Method for producing nickel powder

Abstract

To provide a method for producing nickel powder with a lower coarse particle content, having a nickel hydroxide as a principal component, in particular, to provide a method for producing nickel powder by a wet method allowing simpler and easier production of nickel powder.SOLUTION: A method for producing nickel powder includes: a crystallization step of conducting reductive reaction in a reaction liquid containing a water-soluble nickel salt, a salt of a metal nobler than nickel, a reductant, an alkali hydroxide and water mixed together, to obtain nickel powder slurry as a reaction termination liquid containing nickel crystallization powder; a wash-filtration step of washing and filtering out the nickel crystallization powder in the nickel powder slurry to obtain a nickel powder cake; and a desiccation step of desiccating the nickel powder cake to obtain nickel powder. A temperature of the nickel powder cake is held within a range of 0 to 20°C after the filtering out in the wash-filtration step to a start of desiccation at the desiccation step.SELECTED DRAWING: Figure 1

Description

Technical Field

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

Background Art

[0002] Nickel powder is used as a material for capacitors in electronic circuits, and particularly as a material for thick-film conductors constituting internal electrodes of laminated ceramic components such as multilayer ceramic capacitors (MLCC) and multilayer ceramic substrates.

[0003] In recent years, the capacitance of multilayer ceramic capacitors has been increasing, and the amount of internal electrode paste used for forming internal electrodes of multilayer ceramic capacitors has also increased significantly. For this reason, inexpensive base metals such as nickel are mainly used instead of the use of expensive noble metals as the metal powder for the internal electrode paste constituting the thick-film conductor.

[0004] In the process of manufacturing a multilayer ceramic capacitor, an internal electrode paste obtained by kneading nickel powder, a binder resin such as ethyl cellulose, and an organic solvent such as terpineol is screen-printed on a dielectric green sheet. Then, the dielectric green sheet on which the internal electrode paste is printed and dried is laminated so that the internal electrode paste printing layer and the dielectric green sheet overlap alternately, and further pressed to obtain a laminate.

[0005] This laminate is cut into a predetermined size, and then the binder resin is removed by heat treatment (debinding treatment). Further, the laminate after the debinding treatment is fired at a high temperature of about 1300 ° C. to obtain a ceramic molded body.

[0006] Then, an external electrode is attached to the obtained ceramic compact to obtain a multilayer ceramic capacitor. Since base metals such as nickel are used as the metal powder in the internal electrode paste that becomes the internal electrode, the debinding process of the laminate is performed in an atmosphere with an extremely low oxygen concentration, such as an inert atmosphere, so that these base metals do not oxidize.

[0007] With the miniaturization and large capacitance of multilayer ceramic capacitors, both the internal electrodes and the dielectric are being made thinner. Along with this, the particle size of the nickel powder used in the internal electrode paste has also been refined, and nickel powder with an average particle size of 0.4 μm or less is required. In particular, the use of nickel powder with an average particle size of 0.3 μm or less has become mainstream.

[0008] The manufacturing methods of nickel powder are roughly classified into a vapor phase method and a wet method. As the vapor phase method, for example, there are a method of producing nickel powder by reducing nickel chloride vapor described in Patent Document 1 with hydrogen, and a method of producing nickel powder by vaporizing nickel metal in plasma described in Patent Document 2. As the wet method, for example, there is a method of producing nickel powder by adding a reducing agent to a nickel salt solution described in Patent Document 3.

[0009] The vapor phase method is an effective means for obtaining high-performance nickel powder with excellent crystallinity because it is a high-temperature process of about 1000 °C or higher, but there is a problem that the particle size distribution of the obtained nickel powder becomes wide. As described above, in the thinning of the internal electrode, nickel powder with an average particle size of 0.4 μm or less that does not contain coarse particles and has a relatively narrow particle size distribution is required. Therefore, in order to obtain such nickel powder by the vapor phase method, classification treatment of the nickel powder by introducing an expensive classification device is essential.

[0010] In the classification process, it is possible to remove coarse particles larger than the classification point with an arbitrary value of about 0.6 μm to 2 μm in particle size as a guide. However, since a part of the particles smaller than the classification point is also removed at the same time, there is also a problem that the actual product yield is significantly reduced. Therefore, in the gas phase method, an increase in the product cost is inevitable, including the introduction of the above-mentioned expensive equipment.

[0011] Furthermore, in the gas phase method, when using nickel powder with an average particle size of 0.2 μm or less, particularly when the average particle size is 0.1 μm or less, it becomes difficult to remove coarse particles by classification treatment, and thus it is impossible to cope with further thinning of the internal electrode in the future.

[0012] On the other hand, the wet method has the advantage that the particle size distribution of the obtained nickel powder is narrower compared to the gas phase method. In particular, in the method of producing nickel powder by crystallization in which a solution containing hydrazine as a reducing agent is added to a solution containing a copper salt as a nickel salt described in Patent Document 3, in the presence of a salt of a metal more precious than nickel (nucleating agent), the nickel salt (precisely, nickel ions (Ni 2+ ) or nickel complex ions) are reduced by hydrazine, so the number of nuclei generated is controlled (that is, the particle size is controlled), and the nucleation and particle growth become uniform, and it is known that fine nickel powder with a narrower particle size distribution (hereinafter, the nickel powder generated in the reaction solution may be referred to as nickel crystallization powder) can be obtained.

[0013] By the way, when applying the nickel powder obtained by the above wet method to a multilayer ceramic capacitor, in order to prevent short - circuit (electrode short - circuit) between electrodes in the laminate composed of the internal electrode layer and the dielectric layer described above, a high flatness is required for the nickel - paste dry film (a dry film obtained by printing and drying nickel paste) mainly composed of nickel powder and resin. In particular, in order to cope with the thinning of the internal electrode layer (film thickness of about 0.5 μm to 1.0 μm) accompanying the increase in capacitance of recent multilayer ceramic capacitors, fine nickel powder with an average particle size of 0.3 μm or less, preferably 0.2 μm or less, is used. Also, coarse particles with a size of about the same as the film thickness of the internal electrode layer contained in the nickel powder (for example, 0.8 μm to 1.2 μm) are required to be reduced to the limit.

[0014] Therefore, Patent Document 4 discloses a method for producing nickel powder using a wet method. In the crystallization step of performing a reduction reaction in a strongly alkaline reaction solution to enhance the reducing power of hydrazine, 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 to form coarse particles caused by the connection of nickel particles, thereby inexpensively obtaining high - performance nickel powder with a very low content of coarse particles.

Prior Art Documents

Patent Documents

[0015]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0016] In the method for producing nickel powder using the wet method as described above, hydrazine is often used as a reducing agent. Therefore, as is clear from Patent Document 4 and the like, the reaction solution generally becomes strongly alkaline. In the crystallization step shown in FIG. 1, a nickel powder slurry in which nickel crystallized powder is generated in this strongly alkaline reaction solution is obtained by the reduction reaction of hydrazine. A washing and filtration step of washing the nickel crystallized powder with pure water and filtering and recovering it to obtain a nickel powder cake, and a drying step of drying this nickel powder cake (by vacuum drying or the like) to obtain dried nickel crystallized powder (nickel powder) are carried out subsequent to the crystallization step.

[0017] Since the above washing and filtration step is usually carried out in the air, the obtained nickel powder cake is inevitably exposed to the air to some extent during the washing and filtration step or until the drying in the drying step is carried out, and oxidation of the nickel crystallized powder occurs. In particular, when carrying out the crystallization reaction on a mass production scale, unlike the handling of a small amount at the laboratory level, it takes time to handle the nickel powder cake, so the oxidation of the nickel crystallized powder due to the above air exposure easily progresses.

[0018] In the nickel powder cake obtained from the above strongly alkaline nickel powder slurry through the washing and filtration step, when oxidation of the above nickel crystallized powder occurs, coarse particles 10 mainly composed of nickel hydroxide firmly solidified with nickel hydroxide 2 generated by oxidation between nickel crystallized powders 1 (particle size of 0.4 μm or less) may be generated (see FIG. 2). In many cases, the particle size of the coarse particles 10 is 0.8 μm or more, which may cause problems such as in multilayer ceramic capacitors, and thus an effective countermeasure for suppressing them has been demanded.

[0019] Therefore, an object of the present invention is to provide a method for producing nickel powder having a low content of coarse particles mainly composed of nickel hydroxide, and in particular, a method for producing nickel powder by a wet method that can produce the above nickel powder more simply and easily.

Means for Solving the Problems

[0020] In the method for producing nickel powder by a wet process, which is used for laminated ceramic capacitors and the like, the inventors have found that by controlling the temperature of the nickel cake within a predetermined range during the washing and filtration process of washing and filtering nickel crystallized powder (nickel powder) from a strongly alkaline nickel powder slurry that is the reaction final solution of crystallization, the generation of coarse particles mainly composed of nickel hydroxide can be effectively suppressed. The present invention has been completed based on such findings.

[0021] In order to solve the above problems, the method for producing nickel powder of the present invention includes a crystallization step of performing a reduction reaction in a reaction solution obtained by mixing a water-soluble nickel salt, a salt of a metal nobler than nickel, a reducing agent, an alkali hydroxide, and water to obtain a nickel powder slurry as a reaction final solution containing nickel crystallized powder, a washing and filtration step of washing and filtering the nickel crystallized powder in the nickel powder slurry to obtain a nickel cake, and a drying step of drying the nickel cake to obtain nickel powder. During the period from after filtration in the washing and filtration step to the start of drying in the drying step, the temperature of the nickel cake is maintained in the range of 0 to 20°C. This is the method for producing nickel powder.

