Nickel powder manufacturing method

Abstract

To provide a manufacturing method of nickel powder capable of obtaining nickel powder with less coarse particles.SOLUTION: A manufacturing method of nickel powder includes: a wet-disintegration step of wet-disintegrating, in a reaction liquid containing water-soluble nickel salt, metal salt of a metal nobler than nickel, hydrazine, alkali hydroxide and water, slurry of nickel crystallization powder obtained by crystallizing the water-soluble nickel salt by a reductive reaction by the hydrazine so as to obtain nickel powder slurry with a reduced amount of coarse particles; an acid cleaning step of feeding an acid to the nickel powder slurry after the wet-disintegration step to adjust a pH of the nickel powder slurry to a value 2 to 7 and dissolve nickel hydroxide in the nickel powder slurry; a solvent substitution step of substituting a solvent of the nickel powder slurry after the acid cleaning step with a water-soluble organic solvent; a solid-liquid separation step of solid-liquid separating the nickel powder slurry after the solvent substitution step to obtain a nickel powder cake; and a desiccation step of desiccating the nickel powder cake after the solid-liquid separation step to obtain nickel powder.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a method for producing inexpensive, high-performance nickel powder used as an electrode material for multilayer ceramic components, and more particularly to a method for producing inexpensive, high-performance nickel powder obtained by a wet method. [Background technology]

[0002] Conventionally, conductive pastes mainly composed of conductive powder, resin and organic solvent are used to form conductive layers, electrode layers, interlayer connecting materials constituting wiring boards of electric circuits, electrode layers of electronic components, etc. As these wiring boards and electronic components become smaller and denser, the width and thickness of the conductive layers and electrode layers tend to be reduced. For this reason, there is a demand for the conductive powder, which is a material of the conductive paste used to form them, to be smaller in diameter. Powders usually contain coarse particles that exceed a specific particle size, but as the conductive powder becomes smaller in diameter, the presence of coarse particles in the conductive powder has a negative effect on the properties of the conductive powder, which has become a problem. For this reason, the conductive powder is required to have the property of not containing coarse particles or to have very few coarse particles.

[0003] The problem of the presence of coarse particles in the powder is particularly problematic in the field of conductive paste for the internal electrodes of multilayer ceramic capacitors (MLCCs). Multilayer ceramic capacitors are manufactured, for example, by screen-printing a conductive paste in which nickel powder is dispersed as the conductive powder on a dielectric green sheet to form internal electrodes, stacking and pressing a plurality of green sheets on which the internal electrodes are printed so that the internal electrodes overlap each other to obtain a laminate, cutting the laminate to a specified size, removing the binder, and then firing at a high temperature of up to 1300°C to obtain a ceramic sintered body, and finally attaching external electrodes to the ceramic sintered body.

[0004] As multilayer ceramic capacitors become smaller and have larger capacitance, the thickness of both the internal electrodes and dielectrics is being reduced. Accordingly, the particle size of the nickel powder used in the internal electrode paste is also becoming finer, and nickel powder with an average particle size of 0.4 μm or less is required, and nickel powder with an average particle size of 0.3 μm or less is the mainstream. In addition, although it depends on the thickness of the internal electrode, in conductive powders, a particle size of about 3 to 5 times or more of the number average particle size of the entire powder is usually set as the reference particle size, and particles larger than this reference particle size are considered to be coarse particles. Here, when the conductive powder is mainly composed of primary particles, the coarse particles include not only particles with a primary particle size larger than the reference size, but also secondary particles in which the primary particles are firmly connected or aggregated and cannot be easily disintegrated, and have a particle size larger than the reference particle size.

[0005] Nickel powder manufacturing methods can be roughly divided into gas phase methods and wet methods. Gas phase methods include, for example, a method described in Patent Document 1 in which nickel chloride vapor is reduced with hydrogen to produce nickel powder (gas phase nickel powder), and a method described in Patent Document 2 in which nickel metal is vaporized in plasma to produce nickel powder (gas phase nickel powder). Wet methods include, for example, a method described in Patent Document 3 in which a reducing agent is added to a nickel salt solution to produce nickel powder (hereinafter, nickel powder obtained by a wet method may be referred to as "wet nickel powder").

[0006] The gas phase method is an effective means for obtaining nickel powder with excellent crystallinity and high properties because it is a high-temperature process of about 1000℃ or more, but the particle size distribution of the obtained nickel powder is broad. As mentioned above, nickel powder with an average particle size of 0.4μm or less that does not contain coarse particles and has a relatively narrow particle size distribution is required for thinning of internal electrodes, so in order to obtain such nickel powder by the gas phase method, classification processing using expensive classification equipment is essential.

[0007] In the classification process, it is possible to remove coarse particles larger than the classification point, which is an arbitrary value of about 0.6 μm to 2 μm, but there is also the problem that some particles smaller than the classification point are also removed at the same time, resulting in a significant decrease in actual product yield. Therefore, in the gas phase method, an increase in product costs is unavoidable, including the introduction of the above-mentioned expensive equipment.

[0008] Furthermore, when using nickel powder with an average particle size of 0.2 μm or less, especially 0.1 μm or less, the removal of coarse particles by classification becomes difficult, making the vapor phase method unable to accommodate future efforts to make internal electrodes even thinner.

[0009] On the other hand, the wet method has the advantage that the particle size distribution of the resulting nickel powder is narrower than that of the gas phase method. In particular, in the method 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 (copper salt) of a metal more noble than nickel acts as a nucleating agent, and nickel salt (more precisely, nickel ions (Ni 2+ ), or nickel complex ions) are reduced with hydrazine. Therefore, it is known that 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, fine nickel powder can be obtained with a narrower particle size distribution than the gas phase method.

[0010] Incidentally, when the nickel powder obtained by the above-mentioned wet method is applied to a multilayer ceramic capacitor, in order to prevent short circuits between electrodes in the laminate consisting of the internal electrode layer and the dielectric layer, a high flatness is required for the nickel paste dry film (dried film obtained by printing and drying the nickel paste) mainly composed of nickel powder and resin. In particular, in order to respond to the thinning of the internal electrode layer (about 0.5 μm to 1.0 μm) accompanying the high capacity of the multilayer ceramic capacitor in recent years, 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 to reduce coarse particles of the same size as the film thickness of the internal electrode layer contained in the nickel powder (for example, 0.8 μm to 1.2 μm) to the utmost limit.

[0011] Therefore, Patent Document 4 discloses a method for producing nickel powder using a wet method, in which a crystallization process is carried out in a strongly alkaline reaction solution to increase the reducing power of hydrazine, and in this process, a very small amount of a specific amine compound or sulfide compound is added to the reaction solution to significantly suppress the self-decomposition reaction of hydrazine and make it difficult for nickel particles to bond together to form coarse particles, thereby inexpensively obtaining high-performance nickel powder with an extremely low content of coarse particles.

[0012] As a means for reducing coarse particles in nickel powder, the above-mentioned classification process is used to remove coarse particles in gaseous nickel powder, but as described in Patent Document 5, a method of suppressing the formation of aggregates in gaseous nickel powder by using glutamic acid or the like in the nickel powder slurry to be classified is also used. On the other hand, a crushing process or the like has been proposed as a means for reducing coarse particles in wet nickel powder. For example, Patent Document 6 discloses a wet nickel powder having a specific particle size distribution and a specific range of average primary particle diameter by a specific crushing process, and a method for producing the same.

[0013] However, when the nickel powder with an average particle size of 0.2 μm or less, especially 0.1 μm or less, is used, the removal of coarse particles or the crushing itself becomes increasingly difficult. In particular, when nickel powder with a particle size of 0.15 μm or less is subjected to dry crushing, there is a risk of insufficient crushing force due to a decrease in kinetic energy caused by a decrease in the mass of each nickel particle, and there is a risk of heat generation and fire due to an increase in oxidation heat on the newly crushed surface caused by an increase in the specific surface area. Therefore, it is becoming increasingly difficult to respond to the future thinning of internal electrodes using classification and crushing.

[0014] On the other hand, as a method for producing nickel powder with excellent dispersibility, for example, Patent Document 7 reports a method using a hydrothermal synthesis method. According to this proposed method, it is disclosed that the resultant product after the hydrothermal reaction is washed with deionized water and ethanol, but the degree of aggregation of the obtained nickel powder is judged only by SEM observation, and the content of coarse particles is not quantitatively evaluated. Therefore, it is not clear whether the hydrothermal synthesis method is effective from the viewpoint of suppressing the generation of coarse particles.

[0015] In addition, Patent Document 8 reports a method in which after crystallization by a wet method, the post-reaction liquid is diluted and removed with water by decantation, and the water is similarly diluted and removed with a water-soluble organic solvent. According to this proposed method, it is described that the obtained nickel powder has a low oxygen content and that aggregation is suppressed, but the evaluation of coarse particles is only by SEM observation, and no quantified results have been reported. Therefore, it is not clear whether this method is effective in terms of suppressing the generation of coarse particles.

[0016] FIG. 1 shows the conventional wet nickel powder manufacturing process. In the solid-liquid separation process or drying process, for example, if the nickel powder cake (nickel crystallized powder + adhesion liquid) obtained in the solid-liquid separation process is exposed to air for a long time during handling, nickel particles are oxidized to generate nickel hydroxide, which often forms coarse particles of about 1 μm or more in size that envelop multiple nickel particles (hereinafter, these coarse particles may be referred to as "nickel hydroxide-containing coarse particles"). In addition, as the nickel powder cake is dried, nickel particles are often strongly attracted to each other due to the high surface tension of the adhesion liquid (mainly water), resulting in strong aggregation (dry aggregation). In these cases, not only does the nickel hydroxide-containing coarse particles and dry aggregation simply increase the number of coarse particles in the wet nickel powder, but also causes a problem of reduced dispersibility of nickel particles when a conductive paste is manufactured using the wet nickel powder.

