JP6003880B2 - Silver powder manufacturing method - Google Patents

Description

  The present invention relates to a method for producing silver powder, and more particularly, to a method for producing silver powder that is a main component of resin-type silver paste and fired-type silver paste used for forming wiring layers and electrodes of electronic devices. .

  Silver pastes such as resin-type silver paste and fired-type silver paste are frequently used for forming wiring layers, electrodes, and the like in electronic devices. These silver pastes are applied or printed and then heat-cured or fired to form a conductive film that becomes a wiring layer, an electrode, or the like.

  For example, a resin-type silver paste is made of silver powder, resin, curing agent, solvent, etc., printed on a conductor circuit pattern or terminal, and cured by heating at 100 ° C. to 200 ° C. to form a conductive film. Form. The fired silver paste is made of silver powder, glass, solvent, etc., printed on a conductor circuit pattern or terminal, and heated and fired at 600 ° C. to 800 ° C. to form a conductive film to form wirings and electrodes. In wirings and electrodes formed of these silver pastes, electrically connected current paths are formed by continuous silver powder.

  The silver powder used in the silver paste has a particle size of 0.1 μm to several μm, and the particle size of the silver powder used varies depending on the thickness of the wiring to be formed and the thickness of the electrode. Further, by uniformly dispersing silver powder in the paste, it is possible to form a wiring having a uniform thickness and an electrode having a uniform thickness.

  The characteristics required for silver powder for silver paste vary depending on the application and use conditions, but the general and important point is that the particle size is uniform, there is little aggregation, and the dispersibility in the paste is high. is there. This is because if the particle size is uniform and the dispersibility in the paste is high, curing or firing proceeds uniformly, and a conductive film having low resistance and high strength can be formed. If the particle size is non-uniform and the dispersibility is poor, silver particles will not be uniformly present in the printed film, resulting in non-uniform thickness and thickness of wiring and electrodes, as well as non-uniform curing or firing. The resistance of the conductive film tends to increase, and the conductive film tends to be brittle and weak.

  Further, as a matter required for silver powder for silver paste, it is also important that the manufacturing cost is low. This is because silver powder is a major component of the paste and therefore has a large proportion of the paste price. In order to reduce the manufacturing cost, it is important not only to lower the unit price of raw materials and materials to be used, but also to reduce the cost of treating waste liquid and exhaust gas.

  In the production of silver powder used in the above-described silver paste, the raw material used as the silver source is generally silver nitrate. For example, Patent Document 1 discloses a method in which a solution containing a silver ammine complex in which silver nitrate is dissolved in ammonia and a reducing agent solution are continuously mixed and reduced to obtain uniform silver powder.

  According to the production method shown in Patent Document 1, it is said that a granular silver powder having an average particle diameter of 0.1 μm to 1 μm and uniform and having little aggregation is obtained. However, silver nitrate generates toxic nitrous acid gas in the process of dissolution in aqueous ammonia, and a device for recovering this is required. Further, since a large amount of nitrate nitrogen and ammonia nitrogen is contained in the wastewater, an apparatus for the treatment is also required. In addition, silver nitrate is dangerous and deleterious, so it must be handled with care. As described above, when silver nitrate is used as a raw material for silver powder, there is a problem that the influence and risk on the environment are larger than those of other silver compounds.

  Therefore, a method for producing silver powder by reducing silver chloride without using silver nitrate as a raw material has been proposed. When silver chloride is used, since nitrous acid gas is not generated when dissolved in aqueous ammonia, the processing cost is low and the environmental risk is low. Furthermore, silver chloride does not correspond to a dangerous substance or a deleterious substance and has an advantage that it is a silver compound that is relatively easy to handle although it needs to be shielded from light. Silver chloride also exists as an intermediate product in the silver refining process, and has sufficient purity for the electronics industry.

  Patent Document 2 discloses a method of obtaining silver powder by adding hydrazine as a reducing agent to a silver solution obtained by adding a dispersing agent and a silver fine particle slurry to a silver solution obtained by dissolving silver chloride in ammonia water. However, the particle size of the silver powder obtained by this method is 0.2 μm to 3 μm, and there is a problem in uniformity.

  Furthermore, in Patent Document 3, a nucleus-containing reducing agent solution and a silver solution for particle growth containing a silver complex are continuously mixed to obtain a reaction solution, and the silver complex is reduced in the reaction solution to grow silver particles. A method for producing silver powder is disclosed. However, although this method can obtain silver powder having excellent particle size uniformity, there is a problem that the particle size of the obtained silver powder deviates from the intended particle size, and the silver particle cannot be obtained by stabilizing the particle size. It was.

