Metal powder manufacturing method

Abstract

To provide a manufacturing method of metal powder by an atomization method utilizing a metal containing an organic compound such as a classification residue in which hot water blur or clogging of a tap nozzle are suppressed, and a long term operation is possible.SOLUTION: A method of manufacturing a metal powder in which a molten raw material metal is heated and melted to a molten metal, the molten metal is dropped, a fluid is blown to a flow of the dropping molten metal to crash and coagulate the molten metal to obtain metal powder, in which an amount of carbon of the molten raw material metal is 0.01 mass% or less, and the molten metal raw material metal contains a carbon reduction raw material metal, the carbon reduction raw material metal is obtained by reducing its carbon amount by heating a raw material metal having a carbon amount larger than 0.01 mass% under an acidic atmosphere.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing metal powder. [Background technology]

[0002] Metal powders are important materials in industry and are used in a variety of applications, including electronic materials, catalysts, active materials in batteries, tools, pharmaceuticals, and jewelry, depending on their properties.

[0003] There are various methods for producing metal powders, such as wet reduction, atomization, and plasma methods, and the optimum method is selected from the viewpoints of the intended use of the metal powder and acceptable costs.

[0004] Of the elements contained in metal powder, carbon may need to be reduced depending on the application of the metal type, and conventionally, molten steel is decarburized before being used to produce powder, or powder is first decarburized (Patent Documents 1 and 2). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 7-157803 [Patent Document 2] Japanese Patent Application Publication No. 1-234505 Summary of the Invention [Problem to be solved by the invention]

[0006] Generally, in the production of metal powder by atomization, raw metal is heated and melted in a furnace to produce molten metal, which is then dropped from a nozzle at the bottom of the furnace. A fluid such as water is sprayed onto the falling flow of the molten metal to pulverize and solidify the molten metal into particles, and the resulting metal powder is then recovered. The resulting metal powder is then subjected to a classification process to achieve the desired particle size distribution, resulting in the final metal powder product. As a by-product of the classification, metal powder that is rejected from the final product is generated. In this specification, the metal powder rejected from the final product is referred to as "classification residue."

[0007] In order to improve the fluidity of metal powders, surface treatment is sometimes performed by coating the particle surfaces with an organic compound. If the powder does not have high fluidity, it may not be possible to successfully classify it in the classification process. Therefore, the surface treatment is generally performed before the classification process.

[0008] Although it is conceivable that the classification residue (coated with organic compounds) generated during the classification process would be disposed of, if it could be reused as a raw material for molten metal, this would be advantageous in terms of both the production cost of metal powder and the environmental impact. The inventors reused the classification residue to prepare molten metal and performed continuous atomization operations, but melt spillage and clogging of the discharge nozzle occurred during the operation, making long-term continuous atomization operations impossible. The inability to perform long-term continuous operation is a problem in terms of the production cost and production efficiency of metal powder. Melt spillage refers to the phenomenon in which at least a portion of the molten metal emerging from the discharge nozzle does not fall vertically but splashes sideways.

[0009] The present invention aims to provide a method for producing metal powder by an atomization method that utilizes metal containing organic compounds, such as classification residue, in which melt spillage and clogging of the discharge nozzle are suppressed and long-term continuous operation is possible. [Means for solving the problem]

[0010] In order to solve the above problems, the inventor first investigated the causes of the melt runout and clogging of the discharge nozzle. Analysis of the material clogging the discharge nozzle revealed that it contained a large amount of carbon. The inventor speculated that this was a carbide of organic compounds adhering to the surface of the classification residue particles, and came up with the idea of ​​removing the organic compounds before preparing the molten metal to prevent this from occurring. After actual testing, the inventor completed the present invention. In this invention, the amount of carbon in the classification residue, etc. is used as an indicator of the amount of organic compounds it contains.