[0017] Patent Document 9 reports a method of obtaining nickel powder by carrying out solid-liquid separation after crystallization by a wet method, reslurrying the mixture with a water-soluble organic solvent, and then carrying out solid-liquid separation and drying. This proposed method makes it possible to suppress the generation of nickel hydroxide-containing coarse particles and the drying aggregation of nickel particles. It also describes that the obtained nickel powder can be subjected to dry crushing methods such as spiral jet crushing and counter jet mill crushing, wet crushing methods such as high-pressure fluid collision crushing, and other general-purpose crushing methods as necessary. However, in the examples, only the case where the dry crushing method is used is described, and as mentioned above, it is difficult to apply the dry crushing method to nickel powder with an average particle size of 0.15 μm or less, so wet processing is preferable, but no specific method is mentioned for obtaining dried nickel powder after crushing when the wet crushing method is applied. [Prior art documents] [Patent documents]

[0018] [Patent Document 1] Japanese Patent Application Publication No. 4-365806 [Patent Document 2] Special Publication No. 2002-530521 [Patent Document 3] JP 2002-53904 A [Patent Document 4] WO2017 / 069067 issue [Patent Document 5] JP 2006-219688 A [Patent Document 6] JP 2001-247903 A [Patent Document 7] Patent No. 5421339 [Patent Document 8] Patent No. 6065699 [Patent Document 9] JP 2019-44268 A Summary of the Invention [Problem to be solved by the invention]

[0019] An object of the present invention is to provide a method for producing nickel powder that produces nickel powder with fewer coarse particles when nickel powder is produced using a wet method based on an aqueous solution. [Means for solving the problem]

[0020] In order to solve the above problems, the method for producing nickel powder of the present invention includes a wet-grinding step in which a slurry of nickel crystallized powder obtained by crystallizing the water-soluble nickel salt by a reduction reaction with hydrazine in a reaction liquid containing a water-soluble nickel salt, a metal salt of a metal nobler than nickel, hydrazine, an alkali hydroxide, and water is wet-grinded to obtain a nickel powder slurry with reduced coarse particles; an acid washing step in which an acid is added to the nickel powder slurry after the wet-grinding step to adjust the pH of the nickel powder slurry to a value of 2 to 7 and dissolve the nickel hydroxide in the nickel powder slurry; a solvent replacement step in which the solvent of the nickel powder slurry after the acid replacement step is replaced with a water-soluble organic solvent; a solid-liquid separation step in which the nickel powder slurry after the solvent replacement step is subjected to solid-liquid separation to obtain a nickel powder cake; and a drying step in which the nickel powder cake after the solid-liquid separation step is dried to obtain nickel powder. Effect of the Invention

[0021] As described above, according to the present invention, when nickel powder is produced using a wet method based on an aqueous solution, a method for producing nickel powder that can produce nickel powder with fewer coarse particles can be provided. [Brief description of the drawings]

[0022] [Figure 1] FIG. 1 is a schematic diagram showing an example of a conventional wet nickel powder manufacturing process. [Diagram 2] FIG. 1 is a schematic diagram showing nickel hydroxide-containing coarse particles found in a conventional wet nickel powder. [Diagram 3] FIG. 2 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. [Figure 4] FIG. 1 is a schematic diagram of a suction filtration device used for evaluating the content of coarse particles in nickel powders obtained by the production methods shown in Examples 1 to 8, Comparative Example, and Reference Example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] The method for producing nickel powder according to the present invention will be described below with reference to the drawings in the following order. Note that the present invention is not limited to the following examples, and can be modified as desired without departing from the gist of the present invention.

[0024] Nickel powder manufacturing method 1.Crystallization process 1-1. Chemicals used in the crystallization process 1-2. Crystallization reaction procedure (crystallization procedure) 1-3. Crystallization reaction (reduction reaction, hydrazine self-decomposition reaction) 1-4. Crystallization conditions (reaction start temperature) 2.Wet crushing process 3. Acid washing process 4. Solvent replacement process 5.Solid-liquid separation process 6.Drying process

[0025] <Method of manufacturing nickel powder> First, a method for producing nickel powder according to one embodiment of the present invention will be described. Fig. 3 shows a schematic diagram of an example of the production process in the wet nickel powder production method according to one embodiment of the present invention. The wet nickel powder production method according to one embodiment of the present invention includes a crystallization process, a wet crushing process, an acid washing process, a solvent substitution process, a solid-liquid separation process, and a drying process.

[0026] If desired, a sulfur compound may be added to the reaction solution or cleaning solution containing the nickel crystallized powder, and the surface of the nickel crystallized powder may be modified with a sulfur component to obtain wet nickel powder (nickel crystallized powder). By surface-treating the wet nickel powder in this way, it is possible to weaken the activity of nickel to decompose organic matter, and for example, deterioration of the resin coexisting with the wet nickel powder in the conductive paste can be suppressed. In the present invention, "powder" and "powder" refer to a state in which a large number of particles are gathered together to form an aggregate, and crystallized nickel particles dispersed in a slurry form or dried to form a solid aggregate correspond to nickel powder or nickel powder. For example, "nickel crystallized powder" is nickel powder in a reduced state in the crystallization process.

[0027] (1. Crystallization process) In the crystallization step, the nickel salt (more precisely, nickel ion or nickel complex ion) can be reduced by a reduction reaction using a reducing agent such as hydrazine in a reaction solution containing at least a water-soluble nickel salt, a salt of a metal nobler than nickel, a reducing agent, an alkali hydroxide, and water. In the present invention, when hydrazine is used, an amine compound or a sulfur-containing compound can be mixed with this reaction solution as necessary, and the nickel salt can be reduced in the presence of the amine compound or the sulfur-containing compound while suppressing the decomposition of hydrazine as a reducing agent.

[0028] (1-1. Chemicals used in the crystallization process) In the crystallization step of the present invention, a reaction liquid containing various chemicals such as nickel salt, salt of a metal nobler than nickel, a reducing agent, an alkali hydroxide, and, if necessary, an amine compound or a sulfur-containing compound, and water is used. From the viewpoint of reducing the amount of impurities in the obtained nickel powder, the water used as the solvent is preferably high-purity water such as ultrapure water (electrical conductivity: ≦0.06 μS / cm (microsiemens per centimeter)) or pure water (electrical conductivity: ≦1 μS / cm), and among these, it is preferable to use pure water, which is inexpensive and easily available. Each of the above-mentioned various chemicals will be described in detail below.

[0029] (a) Water-soluble nickel salts The water-soluble nickel salt is not particularly limited as long as it is a nickel salt that is easily 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, it is more preferable to use nickel chloride, nickel sulfate, or a mixture thereof.

[0030] (b) A metal salt of a metal more noble than nickel. A salt of a metal more noble than nickel has a lower ionization tendency than nickel, and is therefore reduced before nickel when nickel is reduced and precipitated. Therefore, when a salt of a metal more noble than nickel is contained in a nickel salt solution, the metal more noble than nickel is reduced first when nickel is reduced and precipitated, and acts as a nucleating agent that becomes an initial nucleus, so that it becomes easy to control the particle size and make the nickel crystallized powder (nickel powder) obtained by particle growth of the initial nucleus.

[0031] The metal salt of a metal more noble than nickel may be any metal salt that is water-soluble and has a lower ionization tendency than nickel, and examples thereof include water-soluble copper salts, and water-soluble noble metal salts such as gold salts, silver salts, platinum salts, palladium salts, rhodium salts, and iridium salts. For example, the water-soluble copper salt may be copper sulfate, the water-soluble silver salt may be silver nitrate, and the water-soluble palladium salt may be sodium palladium(II) chloride, ammonium palladium(II) chloride, palladium(II) nitrate, palladium(II) sulfate, etc., but are not limited thereto.

[0032] As the metal salt of a metal more noble than nickel, it is preferable to use the above-mentioned palladium salt, since although the particle size distribution becomes somewhat broad, it is possible to more finely control the particle size of the resulting nickel powder. When a palladium salt is used, the ratio of palladium salt to nickel [mol ppm] (moles of palladium salt / moles of nickel × 10 6) can be appropriately selected according to the target number average particle diameter of the wet nickel powder. For example, if the number average particle diameter of the wet nickel powder is set to 0.05 μm to 0.5 μm, it is preferable that the ratio of palladium salt to nickel is within the range of 0.2 mol ppm to 100 mol ppm, preferably within 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 diameter of the produced wet 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 producing the wet nickel powder.

[0033] (c) Reducing agent The reducing agent used in the crystallization step of the present invention is not particularly limited, but for example, hydrazine (N2H4, molecular weight: 32.05) can be mentioned. In addition to anhydrous hydrazine, hydrazine hydrate (N2H4·H2O, molecular weight: 50.06) can be used as hydrazine. Either can be used. The reduction reaction of hydrazine is as shown in the formula (2) described later, and has the following characteristics: it is particularly alkaline and has high reducing power, the by-products of the reduction reaction are nitrogen gas and water, so that impurity components due to the reduction reaction are not generated in the reaction solution, the amount of impurities in hydrazine is small to begin with, and it is easy to obtain. Therefore, hydrazine is suitable as a reducing agent, and for example, commercially available industrial grade 60% by mass hydrazine hydrate can be used.