  Thus, in the conventional process of reducing silver salt as a raw material, there is a problem in particle uniformity. In particular, when producing silver powder having a small particle size of 0.5 μm or less, it is more difficult to stabilize the particle size, and the silver concentration in the reaction solution must be lowered. On the other hand, when the silver concentration in the reaction solution is lowered, there arises a problem that productivity is deteriorated.

JP 2010-070793 A JP 2010-043337 A International Publication No. 2013/133103

  Then, in view of such a conventional situation, this invention aims at providing the manufacturing method of silver powder which can manufacture silver powder which has a uniform and desired particle size stably with high productivity. .

  As a result of intensive investigations to achieve the above object, the present inventors have determined that the solid particles contained in the silver solution for generating nuclei and / or the silver solution for growing nuclei have a particle diameter of the obtained silver nuclei. It has a great influence, and it is possible to control the number of silver particles that grow by reducing the content of the solid particles. As a result, even when the silver concentration in the reaction solution is high, the particle size is uniform and desired. The present inventors have found that silver powder having a high yield can be easily obtained with high productivity.

  That is, the method for producing silver powder according to the present invention comprises continuously mixing a silver solution containing a silver complex and a reducing agent solution into a reaction solution, and reducing the silver complex in the reaction solution to obtain a silver particle slurry. Then, a silver powder production method for producing silver powder through filtration, washing, and drying steps, comprising mixing a nucleation silver solution containing a silver complex, a solution containing a strong reducing agent, and a dispersant. A core containing reducing agent prepared by mixing a silver core solution preparing step for obtaining a silver core solution, a silver core solution obtained by the above silver core solution preparing step, and a weak reducing agent having a higher standard electrode potential than the above strong reducing agent. A core-containing reducing agent solution preparation step for obtaining a solution, a core-containing reducing agent solution obtained by the core-containing reducing agent solution preparation step, and a solid particle content of 20 mass ppm or less with respect to the amount of silver, A continuous solution is mixed with a silver solution for particle growth containing a complex to form a reaction solution. A particle growth step of growing silver particles by reducing the complex, and the equivalent of the strong reducing agent with respect to the amount of silver in the silver solution for nucleation is 1.0 equivalent or more and less than 4.0 equivalents, The standard electrode potential of the strong reducing agent is 0.056 V or less, and the silver concentration in the silver solution for nucleation is 0.1 g / L to 6.0 g / L.

  The nucleation silver solution preferably has a solid particle content of 20 ppm by mass or less based on the silver content.

  In the production method, it is preferable to further include a filtration step of ultrafiltration of the nucleation silver solution before mixing with the solution containing the strong reducing agent, and the molecular weight cut-off of the ultrafiltration is 150, 000 or less is preferable.

  In the production method, it is preferable to further include a filtration step of ultrafiltration of the silver solution for particle growth before mixing with the nucleus-containing reducing agent solution, and the molecular weight cut-off of the ultrafiltration is 150,000. The following is preferable.

  Further, it is preferable that the difference in standard electrode potential between the strong reducing agent and the weak reducing agent is 1.0 V or more. Specifically, it is preferable to use hydrazine monohydrate as the strong reducing agent and ascorbic acid as the weak reducing agent.

  The silver concentration in the silver solution for particle growth is preferably 20 g / L to 90 g / L.

  The silver complex is preferably a silver ammine complex obtained by dissolving silver chloride in aqueous ammonia, and the ammonia amount relative to the silver amount in the nucleation silver solution is 20 to 100 in molar ratio. Preferably there is.

  Further, the mixing amount of the dispersant is preferably 1% by mass to 30% by mass with respect to the amount of silver in the particle growth silver solution after mixing the nucleus-containing reducing agent solution and the particle growth silver solution. . As the dispersant, it is preferable to use at least one selected from polyvinyl alcohol, polyvinyl pyrrolidone, a modified silicone oil surfactant, and a polyether surfactant.

  Further, in the mixing of the nucleus-containing reducing agent solution and the silver solution for particle growth, it is preferable that each solution is individually supplied to a reaction tube and mixed by a static mixer disposed in the reaction tube.

  According to the method for producing silver powder according to the present invention, silver powder having a uniform particle size not containing fine particles can be stably produced with a desired particle size. Therefore, according to the silver powder manufactured by this method, it can be suitably used as a silver powder for a paste such as a resin-type silver paste or a fired-type silver paste used for forming a wiring layer or an electrode of an electronic device.

  In addition, the silver powder production method according to the present invention is easy and stable in controlling the particle size of the silver powder, and thus is excellent in mass productivity and has an extremely large industrial value.