[0011] That is, the present invention is as follows. [1] A method for producing metal powder by heating and melting a molten raw material metal to form a molten metal, dropping the molten metal, and spraying a fluid onto the falling flow of the molten metal to pulverize and solidify the molten metal, wherein the carbon content of the molten raw material metal is 0.01 mass% or less, and the molten raw material metal includes a reduced-carbon raw material metal, and the reduced-carbon raw material metal is obtained by heating a raw material metal with a carbon content of more than 0.01 mass% in an oxidizing atmosphere to reduce the carbon content.

[0012] [2] The method for producing a metal powder according to [1], wherein the raw metal having a carbon content of more than 0.01% by mass has a volume-based cumulative 50% particle size (D50) of 0.1 to 50 μm as measured by a laser diffraction particle size analyzer, and is a metal powder containing an organic compound, or a metal piece having an organic compound adhered to its surface.

[0013] [3] The method for producing a metal powder according to [2], wherein the raw material metal having a carbon content of more than 0.01% by mass is heated at 100 to 400°C.

[0014] [4] The method for producing metal powder according to any one of [1] to [3], wherein the carbon-reduced raw metal is obtained by heating the raw metal having a carbon content of more than 0.01% by mass in an oxidizing atmosphere to reduce the carbon content to 0.01% by mass or less.

[0015] [5] The method for producing a metal powder according to any one of [1] to [4], wherein the carbon content of the raw material metal having a carbon content of more than 0.01% by mass is 1% by mass or less.

[0016] [6] The method for producing a metal powder according to any one of [1] to [5], wherein the amount of the reduced-carbon raw metal in the molten raw metal is 5 to 100 mass %.

[0017] [7] The method for producing a metal powder according to any one of [1] to [6], wherein the raw metal having a carbon content of more than 0.01% by mass is a metal powder produced by an atomization method and surface-treated with an organic compound.

[0018] [8] The constituent metal of the raw material metal having a carbon content of more than 0.01% by mass has a free energy of oxide formation ΔG M and the free energy of formation of carbon oxides at that temperature, ΔG C The difference (ΔG M -ΔG C ) is greater than 0 kJ / molO2. [Effects of the Invention]

[0019] According to the present invention, there is provided a method for producing metal powder by an atomization method that utilizes metals containing organic compounds, such as classification residue, which suppresses melt spillage and clogging of the discharge nozzle and enables long-term continuous operation. DETAILED DESCRIPTION OF THE INVENTION

[0020] Hereinafter, each configuration of the embodiment of the method for producing metal powder of the present invention will be described in detail.

[0021] [Molten metal raw material] In the present invention, a molten metal raw material having a carbon content of 0.01% by mass or less is used as the raw material for preparing the molten metal. When the molten metal is prepared from a molten metal raw material having such a low carbon content, i.e., a low content of organic compounds, melt bleeding and nozzle clogging are less likely to occur during atomization. From the viewpoint of preventing melt bleeding and the like, the carbon content of the molten metal raw material is preferably 0.008% by mass or less (80 ppm or less). The detection limit of current carbon measurement devices is about 10 ppm, but there are also metals whose carbon content is below this detection limit (i.e., 10 ppm or less). When the proportion of the carbon-reduced raw material metal described below in the molten metal raw material is low, the carbon content of the molten metal raw material as a whole may be below the detection limit.

[0022] While the metal species of the molten metal raw material is not particularly limited, specific examples include one or more elements from Groups 2 to 15 of the Periodic Table. Given the wide range of demand for these metals, such as electronic materials, they are preferably one or more of the following: Au (gold), Ag (silver), Cu (copper), Pd (palladium), Pt (platinum), Ni (nickel), Co (cobalt), Al (aluminum), Si (silicon), P (phosphorus), B (boron), Ti (titanium), Cr (chromium), Fe (iron), Zn (zinc), In (indium), Sn (tin), Te (tellurium), Bi (bismuth), Mg (magnesium), and Mn (manganese). These metals may be used alone to form a single metal powder, or multiple metals may be used to form an alloy powder. Furthermore, trace amounts of other metals may be added to one or multiple metals as the main metals to impart desired properties to the resulting metal (alloy) powder.

