Method for cleaning ultrafine metal powders

Abstract

To provide a cleaning method of metal ultrafine powder in which formation of aggregates is suppressed during cleaning.SOLUTION: In a cleaning method of metal ultrafine powder, the metal ultrafine powder obtained by vapor phase reduction of a metal halide is cleaned using a buffer solution having a pH of 3.0-6.0, and the metal ultrafine powder cleaned using the buffer solution is further cleaned using a solution having a pH of less than 10.0.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for cleaning ultrafine metal powders. [Background technology]

[0002] Metal ultrafine powders are used in a variety of applications. For example, they are used as materials for electronic components (e.g., internal electrodes in ceramic capacitors), magnetic devices, and circuit materials. In the case of such ultrafine metal powders, the miniaturization and high integration of electronic devices in recent years have led to a demand for further fineness and higher reliability.

[0003] A known method for producing ultrafine metal powder involves gas-phase reduction of metal halide vapor. However, it is known that halogens remain in the resulting ultrafine metal powder. For this reason, conventional methods have involved washing the resulting ultrafine metal powder. For example, Patent Document 1 discloses a method for washing metal powder obtained by gas-phase reduction of metal halide vapor using an aqueous solution containing an organic acid. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 11-189813 [Overview of the project] [Problems that the invention aims to solve]

[0005] As mentioned above, in recent years there has been a demand for further miniaturization of metal ultrafine powders. When the inventors investigated the application of cleaning with an aqueous solution containing an organic acid to metal ultrafine powders of a recently required standard, they encountered the problem of the metal ultrafine powders agglomerating during cleaning. The aggregates formed by the agglomeration of metal ultrafine powders during cleaning were difficult to completely disintegrate in subsequent processes. The presence of aggregates in ultrafine metal powders can lead to a decrease in the classification yield in subsequent classification processes, making the suppression of aggregate formation desirable. Furthermore, the presence of aggregates in ultrafine metal powders can cause short circuits between electrodes during the formation of thin internal electrodes in ceramic capacitors, potentially leading to product defects and failing to meet the quality standards required in recent years.

[0006] Therefore, the object of the present invention is to provide a method for cleaning metal ultrafine powders in which the formation of aggregates is suppressed during cleaning. [Means for solving the problem]

[0007] The inventors of the present invention conducted diligent research to solve the above problems and discovered that the formation of aggregates is suppressed when washing with a buffer solution of a predetermined pH and then washing again with a liquid of a predetermined pH, thus completing the present invention. In other words, they found that the above problems are solved by the following configuration.

[0008] [1] A method for washing metal ultrafine powder, comprising washing the metal ultrafine powder obtained by gas-phase reduction of metal halide vapor with a buffer solution having a pH of 3.0 to 6.0, and further washing the metal ultrafine powder washed with the buffer solution with a liquid having a pH of less than 10.0. [2] The method for cleaning metal ultrafine powder according to [1], wherein the metal ultrafine powder is one or more selected from the group consisting of nickel ultrafine powder, nickel alloy ultrafine powder, copper ultrafine powder, and copper alloy ultrafine powder. [3] Median diameter D based on the number of the above-mentioned ultrafine metal powders 50 A method for washing metal ultrafine powder as described in [1] or [2], wherein the particle size is 0.05 to 0.25 μm. [4] The method for washing metal ultrafine powder according to any one of [1] to [3], wherein the buffer solution contains a component capable of forming a chelate complex with metal ions of the metal contained in the metal ultrafine powder. 〔5〕The washing method of the metal ultrafine powder according to any one of 〔1〕~〔4〕, wherein the buffer solution is selected from the group consisting of an aqueous solution of citric acid - sodium citrate, an aqueous solution of tartaric acid - sodium tartrate, an aqueous solution of succinic acid - sodium succinate, and an aqueous solution of glutamic acid - sodium glutamate.

Advantages of the Invention

[0009] According to the present invention, it is possible to provide a method for washing a metal ultrafine powder in which the formation of aggregates is suppressed during washing.

Modes for Carrying Out the Invention

[0010] Hereinafter, the present invention will be described in detail. The description of the constituent elements described below may be made based on representative embodiments of the present invention, but the present invention is not limited to such embodiments.

[0011] Hereinafter, the meaning of each description in this specification will be described. In this specification, a numerical range represented by "~" means a range including the numerical values described before and after "~" as the lower limit value and the upper limit value. In this specification, the "metal ultrafine powder" refers to an aggregate of particles mainly containing a metal. Mainly containing a metal means that it contains 80 to 100% by mass of metal atoms in a metallic state with respect to the total mass of the particles. Note that the metal atoms in a metallic state refer to those with an oxidation number of 0 for the metal atoms.