It is process drawing of the manufacturing method of the silver powder to which this invention is applied. 2 is an SEM image of silver nuclei obtained in Example 1. 2 is an SEM image of silver powder obtained in Example 1. 3 is a SEM image of silver nuclei obtained in Example 2. 3 is a SEM image of silver powder obtained in Example 2. 4 is an SEM image of silver nuclei obtained in Example 4. 4 is a SEM image of silver powder obtained in Example 4. 6 is a SEM image of silver nuclei obtained in Example 5. 6 is a SEM image of silver powder obtained in Example 5. 7 is a SEM image of silver nuclei obtained in Example 6. It is a SEM image of the silver powder obtained in Example 6. 7 is a SEM image of silver nuclei obtained in Example 7. 7 is a SEM image of silver powder obtained in Example 7. 10 is a SEM image of silver nuclei obtained in Example 8. It is a SEM image of the silver powder obtained in Example 8. 10 is a SEM image of silver nuclei obtained in Example 9. 10 is a SEM image of silver powder obtained in Example 9. 2 is a SEM image of silver powder obtained in Comparative Example 1. 10 is a SEM image of silver nuclei obtained in Comparative Example 3. 6 is a SEM image of silver nuclei obtained in Comparative Example 4. 6 is a SEM image of silver powder obtained in Comparative Example 4.

  Hereinafter, specific embodiments of the method for producing silver powder according to the present invention will be described in detail. Note that the present invention is not limited to the following embodiments, and can be modified as appropriate without departing from the gist of the present invention.

  In the method for producing silver powder according to the present embodiment, a silver solution containing a silver complex and a reducing agent solution are continuously mixed to form a reaction solution, and the silver complex in the reaction solution is reduced to obtain a silver particle slurry. After that, it is a method for producing silver powder through the steps of filtration, washing, and drying, and by containing silver nuclei in the reducing agent solution, a silver powder having a uniform and desired particle diameter can be obtained. is there.

  In addition, the method for producing silver powder according to the present embodiment reduces the solid particles contained in the particle growth silver solution for growing nuclei, thereby suppressing the generation of silver particles growing from the solid particles and growing. The number of silver particles can be controlled. As a result, even under conditions where the silver concentration in the reaction solution is high, the subsequent particle growth can be easily controlled to a uniform and desired particle size by the standard electrode potential of the reducing agent. Can do.

  Furthermore, the method for producing silver powder according to the present embodiment can produce a silver nucleus solution having a small particle size and a large number of nuclei by reducing the content of solid particles contained in the silver solution for generating nuclei, As a result, silver particles can be uniformly grown from a large number of silver nuclei, and this is particularly effective in obtaining silver powder having a small particle size.

  Also, a silver nucleus solution obtained by mixing a solution containing a strong reducing agent, a silver solution for nucleation containing a silver complex, and a dispersing agent, and a weak reducing agent having a higher standard electrode potential than that of the strong reducing agent are mixed. To make a nucleus-containing reducing agent solution. Then, this nucleus-containing reducing agent solution and a silver solution for particle growth containing a silver complex are mixed and reduced. Thereby, the silver powder which has a uniform particle size can be obtained.

  Here, the strong reducing agent means a reducing agent having a strong reducing power, and the weak reducing agent means that the standard electrode potential is higher than that of the strong reducing agent, that is, a reducing agent having a weak reducing power. means.

  Moreover, the method for producing silver powder according to the present embodiment includes a nucleus-containing reducing agent solution obtained by mixing a silver nucleus solution containing silver nuclei and a reducing agent, and a silver solution for particle growth containing a silver complex. Quantitatively and continuously supplied to a certain space and mixed to cause a reduction reaction, and the reduced solution after completion of the reduction reaction, that is, a silver particle slurry, is quantitatively and continuously discharged. Thus, by supplying each solution quantitatively and continuously and reducing it, the concentration of the silver complex and the concentration of the reducing agent in the reduction reaction field can be kept constant, and constant particle growth can be achieved. As a result, silver powder having a uniform size and a sharp particle size distribution can be obtained. Further, by continuously supplying the silver solution and the reducing agent solution and discharging the silver particle slurry, the silver powder can be continuously obtained, and the silver powder can be produced with high productivity.

  In this silver powder production method, it is particularly preferable to use silver chloride as a starting silver compound, for example, a silver ammine complex obtained by dissolving silver chloride in aqueous ammonia or the like. By using silver chloride as a starting material in this way, it is possible to reduce the production cost by eliminating the need for a nitrite gas recovery device that is required when silver nitrate is used as the starting material, and having a low environmental impact. Can do. In view of the above, it is preferable to use silver chloride in both the nucleation silver solution and the particle growth silver solution.

