Method for producing nickel powder

Abstract

The present invention provides a method for producing nickel powder that suppresses the increase in coarse particles after dry crushing treatment following the nickel powder slurry process in the nickel powder manufacturing process. [Solution] A method for producing nickel powder includes a solid-liquid separation step of obtaining a nickel powder cake by solid-liquid separation of a nickel powder slurry in which nickel powder is dispersed in water, and a drying step of drying the nickel powder cake to obtain dried nickel powder. Furthermore, a method for producing nickel powder includes a water-soluble organic solvent treatment step of contacting dried nickel powder with a water-soluble organic solvent to obtain water-soluble organic solvent treated nickel powder, and a dry crushing step of dry crushing the water-soluble organic solvent treated nickel powder to obtain crushed nickel powder.

Description

[Technical Field]

[0001] This invention relates to a method for producing nickel powder used as an electrode material for multilayer ceramic components. [Background technology]

[0002] Conventionally, conductive pastes, primarily composed of conductive powder, resin, and organic solvents, have been used to form conductive layers, electrode layers, and interlayer connectors that constitute wiring boards for electrical circuits, as well as electrode layers for electronic components. With the miniaturization and increased density of these wiring boards and electronic components, the width and thickness of conductive and electrode layers tend to decrease. Therefore, the conductive powder used in the conductive pastes to form these layers also needs to be smaller in diameter. Powders typically contain coarse particles exceeding a certain particle size, but with the miniaturization of conductive powders, the adverse effects of these coarse particles on the properties of the conductive powder have become a problem. Therefore, conductive powders are required to contain no coarse particles, or very few coarse particles.

[0003] The problem caused by the presence of coarse particles in this powder is particularly problematic in the field of conductive pastes for the internal electrodes of multilayer ceramic capacitors (MLCCs). Multilayer ceramic capacitors are manufactured by, for example, screen printing a conductive paste, mainly nickel powder dispersed as conductive powder, onto a dielectric green sheet to form the internal electrodes; stacking and pressing together multiple green sheets on which the internal electrodes are printed so that the internal electrodes overlap alternately to obtain a laminate; cutting this laminate to a predetermined size, debinding, and then firing it at a high temperature of 1300°C to obtain a ceramic sintered body; and finally, attaching external electrodes to this ceramic sintered body.

[0004] With the miniaturization and increased capacitance of multilayer ceramic capacitors, both the internal electrodes and dielectrics are being made thinner. Consequently, the particle size of the nickel powder used in the internal electrode paste is also being made finer, requiring nickel powder with an average particle size of 0.4 μm or less, and in particular, nickel powder with an average particle size of 0.3 μm or less is becoming mainstream. In addition, depending on the film thickness of the internal electrodes, in conductive powders, a particle size of 3 to 5 times or more the number average particle size of the entire powder is usually used as the standard particle size, and particles larger than this standard particle size are considered coarse particles. Here, when the conductive powder is mainly composed of primary particles, coarse particles include not only particles with a primary particle size larger than the standard, but also secondary particles in which the primary particles are firmly bound or aggregated and cannot be easily broken down, and which have a particle size larger than the standard particle size.

[0005] There are two main methods for producing nickel powder: the vapor phase method and the wet method. Examples of vapor phase methods include the method described in Patent Document 1, which involves reducing nickel chloride vapor with hydrogen to produce nickel powder (hereinafter, nickel powder obtained by the vapor phase method may be referred to as "vapor phase nickel powder"), and the method described in Patent Document 2, which involves vaporizing nickel metal in a plasma to produce nickel powder (vapor phase nickel powder). Examples of wet methods include the method described in Patent Document 3, which involves adding a reducing agent to a nickel salt solution to produce nickel powder (hereinafter, nickel powder obtained by the wet method may be referred to as "wet nickel powder").

[0006] The vapor phase method has the problem of producing nickel powder with a broad particle size distribution. As mentioned above, thinning the internal electrode layer requires nickel powder that does not contain coarse particles and has a relatively narrow particle size distribution with an average particle size of 0.4 μm or less. Therefore, obtaining such nickel powder by the vapor phase method requires classification treatment using expensive classification equipment.

[0007] Furthermore, while classification allows for the removal of coarse particles larger than a classification point (approximately 0.6 μm to 2 μm), some particles smaller than the classification point are also removed simultaneously, resulting in a significant decrease in actual product yield. Consequently, gas-phase methods inevitably lead to increased product costs, including the aforementioned high cost of equipment.

[0008] Furthermore, in the gas phase method, when using nickel powder with an average particle size of 0.2 μm or less, especially 0.1 μm or less, it becomes difficult to remove coarse particles through classification, making it impossible to accommodate further thinning of the internal electrode in the future.

[0009] Furthermore, nickel powder obtained by the gas phase method may contain residual foreign substances such as halogens originating from the starting materials, so washing with pure water should also be considered.

[0010] On the other hand, the wet method has the advantage of producing nickel powder with a narrower particle size distribution compared to the gas-phase method. In particular, in the method for producing nickel powder described in Patent Document 3, in which a solution containing hydrazine as a reducing agent is added to a solution containing nickel salt and copper salt, a metal salt of a metal nobler than nickel (copper salt) acts as a nucleating agent, and in the presence of this nucleating agent, the nickel salt (more precisely, nickel ions (Ni)) 2+ Nickel ions (or nickel complex ions) are reduced by hydrazine. Therefore, the particle size can be controlled by controlling the number of nuclei generated, and since the generation of nuclei and the growth of nickel particles are uniform, it is known that fine nickel powder can be obtained with a narrower particle size distribution than in the gas phase method.

[0011] Incidentally, when applying nickel powder obtained by the above wet method to multilayer ceramic capacitors, high flatness is required for the nickel paste dried film (a dried film obtained by printing and drying nickel paste) which mainly consists of nickel powder and resin, in order to prevent short circuits between electrodes in the laminate consisting of the internal electrode layer and the dielectric layer mentioned above. In particular, in order to cope with the thinning of the internal electrode layer (approximately 0.5 μm to 1.0 μm) accompanying the recent increase in capacitance of multilayer ceramic capacitors, fine nickel powder with an average particle size of 0.3 μm or less, preferably 0.2 μm or less, is used, and it is required that coarse particles of a size similar to the thickness of the internal electrode layer (for example, 0.8 μm to 1.2 μm) contained in the nickel powder be reduced to the absolute minimum.

[0012] As a means of reducing coarse particles in nickel powder, in the case of gas-phase nickel powder, the removal of coarse particles by the classification process described above can be used. However, as described in Patent Document 5, methods such as suppressing aggregate formation of gas-phase nickel powder and removing halogens by using glutamic acid or the like in the nickel powder slurry undergoing purification treatment are also used. On the other hand, in the case of wet nickel powder, crushing treatment has been proposed as a means of reducing coarse particles. For example, Patent Document 6 discloses a wet nickel powder having a specific particle size distribution and an average primary particle size within a specific range, obtained by a specific crushing treatment, and a method for producing the same.

[0013] However, the above classification and crushing processes become increasingly difficult when dealing with nickel powder with an average particle size of 0.2 μm or less, especially 0.1 μm or less, as removing or crushing coarse particles becomes increasingly challenging. In particular, when dry crushing is applied to nickel powder with a particle size of 0.15 μm or less, there is a risk of insufficient crushing power due to a decrease in kinetic energy caused by the reduction in the mass of individual nickel particles, and a risk of heat generation and ignition due to increased oxidation heat on the newly crushed surface resulting from the increase in specific surface area. Therefore, the difficulty of addressing the future thinning of internal electrodes through classification and crushing processes is increasing even further. [Prior art documents] [Patent Documents]

[0014] [Patent Document 1] Japanese Patent Application Laid-Open No. 4-365806 [Patent Document 2] Japanese Patent Application Laid-Open No. 2002-530521 [Patent Document 3] Japanese Patent Application Laid-Open No. 2002-53904 [Patent Document 5] Japanese Patent Application Laid-Open No. 2006-219688 [Patent Document 6] Japanese Patent Application Laid-Open No. 2001-247903 [Summary of the Invention] [Problems to be Solved by the Invention]

[0015] An object of the present invention is to provide a method for producing nickel powder that suppresses an increase in coarse particles after dry crushing treatment after passing through a nickel powder slurry in the nickel powder production process. [Means for Solving the Problems]

[0016] In order to solve the above problems, the method for producing nickel powder of the present invention includes a solid-liquid separation step of solid-liquid separating a nickel powder slurry in which nickel powder is dispersed in water to obtain a nickel powder cake, a drying step of drying the nickel powder cake to obtain a dried nickel powder, a water-soluble organic solvent treatment step of bringing the dried nickel powder into contact with a water-soluble organic solvent to obtain a water-soluble organic solvent-treated nickel powder, and a dry crushing step of dry crushing the water-soluble organic solvent-treated nickel powder to obtain a crushed nickel powder.

[0017] Further, the water-soluble organic solvent may contain at least one of alcohol, ketone, or ether as a main component.

[0018] Further, the water-soluble organic solvent may have a boiling point of 50°C to 140°C.

[0019] Further, the water-soluble organic solvent may contain alcohols selected from at least any one of methanol, ethanol, 1-propanol, 2-propanol, and denatured alcohol.

[0020] Further, the water-soluble organic solvent may contain 90% by mass or more of alcohols selected from at least any one of methanol, ethanol, 1-propanol, 2-propanol, and denatured alcohol.

[0021] Further, the water-soluble organic solvent treatment step may include a water-soluble organic solvent slurrying treatment step of slurrying the dried nickel powder in the water-soluble organic solvent to obtain a water-soluble organic solvent slurry, and a drying step of drying the water-soluble organic solvent slurry to remove the water-soluble organic solvent.

[0022] Further, the water-soluble organic solvent treatment step may include a water-soluble organic solvent vapor treatment step of exposing the dried nickel powder to the vapor of the water-soluble organic solvent.

[0023] Further, the water-soluble organic solvent treatment step may include a water-soluble organic solvent contact treatment step of shaking the dried nickel powder and the water-soluble organic solvent together.

[0024] Further, the solid-liquid separation step may be a step of solid-liquid separating a slurry of crystallized nickel powder obtained by crystallizing the water-soluble nickel salt by a reduction reaction with hydrazine in a reaction solution containing a water-soluble nickel salt, a metal salt of a metal nobler than nickel, hydrazine, an alkali hydroxide, and water to obtain the nickel powder cake.

[0025] Further, the nickel powder may have a number average particle size of 0.03 μm to 0.3 μm.

Advantages of the Invention

[0026] As described above, the present invention provides a method for producing nickel powder that suppresses the increase in coarse particles after the dry crushing treatment following the nickel powder slurry in the nickel powder manufacturing process. [Brief explanation of the drawing]

[0027] [Figure 1] This is a schematic diagram showing coarse particles containing nickel hydroxide, which are found in conventional wet nickel powder. [Figure 2] This is a schematic diagram showing an example of the manufacturing process in a method for producing nickel powder according to one embodiment of the present invention. [Figure 3] This is a schematic diagram showing an example of a manufacturing process in a method for producing nickel powder according to one embodiment of the present invention, which differs from the manufacturing process shown in Figure 2. [Figure 4] This is a schematic diagram showing an example of a manufacturing process in a method for producing nickel powder according to one embodiment of the present invention, which differs from the manufacturing process shown in Figures 2 and 3. [Figure 5] This is a schematic diagram of a suction filtration apparatus used to evaluate the coarse particle content of wet nickel powder obtained by the manufacturing methods shown in the Examples, Comparative Examples, and Reference Examples. [Modes for carrying out the invention]

[0028] The method for producing nickel powder according to the present invention will be described below with reference to the drawings.

[0029] There are two main methods for synthesizing nickel powder particles, the gas-phase method and the wet method, which are described below, and the process follows this classification. In the nickel powder manufacturing method according to the present invention, nickel particles synthesized by the gas-phase method or the wet method are slurryed due to the synthesis process, and become nickel powder through the first drying step and dry crushing step described later. Each step will be described in the following order.

[0030] 1-A. Synthesis of nickel particles and gas-phase nickel powder slurry by gas-phase method 1-B. Crystallization of nickel particles by wet method and crystallized nickel powder slurry 1-B-1. Crystallization process 1-B-1-1. Chemicals used in the crystallization process 1-B-1-2. Crystallization Reaction Procedure (Crystallization Procedure) 1-B-1-3. Crystallization reactions (reduction reactions, hydrazine autodecomposition reactions) 1-B-1-4. Crystallization conditions (reaction initiation temperature) 1-2. Solid-liquid separation process 1-2-A. Solid-liquid separation process of gas-phase nickel powder slurry 1-2-B. Solid-liquid separation process of crystallized nickel powder slurry 1-3.First drying process (drying process) 1-4. Water-soluble organic solvent treatment process (surface treatment process) [Solvents used in water-soluble organic solvent processing steps] [Types of water-soluble organic solvent treatment processes] 1-4-A. Water-soluble organic solvent treatment process (Part 1) (1-4-A-1. Preparation) (1-4-A-2. Water-soluble organic solvent slurry preparation step) (1-4-A-3. 2nd solid-liquid separation stage) (1-4-A-4. Second drying stage) 1-4-B. Water-soluble organic solvent treatment process (part 2) (1-4-B-1. Preparation) (1-4-B-1-1. Preparation of water-soluble organic solvents) (1-4-B-1-2. Preparation of dried nickel powder) (1-4-B-2. Water-soluble organic solvent vapor treatment step) 1-4-C. Water-soluble organic solvent treatment process (part 3) (1-4-C-1. Preparation) (1-4-C-1-1. Preparation of water-soluble organic solvents) (1-4-C-1-2. Preparation of dried nickel powder) (1-4-C-2. Water-soluble organic solvent contact treatment step) (1-4-C-3. Recovery of nickel powder) 1-5.Dry crushing process 2. Nickel powder obtained by the dry crushing process 3. Effects on conductive paste (nickel paste) for internal electrodes

[0031] Here, the terms are defined as follows: Powder refers to an aggregate of particles. Gas-phase nickel powder refers to an aggregate of nickel particles synthesized by the gas-phase method that has not undergone the first drying process. Crystallized nickel powder refers to an aggregate of nickel particles that have been reduced and crystallized by the wet process that has not undergone the first drying process. Dry nickel powder refers to powder that has undergone the first drying process. Nickel powder refers to powder that can be processed into nickel paste after a dry crushing process. Gas-phase nickel powder slurry is a slurry of gas-phase nickel particles that has not undergone the first drying process. Crystallized nickel powder slurry is a slurry of crystallized nickel particles synthesized by the wet process that has not undergone the first drying process.