[0022] The water content of the nickel cake may be 5% by mass or more and less than 30% by mass.

[0023] In the washing and filtration step, the temperature of the pure water used for washing the nickel crystallized powder may be 0°C or higher and 20°C or lower.

[0024] The nickel powder may have a number average particle size of 0.03 μm to 0.4 μm, the content of coarse particles mainly composed of nickel hydroxide having a particle size exceeding 0.8 μm may be 200 ppm by mass or less, and the content of coarse particles mainly composed of nickel hydroxide having a particle size exceeding 1.2 μm may be 100 ppm by mass or less.

Advantages of the Invention

[0025] A method for producing nickel powder with a low content of coarse particles mainly composed of nickel hydroxide, and in particular, a method for producing nickel powder by a wet method that can produce the above nickel powder more simply and easily can be provided.

Brief Description of the Drawings

[0026]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0027] Hereinafter, the method for producing nickel powder according to this embodiment will be described. This embodiment is not limited to these, and can be arbitrarily changed without departing from the gist of this embodiment.

[0028] 1. Nickel powder 2. Method for producing nickel powder using a wet method 2-1. Crystallization step 2-1-1. Chemicals used in the crystallization step 2-1-2. Crystallization procedure 2-1-3. Reduction reaction 2-1-4. Reaction start temperature 2-2. Washing and filtration step 2-2-1. Methods and procedures for washing and filtration 2-3. Drying step 2-4. Crushing step (post-treatment step)

[0029] <1. Nickel powder> The nickel powder obtained by the manufacturing method of the present invention has a very low content of coarse particles mainly composed of nickel hydroxide with a particle size exceeding 0.8 μm, and can achieve high flatness in the nickel paste dry film. Therefore, it is possible to effectively prevent short circuits (short circuits) between electrodes in a laminate composed of an internal electrode layer and a dielectric layer, and it is suitable for use as an internal electrode of a multilayer ceramic capacitor.

[0030] The nickel powder has a substantially spherical particle shape, and its average particle size is 0.03 μm to 0.4 μm from the viewpoint of corresponding to the thinning of the internal electrode of a multilayer ceramic capacitor in recent years. The substantially spherical shape includes not only a perfect sphere but also an ellipsoid having an elliptical shape in which the ratio of the minor axis to the major axis (minor axis / major axis) of a predetermined cross-section is 0.8 to 1.0.

[0031] Note that the average particle size in this embodiment is the number average particle size obtained from a scanning electron microscope photograph (SEM image) of the nickel powder. Further, for the nickel powder manufactured by the wet method of this embodiment described later, it has a sufficiently narrow particle size distribution even without classification treatment. That is, in this case, the CV value, which is the value obtained by dividing the standard deviation of the particle size by its average particle size, is 0.2 or less, and the nickel powder has a uniform particle size.

[0032] Here, the coarse particles mainly composed of nickel hydroxide are solids in which nickel hydroxide generated on the surface of nickel crystallized powder entraps and firmly binds the nickel crystallized powder in the process from the washing and filtration process to the drying process described later. Even if the crushing treatment described later is performed, these are not easily loosened. Note that the coarse particles mainly composed of nickel hydroxide can be identified by separating the coarse particles from the crushed nickel powder using a filter such as a membrane filter, and analyzing the coarse particles by EDS (energy dispersive X-ray analysis), XPS (X-ray photoelectron spectroscopy), etc. and observing the shape by a scanning electron microscope (SEM), etc.

[0033] The nickel powder of the present invention can have a content of coarse particles mainly composed of nickel hydroxide exceeding 0.8 μm of 200 mass ppm or less, and further 100 mass ppm or less. The content of coarse particles mainly composed of nickel hydroxide having a particle size exceeding 1.2 μm can be 100 mass ppm or less, and further 50 mass ppm or less.

[0034] Of course, the influence of the coarse particles depends on the film thickness of the internal electrode layer of the multilayer ceramic capacitor in which the nickel powder is used. However, in recent years' thinner internal electrode layers, if the content of coarse particles mainly composed of nickel hydroxide with a particle size exceeding 0.8 μm exceeds 200 mass ppm, or if the content of coarse particles mainly composed of nickel hydroxide with a particle size exceeding 1.2 μm exceeds 100 mass ppm, the occurrence of short circuits between electrodes may become remarkable. Needless to say, the smaller the content of coarse particles mainly composed of nickel hydroxide, the better. If the content of coarse particles mainly composed of nickel hydroxide with a particle size exceeding 0.8 μm is 100 mass ppm or less, or if the content of coarse particles mainly composed of nickel hydroxide with a particle size exceeding 1.2 μm is 50 mass ppm or less, the occurrence rate of short circuits between electrodes can be sufficiently reduced. The particle size of the coarse particles mainly composed of nickel hydroxide may be the minor axis diameter obtained from the SEM image.

[0035] Incidentally, the coarse particles mainly composed of nickel hydroxide can be identified by separating the crushed nickel powder using a filter such as a membrane filter to separate the coarse particles, and then analyzing the coarse particles by EDS (energy dispersive X-ray analysis), XPS (X-ray photoelectron spectroscopy), etc. and observing the shape by a scanning electron microscope (SEM), etc.

[0036] Generally, nickel powder may contain a small amount of impurities. For example, nickel powder obtained by the wet process may contain trace amounts of oxygen due to surface oxidation of nickel particles, chlorine presumably due to nickel chloride as the nickel raw material, and alkali metals such as sodium due to sodium hydroxide. Also, nickel powder obtained by the vapor phase method may contain trace amounts of chlorine in the case of nickel powder obtained by reducing nickel chloride vapor with hydrogen. These impurities may cause defects in the internal electrodes during the manufacture of multilayer ceramic capacitors, so it is preferable to reduce them as much as possible. For example, for chlorine and alkali metals, it is preferable that the content in the nickel powder is 0.01 mass% or less.

[0037] Nickel powder applicable to the internal electrodes of multilayer ceramic capacitors, such as the nickel powder of this embodiment, usually may contain a small amount of sulfur in order to suppress its catalytic activity. This is because the surface of nickel particles has high catalytic activity. For example, if it is used as it is without containing sulfur or the like, during the debinding process in the manufacture of multilayer ceramic capacitors, it promotes the thermal decomposition of binder resins such as ethyl cellulose resin contained in the internal electrode paste, and the binder resin is decomposed from a low temperature, resulting in a significant decrease in the strength of the laminate. At the same time, a large amount of decomposition gas is generated and cracks are likely to occur in the laminate.

[0038] As described above, in order to incorporate sulfur into nickel powder, a surface treatment for attaching sulfur to the surface of nickel particles is carried out. From the viewpoint of expressing the decomposition suppression effect of the binder resin and reducing the influence of sulfur as an impurity on the characteristics of the multilayer ceramic capacitor, it is most preferable that the entire surface of the nickel particles is thinly and uniformly modified (coated) with sulfur. However, as long as the effect of suppressing the decomposition of the binder resin can be exerted, even if the entire surface of the nickel particles is not modified (coated), a modified (coated) state in which a part of the surface is modified (coated) may be sufficient. In the present embodiment, the concept including such overall modification (coating) and partial modification (coating) of nickel particles is referred to as "surface treatment". In addition, in the method for producing nickel powder using the wet method of the present embodiment, since the reaction final solution of crystallization contains an excessive amount of alkali hydroxide and exhibits strong alkalinity, even if the pH of the solution slightly decreases due to the surface treatment with sulfur as described above, the pH of the nickel powder slurry or its diluted solution does not become 7.0 to pH 9.0, and this surface treatment does not correspond to neutralization as referred to in the present embodiment.

[0039] When the sulfur content in the nickel powder exceeds 0.5% by mass, internal electrode defects caused by sulfur may occur. Preferably, it is 0.3% by mass or less, and more preferably 0.2% by mass or less. The lower limit of the sulfur content is not particularly limited, and it may be an extremely small content that results in a measurement result below the detection limit by an analytical instrument used for the analysis of the content, for example, a sulfur analyzer by combustion method or an ICP analyzer.

[0040] <2. Method for Producing Nickel Powder Using Wet Method> Next, a method for producing nickel powder using a wet method according to an embodiment of the present embodiment will be described. The method for producing nickel powder using a wet method according to an embodiment of the present embodiment is carried out in a strongly alkaline reaction solution obtained by mixing at least a water-soluble nickel salt, a salt of a metal nobler than nickel, a reducing agent (e.g., hydrazine), an alkali hydroxide as a pH adjuster, and water. In this solution, nickel is crystallized by a reduction reaction with hydrazine or the like to obtain a strongly alkaline nickel powder slurry, which is a reaction end solution containing nickel crystallized powder. The method includes a crystallization step of obtaining the slurry, a washing and filtration step of separating the nickel crystallized powder while washing the nickel powder slurry to obtain a nickel powder cake, and a drying step of drying the nickel powder cake to obtain nickel crystallized powder (nickel powder). Further, if necessary, an amine compound or a sulfur-containing compound may be added to the reaction solution to act as a self-decomposition inhibitor (amine compound, sulfur-containing compound) of hydrazine and an accelerator (complexing agent) (amine compound) for the reduction reaction. Further, a comminution step, which may be performed as needed, may be added as a post-treatment step.

[0041] Optionally, a very small amount of a sulfur compound may be added to the strongly alkaline nickel powder slurry, which is the reaction end solution containing nickel crystallized powder, or its dilution or washing solution, so that the sulfur component modifies the surface of the nickel particles to a content of 0.5% by mass or less in the nickel powder (sulfur coating treatment). Further, the obtained nickel powder can be heat-treated, for example, at about 200°C to 300°C in an inert atmosphere or a reducing atmosphere to obtain nickel powder. This sulfur coating treatment and heat treatment can control the binder removal behavior in the internal electrodes during the production of the multilayer ceramic capacitor and the sintering behavior of the nickel powder, and thus are very effective when used within an appropriate range.

