer to prepare mixed powders with different CO gasification catalyst powder content. Table 2 shows the respective content (parts by mass) of calcium hydroxide and sodium carbonate per 100 parts by mass of coke powder, and the total content (parts by mass) of both. Next, reduced iron powder was obtained using the obtained mixed powder by the same method as described above. Next, the reduction rate of the obtained reduced iron powder was calculated using the same method as described above. Table 2 shows the calculated reduction rates of the reduced iron powder.

[0082] Furthermore, for the samples shown in Tables 1 and 2, the relationship between the CO gasification catalyst powder content (parts by mass) per 100 parts by mass of carbonaceous reduction powder and the reduction rate of reduced iron powder is shown in the graph in Figure 3. Here, for the samples shown in Table 2, the horizontal axis of the graph in Figure 3 represents the total content (parts by mass), which is the sum of the respective content (parts by mass) of calcium hydroxide and sodium carbonate.

[0083]

[0084] As shown in Tables 1 and 2, and Figure 3, in the invention example where the lower of the thermal decomposition temperature or melting point was less than 1000°C, and the content of CO gasification catalyst powder in the mixed powder was 0.10 parts by mass or more and 30.00 parts by mass or less per 100 parts by mass of coke powder, the reduced iron powder obtained from sponge iron showed a high reduction rate of 40% or more. In particular, in the invention example shown in Table 1, where the content of CO gasification catalyst powder was 0.50 parts by mass or more and 25.00 parts by mass or less, the reduction rate of the reduced iron powder was 60% or more, which is particularly high compared to the other invention examples. The same is true for the invention example shown in Table 2, where the total content of CO gasification catalyst powder was 5.00 parts by mass or more and 25.00 parts by mass or less.

[0085] On the other hand, the reduction rate of the reduced iron powder in the comparative example where the CO gasification catalyst powder content was less than 0.10 parts by mass was less than 40%. The reduction rate of the reduced iron powder in these comparative examples showed almost no improvement compared to the reduction rate of 33% of the reference example reduced iron powder obtained from carbonaceous reduced powder without the CO gasification catalyst powder. This is thought to be because the effect of promoting the CO gasification of coke powder was small, and the effect of increasing the reduction rate of iron oxide powder was poor. Furthermore, the reduction rate of the reduced iron powder in the comparative example where the CO gasification catalyst powder content was more than 30.00 parts by mass was also less than 40%. This is thought to be because, although the CO gasification of coke powder is promoted in the temperature range below 1000°C, the total amount of CO gas generated decreases because the weight of coke powder charged into the container decreases, and the reduction rate of iron oxide powder decreases.

[0086] Furthermore, the reduction rate of the reduced iron powder in the comparative example using commercially available strontium carbonate as the CO gasification catalyst powder showed almost no improvement compared to the reduction rate of the reference example reduced iron powder obtained from carbonaceous reduced powder without the CO gasification catalyst powder. The thermal decomposition temperature of strontium carbonate is 1000°C and its melting point is 1497°C, and the lower of the two, 1000°C, is higher than the range of the present invention. For this reason, it is thought that in the reduced iron powder of these comparative examples, the amount of CO gas generated during the heating process until the container temperature reached the temperature necessary for the reduction of the iron oxide powder was small, and the reduction of the iron oxide powder did not proceed easily.

[0087] [Example 2] The same iron oxide powder and coke powder used in Example 1 were prepared. Next, several types of calcium carbonate and calcium hydroxide with different crystallinity were prepared as CO gasification catalyst powders, and CuK αPowder X-ray diffraction was performed using a beam. The crystallinity of these compounds was adjusted by heating them for a predetermined time at a temperature lower than the lower of the thermal decomposition temperature and the melting point. A SmartLab ("SmartLab" is a registered trademark) manufactured by Rigaku Corporation was used for powder X-ray diffraction, and a Cu target was used as the radiation source. The scanning speed of the diffraction angle 2θ of the counter tube fixed to the goniometer was set to 4 degrees per minute. Among the obtained diffraction patterns, the peak with the maximum diffraction intensity was the (10⁴) peak at a diffraction angle 2θ of approximately 29 degrees for calcium carbonate, and the (10¹) peak at a diffraction angle 2θ of approximately 34 degrees for calcium hydroxide. After background processing of these peaks, the full width at half maximum (FWHM) of the peaks was determined. Table 3 shows the D50 measured by the method described above, the thermal decomposition temperature and melting point as stated in the safety data sheet, and the obtained FWHM for these compounds.

[0088] Next, 100 parts by mass of coke powder and calcium compounds in the amounts (parts by mass) shown in Table 3 were mixed to prepare mixed powders with different types of calcium compounds, full width at half maximum, and content, using the same method as in Example 1. Then, using the obtained mixed powders and iron oxide powder, reduced iron powder was prepared using the same method as in Example 1, and the reduction rate of the obtained reduced iron powder was calculated using the same method as in Example 1. The reduction rates of the obtained reduced iron powder are shown in Table 3.