[0032] 1-A. Synthesis of nickel particles and gas-phase nickel powder slurry by gas-phase method An example of a method for synthesizing nickel particles by the gas phase method is described below. The reaction vessel for producing nickel particles by the gas phase method is cylindrical and made of ceramic or the like. One end of the reaction vessel is equipped with an inert gas supply pipe for supplying argon or the like to deliver the nickel halide compound (e.g., nickel chloride), which is the raw material placed inside the reaction vessel, to the reaction section, and a reducing gas supply pipe for supplying hydrogen or the like as a reducing gas to the reaction section. The other end is equipped with a nickel powder recovery section. The reaction vessel is surrounded by a heat source such as an electric heater for sublimating the nickel halide compound and an electric heater for promoting the reduction reaction in the reaction section. Individual nickel particles formed in the reaction section are transported by an inert gas or the like and recovered in the recovery section of the reaction vessel. A cooling section may be provided between the reaction section and the recovery section of the reaction vessel to cool the obtained nickel particles.

[0033] The sublimated nickel halide vapor is transported to the reaction chamber by an inert gas, where a gas-phase chemical reaction occurs with the reducing gas due to heat. In this gas-phase chemical reaction, monomers of metal atoms or clusters are generated the moment the nickel halide vapor comes into contact with the reducing gas, and fine particles are generated by the collision and aggregation of these monomers. Furthermore, it is thought that particle growth occurs through collision and coalescence. Linked particles, formed by the linkage of grown particles, may also be produced. Here, spherical nickel particles can be obtained by maintaining a temperature of 1000°C or higher in the reaction chamber.

[0034] Incidentally, nickel particles manufactured using this vapor phase method and recovered in the recovery unit contain residual halogens due to the nickel halide compounds used as raw materials. These residual halogens can lead to malfunctions in multilayer ceramic capacitors and therefore need to be removed.

[0035] Therefore, the nickel particles recovered in the recovery section are washed with pure water to remove halogens, becoming a gas-phase nickel powder slurry (nickel powder slurry). Then, through a solid-liquid separation process and a dry crushing process, it becomes nickel powder.

[0036] 1-B. Crystallization of nickel particles by wet method and crystallized nickel powder slurry This section explains the process of synthesizing (crystallizing) nickel particles using the wet method.

[0037] Figures 2 to 4 show schematic diagrams of an example of the manufacturing process in a method for producing nickel powder according to one embodiment of the present invention. The method for producing nickel powder according to one embodiment of the present invention includes a crystallization step.

[0038] (1-B-1. Crystallization process) In the crystallization process, a nickel salt (more precisely, nickel ions or nickel complex ions) is reduced in a reaction solution containing at least a water-soluble nickel salt, a salt of a metal nobler than nickel, a reducing agent, alkali hydroxide, and water, by a reduction reaction using a reducing agent such as hydrazine, to obtain a crystallized nickel powder slurry (nickel powder slurry). In the present invention, when hydrazine is used, an amine compound or a sulfur-containing compound may be mixed into the reaction solution as needed, and the nickel salt can be reduced while suppressing the decomposition of hydrazine as a reducing agent in the presence of the amine compound or sulfur-containing compound. If desired, a sulfur compound may be added to the reaction solution containing the crystallized nickel powder or to the washing solution to perform a surface treatment (sulfur coating treatment) to modify the surface of the crystallized nickel powder with the sulfur component and obtain surface-treated nickel powder. By performing this surface treatment, it is possible to weaken the activity of nickel in decomposing organic matter, and for example, the deterioration of resin coexisting with nickel powder in a conductive paste can be suppressed.

[0039] (1-B-1-1. Chemicals used in the crystallization process) In the crystallization process of the present invention, a reaction solution containing water, various agents such as nickel salts, salts of metals nobler than nickel, reducing agents, alkali hydroxides, and optionally amine compounds and sulfur-containing compounds, is used. From the viewpoint of reducing the amount of impurities in the resulting nickel powder, highly pure water such as ultrapure water (conductivity: ≤0.06 μS / cm (microsiemens per centimeter)) or pure water (conductivity: ≤1 μS / cm) is preferred as the solvent, and among these, pure water is preferable because it is inexpensive and readily available. The various agents described above will be explained in detail below.

[0040] (a) Water-soluble nickel salt The water-soluble nickel salt is not particularly limited as long as it is readily soluble in water, and for example, one or more selected from nickel chloride, nickel sulfate, and nickel nitrate can be used. Among these nickel salts, nickel chloride, nickel sulfate, or mixtures thereof are more preferable.

[0041] (b) Metal salts of metals nobler than nickel Salts of metals nobler than nickel have a lower ionization tendency than nickel, and therefore are reduced before nickel during the reduction deposition of nickel. Consequently, when salts of metals nobler than nickel are included in a nickel salt solution, they act as nucleation agents, being reduced first during the reduction deposition of nickel. This allows for easy particle size control and refinement in the crystallized nickel powder obtained from these initial nuclei.

[0042] As a metal salt of a metal nobler than nickel, any metal salt of a metal that is water-soluble and has a lower ionization tendency than nickel is acceptable. Examples include water-soluble copper salts, gold salts, silver salts, platinum salts, palladium salts, rhodium salts, iridium salts, and other water-soluble precious metal salts. For example, copper sulfate can be used as a water-soluble copper salt, silver nitrate as a water-soluble silver salt, and sodium palladium(II) chloride, ammonium palladium(II) chloride, palladium(II) nitrate, and palladium(II) sulfate can be used as water-soluble palladium salts, but are not limited to these.

[0043] As a metal salt of a metal nobler than nickel, using the palladium salt mentioned above is preferable because, although the particle size distribution becomes somewhat broader, it allows for finer control of the particle size of the resulting crystallized nickel powder. The ratio of palladium salt to nickel [moles in ppm] when using palladium salt is calculated as follows: (moles of palladium salt / moles of nickel × 10⁻¹⁰) 6The number average particle size of the crystallized nickel powder can be appropriately selected depending on the desired number average particle size. For example, if the number average particle size of the crystallized nickel powder is set to 0.05 μm to 0.5 μm, it is preferable to set the ratio of palladium salt to nickel in the range of 0.2 mol ppm to 100 mol ppm, preferably in the range of 0.5 mol ppm to 80 mol ppm. If this ratio is less than 0.2 mol ppm, the number average particle size of the manufactured crystallized nickel powder may exceed 0.4 μm. On the other hand, if this ratio exceeds 100 mol ppm, a large amount of expensive palladium salt will be used, which may lead to an increase in the cost of manufacturing the crystallized nickel powder.

[0044] (c) Reducing agent The reducing agent used in the crystallization step of the present invention is not particularly limited, but examples include hydrazine (N2H4, molecular weight: 32.05). In addition to anhydrous hydrazine, there is also hydrated hydrazine (N2H4·H2O, molecular weight: 50.06), and either can be used. The reduction reaction of hydrazine is as shown in formula (2) described later, and it has the following characteristics: it has high reducing power in alkaline conditions, the by-products of the reduction reaction are nitrogen gas and water, so no impurity components are generated in the reaction solution due to the reduction reaction, hydrazine itself contains few impurities, and it is readily available. For these reasons, hydrazine is suitable as a reducing agent, and for example, commercially available industrial grade 60% by mass hydrated hydrazine can be used.

[0045] (d) Alkali hydroxide The reducing power of hydrazine increases with increasing alkalinity of the reaction solution (see equation (2) described later); therefore, alkali hydroxide can be used as a pH adjuster to increase alkalinity in the crystallization process. While there are no particular limitations on the alkali hydroxide, alkali metal hydroxides are preferred due to their availability and cost. Specifically, it is more preferable to use one or more selected from sodium hydroxide and potassium hydroxide.

[0046] The amount of alkali hydroxide added should be determined so that the pH of the reaction solution is 9.5 or higher, preferably 10 or higher, and more preferably 10.5 or higher, at the reaction temperature, in order to sufficiently increase the reducing power of hydrazine as a reducing agent. For example, comparing the pH of the reaction solution at 25°C and 70°C, the pH at the higher temperature of 70°C will be lower.

[0047] (e) Amine compounds As described above, the amine compound of the present invention has the effects of inhibiting the self-decomposition of hydrazine, promoting the reduction reaction, and inhibiting the linkage between nickel particles, and may be included in the reaction solution as needed. The above amine compound is a compound that contains two or more functional groups selected from primary amino groups (-NH2) or secondary amino groups (-NH-) in its molecule, and for example, at least one alkyleneamine or alkyleneamine derivative can be used. As an example, it is preferable to use an amine compound that has at least the structure of the following formula A in which the nitrogen atom of the amino group in the molecule is bonded via a carbon chain having 2 carbon atoms.

[0048] [ka]

[0049] More specifically, one or more alkyleneamines can be selected from ethylenediamine (H2NC2H4NH2), diethylenetriamine (H2NC2H4NHC2H4NH2), triethylenetetramine (H2N(C2H4NH)2C2H4NH2), tetraethylenepentamine (H2N(C2H4NH)3C2H4NH2), pentaethylenehexamine (H2N(C2H4NH)4C2H4NH2), and propylenediamine (also known as 1,2-diaminopropane, 1,2-propanediamine) (CH3CH(NH2)CH2NH2). Furthermore, alkyleneamine derivatives include tris(2-aminoethyl)amine (N(C2H4NH2)3), N-(2-aminoethyl)ethanolamine (also known as 2-(2-aminoethylamino)ethanol (H2NC2H4NHC2H4OH), N-(2-aminoethyl)propanolamine (also known as 2-(2-aminoethylamino)propanol) (H2NC2H4NHC3H6OH), and L (or D, DL)-2,3-diaminopropionic acid (also known as: 3-amino-L (or D,DL)-alanine) (H2NCH2CH(NH)COOH), ethylenediamine-N,N'-diacetic acid (also known as ethylene-N,N'-diglycine) (HOOCCH2NHC2H4NHCH2COOH), N,N'-diacetylethylenediamine (also known as N,N'-ethylenebisacetamide) (CH3CONHC2H4NHCOCH3), 1,2-cyclohexanediamine (also known as 1,2-diaminocyclohexane) (H2NC6H 10 One or more alkyleneamines and alkyleneamine derivatives selected from NH2, N,N'-dimethylethylenediamine (CH3NHC2H4NHCH3), N,N'-diethylethylenediamine (C2H5NHC2H4NHC2H5), and N,N'-diisopropylethylenediamine (CH3(CH3)CHNHC2H4NHCH(CH3)CH3) can be used. These alkyleneamines and alkyleneamine derivatives are water-soluble, and among them, ethylenediamine and diethylenetriamine are preferred because they have a relatively strong inhibitory effect on the self-decomposition of hydrazine, are readily available and inexpensive.

[0050] The action of the above amine compound as a reducing reaction accelerator is due to nickel ions (Ni) in the reaction solution.2+ This is thought to be due to its function as a complexing agent, which complexes ) to form nickel complex ions. Furthermore, its effects as an inhibitor of hydrazine self-decomposition and an inhibitor of linkage between nickel particles are presumed to be due to the interaction between the primary amino group (-NH2) and secondary amino group (-NH-) within the amine compound molecule and the surface of hydrazine or crystallized nickel powder.

[0051] Furthermore, it is preferable that the alkyleneamine or alkyleneamine derivative, which is an amine compound, has the structure of formula A above, in which the nitrogen atom of the amino group in the molecule is bonded via a carbon chain with 2 carbon atoms. This is because it enhances the effect of suppressing the decomposition of hydrazine molecules. For example, if the nitrogen atom of the amino group that is strongly adsorbed to crystallized nickel powder is bonded via a carbon chain with 3 or more carbon atoms, it is thought that the longer carbon chain increases the degree of freedom of movement (molecular flexibility) of the carbon chain portion of the amine compound molecule. As a result, it becomes difficult to effectively prevent contact of hydrazine molecules with crystallized nickel powder, leading to an increase in the number of hydrazine molecules that self-decompose due to the catalytic activity of nickel, and thus reducing the effect of suppressing the self-decomposition of hydrazine.