[0042] Furthermore, if necessary, a comminution step (post-treatment step) of subjecting the nickel crystallized powder to a comminution treatment is preferably added to obtain nickel powder with reduced coarse particles or the like caused by the connection of nickel particles generated during the nickel particle generation process in the crystallization step.

[0043] By performing such a crystallization step, a washing / filtration step, a drying step, and, if necessary, a crushing step, nickel powder having a substantially spherical particle shape and an average particle diameter of 0.03 μm to 0.4 μm can be obtained.

[0044] (2-1. Crystallization step) In the crystallization step, nickel salt (precisely, nickel ions or nickel complex ions) can be reduced in a reaction solution in which at least a water-soluble nickel salt, a salt of a metal nobler than nickel, a reducing agent, an alkali hydroxide, and water are mixed, by a reduction reaction using a reducing agent such as hydrazine. In this embodiment, when hydrazine is used, an amine compound or a sulfur-containing compound may be mixed with this reaction solution as necessary, and in the presence of the amine compound or the sulfur-containing compound, the nickel salt can be reduced while suppressing the decomposition of hydrazine as a reducing agent.

[0045] (2-1-1. Chemicals used in the crystallization step) In the crystallization step of this embodiment, a reaction solution containing various chemicals such as a nickel salt, a 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. Water as a solvent is preferably of high purity such as ultrapure water (conductivity: ≦0.06 μS / cm (micro Siemens per centimeter)) or pure water (conductivity: ≦1 μS / cm) from the viewpoint of reducing the amount of impurities in the obtained nickel powder, and among them, it is preferable to use pure water which is inexpensive and easily available. Hereinafter, each of the above-mentioned various chemicals will be described in detail.

[0046] (a) Nickel salt The nickel salt used in this embodiment 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.

[0047] (b) Salt of a metal nobler than nickel Salts of metals more noble than nickel are reduced prior to nickel when nickel is reduced and deposited because they have a lower ionization tendency than nickel. 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 deposited, acting as a nucleating agent that forms initial nuclei. As a result, in the nickel crystallization powder (nickel powder) obtained by the growth of these initial nuclei into particles, it becomes possible to easily control the particle size and make it finer.

[0048] As the salt of a metal more noble than nickel, any metal salt that is water-soluble and has a lower ionization tendency than nickel may be used. For example, 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 can be mentioned. For example, copper sulfate can be used as the water-soluble copper salt, silver nitrate can be used as the water-soluble silver salt, and sodium palladium(II) chloride, ammonium palladium(II) chloride, palladium(II) nitrate, palladium(II) sulfate, etc. can be used as the water-soluble palladium salt, but it is not limited to these.

[0049] When using the above-mentioned palladium salt in particular as the salt of a metal more noble than nickel, although the particle size distribution becomes somewhat wider, it is preferable because it becomes possible to more finely control the particle size of the obtained nickel powder. The ratio [mol ppm] of the palladium salt to nickel (number of moles of palladium salt / number of moles of nickel × 10 6 ) can be appropriately selected according to the target number average particle size of the nickel powder. For example, if the average particle size of the nickel powder is set to 0.03 μm to 0.4 μm, the ratio of the palladium salt to nickel is preferably in the range of 0.2 mol ppm to 100 mol ppm, more preferably in the range of 0.5 mol ppm to 25 mol ppm. If the above ratio is less than 0.2 mol ppm, the average particle size of the obtained 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 the nickel powder.

[0050] (c) Reducing agent The reducing agent used in the crystallization step of this embodiment is not particularly limited, and examples thereof include hydrazine (N2H4, molecular weight: 32.05). Note that in addition to anhydrous hydrazine, hydrazine hydrate, hydrated hydrazine (N2H4·H2O, molecular weight: 50.06), may be used, and either one can be used. The reduction reaction of hydrazine is as shown in formula (2) described later. In particular, due to its high reducing power in an alkaline environment, the by-products of the reduction reaction being nitrogen gas and water, no impurity components are generated in the reaction solution due to the reduction reaction, the impurities in hydrazine being few in the first place, and its easy availability, hydrazine has the characteristics described above. Therefore, hydrazine is suitable as a reducing agent, and for example, commercially available industrial-grade 60% by mass hydrated hydrazine can be used.

[0051] (d) Alkali hydroxide As shown in formula (2) described later, the reducing power of hydrazine increases as the alkalinity of the reaction solution becomes stronger. Therefore, in this embodiment, in the crystallization step, alkali hydroxide can be used as a pH adjuster to increase the alkalinity. The alkali hydroxide is not particularly limited, but from the viewpoints of easy availability and cost, it is preferable to use an alkali metal hydroxide. Specifically, it is more preferable to use one or more selected from sodium hydroxide (NaOH) and potassium hydroxide (KOH).

[0052] The blending amount of the alkali hydroxide is determined such that the pH of the reaction solution is 9.5 or higher, preferably 10 or higher, and more preferably 10.5 or higher at the reaction temperature so that the reducing power of hydrazine as a reducing agent is sufficiently increased. When comparing, for example, around 25°C and 70°C, the pH of the reaction solution is slightly lower at the higher temperature of 70°C.

[0053] (e) Amine compound Since the amine compound has the functions of a self-decomposition inhibitor of hydrazine, a reduction reaction accelerator, and further a linking inhibitor between nickel particles as described above, it may be added to the reaction solution as necessary. The above amine compound is a compound containing two or more functional groups selected from primary amino groups (-NH2) or secondary amino groups (-NH-) in the molecule. For example, at least one of alkylene amines or alkylene amine derivatives 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 2 carbon atoms.

[0054]

Chemical formula

[0055] More specifically, as the alkylene amine, one or more selected from ethylenediamine (H2NC2H4NH2), diethylenetriamine (H2NC2H4NHC2H4NH2), triethylenetetramine (H2N(C2H4NH)2C2H4NH2), tetraethylenepentamine (H2N(C2H4NH)3C2H4NH2), pentaethylenehexamine (H2N(C2H4NH)4C2H4NH2), propylenediamine (CH3CH(NH2)CH2NH2) can be used. Further, as the alkylene amine derivative, tris(2-aminoethyl)amine (N(C2H4NH2)3), N-(2-aminoethyl)ethanolamine (H2NC2H4NHC2H4OH), N-(2-aminoethyl)propanolamine (H2NC2H4NHC3H6OH), 2,3-diaminopropionic acid (H2NCH2CH(NH)COOH), 1,2-cyclohexanediamine (H2NC6H 10NH2), ethylenediamine-N,N'-diacetic acid (alternative name: ethylene-N,N'-diglycine, HOOCCH2NHC2H4NHCH2COOH), N,N'-diacetyl ethylenediamine (CH3CONHC2H4NHCOCH3), N,N'-dimethyl ethylenediamine (CH3NHC2H4NHCH3), N,N'-diethyl ethylenediamine (C2H5NHC2H4NHC2H5), N,N'-diisopropyl ethylenediamine (CH3(CH3)CHNHC2H4NHCH(CH3)CH3), 1,2-cyclohexanediamine (H2NC6H 10 One or more selected from the following can be used. These alkyleneamines and alkyleneamine derivatives are water-soluble. Among them, ethylenediamine and diethylenetriamine are preferred because they are easily available and inexpensive.

[0056] The action as a reducing reaction accelerator of the above amine compound is considered to be due to its function as a complexing agent that complexes nickel ions (Ni 2+ ) in the reaction solution to form nickel complex ions. Also, regarding the action as a self-decomposition inhibitor of hydrazine and an inhibitor for suppressing the connection between nickel particles, it is presumed that the action is manifested by the interaction between the primary amino group (-NH2) or secondary amino group (-NH-) in the amine compound molecule and the surface of the nickel crystallization powder in the reaction solution.

[0057] 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 having 2 carbon atoms. This is because the effect of suppressing the decomposition of hydrazine molecules becomes greater. For example, if the nitrogen atom of the amino group that strongly adsorbs to the nickel crystallization powder is bonded via a carbon chain having 3 or more carbon atoms, it is considered that the degree of freedom of movement (molecular flexibility) of the carbon chain portion of the amine compound molecule increases as the carbon chain becomes longer. As a result, the contact of hydrazine molecules with the nickel crystallization powder cannot be effectively blocked, and the number of hydrazine molecules that self-decompose due to the catalytic activity of nickel increases, which is considered to reduce the inhibitory effect of hydrazine self-decomposition.

[0058] Actually, compared with ethylenediamine (H2NC2H4NH2) in which the nitrogen atom of the amino group in the molecule is bonded via a carbon chain with 2 carbon atoms, or propylenediamine (also known as 1,2-diaminopropane, 1,2-propanediamine) (CH3CH(NH2)CH2NH2), trimethylenediamine (also known as 1,3-diaminopropane, 1,3-propanediamine) (H2NC3H6NH2) in which the nitrogen atom of the amino group in the molecule is bonded via a carbon chain with 3 carbon atoms has been confirmed to have inferior self-decomposition inhibitory effect on hydrazine.