[0012] <Method for Washing Metal Ultrafine Powder> The method for washing a metal ultrafine powder of the present invention is a method for treating a metal ultrafine powder obtained by vapor-phase reduction of a metal halide. Hereinafter, the metal ultrafine powder to be subjected to the method for washing a metal ultrafine powder of the present invention is also referred to as "untreated powder". Further, hereinafter, the method for washing a metal ultrafine powder of the present invention is also simply referred to as "the washing method of the present invention". The cleaning method of the present invention is to clean the untreated powder using a buffer solution with a pH of 3.0 to 6.0, and then further clean the cleaned untreated powder using a liquid with a pH of less than 10.0. Hereinafter, the step of cleaning the untreated powder using a buffer solution with a pH of 3.0 to 6.0 is also referred to as the "first cleaning step", and the step of further cleaning using a liquid with a pH of less than 10.0 is also referred to as the "second cleaning step".

[0013] In the cleaning method of the present invention, the mechanism by which the formation of aggregates is suppressed is not necessarily clear, but the present inventors speculate as follows. Hereinafter, the mechanism in the case of using metal chloride as the metal halide will be described, but it is considered that the formation of aggregates is suppressed by the same mechanism in other metal halides. When used in the manufacture of electronic components and the like, it is required to remove impurities adhering to the surface of the metal ultrafine powder. Here, in the cleaning method of the present invention, first, the first cleaning step is carried out to dissolve and remove the metal chloride adhering to the untreated powder. Next, in the second cleaning step, at least one of the metal chloride of the cleaned untreated powder remaining without being removed in the first cleaning step and the component derived from the metal chloride reattached after dissolution to the cleaned untreated powder is removed. Also, since it may not be possible to completely separate the buffer solution containing the component derived from the metal chloride used in the first cleaning step from the cleaned untreated powder, in the second cleaning step, a part of the buffer solution containing the component derived from the metal chloride is also removed from the cleaned untreated powder. In the present invention, a liquid with a pH of less than 10.0 is used in the second cleaning step. The metal chloride adhering to the surface of the untreated powder is considered to be easily soluble, but it is considered to adhere strongly to the surface due to the heat history of the gas-phase reduction reaction. Therefore, in order to efficiently remove the metal chloride adhering to the surface of the untreated powder, it is considered preferable to use an acidic aqueous solution with a high dissolution rate of the metal chloride. On the other hand, if the pH of the acidic aqueous solution is too low, the dissolution rate of the metal portion of the untreated powder will also increase, and the washed slurry of the untreated powder will contain metal ions derived from the metal portion of the untreated powder in addition to metal ions derived from metal chlorides. Therefore, if pH fluctuations are suppressed in the first washing step by using a buffer solution within a predetermined pH range, the slurry of the washed untreated powder will be less likely to contain excess metal ions derived from the metal portion of the untreated powder. In the untreated powder slurry obtained in the first washing step, readily soluble metal chlorides, metal ions derived from the untreated powder, chloride ions, and ions derived from the buffer solution may remain. When metal ions are brought into contact with a liquid with a pH of 10.0 or higher, they tend to form metal hydroxide particles (e.g., colloidal particles) with low solubility and small particle size. These small metal hydroxide particles (e.g., colloidal metal hydroxide particles) are prone to agglomeration and can trigger the agglomeration of untreated powder. Therefore, it is hypothesized that using a liquid with a pH below 10.0 will result in ultrafine metal powder with suppressed aggregate formation. Furthermore, if a buffer solution within a specified pH range is not used in the first washing step, a slurry containing a large amount of metal ions derived from the untreated powder will remain, which is thought to promote the formation of metal hydroxides and make aggregation more likely.

[0014] [Unprocessed powder] The metal ultrafine powder (untreated powder) used in the cleaning method of the present invention is obtained by gas-phase reduction of metal halide vapor. A known method can be used to obtain the untreated powder. Methods for obtaining unprocessed powder include, for example, the following: First, a vapor of the metal halide is generated and introduced into the reaction vessel. Next, a reducing gas (e.g., hydrogen gas and ammonia gas) is introduced into the reaction vessel and brought into contact with the metal halide vapor to reduce the metal halide and produce metal ultrafine powder. The generated metal ultrafine powder is discharged from the reaction vessel and recovered by methods such as filtering and trapping with liquid. The recovered metal ultrafine powder is then subjected to appropriate operations such as washing, classification, and drying. Furthermore, the particle size of the metal ultrafine powder can be controlled by controlling the amount and partial pressure of the metal halide vapor introduced into the reaction vessel, the amount and partial pressure of the reducing gas introduced, the residence time in the reaction vessel, the reaction temperature, the cooling rate, etc. As described above, metal ultrafine powder (untreated powder) obtained by gas-phase reduction of metal halide vapor often has unreacted metal halides attached to it. Therefore, the cleaning method of the present invention is carried out to remove the metal halides attached to the metal ultrafine powder (untreated powder). Examples of metal halides include metal fluorides, metal chlorides, metal bromides, and metal iodides, with metal chlorides being preferred.