[0052] In fact, compared to ethylenediamine (H2NC2H4NH2) and propylenediamine (also known as 1,2-diaminopropane, 1,2-propanediamine) (CH3CH(NH2)CH2NH2), in which the nitrogen atom of the amino group in the molecule is bonded via a carbon chain with 2 carbon atoms, trimethylenediamine (also known as 1,3-diaminopropane, 1,3-propanediamine) (H2NC3H6NH2), in which the nitrogen atom of the amino group in the molecule is bonded via a carbon chain with 3 carbon atoms, has been confirmed to have a less effective inhibitory effect on the self-decomposition of hydrazine.

[0053] Here, the ratio of the amine compound to nickel in the reaction solution [mol%] ((moles of amine compound / moles of nickel) × 100) is preferably in the range of 0.01 mol% to 5 mol%, more preferably in the range of 0.03 mol% to 2 mol%. If the above ratio is less than 0.01 mol%, the amount of the amine compound is too small, and it may not be possible to obtain the effects of a hydrazine self-decomposition inhibitor, a reduction reaction accelerator, or an inhibitor of the linkage between nickel particles. On the other hand, if the above ratio exceeds 5 mol%, the amine compound's function as a complexing agent for forming nickel complex ions becomes too strong, which may result in abnormal particle growth of the crystallized nickel powder. This may lead to a loss of granularity and sphericity of the crystallized nickel powder, resulting in an irregular shape, or the formation of many coarse particles where nickel particles are linked together, thus degrading the properties of the crystallized nickel powder.

[0054] (f) Sulfur-containing compounds (hydrazine autodegradation inhibitors) The reaction solution may contain a sulfur-containing compound. The sulfur-containing compound used in this invention is a compound applied as a brightener for nickel plating and as a stabilizer for plating baths. Unlike the amine compound mentioned above, when used alone, it does not have a very strong inhibitory effect on the self-decomposition of hydrazine. However, it has interactions with the surface of nickel particles, such as adsorption, and when used in combination with the amine compound, it has the effect of a hydrazine self-decomposition inhibitory aid, which can significantly enhance the inhibitory effect on the self-decomposition of hydrazine. Therefore, it is good to add it to the reaction solution as needed. The sulfur-containing compound is a compound that contains at least one of the following groups in its molecule: sulfide group (-S-), sulfonyl group (-S(=O)2-), sulfonic acid group (-S(=O)2-O-), or thioketone group (-C(=S)-). Furthermore, in addition to its effect as a hydrazine self-decomposition inhibitory aid, the sulfur-containing compound also has the effect of inhibiting the linkage between nickel particles. When used in combination with the amine compound, it can more effectively reduce the amount of coarse particles formed when nickel particles are linked together.

[0055] As for sulfur-containing compounds, for example, in the case of sulfide compounds having a sulfide group (-S-) in the molecule, high water solubility is desirable. Therefore, it is preferable that the compound is one of a carboxyl group-containing sulfide compound, a hydroxyl group-containing sulfide compound, or an amino group-containing sulfide compound, which further contains at least one of a carboxyl group (-COOH), a hydroxyl group (-OH), or an amino group (primary: -NH2, secondary: -NH-, tertiary: -N<) in the molecule. Thiazole ring-containing sulfide compounds, which contain at least one thiazole ring (C3H3NS), are also applicable, although they do not have high water solubility. More specifically, L(or D, DL)-methionine (CH3SC2H4CH(NH2)COOH), L(or D, DL)-ethionine (C2H5SC2H4CH(NH2)COOH), N-acetyl-L(or D, DL)-methionine (CH3SC2H4CH(NH(COCH3))COOH), lanthionine (also known as 3,3'-thiodialanine) (HOOCCH(NH2)CH2SCH2CH(NH2)COOH), thiodipropionic acid (also known as 3,3'-thiodipropionic acid) (HOOCC2H4SC One or more of the following are preferred: 2H4COOH, thiodiglycolic acid (also known as 2,2'-thiodiglycolic acid, 2,2'-thiodiacetic acid, 2,2'-thiobisacetic acid, mercaptodiacetic acid) (HOOCCH2SCH2COOH), methionol (also known as 3-methylthio-1-propanol) (CH3SC3H6OH), thiodiglycol (also known as 2,2'-thiodiethanol) (HOC2H5SC2H5OH), thiomorpholine (C4H9NS), thiazole (C3H3NS), and benzothiazole (C7H5NS). Among these, methionine and thiodiglycolic acid are preferred because they are excellent at assisting in the inhibition of hydrazine autodegradation, are readily available, and are inexpensive. Note that "L (or D, DL)-methionine" means "L-methionine, D-methionine, or DL-methionine," and the same applies to other compounds that have the notation L (or D, DL).

[0056] As sulfur-containing compounds other than sulfide compounds, more specifically, saccharin (alternative name: o-benzoic acid sulfimide, o-sulfobenzoimide) (C7H5NO3S), sodium dodecyl sulfate (C 12 H 25 OS(O)2ONa), dodecylbenzenesulfonic acid (C 12 H 25 C6H4S(O)2OH), sodium dodecylbenzenesulfonate (C 12 H 25 C6H4S(O)2ONa), sodium bis(2-ethylhexyl) sulfosuccinate (alternative name: sodium di-2-ethylhexyl sulfosuccinate, sodium dioctyl sulfosuccinate) (NaOS(O)2CH(COOCH2CH(C2H5)C4H9)CH2(COOCH2CH(C2H5)C4H9)), thiourea (H2NC(S)NH2), and one or more selected therefrom. These sulfur-containing compounds are water-soluble. Among them, saccharin and thiourea are preferred because they have an excellent auxiliary effect on suppressing the self-decomposition of hydrazine and are easily available and inexpensive.

[0057] Regarding the action of the sulfur-containing compound as an auxiliary agent for suppressing the self-decomposition of hydrazine and as an agent for suppressing the connection between nickel particles, it can be speculated as follows. That is, the sulfur-containing compound has a sulfide group (-S-), a sulfonyl group (-S(=O)2-), a sulfonic acid group (-S(=O)2-O-), and a thioketone group (-C(=S)-) in the molecule, which are adsorbed on the nickel surface of the nickel particles by intermolecular forces. However, by itself, the action of covering and protecting the crystallized nickel powder like the amine compound molecules described above is not enhanced. On the other hand, when an amine compound and a sulfur-containing compound are used in combination, when the amine compound molecules strongly adsorb and cover the surface of the crystallized nickel powder, there is a high possibility that tiny regions that cannot be completely covered by the amine compound molecules will occur. However, the sulfur-containing compound molecules adsorb and supplementarily cover this part, so that the contact between hydrazine molecules in the reaction solution and the crystallized nickel particles is more effectively prevented, and furthermore, the aggregation of the crystallized nickel particles can be more powerfully prevented, and the above action is manifested.

[0058] Here, the ratio of the sulfur-containing compound to nickel in the reaction solution [mol%] ((moles of sulfur-containing compound / moles of nickel) × 100) is preferably in the range of 0.01 mol% to 5 mol%, more preferably 0.03 mol% to 2 mol%, and more preferably 0.05 mol% to 1 mol%. If the above ratio is less than 0.01 mol%, there is too little of the sulfur-containing compound, and the effects of the hydrazine self-decomposition inhibitor and the nickel particle linkage inhibitor may not be obtained. On the other hand, even if the above ratio exceeds 5 mol%, no improvement in the above effects is observed, and it simply increases the amount of sulfur-containing compound used, which increases the cost of the chemicals. At the same time, the amount of organic components in the reaction solution increases, which increases the chemical oxygen demand (COD) of the reaction wastewater from the crystallization process, resulting in increased wastewater treatment costs.

[0059] (g) Other contents In addition to the above, the reaction solution in the crystallization process may contain various additives such as dispersants, complexing agents, and defoaming agents. For example, using appropriate dispersants and complexing agents in appropriate amounts can improve the granularity (sphericity) and surface smoothness of the crystallized nickel powder, and reduce coarse particles. Similarly, using appropriate defoaming agents in appropriate amounts can suppress foaming during the crystallization process caused by nitrogen gas (see equations (2) to (4) below) generated during the crystallization reaction, thereby preventing, for example, the reaction solution from overflowing from the container. Known substances can be used as dispersants, such as alanine (CH3CH(COOH)NH2), glycine (H2NCH2COOH), triethanolamine (N(C2H4OH)3), and diethanolamine (also known as iminodiethanol) (NH(C2H4OH)2). Furthermore, known substances can be used as complexing agents, including hydroxycarboxylic acids, carboxylic acids (organic acids containing at least one carboxyl group), hydroxycarboxylic acid salts and hydroxycarboxylic acid derivatives, carboxylate salts and carboxylic acid derivatives, specifically tartaric acid, citric acid, malic acid, ascorbic acid, formic acid, acetic acid, pyruvic acid, and their salts and derivatives. In addition, the defoaming agent is not particularly limited as long as it exhibits excellent defoaming properties under alkaline conditions, and oil-type, solvent-type, silicone-based, or non-silicone-based defoaming agents can be used.

[0060] (1-B-1-2. Crystallization Reaction Procedure (Crystallization Procedure)) In the crystallization process, a nickel salt solution is prepared by dissolving at least a water-soluble nickel salt and a salt of a metal nobler than nickel in water, a reducing agent solution is prepared by dissolving a reducing agent (e.g., hydrazine) in water, and an alkali hydroxide solution is prepared by dissolving alkali hydroxide in water. These are added and mixed to prepare the reaction solution. Then, a crystallization reaction is carried out in this reaction solution to crystallize nickel particles by a reduction reaction to obtain crystallized nickel powder. If necessary, amine compounds and sulfur-containing compounds can be added and mixed in one of the above solutions or a mixture thereof before preparing the reaction solution, or they can be added and mixed in the reaction solution after it has been prepared. At room temperature, the reduction reaction begins as soon as the reaction solution is prepared.

[0061] Here, there are two specific crystallization procedures: one involves adding a reducing agent (e.g., hydrazine) obtained by pre-mixing a reducing agent solution and an alkali hydroxide solution to a nickel salt solution containing the nickel salt to be reduced and a salt of a metal nobler than nickel, and mixing to prepare the reaction solution; and the other involves adding an alkali hydroxide solution to a nickel salt / reducing agent solution obtained by adding a reducing agent solution (e.g., hydrazine solution) to the above nickel salt solution and mixing to prepare the reaction solution. In the former, a reducing agent (e.g., hydrazine) whose alkalinity and reducing power have been increased by alkali hydroxide is added and mixed to the nickel salt solution containing the substance to be reduced, while in the latter, the reducing agent (e.g., hydrazine) is pre-mixed to the nickel salt solution containing the substance to be reduced, and then the pH is adjusted (increased) with alkali hydroxide to increase the reducing power.

[0062] In the former case (when a nickel salt solution is added and mixed with a reducing agent / alkali hydroxide solution), depending on the temperature at the time the reaction solution is prepared, i.e., when the reduction reaction begins (hereinafter sometimes referred to as the reaction start temperature), if the time required to add and mix the nickel salt solution (a solution containing nickel salt and a salt of a metal nobler than nickel) with the reducing agent / alkali hydroxide solution, whose reducing power is increased by raising its alkalinity with alkali hydroxide (hereinafter sometimes referred to as the raw material mixing time), the alkalinity increases locally in the region where the nickel salt solution and the reducing agent / alkali hydroxide solution are added and mixed, increasing the reducing power of hydrazine, and causing nucleation due to the salt of a metal nobler than nickel, which is the nucleating agent. Therefore, the nucleating effect of the added nucleating agent weakens towards the end of the raw material mixing time, and the dependence of nucleation on the raw material mixing time becomes greater, making it difficult to finely mill the crystallized nickel powder or obtain a narrow particle size distribution. This tendency is more pronounced when an alkaline reducing agent / alkali hydroxide solution is added and mixed with a weakly acidic nickel salt solution. The above trend can be suppressed by shortening the mixing time of the raw materials, so short mixing times are desirable. However, considering constraints on mass production equipment, the mixing time of the raw materials is preferably 10 to 180 seconds, more preferably 20 to 120 seconds, and even more preferably 30 to 80 seconds.

[0063] On the other hand, in the latter case (when an alkali hydroxide solution is added and mixed with a nickel salt solution and a reducing agent solution to form a nickel salt / reducing agent solution), the hydrazine reducing agent is pre-added and mixed into the nickel salt / reducing agent solution, which contains nickel salt, a salt of a metal nobler than nickel, and a reducing agent, resulting in a uniform concentration. Therefore, the dependence of nucleation that occurs when adding and mixing the alkali hydroxide solution on the raw material mixing time of the alkali hydroxide is not as large as in the former case, and it is easier to refine the crystallized nickel powder and obtain a narrow particle size distribution. However, for the same reasons as in the former case, a short mixing time for the alkali hydroxide solution is desirable, and considering constraints on mass production equipment, such a mixing time is preferably 10 to 180 seconds, more preferably 20 to 120 seconds, and even more preferably 30 to 80 seconds.

[0064] Regarding the addition and mixing of the amine compound and sulfur-containing compound of the present invention, as described above, there are two types of procedures: one in which the compound is added to the reaction solution before it is prepared, and another in which the compound is added and mixed after the reaction solution has been prepared and the reduction reaction has started.

[0065] In the former case (where the amine compound or sulfur-containing compound is added to the reaction solution before it is prepared), the amine compound or sulfur-containing compound is added to the reaction solution beforehand. This has the advantage that the various effects of the amine compound or sulfur-containing compound are expressed from the moment nucleation begins, which is caused by a salt of a metal nobler than nickel (nucleating agent). On the other hand, the interaction between the amine compound or sulfur-containing compound and the nickel particle surface, such as adsorption, may be involved in nucleation and may affect the particle size and particle size distribution of the resulting crystallized nickel powder.

