Method for producing nickel powder

Abstract

To provide a method for producing nickel powder capable of suppressing generation of aggregated particles without inhibiting a reduction reaction of a nickel salt in a liquid-phase synthesis method.SOLUTION: Provided is a method for producing nickel powder, includes a step of initiating a nickel precipitation reaction in a reaction solution obtained by mixing a nickel salt, a metal more noble than nickel or a salt thereof, a reducing agent, an alkali metal hydroxide, a sulfonium salt, and water, the sulfonium salt being used in an amount of 100 mol% or more relative to nickel in the reaction solution.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Nickel powder is used as a material for forming the internal electrodes of multilayer ceramic capacitors. Liquid-phase synthesis, one method for producing nickel powder, can produce nickel powder with a narrow primary particle size distribution. However, nickel particles tend to aggregate during the reaction process, resulting in a high content of coarse particles (connected particles). These coarse particles (connected particles) are problematic as they can cause short circuits and performance degradation in multilayer ceramic capacitors. Patent Document 1 therefore discloses a method for producing nickel powder with fewer connected particles by adding a sulfide compound to the liquid-phase synthesis method. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-150073 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in the method disclosed in Patent Document 1, the amount of sulfide compound added is limited to an extremely small amount because there is a risk that the sulfide compound may inhibit the reduction reaction of the nickel salt, and there is room for improvement in terms of suppressing the generation of linked particles.

[0005] An object of the present invention is to provide a method for producing nickel powder that does not inhibit the reduction reaction of nickel salt and can suppress the generation of linked particles. [Means for solving the problem]

[0006] The present invention includes the following. [1] A method for producing nickel powder, comprising: The method includes a step of initiating a nickel precipitation reaction in a reaction solution obtained by mixing a nickel salt, a metal nobler than nickel or a salt thereof, a reducing agent, an alkali metal hydroxide, a sulfonium salt, and water, The method for producing nickel powder, wherein the sulfonium salt is used in an amount of 100 mol % or more relative to the nickel in the reaction solution. [2] The method for producing nickel powder according to [1], wherein the sulfonium salt is trimethylsulfonium iodide. [3] The method for producing nickel powder according to [1] or [2], wherein the sulfonium salt is used in an amount of 280 mol% or more relative to the nickel in the reaction solution. [Effects of the Invention]

[0007] According to the present invention, the reduction reaction of the nickel salt is not inhibited, and the generation of linked particles can be suppressed. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a scanning electron microscope photograph (SEM image) of the nickel powder according to Example 1. [Figure 2] 1 is a scanning electron microscope photograph (SEM image) of nickel powder according to Comparative Example 3. DETAILED DESCRIPTION OF THE INVENTION

[0009] The method for producing nickel powder of the present invention may use a liquid-phase synthesis method, for example, by preparing a nickel salt solution containing a nickel salt, a metal nobler than nickel or its salt, a sulfonium salt, and water, and a reducing agent solution containing a reducing agent, an alkali metal hydroxide, and water, and then mixing the nickel salt solution with the reducing agent solution to initiate a nickel precipitation reaction in the reaction solution. More specifically, the method may include the steps of preparing the nickel salt solution, preparing the reducing agent solution, and mixing the nickel salt solution with the reducing agent solution, thereby initiating a nickel precipitation reaction in the reaction solution. As the reaction progresses, metallic nickel powder precipitates. Hereinafter, the "step of initiating a nickel precipitation reaction in the reaction solution and precipitating metallic nickel powder" will also be referred to as the "precipitation step."

[0010] In particular, the method for producing nickel powder of the present invention is characterized in that the sulfonium salt is used in an amount of 100 mol % or more relative to the nickel in the reaction solution.

[0011] [Nickel salt solution] The nickel salt solution contains, for example, a nickel salt, a metal nobler than nickel or a salt thereof, a sulfonium salt, and water.