[0023] <Reduced carbon raw metal> The molten metal raw material described above includes the reduced-carbon raw material metal described below. The molten metal raw material may be entirely or partially reduced-carbon raw material metal. This reduced-carbon raw material metal is a raw material metal with a carbon content of more than 0.01% by mass, which has been reduced in carbon content by heating in an oxidizing atmosphere. The amount (usage ratio) of the reduced-carbon raw material metal in the molten metal raw material is determined according to the carbon content of this reduced-carbon raw material metal, so that the carbon content of the entire molten metal raw material is 0.01% by mass or less (preferably within the preferred range described above).

[0024] As explained below, the raw material metals that form the basis of the reduced-carbon raw material metals are typically classified residues and other waste materials or recycled raw materials, which are cost-effective. From this cost perspective, preferably 5 to 100 mass% of the molten raw material metal is reduced-carbon raw material metal, more preferably 15 to 100 mass% is reduced-carbon raw material metal, even more preferably 30 to 100 mass% is reduced-carbon raw material metal, and particularly preferably 40 to 100 mass% is reduced-carbon raw material metal. When the proportion of reduced-carbon raw material metal is less than 100 mass%, the remaining metals that make up the molten raw material metal include metal shot such as silver shot and copper phosphorus shot, and electrolytic metal.

[0025] To enable the use of reduced-carbon raw metal in the above-mentioned proportions, it is preferable to reduce the carbon content of the raw metal to 0.01% by mass or less, particularly from the viewpoint of reliably suppressing melt spillage and clogging of the tapping nozzle and enabling continuous atomization operation even when the molten raw metal is composed solely of reduced-carbon raw metal. Furthermore, from the viewpoint of continuous atomization operation, it is more preferable that the carbon content of the reduced-carbon raw metal obtained by heating the raw metal be 0.009% by mass or less (90 ppm or less). However, since reducing the carbon content to below the detection limit is not practical from the viewpoint of cost, etc., the carbon content of the reduced-carbon raw metal is preferably 0.002% by mass or more (20 ppm or more).

[0026] (raw metal) As mentioned above, the raw material metal has a carbon content of more than 0.01 mass %. There is no upper limit to the carbon content, but it is usually 1 mass % or less.

[0027] The constituent metal species of the raw metal are basically the same as the metal species of the molten metal described above. Even if oxidation of the metal occurs when the carbon content is reduced by heating, the resulting carbon-reduced raw metal can be reduced, or a molten metal prepared from the molten metal containing the carbon-reduced raw metal can be reduced, but this increases the cost of preparing the molten metal. Because such metal oxidation is unlikely to occur, the constituent metal species of the raw metal having a carbon content of more than 0.01 mass% is preferably a metal M that satisfies the following condition (1): Condition (1): The free energy ΔG of formation of oxides of the constituent metals of the raw material metal at the heating temperature M and the free energy of formation of carbon oxides at that temperature, ΔG C The difference (ΔG M -ΔG C ) is greater than 0 kJ / molO2.

[0028] The smaller the free energy of oxide formation ΔG, the more stable the oxide, and can be used as an index of the ease with which an oxidation reaction occurs. The metal M is less likely to oxidize than carbon at the heating temperature. The difference (ΔG M -ΔG C ) has no particular upper limit, but the upper limit is, for example, about 700 kJ / molO2.

[0029] The free energy of formation of carbon oxides under the above condition (1), ΔG CRegarding the reaction in which carbon and oxygen react to produce carbon dioxide, the free energy of formation of carbon dioxide is constant at approximately -390 kJ / molO2 at any temperature. On the other hand, the free energy of formation of carbon monoxide in the reaction in which carbon and oxygen react to produce carbon monoxide decreases as the temperature increases, and becomes the same as the free energy of formation of carbon dioxide (approximately -390 kJ / molO2) at about 700°C, and becomes smaller than the free energy of formation of carbon dioxide as the temperature increases further. In the present invention, the free energy of formation of carbon oxides, ΔG C is the free energy of formation of carbon monoxide in a reaction that produces -390 kJ / molO2 at temperatures below 700°C and carbon monoxide at temperatures above 700°C.