[0066] Conversely, in the latter case (when the reaction solution is prepared and the reduction reaction is started before the amine compound or sulfur-containing compound is added and mixed into the reaction solution after the very early stages of the crystallization process, in which nucleation caused by the nucleating agent occurs. Therefore, although the action of the amine compound or sulfur-containing compound described above is somewhat delayed, the amine compound or sulfur-containing compound does not participate in nucleation, so the particle size and particle size distribution of the resulting crystallized nickel powder are less affected by the amine compound or sulfur-containing compound, and it is easier to control them. Here, the mixing time for adding the amine compound or sulfur-containing compound to the reaction solution in this procedure can be either added all at once within a few seconds, or added in installments or dropwise over a period of several minutes to 30 minutes. As mentioned above, amine compounds have the effect of promoting the reduction reaction (complexing agent). Therefore, adding them slowly allows crystal growth to proceed slowly, resulting in highly crystalline crystallized nickel powder, but the self-decomposition of hydrazine is also suppressed gradually, and the effect of reducing hydrazine consumption decreases. Therefore, the above mixing time should be determined appropriately while considering the balance between these two factors. Furthermore, the timing of adding and mixing amine compounds and sulfur-containing compounds in the former procedure can be appropriately selected based on a comprehensive assessment of the purpose.

[0067] When adding and mixing a nickel salt solution with a reducing agent / alkali hydroxide solution, or adding and mixing a nickel salt solution with a reducing agent solution, or adding and mixing an alkali hydroxide solution to a nickel salt / reducing agent solution, stirring is preferable. Good stirring reduces heterogeneity depending on the location of nucleation, and also reduces the dependence of nucleation on the raw material mixing time and the alkali hydroxide mixing time, as described above, making it easier to refine the crystallized nickel powder and obtain a narrow particle size distribution. Any known method can be used for stirring and mixing, and it is preferable to use stirring blades from the standpoint of controlling stirring and mixing efficiency and equipment costs.

[0068] (1-B-1-3. Crystallization reactions (reduction reactions, hydrazine autodecomposition reactions)) In the crystallization process, nickel salts are reduced with hydrazine in the reaction solution in the presence of alkali hydroxide and a salt of a metal nobler than nickel to obtain crystallized nickel powder. Furthermore, if necessary, the self-decomposition of hydrazine can be significantly suppressed by the action of trace amounts of specific amine compounds or sulfur-containing compounds to enable the reduction reaction.

[0069] First, let's explain the reduction reaction in the crystallization process. Nickel ions (Ni 2+ The reaction when ) crystallizes to form nickel (Ni) is a two-electron reaction as shown in equation (1) below. The reaction with hydrazine (N2H4) is a four-electron reaction as shown in equation (2) below. For example, as mentioned above, when nickel chloride (NiCl2) is used as the nickel salt and sodium hydroxide (NaOH) is used as the alkali hydroxide, the entire reduction reaction is represented as equation (3) below, in which nickel hydroxide (Ni(OH)2) produced by the neutralization reaction of nickel chloride and sodium hydroxide is reduced by hydrazine. Stoichiometrically (theoretically), 0.5 moles of hydrazine (N2H4) are required for every 1 mole of nickel (Ni).

[0070] From the reduction reaction of hydrazine in equation (2), it can be seen that the reducing power of hydrazine increases as the alkalinity increases. The alkali hydroxide mentioned above is used as a pH adjuster to increase alkalinity and plays a role in promoting the reduction reaction of hydrazine.

[0071] [C2] Ni 2+ +2e - →Ni↓ (2-electron reaction) ···(1) N2H4→N2↑+4H + +4e - (4-electron reaction) ... (2) 2NiCl2 + N2H4 + 4NaOH → 2Ni(OH)2 + N2H4 + 4NaCl →2Ni↓+N2↑+4NaCl+4H2O ···(3)

[0072] As described above, in conventional crystallization processes, the active surface of the crystallized nickel powder acts as a catalyst, accelerating the autodecomposition reaction of hydrazine shown in equation (4) below, and in some cases, a large amount of hydrazine, which is a reducing agent, is consumed in addition to being used for reduction. For this reason, depending on the crystallization conditions such as the reaction start temperature, for example, about 2 moles of hydrazine per 1 mole of nickel was commonly used, which is about four times the theoretical value required for reduction mentioned above. Furthermore, as shown in equation (4), a large amount of ammonia is produced as a by-product in the autodecomposition of hydrazine, resulting in a high concentration of ammonia in the reaction solution and the generation of nitrogen-containing wastewater. Thus, the use of excessive amounts of hydrazine, which is an expensive chemical, and the costs of treating nitrogen-containing wastewater are factors that increase the manufacturing cost of nickel powder by the wet process (wet nickel powder).

[0073] [C3] 3N2H4→N2↑+4NH3···(4)

[0074] In the crystallization process of the crystallized nickel powder of the present invention, it is preferable to add a minute amount of a specific amine compound or sulfur-containing compound to the reaction solution to significantly suppress the autodecomposition reaction of hydrazine and to greatly reduce the amount of hydrazine used, which is an expensive chemical. The reasons why the above-mentioned specific amine compound can suppress the autodecomposition of hydrazine are thought to be (I) that the molecules of the above-mentioned specific amine compound or sulfur-containing compound are adsorbed onto the surface of the crystallized nickel particles in the reaction solution, physically interfering with the contact between the active surface of the nickel particles and the hydrazine molecules, and (II) that the molecules of the specific amine compound or sulfur-containing compound act on the surface of the crystallized nickel particles, deactivating the catalytic activity of the surface.

[0075] In addition, in conventional wet crystallization processes, nickel ions (Ni) are used in tartaric acid and citric acid to shorten the reduction reaction time (crystallization reaction time) to a practical range. 2+ It is common to use complexing agents, which form complex ions with tartaric acid and citric acid to increase the ionic nickel concentration, as accelerators for the reduction reaction. However, these complexing agents, such as tartaric acid and citric acid, do not have the same effect as hydrazine autodegradation inhibitors as the specific amine compounds and sulfur-containing compounds mentioned above.

[0076] On the other hand, the specific amine compounds mentioned above also act as complexing agents, similar to tartaric acid and citric acid, and have the advantage of combining the effects of inhibiting the self-decomposition of hydrazine and promoting the reduction reaction.

[0077] (1-B-1-4. Crystallization conditions (reaction initiation temperature)) The crystallization reaction in the crystallization step begins, for example, in a reaction solution obtained by adding and mixing a solution containing a reducing agent (e.g., hydrazine) and an alkali hydroxide (reducing agent / alkali hydroxide solution) to a solution containing at least a water-soluble nickel salt and a salt of a metal nobler than nickel (nickel salt solution). In this case, the reaction start temperature for the crystallization reaction is preferably 40°C to 95°C, more preferably 50°C to 80°C, and even more preferably 60°C to 70°C. The temperatures of the nickel salt solution and the reducing agent / alkali hydroxide solution can be freely set, as long as the temperature of the mixture obtained by pre-mixing them, i.e., the reaction start temperature, falls within the above temperature range.

[0078] While a higher reaction initiation temperature promotes the reduction reaction and tends to result in highly crystallized nickel powder, it also accelerates the autodecomposition of hydrazine, leading to increased hydrazine consumption and more vigorous foaming of the reaction solution. Therefore, if the reaction initiation temperature is too high, hydrazine consumption may increase significantly, or excessive foaming may prevent the crystallization reaction from continuing. Conversely, if the reaction initiation temperature is too low, the crystallinity of the crystallized nickel powder may decrease significantly, or the reduction reaction may slow down, greatly extending the crystallization process time and reducing the productivity of nickel powder. For these reasons, by using the above temperature range, it is possible to suppress hydrazine consumption, maintain high productivity, and produce high-performance nickel powder at a low cost.

[0079] The crystallization step is not an essential step in the manufacturing method of the present invention. Alternatively, crystallized nickel powder can be obtained by purchasing it, for example, by crystallizing the water-soluble nickel salt by a reduction reaction with hydrazine in a reaction solution containing a water-soluble nickel salt, a metal salt of a metal nobler than nickel, hydrazine, alkali hydroxide, and water, and then forming it into a slurry, or by purchasing a slurry of the crystallized nickel powder after the crystallization step, and then performing the dry crushing step described later.

[0080] Furthermore, in the crystallized nickel powder slurry obtained by the crystallization process, or in the slurry of the obtained crystallized nickel powder, the number-average particle size of the crystallized nickel powder is preferably 0.03 μm or more and 0.3 μm or less, more preferably 0.03 μm or more and 0.2 μm or less, and even more preferably 0.03 μm or more and 0.15 μm or less, considering that it will be used as a material for MLCCs.

[0081] [Process up to dry crushing] The nickel powder ultimately obtained from the aforementioned gas-phase nickel powder slurry or crystallized nickel powder slurry requires a fine particle size with an average particle size of 0.3 μm or less, preferably 0.2 μm or less, for applications such as multilayer ceramic capacitors. Therefore, in this invention, a dry crushing step is performed to dry crush the nickel powder. Prior to the dry crushing step, a solid-liquid separation step, a drying step (first drying step), and a water-soluble organic solvent treatment step (surface treatment step) are performed. In other words, the method for producing nickel powder according to this invention includes a solid-liquid separation step, a drying step (first drying step), a water-soluble organic solvent treatment step (surface treatment step), and a dry crushing step.

[0082] 1-2. Solid-liquid separation process In the solid-liquid separation process, a nickel powder cake is obtained by solid-liquid separation of a gas-phase nickel powder slurry or a crystallized nickel powder slurry (a nickel powder slurry in which nickel powder is dispersed in water). The solid-liquid separation process includes a solid-liquid separation process performed on the gas-phase nickel powder slurry obtained in the gas-phase method described above, and a solid-liquid separation process performed on the crystallized nickel powder slurry obtained in the wet method described above.

[0083] 1-2-A. Solid-liquid separation process of gas-phase nickel powder slurry As described above, the vapor-phase nickel powder, manufactured by the vapor phase method and with halogens removed by washing, is in the state of vapor-phase nickel powder slurry. Therefore, solid-liquid separation is performed by suction filtration to obtain nickel powder cake.

[0084] 1-2-B. Solid-liquid separation process of crystallized nickel powder slurry The crystallized nickel powder produced in the reaction solution by the reduction reaction with hydrazine is in the state of a crystallized nickel powder slurry. Therefore, as mentioned above, if necessary, it can be treated with a sulfur coating using sulfur compounds such as mercapto compounds or disulfide compounds, and then separated from the reaction solution using known procedures to obtain a nickel powder cake. Specifically, the crystallized nickel powder can be separated from the reaction solution using a Denver filter, filter press, centrifuge, decanter, etc. Furthermore, by thoroughly washing the crystallized nickel powder with high-purity water such as pure water (conductivity: ≤1 μS / cm) by decantation before or after the solid-liquid separation process, any remaining unreacted substances and impurities can be removed, and the adhesion of the crystallized nickel powder due to impurity concentration during drying can be suppressed.

[0085] 1-3.First drying process (drying process) In the first drying step, the nickel powder cake obtained in the solid-liquid separation step is dried to obtain dried nickel powder. Specifically, using a general-purpose drying apparatus such as an inert gas atmosphere dryer or a vacuum dryer, the drying temperature is set to 30°C to 300°C, preferably 50°C to 200°C, and more preferably 80°C to 150°C, for a predetermined time to remove moisture from the nickel powder cake and obtain dried nickel powder. Here, the dried nickel powder obtained from the nickel powder cake obtained by washing the nickel powder produced by the gas phase method is referred to as "washed gas phase nickel powder." Nickel powder produced by the wet method is referred to as "dried wet nickel powder."

[0086] Furthermore, with crystallized nickel powder, drying in an inert, reducing, or vacuum atmosphere at approximately 200°C to 300°C can yield a post-dried wet nickel powder that has undergone heat treatment in addition to simple drying. Heat treatment can alter the surface composition of the oxide film formed on the surface of the nickel particles (for example, the ratio of nickel metal, nickel oxide, and nickel hydroxide). Specifically, this results in an increase in the proportion of nickel oxide and a decrease in the proportion of nickel hydroxide in the oxide film. In addition, since heat treatment promotes crystal growth, higher drying temperatures result in a post-dried wet nickel powder with a larger crystallite size.

[0087] 1-4. Water-soluble organic solvent treatment process (surface treatment process) While dried nickel powders, such as gas-phase nickel powder after washing or wet-processed nickel powder after drying, can be processed into conductive pastes for internal electrodes in their current state, newly produced dried nickel powder is highly chemically active, and aggregation can occur due to water used in washing or wet processing. Figure 1 is a schematic diagram showing coarse nickel hydroxide-containing particles found in conventional wet-processed nickel powder. As shown in Figure 1, the dried nickel powder after production may contain aggregated coarse nickel hydroxide-containing particles formed by multiple nickel particles 30 and grown nickel hydroxide 40. Furthermore, linked particles may be generated in dried nickel powder obtained via the gas-phase method. Also, linked particles may be generated during crystallization in dried nickel powder obtained via the wet method.