[0012] The nickel salt is not particularly limited as long as it is a nickel salt that is easily soluble in water, and examples thereof include nickel chloride, nickel sulfate, nickel nitrate, nickel acetate, etc. From the viewpoint of being inexpensive and easily procurable, nickel chloride, nickel sulfate, or a mixture thereof is preferred as the nickel salt.

[0013] A metal more noble than nickel or its salt is added to the nickel salt solution to function as a nucleating agent for generating crystal nuclei in the precipitation step. The metal ions of the metal more noble than nickel are reduced before nickel to form initial nuclei, and these initial nuclei grow into particles, thereby producing a fine nickel powder.

[0014] Examples of metals more noble than nickel or salts thereof include copper, silver, gold, platinum, palladium, rhodium, iridium, etc., or salts of these metals, and specific examples include copper sulfate, silver nitrate, sodium palladium(II) chloride, ammonium palladium(II) chloride, palladium(II) nitrate, palladium(II) sulfate, hexachloroplatinic acid, or hydrates of these salts. One or more types of metals more noble than nickel or salts thereof can be used.

[0015] The amount of the metal or salt thereof more noble than nickel is not particularly limited, but the amount of the metal serving as a nucleating agent is preferably 20 ppm by mass or more and 1000 ppm by mass or less relative to the amount of nickel powder produced, i.e., the amount of nickel contained in the nickel salt solution. If the amount is less than 20 ppm by mass, the particle size of the nickel powder tends to be large, and if the amount is more than 1000 ppm by mass, the particle size becomes finer but the particle size distribution tends to be broader.

[0016] A sulfonium salt is a salt consisting of a sulfonium ion, which is a cationic component, and an anionic component.

[0017] The cationic component constituting the sulfonium salt is a sulfonium ion having a positive charge on the sulfur atom, and may be either an organic sulfonium ion or an inorganic sulfonium ion, with an organic sulfonium ion being preferred.

[0018] Examples of the organic sulfonium ion include alkylsulfonium ions such as trialkylsulfonium ions, dialkylsulfonium ions, and monoalkylsulfonium ions, with trialkylsulfonium ions being preferred.

[0019] The number of carbon atoms in the alkyl group of the alkylsulfonium ion is usually 1 or more and 20 or less, preferably 1 or more and 10 or less, more preferably 1 or more and 6 or less, even more preferably 1 or more and 4 or less, and still more preferably 1 or 2.

[0020] The anion component constituting the sulfonium salt may be either an organic anion or an inorganic anion, but is preferably an inorganic anion.

[0021] Examples of inorganic anions include halide ions, hydroxide ions, sulfate ions, carbonate ions, nitrate ions, tetrafluoroborate ions, and hexafluorophosphate ions, with halide ions being preferred.

[0022] The sulfonium salt is preferably a salt composed of a trialkylsulfonium ion and a halide ion, and specific examples include trimethylsulfonium iodide, trimethylsulfonium bromide, trimethylsulfonium chloride, etc. Among these, trimethylsulfonium iodide is preferred because it is available at a relatively low cost.

[0023] During the precipitation process, sulfonium salts (sulfonium ions) adsorb onto the surface of nickel particles precipitated in the reaction solution, preventing the nickel particles from agglomerating and thereby suppressing the generation of linked particles. Furthermore, because sulfonium ions are positively charged cationic ions, they are less likely to form complexes with nickel ions in the reaction solution. Therefore, because they do not inhibit the reduction reaction of nickel salts, even when incorporated in large amounts, they can suppress the generation of linked particles without reducing reactivity.

[0024] The amount of sulfonium salt to be added is 100 mol% or more relative to the nickel in the reaction solution, and from the viewpoint of suppressing the generation of linked particles, it is preferably 150 mol% or more, more preferably 200 mol% or more, and even more preferably 280 mol% or more. If the amount of sulfonium salt to be added is less than 100 mol%, the generation of linked particles cannot be sufficiently suppressed. The sulfonium salt may be added to the reducing agent solution described below.