[0034] (d) Alkali hydroxide The reducing power of hydrazine increases as the alkalinity of the reaction solution increases (see formula (2) described later), so an alkali hydroxide can be used as a pH adjuster to increase the alkalinity in the crystallization step. The alkali hydroxide is not particularly limited, but it is preferable to use an alkali metal hydroxide in terms of availability and cost. Specifically, it is more preferable to use one or more selected from sodium hydroxide and potassium hydroxide.

[0035] The amount of alkali hydroxide to be added should be determined so that the pH of the reaction solution at the reaction temperature is 9.5 or more, preferably 10 or more, and more preferably 10.5 or more, so that the reducing power of hydrazine as a reducing agent is sufficiently increased. For example, when comparing the pH of the reaction solution at 25°C and around 70°C, the pH at the higher temperature of 70°C is lower.

[0036] (e) Amine compounds As described above, the amine compound of the present invention has the functions of an autolysis inhibitor for hydrazine, a reduction reaction promoter, and an inhibitor for linking nickel particles with each other, and therefore may be included in the reaction solution as necessary. The amine compound is a compound containing a total of two or more functional groups selected from a primary amino group (-NH2) or a secondary amino group (-NH-) in the molecule, and at least one of an alkyleneamine or an alkyleneamine derivative can be used. As an example, it is preferable to use an amine compound having 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 two carbon atoms.

[0037] [ka]

[0038] More specifically, the alkyleneamine may be one or more 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). Examples of 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), 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 selected from the group consisting of 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 effect of inhibiting the self-decomposition of hydrazine, are easily available, and are inexpensive.

[0039] The above amine compound acts as a reduction reaction accelerator by absorbing nickel ions (Ni2+ It is believed that this is due to its function as a complexing agent that complexes hydrazine with hydrazine and forms nickel complex ions. In addition, it is speculated that the actions of the amine compound as an inhibitor of the self-decomposition of hydrazine and as an inhibitor of the bonding between nickel particles are manifested by the interaction between the primary amino group (-NH2) and secondary amino group (-NH-) in the amine compound molecule and the surface of hydrazine and nickel crystallized powder.

[0040] In addition, it is preferable that the alkyleneamine or alkyleneamine derivative, which is an amine compound, has the structure of the above formula A in which the nitrogen atom of the amino group in the molecule is bonded via a carbon chain with two carbon atoms. This is because the effect of suppressing the decomposition of hydrazine molecules is increased. For example, if the nitrogen atom of the amino group that strongly adsorbs to nickel crystallized powder is bonded via a carbon chain with three or more carbon atoms, it is considered that the carbon chain becomes longer and the degree of freedom of movement (molecular flexibility) of the carbon chain part of the amine compound molecule increases. As a result, it is considered that the contact of hydrazine molecules with nickel crystallized powder cannot be effectively hindered, and the number of hydrazine molecules that self-decompose due to the catalytic activity of nickel increases, thereby reducing the effect of suppressing the self-decomposition of hydrazine.

[0041] In fact, it has been confirmed that trimethylenediamine (also known as 1,3-diaminopropane, 1,3-propanediamine) (H2NC3H6NH2), in which the nitrogen atoms of the amino groups in the molecule are bonded via a carbon chain with three carbon atoms, has inferior inhibitory effect on the self-decomposition of hydrazine compared to ethylenediamine (H2NC2H4NH2) and propylenediamine (also known as 1,2-diaminopropane, 1,2-propanediamine) (CH3CH(NH2)CH2NH2).

[0042] Here, the ratio of the amine compound to nickel in the reaction solution [mol %] ((moles of amine compound / moles of nickel) x 100) is in the range of 0.01 mol % to 5 mol %, preferably in the range of 0.03 mol % to 2 mol %. If the ratio is less than 0.01 mol %, the amount of the amine compound is too small, and it may not be possible to obtain each function as an inhibitor of self-decomposition of hydrazine, an accelerator of reduction reaction, or an inhibitor of linkage between nickel particles. On the other hand, if the ratio exceeds 5 mol %, the amine compound may act too strongly as a complexing agent for forming nickel complex ions, which may cause abnormal growth of the particles of the nickel crystallized powder, and the granularity or sphericity of the nickel powder may be lost, resulting in an irregular shape, or many coarse particles formed by linking nickel particles together may be formed, which may cause deterioration of the properties of the nickel powder.

[0043] (f) Sulfur-containing compounds (hydrazine self-decomposition inhibitors) The reaction solution may contain a sulfur-containing compound. The sulfur-containing compound used in the present invention is a compound that is applied to a brightener for nickel plating or a stabilizer for plating baths, and unlike the above-mentioned amine compound, when used alone, the self-decomposition suppression effect of hydrazine is not so large. However, it has an interaction with the nickel particle surface such as adsorption, and when used in combination with the above-mentioned amine compound, it has the effect of a hydrazine self-decomposition suppression assistant that can significantly strengthen the self-decomposition suppression effect of hydrazine. Therefore, it is recommended to add it to the reaction solution as necessary. And the above-mentioned sulfur-containing compound is a compound that contains at least one of 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. Furthermore, the above-mentioned sulfur-containing compound has the effect of a hydrazine self-decomposition suppression assistant, as well as a bond suppression agent between nickel particles, and when used in combination with the above-mentioned amine compound, it can more effectively reduce the amount of coarse particles generated by nickel particles bonded to each other.

[0044] As the sulfur-containing compound, for example, in the case of a sulfide compound having a sulfide group (-S-) in the molecule, it is desirable that the compound has high water solubility, and therefore, any of a carboxyl group-containing sulfide compound, a hydroxyl group-containing sulfide compound, and an amino group-containing sulfide compound, which further contain at least one of a carboxyl group (-COOH), a hydroxyl group (-OH), and an amino group (primary: -NH2, secondary: -NH-, tertiary: -N<) in the molecule, is suitable. Thiazole ring-containing sulfide compounds, which contain at least one thiazole ring (C3H3NS), are also applicable, although they are not highly water soluble. 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 2H4COOH), thiodiglycolic acid (other names: 2,2'-thiodiglycolic acid, 2,2'-thiodiacetic acid, 2,2'-thiobisacetic acid, mercaptodiacetic acid) (HOOCCH2SCH2COOH), methionol (other names: 3-methylthio-1-propanol) (CH3SC3H6OH), thiodiglycol (other names: 2,2'-thiodiethanol) (HOC2H5SC2H5OH), thiomorpholine (C4H9NS), thiazole (C3H3NS), and benzothiazole (C7H5NS) are preferred. Among these, methionine and thiodiglycolic acid are preferred because they are excellent in assisting in suppressing the self-decomposition of hydrazine, and are easily available and inexpensive. It should be noted that "L (or D, DL)-methionine" means "L-methionine, D-methionine or DL-methionine," and the same applies to other compounds designated by L (or D, DL).

[0045] More specifically, sulfur-containing compounds other than sulfide compounds include saccharin (also known as o-benzoic acid sulfimide, o-sulfobenzimid) (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 The sulfur-containing compounds are water-soluble, and among them, saccharin and thiourea are preferred because they are excellent at assisting in the inhibition of the self-decomposition of hydrazine, and are easily available and inexpensive.

[0046] The action of the sulfur-containing compound as an auxiliary agent for inhibiting the self-decomposition of hydrazine and as an inhibitor of the connection between nickel particles can be speculated as follows. That is, the sulfide group (-S-), sulfonyl group (-S(=O)2-), sulfonic acid group (-S(=O)2-O-), and thioketone group (-C(=S)-) in the molecule of the sulfur-containing compound are adsorbed to the nickel surface of the nickel particles by intermolecular forces, but the action of covering and protecting the nickel crystallized powder is not as great as that of the amine compound molecule described above when the amine compound molecule is used in combination with the sulfur-containing compound. On the other hand, when the amine compound molecule is strongly adsorbed to the surface of the nickel crystallized powder to cover and protect it, there is a high possibility that a minute area that cannot be completely covered by the amine compound molecules is generated, but the sulfur-containing compound molecule covers this part by auxiliary adsorption, which more effectively prevents the contact between the hydrazine molecule in the reaction solution and the nickel crystallized powder, and furthermore, the nickel crystallized powder can be more strongly prevented from coalescing with each other, thereby manifesting the above action.

[0047] Here, the ratio [mol %] of the sulfur-containing compound to nickel in the reaction solution ((moles of sulfur-containing compound / moles of nickel) x 100) is in the range of 0.01 mol % to 5 mol %, preferably 0.03 mol % to 2 mol %, and more preferably 0.05 mol % to 1 mol %. If the ratio is less than 0.01 mol %, the amount of the sulfur-containing compound is too small, and the effects of the hydrazine self-decomposition inhibitor and the nickel particle bond inhibitor may not be obtained. On the other hand, even if the ratio exceeds 5 mol %, the above effects are not improved, and the amount of the sulfur-containing compound used simply increases, increasing the chemical cost and the amount of organic components blended in the reaction solution, increasing the chemical oxygen demand (COD) of the reaction wastewater in the crystallization process, resulting in an increase in wastewater treatment costs.