[0088] Nickel powder containing such aggregated particles cannot fully exhibit the properties of conductive paste for internal electrodes. Therefore, it is necessary to perform a dry crushing process on the washed gas-phase nickel powder or the dried wet nickel powder to loosen the aggregated particles. However, even if the washed gas-phase nickel powder or the dried wet nickel powder is crushed as is, the liquid crosslinking force caused by water molecules remaining on the surface of the nickel particles may cause the particles to stick together due to the force of crushing, resulting in coarse particles. Therefore, in the nickel powder manufacturing method of this embodiment, before performing the dry crushing process on the washed gas-phase nickel powder or the dried wet nickel powder, a water-soluble organic solvent treatment process is performed to remove water molecules by replacing the water molecules on the surface of the nickel particles with a water-soluble organic solvent, or to reduce the liquid crosslinking force caused by water molecules by attaching a water-soluble organic solvent. In other words, in the nickel powder manufacturing method of this embodiment, in addition to the solid-liquid separation process and drying process described above, the water-soluble organic solvent treatment process and the dry crushing process described later are performed.

[0089] [Solvents used in water-soluble organic solvent processing steps] The solvent used in the water-soluble organic solvent treatment process is preferably a solvent containing 90% by mass or more, preferably 95% by mass or more, of a water-soluble organic solvent. A water-soluble organic solvent content of 90% by mass or more allows for efficient penetration into the interparticle gaps, effectively mitigating drying aggregation. If the purity of the water-soluble organic solvent in the solvent is low, the drying aggregation mitigation effect may be insufficient. The solvent used in the water-soluble organic solvent treatment process may also consist of water and an alcohol component.

[0090] Furthermore, it is desirable that the water-soluble organic solvent has a boiling point of 50°C to 140°C, preferably 50°C to 120°C, more preferably 50°C to 100°C, and even more preferably 50°C to 90°C. A boiling point below 50°C is undesirable because it is too volatile, making it difficult to handle and increasing the risk of flammability. If it exceeds 120°C, it may be difficult to volatilize the water-soluble organic solvent in subsequent processes such as the second drying stage and the dry crushing stage. If the boiling point of the water-soluble organic solvent is too high, it will take too long to dry the nickel powder, significantly reducing drying efficiency and potentially resulting in cost disadvantages. However, in subsequent processes such as the second drying stage, the dry crushing stage, and the water-soluble organic solvent treatment process (part 3) described later, which does not require the generation of water-soluble organic solvent vapor, the nickel powder can be processed without problems even if a water-soluble organic solvent with a boiling point between 120°C and 140°C is used.

[0091] Furthermore, the water-soluble organic solvent is preferably a solvent whose main component is at least one of alcohol, ketone, or ether. That is, the water-soluble organic solvent may have alcohol, ketone, or ether as its main component, or it may have a mixture of two or more alcohols, ketones, or ethers as its main component. Among these, alcohol-based organic solvents are preferred when considering volatility and safety (harmfulness, flammability). The main component refers to, for example, a component that is present in the water-soluble organic solvent at a concentration of 50% by mass or more. In addition, the water-soluble organic solvent may contain alcohols at a concentration of 90% by mass or more.

[0092] When using alcohols as the water-soluble organic solvent, specifically, one or more organic solvents selected from methanol (boiling point: 64.7°C), ethanol (boiling point: 78.3°C), 1-propanol (also known as n-propyl alcohol) (boiling point: 97.2°C), and 2-propanol (also known as isopropyl alcohol) (boiling point: 82.4°C), 1-butanol (boiling point: 117°C), and 1-pentanol (boiling point: 137°C), or organic solvents containing these as main components (for example, various denatured alcohols) can be used. Furthermore, considering cost, denatured alcohol, which is a mixed solvent mainly composed of ethanol and containing two or more other substances such as methanol, isopropyl alcohol, n-propyl alcohol, and water, is most preferred.

[0093] [Types of water-soluble organic solvent treatment processes] There are three types of water-soluble organic solvent treatment: water-soluble organic solvent treatment step (1), which includes a water-soluble organic solvent slurry treatment step as shown in Figure 2; water-soluble organic solvent treatment step (2), which includes a water-soluble organic solvent vapor treatment step as shown in Figure 3; and water-soluble organic solvent treatment step (3), which includes a water-soluble organic solvent contact treatment step as shown in Figure 4.

[0094] 1-4-A. Water-soluble organic solvent treatment process (Part 1) The water-soluble organic solvent treatment process (part 1) shown in Figure 2 involves dispersing the washed gas-phase nickel powder or the dried wet nickel powder (dried nickel powder) in a water-soluble organic solvent as follows to obtain a nickel water-soluble organic solvent slurry (water-soluble organic solvent slurry), and then separating the solvent from the nickel water-soluble organic solvent slurry to obtain water-soluble organic solvent slurry-treated nickel powder (water-soluble organic solvent-treated nickel powder). The water-soluble organic solvent treatment process (part 1) includes a water-soluble organic solvent slurry formation step and a second drying step, and is performed in the following order.

[0095] (1-4-A-1. Preparation) Prepare the water-soluble organic solvents mentioned above. For example, if using one type of organic solvent such as methanol, ethanol, or 1-propanol, these organic solvents can be used as is without any special preparation. On the other hand, if using organic solvents such as denatured alcohol, preparation is necessary, for example, by adding a denaturing agent such as methanol to ethanol.

[0096] (1-4-A-2. Water-soluble organic solvent slurry preparation step) Washed gas-phase nickel powder or dried wet nickel powder is dispersed in the prepared water-soluble organic solvent. This yields a nickel water-soluble organic solvent slurry (water-soluble organic solvent slurry).

[0097] (1-4-A-3. 2nd solid-liquid separation stage) Next, the solvent is separated from the nickel-soluble organic solvent slurry. Specific methods include filtration, and while filter paper and equipment such as a Nutsche filter, Denver filter, filter press, centrifuge, or decanter can be used, they are not limited to these.

[0098] (1-4-A-4. Second drying stage) After separating the solvent, nickel powder treated with a water-soluble organic solvent slurry can be obtained by drying the nickel powder under reduced pressure at, for example, 25°C to 100°C. However, the drying method in the drying stage is not limited to this.

[0099] As described above, after forming a water-soluble organic solvent slurry by slurrying the dried nickel powder with a water-soluble organic solvent, the water-soluble organic solvent slurry is dried to remove the water-soluble organic solvent. As a result, the water molecules present on the surface of each nickel particle are replaced by water-soluble organic solvent molecules, and then the water-soluble organic solvent molecules volatilize from the surface of the nickel particles.

[0100] Incidentally, it is also conceivable to use a water-soluble organic solvent instead of water during the solid-liquid separation process to slurry the nickel powder without performing the first drying process, and this method can also obtain the same effect as the crushing results in this embodiment. However, the nickel powder slurry after the crystallization process is a slurry of nickel powder in water, and also contains impurities other than water and nickel powder, so it is necessary to repeatedly wash the nickel powder to remove the impurities. If a water-soluble organic solvent is used instead of water for this washing, the amount of water-soluble organic solvent used from the crystallization process to the solid-liquid separation process may be more than 10 times greater than when the water-soluble organic solvent is used only in the water-soluble organic solvent slurrying process, as in this embodiment. In this embodiment, by using a water-soluble organic solvent in the water-soluble organic solvent slurrying process and suppressing the total amount used, the burden on the environment can be reduced and resources can be saved.

[0101] 1-4-B. Water-soluble organic solvent treatment process (part 2) Unlike the water-soluble organic solvent treatment process (1), the water-soluble organic solvent treatment process (2) includes a water-soluble organic solvent vapor treatment step (see Figure 3) in which the washed gas-phase nickel powder or the dried wet nickel powder (these powders are referred to as "dried nickel powder") is brought into contact with water-soluble organic solvent vapor to obtain nickel powder treated with water-soluble organic solvent vapor. To bring the washed gas-phase nickel powder or the dried wet nickel powder into contact with water-soluble organic solvent vapor, for example, the washed gas-phase nickel powder or the dried wet nickel powder can be placed in a container, water-soluble organic solvent vapor can be introduced into the container, and the powder can be exposed to the water-soluble organic solvent vapor. The water-soluble organic solvent treatment process (2) is performed in the following order.

[0102] (1-4-B-1. Preparation) As preparation, prepare for the water-soluble organic solvent treatment and prepare the dried nickel powder, as in (1-4-A-1. Preparation).

[0103] (1-4-B-1-1. Preparation of water-soluble organic solvents) Prepare the water-soluble organic solvent as described above. For example, if using one type of organic solvent such as methanol, ethanol, or 1-propanol, these organic solvents can be used as is without any special adjustments. On the other hand, if using an organic solvent such as denatured alcohol, adjustments are made, for example, by adding a denaturing agent such as methanol to ethanol. Then, the water-soluble organic solvent is evaporated to vapor and mixed with an inert gas such as N2 gas or dry air. Note that water-soluble organic solvents volatilize even at room temperature of 25°C, but in this embodiment, it is preferable that the water-soluble organic solvent vapor is evaporated at a temperature of 40°C to 70°C. If the temperature of the water-soluble organic solvent vapor is below 40°C, it is not possible to remove the water molecules remaining on the surface of the gas-phase nickel powder after washing or the wet nickel powder after drying, and the dry crushing process will yield the same results as powder that has not undergone the water-soluble organic solvent treatment process. On the other hand, if the temperature of the water-soluble organic solvent vapor exceeds 70°C, some particles of the gas-phase nickel powder after washing or the wet nickel powder after drying will reach a temperature above 70°C, leading to strong aggregation that is difficult to crush even when treated in the dry crushing process. In particular, when a dry crushing process is carried out continuously, if there are some firmly aggregated particles that are difficult to crush, these particles will act as nuclei to generate even larger aggregated particles. Therefore, the water-soluble organic solvent vapor in this embodiment is preferably a water-soluble organic solvent vapor that has been vaporized at a temperature of 40°C to 70°C.

[0104] (1-4-B-1-2. Preparation of dried nickel powder) Washed gas-phase nickel powder and dried wet nickel powder are placed in a processing container and kept at room temperature between 0°C and 30°C. Examples of processing containers include, but are not limited to, drum shakers, dispersion and stirring devices, mixing and stirring machines, dispersion and mixing kneaders, planetary mixers, and pot-type mixers.

[0105] (1-4-B-2. Water-soluble organic solvent vapor treatment step) In a processing container containing the washed gaseous nickel powder or the dried wet nickel powder, a vapor of a water-soluble organic solvent mixed with an inert gas such as N2 gas or dry air (mixed gas) is introduced and brought into contact with the washed gaseous nickel powder or the dried wet nickel powder. Here, the mixed gas is a gas obtained by mixing an inert gas or dry air with the vapor of a water-soluble organic solvent.

[0106] The amount of water-soluble organic solvent in the mixed gas is 0.1 g to 1 g per liter, preferably 0.1 g to 0.5 g. If the amount of water-soluble organic solvent per liter is less than 0.1 g, the water-soluble organic solvent vapor treatment step will be insufficient and will interfere with the dry crushing step, and even if it exceeds 1 g, the effect of the water-soluble organic solvent vapor treatment step will not improve. Furthermore, the water-soluble organic solvent vapor treatment step can be carried out at room temperature and atmospheric pressure, and the water-soluble organic solvent vapor may be circulated and introduced into the treatment container on which the washed gas phase nickel powder or the dried wet nickel powder is placed.

[0107] In the water-soluble organic solvent vapor treatment stage, water molecules on the surface of each nickel particle are replaced by water-soluble organic solvent molecules, which then volatilize from the surface of the nickel particles. This yields water-soluble organic solvent vapor-treated nickel powder (water-soluble organic solvent treated nickel powder). The volatilization of water-soluble organic solvent molecules from the surface of the nickel particles reduces the amount of hard, aggregated particles that are difficult to crush. Furthermore, the fact that the water-soluble organic solvent molecules that replaced water on the surface of each nickel particle treated with the water-soluble organic solvent volatilize from the surface of the nickel particles is also important from another perspective. That is, since the water-soluble organic solvent molecules are dried and removed from the surface of the nickel particles, dry crushing can be performed in a state where the water-soluble organic solvent molecules have volatilized, allowing for safer dry crushing even for large quantities of dried nickel powder. Note that in the water-soluble organic solvent vapor treatment stage, only the water-soluble organic solvent vapor comes into contact with the washed gas-phase nickel powder or the dried wet nickel powder; a second solid-liquid separation stage or a second drying stage, such as in the water-soluble organic solvent slurry treatment stage, is unnecessary.

[0108] Furthermore, the amount of water-soluble organic solvent used in the water-soluble organic solvent vapor treatment step of this embodiment can be 0.1 times or less of that used in the water-soluble organic solvent slurry treatment step of this embodiment. The water-soluble organic solvent vapor treatment step of this embodiment contributes to even greater resource conservation than the water-soluble organic solvent slurry treatment step of this embodiment.

[0109] 1-4-C. Water-soluble organic solvent treatment process (part 3) Unlike the water-soluble organic solvent treatment process (1) and the water-soluble organic solvent treatment process (2), the water-soluble organic solvent treatment process (3) includes a water-soluble organic solvent contact treatment step (see Figure 4) in which the washed gas-phase nickel powder or the dried wet nickel powder (dried nickel powder) is brought into contact with a water-soluble organic solvent to obtain nickel powder treated with a water-soluble organic solvent. To bring the washed gas-phase nickel powder or the dried wet nickel powder into contact with a water-soluble organic solvent, for example, the washed gas-phase nickel powder or the dried wet nickel powder can be placed in a container, the water-soluble organic solvent can be added to the container, and the water-soluble organic solvent can be shaken to mix with the powder. The water-soluble organic solvent treatment process (3) is performed in the following order.