[0059] Here, the ratio [mol%] of the above amine compound to nickel in the reaction solution ((number of moles of amine compound / number of moles of nickel) × 100) is preferably in the range of 0.01 mol% to 5 mol%, more preferably in the range of 0.03 mol% to 2 mol%. If the above ratio is less than 0.01 mol%, the amount of the above amine compound is too small, and each effect as a self-decomposition inhibitor of hydrazine, a reduction reaction accelerator, or a linking inhibitor between nickel particles may not be obtained. On the other hand, if the above ratio exceeds 5 mol%, the function of the amine compound as a complexing agent for forming nickel complex ions becomes too strong, and as a result, abnormalities may occur in the particle growth of nickel crystallization powder, the granularity and sphericity of the nickel powder may be lost and become irregular shapes, or many coarse particles in which nickel particles are linked to each other may be formed, etc., and there is a risk of deterioration of the properties of the nickel powder.

[0060] (f) Sulfur-containing compound (auxiliary agent for inhibiting self-decomposition of hydrazine) A sulfur-containing compound is a compound applied to a brightener for nickel plating or a stabilizer for a plating bath. Different from the above amine compound, when used alone, the self-decomposition inhibitory effect on hydrazine is not so great. However, it has an interaction such as adsorption with the surface of nickel particles and has the effect of an auxiliary agent for inhibiting the self-decomposition of hydrazine, which can greatly enhance the self-decomposition inhibitory effect of hydrazine when used in combination with the above amine compound. Therefore, it may be added to the reaction solution as necessary. And the above sulfur-containing compound is a compound containing 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, in addition to the effect of an auxiliary agent for inhibiting the self-decomposition of hydrazine, the above sulfur-containing compound also has the effect as an inhibitor for suppressing the connection between nickel particles, and when used in combination with the above amine compound, it can also more effectively reduce the amount of generation of coarse particles in which nickel particles are connected to each other.

[0061] As sulfur-containing compounds, for example, in the case of sulfide compounds having a sulfide group (-S-) in the molecule, it is desirable that the water solubility is high. Therefore, it is preferably any one of a carboxy group-containing sulfide compound, a hydroxyl group-containing sulfide compound, and an amino group-containing sulfide compound that further contains at least one or more of a carboxy group (-COOH), a hydroxyl group (-OH), and an amino group (primary: -NH2, secondary: -NH-, tertiary: -N<) in the molecule. Also, a thiazole ring-containing sulfide compound containing at least one or more thiazole rings (C3H3NS) has low water solubility but is applicable. More specifically, L (or D, or DL)-methionine (CH3SC2H4CH(NH2)COOH), L (or D, or DL)-ethionine (C2H5SC2H4CH(NH2)COOH), N-acetyl-L (or D, or DL)-methionine (CH3SC2H4CH(NH(COCH3))COOH), lanthionine (alternative name: 3,3'-thiodialanine) (HOOCCH(NH2)CH2SCH2CH(NH2)COOH), thiodipropionic acid (alternative name: 3,3'-thiodipropionic acid) (HOOCC2H4SC2H4COOH), thioglycolic acid (alternative name: 2,2'-thiodiglycolic acid, 2,2'-thiodiacetic acid, 2,2'-thiobisacetic acid, mercaptodiacetic acid) (HOOCCH2SCH2COOH), methionol (alternative name: 3-methylthio-1-propanol) (CH3SC3H6OH), thiodiglycol (alternative name: 2,2'-thiodiethanol) (HOC2H5SC2H5OH), thiomorpholine (C4H9NS), thiazole (C3H3NS), benzothiazole (C7H5NS), and one or more selected therefrom are preferable. Among these, methionine and thioglycolic acid are preferable because they are excellent in the auxiliary action of suppressing the self-decomposition of hydrazine and are easily available and inexpensive.

[0062] As sulfur-containing compounds other than sulfide compounds, more specifically, saccharin (alternative name: o-benzoic acid sulfimide, o-sulfobenzoimide) (C7H5NO3S), sodium dodecyl sulfate (C 12 H 25 OS(O)2ONa), dodecylbenzenesulfonic acid (C12 H 25 C6H4S(O)2OH), sodium dodecylbenzenesulfonate (C 12 H 25 C6H4S(O)2ONa), bis(2-ethylhexyl) sodium sulfosuccinate (other names: di-2-ethylhexyl sodium sulfosuccinate, dioctyl sodium sulfosuccinate) (NaOS(O)2CH(COOCH2CH(C2H5)C4H9)CH2(COOCH2CH(C2H5)C4H9)), thiourea (H2NC(S)NH2), and one or more selected therefrom are preferred. These sulfur-containing compounds are water-soluble. Among them, saccharin and thiourea are preferred because they are excellent in the auxiliary action of suppressing the self-decomposition of hydrazine and are easily available and inexpensive.

[0063] Regarding the action of the sulfur-containing compound as an auxiliary agent for suppressing the self-decomposition of hydrazine and as an inhibitor for suppressing the connection between nickel particles, it can be speculated as follows. That is, the sulfur-containing compound has a sulfide group (-S-), a sulfonyl group (-S(=O)2-), a sulfonic acid group (-S(=O)2-O-), and a thioketone group (-C(=S)-) in the molecule, which are adsorbed on the nickel surface of the nickel particles by intermolecular forces. However, by itself, it does not have a great effect of covering and protecting the nickel crystallization powder like the amine compound molecules described above. On the other hand, when an amine compound and a sulfur-containing compound are used in combination, when the amine compound molecules are strongly adsorbed on the surface of the nickel crystallization powder to cover and protect it, there is a high possibility that tiny regions that cannot be completely covered by the amine compound molecules will occur. However, the sulfur-containing compound molecules adsorb and supplementarily cover that part, so that the contact between the hydrazine molecules and the nickel crystallization powder in the reaction solution can be more effectively prevented. Furthermore, the combination of the nickel crystallization powders can be more strongly prevented, and the above-mentioned action is manifested.

[0064] Here, the ratio [mol%] of the sulfur-containing compound to nickel in the reaction solution ((number of moles of sulfur-containing compound / number of moles of nickel) × 100) is in the range of 0.01 mol% to 5 mol%, preferably 0.03 mol% to 2 mol%, more preferably 0.05 mol% to 1 mol%. If the above ratio is less than 0.01 mol%, the sulfur-containing compound is too little, and there is a risk that each function of the self-decomposition inhibitor of hydrazine and the inhibitor of the connection between nickel particles cannot be obtained. On the other hand, even if the above ratio exceeds 5 mol%, no improvement in the above functions is observed. Therefore, only the amount of the sulfur-containing compound used increases, the chemical cost increases, and at the same time, the amount of the organic component in the reaction solution increases, resulting in an increase in the chemical oxygen demand (COD) of the reaction waste liquid in the crystallization process, thus increasing the waste liquid treatment cost.

[0065] (g) Other inclusions In the reaction solution for the crystallization process, in addition to the above-mentioned nickel salt, salts of metals nobler than nickel, reducing agents (e.g., hydrazine), alkali hydroxides, and amine compounds, various additives such as a dispersant, a complexing agent, and an antifoaming agent may be contained in a small amount. For example, if an appropriate dispersant or complexing agent is used in an appropriate amount, it may be possible to improve the granularity (sphericity) and surface smoothness of the nickel crystallization powder or reduce coarse particles. Also, if an appropriate antifoaming agent is used in an appropriate amount, by suppressing the foaming in the crystallization process caused by the nitrogen gas generated in the crystallization reaction (see the following formulas (2) to (4)), it is possible to prevent, for example, the aqueous solution from overflowing from the container. As the dispersant, known substances can be used, and examples include alanine (CH3CH(COOH)NH2), glycine (H2NCH2COOH), triethanolamine (N(C2H4OH)3), diethanolamine (also known as iminodiethanol) (NH(C2H4OH)2), etc. Also, as the complexing agent, known substances can be used, and examples include hydroxycarboxylic acids, carboxylic acids (organic acids containing at least one carboxyl group), hydroxycarboxylate salts and hydroxycarboxylic acid derivatives, carboxylate salts and carboxylic acid derivatives. Specifically, tartaric acid, citric acid, malic acid, ascorbic acid, formic acid, acetic acid, pyruvic acid, and their salts and derivatives, etc. can be mentioned. Furthermore, as the antifoaming agent, as long as it has excellent defoaming properties under alkaline conditions, it is not particularly limited, and oil-type or solvent-type silicone-based or non-silicone-based antifoaming agents can be used.

[0066] (2-1-2. Crystallization Procedure) In the crystallization process, at least a nickel salt solution obtained by dissolving a water-soluble nickel salt and a salt of a metal nobler than nickel in water, a reducing agent solution obtained by dissolving a reducing agent (e.g., hydrazine) in water, and an alkali hydroxide solution obtained by dissolving an alkali hydroxide in water are prepared, and these are added and mixed to prepare a reaction solution. Then, a crystallization reaction is carried out in which nickel particles are crystallized in this reaction solution by a reduction reaction to obtain nickel crystallized powder. In addition, an amine compound or a sulfur-containing compound added as necessary can be added and mixed to any one of the above solutions or a mixture thereof before preparing the reaction solution, or can be added and mixed to the reaction solution after preparing the reaction solution. Note that at room temperature, the reduction reaction starts when the reaction solution is prepared.

[0067] Here, as specific crystallization procedures, there are two types: a procedure in which a reducing agent·alkali hydroxide solution containing a reducing agent (e.g., hydrazine) and an alkali hydroxide, which is obtained by previously mixing a reducing agent solution and an alkali hydroxide solution, is added and mixed to a nickel salt solution containing a nickel salt as a substance to be reduced and a salt of a metal nobler than nickel to prepare a reaction solution; and a procedure in which an alkali hydroxide solution is added and mixed to a nickel salt·reducing agent solution obtained by adding and mixing a reducing agent solution (e.g., hydrazine solution) to the above nickel salt solution to prepare a reaction solution. The former is to add and mix a reducing agent (e.g., hydrazine) with a high alkalinity and enhanced reducing power due to an alkali hydroxide to a nickel salt solution containing a substance to be reduced, while the latter is different in that a reducing agent (e.g., hydrazine) is previously mixed with a nickel salt solution containing a substance to be reduced, and then the pH is adjusted (raised) by an alkali hydroxide to enhance the reducing power.