[0048] (f) Other Ingredients In addition to the above, various additives such as dispersants, complexing agents, and antifoaming agents may be contained in the reaction solution in the crystallization step. For example, if an appropriate dispersant or complexing agent is used in an appropriate amount, it may be possible to improve the granularity (sphericity) of the nickel crystallized powder and the smoothness of the particle surface of the nickel crystallized powder, and to reduce coarse particles. In addition, if an appropriate antifoaming agent is used in an appropriate amount, it is possible to suppress foaming in the crystallization step caused by nitrogen gas (see formulas (2) to (4) below) generated in the crystallization reaction, thereby preventing, for example, the reaction solution from overflowing from the container. As the dispersant, a known substance can be used, such as alanine (CH3CH(COOH)NH2), glycine (H2NCH2COOH), triethanolamine (N(C2H4OH)3), diethanolamine (also known as iminodiethanol) (NH(C2H4OH)2), etc. As the complexing agent, known substances can be used, and examples thereof include hydroxycarboxylic acid, carboxylic acid (organic acid containing at least one carboxyl group), hydroxycarboxylic acid salts and hydroxycarboxylic acid derivatives, carboxylates and carboxylate derivatives, specifically, tartaric acid, citric acid, malic acid, ascorbic acid, formic acid, acetic acid, pyruvic acid, and salts and derivatives thereof. Furthermore, as the defoaming agent, there is no particular limitation so long as it has excellent foam breaking properties under alkaline conditions, and oil-type or solvent-type silicone-based or non-silicone-based defoaming agents can be used.

[0049] (1-2. Crystallization reaction procedure (crystallization procedure)) In the crystallization step, a nickel salt solution in which at least a water-soluble nickel salt and a salt of a metal nobler than nickel are dissolved in water, a reducing agent solution in which a reducing agent (e.g., hydrazine) is dissolved in water, and an alkali hydroxide solution in which an alkali hydroxide is dissolved in water are prepared, and these are added and mixed to prepare a reaction liquid. Then, a crystallization reaction is performed in which nickel particles are crystallized in this reaction liquid by a reduction reaction to obtain nickel crystallized powder. Note that an amine compound or a sulfur-containing compound, which is added as necessary, can be added and mixed to any of the above solutions or a mixture of them before preparing the reaction liquid, or can be added and mixed to the reaction liquid after preparing the reaction liquid. Note that, in a room temperature environment, the reduction reaction starts at the time when the reaction liquid is prepared.

[0050] Here, there are two specific crystallization procedures: a procedure in which a reducing agent (e.g., hydrazine) and a reducing agent-alkali hydroxide solution containing an alkali hydroxide are added to a nickel salt solution containing nickel salt as a material to be reduced and a salt of a metal nobler than nickel, and mixed to prepare a reaction solution, and a procedure in which an alkali hydroxide solution is added to the nickel salt-reducing agent solution obtained by adding a reducing agent solution (e.g., hydrazine solution) to the above nickel salt solution and mixing it to prepare a reaction solution. The former procedure involves adding and mixing a reducing agent (e.g., hydrazine) that has been made highly alkaline and has increased reducing power due to the alkali hydroxide to a nickel salt solution containing the material to be reduced, while the latter procedure involves mixing a reducing agent (e.g., hydrazine) with a nickel salt solution containing the material to be reduced in advance, and then adjusting (raising) the pH with an alkali hydroxide to increase the reducing power.

[0051] In the former case (when nickel salt solution and reducing agent / alkali hydroxide solution are added and mixed), depending on the temperature at the time when the reaction solution is prepared, that is, at the time when the reduction reaction starts (hereinafter, sometimes referred to as the reaction start temperature), if the time required for adding and mixing the nickel salt solution (a solution containing nickel salt and a salt of a metal nobler than nickel) and the reducing agent / alkali hydroxide solution, which has been made more alkaline by using an alkali hydroxide to increase the reducing power, becomes long, the alkalinity increases locally in the nickel salt solution and reducing agent / alkali hydroxide solution addition and mixing area from the middle of the addition and mixing stage, and the reducing power of hydrazine increases, causing nucleation due to the salt of a metal nobler than nickel, which is the nucleating agent. Therefore, the closer to the end of the raw material mixing time, the greater the dependency of nucleation on the raw material mixing time becomes, which means that the nucleation effect of the added nucleating agent weakens, and there is a tendency that it becomes difficult to refine the nickel crystallized powder or obtain a narrow particle size distribution. This tendency is more noticeable when an alkaline reducing agent / alkali hydroxide solution is added and mixed with a weakly acidic nickel salt solution. The above tendency can be suppressed more easily as the mixing time of the raw materials is shorter, so mixing for a short period of time is desirable. However, taking into consideration 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.

[0052] On the other hand, in the latter case (when an alkali hydroxide solution is added and mixed into a nickel salt-reducing agent solution obtained by adding and mixing a nickel salt solution and a reducing agent solution), the reducing agent hydrazine is added and mixed in advance in the nickel salt-reducing agent solution containing nickel salt, a salt of a metal nobler than nickel, and a reducing agent, to obtain a uniform concentration. Therefore, the dependency of nucleation that occurs when adding and mixing the alkali hydroxide solution on the mixing time of the raw materials is not as large as in the former case, and it is easy to refine the nickel crystallized powder and obtain a narrow particle size distribution. However, for the same reason as in the former case, it is desirable to mix the alkali hydroxide solution for a short time, and considering the constraints of mass production equipment, the mixing time is preferably 10 seconds to 180 seconds, more preferably 20 seconds to 120 seconds, and even more preferably 30 seconds to 80 seconds.

[0053] As described above, there are two types of adding and mixing the amine compound and the sulfur-containing compound of the present invention: a procedure in which the amine compound and the sulfur-containing compound are added to the reaction liquid before the reaction liquid is prepared, and a procedure in which the amine compound and the sulfur-containing compound are added and mixed after the reaction liquid is prepared and the reduction reaction is started.

[0054] In the former case (when the amine compound or sulfur-containing compound is added to the reaction solution before the reaction solution is prepared), the amine compound or sulfur-containing compound is added to the reaction solution beforehand, so that the various actions of the amine compound or sulfur-containing compound are manifested from the start of nucleation caused by the salt (nucleating agent) of a metal more noble than nickel. On the other hand, the interaction of the amine compound or sulfur-containing compound with the nickel particle surface, such as adsorption, is involved in nucleation, and may affect the particle size and particle size distribution of the resulting nickel crystallized powder.

[0055] On the other hand, in the latter case (when the reaction solution is prepared and added and mixed after the start of the reduction reaction), the amine compound or sulfur-containing compound is added and mixed into the reaction solution after the very initial stage of the crystallization process in which nucleation due to the nucleating agent occurs. Therefore, although the action of the amine compound or sulfur-containing compound described above is somewhat delayed, since the amine compound or sulfur-containing compound is no longer involved in nucleation, the particle size and particle size distribution of the obtained nickel crystallized powder are less likely to be affected by the amine compound or sulfur-containing compound, and there is an advantage that they can be easily controlled. Here, the mixing time for adding and mixing the amine compound or sulfur-containing compound into the reaction solution in this procedure may be added all at once within a few seconds, or may be added in portions or dropwise over a period of several minutes to about 30 minutes. As mentioned above, the amine compound acts as a reduction reaction accelerator (complexing agent). Therefore, adding slowly will cause the crystal growth to proceed slowly and the nickel crystallized powder will become highly crystalline, but the inhibition of the self-decomposition of hydrazine will also gradually work, and the effect of reducing the amount of hydrazine consumed will decrease, so the mixing time may be appropriately determined while taking into account the balance between these two. In the former procedure, the timing of adding and mixing the amine compound and the sulfur-containing compound can be appropriately selected based on a comprehensive judgment depending on the purpose.

[0056] When adding and mixing the nickel salt solution and the reducing agent / alkali hydroxide solution, when adding and mixing the nickel salt solution and the reducing agent solution, and when adding and mixing the alkali hydroxide solution to the nickel salt / reducing agent solution, it is preferable to use stirring and mixing in which the solutions are mixed while being stirred. If the stirring and mixing properties are good, unevenness is reduced depending on the location of nucleation, and the dependency of nucleation on the raw material mixing time and the alkali hydroxide mixing time as described above is reduced, making it easier to refine the nickel crystallized powder and obtain a narrow particle size distribution. Any known method may be used for the stirring and mixing method, and it is preferable to use a stirring blade in terms of controlling the stirring and mixing properties and equipment costs.

[0057] (1-3. Crystallization reaction (reduction reaction, hydrazine self-decomposition reaction)) In the crystallization process, nickel salt is reduced with hydrazine in the presence of an alkali hydroxide and a salt of a metal nobler than nickel in the reaction solution to obtain nickel crystallized powder. If necessary, the self-decomposition of hydrazine can be significantly suppressed by the action of a trace amount of a specific amine compound or sulfur-containing compound to carry out the reduction reaction.

[0058] First, the reduction reaction in the crystallization process will be explained. 2+ The reaction in which nickel (Ni) crystallizes is a two-electron reaction as shown in formula (1) below. The reaction with hydrazine (N2H4) is a four-electron reaction as shown in formula (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 overall reduction reaction is expressed as formula (3) below, in which nickel hydroxide (Ni(OH)2), which is produced by the neutralization reaction of nickel chloride and sodium hydroxide, is reduced by hydrazine. Stoichiometrically (theoretical value), 0.5 moles of hydrazine (N2H4) are required for 1 mole of nickel (Ni).

[0059] Here, from the reduction reaction of hydrazine in formula (2), it can be seen that the stronger the alkalinity of hydrazine, the greater its reducing power. The above-mentioned alkali hydroxide is used as a pH adjuster to increase alkalinity, and plays a role in promoting the reduction reaction of hydrazine.