[0110] (1-4-C-1. Preparation) As preparation, prepare for the water-soluble organic solvent treatment and prepare the dried nickel powder, as in (1-4-A-1. Preparation).

[0111] (1-4-C-1-1. Preparation of water-soluble organic solvents) Prepare the water-soluble organic solvents mentioned above. For example, when using one type of organic solvent such as methanol, ethanol, or 1-propanol, these organic solvents can be used as is without any special preparation. On the other hand, when using organic solvents such as denatured alcohol, preparation is necessary, for example, by adding a denaturing agent such as methanol to ethanol.

[0112] (1-4-C-1-2. Preparation of dried nickel powder) The dried nickel powder is stored in a processing container and kept at room temperature between 0°C and 30°C. The processing container is one that agitates the particles constituting the gaseous nickel powder after washing and the wet nickel powder after drying without applying shear to them. Examples of such containers include those used in processing equipment such as tumbler mixers, V-type mixers, and pot-type mixers. However, the processing container is not limited to these.

[0113] (1-4-C-2. Water-soluble organic solvent contact treatment step) Dry nickel powder and the aforementioned water-soluble organic solvent are placed in the container of the processing apparatus described above and shaken to cause the water-soluble organic solvent to adhere to the nickel powder. In the water-soluble organic solvent contact treatment stage, the particles constituting the washed gas-phase nickel powder and the particles constituting the dried wet nickel powder stored in the container of the processing apparatus are shaken without applying shear to stir each particle and bring the water-soluble organic solvent into contact with each particle. Through this operation, water-soluble organic solvent-attached nickel powder is obtained. Note that the processing apparatus is not limited to tumbler mixers, V-type mixers, or pot-type mixers; any processing apparatus capable of shaking the particles constituting the nickel powder in the container without applying shear can be used.

[0114] The amount of water-soluble organic solvent added is preferably 0.02 parts by mass or more and 2 parts by mass or less per 100 parts by mass of dry nickel powder, more preferably 0.045 parts by mass or more and 2 parts by mass or less, and even more preferably 0.07 parts by mass or more and 2 parts by mass or less. In the water-soluble organic solvent contact treatment stage, the amount of water-soluble organic solvent adhering to the dry nickel powder varies depending on the type of alcohol. Also, although it varies depending on the specific surface area of ​​the dry nickel powder, there is a fixed amount of water-soluble organic solvent that can adhere to the dry nickel powder, and once a certain amount of water-soluble organic solvent is added, the amount of water-soluble organic solvent adhering to the nickel powder will not increase beyond that fixed value. If sufficient water-soluble organic solvent adheres, continuous operation in the dry crushing process can be ensured, and properties such as surface roughness of the dried film of the conductive paste obtained after processing can also be satisfied.

[0115] Furthermore, the water-soluble organic solvent treatment process (part 3) can reduce the amount of water-soluble organic solvent used compared to the water-soluble organic solvent treatment process (part 2), thus contributing to greater resource conservation.

[0116] (1-4-C-3. Recovery of nickel powder) Next, the nickel powder is collected from the container. At this point, since the nickel powder contains almost no liquid components, a drying process is not necessary.

[0117] As described above, by attaching a small amount of water-soluble organic solvent to the dried nickel powder, some of the water molecules present on the surface of each nickel particle are replaced by the water-soluble organic solvent.

[0118] 1-5.Dry crushing process The dry crushing process can be performed on nickel powder treated with a water-soluble organic solvent slurry, nickel powder treated with water-soluble organic vapor, or nickel powder treated with a water-soluble organic solvent contact process, using known dry crushing methods such as spiral jet crushing or counter-jet mill crushing. Furthermore, it is preferable that the gas supply pressure during the dry crushing method be between 0.2 MPa and 0.9 MPa.

[0119] As mentioned above, if the dry crushing process is carried out without a water-soluble organic solvent treatment process after the drying process, the washed gas-phase nickel powder and dried wet nickel powder may, even if crushed as is, collide and combine with other nickel particles due to the force of crushing, caused by water molecules remaining on the surface of the nickel particles. When nickel particles combine to form coarse particles during the dry crushing process, these coarse particles grow into even larger coarse particles due to the nickel powder supplied to the dry crushing process. When coarse particles are generated in this way during the dry crushing process, the nickel powder will contain defective products that cannot be used as conductive paste for internal electrodes, and the dry crushing process must be stopped. This generation of coarse particles tends to be more pronounced when dry crushing is performed on a large scale, such as when the washed gas-phase nickel powder or dried wet nickel powder subject to dry crushing is 10 kg or more. In other words, when dry crushing is performed continuously on nickel powder weighing 10 kg or more that has not undergone the water-soluble organic solvent treatment step of this embodiment, coarse particles often occur during the process. Furthermore, when coarse particles occur, new coarse particles tend to be generated around them, thus promoting the generation of coarse particles, making it unavoidable to stop the dry crushing process, and significantly reducing work efficiency. Moreover, as mentioned above, nickel powder containing coarse particles is a defective product, and it is not practical to return this nickel powder containing coarse particles to the state of water-soluble nickel salt and repeat the manufacturing method from the crystallization step again. Therefore, the generation of coarse particles results in material waste.

[0120] In contrast, in this embodiment, by applying a water-soluble organic solvent treatment, the water molecules on the surface of each nickel particle are replaced with water-soluble organic solvent molecules, and then, as described above, the water-soluble organic solvent molecules volatilize from the surface of the nickel particles. As a result of the water-soluble organic solvent treatment in this embodiment, water molecules and water-soluble organic solvent molecules are removed from the surface of the nickel particles, so drying aggregation between nickel particles is greatly suppressed, and even if dried aggregated particles are formed, the bonding strength is weak. Therefore, even if the dry crushing treatment is performed continuously, coarse particles are not generated.

[0121] In other words, in the nickel powder manufacturing method of this embodiment, even when 10 kg or more of nickel powder is continuously subjected to dry crushing treatment, nickel powder crushed into nickel particles can be obtained without problems such as the generation of coarse particles. For this reason, in the nickel powder manufacturing method of this embodiment, the dry crushing treatment can be run for a long time without interruption for large quantities of dry nickel powder, and it can be used for mass production of nickel powder.

[0122] Furthermore, the significant suppression of drying aggregates and the weak bonding strength of any dried aggregated particles are advantageous for the disintegration of linked particles generated during the crystallization process of nickel particles. In other words, in the dry disintegration process of this embodiment, drying aggregates between nickel particles are significantly suppressed, and even if dried aggregated particles are generated, the bonding strength is weak. Therefore, the disintegration force acts on the linked particles generated during the crystallization process of nickel particles, promoting the disintegration of these linked particles.

[0123] As a result of these actions and effects, the nickel powder manufacturing method of this embodiment can produce nickel powder with reduced coarse particles by going through a dry crushing process after the water-soluble organic solvent treatment process.

[0124] 2. Nickel powder obtained by the dry crushing process Considering that the nickel powder obtained by the manufacturing method of this embodiment will be used as a material for MLCCs, the number-average particle size is preferably 0.03 μm or more and 0.3 μm or less, more preferably 0.03 μm or more and 0.2 μm or less, and even more preferably 0.03 μm or more and 0.15 μm or less. If the number-average particle size of the nickel powder exceeds 0.3 μm, it will not contribute to thinning the internal electrodes of the MLCC. On the other hand, if the number-average particle size of the nickel powder is less than 0.03 μm, it may make handling difficult due to the viscosity of the internal electrode paste and the sintering behavior of the nickel powder.

[0125] The number-average particle size of nickel powder can be determined by statistically processing particle sizes measured using known laser diffraction / scattering particle size distribution analyzers or from electron microscope images.

[0126] 3. Effects on conductive paste for internal electrodes By using nickel powder produced by the nickel powder production method of this embodiment, the surface roughness of the dried film obtained by printing and drying the conductive paste for internal electrodes can be reduced. The ability to reduce the surface roughness of the dried film of the conductive paste for internal electrodes is due in part to the fact that the nickel powder produced by the nickel powder production method of this embodiment contains fewer coarse particles. Because the conductive paste for internal electrodes using nickel powder produced by the nickel powder production method of this embodiment contains fewer coarse particles of nickel powder, it also contributes to preventing short circuits in multilayer ceramic capacitors.

[0127] The above description of the nickel powder manufacturing method explains the process from the production of dried nickel powder by gas-phase or wet methods to the dry crushing process. However, this is merely an example. For example, the nickel powder manufacturing method may begin with the solid-liquid separation process described above, after obtaining dried nickel powder produced by the gas-phase or wet methods in advance. [Examples]

[0128] The present invention will be described in more detail below using examples, but the present invention is not limited in any way to the following examples. The evaluation items and methods used in the examples are as follows.

[0129] <Manufacturing of wet nickel powder> A total of 105 kg of wet nickel powder was prepared using the following procedure.

[0130] [Preparation of solutions of nickel salts and metal salts of metals nobler than nickel] An aqueous solution of nickel chloride hexahydrate (NiCl2·6H2O, molecular weight: 237.69) was prepared by dissolving 20.5 kg of Ni metal in 205 L of pure water (referred to as "nickel chloride aqueous solution"), and an aqueous solution of palladium(II) ammonium chloride (also known as tetrachloropalladium(II)ate ammonium) ((NH4)2PdCl4, molecular weight: 284.31), a metal salt of a metal nobler than nickel, was prepared by dissolving 1.2 g of Pd metal in 1 L of pure water (referred to as "1.2 g-Pd / L aqueous solution"). Then, 205 L of nickel chloride aqueous solution, 854.9 mL of 1.2 g-Pd / L aqueous solution, and 260.4 g of L-methionine (CH3SC2H4CH(NH2)COOH, molecular weight: 149.21), which contains one sulfide group (-S-) in its molecule as a sulfur-containing compound and acts as an aid in inhibiting self-decomposition, were dissolved in 180 L of pure water to prepare a nickel salt nucleating agent-containing aqueous solution, which contains nickel salt, sulfur-containing compound, and a nucleating agent, which is a metal salt of a metal nobler than nickel, as its main components. In this nickel salt nucleating agent-containing solution, the sulfide compound L-methionine was present in a trace amount of 0.005 (0.5 mol%) molar ratio to nickel, and palladium (Pd) was present in 50 mass ppm (27.58 mol ppm) relative to nickel (Ni).

[0131] [Preparation of reducing agent solution] As a reducing agent, 42.435 kg of commercially available industrial-grade 60% hydrated hydrazine (manufactured by MGC Otsuka Chemical Co., Ltd.), which was diluted 1.67 times with pure water, was weighed out to prepare a reducing agent solution that was an aqueous solution containing hydrazine as the main component and did not contain alkali hydroxide.

[0132] [Alkali hydroxide solution] As an alkali hydroxide, sodium hydroxide (NaOH, molecular weight: 40.0) was dissolved in pure water to prepare 155.185 L of an alkali hydroxide solution containing sodium hydroxide at a concentration of 382 g / L.

[0133] [Amine compound solution] As an amine compound, 209.1 g of ethylenediamine (abbreviated as EDA) (H2NC2H4NH2, molecular weight: 60.1), an alkyleneamine containing two primary amino groups (-NH2) in its molecule, was dissolved in 3.69 L of pure water to prepare an amine compound solution containing ethylenediamine.

[0134] Furthermore, with the exception of 60% hydrated hydrazine, the reagents used in the nickel salt nucleating agent solution, reducing agent solution, alkali hydroxide solution, and amine compound solution were all manufactured by Wako Pure Chemical Industries, Ltd.

[0135] [Crystallization process] A nickel salt nucleating agent-containing solution was placed in a Teflon®-coated stainless steel container equipped with stirring blades and heated while stirring until the liquid temperature reached 85°C. Then, a reducing agent solution at 27°C was added and mixed for 10 seconds to obtain a nickel salt / reducing agent-containing solution, with a molar ratio of Ni metal to hydrazine water encapsulation of 1:1.46. To this nickel salt / reducing agent-containing solution, an alkali hydroxide solution at 27°C was added and mixed for 120 seconds to obtain a molar ratio of Ni metal to sodium hydroxide of 1:3.54. A reaction solution (nickel chloride + palladium salt + hydrazine + sodium hydroxide) at 70°C was prepared, and the reduction reaction (crystallization reaction) was started (reaction start temperature 70°C). From 8 minutes after the start of the reaction to 28 minutes after the start of the reaction, the above amine compound solution was added dropwise to the reaction solution over a period of 20 minutes, so that the molar ratio of Ni metal to ethylenediamine was 1:0.01 (1.0 mol%). The reduction reaction proceeded while suppressing the autodecomposition of hydrazine, causing crystallized nickel powder to precipitate in the reaction solution, and a slurry containing crystallized nickel powder (nickel powder slurry) was obtained. The reduction reaction was completed within 60 minutes from the start of the reaction, and the supernatant of the reaction solution was clear, confirming that all nickel components in the reaction solution were reduced to metallic nickel and became crystallized nickel powder.

[0136] [Solid-liquid separation process] Using pure water with an electrical conductivity of 1 μS / cm, the slurry containing crystallized nickel powder was filtered to separate the solid and liquid components. The filtered filtrate was washed until its electrical conductivity was 15 μS / cm or less to obtain a nickel powder cake.

[0137] [First drying process] The nickel powder cake was dried in a vacuum dryer set to a temperature of 120°C for 8 hours to obtain wet nickel powder.