[0015] The untreated powder consists of particles containing metal. The untreated powder may be a metal ultrafine powder containing one type of metal element, a metal ultrafine powder containing two or more types of metal elements, or a metal ultrafine powder containing one or more metal elements and nonmetallic elements other than halogen elements (e.g., chlorine). Examples of metal elements contained in the untreated powder include one or more metal elements selected from the group consisting of iron (Fe), cobalt (Co), tin (Sn), nickel (Ni), and copper (Cu). Nonmetallic elements other than halogen elements (e.g., chlorine) contained in the untreated powder include, for example, one or more elements selected from the group consisting of oxygen (O), silicon (Si), and sulfur (S), with Si being a preferred example.

[0016] Specific examples of untreated powders include Fe ultrafine powder, Fe alloy ultrafine powder, Co ultrafine powder, Co alloy ultrafine powder, Ni ultrafine powder, Ni alloy ultrafine powder, Cu ultrafine powder, and Cu alloy ultrafine powder. Among these, it is preferable that the untreated powder be one or more selected from the group consisting of Ni ultrafine powder, Ni alloy ultrafine powder, Cu ultrafine powder, and Cu alloy ultrafine powder. Examples of alloys that make up Fe alloy ultrafine powder include Fe-Si alloy, Fe-Ni alloy, Fe-Sn (tin) alloy, Fe-Cr alloy, and Fe-Cr-Si alloy. Examples of alloys that make up Co alloy ultrafine powder include Co-Si alloys, Co-Sn alloys, Co-Ni alloys, and Co-Cr alloys. Examples of alloys that make up Ni alloy ultrafine powder include Ni-Si alloys, Ni-Sn alloys, and Ni-Cr alloys. Examples of alloys that make up Cu alloy ultrafine powder include Cu-Ni alloys and Cu-Si alloys.

[0017] The halogen element content (e.g., chlorine) in untreated powder is typically 100 to 30,000 ppm by mass, and is often between 5,000 and 30,000 ppm by mass, relative to the total mass of the untreated powder. The halogen element content (e.g., chlorine) in untreated powder can be measured by contacting a halogen element-containing gas generated by heating with a specific electrode and observing the change in the electrode's potential and the amount of charge reacted at the electrode. An example of a device for performing the above measurement is the TOX-2100H manufactured by Nitto Seiko Analytech Co., Ltd.

[0018] Median diameter (D) based on the number of untreated powder particles 50 ) is preferably 0.25 μm or less. As mentioned above, metal ultrafine powders tend to aggregate as they are atomized, but according to the washing method of the present invention, the metal ultrafine powder is as described above D 50 Even within this range, aggregate formation can be suppressed. D based on the number of ultrafine particles 50 The particle size is more preferably 0.20 μm or less, even more preferably 0.15 μm or less, and particularly preferably 0.10 μm or less. On the other hand, the lower limit is not particularly limited, but from a practical standpoint, D is based on the number of unprocessed powder particles. 50 The particle size is preferably 0.05 μm or larger. Note that D is based on the number of unprocessed powders. 50This is obtained by measurement using a scanning electron microscope (SEM). Specifically, first, when observing untreated powder particles with an SEM, a magnification is selected such that the number of particles observed in one field of view is between 200 and 600, and imaging is performed. Then, imaging is repeated by changing the field of view until the number of particles in the acquired SEM image exceeds 10,000, and the equivalent diameter in each field of view is measured to obtain a particle size distribution curve based on the number of particles, and the D value based on the number of particles is obtained. 50 The value of D of the untreated powder is calculated. 50 This represents the particle size that corresponds to 50% of the cumulative distribution based on particle count.

[0019] The shape of the unprocessed powder is not particularly limited. Examples of the untreated powder's shape include spherical, plate-like, columnar, needle-like, and polyhedral forms. Among these, spherical or polyhedral forms are preferred.

[0020] [First Washing Process] The present invention's washing method involves a first washing step in which untreated powder is washed using a buffer solution with a pH of 3.0 to 6.0 (hereinafter also referred to as "specific buffer solution"). In the first washing step, the main purpose is to remove halogenated compounds (e.g., metal chlorides) contained in (or adhering to) the untreated powder.