[0060] [C2] Ni 2+ +2e - →Ni↓ (two-electron reaction) ···(1) N2H4→N2↑+4H + +4e - (4-electron reaction) (2) 2NiCl2+N2H4+4NaOH →2Ni(OH)2+N2H4+4NaCl →2Ni↓+N2↑+4NaCl+4H2O ···(3)

[0061] As described above, in the conventional crystallization process, the active surface of the nickel crystallization powder acts as a catalyst to promote the self-decomposition reaction of hydrazine shown in the following formula (4), and hydrazine as a reducing agent may be consumed in large quantities other than for reduction. For this reason, although it depends on the crystallization conditions such as the reaction start temperature, for example, about 2 moles of hydrazine per mole of nickel, which is about four times the theoretical value required for the reduction described above, was generally used. Furthermore, as shown in formula (4), a large amount of ammonia is by-produced in the self-decomposition of hydrazine, and ammonia is contained in the reaction liquid at a high concentration, resulting in the generation of nitrogen-containing waste liquid. Thus, the use of an excessive amount of hydrazine, which is an expensive chemical, and the generation of the cost of treating the nitrogen-containing waste liquid are factors that increase the manufacturing cost of nickel powder by the wet method (wet nickel powder).

[0062] [C3] 3N2H4→N2↑+4NH3...(4)

[0063] In the method for producing nickel powder of the present invention, it is preferable to add a trace amount of a specific amine compound or sulfur-containing compound to the reaction solution to significantly suppress the self-decomposition reaction of hydrazine, and to significantly reduce the amount of hydrazine used, which is expensive as a drug. The specific amine compound can suppress the self-decomposition of hydrazine because (I) the molecules of the specific amine compound or sulfur-containing compound are adsorbed on the surface of the nickel crystallized powder in the reaction solution, physically preventing the contact between the active surface of the nickel crystallized powder and the hydrazine molecules, and (II) the molecules of the specific amine compound or sulfur-containing compound act on the surface of the nickel crystallized powder, deactivating the catalytic activity of the surface.

[0064] In the conventional wet crystallization process, nickel ions (Ni 2+ In general, complexing agents that form complex ions with hydrazine to increase the concentration of ionic nickel are used as reduction reaction accelerators. However, these complexing agents, such as tartaric acid and citric acid, do not have the effect of suppressing the self-decomposition of hydrazine, unlike the above-mentioned specific amine compounds and sulfur-containing compounds.

[0065] On the other hand, the specific amine compounds also act as complexing agents like tartaric acid and citric acid, and have the advantage of acting both as an agent for inhibiting the self-decomposition of hydrazine and as an agent for promoting the reduction reaction.

[0066] (1-4. Crystallization conditions (reaction start temperature)) The crystallization reaction in the crystallization step is initiated 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 (nickel salt solution) containing at least a water-soluble nickel salt and a salt of a metal nobler than nickel. In this case, the reaction initiation temperature of 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 are not particularly limited and can be freely set as long as the temperature of the mixed solution obtained by premixing them, i.e., the reaction initiation temperature, is within the above temperature range.

[0067] The higher the reaction initiation temperature, the more the reduction reaction is promoted and the nickel crystallized powder tends to be highly crystallized, but on the other hand, the self-decomposition reaction of hydrazine is promoted even more, so the consumption of hydrazine increases and the foaming of the reaction solution tends to become more intense. Therefore, if the reaction initiation temperature is too high, the consumption of hydrazine may increase significantly or the crystallization reaction may not be able to continue due to a large amount of foaming. On the other hand, if the reaction initiation temperature is too low, the crystallinity of the nickel crystallized powder may decrease significantly, the reduction reaction may slow down, the time of the crystallization process may be significantly extended, and the productivity of the nickel powder may decrease. For the above reasons, by setting the temperature within the above range, it is possible to produce high-performance nickel powder at a low cost while suppressing the consumption of hydrazine and maintaining high productivity.

[0068] The crystallization step is not an essential step in the production method of the present invention. A nickel crystallized powder obtained by crystallizing a water-soluble nickel salt through a reduction reaction with hydrazine in a reaction liquid containing a water-soluble nickel salt, a metal salt of a metal nobler than nickel, hydrazine, an alkali hydroxide, and water may be obtained by purchasing the nickel crystallized powder and making it into a slurry, or a slurry of the nickel crystallized powder after the crystallization step may be obtained by purchasing the nickel crystallized powder and then subjected to the wet crushing step described below.

[0069] In addition, in the nickel crystallized powder slurry obtained in the crystallization step or the obtained nickel crystallized powder slurry, the number average particle size of the nickel crystallized powder is preferably 0.03 μm to 0.15 μm, considering that the nickel crystallized powder is used as a material for MLCC.

[0070] (2. Wet crushing process) Although the nickel crystallized powder produced in the reaction solution by the reduction reaction with hydrazine described above can suppress the content of coarse particles to a low level, when it is desired to reduce the coarse particles to the minimum, or when the content ratio of coarse particles due to the bonding of particles during crystallization becomes somewhat large and problematic depending on the crystallization procedure and conditions, a wet crushing process must be provided to break the bonded nickel particles at the bonding parts and reduce the coarse particles. As described above, it is difficult to apply a dry crushing method to nickel powder with an average particle size of 0.15 μm or less, so as to the crushing process, it is recommended to apply wet crushing methods such as high-pressure fluid collision crushing processing in which raw material on a slurry pressurized to ultra-high pressure is sprayed and collided at high speed to crush, or supersonic droplet collision dispersion method in which raw material slurry is supplied to a gas flow exceeding the speed of sound to crush by collision, or other general-purpose crushing methods. Specific examples of wet atomization devices include a Starburst used in high-pressure fluid collision disintegration treatment and a G-smasher used in supersonic droplet collision dispersion method. As described above, the nickel crystallized powder before the wet disintegration step may be sulfur-coated with a sulfur compound such as a mercapto compound or a disulfide compound, if necessary, and then thoroughly washed with high-purity water such as pure water (electrical conductivity: ≦1 μS / cm) to remove remaining unreacted substances and impurities. The nickel concentration of the slurry used in the wet disintegration step is desirably 2% by mass or more, preferably 5% by mass or more, and more preferably 13% by mass or more.

[0071] (3. Acid washing process) In the nickel slurry obtained through the wet crushing process, nickel hydroxide is generated during the crushing process, and as the nickel hydroxide grows, it becomes a cause of nickel hydroxide-containing coarse particles. In order to prevent the generation of nickel hydroxide-containing coarse particles, an acid is continuously dropped into the slurry to lower the pH to neutral or weakly acidic, and the pH is maintained for a certain period of time to dissolve and remove the nickel hydroxide. The maintained pH is pH 2 to 7, preferably pH 4 to 6, and the retention time may be set according to the amount of nickel hydroxide to be dissolved, and is not particularly limited, but for example, the retention time can be set to 5 minutes to 90 minutes, preferably 10 minutes to 40 minutes.

[0072] The acid used for the acid washing is not particularly limited as long as it is water-soluble, and one or more inorganic acids selected from sulfuric acid, hydrochloric acid, and nitric acid, or organic acids selected from ascorbic acid, citric acid, and acetic acid can be used.

[0073] It is important that this step is carried out between the wet disintegration step and the solvent replacement step. This step is carried out after the wet disintegration step, since it is carried out in order to dissolve nickel hydroxide generated during or after the disintegration treatment in the wet disintegration step and to suppress the generation of coarse particles. In addition, since the acid used in the acid washing is water-soluble, it is difficult to carry out acid washing on the slurry after the solvent has been replaced with an organic solvent after the solvent replacement step, so this step is carried out before the solvent replacement step.

[0074] (4. Solvent replacement process) The solvent replacement process is a process in which the solvent of the nickel powder slurry is replaced with a water-soluble organic solvent to replace the water contained in the slurry. The replacement method may be performed by a known procedure such as repeating decantation and diluting the nickel powder with a solvent containing a water-soluble organic solvent, or using a solid-liquid separation facility in a subsequent process.

[0075] If the nickel crystallized powder is dried in the form of nickel powder cake containing moisture, the high surface tension of moisture (approximately 73 mN / m; 20°C) causes the nickel crystallized powder to dry and aggregate, increasing the number of coarse particles, and furthermore, when the nickel powder cake comes into contact with air during handling, the nickel crystallized powder is oxidized, making it easier to form coarse particles containing nickel hydroxide, which may not only increase the number of coarse particles but also reduce the dispersibility of the wet nickel powder. Therefore, by diluting the moisture contained in the nickel powder cake with a water-soluble organic solvent through solvent replacement, and then filtering and removing the water-soluble organic solvent containing moisture in the subsequent solid-liquid separation process, the nickel powder cake mainly contains the water-soluble organic solvent, it is possible to remove most of the moisture from the nickel powder cake.

[0076] As a result, in the subsequent drying process in which the nickel powder cake containing mainly the water-soluble organic solvent is dried, the nickel powder cake contains almost no moisture, so drying aggregation caused by the high surface tension of water is suppressed.Furthermore, even if the nickel crystallized powder comes into contact with air and oxidation progresses, nickel hydroxide does not grow in the water-soluble organic solvent liquid so that the nickel hydroxide particles wrap around multiple nickel crystallized powders, so that nickel hydroxide-containing coarse particles can also be suppressed, and an increase in coarse particles and a decrease in the dispersibility of the wet nickel powder can be prevented.

[0077] In this case, in order to reduce the amount of the water-soluble organic solvent used, the nickel powder cake may be washed in advance with high-purity water such as pure water (electric conductivity: ≦1 μS / cm) and a certain percentage of the water removed by solid-liquid separation before replacement with the water-soluble organic solvent. Furthermore, water can be efficiently replaced with the water-soluble organic solvent by repeating solid-liquid separation and replacement with the water-soluble organic solvent multiple times.