[0138] Then, the entire process from [preparation of solutions of nickel salts and metal salts of metals nobler than nickel] to [first drying step] was carried out in 21 batches, and the dried wet nickel powder from the 21 batches was mixed in a container to obtain a total of 105 kg of dried wet nickel powder. The number-average particle size of the obtained dried wet nickel powder was 0.15 μm.

[0139] In Examples 1, 2, and Comparative Example 1, described later, 30 kg was taken from the total of 105 kg of dried wet nickel powder for the experiments. In other words, there was no variation in properties among the dried wet nickel powders used in Examples 1, 2, and Comparative Example.

[0140] [Example 1] [Water-soluble organic solvent treatment process (Part 1) (including the water-soluble organic solvent slurry formation step)] To 30 kg of dried wet nickel powder, 12 kg of denatured alcohol, a mixture of 1% methanol, 86% ethanol, and 13% 2-propanol as a water-soluble organic solvent, was added to form a slurry. This slurry was then placed in a Nutsch filter, and filtration was continued until the solid content concentration reached 70% by mass or more to separate the solids and liquids, thereby obtaining a nickel powder cake treated with a water-soluble organic solvent.

[0141] [Second drying process] A nickel powder cake treated with a water-soluble organic solvent was dried in a vacuum dryer set to 50°C for 12 hours to obtain nickel powder treated with a water-soluble organic solvent slurry.

[0142] [Dry crushing process] The obtained water-soluble organic solvent slurry-treated nickel powder was subjected to dry crushing treatment using a dry crushing device (spiral jet mill) to obtain wet nickel powder (crushed nickel powder) according to Example 1, which was dry crushed. Specifically, 30 kg of the dried wet nickel powder was introduced into the main container of the spiral jet mill, and crushing treatment was carried out for 6 hours using gas introduced at a supply pressure of 0.2 MPa to 0.9 MPa.

[0143] [Example 2] [Water-soluble organic solvent treatment process (part 2) (including water-soluble organic solvent vapor treatment step)] 30 kg of dried wet nickel powder was placed in a planetary mixer (Hibismix 2P-1 model, manufactured by PRIMIX) and stirred. While stirring, a mixed gas was introduced into the planetary mixer at a flow rate of 5 L / min. This mixed gas was obtained by heating denatured alcohol (a mixture of 1% methanol, 86% ethanol, and 13% 2-propanol as a water-soluble organic solvent) to 50°C and mixing it with N2 gas. The mixed gas contained 0.23 g of denatured alcohol per liter. A total of 975 g of denatured alcohol was introduced. The dried wet nickel powder was then brought into contact with this denatured alcohol vapor for 1 hour to obtain water-soluble organic solvent vapor-treated nickel powder.

[0144] [Dry crushing process] The obtained water-soluble organic solvent vapor-treated nickel powder was crushed using the same dry crushing apparatus as in Example 1 to obtain the crushed wet nickel powder (crushed nickel powder) according to Example 2. Specifically, 30 kg of the dried wet nickel powder was introduced into the main container of a spiral jet mill, and crushing treatment was carried out for 6 hours using gas introduced at a supply pressure of 0.2 MPa to 0.9 MPa.

[0145] [Comparative Example 1] 30 kg of dried wet nickel powder was subjected to the same dry crushing process as in Examples 1 and 2, without performing the water-soluble organic solvent treatment step in Examples 1 and 2.

[0146] <Evaluation and Results> (number average particle size) The obtained wet nickel powder was observed using a scanning electron microscope (SEM, JEOL Ltd., JSM-7100F). By processing the SEM images, the area of ​​100 to 200 particles whose overall shape could be confirmed was measured. The diameter of each particle was calculated from the measured area by converting it to a perfect circle, and the average of these calculated diameters was then calculated to determine the number-average particle size. From the perspective of responding to the thinning of internal electrodes in recent multilayer ceramic capacitors, the target was set for the number-average particle size of the wet nickel powder to be 0.15 μm or less. As a result, as shown in Table 1, the number-average particle size of the nickel powder in Examples 1 and 2 and Comparative Example 1 was all 0.15 μm.

[0147] (Content of coarse particles) For the crushed wet nickel powder, particles with a particle size exceeding 0.6 μm, which is approximately four times the number-average particle size, were defined as coarse particles, and their content was measured.

[0148] First, in evaluating the coarse particles, a dispersion step was performed to obtain a slurry in which nickel particles were dispersed in a dispersion medium. Specifically, 0.03 g of wet nickel powder from Examples 1 and 2 and Comparative Example 1, and 100 ml of 0.1% by mass sodium hexametaphosphate aqueous solution as a dispersion medium were stirred and mixed in a 250 ml glass beaker. Next, the beaker was placed in the tank of an ultrasonic cleaner, and ultrasonic waves (26 kHz, 300 W) were applied for 3 minutes to disperse the wet nickel powder in the sodium hexametaphosphate aqueous solution. Through the above dispersion step, a slurry in which nickel particles were dispersed in a dispersion medium was obtained.

[0149] Next, the slurry was subjected to a filtration process using a suction filtration apparatus with the same configuration as the suction filtration apparatus 100 shown in a schematic diagram in Figure 5. In this case, a regenerated cellulose membrane filter (Fujifilm Corporation, MICRO FILTER) with a diameter of 90 mm and a pore size of 0.6 μm was attached to the suction filtration apparatus as the membrane filter 1. Specifically, the membrane filter 1 was placed on the filter holder 2, a suction cup 3 as a slurry input container was attached to the filter holder 2, and the filter holder 2 was then placed in the filtrate recovery container 4. With the vacuum pump 5 running, the slurry 10 in which nickel particles 10b were dispersed in the dispersion medium 10a was poured into the suction cup 3 and suction filtration was performed. Furthermore, 50 ml of 0.1 mass% sodium hexametaphosphate aqueous solution, which was used as the dispersion medium, was poured into the suction cup 3 into the 250 ml capacity beaker that contained the slurry, washing the walls of the beaker, and suction filtration was performed. Through the above filtration process, a filtrate 20 was obtained containing a membrane filter 1 with coarse particles larger than the filter pore size of 0.6 μm attached as residue, a dispersion medium 20a that passed through the membrane filter 1, and nickel particles 20b that passed through the membrane filter 1.

[0150] The nickel particles constituting the above residue were defined as coarse particles larger than the filter pore size. The mass of these coarse particles was calculated using inductively coupled plasma atomic emission spectroscopy (ICP), and the coarse particle content in the wet nickel powder was determined. Specifically, the membrane filter with the residue attached, obtained from the above filtration process, was air-dried, then boiled in aqua regia to dissolve the coarse particles, and the aqueous solution obtained by removing the membrane filter was used as the sample solution. This sample solution was subjected to ICP spectroscopy using an ICP spectrometer (Agilent Technologies, Inc., ICP720) to quantify the nickel. Furthermore, the total mass of nickel in the residue collected as coarse particles was calculated from the quantified nickel value, and the coarse particle content in the wet nickel powder (0.03g) used for evaluation was determined.

[0151] (Rate of change of coarse particles) The time-dependent changes in coarse particles in wet nickel powder immediately after crushing and 6 hours after crushing were evaluated. Specifically, the ratio of the coarse particle content in wet nickel powder immediately after crushing and 6 hours after crushing, obtained during the process of measuring the coarse particle content as described above, was calculated using (Equation 1).

[0152] Change in coarse particle content (%) = Coarse particle content 6 hours after crushing treatment / Coarse particle content immediately after crushing treatment ... (Equation 1)

[0153] (Evaluation results) Table 1 shows the solvent used, the amount of solvent used, the coarse particle content, the percentage change in coarse particles immediately after the crushing treatment and 6 hours after the crushing treatment, and the number-average particle size of the obtained wet nickel powder in Example 1, Example 2, and Comparative Example 1.

[0154] In all three examples (Example 1, Example 2, and Comparative Example 1), crystallized nickel powder was obtained by a crystallization process under the same conditions. Therefore, the number-average particle size of the wet nickel powder obtained immediately after the start of the crushing process in all three examples (Example 1, Example 2, and Comparative Example 1) was 0.15 μm.

[0155] The coarse particle content of the wet nickel powder in Example 1 was 200 ppm by mass for particles larger than 0.6 μm immediately after dry crushing treatment, and 180 ppm by mass for particles larger than 0.6 μm 6 hours after dry crushing treatment. The rate of change in coarse particles was -10%.

[0156] The coarse particle content of the wet nickel powder in Example 2 was 200 ppm by mass for particles larger than 0.6 μm immediately after dry crushing treatment, and 200 ppm by mass for particles larger than 0.6 μm 6 hours after crushing treatment. The rate of change in coarse particles was 0%.

[0157] The coarse particle content of the wet nickel powder in Comparative Example 1 was 400 ppm by mass for particles larger than 0.6 μm immediately after dry crushing and 20,000 ppm by mass for particles larger than 0.6 μm 6 hours after crushing. The rate of change in coarse particles was 5000%. In Comparative Example 1, the amount of coarse particles increased after 6 hours of continuous dry crushing, indicating that coarse particles are produced during the dry crushing process. In actual operation, the crushing process is stopped before the crushing result of 6 hours in Comparative Example 1 is reached.

[0158] Based on the above, by subjecting the dried wet nickel powder to a water-soluble organic solvent treatment as in Examples 1 and 2, it is possible to suppress the generation of coarse particles and the drying aggregation of nickel particles. Furthermore, it was possible to obtain a wet nickel powder that could suppress the generation of coarse particles and the drying aggregation of nickel particles over time, from immediately after the dry crushing process to 6 hours after the crushing process. In addition, compared to the slurrying method of Example 1, the water-soluble organic solvent vapor treatment of Example 2 was able to significantly reduce the amount of water-soluble organic solvent used.

[0159] [Table 1]

[0160] <Preparation of conductive paste and evaluation of dried film> [Example 3] A mixture of 60.3% by mass of wet nickel powder, 1.7% by mass of ethyl cellulose resin, 0.7% by mass of polyvinyl butyral resin, and 37.3% by mass of terpineol was prepared according to Example 1. The mixture was then compressed and sheared using a three-roll kneading device to obtain the conductive paste according to Example 3. The ethyl cellulose resin and polyvinyl butyral resin were dissolved in terpineol to form a vehicle before being used in the preparation of the paste.

[0161] A conductive paste according to Example 3 was screen printed onto a 2.54 cm (1 inch) square piece of heat-resistant tempered glass, and dried in air at 120°C for 1 hour to produce a dried film with a thickness of 20 mm square and a thickness of 2 μm. The surface roughness Sa (arithmetic mean height) and Sz (maximum height) of the dried film were measured according to the ISO 25178 standard. Specifically, a non-contact surface property measuring device was prepared, and the measurement surface was determined by performing a surface filter treatment on the original surface of the paste. Twenty evaluation regions were designated within the measurement surface, and the arithmetic mean height and maximum height were calculated for each evaluation region, and the average value was calculated. The results are shown in Table 2.

[0162] [Comparative Example 2] A conductive paste according to Comparative Example 2 was prepared and evaluated using the same procedure as in Example 3, except that wet nickel powder was used immediately after the start of the dry crushing process in Comparative Example 1. The results are shown in Table 2.

[0163] [Table 2]

[0164] (Evaluation results) In both Example 3 and Comparative Example 2, crystallized nickel powder was obtained using the same crystallization process, and therefore the number-average particle size was the same. However, the dry coating surface roughness (arithmetic mean height) of the wet nickel powder in Example 3 was 66 nm, and the dry coating surface roughness (maximum height) was 1.59 μm, while the dry coating surface roughness (arithmetic mean height) of the wet nickel powder in Comparative Example 2 was 79 nm, and the dry coating surface roughness (maximum height) was 2.22 μm.

[0165] From the results of the dried coating surface roughness (arithmetic mean height), it can be seen that in Example 3, the surface of the dried coating is smoother even when the average of multiple evaluation areas is taken, compared to Comparative Example 2. Furthermore, from the results of the dried coating surface roughness (maximum height), it can be seen that in Example 3, the difference in height is smaller even when the average of multiple evaluation areas is taken, compared to Comparative Example 2. In conclusion, by subjecting wet nickel powder to water-soluble organic solvent treatment, it was possible to obtain nickel powder that can improve the surface roughness of the dried film of conductive paste prepared using this powder.

[0166] <Wet Nickel Powder Manufacturing 2> Furthermore, to verify the water-soluble organic solvent treatment process (part 3), 125 kg of dried wet nickel powder was prepared. The method for producing this dried wet nickel powder was the same as that used in Examples 1 and 2 and Comparative Example 1, although the input quantities differed. The same dried wet nickel powder was used throughout all the examples.

[0167] [Preparation of solutions of nickel salts and metal salts of metals nobler than nickel] An aqueous solution of nickel chloride hexahydrate (NiCl2·6H2O, molecular weight: 237.69) was prepared by dissolving 25.0 kg of Ni metal in 250 L of pure water (referred to as "nickel chloride aqueous solution"), and an aqueous solution of palladium(II) ammonium chloride (also known as tetrachloropalladium(II)ate ammonium) ((NH4)2PdCl4, molecular weight: 284.31), a metal salt of a metal nobler than nickel, was prepared by dissolving 1.5 g of Pd metal in 1.25 L of pure water (referred to as "1.2 g-Pd / L aqueous solution"). Then, 250 L of nickel chloride aqueous solution, 1042.6 mL of 1.2 g-Pd / L aqueous solution, and 317.6 g of L-methionine (CH3SC2H4CH(NH2)COOH, molecular weight: 149.21), which contains one sulfide group (-S-) in its molecule as a sulfur-containing compound and acts as an aid in inhibiting self-decomposition, were dissolved in 219.5 L of pure water to prepare a nickel salt nucleating agent-containing solution, which is an aqueous solution containing a nickel salt, a sulfur-containing compound, and a nucleating agent that is a metal salt of a metal nobler than nickel as its main components. In this nickel salt nucleating agent-containing solution, the sulfide compound L-methionine was present in a trace amount of 0.005 (0.5 mol%) molar ratio to nickel, and palladium (Pd) was present in an amount of 50 mass ppm (27.58 mol ppm) relative to nickel (Ni).