[0068] In the former case (when adding and mixing a nickel salt solution, a reducing agent, and an alkaline hydroxide solution), depending on the temperature at the time when the reaction solution is prepared, that is, the temperature at the start of the reduction reaction (hereinafter sometimes referred to as the reaction start temperature), although it also depends on the time required for adding and mixing the reducing agent - alkaline hydroxide solution with increased alkalinity and enhanced reducing power by the nickel salt solution (a solution containing a nickel salt and a salt of a metal nobler than nickel) and the alkaline hydroxide (hereinafter sometimes referred to as the raw material mixing time), when this time becomes longer, from the middle stage of the addition and mixing, the alkalinity rises locally in the addition and mixing region of the nickel salt solution and the reducing agent - alkaline hydroxide solution, the reducing power of hydrazine increases, and nucleation occurs due to the salt of a metal nobler than nickel, which is the nucleating agent. Therefore, as the end of the raw material mixing time approaches, the nucleation time dependence of the nucleating agent added becomes larger, that is, the nucleation effect weakens, and there is a tendency that it becomes difficult to refine the nickel crystallization powder or obtain a narrow particle size distribution. This tendency is more prominent when adding and mixing an alkaline reducing agent - alkaline hydroxide solution to a weakly acidic nickel salt solution. Since the above tendency can be suppressed as the mixing time of the raw materials is shorter, short-time mixing is desirable. However, considering restrictions on mass production equipment, etc., the mixing time of the raw materials is preferably 10 seconds to 180 seconds, more preferably 20 seconds to 120 seconds, and even more preferably 30 seconds to 80 seconds.

[0069] On the other hand, in the latter case (when adding and mixing an alkaline hydroxide solution to a nickel salt - reducing agent solution obtained by adding and mixing a nickel salt solution and a reducing agent solution), in the nickel salt - reducing agent solution containing a nickel salt, a salt of a metal nobler than nickel, and a reducing agent, hydrazine, the reducing agent, is pre-added and mixed to have a uniform concentration. Therefore, the raw material mixing time dependence of the nucleation occurring when adding and mixing the alkaline hydroxide solution does not increase as much as in the former case, and it has the characteristic that it is easy to refine the nickel crystallization powder or obtain a narrow particle size distribution. However, for the same reason as in the former case, although a short mixing time of the alkaline hydroxide solution is desirable, considering restrictions on mass production equipment, etc., such a 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.

[0070] Regarding the addition and mixing of the amine compound and sulfur-containing compound of the present embodiment as well, as described above, there are two types of procedures: a procedure in which the amine compound and sulfur-containing compound are premixed in the reaction solution before the reaction solution is prepared, and a procedure in which the reaction solution is prepared and then added and mixed after the start of the reduction reaction.

[0071] In the former case (when the amine compound and sulfur-containing compound are premixed in the reaction solution before the reaction solution is prepared), since the amine compound and sulfur-containing compound are premixed in the reaction solution, there is an advantage that various actions of the amine compound and sulfur-containing compound are exhibited from the start point of nucleation caused by a salt of a metal more noble than nickel (nucleating agent). On the other hand, the interaction between the nickel particle surface and the amine compound and sulfur-containing compound, such as adsorption, may be involved in nucleation, which may affect the particle size and particle size distribution of the obtained nickel crystallized powder.

[0072] Conversely, in the latter case (when the reaction solution is prepared and an amine compound or a sulfur-containing compound is added and mixed after the start of the reduction reaction), after passing through the very initial stage of the crystallization step where nucleation due to the nucleating agent occurs, the amine compound or the sulfur-containing compound is added and mixed into the reaction solution. Therefore, although the actions of the amine compound and the sulfur-containing compound described above are somewhat delayed, since their involvement in nucleation is eliminated, the particle size and particle size distribution of the obtained nickel crystallization powder are less likely to be affected by the amine compound and the sulfur-containing compound, and there is an advantage that they are easier to control. Here, the mixing time for adding and mixing the amine compound and the sulfur-containing compound into the reaction solution in this procedure may be added all at once within several seconds, or may be added in portions or dropwise over several minutes to about 30 minutes. As described above, the amine compound has an action as a reduction reaction accelerator (complexing agent). Therefore, adding it slowly allows the crystal growth to proceed slowly and the nickel crystallization powder to become highly crystalline, but the self-decomposition inhibition of hydrazine also gradually takes effect, and the effect of reducing the hydrazine consumption decreases. Therefore, the above mixing time may be appropriately determined while considering the balance between these two. Regarding the timing of adding and mixing the amine compound and the sulfur-containing compound in the former procedure, it can be appropriately selected by comprehensively judging according to the purpose.

[0073] For the addition and mixing of the nickel salt solution and the reducing agent / alkali hydroxide solution, the addition and mixing of the nickel salt solution and the reducing agent solution, and the addition and mixing of the alkali hydroxide solution to the nickel salt / reducing agent solution, stirring and mixing while stirring the solution is preferred. When the stirring and mixing property is good, the frequency of non-uniform nucleation depending on the location of nucleation decreases, and the nucleation raw material mixing time dependence and alkali hydroxide mixing time dependence as described above decrease. Therefore, it becomes easy to refine the nickel crystallization powder or obtain a narrow particle size distribution. As the method of stirring and mixing, a known method may be used, and it is preferable to use a stirring blade from the viewpoints of controlling the stirring and mixing property and equipment cost.

[0074] (2-1-3. Reduction Reaction) In the crystallization process, nickel crystal powder is obtained by reducing a nickel salt with hydrazine in a reaction solution in the co - existence of an alkali hydroxide and a salt of a metal more precious than nickel. Further, if necessary, by the action of a very small amount of a specific amine compound or a sulfur - containing compound, the self - decomposition of hydrazine can be significantly suppressed, and the reduction reaction can be carried out.

[0075] First, the reduction reaction in the crystallization process will be described. When nickel ions (Ni 2+ ) are crystallized to become nickel (Ni), the reaction is a 2 - electron reaction of the following formula (1). Also, the reaction of hydrazine (N2H4) is a 4 - electron reaction of the following formula (2). For example, as described above, when nickel chloride (NiCl2) is used as the nickel salt and sodium hydroxide (NaOH) is used as the alkali hydroxide, the entire reduction reaction is represented by the reaction in which nickel hydroxide (Ni(OH)2) generated by the neutralization reaction of nickel chloride and sodium hydroxide is reduced by hydrazine as shown in the following formula (3). Stoichiometrically (as a theoretical value), 0.5 mol of hydrazine (N2H4) is required for 1 mol of nickel (Ni).

[0076] 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 alkali hydroxide is used as a pH adjuster to increase the alkalinity of the reaction solution and plays a role in promoting the reduction reaction of hydrazine.

[0077] [Chemical formula 2] Ni 2+ +2e - →Ni↓ (2 - electron reaction) ···(1) N2H4→N2↑+4H + +4e - (4 - electron reaction) ···(2) 2NiCl2+N2H4+4NaOH →2Ni(OH)2+N2H4+4NaCl →2Ni↓+N2↑+4NaCl+4H2O ···(3)

[0078] As described above, in the conventional crystallization process, the active surface of the nickel crystallized powder serves as a catalyst, promoting the self-decomposition reaction of hydrazine represented by the following formula (4), and a large amount of hydrazine as a reducing agent may be consumed other than for reduction. Therefore, depending on the crystallization conditions such as the reaction start temperature, for example, about 2 moles of hydrazine per 1 mole of nickel, which is about four times the theoretical value required for the above reduction, has generally been used. Further, 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 solution at a high concentration, resulting in a nitrogen-containing waste liquid. Thus, the excessive use of expensive hydrazine and the treatment cost of nitrogen-containing waste liquid are factors that increase the production cost of nickel powder (wet nickel powder) by the wet method.

[0079] [Chemical formula 3] 3N2H4→N2↑+4NH3···(4)

[0080] Therefore, in the method for producing nickel powder of the present embodiment, it is preferable to add a very small amount of a specific amine compound or sulfur-containing compound to the reaction solution to significantly suppress the self-decomposition reaction of hydrazine and greatly reduce the amount of expensive hydrazine used as a chemical. The reason why the above specific amine compound can suppress the self-decomposition of hydrazine is that (I) the molecules of the above specific amine compound or sulfur-containing compound are adsorbed on the surface of the nickel crystallized powder in the reaction solution, physically interfering with the contact between the active surface of the nickel crystallized powder and hydrazine molecules, and (II) the molecules of the specific amine compound or sulfur-containing compound act on the surface of the nickel crystallized powder, inactivating the catalytic activity of the surface, etc. are conceivable.

[0081] In addition, in the crystallization process by the wet method conventionally, in order to shorten the reduction reaction time (crystallization reaction time) to a practical range, nickel ions such as tartaric acid and citric acid (Ni 2+) It is common to use a complexing agent that forms a complex ion with [ion] to increase the ionic nickel concentration as a reduction reaction accelerator. However, these complexing agents such as tartaric acid and citric acid do not have the function of a hydrazine self-decomposition inhibitor like the above-specified amine compound or sulfur-containing compound, or the function as a linkage inhibitor that makes it difficult to form coarse particles generated by the linkage of nickel particles during crystallization.

[0082] On the other hand, the above-specified amine compound also functions as a complexing agent like tartaric acid and citric acid, and has the advantage of having the functions of a hydrazine self-decomposition inhibitor, a linkage inhibitor, and a reduction reaction accelerator.