  Hereinafter, the manufacturing method of the silver powder which concerns on this Embodiment is demonstrated in detail for every process.

  As shown in the process diagram of FIG. 1, the silver powder production method according to the present embodiment includes a silver nucleus solution preparation step S1 for obtaining a silver nucleus solution, and a silver nucleus solution and reduction obtained by the silver nucleus solution preparation step S1. A core-containing reducing agent solution preparation step S2 for mixing the agent to obtain a core-containing reducing agent solution, a core-containing reducing agent solution obtained by the core-containing reducing agent solution preparation step S2, and a silver solution for particle growth containing a silver complex; And a particle growth step S3 in which the silver complex is reduced to grow silver particles.

  In this silver powder production method, it is important to generate silver nuclei with a strong reducing agent and to perform particle growth with a weak reducing agent, and to separate the silver nucleation from the particle growth. It is important to use reducing agents having different standard electrode potentials for silver nucleation and grain growth. If a strong reducing agent and a weak reducing agent are added to the silver solution at the same time, nucleation and particle growth cannot be sufficiently separated, resulting in new nucleation during particle growth from silver nuclei and the inclusion of fine particles. As a result, silver particles having sufficient uniformity in particle size cannot be obtained. On the other hand, after generating nuclei having a uniform particle size with a strong reducing agent, a weak reducing agent is added to form a reducing agent solution, and the reducing agent solution and the silver solution are mixed to cause particle growth. Thus, silver particles having a uniform particle diameter can be obtained.

[Silver core solution preparation process]
In the silver nucleus solution preparation step S1, a solution of silver nuclei that becomes the nucleus of particle growth is generated. Specifically, in this silver nucleus solution preparation step S1, a nucleation silver solution containing a silver complex in a solution containing a strong reducing agent and a dispersing agent obtained by mixing a dispersing agent and a solution containing a strong reducing agent. To obtain a silver nucleus solution. Moreover, after mixing the silver solution for nucleation containing a silver complex and a dispersing agent beforehand, you may reduce by adding the solution containing a strong reducing agent. The dispersing agent only needs to be present in the solution at the time of silver nucleation, and may be mixed with at least one of a nucleation silver solution or a solution containing a strong reducing agent. You may mix a dispersing agent at the time of mixing of the solution containing this.

  As described above, the strong reducing agent is a reducing agent having a strong reducing power, and is preferably a reducing agent having a standard electrode potential of 0.056 V or less, and specifically, hydrazine (−1.15 V) or formalin. (0.056V) etc. can be used preferably. Among these, it is preferable to use hydrazine and its hydrate having particularly strong reducing power, and it is more preferable to use hydrazine monohydrate. Thus, by using a reducing agent having a standard electrode potential of 0.056 V or less and a strong reducing power, fine and uniform silver fine particles suitable as a nucleus can be obtained. When a reducing agent with a weak reducing power exceeding the standard electrode potential of 0.056 V is used, the reduction rate is slowed down, so that particle growth may proceed simultaneously with nucleation, and nuclei with a uniform particle size can be obtained. In addition, the particle size becomes large and silver fine particles preferable as nuclei cannot be obtained.

  Further, the mixing amount of the strong reducing agent is preferably 1.0 equivalent or more and less than 4.0 equivalent with respect to the silver amount in the nucleation silver solution, and is 2.0 equivalents or more and less than 4.0 equivalents. More preferably. By setting the mixing amount of the strong reducing agent in such a range, silver nuclei that are uniform and do not precipitate in the silver nuclei solution can be formed. Then, as will be described later, a silver powder having a uniform particle size is obtained by mixing a reducing agent solution obtained by mixing a weak reducing agent with the silver nucleus solution and a filtered silver solution for particle growth. be able to. Further, it is more preferable that the strong reducing agent is mixed in a range of 2.0 equivalents or more and less than 4.0 equivalents with respect to the amount of silver in the nucleation silver solution, so that finer and more uniform particle size is obtained. Silver nuclei can be obtained.

  When the mixing amount of the strong reducing agent is less than 1.0 equivalent with respect to the silver amount in the silver solution for nucleation, the silver nuclei particles are liable to be connected and precipitate, so the number of nuclei during particle growth is constant. In other words, the particle size may not be sufficiently controlled. In addition, silver nuclei having non-uniform particle sizes may cause non-uniform growth during particle growth, and silver powder having a uniform particle size may not be obtained. On the other hand, when the mixing amount of the strong reducing agent is 4.0 equivalents or more, coarse particles may be generated in the silver nucleus solution, which is not preferable.