[0025] The water to be added to the nickel salt solution is preferably pure water from the viewpoint of reducing the amount of impurities in the nickel powder. Further, a water-soluble organic solvent such as alcohol may be added together with water.

[0026] In addition to the nickel salt, the metal nobler than nickel or its salt, and the sulfonium salt, the nickel salt solution preferably contains a complexing agent. The complexing agent stabilizes the nickel ion concentration by forming a complex with nickel ions in the nickel salt solution. Therefore, the reaction rate of the reduction reaction of the nickel salt can be controlled in the precipitation step.

[0027] As the complexing agent, it is preferable to use a hydroxycarboxylic acid, a salt thereof, or a derivative thereof, or a carboxylic acid, a salt thereof, or a derivative thereof, and specific examples thereof include tartaric acid, citric acid, malic acid, ascorbic acid, formic acid, acetic acid, pyruvic acid, and salts and derivatives thereof.

[0028] The nickel salt solution may further contain a dispersant as long as the dispersant does not inhibit the reduction reaction of the nickel salt. Known dispersants may be used, and specific examples include amines such as triethanolamine, diethanolamine, and oxyethylene alkylamine, as well as salts and derivatives thereof, and amino acids such as alanine and glycine, as well as salts and derivatives thereof.

[0029] In the method for producing nickel powder of the present invention, the order in which the components to be mixed in the nickel salt solution are mixed is not particularly limited.

[0030] [Reducing agent solution] The reducing agent solution includes, for example, a reducing agent, an alkali metal hydroxide, and water.

[0031] Examples of the reducing agent that can be used include hydrazine, sodium borohydride, alcohol, ascorbic acid, or a salt thereof. Hydrazine is preferred as a reducing agent because it has high reducing power, does not produce by-products in the reaction solution during the reduction reaction, contains few impurities, and is easily available. Hydrazine includes anhydrous hydrazine and hydrazine hydrate, which is a hydrazine hydrate, and either may be used. Specifically, commercially available industrial-grade 60% by mass hydrazine hydrate can be used as the hydrazine.

[0032] Since the reducing power of the reducing agent is particularly enhanced in an alkaline solution, an alkali metal hydroxide is added to the reducing agent solution as a pH adjuster. Examples of alkali metal hydroxides include sodium hydroxide, potassium hydroxide, and lithium hydroxide. One or more alkali metal hydroxides can be used. Alternatively, an alkali metal hydroxide can be added to a nickel salt solution.

[0033] The water to be added to the reducing agent solution is preferably pure water from the viewpoint of reducing the amount of impurities in the nickel powder. Further, a water-soluble organic solvent such as alcohol may be added together with water.

[0034] The reducing agent solution may contain a complexing agent, a dispersing agent, etc., similar to the nickel salt solution. The mixing order of the components contained in the reducing agent solution is not particularly limited.

[0035] [Precipitation process] In the method for producing nickel powder of the present invention, for example, a nickel salt solution and a reducing agent solution are mixed to prepare a reaction solution, and nickel is precipitated in the reaction solution by the reducing agent, thereby obtaining nickel powder. The order in which the solutions are mixed when preparing the reaction solution is not particularly limited. That is, the reducing agent solution may be added to the nickel salt solution, or conversely, the nickel salt solution may be added to the reducing agent solution. Furthermore, the solutions may be added all at once, in multiple installments, or continuously dropwise.

[0036] When a nickel salt solution and a reducing agent solution are mixed, the nickel salt (nickel ions) in the reaction solution are reduced by the reducing agent, resulting in the precipitation of nickel powder. When the reaction begins, the color of the reaction solution changes from green to gray due to nucleation by the nucleating agent. Next, the reduction reaction of the nickel salt and the decomposition reaction of the reducing agent progress, and the reaction solution begins to foam due to these reactions. When all the nickel ions in the reaction solution are reduced, the supernatant liquid turns colorless and transparent, and the reaction is complete. As described above, the progress of the reaction can be confirmed from the change in color of the reaction solution and the foaming state.