[0168] [Preparation of reducing agent solution] As a reducing agent, 51.707 kg of commercially available industrial-grade 60% hydrated hydrazine (manufactured by MGC Otsuka Chemical Co., Ltd.), which was diluted 1.67 times with pure water, was weighed out to prepare a reducing agent solution that was an aqueous solution containing hydrazine as the main component and did not contain alkali hydroxide.

[0169] [Alkali hydroxide solution] As an alkali hydroxide, sodium hydroxide (NaOH, molecular weight: 40.0) was dissolved in pure water to prepare 189.250 L of an alkali hydroxide solution containing sodium hydroxide at a concentration of 382 g / L.

[0170] [Amine compound solution] As an amine compound, 255.0 g of ethylenediamine (abbreviated as EDA) (H2NC2H4NH2, molecular weight: 60.1), an alkyleneamine containing two primary amino groups (-NH2) in its molecule, was dissolved in 4.50 L of pure water to prepare an amine compound solution containing ethylenediamine.

[0171] Furthermore, with the exception of 60% hydrated hydrazine, the reagents used in the nickel salt nucleating agent solution, reducing agent solution, alkali hydroxide solution, and amine compound solution were all manufactured by Wako Pure Chemical Industries, Ltd.

[0172] [Crystallization process] A nickel salt nucleating agent-containing solution was placed in a Teflon®-coated stainless steel container equipped with stirring blades and heated while stirring until the liquid temperature reached 85°C. Then, a reducing agent solution at 27°C was added and mixed for 10 seconds to obtain a nickel salt / reducing agent-containing solution, with a molar ratio of Ni metal to hydrazine water encapsulation of 1:1.46. To this nickel salt / reducing agent-containing solution, an alkali hydroxide solution at 27°C was added and mixed for 120 seconds to obtain a molar ratio of Ni metal to sodium hydroxide of 1:3.54. A reaction solution (nickel chloride + palladium salt + hydrazine + sodium hydroxide) at 70°C was prepared, and the reduction reaction (crystallization reaction) was started (reaction start temperature 70°C). From 8 minutes after the start of the reaction to 28 minutes after the start of the reaction, the above amine compound solution was added dropwise to the reaction solution over a period of 20 minutes, so that the molar ratio of Ni metal to ethylenediamine was 1:0.01 (1.0 mol%). The reduction reaction proceeded while suppressing the autodecomposition of hydrazine, causing crystallized nickel powder to precipitate in the reaction solution, and a slurry containing crystallized nickel powder (nickel powder slurry) was obtained. The reduction reaction was completed within 60 minutes from the start of the reaction, and the supernatant of the reaction solution was clear, confirming that all nickel components in the reaction solution were reduced to metallic nickel and became crystallized nickel powder.

[0173] [Solid-liquid separation process] Using pure water with an electrical conductivity of 1 μS / cm, the slurry containing crystallized nickel powder was filtered to separate the solid and liquid components. The filtered filtrate was washed until its electrical conductivity was 15 μS / cm or less to obtain a nickel powder cake.

[0174] [First drying process] The nickel powder cake was dried in a vacuum dryer set to a temperature of 120°C for 8 hours to obtain wet nickel powder.

[0175] Then, the entire process from [preparation of solutions of nickel salts and metal salts of metals nobler than nickel] to [first drying step] was carried out in 5 batches, and the dried wet nickel powder from the 5 batches was mixed in a container to obtain a total of 125 kg of dried wet nickel powder. The number-average particle size of the obtained dried wet nickel powder was 0.15 μm.

[0176] In Examples 4-7 and Comparative Example 3, described later, 25 kg was taken from the total of 125 kg of dried wet nickel powder for the experiments. In other words, there was no variation in properties among the dried wet nickel powder used in Examples 4-7 and Comparative Example 3.

[0177] [Example 4] In a 65L stainless steel container, 25 kg of dried wet nickel powder and 6 g of denatured alcohol, which is a mixture of 1% methanol, 86% ethanol, and 13% 2-propanol, were added. The container was then rotated at 15 rpm for 60 minutes using a tumbler mixer (TMH-65S, manufactured by Eishin Co., Ltd.) to shake and mix the dried wet nickel powder and the alcohol, thus performing a water-soluble organic solvent contact treatment step.

[0178] [Example 5] In a 65L stainless steel container, 25 kg of dried wet nickel powder and 12 g of denatured alcohol, which was a mixture of 1% methanol, 86% ethanol, and 13% 2-propanol, were added. The container was then rotated at 15 rpm for 60 minutes using a tumbler mixer (TMH-65S, manufactured by Eishin Co., Ltd.) to shake and mix the dried wet nickel powder and the alcohol, thus performing a water-soluble organic solvent contact treatment step.

[0179] [Example 6] In a 65L stainless steel container, 25 kg of dried wet nickel powder and 18 g of denatured alcohol, which was a mixture of 1% methanol, 86% ethanol, and 13% 2-propanol, were added. The container was then rotated at 15 rpm for 60 minutes using a tumbler mixer (TMH-65S, manufactured by Eishin Co., Ltd.) to shake and mix the dried wet nickel powder and the alcohol, thus performing a water-soluble organic solvent contact treatment step.

[0180] [Example 7] In a 65L stainless steel container, 25 kg of dried wet nickel powder and 24 g of denatured alcohol, which was a mixture of 1% methanol, 86% ethanol, and 13% 2-propanol, were added. The container was then rotated at 15 rpm for 60 minutes using a tumbler mixer (TMH-65S, manufactured by Eishin Co., Ltd.) to shake and mix the dried wet nickel powder and the alcohol, thus performing a water-soluble organic solvent contact treatment step.

[0181] [Dry crushing process] The obtained alcohol-contact treated nickel powder was subjected to dry crushing using a dry crushing device (spiral jet mill) to obtain the wet nickel powder (crushed nickel powder) according to Examples 4 to 7. Specifically, 25 kg of the dried wet nickel powder from Examples 4 to 7 was introduced into the main container of the spiral jet mill, and crushing was performed for 5 hours using gas introduced at a supply pressure of 0.2 MPa to 0.9 MPa.

[0182] [Comparative Example 3] 25 kg of wet nickel powder after the first drying process was subjected to the same dry crushing process as in Examples 4-7, without performing the alcohol contact treatment process described in Examples 4-7.

[0183] The nickel powders from Examples 4-7 and Comparative Example 3 were evaluated in the same manner as in Example 1.

[0184] [Table 3]

[0185] (Evaluation results) Table 3 shows the number-average particle size of the wet nickel powder in Examples 4-7 and Comparative Example 3, the solvent used in the water-soluble organic solvent contact treatment step, the amount of solvent used, and the content of coarse particles.

[0186] In all of Examples 4-7 and Comparative Example 3, the wet nickel powder after drying was produced by the same method, and the number-average particle size of the wet nickel powder obtained immediately after the start of the crushing process was 0.15 μm in all cases.

[0187] The content of coarse particles larger than 0.6 μm in the wet nickel powders of Examples 4 to 7 was 10,000 ppm by mass or less, which was significantly lower than the result in Comparative Example 3. In particular, the result for Examples 5 to 7 was 200 ppm by mass or less.

[0188] Based on the above, it was found that by applying the water-soluble organic solvent treatment (part 3), it is possible to suppress the content of coarse particles by using a small amount of water-soluble organic solvent.

[0189] <Preparation of conductive paste and evaluation of dried film> A conductive paste was prepared using the wet nickel powders described in Examples 4-7 and Comparative Example 3. A dried film was then manufactured from the conductive paste, and the surface roughness of the dried film was evaluated.

[0190] The nickel paste was prepared by wet dispersion using a thin-film swirling mixing apparatus, with a composition of 60.3% by mass of each nickel powder, 1.7% by mass of ethylcellulose resin, 0.7% by mass of polyvinyl butyral resin, and 37.3% by mass of terpineol. The ethylcellulose resin and polyvinyl butyral resin were dissolved in terpineol to form a vehicle before being used in the preparation of the conductive paste.

[0191] Using the conductive pastes of Examples 4-7 and Comparative Example 3, these conductive pastes were printed onto a 2.54 cm (1 inch) square piece of heat-resistant tempered glass using a doctor blade to achieve a coating thickness of 10 μm. The resulting coatings were dried in air at 120°C for 1 hour to produce dried films. The arithmetic mean height Sa of each dried film was measured using a laser microscope image (×3000x magnification), and each measured value was divided by the arithmetic mean height Sa of Comparative Example 3 to evaluate the dry film roughness index in comparison to Comparative Example 3. The results are shown in Table 4.

[0192] [Table 4]

[0193] (Evaluation results) By subjecting the dried wet nickel powder to a water-soluble organic solvent treatment process, including a water-soluble organic solvent contact treatment step, the dried film of the conductive paste made from this nickel powder as a raw material showed improved smoothness compared to cases where this process was not performed (Table 4). In particular, smoothness was even higher in Examples 5 to 7 (Table 4).

[0194] [summary] In this invention, by treating nickel powder with a water-soluble organic solvent, the generation of coarse particles formed by the drying and aggregation of nickel particles can be suppressed, and the bonding strength between nickel particles can be relaxed. As a result, nickel powder with excellent dispersibility and fewer coarse particles can be obtained. Therefore, high-performance nickel powder suitable as a material for internal electrodes of multilayer ceramic capacitors can be provided at low cost.

[0195] In particular, performing the water-soluble organic solvent treatment step (part 3), i.e., the water-soluble organic solvent contact treatment step, resulted in a reduction in the amount of water-soluble organic solvent used and suppression of coarse particles. In particular, the nickel powders according to Examples 5 to 7 showed excellent results in terms of coarse particle content and surface roughness of the dried film of the conductive paste.

[0196] Although preferred embodiments of the present invention have been described in detail above, the present invention is not limited to these examples. It is clear to any person with ordinary skill in the art to which the present invention belongs that various modifications or alterations can be conceived within the scope of the technical idea described in the claims, and these are also understood to fall within the technical scope of the present invention. [Explanation of Symbols]

[0197] 1. Membrane filter 2. Filter folder 3 Suction cup 4 Filtrate collection container 5. Pressure reducing pump 10 Slurry 10a Dispersion medium 10b Nickel particles 20 filtrate 20a Dispersion medium 20b Nickel particles 30 nickel particles 40 Nickel hydroxide 100 Suction Filtration Device

Claims

1. A solid-liquid separation step to obtain a nickel powder cake by solid-liquid separation of a nickel powder slurry in which nickel powder is dispersed in water, A drying step to obtain dried nickel powder by drying the nickel powder cake, A water-soluble organic solvent treatment step is performed by contacting the dried nickel powder with a water-soluble organic solvent to obtain water-soluble organic solvent-treated nickel powder. A method for producing nickel powder, comprising a dry crushing step of dry crushing the nickel powder treated with the water-soluble organic solvent to obtain nickel powder after crushing.

2. The method for producing nickel powder according to claim 1, wherein the water-soluble organic solvent mainly consists of at least one of an alcohol, a ketone, or an ether.

3. The method for producing nickel powder according to claim 2, wherein the water-soluble organic solvent has a boiling point of 50°C to 140°C.

4. The method for producing nickel powder according to claim 2, wherein the water-soluble organic solvent comprises alcohols selected from at least one of methanol, ethanol, 1-propanol, 2-propanol, and denatured alcohol.

5. The method for producing nickel powder according to claim 4, wherein the water-soluble organic solvent contains 90% by mass or more of alcohols selected from at least one of methanol, ethanol, 1-propanol, 2-propanol, and denatured alcohol.

6. The aforementioned water-soluble organic solvent treatment step is A water-soluble organic solvent slurry preparation step is performed by slurrying the dried nickel powder in the water-soluble organic solvent to obtain a water-soluble organic solvent slurry. A drying step in which the aqueous organic solvent slurry is dried to remove the aqueous organic solvent, A method for producing nickel powder according to claim 1, including the following:

7. The method for producing nickel powder according to claim 1, wherein the water-soluble organic solvent treatment step includes a water-soluble organic solvent vapor treatment step of exposing the dried nickel powder to the vapor of the water-soluble organic solvent.

8. The method for producing nickel powder according to claim 1, wherein the water-soluble organic solvent treatment step includes a water-soluble organic solvent contact treatment step of shaking the dried nickel powder with the water-soluble organic solvent.

9. The method for producing nickel powder according to claim 1, wherein the solid-liquid separation step is a step of obtaining a nickel powder cake by solid-liquid separation of a slurry of crystallized nickel powder obtained by crystallizing the water-soluble nickel salt by a reduction reaction with hydrazine in a reaction solution containing a water-soluble nickel salt, a metal salt of a metal nobler than nickel, hydrazine, alkali hydroxide, and water.

10. The method for producing nickel powder according to claim 1, wherein the nickel powder has a number average particle size of 0.03 μm to 0.3 μm.

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