[0089]

[0090] As shown in Table 3, in the inventive example of reduced iron powder in which the full width at half maximum (FWHM) of the calcium compound was 0.05 degrees or more and 5.50 degrees or less, and the content of the calcium compound in the mixed powder was 0.10 parts by mass or more and 30.00 parts by mass or less per 100 parts by mass of coke powder, the reduction rate was a very high value of over 40%. On the other hand, the reduction rate of the inventive example of reduced iron powder in which the FWHM was less than 0.05 degrees was 40%, which was hardly improved compared to the reduction rates of the other inventive examples of reduced iron powder. This is thought to be because the calcium compound was difficult to thermally decompose due to its excessively high crystallinity, resulting in a low catalytic effect and making it difficult for the carbonaceous reduced powder to gasify CO. Furthermore, the reduction rate of the inventive example of reduced iron powder in which the FWHM was greater than 5.50 degrees was also 40%. This is thought to be because, due to the excessively low crystallinity, the calcium compounds are easily thermally decomposed, causing CO gasification of the carbonaceous reduction powder to occur at a considerably lower temperature than the temperature at which reduction of iron oxide powder begins, and thus failing to secure a sufficient amount of CO gas for the reduction of iron oxide powder.

[0091] [Example 3] Calcium carbonate and calcium hydroxide with different crystallinity were prepared using the same method as in Example 2. For these compounds, the D50 measured by the method described above, the thermal decomposition temperature and melting point as stated in the safety data sheet, and the full width at half maximum obtained using the same method as in Example 2 are shown in Table 4.

[0092] Next, 100 parts by mass of coke powder and 12.00 parts by mass of calcium carbonate or 8.00 parts by mass of calcium hydroxide were mixed to prepare mixed powders with different full widths at half maximum of the CO gasification catalyst powder using the same method as in Example 1. Next, reduced iron powder was prepared using the obtained mixed powder and iron oxide powder using the same method as in Example 1, and the reduction rate of the obtained reduced iron powder was calculated using the same method as in Example 1. The reduction rates of the obtained reduced iron powder are shown in Table 4. Furthermore, the relationship between the full width at half maximum of the X-ray diffraction peak of the CO gasification catalyst powder and the reduction rate of the reduced iron powder for the samples shown in Table 4 is shown in the graph in Figure 4.

[0093]

[0094] As shown in Table 4 and Figure 4, in the inventive example where the full width at half maximum (FWHM) of the calcium compound was between 0.05° and 5.50°, the reduction rate was a very high value of over 40%. In particular, among the inventive examples shown in Table 4, in the inventive example where the FWHM of the calcium compound was between 1.00° and 2.00°, the reduction rate of the reduced iron powder was over 60%, which is particularly high compared to the other inventive examples.

[0095] 1. Container 2. Iron oxide powder 3. Mixed powder (invention) or carbonaceous reduced powder (prior art) 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 mixing carbonaceous reduction powder and CO gasification catalyst powder to form a mixed powder, and then heating the mixture of iron oxide powder and iron oxide powder in a container to reduce the iron oxide powder, wherein the CO gasification catalyst powder is one or more selected from the group consisting of alkali metal compounds and alkaline earth metal compounds, the lower of the thermal decomposition temperature and melting point of the CO gasification catalyst powder is less than 1000°C, and the content of the CO gasification catalyst powder in the mixed powder is 0.10 parts by mass or more and 30.00 parts by mass or less per 100 parts by mass of carbonaceous reduction powder.

2. The method for producing sponge iron according to claim 1, wherein the content of the CO gasification catalyst powder in the mixed powder is 0.50 parts by mass or more and 25.00 parts by mass or less per 100 parts by mass of the carbonaceous reduction powder.

3. The method for producing sponge iron according to claim 1 or 2, wherein the lower of the thermal decomposition temperature and melting point of the CO gasification catalyst powder is 400°C or higher.

4. The method for producing sponge iron according to any one of claims 1 to 3, wherein the CO gasification catalyst powder is one or more selected from the group consisting of alkaline earth metal compounds.

5. The alkaline earth metal is calcium, and the CO gasification catalyst powder contains CuK α A method for producing sponge iron according to claim 4, wherein powder X-ray diffraction is performed using a line, and the full width at half maximum of the peak with the maximum diffraction intensity among the obtained diffraction patterns is 0.05 degrees or more and 5.50 degrees or less.

6. The method for producing sponge iron according to claim 5, wherein the full width at half maximum is 0.10 degrees or more and 5.00 degrees or less.

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