[0021] The specific buffer solution is not particularly limited as long as it has buffering properties and its pH is within the above range; any known buffer solution can be used. A buffer solution is typically an aqueous solution containing a weak acid and its salt. Examples of weak acids included in the specific buffer include organic acids and inorganic acids. Preferred organic acids include carboxylic acids having a carboxyl group (-COOH), and more preferably polycarboxylic acids having multiple carboxyl groups within the molecule. Preferred polycarboxylic acids are dicarboxylic acids having two carboxyl groups within the molecule, or tricarboxylic acids having three carboxyl groups within the molecule. Examples of weak acids included in the specific buffer include acetic acid, oxalic acid, malonic acid, succinic acid, glutaric acid, phthalic acid, lactic acid, malic acid, tartaric acid, citric acid, gluconic acid, glutamic acid, phosphoric acid, and boric acid, with succinic acid, tartaric acid, citric acid, and glutamic acid being preferred. Examples of cations included in the salt with the weak acid include alkali metal ions and alkaline earth metal ions, with alkali metal ions being preferred and sodium ions being more preferred. More specifically, in order to suppress the excessive dissolution of untreated powder in the first washing step and to further suppress the formation of aggregates, a buffer selected from the group consisting of citric acid-sodium citrate aqueous solution, tartaric acid-sodium tartrate aqueous solution, succinic acid-sodium succinate aqueous solution, and glutamic acid-sodium glutamate aqueous solution is more preferred as the buffer, a buffer selected from the group consisting of citric acid-sodium citrate aqueous solution, tartaric acid-sodium tartrate aqueous solution, succinic acid-sodium succinate aqueous solution, and glutamic acid-sodium glutamate aqueous solution is even more preferred, and a buffer selected from the group consisting of citric acid-sodium citrate aqueous solution and tartaric acid-sodium tartrate aqueous solution is even more preferred. For example, "sodium citrate" refers to the sodium salt of citric acid, and may be monosodium citrate, disodium citrate, or trisodium citrate. Among these, trisodium citrate is preferred. Furthermore, the sodium salts of the polyhydric acids described above may be any salts with different numbers of sodium ions, but salts in which the number of acid groups in the polyhydric acid and the number of sodium ions contained are the same (normal salts) are preferred. Specifically, disodium tartrate is preferred as sodium tartrate, disodium succinate is preferred as sodium succinate, and disodium glutamate is preferred as monosodium glutamate. Furthermore, "citric acid-sodium citrate aqueous solution" refers to an aqueous solution containing citric acid and sodium citrate.

[0022] The buffering agent content (e.g., a weak acid and its salt) in the specific buffer solution is preferably 0.001 to 5.0 mol / L, and more preferably 0.01 to 1.0 mol / L.

[0023] Furthermore, it is preferable that the specific buffer solution contains a component (chelating agent) capable of forming a chelate complex with metal ions of the metal contained in the untreated powder, as this further suppresses the formation of aggregates. Examples of chelating agents include citric acid, tartaric acid, succinic acid, glutamic acid, and gluconic acid, as well as their salts. The chelating agent may also function as a weak acid that exhibits buffering action, and it is preferable that the chelating agent be included in the specific buffer solution as a weak acid that exhibits buffering action and its salt.

[0024] The pH of the specific buffer solution is between 3.0 and 6.0, but in this specification, the pH is measured using a known pH meter. Note that the pH is the value at 25°C.

[0025] In the first washing step, the untreated powder is washed using a specific buffer solution, but the method of washing is not particularly limited as long as it is performed. For example, in the first washing step, the untreated powder can be brought into contact with the specific buffer solution. Methods for bringing untreated powder into contact with a specific buffer include, for example, dispersing the untreated powder in the specific buffer and passing the specific buffer through the untreated powder, with the method of dispersing the untreated powder in the specific buffer being preferred.

[0026] One method for dispersing untreated powder in a specific buffer solution is to add the untreated powder to the specific buffer solution and then agitate the specific treatment solution. The agitation of the specific buffer solution can be carried out by known methods, such as mechanically agitating the specific buffer solution, for example, by rotating an agitator installed inside a container holding the specific buffer solution. It is also preferable that baffle plates be installed on the sides of the container holding the specific buffer solution.

[0027] The temperature of the specific buffer solution used in the first washing step can be adjusted as appropriate. The temperature is, for example, 80°C or lower, but preferably less than 60°C, more preferably 50°C or lower, and even more preferably 40°C or lower. The lower limit is not particularly limited, but for example, the temperature is 5°C or higher, and preferably 10°C or higher. The time required to perform the first washing step can be adjusted as appropriate along with the temperature. Examples of such times include 30 seconds to 2 hours, with 5 to 60 minutes being preferable and 10 to 30 minutes more preferable in order to more reliably dissolve the metal halides adhering to the untreated powder.