[0083] (2-1-4. Reaction start temperature) The crystallization reaction in the crystallization step starts, for example, in a reaction solution obtained by adding and mixing a solution containing at least a water-soluble nickel salt and a salt of a metal nobler than nickel (nickel salt solution) with a solution containing a reducing agent (e.g., hydrazine) and an alkali hydroxide (reducing agent·alkali hydroxide solution). In this case, the reaction start 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. Note that the temperature of each of the nickel salt solution and the reducing agent·alkali hydroxide solution is not particularly restricted as long as the temperature of the mixed solution obtained by pre-mixing them, that is, the reaction start temperature, is within the above temperature range, and can be freely set.

[0084] The higher the reaction start temperature, the more the reduction reaction is promoted, and the nickel crystallization powder tends to be highly crystallized. On the other hand, since there is an aspect that the self-decomposition reaction of hydrazine is further promoted, the consumption of hydrazine increases, and the foaming of the reaction solution tends to become intense. Therefore, if the reaction start 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 start temperature becomes too low, the crystallinity of the nickel crystallization powder will be significantly reduced, or the reduction reaction will be slow, resulting in a significant extension of the crystallization process time and a tendency for the productivity of nickel powder to decrease. For the above reasons, by setting the temperature range as described above, it is possible to manufacture high-performance nickel powder at low cost while suppressing the consumption of hydrazine and maintaining high productivity.

[0085] (2-2. Washing and Filtration Process) In the method for producing nickel powder using the wet method as described above, since hydrazine is often used as a reducing agent, as is clear from Patent Document 4 etc., the reaction solution generally becomes strongly alkaline (for example, about pH 14). As shown in FIG. 1, in the crystallization process, a nickel powder slurry in which nickel crystallization powder is generated in this strongly alkaline reaction solution is obtained by the reduction reaction of hydrazine. A washing and filtration process in which the nickel crystallization powder is washed with pure water and filtered and recovered to obtain a nickel powder cake, and a drying process in which this nickel powder cake is dried (such as vacuum drying) to obtain nickel crystallization powder (nickel powder) are carried out subsequent to the crystallization process.

[0086] Since the above washing and filtration process is usually carried out in the air, the obtained nickel powder cake will inevitably be exposed to the air to some extent during the washing and filtration process or until the drying in the drying process is completed, resulting in oxidation of the nickel crystallization powder. In particular, when the crystallization reaction is carried out on a mass production scale, different from the handling of a small amount at the laboratory level, since it takes time to handle the nickel powder cake, the oxidation of the nickel crystallization powder due to the above air exposure tends to progress.

[0087] In the nickel powder cake obtained through a washing and filtration process in which strongly alkaline nickel powder slurry (for example, with a pH of about 14) is washed with pure water and separated by filtration while being washed, although the washing and removal of alkaline hydroxide showing strong alkalinity is attempted, a trace amount of alkaline hydroxide may remain and stay in the nickel powder cake as weakly alkaline adhering liquid (for example, with a pH of about 11). Therefore, when oxidation of the nickel crystallization powder occurs in this state, nickel ions (Ni 2+ ) formed by oxidation are likely to form nickel hydroxide (Ni(OH)2) according to formula (5). At locations where the amount of formed nickel hydroxide is large, as a result, coarse particles 10 (coarse particles mainly composed of nickel hydroxide) (in many cases, with a particle size of 0.8 μm or more, see Figure 2) in which nickel crystallization powders 1 (with a particle size of 0.4 μm or less) are firmly solidified with nickel hydroxide 2 are likely to occur.

[0088] [Chemical Formula 5] Ni 2+ +2OH - →Ni(OH)2 ···(5)

[0089] Therefore, in this embodiment, from after the separation by filtration in the washing and filtration process until the drying of the nickel powder is started in the drying process described later, the temperature of the nickel powder cake is maintained in the range of 0 to 20°C. More preferably, the temperature of the nickel powder cake is maintained in the range of 5 to 20°C. By maintaining the temperature of the nickel powder cake in the range of 0 to 20°C from after the separation by filtration until the start of drying, since the temperature of the nickel powder cake is low, oxidation of the nickel powder hardly progresses, and as a result, nickel hydroxide (Ni(OH)2) hardly grows. If the temperature of the nickel powder cake is less than 0°C, there is a risk that the nickel powder cake will freeze and become difficult to handle. Also, if the temperature of the nickel powder cake exceeds 20°C, the coarse particles in the nickel powder increase.

[0090] Furthermore, it is desirable that the water content of the nickel powder cake be less than 30% by mass from the time of filtration until the start of drying, and more desirably 5% by mass or more and less than 30% by mass. When the water content of the nickel powder cake is less than 30% by mass, the drying time in the subsequent drying process can be shortened. Also, in order to reduce the water content of the nickel powder cake to less than 5% by mass, the filtration time becomes long, and even if it is less than 5% by mass, the drying time is not significantly shortened.

[0091] Also, it is preferable that the temperature of the pure water used for washing the nickel crystallized powder in the washing and filtration process is 0°C or higher and 20°C or lower. Using pure water at 0°C or higher and 20°C or lower, a step of washing the nickel crystallized powder with pure water is performed so that the conductivity of the liquid containing the nickel crystallized powder is 30 μS / cm to 1000 μS / cm, preferably 30 μS / cm to 500 μS / cm, and more preferably 30 μS / cm to 100 μS / cm (washing step). Thereafter, the nickel crystallized powder is filtered off from the liquid to obtain a nickel powder cake containing the adhering liquid (filtration step). By going through these steps, the residual amount of impurities (such as Na, Cl, etc.) caused by the chemicals used in the crystallization process can be significantly reduced. At the same time, the alkalinity of the adhering liquid of the nickel powder cake is weakened, making it difficult to form nickel hydroxide (Ni(OH)2), and the generation of coarse particles mainly composed of nickel hydroxide as described above can be significantly suppressed.

[0092] In particular, for example, after obtaining a liquid containing nickel crystallized powder with the reaction start temperature of the crystallization reaction being 40°C to 95°C, the liquid containing the nickel crystallized powder is washed with pure water at 0°C or higher and 20°C or lower until the conductivity of the liquid containing the nickel crystallized powder reaches 30 μS / cm to 1000 μS / cm, and the temperature of the nickel powder cake is 0°C or higher and 20°C or lower. Therefore, the temperature of the nickel powder can be maintained at a low level, making it difficult for the oxidation of the nickel powder to proceed and difficult to form nickel hydroxide (Ni(OH)2).

[0093] In addition, in this embodiment, before the cleaning process of the cleaning and filtration process, a step of neutralizing a strongly alkaline nickel powder slurry or its diluted solution to a pH of 7.0 to 9.0 with an inorganic acid or an organic acid (neutralization step) can be performed. After the neutralization step, it is also possible to wash with pure water at 0°C or higher and 20°C or lower until the conductivity of the liquid containing nickel crystallized powder becomes 30 μS / cm to 1000 μS / cm.

[0094] In the neutralization step, the pH when neutralizing the nickel powder slurry or its diluted solution with an inorganic acid or an organic acid is preferably 7.0 to 9.0. When the pH is less than 7.0 in the neutralization treatment, that is, when an excessive amount of inorganic acid or organic acid is added, the generation of coarse particles mainly composed of nickel hydroxide may increase. As the strongly alkaline nickel powder slurry or its diluted solution is neutralized to lower the pH, the generation of coarse particles mainly composed of nickel hydroxide is suppressed, but if the pH is made 9.0 or less by the neutralization treatment, the content of coarse particles mainly composed of nickel hydroxide in the nickel powder can be sufficiently reduced.

[0095] The agent used for neutralizing the nickel powder slurry or its diluted solution may be one or more selected from inorganic acids or organic acids, and there is no particular limitation as long as it is easily soluble in water. Specifically, as the inorganic acid, hydrochloric acid (HCl), nitric acid (HNO3), sulfuric acid (H2SO4), carbonic acid (H2CO3), etc. can be used, and as the organic acid, acetic acid (CH3COOH), citric acid (C(OH)(CH2COOH)2COOH), ascorbic acid (C6H8O6), etc. can be used. These inorganic acids and organic acids may be used for neutralization as they are, but considering the ease of handling, it is more preferable to use them in the form of an aqueous solution. For example, when using carbonic acid, carbon dioxide gas (CO2) may be blown into the liquid (nickel powder slurry or its diluted solution) to cause the neutralization action as carbonic acid in the liquid. Also, if an organic acid that forms a complex ion with nickel, such as citric acid, is used, an effect of preventing the oxidation of nickel crystallized powder can also be expected.

[0096] When the nickel powder slurry is, for example, strongly alkaline at about pH 14, even if it is washed directly with pure water until the conductivity of the washing liquid is reduced to about 100 μS / cm, the pH of the washing liquid can remain strongly alkaline at about 11. However, in the method of neutralization with inorganic acids or organic acids according to this embodiment, when the conductivity of the washing liquid is reduced to about the same 100 μS / cm, the pH of the washing liquid can be reduced to at most about 9 or less.

[0097] Also, inorganic acids or organic acids can be directly mixed and added to a strongly alkaline nickel powder slurry (for example, about pH 14). However, in that case, a large amount of inorganic acids or organic acids is required for neutralization, which may be disadvantageous in terms of cost and workability. Therefore, preferably, the supernatant liquid of the nickel powder slurry is first removed by a method such as decantation to reduce the amount of alkaline components in the nickel powder slurry, and then inorganic acids, organic acids, and their aqueous solutions are mixed and added for neutralization.