[0037] The temperature of the reaction solution at the time when nickel precipitation begins after preparation, i.e., the reaction initiation temperature, is preferably 40°C or higher and 95°C or lower, and more preferably 50°C or higher and 90°C or lower. Because nickel precipitation begins immediately after preparation of the reaction solution, the reaction initiation temperature can be considered the temperature of the reaction solution at the time of preparation. The higher the reaction initiation temperature, the faster the reduction reaction rate. However, if the reaction initiation temperature exceeds 95°C, problems such as difficulty in controlling the particle size of the nickel powder or the reaction solution boiling over from the reaction vessel due to an inability to control the reaction rate may occur. Furthermore, if the reaction initiation temperature is lower than 40°C, the reduction reaction rate decreases, the time required for the precipitation process increases, and productivity decreases. For these reasons, if the reaction initiation temperature is set in the range of 40°C or higher and 95°C or lower, nickel powder can be produced with easy control of particle size and particle size distribution while maintaining high productivity.

[0038] For the reasons described above, it is preferable to heat at least one of the nickel salt solution and the reducing agent solution to a reduction temperature by the reducing agent before preparing the reaction solution in the precipitation step. The "reduction temperature by the reducing agent" refers to the temperature at which nickel is precipitated by reducing the nickel salt with the reducing agent, and is preferably 40°C or higher and 95°C or lower.

[0039] If the nickel salt solution and the reducing agent solution are heated to the reduction temperature before mixing, a sudden temperature change does not occur during mixing, and the temperature at which the precipitation reaction begins immediately after mixing can be kept at approximately the same temperature as the reduction temperature.

[0040] In the precipitation step, it is preferable to set the pH of the prepared reaction solution to 11 or higher. As mentioned above, the reducing power of the reducing agent increases in a more alkaline solution. When the pH of the reaction solution is 11 or higher, nuclei of a metal more noble than nickel are generated in the reaction solution with the increased reducing power of the reducing agent, allowing many initial nuclei to be formed uniformly, resulting in nickel powder with a fine particle size and a narrow particle size distribution.

[0041] The pH can be adjusted using a reducing agent solution containing the above-mentioned alkali metal hydroxide, and if necessary, a pH adjuster can also be added when preparing the reaction solution.

[0042] The amount of reducing agent added to the reaction solution in the precipitation step can be adjusted as appropriate, as long as the amount is such that the entire amount of nickel contained in the reaction solution is reduced. For example, when the reducing agent is hydrazine, the molar ratio relative to the nickel contained in the reaction solution may be in the range of 0.5 to 3.25. If the amount of hydrazine added relative to the nickel contained in the reaction solution is less than 0.5, there is a possibility that the entire amount of nickel in the reaction solution will not be reduced. On the other hand, if the amount of hydrazine added relative to the nickel contained in the reaction solution is more than 3.25, an excess of hydrazine will be used, which is economically disadvantageous.

[0043] When mixing the nickel salt solution, reducing agent solution, reaction solution, and other solutions, it is preferable to stir these solutions. Stirring can make the precipitation reaction uniform, allowing nickel powder with a narrow particle size distribution to be obtained. Any known stirring method can be used, and it is preferable to use a stirring blade from the standpoint of controllability and equipment manufacturing costs. As the stirring blade, commercially available products such as paddle blades, turbine blades, Max Blend blades, and Full Zone blades can be used, and measures such as installing baffles or baffle rods in the reaction vessel can also be taken to improve stirring and mixing.

[0044] [Recovery of nickel powder] Only the nickel powder is separated from the nickel powder slurry containing the nickel powder obtained in the above precipitation step by a known procedure such as washing, solid-liquid separation, and drying.