[0028] In a method for dispersing untreated powder in a specific buffer solution, the mass of untreated powder relative to the volume of the specific buffer solution can be adjusted as appropriate, but examples include 0.01 to 2 kg / L, with 0.1 to 1 kg / L being preferred.

[0029] The first washing step is preferably carried out in an inert gas atmosphere. A method for carrying out the first washing step in an inert gas atmosphere is, for example, to replace the container holding the specific buffer solution with an inert gas, introduce the specific buffer solution and untreated powder into the container, and wash the untreated powder while supplying the inert gas to the gas-filled portion of the container. Carrying out the first washing step in an inert gas atmosphere suppresses oxidation of the untreated powder. Examples of preferred inert gases include nitrogen gas and argon gas. Furthermore, it is preferable to remove dissolved oxygen from the specific buffer solution before washing the untreated powder. The method for removing dissolved oxygen is not particularly limited, and known methods can be applied. Examples of methods for removing dissolved oxygen include contact with an inert gas, reduced pressure, and contact with an oxygen scavenger, with contact with an inert gas being preferred.

[0030] The first cleaning step may be performed only once or repeatedly, but it is preferable to perform it only once and then proceed to the second cleaning step described later.

[0031] Furthermore, it is preferable to perform a step of separating the specific buffer solution from the washed untreated powder between the first washing step and the second washing step, which will be described later. The above process can be carried out using methods of solid-liquid separation commonly practiced in this field, such as pressure filtration, vacuum filtration, centrifugation, and decantation. In addition, it is not necessary to completely separate the buffer solution from the washed untreated powder in the above process. For example, it may be in a moist state (e.g., a cake obtained by pressure filtration or vacuum filtration).

[0032] [Second washing process] The cleaning method of the present invention involves performing a second cleaning step in which the untreated powder cleaned in the first cleaning step (hereinafter also referred to as "first cleaning powder") is further cleaned using a liquid with a pH of less than 10.0 (hereinafter also referred to as "specific cleaning solution"). In the second washing step, halogen elements (e.g., metal chlorides), metal ions, and components contained in the specific buffer solution attached to the first washing powder are mainly removed, resulting in a metal ultrafine powder in which aggregate formation is suppressed.

[0033] The specific cleaning solution is not particularly limited as long as its pH is less than 10.0, but it is preferable that it is mainly composed of water. Furthermore, a solution with a low ion concentration (10 μS / cm or less) that does not promote coagulation is preferred. The pH of the specific cleaning solution is preferably less than 9.0, and more preferably less than 8.0. On the other hand, the pH of the specific cleaning solution is preferably greater than 6.0. In particular, pure water is preferred as the specific cleaning solution. Pure water can be obtained by known methods, such as distillation, ion exchange, and reverse osmosis. Pure water refers to water with an electrical resistivity of 1.0 MΩ·cm or higher.

[0034] In the second cleaning step, the first cleaning powder is cleaned using a specific cleaning solution, but the method of cleaning is not particularly limited as long as it is performed. For example, in the second cleaning step, the first cleaning powder can be brought into contact with the specific cleaning solution. The method for bringing the first cleaning powder into contact with the specific cleaning solution is the method described in the first cleaning step, and the preferred embodiment (for example, the temperature of the specific cleaning solution) is the same as in the first cleaning step.

[0035] The second cleaning step is preferably carried out in an inert gas atmosphere. Carrying out the second cleaning step in an inert gas atmosphere suppresses oxidation of the resulting metal ultrafine powder. An example of how to carry out the second cleaning step in an inert gas atmosphere, and an example of an inert gas, are the same as those described in the first cleaning step. Furthermore, it is preferable to remove dissolved oxygen from the specific cleaning solution before cleaning the untreated powder. The method for removing dissolved oxygen is not particularly limited, and known methods can be applied. An example of a method for removing dissolved oxygen is the same as that described in the first cleaning step.