[0030] Preferable metal species for the raw material metal are Au, Ag, Cu, Pd and Pt, because they can reduce the carbon content while suppressing oxidation even when heated at a relatively low temperature, as will be described later.

[0031] Because the carbon content of the raw metal can be reduced by heating at a relatively low temperature (described below), the raw metal is preferably a metal powder containing an organic compound and having a volume-based cumulative 50% particle size (D50) of 0.1 to 50 μm as measured by a laser diffraction particle size analyzer, or a metal piece having an organic compound attached to its surface. Because the carbon content of the metal powder can be sufficiently reduced by heating at a low temperature, the D50 is more preferably 0.3 to 40 μm, even more preferably 0.5 to 30 μm, particularly preferably 0.5 to 20 μm, and most preferably 0.5 to 15 μm. The size of the metal pieces is not particularly limited, but for example, the dimensions (the average major axis of the planar shape of each metal piece when photographed) are 5 mm or less (usually 0.1 mm or more).

[0032] Examples of the raw metals described above include metal powders produced by wet reduction reactions, metal powders that have been surface-treated with organic compounds to improve the flowability of the powder, and metal pieces at least partially coated with resin.

[0033] Regarding the metal powder produced by the wet reduction reaction, an organic compound may be added to the reaction system in order to achieve a desired effect, and in this case, the organic compound may adhere to the surface of the powder particles or become entrapped inside the particles.

[0034] The metal powder surface-treated with the organic compound is typically a residue that is removed by classification from metal powder that has been produced by an atomization method such as gas atomization or water atomization, as described in the section [Problem to be Solved by the Invention], and surface-treated with an organic compound. The organic compound in this case is, for example, an alcohol, amine, fatty acid, or thiol having 2 to 36 carbon atoms.

[0035] Examples of metal pieces at least partially coated with the resin include metal pieces produced by recycling discarded electrical appliances and electronic components. These are called "nuggets" and are inexpensive, making them suitable as raw materials for preparing molten metal. Resin is an organic compound, and using these metal pieces as raw materials for preparing molten metal can cause melt spillage and clogging of the discharge nozzle. However, the application of the present invention suppresses these problems, enabling long-term continuous operation of metal powder production by the atomization method.

[0036] (heating raw metal) The carbon content of the raw metal described above is reduced (preferably to the range described above) by heating it in an oxidizing atmosphere. Carbon is thought to exist in the raw metal in the form of organic compounds, and heating it in an oxidizing atmosphere causes thermal decomposition and oxidation of the organic compounds, resulting in low-molecular-weight compounds such as carbon dioxide, which are then released from the raw metal.

[0037] The oxidizing atmosphere contains oxygen. The oxidizing atmosphere may be an atmosphere containing 100% oxygen by volume, or may be a mixed atmosphere containing oxygen and other gaseous elements. The other gaseous elements are not particularly limited as long as they do not inhibit the oxidizing action of oxygen and are inert to the raw material metal. From the viewpoint of effectively reducing the carbon content of the raw material metal, the oxidizing atmosphere preferably contains 5 to 50% by volume of oxygen, more preferably 8 to 40% by volume. Air satisfies these conditions and is inexpensive, making it highly preferable.

[0038] The heating temperature for heating the raw metal is not particularly limited as long as it can reduce the carbon content, but when the raw metal is a metal powder containing an organic compound or a metal piece with an organic compound attached to its surface, as described above, with a D50 of 0.1 to 50 μm, the carbon content can be reduced by heating at a relatively low temperature of 100 to 400° C., which is very preferable in terms of heat cost. Furthermore, no equipment such as a converter that can withstand high heat treatment as in Patent Document 1 is required for heating, which is very preferable in terms of equipment cost.