[0098] (2-2-1. Washing and Filtration Methods and Procedures) In this embodiment, in the process of washing nickel crystallized powder from a strongly alkaline nickel powder slurry (for example, about pH 14) obtained in the crystallization step using a general washing method, the strongly alkaline nickel powder slurry or its diluted solution is washed with pure water at 0°C or higher and 20°C or lower. Finally, the nickel crystallized powder is recovered by filtration as a nickel powder cake containing adhering liquid and having a temperature of 0°C or higher and 20°C or lower using a general solid-liquid separation device.

[0099] Specific examples of the general washing method include, but are not limited to, repeated decantation and pure water dilution, repeated concentration of nickel crystallized powder (increasing slurry concentration or forming a cake) by a solid-liquid separation device and re-slurrying with pure water (re-slurrying by adding pure water), and through-washing of pure water to the concentrate (cake) of nickel crystallized powder in the solid-liquid separation device. Specific examples of the general solid-liquid separation device include, but are not limited to, a Denver filter, a filter press, a centrifuge, a decanter, etc.

[0100] The pure water used for the above washing is generally preferably of high purity with a conductivity of 1 μS / cm or less. Instead of pure water, distilled water or ultrapure water (conductivity: ≤ 0.06 μS / cm) can also be used, but it is preferable to use pure water that is inexpensive and easily available.

[0101] As a washing procedure for the nickel powder slurry, pure water can be directly mixed and added to the strongly alkaline nickel powder slurry (for example, about pH 14), but in that case, a large amount of pure water is required, which may be disadvantageous in terms of workability. Therefore, preferably, the supernatant of the nickel powder slurry is first removed by a method such as decantation to reduce the amount of alkaline components in the nickel powder slurry, and then pure water is added.

[0102] (2-3. Drying process) The nickel powder cake containing the neutralized adhering liquid obtained in the washing and filtration process can be dried at 50 to 300 °C, preferably 80 to 150 °C, using a general-purpose drying device such as an air dryer, a hot air dryer, an inert gas atmosphere dryer, and a vacuum dryer to obtain nickel crystallized powder (nickel powder). If necessary, after replacing the adhering water in the nickel powder cake with a low-temperature volatile organic solvent such as ethanol, it can be dried with the above-mentioned inert gas atmosphere dryer or vacuum dryer to weaken the drying aggregation between nickel particles caused by the large surface tension of water during drying.

[0103] In addition, when the nickel powder cake is dried at about 200 °C to 300 °C in an inert atmosphere, a reducing atmosphere, or a vacuum atmosphere using a drying device such as an inert gas atmosphere dryer or a vacuum dryer, it is possible to obtain heat-treated nickel powder in addition to simple drying.

[0104] (2-4. Crushing process (post-treatment process)) The nickel crystallized powder (nickel powder) obtained through the crystallization process, washing and filtration process, and drying process, as described above, when amine compounds or sulfur-containing compounds are added as necessary, they act as inhibitors for connecting nickel particles during the crystallization of nickel. Therefore, the content ratio of the coarse particles formed by the connection of nickel particles to each other during the reduction precipitation process (hereinafter, in order to distinguish from the "coarse particles mainly composed of nickel hydroxide", may also be referred to as "connected coarse particles") is not so large in the first place. However, depending on the crystallization procedure and crystallization conditions, the content ratio of the connected coarse particles may become somewhat large and become a problem. In this case, a crushing process can be provided following the crystallization process to divide the connected coarse particles where the nickel particles are connected at the connection part to reduce the connected coarse particles. In the crushing treatment process, dry crushing methods such as spiral jet crushing treatment and counter jet mill crushing treatment, wet crushing methods such as high-pressure fluid collision crushing treatment, and other general-purpose crushing methods can be applied.

[0105] In addition, the coarse particles mainly composed of nickel hydroxide, in which the above-mentioned nickel crystallized powders (particle size of 0.4 μm or less) are firmly solidified with nickel hydroxide, cannot be easily crushed even by using the above-mentioned various general-purpose crushing methods. Therefore, effective countermeasures for suppressing the coarse particles mainly composed of nickel hydroxide are very important, and this embodiment is extremely useful from this perspective as well.

Examples

[0106] Hereinafter, the present invention will be described more specifically using examples, but the present invention is not limited to the following examples. As the characteristics of the nickel powder, the average particle size and the content of the coarse particles are evaluated as follows.

[0107] (Average particle size) The nickel powder obtained in the present invention has a substantially spherical particle shape, and its average particle size is the number average particle size based on the particle size obtained from the image analysis of the observation image (SEM image) using a scanning electron microscope (SEM, manufactured by JEOL Ltd., JSM-7100F) of the nickel powder.

[0108] (Content of coarse particles) 0.05 g of nickel powders of Example 1, Example 2, Comparative Example 1, and Comparative Example 2 obtained by the method shown below were ultrasonically dispersed in 100 mL of an aqueous solution of 0.1 mass% sodium hexametaphosphate, and the obtained nickel powder dispersion was filtered through a membrane filter (pore diameter: 0.8 μm, pore diameter: 1.2 μm) to capture coarse particles having a size larger than the pore diameter (0.8 μm, 1.2 μm) on the filter. The amount of the coarse particles was calculated by ICP emission spectrometry (high-frequency inductively coupled plasma emission spectrometry) of a solution in which the total amount of the coarse particles was dissolved in acid, and the content of the coarse particles contained in the nickel powder (when the particle size exceeds 0.8 μm and when the particle size exceeds 1.2 μm) was determined.

[0109] In principle, the content of the coarse particles includes all cases ((1) to (3)) of coarse particles: (1) Coarse nickel particles having a particle size exceeding 0.8 μm or 1.2 μm alone; (2) Linked coarse particles formed by nickel particles (nickel crystallization powder) having a particle size of 0.4 μm or less being linked on the surface of nickel particles during the crystallization process (reduction precipitation process) to have a size exceeding 0.8 μm or 1.2 μm; (3) Coarse particles mainly composed of nickel hydroxide formed by strong solidification of nickel particles (nickel crystallization powder) having a particle size of 0.4 μm or less due to oxidation, with a size exceeding 0.8 μm or 1.2 μm (see Figure 2).

[0110] However, in the present invention, as described above, since wet nickel powder having a narrow particle size distribution is used, large nickel particles having a particle size exceeding 0.8 μm or 1.2 μm are not generated alone, and the case of (1) above does not need to be considered. Further, the linked coarse particles formed by the linking of nickel particles (nickel crystallization powder) having a particle size of 0.4 μm or less on the surface of the nickel particles during the crystallization process (the process of reduction precipitation) have a content that is not so large in the first place if appropriate crystallization conditions are adopted, and the linking strength between the nickel particles is not so strong either. Therefore, as described above, if a crushing treatment is performed after crystallization, even if the content ratio of the linked coarse particles is somewhat large, the linked coarse particles can be divided at the linking part to make the size of the linked coarse particles 0.8 μm or less, so the case of (2) above does not need to be considered either. Therefore, only the case of (3) is considered as the coarse particles, and the content of the coarse particles having a particle size exceeding 0.8 μm (or a particle size of 1.2 μm) in the present invention may be considered as the content of the coarse particles mainly composed of nickel hydroxide having a particle size exceeding 0.8 μm (or a particle size of 1.2 μm). Coarse particles mainly composed of nickel hydroxide having a size exceeding 0.8 μm or 1.2 μm are coarse particles formed by enclosing and firmly binding a plurality of nickel particles (nickel crystallization powder) by the nickel hydroxide generated on the surface of the nickel crystallization powder. Therefore, even if the above-mentioned crushing treatment is performed, they will not be loosened and will remain in the nickel powder as coarse particles of the original size.

[0111] (Coarse particles mainly composed of nickel hydroxide) As described above, coarse particles with a size exceeding 0.8 μm or 1.2 μm trapped on the membrane filter have a structure in which a plurality of nickel particles (nickel crystallization powder) trapped in a matrix made of nickel hydroxide are firmly bonded through the nickel hydroxide matrix, as shown in Fig. 2. It should be noted that the fact that the above matrix (reference numeral 2 in Fig. 2) is mainly composed of nickel hydroxide (Ni(OH)₂) is confirmed by the following: (a) In the measurement of the matrix part by energy-dispersive X-ray spectroscopy (SEM-EDX) of a scanning electron microscope for coarse particles with a size exceeding 0.8 μm or 1.2 μm trapped on the membrane filter, many places where the molar ratio of nickel to oxygen is about Ni:O = 1:2 are observed; (b) In the measurement results by X-ray photoelectron spectroscopy (XPS) of coarse particles with a size exceeding 0.8 μm or 1.2 μm recovered from the membrane filter, nickel hydroxide (Ni(OH)₂) is detected as the main component.

[0112] (Example 1) [Preparation of Solutions of Nickel Salt and Salt of Metal Nobler than Nickel] 405 g of nickel chloride hexahydrate (NiCl₂·6H₂O, molecular weight: 237.69) as a nickel salt, 1.271 g 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 auxiliary agent for suppressing the self-decomposition of hydrazine, and 0.134 mg of ammonium palladium(II) chloride (alias: ammonium tetrachloropalladium(II) acid) ((NH₄)₂PdCl₄, molecular weight: 284.31) as a salt of a metal nobler than nickel were dissolved in 1880 mL of pure water to prepare a nickel salt solution, which is an aqueous solution containing a nickel salt as the main component, a sulfur-containing compound, and a nucleating agent that is a metal salt of a metal nobler than nickel. Here, in the nickel salt solution, L-methionine, which is a sulfide compound, is in a trace amount of 0.5 mol% (molar ratio of 0.005) with respect to nickel, and palladium is 0.28 mol ppm (0.50 mass ppm) with respect to nickel.