[0036] Furthermore, it is preferable to repeat the second cleaning step. When the second washing process is repeated, the first washing powder can be washed again by separating it from the specific washing solution to obtain the first washing powder (hereinafter also referred to as "second washing powder"), and then washing the second washing powder again by contacting it with the specific washing solution. The method for separating the second washing powder from the specific washing solution is the same as the method for separating the specific buffer solution from the washed untreated powder described above. Note that when separating the second washing powder from the specific washing solution, it is not necessary to completely separate the specific washing solution from the second washing powder. For example, they may be separated in a wet state (e.g., a cake obtained by pressure filtration or vacuum filtration). Furthermore, it is preferable to contact the second washing powder with the specific buffer solution again to separate the second washing powder from the specific washing solution, and then contact the second washing powder obtained from this separation with the specific buffer solution again. The second washing powder obtained from separating the second washing powder from the specific washing solution is also referred to as "second washing powder." When the second cleaning step is repeated, the specific cleaning solutions used may be the same or different. In particular, when the second cleaning step is repeated, it is preferable that the specific cleaning solutions used are pure water. Here, if the specific cleaning solution used in the repeatedly performed second cleaning step is pure water, it is preferable to measure the conductivity of the specific cleaning solution separated from the second cleaning powder (e.g., the filtrate in pressure filtration or vacuum filtration) and repeat the second cleaning step until the conductivity is 100 μS / cm or less. By monitoring the conductivity of the specific cleaning solution separated from the second cleaning powder, the degree of cleaning of the second cleaning powder can be determined. The conductivity is thought to correspond to halogen elements (e.g., metal chlorides) that may be contained in the second cleaning powder. Therefore, if the second cleaning step is performed until the conductivity falls within the above range, halogen elements (e.g., metal chlorides) that may be contained in the second cleaning powder are sufficiently removed.

[0037] Furthermore, it is preferable not to allow other liquids to come into contact with the first cleaning powder between the first cleaning step and the second cleaning step. In other words, it is preferable not to perform any other cleaning steps between the first cleaning step and the second cleaning step. Not performing any other cleaning steps means that the first cleaning powder obtained in the first cleaning step does not come into contact with any liquid (for example, an alkaline solution with a pH of 10.0 or higher) between the first cleaning step and the second cleaning step.

[0038] [Other processes] The cleaning method of the present invention may include other steps besides those described above. Other processes include, for example, modification processes, drying processes, crushing processes, and classification processes.

[0039] The surface modification process refers to the process of modifying the surface of a metal ultrafine powder with a desired compound. The surface modification process can be carried out, for example, during or after the second cleaning process. Examples of compounds used to modify the surface of metal ultrafine powders in the surface modification process include dispersants and antioxidants.

[0040] The drying process is preferably carried out on the metal ultrafine powder obtained by the second washing process. The drying method is not particularly limited, and known methods can be applied. Examples of the drying method include heat drying, vacuum drying, hot air drying, spray drying, and combinations thereof. Further, drying may be performed while replacing the liquid used in the second washing step with another liquid (for example, an alcohol-based solvent, a ketone-based solvent, etc.) different from the liquid used in the second washing step. The drying step is preferably carried out in an inert gas atmosphere or a vacuum atmosphere.

[0041] The crushing step is preferably carried out on the metal ultrafine powder obtained by carrying out the above drying step. The crushing method is not particularly limited, and known methods can be applied. Examples of the crushing method include methods using a pin mill, a jet mill, a ball mill, a bead mill, etc. The crushing step is also preferably carried out in an inert gas atmosphere.

[0042] The classification step can be carried out at any timing. Coarse particles may be removed or fine particles may be removed by the classification step. The classification method is not particularly limited, and known methods can be applied. The classification method may be dry classification or wet classification. Examples of the classification method include gravity classification, inertial classification, and centrifugal classification. Dry classification is preferably carried out in an inert gas atmosphere. Wet classification is preferably carried out using a solution from which dissolved oxygen has been removed.

[0043] <Properties of the obtained metal ultrafine powder> According to the cleaning method of the present invention, a metal ultrafine powder in which the formation of aggregates is suppressed can be obtained. Hereinafter, the preferable properties of the obtained metal ultrafine powder will be described.​​​​​​​​​​50 The measurement method is the same as that used for the previous measurement (measurement by SEM).

[0045] Furthermore, the formation of aggregates is suppressed in the resulting ultrafine metal powder. The degree of aggregate formation in metal ultrafine powders is obtained by laser diffraction. 95L The particle size corresponding to 95% of the cumulative distribution based on volume can be used as an indicator for determination. The measurement method follows the procedure in the examples described later. D when measuring metal ultrafine powder using laser diffraction. 95L The particle size is preferably less than 5.0 μm, and more preferably less than 3.0 μm. The lower limit is not particularly limited, D 95L For example, this is 0.5 μm or larger.

[0046] The halogen element (e.g., chlorine) content of the resulting metal ultrafine powder is preferably 100 ppm by mass or less, relative to the total mass of the resulting metal ultrafine powder. The lower limit is not particularly limited, for example, 0.1 ppm by mass. The method for measuring the halogen element (e.g., chlorine) content is the same as the method for measuring the halogen element (e.g., chlorine) content of untreated powder. [Examples]

[0047] The present invention will be described in more detail below based on examples. The materials, quantities, proportions, processing details, and processing procedures shown in the following examples can be modified as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be interpreted as being limited by the following examples.