[0039] From the viewpoint of sufficiently reducing the heat cost and the carbon content of the metal powder or metal pieces, the heating temperature is preferably 120 to 330°C, and more preferably 140 to 280°C.

[0040] There is no particular limitation on the pressure under which the raw material metal is heated, and the heating may be carried out under atmospheric pressure, or under reduced pressure to facilitate the removal of carbon.

[0041] The heating time is appropriately set so as to sufficiently reduce the carbon content of the raw metal. Note that an excessively long heating time is undesirable from the viewpoint of the production efficiency of the carbon-reduced raw metal, and therefore the heating time is preferably 1 to 25 hours, more preferably 1 to 18 hours.

[0042] <Supply of molten metal (molten metal preparation process)> In the present invention, a molten metal is prepared by heating and melting a molten metal source material containing the carbon-reduced source metal described above. When the molten metal source material is substantially composed of a single metal, the heating and melting is carried out at a temperature equal to or higher than the melting point of that metal. When the molten metal source material is composed of multiple different constituent metals, the heating and melting is carried out at a temperature equal to or higher than the melting point of the constituent metal with the highest melting point. When the constituent metals are an alloy, the melting point of the constituent metals is the melting point of the alloy.

[0043] When preparing the molten metal, it is preferable to prepare the molten metal in an atmosphere of a non-oxidizing gas (an inert gas such as He, Ar, or N2, or a reducing gas such as H2 or CO) in order to prevent oxygen from being mixed into the molten metal. If the raw material metal for the molten metal is a metal or alloy in which carbon does not dissolve, carbon black or charcoal may be added to the molten metal to prevent oxidation or to reduce the oxidized metal if oxidation of the metal occurs during the production of the reduced-carbon raw material metal.

[0044] <Fluid spraying (atomization process)> The prepared molten metal is dropped, for example, from a discharge nozzle of a furnace, and a fluid is sprayed onto the flow of the falling molten metal, thereby pulverizing and solidifying the molten metal to obtain metal powder.

[0045] The fluid sprayed onto the falling stream of molten metal can be water or a gas at a temperature below the melting point of the molten metal. The gas must be inert to the molten metal.

[0046] The temperature of the molten metal when spraying the fluid is preferably 50 to 800°C higher than the melting point of the molten metal, and more preferably 100 to 700°C higher, from the viewpoint of reducing the viscosity to increase the crushing force of the fluid to obtain fine metal powder, and from the viewpoint of the load on the furnace and heat costs. Regarding the melting point of the molten metal, when the molten metal is an alloy molten metal, the melting point of the molten metal is the melting point of the alloy.

[0047] The fluid may be sprayed in an air atmosphere, but to prevent oxidation of the molten metal and the resulting metal powder, it may be sprayed in a non-oxidizing gas atmosphere, such as an inert gas such as He, Ar, or N, or a reducing gas such as H or CO.

[0048] <Other processes> In the method for producing metal powder of the present invention, the metal powder obtained by spraying the fluid described above may be subjected to the following optional steps.

[0049] (Solid-liquid separation process) When the fluid is a liquid such as water, a slurry in which the metal powder is dispersed in the liquid is obtained. The metal powder is recovered by subjecting this slurry to solid-liquid separation. A conventionally known method for solid-liquid separation can be used without particular limitation, and for example, the slurry may be pressure-filtered using a filter press or the like. The recovered metal powder may be washed with water or the like.

[0050] (drying process) The metal powder obtained in the solid-liquid separation step may be dried. Drying may be carried out at room temperature (25°C), or at a high temperature (40 to 120°C) to improve the drying rate. Drying may be carried out under atmospheric pressure, but to improve the drying rate, drying may be carried out in a reduced pressure environment of -0.05 MPa or less relative to atmospheric pressure. Drying may also be carried out in a vacuum environment (-0.095 MPa or less).