[0045] To separate the nickel powder from the nickel powder slurry, solid-liquid separation is performed using known means such as a Denver filter, filter press, centrifuge, or decanter, and the nickel powder is thoroughly washed with high-purity water such as pure water or ultrapure water having a conductivity of 1 μS / cm or less. Here, "sufficient washing" refers to, for example, using pure water with a conductivity of about 1 μS / cm, filtering and washing the nickel powder until the conductivity of the filtrate obtained is 10 μS / cm or less. After solid-liquid separation and washing, nickel powder is obtained by drying at a temperature in the range of 50°C to 200°C, preferably 80°C to 150°C, using a general-purpose drying device such as an atmospheric dryer, hot air dryer, inert gas atmosphere dryer, or vacuum dryer.

[0046] If necessary, a sulfur-surface-treated nickel powder can be obtained by adding a sulfur coating agent, which is a water-soluble sulfur compound, to the nickel powder slurry. Examples of the sulfur coating agent include water-soluble sulfur compounds containing either a mercapto group or a disulfide group, such as thiomalic acid, L-cysteine, thioglycerol, and dithiodiglycolic acid.

[0047] [Crushing process (post-processing process)] The nickel powder obtained in the precipitation step can be used as it is as the final nickel powder product, but it is more preferable to subject it to a crushing treatment as necessary to reduce coarse particles and connected particles formed during the nickel precipitation process. As the crushing treatment, dry crushing methods such as spiral jet crushing and counter jet mill crushing, wet crushing methods such as high-pressure fluid collision crushing, and other general-purpose crushing methods can be applied. [Example]

[0048] The present invention will be explained in more detail below by showing examples and comparative examples, but the present invention is not limited to these examples.

[0049] [Evaluation method] (1) Judgment of reaction completion In the precipitation step, the presence of nickel ions in the reaction solution was confirmed by checking the color of the reaction solution and whether or not there was any discoloration of the ion detection paper. The color of the solution was confirmed visually, and the ion detection paper was Nickel Check manufactured by ADVANTEC. If the supernatant was colorless and transparent and the ion detection paper showed no discoloration before the reaction solution stopped bubbling, the reaction was judged to be complete and marked with a "○". If the supernatant was colored or the ion detection paper showed discoloration when the reaction solution stopped bubbling, the reaction was judged to be incomplete and marked with a "×".

[0050] (2) Amount of connected particles 0.5 g of the resulting nickel powder was dispersed in 100 g of 0.1% by mass aqueous sodium hexametaphosphate solution to prepare a nickel powder slurry, which was then passed through an Isopore filter with a pore size of 0.8 μm and a diameter of 47 mm. The nickel powder residue trapped on the filter was completely dissolved using dilute nitric acid, and the nickel ion concentration in the resulting solution was quantified using inductively coupled plasma atomic emission spectroscopy (ICP-AES). The amount of coarse particles and connected particles trapped on the filter was calculated from the resulting nickel ion concentration and the total amount of solution.

[0051] (3) Reduction rate of connected particles Nickel powder was obtained in the same manner as in Example 1, except that no sulfonium salt was added. The amount of linked particles was then measured in the same manner as in (2), and this was used as the reference value. Based on the amount of linked particles (sample value) of the nickel powder obtained in each Example and Comparative Example, the linked particle reduction rate was calculated using the following formula: Connected particle reduction rate (%) = [(reference value - sample value) / reference value] x 100 The reduction rate of connected particles was evaluated according to the following evaluation criteria. ○: 50% or more ×: Less than 50%

[0052] (4) Overall Judgment Based on the results of (1) and (3) above, an overall evaluation was made according to the following evaluation criteria. ○: The reaction completion judgment is "○" and the linked particle reduction rate is evaluated as "○" ×: Reaction completion judged as "×" or linked particle reduction rate evaluated as "×"

[0053] Example 1 [Preparation of nickel salt solution] A nickel salt solution was prepared by stirring and mixing 105.0 g of nickel sulfate hexahydrate as a nickel salt, 15.6 g of a 0.1% aqueous palladium sulfate solution as a metal salt (nucleating agent) of a metal nobler than nickel, 30.0 g of trisodium citrate as a complexing agent, and 228.3 g of trimethylsulfonium iodide (280 mol% relative to nickel) as a sulfonium salt in 285.0 g of pure water.