[0078] The solvent used in the solvent replacement step is preferably a solvent containing 70% by mass or more, preferably 90% by mass or more, of a water-soluble organic solvent. By making the content of the water-soluble organic solvent in the solvent 70% by mass or more, the surface tension of the solvent can be kept at a low level, and the nickel hydroxide-containing coarse particles and dry aggregation can be effectively suppressed. If the purity of the water-soluble organic solvent in the solvent is low, the effect of suppressing the nickel hydroxide-containing coarse particles and dry aggregation may be insufficient. The solvent used in the solvent replacement step may be a solvent composed of water and a water-soluble organic solvent component.

[0079] The boiling point of the water-soluble organic solvent is desirably 50° C. to 120° C., preferably 50° C. to 100° C., and more preferably 50° C. to 90° C. If the boiling point is less than 50° C., it is undesirable because the volatility is too high, making handling difficult and increasing the risk of volatilization and ignition, while if it exceeds 120° C., it becomes difficult to volatilize the water-soluble organic solvent in the subsequent drying step. If the boiling point of the water-soluble organic solvent is too high, the drying step takes too long, significantly reducing the drying efficiency and causing a cost disadvantage.

[0080] If a water-soluble organic solvent is replaced with a non-water-soluble organic solvent, it is difficult to replace the water, and this is not preferred. Note that, depending on the water-soluble organic solvent, an azeotrope with water may be formed, and when such an azeotrope of water and a water-soluble organic solvent is formed, it is desirable that the water concentration in the azeotrope is less than 30 mass%, preferably less than 20 mass%, and more preferably less than 10 mass% (for example, the water concentration in the azeotrope composition of water-ethanol is 4 mass%). As the water-soluble organic solvent that can be removed by evaporation at a relatively low temperature, specifically, one or more types of water-soluble and volatile solvents selected from acetone (boiling point: 56.5°C), 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), 2-propanol (also known as isopropyl alcohol) (boiling point: 82.4°C), 1,4-dioxane (boiling point: 102°C), 1,2-dimethoxyethane (boiling point: 82°C), and tetrahydrofuran (boiling point: 65°C), or organic solvents containing them as the main component (for example, various denatured alcohols, etc.) can be used. Among these, alcohol-based organic solvents are preferable in consideration of volatility and safety (harmfulness, flammability). Also, in consideration of the cost, denatured alcohol, which is a mixed solvent containing ethanol as the main component and two or more of methanol, isopropyl alcohol, n-propyl alcohol, water, etc., is most preferable. The water-soluble organic solvent is preferably a solvent containing alcohol, ketone, or ether as the main component. The water-soluble organic solvent preferably contains any one of methanol, ethanol, 1-propanol, 2-propanol, acetone, 1,4-dioxane, 1,2-dimethoxyethane, tetrahydrofuran, and denatured alcohol.

[0081] (5. Solid-liquid separation process) The solid-liquid separation step may be carried out by separating the nickel slurry from the solvent to obtain a nickel powder cake using known procedures, including but not limited to a Denver filter, a filter press, a centrifuge, a decanter, and the like.

[0082] (6. Drying process) The drying step is a step of drying the nickel powder cake after the solid-liquid separation step to obtain wet nickel powder. Specifically, a general-purpose drying device such as an inert gas atmosphere dryer or a vacuum dryer is used to dry the nickel powder cake for a predetermined time at a drying temperature of 30°C to 300°C, preferably 50°C to 200°C, and more preferably 80°C to 150°C, thereby removing moisture from the nickel powder cake and obtaining dried nickel powder. In addition, when drying is performed at about 200°C to 300°C in an inert atmosphere, reducing atmosphere, or vacuum atmosphere, it is possible to obtain wet nickel powder that has been heat-treated in addition to being simply dried. By performing heat treatment, the surface composition (for example, the ratio of nickel metal, nickel oxide, and nickel hydroxide) in the oxide film formed on the surface of the nickel particles can be changed. Specifically, the proportion of nickel oxide in the oxide film increases and the proportion of nickel hydroxide decreases. In addition, since crystal growth advances due to heat treatment, the higher the drying temperature, the larger the crystallite diameter of the wet nickel powder obtained. EXAMPLES

[0083] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples. The evaluation items and methods used in the examples are as follows.

[0084] Example 1 <Wet nickel powder production> [Preparation of solutions of nickel salts and metal salts of metals more noble than nickel] An aqueous solution of nickel chloride hexahydrate (NiCl2·6H2O, molecular weight: 237.69) was dissolved in 1 L of pure water so that 100 g of Ni metal was present (referred to as the "100 g-Ni / L aqueous solution"). Another aqueous solution of ammonium palladium (II) chloride (also known as ammonium tetrachloropalladate (II)) ((NH4)2PdCl4, molecular weight: 284.31), a metal salt of a metal more noble than nickel, was dissolved in 1 L of pure water so that 1.2 g of Pd metal was present (referred to as the "1.2 g-Pd / L aqueous solution"). Then, 1000mL of 100g-Ni / L aqueous solution, 4.17mL of 1.2g-Pd / L aqueous solution, and 1.27g of L-methionine (CH3SC2H4CH(NH2)COOH, molecular weight: 149.21) containing one sulfide group (-S-) in the molecule as a sulfur-containing compound as an autolysis suppression auxiliary were dissolved in 881mL of pure water to prepare a nickel salt nucleating agent-containing solution, which is an aqueous solution containing nickel salt as the main component, a sulfur-containing compound, and a nucleating agent that is a metal salt of a metal more noble than nickel. Here, in the nickel salt nucleating agent-containing solution, the sulfide compound L-methionine is a trace amount of 0.005 (0.5 mol%) in molar ratio to nickel, and palladium (Pd) is 50 mass ppm (27.58 mol ppm) to nickel (Ni).

[0085] [Preparation of reducing agent solution] As a reducing agent, hydrazine hydrate (N2H4·H2O, molecular weight: 50.06) was diluted 1.67 times with pure water to prepare 207 g of commercially available industrial grade 60% hydrazine hydrate (Otsuka-MGC Chemical Co., Ltd.) to prepare a reducing agent solution, which is an aqueous solution containing hydrazine as the main component and does not contain alkali hydroxide.

[0086] [Alkaline hydroxide solution] As the alkali hydroxide, sodium hydroxide (NaOH, molecular weight: 40.0) was dissolved in pure water to prepare 757 mL of an alkali hydroxide solution containing sodium hydroxide at a concentration of 382 g / L.

[0087] [Amine compound solution] As an amine compound, 1.02 g of ethylenediamine (abbreviation: EDA) (H2NC2H4NH2, molecular weight: 60.1), an alkyleneamine containing two primary amino groups (-NH2) in the molecule, was dissolved in 18 mL of pure water to prepare an amine compound solution, which is an aqueous solution containing ethylenediamine.

[0088] The materials used in the nickel salt nucleating agent-containing solution, reducing agent solution, alkali hydroxide solution, and amine compound solution were all reagents manufactured by Wako Pure Chemical Industries, Ltd., except for 60% hydrazine hydrate.

[0089] [Crystallization process] The nickel salt nucleating agent-containing solution was placed in a Teflon (registered trademark)-coated stainless steel container equipped with a stirring blade, and heated while stirring until the liquid temperature reached 85°C. After that, a reducing agent solution with a liquid temperature of 27°C was added and mixed for 10 seconds so that the molar ratio of Ni metal to hydrazine hydrate became 1:1.46 to prepare a nickel salt-reducing agent-containing solution. An alkali hydroxide solution with a liquid temperature of 27°C was added and mixed for 120 seconds to this nickel salt-reducing agent-containing solution so that the molar ratio of Ni metal to sodium hydroxide became 1:3.54 to prepare a reaction solution (nickel chloride + palladium salt + hydrazine + sodium hydroxide) with a liquid temperature of 70°C, and the reduction reaction (crystallization reaction) was initiated (reaction start temperature 70°C). Over a period of 20 minutes from 8 to 28 minutes after the start of the reaction, the amine compound solution was added dropwise to the reaction solution so that the molar ratio of Ni metal to ethylenediamine was 1:0.01 (1.0 mol%), and the reduction reaction was allowed to proceed while suppressing the self-decomposition of hydrazine, causing nickel crystallized powder to precipitate in the reaction solution. The reduction reaction was completed within 60 minutes after the start of the reaction, and the supernatant liquid of the reaction solution was transparent, confirming that all of the nickel components in the reaction solution had been reduced to metallic nickel and turned into nickel crystallized powder.

[0090] The reaction solution containing the nickel crystallized powder was in the form of a slurry, and an aqueous solution of mercaptoacetic acid (thioglycolic acid) (HSCH2COOH, molecular weight: 92.12) was added to this nickel crystallized powder-containing slurry to subject the nickel crystallized powder to surface treatment (sulfur coating treatment).

[0091] [Wet crushing process] After the surface treatment, the nickel crystallized powder-containing slurry was washed by repeatedly decanting and adding pure water (electrical conductivity 1 μS / cm) until the electrical conductivity of the slurry became 15 μS / cm or less, and the nickel crystallized powder-containing slurry with a nickel concentration of 25 mass% was obtained, and then subjected to wet crushing.

[0092] [Acid washing process] The nickel crystallized powder-containing slurry after the wet disintegration process was neutralized by dropping 1% by mass of sulfuric acid aqueous solution (H2SO4, molecular weight: 98.08) and maintaining the pH at 4 to 5 for 20 minutes. The nickel concentration of the nickel powder slurry at this time was 5% by mass. The neutralization reaction formula is Ni(OH)2+H2SO4→NiSO4+2H2O.