[0051] (Crushing process, surface treatment process, classification process) The particle size distribution of the metal powder may be adjusted by crushing or classifying the metal powder. Simultaneously with crushing, or before or after crushing, the metal powder may be surface-treated with an organic compound. Examples of the organic compound include alcohols, amines, fatty acids, and thiols having 2 to 36 carbon atoms.

[0052] Although the present invention has been described above by way of example, it is understood that the present invention is not limited to such examples. It is clear that a person skilled in the art can conceive of various modifications and alterations within the scope of the technical ideas set forth in the claims, and that such modifications and alterations are also within the technical scope of the present invention. [Example]

[0053] <Generation of classification residue (coarse silver powder with organic compounds attached)> Shot silver was heated to 1600°C in an air atmosphere, and the molten metal was dropped from the outlet nozzle at the bottom of a carbon crucible. A water atomizer sprayed atomized water at a pressure of 103 MPa and a rate of 160 L / min in an air atmosphere, pulverizing and rapidly solidifying the molten metal. The resulting slurry was subjected to solid-liquid separation, and the solid was washed with water and dried. The temperature of the molten metal was measured using an optical fiber thermometer. The same procedure is followed hereafter.

[0054] Oleic acid (0.06 parts by mass per 100 parts by mass of the solid) was added to the dried solid as a surface treatment agent, and the solid was mixed with the surface treatment agent while being crushed to obtain silver powder surface-treated with oleic acid.

[0055] The surface-treated silver powder was then classified to obtain a finer silver powder having an average particle size of 3 μm. The average particle size was measured by dispersing the silver powder using nitrogen gas at a dispersion pressure of 5 bar (0.5 MPa) using a laser diffraction particle size analyzer (SYMPATEC's HELOS&RODOS (airflow dispersion module)) and was measured as a cumulative 50% particle size on a volume basis.

[0056] On the other hand, a by-product of the classification was coarse silver powder (classification residue), which had a larger particle size on the coarse powder side. The carbon content of this classification residue was 0.086 mass%. The carbon content was measured using a carbon / sulfur analyzer (EMIA-22V, manufactured by Horiba, Ltd.). The average particle size of this classification residue was 5.8 μm.

[0057] [Example 1] <Heat treatment of classification residue> 7 kg of the above classification residue was dispensed onto a SUS tray. The SUS tray was placed in a shelf-type dryer (DN610I model, Yamato Scientific Co., Ltd.), and the temperature inside the dryer was raised from room temperature to 150°C at a rate of 5°C / min and maintained at 150°C for 10 hours. During this time, air (atmospheric air) was flowed into the dryer (internal volume: 150 L) at a flow rate of 30 L / min (the pressure inside the dryer was the same as atmospheric pressure). After that, the temperature inside the dryer was allowed to naturally drop to room temperature, and the above classification residue (referred to as "post-heating classification residue"; this is the carbon-reduced raw metal in this invention) was removed from the dryer. This drying was performed multiple times as described below, with a maximum of 12 SUS trays placed in the dryer at one time. The free energy of formation of silver oxide at 150°C, ΔG M is approximately 0 kJ / molO2, and the free energy of formation of carbon oxides at that temperature, ΔG C is about -390kJ / molO2, so the difference between these (ΔG M -ΔG C ) is approximately 390 kJ / molO2.

[0058] The above classification residue was heated in a dryer multiple times to obtain the amount of classification residue after heating required for repeated water atomization operations described below. The carbon content of the obtained classification residue after heating was measured in the same manner as above and was found to be 0.009% by mass.

[0059] <Atomization using classification residue after heating> 19.02 kg of silver shot (carbon content: below the detection limit (below 0.001% by mass)) and 21.00 kg of the post-heating classification residue obtained above were placed in a carbon crucible (the proportion of post-heating classification residue (carbon-reduced raw metal) to the total molten metal raw material was 52.4% by mass, and the carbon content of the molten metal raw material was approximately 0.005% by mass), and melted by heating to 1600°C in a nitrogen atmosphere. Note that the carbon crucible used here was a new one that had been replaced with the carbon crucible used in <Generation of classification residue> above.