[0048] <Cleaning> The untreated powder was washed according to the procedure shown below to obtain ultrafine metal powder. The washing procedures for each example and comparative example are described below. [Example 1] First, we prepared Ni ultrafine powder (hereinafter also referred to as "untreated Ni powder") as untreated powder (metal ultrafine powder obtained by gas-phase reduction of metal chloride vapor). 50 When measured using the method described above (measurement by SEM), the result was 0.1 μm. Next, 50 L of a 0.1 mol / L citric acid aqueous solution (Solution 1 in the table below), prepared with purified water from which dissolved oxygen has been removed, and 50 L of a 0.1 mol / L trisodium citrate aqueous solution (Solution 2 in the table below), prepared with purified water from which dissolved oxygen has been removed, were mixed to prepare a specific buffer solution for the first washing step. The pH of the prepared specific buffer solution was 4.4. The entire amount of the prepared specific buffer solution was placed in a washing container equipped with a stirring blade, and then 25 kg of the untreated Ni powder was placed in the washing container. The stirring blade was rotated and washing was performed for 20 minutes (first washing step). The temperature of the specific buffer solution during washing was 25°C. The washing container was designed to block out outside air, and during washing, nitrogen gas was flowed as an inert gas into the space above the mixture of the specific buffer solution and untreated Ni powder inside the washing container. After performing the first washing step, the mixture of the specific buffer solution and the untreated Ni powder was pressure filtered to obtain a cake of the untreated Ni powder that had undergone the first washing step (hereinafter also referred to as "first-washed Ni powder").

[0049] In a washing container similar to the one used in the first washing step described above, 100 L of pure water (ion-exchanged water, electrical resistivity: 1 MΩ·cm, dissolved oxygen concentration 0.01 mg / L or less, corresponding to a specific washing solution) was placed, and the cake of the first washing Ni powder obtained above was added. The stirring blade of the washing container containing the pure water and the obtained cake of the first washing Ni powder was rotated and washing was performed for 20 minutes (second washing step). The temperature of the pure water during washing was 25°C. Furthermore, the washing container was capable of blocking out the outside air, and during washing, nitrogen gas, as an inert gas, was flowed into the space above the mixture of pure water and the first washing Ni powder inside the washing container. After the second washing step, the mixture of the specific washing solution and the first washing Ni powder was pressure filtered to obtain a Ni ultrafine powder cake. The conductivity of the filtrate obtained by pressure filtration after the second washing step was then measured to confirm whether the conductivity was 100 μS / cm or less. If the conductivity was not 100 μS / cm or less, the second washing step was repeated. When the conductivity of the filtrate after the second washing step was 100 μS / cm or less, the washing was terminated, and the resulting Ni ultrafine powder cake was dried in a vacuum dryer set to 60°C to obtain Ni ultrafine powder.

[0050] [Examples 2-36 and Comparative Examples 1-24] In Examples 2 to 36, washing was performed in the same manner as in Example 1, except that the untreated powder, the components used to prepare the specific buffer solution, and the mixing amounts during the preparation of the specific buffer solution were changed, as shown in the table below. In Comparative Examples 1 to 24, the washing procedure was the same as in Example 1, except that the untreated powder was changed as shown in the table below, and the washing solution shown in the table was used instead of the specific buffer solution. In the table, "Ni-Si" represents ultrafine metal powder of Ni-Si alloy, where the Ni content is 99% by mass and the Si content is 1% by mass relative to the total Ni and Si content. In the table, "Cu-Ni" represents ultrafine metal powder of Cu-Ni alloy, where the Cu content is 67% by mass and the Ni content is 33% by mass relative to the total Cu and Ni content.

[0051] The chlorine content of the untreated powder used in each example and comparative example was in the range of 1,000 to 30,000 ppm by mass.

[0052] <Rating> [Evaluation of aggregates] D obtained by laser diffraction of the metal ultrafine powders obtained in each example and each comparative example. 95L We compared particle sizes (corresponding to 95% of the cumulative distribution based on volume). D of the obtained metal ultrafine powder 95L It is measured using the following procedure. A MicroTrac MT3300 manufactured by Nikkiso Co., Ltd. was used for the measurements. First, a solution was prepared by adding 0.3 g of untreated powder to 100 mL of a 4% by mass sodium hexametaphosphate aqueous solution. This solution was then irradiated with ultrasound at 250 μA (output 75 W) for 6 minutes using a US-300T titanium alloy ultrasound machine with a 26Φ tip diameter to obtain a dispersion for measurement. Next, the above-mentioned dispersion for measurement was introduced into the water circulating in the apparatus to achieve a predetermined transmittance. After introducing the dispersion for measurement, the ultrasonic irradiation mechanism attached to the apparatus was operated for a predetermined time, and then the measurement was performed to obtain a volume-based particle size distribution curve. From the obtained volume-based particle size distribution curve, the particle size (D) corresponding to 95% of the volume-based cumulative distribution was determined. 95L ) was obtained. When aggregates are formed by washing, D 95L The value becomes larger. D of metal ultrafine powder 95L The measurement results are shown in the table below.