[0093] [Solvent replacement process, solid-liquid separation process] After the acid washing step, a filter paper was placed on the Nutsche, and the nickel crystallized powder-containing slurry was added thereto for filtration. Then, pure water with a conductivity of 1 μS / cm was added to the nickel crystallized powder on the filter paper, and the filtrate was filtered and washed until the conductivity of the filtrate after filtration was 30 μS / cm or less. Then, ethanol (boiling point: 78.3 ° C) with a purity of 99.9% or more was added to the Nutsche and passed through, and the solvent in the nickel slurry was replaced from water to ethanol. The ethanol concentration contained in the solvent of the nickel slurry after the solvent replacement was 92.4 mass%, and the remaining 7.6 mass% was water. Here, the ethanol concentration was determined by collecting the final filtrate (last 50 mL) of the solid-liquid separation step, measuring the Karl Fischer moisture content (150 ° C), and calculating "100-moisture content (%) = solvent concentration in the filtrate (%)", and the ethanol concentration was determined as the solvent concentration in the filtrate. It was calculated in the same manner in other examples. After the solvent replacement, filtration was continued until the solid content concentration reached 40 mass % or more to perform solid-liquid separation, and a nickel powder cake was obtained.

[0094] [Drying process] The nickel powder cake was dried in a vacuum dryer set at a temperature of 120° C. for 6 hours to obtain wet nickel powder.

[0095] <Evaluation and its results> (number average particle size) The obtained wet nickel powder was observed with a scanning electron microscope (SEM, JEOL Ltd., JSM-7100F), and the area of ​​100 to 200 particles whose overall shape could be confirmed by image processing of the SEM images was measured, and the diameter of each particle was calculated by converting the measured area into a perfect circle, and the average of the calculated diameters was calculated, which was taken as the number average particle size. From the viewpoint of responding to the recent trend of thinning of the internal electrodes of multilayer ceramic capacitors, the number average particle size of the nickel powder was set as a target of 0.15 μm or less. As a result, as shown in Table 1, the number average particle size in each of the examples, comparative examples, and reference examples was 80 nm.

[0096] (Content of coarse particles) For the crushed wet nickel powder, particles having a particle size exceeding 0.4 μm, which is about five times the number average particle size, were defined as coarse particles.

[0097] First, in the evaluation of coarse particles, a dispersion process was performed to obtain a slurry in which nickel particles were dispersed in a dispersion medium. Specifically, 0.03 g of nickel powder and 100 ml of 0.1 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. The above dispersion process resulted in a slurry in which nickel particles were dispersed in a dispersion medium.

[0098] Next, the above slurry was subjected to a filtration process using a suction filtration apparatus having the same configuration as the suction filtration apparatus 100 shown in the schematic diagram of FIG. 4. For the crushed wet nickel powder, a regenerated cellulose membrane filter (FUJIFILM Corporation, MICRO FILTER) having a diameter of 90 mm and a pore size of 0.4 μm was used. The membrane filter 1 was placed on a filter folder 2, a suction cup 3 was attached to the filter folder 2 as a slurry input container, and the filter folder 2 was further installed in a filtrate recovery container 4. With the vacuum pump 5 in operation, a slurry 10 in which nickel particles 10b were dispersed in a dispersion medium 10a was poured into the suction cup 3, and suction filtration was performed. Furthermore, 50 ml of a 0.1 mass% sodium hexametaphosphate aqueous solution used as a dispersion medium was poured into the suction cup 3 in a manner to wash off the wall surface of the 250 ml beaker in which the slurry was contained, and suction filtration was performed. Through the above filtration process, a filter 1 on which coarse particles larger than the filter pore size of 0.4 μm adhered as residue, and a filtrate 20 containing a dispersion medium 20a and nickel particles 20b were obtained.

[0099] The nickel particles constituting the residue were calculated as coarse particles larger than the filter pore size using high-frequency inductively coupled plasma emission spectroscopy (ICP method), and the content of coarse particles contained in the nickel powder was determined. Specifically, the membrane filter with the residue attached obtained by the above filtration process was naturally dried, then boiled in aqua regia to dissolve the coarse particles, and the aqua regia solution from which the membrane filter was removed was used as a sample solution. This sample solution was subjected to ICP spectroscopy using an ICP spectroscopy analyzer (Agilent Technologies, ICP720) to quantify nickel. Furthermore, the total mass of nickel in the residue collected as coarse particles was calculated from the quantitative value of nickel, and the content of coarse particles contained in the wet nickel powder (0.03 g) used for evaluation was calculated. As a result, the content of coarse particles in the wet nickel powder used for evaluation was 140 mass ppm for sizes exceeding 0.4 μm after crushing treatment.

[0100] (Evaluation of nickel hydroxide-containing coarse particles) The presence or absence of nickel hydroxide-containing coarse particles (see Figure 2) in the wet nickel powder was evaluated by the following method. That is, the membrane filter to which the coarse particles adhered as residue, obtained in the process of measuring the content of the coarse particles described above, was observed with a scanning electron microscope (SEM) at a magnification of 10,000 times in five different fields of view, and the average number of nickel hydroxide-containing coarse particles with a size of 0.4 μm or more per field of view was evaluated (less than 1: ○, 1 to less than 3: △, 3 or more: ×), and the evaluation result was "○".

[0101] Example 2 In the acid washing step, the nickel crystallized powder-containing slurry after wet crushing was neutralized by dropping 1% sulfuric acid (H2SO4, molecular weight: 98.08) and maintaining the pH at 3 to 4 for 20 minutes. Except for this, nickel powder was produced and evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0102] Example 3 In the acid washing step, the nickel crystallized powder-containing slurry after wet crushing was neutralized by dropping 1% sulfuric acid (H2SO4, molecular weight: 98.08) and maintaining the pH at 2 to 3 for 20 minutes. Except for this, nickel powder was produced and evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0103] Example 4 In the solvent replacement step, the ethanol concentration in the solvent of the nickel slurry after the solvent replacement was 84.5 mass %, and the remaining 15.5 mass % was water, but the same operation as in Example 1 was performed. The results are shown in Table 1.

[0104] Example 5 In the solvent replacement step, the ethanol concentration in the solvent of the nickel slurry after the solvent replacement was 72.2 mass %, and the remaining 27.8 mass % was water, but the same operation as in Example 1 was performed. The results are shown in Table 1.

[0105] Example 6 In the acid washing step, the nickel crystallized powder-containing slurry after wet crushing was neutralized by dropping 1% hydrochloric acid (HCl, molecular weight: 36.46) and maintaining the pH at 4 to 5 for 20 minutes. Except for this, nickel powder was produced and evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0106] Example 7 In the acid washing step, the nickel crystallized powder-containing slurry after wet crushing was neutralized by dropping 1% nitric acid (HNO3, molecular weight: 63.01) and maintaining the pH at 4 to 5 for 20 minutes. Except for this, nickel powder was produced and evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0107] Example 8 In the acid washing step, the nickel crystallized powder-containing slurry after wet crushing was neutralized by dropping 1% acetic acid aqueous solution (CH3COOH) and maintaining the pH at 4 to 5 for 20 minutes, but the same operation as in Example 1 was carried out to produce nickel powder and evaluate it. The results are shown in Table 1.

[0108] Example 9 In the acid washing step, the nickel crystallized powder-containing slurry after wet disintegration was neutralized by dropping an aqueous citric acid solution (C(OH)(CH2COOH)2COOH) and maintaining the pH at 4 to 5 for 20 minutes. Except for this, nickel powder was produced and evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0109] Example 10 In the acid washing step, the nickel crystallized powder-containing slurry after wet disintegration was neutralized by dropping an aqueous ascorbic acid solution (C6H8O6) and maintaining the pH at 4 to 5 for 20 minutes, but the same operations as in Example 1 were carried out to produce nickel powder and evaluate it. The results are shown in Table 1.

[0110] (Reference example) In the production of wet nickel powder, the same operation as in Example 1 was performed up to the wet disintegration step, and the acid washing step and subsequent steps were not performed. The nickel crystallized powder-containing slurry obtained in the wet disintegration step was not dried, and the nickel crystallized powder was not exposed to air. The nickel crystallized powder-containing slurry containing 0.03 g of nickel powder in the slurry state was added to 100 ml of 0.1 mass% sodium hexametaphosphate aqueous solution to disperse the nickel crystallized powder, and thereafter the same operation as in Example 1 was performed to determine the content of coarse particles in the wet nickel powder (nickel crystallized powder) when nickel hydroxide-containing coarse particles or dry aggregation did not occur, and the nickel hydroxide-containing coarse particles were evaluated. The content of coarse particles was 40 mass ppm at a size exceeding 0.4 μm, and the evaluation result of the nickel hydroxide-containing coarse particles was "○". In addition, the nickel powder slurry obtained in the wet crushing process was dried for 6 hours in a vacuum dryer set at a temperature of 120°C to obtain wet nickel powder. The obtained wet nickel powder was observed with a scanning electron microscope (SEM, JEOL Ltd., JSM-7100F) to measure the number average particle size, which was 80 nm.

[0111] Comparative Example 1 In the solvent replacement step, the ethanol concentration in the solvent of the nickel slurry after the solvent replacement was 51.0 mass %, and the remaining 49.0 mass % was water, but the same operation as in Example 1 was performed. The results are shown in Table 1, and the average particle size was 80 nm, and the content of coarse particles with a size exceeding 0.4 μm was 2800 mass ppm. The evaluation result of the nickel hydroxide-containing coarse particles was "X".

[0112] Comparative Example 2 In the wet nickel powder production, nickel powder was produced and evaluated in the same manner as in Example 1, except that the acid washing step was omitted. The results are shown in Table 1, and the average particle size was 80 nm, and the content of coarse particles with a size exceeding 0.4 μm was 3400 mass ppm. The evaluation result of the nickel hydroxide-containing coarse particles was "X".