[0060] The molten metal (temperature: 1605°C) was dropped from the outlet nozzle at the bottom of the carbon crucible, and atomized water was sprayed onto it at a water pressure of 92 MPa and a rate of 166 L / min in an air atmosphere using a water atomizer, causing the molten metal to be rapidly cooled and solidified while being crushed. The resulting slurry was subjected to solid-liquid separation, water washing, drying, surface treatment while crushing, and classification in the same manner as above.

[0061] The above series of operations was repeated using the same carbon crucible without replacing it, and was able to be carried out 16 times (16 batches) without any substantial problems (although there was some slight melt fluctuation, all of the molten metal was able to be discharged from the discharge nozzle at the bottom of the tundish).

[0062] [Comparative Example] Water atomization was repeated without replacing the carbon crucible in the same manner as in Example 1, except that classification residue (coarse silver powder) that had not been subjected to the above heat treatment was used instead of the classification residue after heating (the carbon content of this coarse silver powder was 0.086% by mass). As a result, on the second run, melt spillage and clogging of the nozzle occurred before the molten metal was completely discharged from the nozzle. This was managed by cleaning the nozzle (removing the nozzle blockage and preventing melt spillage), but on the fourth run, melt spillage and clogging of the nozzle occurred frequently, making the operation impossible.

[0063] [Reference example] AG-4-8F silver powder (manufactured by DOWA Hi-Tech Co., Ltd. and sold by DOWA Electronics Co., Ltd., D50 1.9 μm, carbon content 0.196 mass%) produced by wet reduction reaction was heated at 200°C for 10 hours using a tray dryer (DN610I type, Yamato Scientific Co., Ltd.) in the same manner as in Example 1. The carbon content of the silver powder after heating was measured and found to be 0.005 mass%.

Claims

1. A method for producing metal powder comprising the steps of heating and melting a molten raw metal to produce a molten metal, dropping the molten metal, and spraying a fluid onto the falling flow of the molten metal to pulverize and solidify the molten metal to obtain a metal powder, the method comprising the steps of: The carbon content of the molten metal raw material is 0.01% by mass or less, and the molten metal raw material includes a reduced-carbon raw material metal, In the method for producing metal powder, the carbon-reduced raw metal is obtained by heating a raw metal having a carbon content of more than 0.01 mass % at 100 to 400° C. in an oxidizing atmosphere to reduce the carbon content.

2. 2. The method for producing a metal powder according to claim 1, wherein the raw metal having a carbon content of more than 0.01 mass% has a volume-based cumulative 50% particle diameter (D50) of 0.1 to 50 μm as measured by a laser diffraction particle size distribution analyzer, and is a metal powder containing an organic compound, or a metal piece having an organic compound adhered to its surface.

3. 3. The method for producing metal powder according to claim 1, wherein the carbon-reduced raw metal is obtained by heating a raw metal having a carbon content of more than 0.01% by mass in an oxidizing atmosphere to reduce the carbon content to 0.01% by mass or less.

4. The method for producing metal powder according to any one of claims 1 to 3, wherein the carbon content of the raw material metal having a carbon content of more than 0.01% by mass is 1% by mass or less.

5. The method for producing metal powder according to any one of claims 1 to 4, wherein the amount of reduced-carbon raw metal in the molten raw metal is 5 to 100 mass %.

6. The method for producing metal powder according to any one of claims 1 to 5, wherein the raw metal having a carbon content of more than 0.01 mass% is a metal powder produced by an atomization method and surface-treated with an organic compound.

7. The constituent metal of the raw material metal having a carbon content of more than 0.01 mass % has a free energy of oxide formation ΔG Ｍ and the free energy of formation of the oxide of carbon at the temperature ΔG Ｃ Difference from (ΔG Ｍ -ΔG Ｃ ) is 0 kJ / molO ２ The method for producing a metal powder according to any one of claims 1 to 6, wherein the metal M is larger than the metal M.