[0054] [Preparation of reducing agent solution] A reducing agent solution was prepared by mixing 64.8 g of 60% hydrazine hydrate, which had been purified by removing organic impurities such as pyrazole, as a reducing agent, and 57.0 g of sodium hydroxide as a pH adjuster, in 271.8 g of pure water with stirring.

[0055] [Precipitation process] After heating the nickel salt solution and reducing agent solution to a temperature of 60°C, the reducing agent solution was added dropwise to the nickel salt solution at a rate of approximately 150 ml / min, initiating the precipitation reaction in the reaction solution. The nickel salt solution and reducing agent solution, each at a temperature of 60°C, were stirred and mixed, generating heat, causing the temperature of the reaction solution to rise to 66°C, so the reaction started at 66°C. Approximately 2 to 3 minutes after the start of the reaction, the reaction solution changed color (from green to gray) due to nucleation by the nucleating agent, but stirring was continued to allow the reduction reaction to proceed, yielding nickel precipitate powder. The supernatant of the reaction solution was colorless and transparent, and no discoloration was observed on ion detection paper, confirming that all of the nickel components in the reaction solution had been reduced to metallic nickel. The reaction liquid containing the obtained nickel precipitate powder was filtered and washed using pure water with a conductivity of 1 μS / cm until the conductivity of the filtrate filtered from the reaction liquid became 10 μS / cm or less. After solid-liquid separation, the filtrate was dried in a vacuum dryer set at 150°C to obtain nickel powder.

[0056] (Example 2, Comparative Examples 1 to 5) The reduction reaction was carried out in the same manner as in Example 1, except that the additives and the amounts added were changed as shown in Table 1.

[0057] The completion of the reaction was determined by the above evaluation method, and the reduction rate of connected particles in the obtained nickel powder was calculated. The results are shown in Table 1. Fig. 1 shows a scanning electron microscope (SEM) image of the nickel powder obtained in Example 1. Fig. 2 shows a scanning electron microscope (SEM) image of the nickel powder obtained in Comparative Example 3.

[0058] In Examples 1 and 2, the reduction rate of linked particles was 50% or more, and the larger the amount of sulfonium salt added, the more the generation of linked particles was suppressed. On the other hand, in Comparative Example 1, the amount of sulfonium salt added was small, so the generation of linked particles could not be suppressed.

[0059] In Comparative Examples 2 and 4, the supernatant was colored when the foaming of the reaction solution stopped, and the reaction was not completed. This is presumably because the use of large amounts of nonionic triethanolamine and anionic thiomalic acid inhibited the reduction reaction, significantly reducing reactivity. In Comparative Examples 3 and 5, the reaction was completed because the amount of additive added was small, but the absence of a sulfonium salt meant that the generation of linked particles could not be suppressed.

[0060] As shown in Figure 1, the nickel powder produced by the method for producing nickel powder of the present invention is a powder with high sphericality and no aggregation of particles. On the other hand, as shown in Figure 2, the comparative example is a powder with low sphericality, with irregularly shaped particles strung together like beads.

[0061] [Table 1]

[0062] The embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. The scope of the present invention is defined by the claims, not by the above description, and is intended to include all modifications within the meaning and scope of the claims.

Claims

1. A method for producing nickel powder, comprising: The method includes a step of initiating a nickel precipitation reaction in a reaction solution obtained by mixing a nickel salt, a metal nobler than nickel or a salt thereof, a reducing agent, an alkali metal hydroxide, a sulfonium salt, and water, The method for producing nickel powder, wherein the sulfonium salt is used in an amount of 100 mol % or more relative to the nickel in the reaction solution.

2. 2. The method for producing nickel powder according to claim 1, wherein the sulfonium salt is trimethylsulfonium iodide.

3. The method for producing nickel powder according to claim 1 or 2, wherein the sulfonium salt is used in an amount of 280 mol% or more relative to the nickel in the reaction solution.

Images