[0113] (Evaluation Results) The solvent replacement rate, the content of coarse particles, the evaluation results of nickel hydroxide-containing coarse particles, and the number average particle size of the resulting nickel powder in Examples 1 to 10, Comparative Examples 1 and 2, and Reference Example are shown in Table 1. Note that the examples in which the solvent replacement rate was >90% are examples in which the solvent replacement was performed under conditions that gave >90%, but the solvent replacement rate was not quantitatively evaluated.

[0114] [Table 1]

[0115] In all of Examples 1 to 10, Comparative Examples 1 and 2, and Reference Example, nickel crystallized powder was obtained by the crystallization process under the same conditions, and the number average particle size was the same. As in Examples 1 to 10, the acid washing process and the solvent replacement process were performed to dissolve and remove nickel hydroxide, which is a source of coarse particles, and the solvent of the nickel powder slurry was replaced with a water-soluble organic solvent, so that it was possible to suppress the generation of nickel hydroxide-containing coarse particles and the dry aggregation of nickel particles, and the content of coarse particles was maintained at a level almost equal to that of the Reference Example in which nickel hydroxide-containing coarse particles and dry aggregation of nickel particles did not occur immediately after the wet crushing process. That is, in Examples 1 to 10, the coarse particles generated in the wet crushing process were eliminated by dissolving nickel hydroxide in the acid washing process, and the water in the solvent was removed and replaced with an organic solvent in the solvent replacement process, which resulted in preventing the dry aggregation of nickel particles after acid washing.

[0116] [summary] The method for producing nickel powder according to the present invention is a method for producing nickel powder by a wet method using hydrazine as a reducing agent, and by carrying out an acid washing step and a solvent replacement step after a wet crushing step, which is one step of the wet method, to dissolve and remove nickel hydroxide, which is a source of coarse particles, and to replace the solvent of the nickel powder slurry with a water-soluble organic solvent, the formation of nickel hydroxide-containing coarse particles is effectively prevented, and the amount and bonding strength of coarse particles formed by drying and agglomeration of nickel particles are suppressed, so that a wet nickel powder with excellent dispersibility and few coarse particles can be obtained. Therefore, a high-performance wet nickel powder suitable for the internal electrodes of multilayer ceramic capacitors can be produced at low cost.

[0117] Although the preferred embodiment of the present invention has been described in detail above, the present invention is not limited to such an example. It is clear that a person having ordinary knowledge in the technical field to which the present invention pertains can conceive of various modified or amended examples within the scope of the technical ideas described in the claims, and it is understood that these also naturally fall within the technical scope of the present invention. [Explanation of symbols]

[0118] 1. Membrane filter 2. Filter Folder 3 Suction cups 4 Filtrate collection container 5. Pressure reducing pump 10 Slurry 10a Dispersion medium 10b Nickel particles 20 Filtrate 20a Dispersion medium 20b Nickel particles 100 Suction filtration device

Claims

1. a wet-grinding step of wet-grinding a slurry of nickel crystallized powder obtained by crystallizing the water-soluble nickel salt through a reduction reaction with hydrazine in a reaction liquid containing a water-soluble nickel salt, a metal salt of a metal nobler than nickel, hydrazine, an alkali hydroxide, and water, to obtain a nickel powder slurry having reduced coarse particles; After the wet disintegration step, an acid washing step is performed in which an acid is added to the nickel powder slurry to adjust the pH of the nickel powder slurry to a value of 2 to 5, and nickel hydroxide in the nickel powder slurry is dissolved; A solvent replacement process for replacing the solvent of the nickel powder slurry after the acid washing process with a water-soluble organic solvent; A solid-liquid separation step of obtaining a nickel powder cake by solid-liquid separation of the nickel powder slurry after the solvent substitution step; A method for producing nickel powder, comprising a drying step of drying the nickel powder cake after the solid-liquid separation step to obtain nickel powder.

2. 2. The method for producing nickel powder according to claim 1, comprising a crystallization step of obtaining the nickel crystallized powder by a reduction reaction with hydrazine in a reaction solution containing a water-soluble nickel salt, a metal salt of a metal more noble than nickel, hydrazine, an alkali hydroxide, and water.

3. The method for producing nickel powder according to claim 2 , wherein the reaction liquid contains an amine compound or a sulfur-containing compound.

4. The method for producing nickel powder according to any one of claims 1 to 3, wherein the nickel crystallized powder has a number average particle size of 0.03 μm to 0.15 μm.

5. The acid washing step is a step of dropping one or more acids selected from sulfuric acid, hydrochloric acid, nitric acid, acetic acid, citric acid, and ascorbic acid into the nickel powder slurry after the wet disintegration step to neutralize the nickel powder slurry and maintain the pH of the nickel powder slurry at 2 to 5. The method for producing nickel powder according to any one of claims 1 to 4.

6. The method for producing nickel powder according to any one of claims 1 to 5, wherein the solvent replacement step is a step of replacing the solvent of the nickel powder slurry with a water-soluble organic solvent using a solvent containing 70 mass% or more of the water-soluble organic solvent.

7. The reaction liquid contains an amine compound, The amine compound is at least one of an alkyleneamine or an alkyleneamine derivative, and is represented by the following formula A in which a nitrogen atom of an amino group in the molecule is bonded via a carbon chain having two carbon atoms: 【Chemistry 1】 The method for producing nickel powder according to any one of claims 1 to 6, wherein the nickel powder has at least the structure:

8. The alkyleneamine is ethylenediamine (H ２ N.C. ２ H ４ N.H. ２ ), diethylenetriamine (H ２ N.C. ２ H ４ N.H.C. ２ H ４ N.H. ２ ), triethylenetetramine (H ２ N (C ２ H ４ N.H. ２ C ２ H ４ N.H. ２ ), tetraethylenepentamine (H ２ N (C ２ H ４ N.H. ３ C ２ H ４ N.H. ２ ), pentaethylenehexamine (H ２ N (C ２ H ４ N.H. ４ C ２ H ４ N.H. ２ ), and propylenediamine (CH ３ CH(NH ２ ) CH ２ N.H. ２ ) is one or more selected from The alkyleneamine derivative is tris(2-aminoethyl)amine (N(C ２ H ４ N.H. ２ ) ３ ), N-(2-aminoethyl)ethanolamine (H ２ N.C. ２ H ４ N.H.C. ２ H ４ OH), N-(2-aminoethyl)propanolamine (H ２ N.C. ２ H ４ N.H.C. ３ H ６ OH), 2,3-diaminopropionic acid (H ２ NCH ２ CH(NH)COOH), 1,2-cyclohexanediamine (H ２ N.C. ６ H １０ N.H. ２ ), ethylenediamine-N,N'-diacetic acid (HOOCCH ２ N.H.C. ２ H ４ N.H.C.H. ２ COOH), N,N'-diacetylethylenediamine (CH ３ CONHC ２ H ４ NHCOCH ３ ), N,N'-dimethylethylenediamine (CH ３ N.H.C. ２ H ４ N.H.C.H. ３ ), N,N'-diethylethylenediamine (C ２ H ５ N.H.C. ２ H ４ N.H.C. ２ H ５ ), and N,N'-diisopropylethylenediamine (CH ３ (CH ３ ) CHNHC ２ H ４ NHCH (CH ３ ) CH ３ 8. The method for producing nickel powder according to claim 7, wherein the nickel powder is one or more selected from the group consisting of: 【Request 9】 The sulfur-containing compound has a sulfide group (-S-), a sulfonyl group (-S(=O) ２ -), sulfonic acid group (-S(=O) ２ The method for producing nickel powder according to claim 3, wherein the nickel powder is one or more compounds selected from the group consisting of compounds containing at least one of a ketone group (-C(=S)-), a ketone group (-O-), and a thioketone group (-C(=S)-). 【Request 10】 The sulfur-containing compound is L (or D, DL)-methionine (CH ３ SC ２ H ４ CH(NH ２ )COOH), L (or D, DL)-ethionine (C ２ H ５ SC ２ H ４ CH(NH ２ )COOH), N-acetyl-L (or D, DL)-methionine (CH ３ SC ２ H ４ CH(NH(COCH) ３ ))COOH), lanthionine (HOOCCH(NH ２ ) CH ２ SCH ２ CH(NH ２ )COOH), thiodipropionic acid (HOOCC ２ H ４ SC ２ H ４ COOH), thiodiglycolic acid (HOOCCH ２ SCH ２ COOH), methionol (CH ３ SC ３ H ６ OH), thiodiglycol (HOC ２ H ５ SC ２ H ５ OH), thiomorpholine (C ４ H ９ NS), thiazole (C ３ H ３ NS), benzothiazole (C ７ H ５ NS), saccharin (C ７ H ５ NO ３ S), sodium dodecyl sulfate (C １２ H ２５ OS (O) ２ ONa), dodecylbenzenesulfonic acid (C １２ H ２５ C ６ H ４ S (O) ２ OH), sodium dodecylbenzenesulfonate (C １２ H ２５ C ６ H ４ S (O) ２ ONa), di-2-ethylhexyl sodium sulfosuccinate (NaOS(O) ２ CH (COOCH ２ CH (C ２ H ５ ) C ４ H ９ ) CH ２ (COOCH ２ CH (C ２ H ５ ) C ４ H ９ ), and thiourea (H ２ NC(S)NH ２ 10. The method for producing nickel powder according to claim 9, wherein the nickel powder is one or more selected from the group consisting of:

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