[0053] [Evaluation of chlorine content] The chlorine content of the metal ultrafine powders obtained in each example and comparative example was measured by the method described above. The results of the chlorine content measurement of the metal ultrafine powder are shown in the table below.

[0054] <Result> The table shows an overview of the cleaning methods for each example and comparative example, as well as the evaluation results of the metal ultrafine powder obtained by cleaning. The pH measurement method shown in the table is as described above.

[0055] [Table 1]

[0056] [Table 2]

[0057] The results shown in the table confirm that when washing is performed using a specific buffer solution with a predetermined pH, and then further washing is performed using a liquid with a pH of less than 10.0, aggregate formation is suppressed. It was also confirmed that the chlorine content is reduced (Examples 1-36). On the other hand, if washing is not performed using a specific buffer solution with a predetermined pH, the same untreated powder is used in the example (type and D 50However, compared to the example using the same untreated powder, the formation of aggregates could not be suppressed (Comparative Examples 1-24). Specifically, for example, if the type of untreated powder is "Ni", D 50 Comparing Examples 1-5 and Comparative Examples 1-2, where the particle size is "0.10 μm", Examples 1-5, which were washed with a specific buffer solution at a predetermined pH, are D 95L The low value indicated that aggregate formation was suppressed. Note that unprocessed D 50 If the particle size is less than 0.20 μm (for example, Examples 1-5 and Comparative Examples 1-2, and Examples 29-30 and Comparative Examples 17-18, where the untreated powder type is "Ni"), the difference between the Examples and Comparative Examples is greater compared to when it is 0.20 μm (for example, Examples 21-22 and Comparative Examples 9-10, where the untreated powder type is "Ni"). 95L The significant difference in this regard indicated that aggregate formation could be more effectively suppressed.

[0058] Furthermore, the cleaning method of the present invention is applicable even if the metal ultrafine powder is of a metal type other than those confirmed in the examples. For example, in the case of metal species such as Fe and Co, the halides (e.g., chlorides) of these metal species are readily soluble in water. Furthermore, the ions of these metal species form complex ions in buffer solutions with a pH of 3.0 to 6.0 and dissolve in aqueous solutions. Therefore, similar to the Ni ultrafine powder and Cu ultrafine powder, as well as the Ni alloy ultrafine powder and Cu alloy ultrafine powder, whose effectiveness was confirmed in the examples, it is believed that the cleaning method of the present invention suppresses the formation of aggregates in metal ultrafine powders such as Fe and Co, as well as alloy ultrafine powders containing Fe and Co, through the mechanism described above. The alloys constituting the alloy ultrafine powders containing Fe and Co are as described above. [Industrial applicability]

[0059] The ultrafine metal powder obtained by the cleaning method of the present invention is suitable for use in electronic components and is expected to be utilized in various industrial fields.

Claims

1. A method for washing metal ultrafine powder, comprising washing the metal ultrafine powder obtained by gas-phase reduction of metal halide vapor with a buffer solution having a pH of 3.0 to 6.0, and further washing the metal ultrafine powder washed with the buffer solution with a liquid having a pH of less than 10.0, A method for cleaning metal ultrafine powder, wherein no other cleaning is performed between cleaning with the buffer solution and cleaning with the liquid with a pH of less than 10.

0.

2. The method for cleaning metal ultrafine powder according to claim 1, wherein the metal ultrafine powder is one or more selected from the group consisting of nickel ultrafine powder, nickel alloy ultrafine powder, copper ultrafine powder, and copper alloy ultrafine powder.

3. The median diameter D based on the number of the aforementioned ultrafine metal powders ５０ The method for washing metal ultrafine powder according to claim 1 or 2, wherein the particle size is 0.05 to 0.25 μm.

4. The method for washing metal ultrafine powder according to claim 1 or 2, wherein the buffer solution contains a component capable of forming a chelate complex with metal ions of the metal contained in the metal ultrafine powder.

5. The method for washing metal ultrafine powder according to claim 1 or 2, wherein the buffer solution is selected from the group consisting of citrate-sodium citrate aqueous solution, tartaric acid-sodium tartrate aqueous solution, succinic acid-sodium succinate aqueous solution, and glutamic acid-sodium glutamate aqueous solution.

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