[0113] [Preparation of Reducing Agent Solution] A commercially available industrial-grade 60% by mass hydrazine hydrate (manufactured by MGC Otsuka Chemical Co., Ltd.) in which hydrazine hydrate (N2H4·H2O, molecular weight: 50.06) as a reducing agent was diluted 1.67 times with pure water was weighed at 207 g, and a reducing agent solution was prepared which is an aqueous solution containing no alkali hydroxide and containing hydrazine as the main component. The hydrazine contained in the reducing agent solution was prepared such that the molar ratio to nickel in the nickel salt solution was 1.46.

[0114] [Alkali hydroxide solution] As the alkali hydroxide, 230 g of sodium hydroxide (NaOH, molecular weight: 40.0) was dissolved in 672 mL of pure water to prepare an alkali hydroxide solution which is an aqueous solution containing sodium hydroxide as the main component. The sodium hydroxide contained in the alkali hydroxide solution was prepared such that the molar ratio to nickel in the nickel salt solution was 5.75.

[0115] [Amine compound solution] As an amine compound as a self-decomposition inhibitor of hydrazine and a reducing reaction accelerator (complexing agent), 1.024 g of ethylenediamine (abbreviation: EDA) (H2NC2H4NH2, molecular weight: 60.1), which is an alkyleneamine containing two primary amino groups (-NH2) in the molecule, was dissolved in 19 mL of pure water to prepare an amine compound solution which is an aqueous solution containing ethylenediamine as the main component. The ethylenediamine contained in the amine compound solution was in a trace amount of 1.0 mol% (molar ratio of 0.01) with respect to nickel in the nickel salt solution. In addition, as the materials used in the above nickel salt solution, reducing agent solution, alkali hydroxide solution, and amine compound solution, reagents manufactured by Wako Pure Chemical Industries, Ltd. were used in all cases except for 60% by mass hydrazine hydrate.

[0116] [Crystallization step] A nickel salt solution prepared by dissolving nickel chloride and a palladium salt in pure water was placed in a Teflon (registered trademark)-coated stainless steel container equipped with a stirring blade, heated while stirring to a liquid temperature of 85 °C, and then a reducing agent solution containing hydrazine and water at a liquid temperature of 25 °C was added and mixed in 20 seconds to obtain a nickel salt-reducing agent-containing liquid. To this nickel salt-reducing agent-containing liquid, an alkali hydroxide solution containing alkali hydroxide and water at a liquid temperature of 25 °C was added and mixed in 80 seconds to prepare a reaction liquid (nickel chloride + palladium salt + hydrazine + sodium hydroxide) at a liquid temperature of 70 °C, and a reduction reaction (crystallization reaction) was started. The reaction start temperature was 63 °C. After the start of the reaction, the amine compound solution was added dropwise and mixed with the reaction liquid over 10 minutes from 8 minutes to 18 minutes after the start of the reaction to proceed with the reduction reaction while suppressing the self-decomposition of hydrazine, and nickel crystallized powder was crystallized in the reaction liquid. It was confirmed that within 60 minutes from the start of the reaction, the reduction reaction of the aforementioned formula (3) was completed, the supernatant of the reaction liquid was transparent, and all the nickel components in the reaction liquid were reduced to metallic nickel.

[0117] With respect to 207 g of 60 mass% water-hydrated hydrazine incorporated in the reducing agent solution, the amount of 60 mass% water-hydrated hydrazine consumed in the crystallization reaction was 171 g, and the molar ratio to nickel was 1.20. Here, since the molar ratio of hydrazine consumed in the reduction reaction to nickel is assumed to be 0.5 from the aforementioned formula (3), it is estimated that the molar ratio of hydrazine consumed in self-decomposition to nickel was 0.70.

[0118] [Washing and Filtration Process] In the crystallization process, a slurry-like strongly alkaline (pH: 14.1) nickel powder slurry containing nickel crystallized powder is obtained as the reaction final solution. An aqueous solution of mercaptoacetic acid (thioglycolic acid) (HSCH2COOH, molecular weight: 92.12) is added to this nickel powder slurry to perform surface treatment (sulfur coating treatment) on the nickel crystallized powder. After that, it is allowed to stand to precipitate the nickel crystallized powder, and about 50% by mass of the supernatant liquid is removed (decantation) from the reaction solution. To the subsequent nickel powder slurry, pure water with a conductivity of 1 μS / cm is added in an amount approximately the same as the removed supernatant liquid for dilution (pH: 13.8). Then, using pure water adjusted to 5°C, suction filtration washing is performed using a Buchner funnel (filter paper: 5C) until the conductivity of the filtrate filtered from the slurry containing the surface-treated nickel crystallized powder reaches 100 μS / cm, and solid-liquid separation is carried out to obtain a nickel powder cake. At this time, the temperature of the nickel powder cake was 5°C, and the water content was 25% by mass. The temperature of the nickel powder cake was maintained at 5°C from after filtration separation in the washing and filtration process until the start of drying in the drying process described later.

[0119] [Drying Process] The above nickel powder cake was dried in a vacuum dryer set at a temperature of 150°C to obtain nickel crystallized powder (nickel powder).

[0120] [Crushing Treatment Process (Post-treatment Process)] Following the crystallization process, washing and filtration process, and drying process, a crushing process was carried out to reduce the coarse particles mainly formed by the connection of nickel particles in the nickel powder. Specifically, the above nickel crystallized powder (nickel powder) obtained in the crystallization process was subjected to spiral jet crushing treatment, which is a dry crushing method. Through the above processes, the nickel powder according to Example 1, which was produced using the wet method, was obtained.

[0121] (Physical Properties of Nickel Powder) Table 1 shows the moisture content and temperature of the nickel powder cake and the ratio of flaky Ni(OH)₂ determined from the results of the above SEM observation. Similarly, the moisture content and temperature of the nickel powder cake and the ratio of flaky Ni(OH)₂ determined from the results of the above SEM observation in Example 2, Comparative Examples 1 and 2 described below are also shown in Table 1.

[0122] (Example 2) In the washing and filtration step, nickel crystal powder (nickel powder) was obtained in the same manner as in Example 1, except that pure water adjusted to 20°C was used, and the temperature of the nickel powder cake was 20°C and the moisture content of the nickel powder cake was 27% by mass. Then, the above nickel crystal powder (nickel powder) was subjected to the same spiral jet disintegration treatment as in Example 1. Through the above steps, nickel powder according to Example 2 prepared by the wet method was obtained.

[0123] (Comparative Example 1) In the washing and filtration step, nickel crystal powder (nickel powder) was obtained in the same manner as in Example 1, except that pure water adjusted to 25°C was used, and the temperature of the nickel powder cake was 25°C and the moisture content of the nickel powder cake was 27% by mass. Then, the above nickel crystal powder (nickel powder) was subjected to the same spiral jet disintegration treatment as in Example 1. Through the above steps, nickel powder according to Comparative Example 1 prepared by the wet method was obtained.

[0124] (Comparative Example 2) In the washing and filtration step, nickel crystal powder (nickel powder) was obtained in the same manner as in Example 1, except that pure water adjusted to 40°C was used, and the temperature of the nickel powder cake was 40°C and the moisture content of the nickel powder cake was 20% by mass. Then, the above nickel crystal powder (nickel powder) was subjected to the same spiral jet disintegration treatment as in Example 1. Through the above steps, nickel powder according to Comparative Example 2 prepared by the wet method was obtained.

[0125]

Table 1

[0126] The nickel powders obtained by the methods of Examples 1 and 2, in which the temperature of the nickel powder cake was lower than that in Comparative Examples 1 and 2, had a lower content of coarse particles than the nickel powders obtained by the methods of Comparative Examples 1 and 2. From this result, it was clarified that by maintaining the temperature of the nickel powder cake in the range of 0 to 20°C from after filtration to the start of drying, the oxidation of the nickel powder was difficult to proceed due to the low temperature of the nickel powder cake, and the growth of nickel hydroxide (Ni(OH)₂) was difficult.

Explanation of Signs

[0127] 1 Nickel particles (nickel crystallization powder) 2 Nickel hydroxide generated by oxidation of nickel crystallization powder 10 Coarse particles mainly composed of nickel hydroxide

Claims

1. In a reaction solution obtained by mixing a water-soluble nickel salt, a salt of a metal nobler than nickel, a reducing agent, an alkali hydroxide, and water, a reduction reaction is carried out to obtain a nickel powder slurry as a reaction final solution containing nickel crystallized powder, a crystallization step; a washing / filtration step of washing and filtering the nickel crystallized powder in the nickel powder slurry to obtain a nickel powder cake; a drying step of drying the nickel powder cake to obtain nickel powder, and a method for producing nickel powder, wherein the temperature of the nickel powder cake is maintained in the range of 0 to 20°C from after filtration in the washing / filtration step until the start of drying in the drying step.

2. The method for producing nickel powder according to claim 1, wherein the water content of the nickel powder cake is 5% by mass or more and less than 30% by mass.

3. The method for producing nickel powder according to claim 1 or 2, wherein in the washing / filtration step, the temperature of the pure water used for washing the nickel crystallized powder is 0°C or higher and 20°C or lower.

4. The nickel powder has a number average particle size of 0.03 µm to 0.4 µm, the content of coarse particles mainly composed of nickel hydroxide having a particle size exceeding 0.8 µm is 200 mass ppm or less, and the content of coarse particles mainly composed of nickel hydroxide having a particle size exceeding 1.2 µm is 100 mass ppm or less. The method for producing nickel powder according to any one of claims 1 to 3.

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