  The dispersant is preferably at least one selected from polyvinyl alcohol, polyvinyl pyrrolidone, modified silicone oil surfactants, and polyether surfactants. If a dispersant is not used, silver nuclei generated by the reduction reaction and silver particles with grown nuclei are aggregated, resulting in poor dispersibility.

  Further, the mixing amount of the dispersing agent is the amount of silver in the particle growth silver solution after mixing the nucleus-containing reducing agent solution and the particle growth silver solution, which will be described later, that is, the amount of silver in the reaction solution to the nucleus-containing reducing agent. It is preferable to set it as 1 mass%-30 mass% with respect to the silver amount used for the particle growth which deducted the silver amount in a solution, and it is more preferable to set it as 1.5 mass%-20 mass%. If the mixing amount is less than 1% by mass, a sufficient aggregation suppressing effect cannot be obtained. On the other hand, even if the mixing amount exceeds 30% by mass, there is no further improvement in the aggregation suppressing effect, such as wastewater treatment. The load will only increase. The amount of silver in the nucleation silver solution is small compared to the amount of silver in the particle growth silver solution. Therefore, by adding the above-described amount of dispersant to the nucleation silver solution in advance. In addition, a sufficient aggregation preventing effect can be obtained even during nucleation.

  Further, when polyvinyl alcohol or polyvinyl pyrrolidone is used as the dispersant, foaming may occur during the reduction reaction, and thus, for example, an antifoaming agent may be added to the silver solution described later.

  The nucleation silver solution is a solution containing a silver complex obtained by dissolving a silver compound with a complexing agent, and generates a silver nucleus by mixing and reducing the above-described strong reducing agent and dispersing agent. Solution.

  Here, it is preferable to use a silver solution for nucleation in which the content of solid particles is 20 ppm by mass or less with respect to the amount of silver in the silver nucleus solution. By suppressing the content of solid particles in the nucleation silver solution, nucleation is performed by spontaneous nucleation by a strong reducing agent, so that the number of silver nuclei produced can be controlled and obtained. It becomes possible to easily control the silver particles to a desired particle size. If the content of the solid particles exceeds 20 mass ppm with respect to the silver content in the silver nucleus solution, the solid particles act as nuclei in the silver nucleus solution preparation step S1, and therefore it is not easy to control the number of nuclei. The particle size of the nucleus cannot be easily controlled. For this reason, although the particle size uniformity of silver powder is improved by the addition of nuclei, there arises a problem that it is not easy to control the particle size of silver powder.

  Furthermore, if the content of solid particles exceeds 20 mass ppm with respect to the amount of silver in the silver nucleus solution, the generation of spontaneous nuclei is suppressed, and a large number of nuclei cannot be generated, so the number of nuclei is small. In particular, it becomes difficult to easily produce a silver powder having a small particle diameter, specifically, a silver powder having an average primary particle diameter of 0.5 μm or less as measured by observation with a scanning battery microscope.

  Therefore, by controlling the content of solid particles contained in the nucleation silver solution to 20 mass ppm or less with respect to the silver amount, the number and particle size of nuclei generated in the silver nucleus solution adjustment step S1 can be controlled. It becomes possible and control of the particle diameter of the silver powder obtained becomes easy.

  Furthermore, by making the content of solid particles contained in the silver solution for nucleation 20 ppm by mass or less with respect to the amount of silver, it is possible to generate a large number of fine nuclei, and it is easy even for silver powder having a small particle size. It is possible to control the particle size. The solid particle content is analyzed by dissolving the silver solution for particle growth with an ultrafilter having a molecular weight cut-off of 10,000 or less, and then dissolving the solid particles collected by ultrafiltration with nitric acid. Can be obtained.

  The nucleation silver solution may have a solid particle content of 20 mass ppm or less based on the amount of silver in the silver solution, but before mixing with a solution containing a dispersant and a strong reducing agent, It is preferred to ultrafilter the silver solution. Thereby, the solid particles contained in the nucleation silver solution can be reduced.

  Although it is necessary to change the filtration accuracy depending on the particle size of impurities contained in the silver solution, for example, when ultrafiltration with a molecular weight cut off of 150,000 is performed, impurity particles of 10 nm or more can be removed. Usually, since the content of solid particles of less than 10 nm is small, the content can be reduced to 20 mass ppm or less with respect to the amount of silver in the silver solution by removing solid particles of 10 nm or more. If the removal of the solid particles is insufficient, ultrafiltration of the molecular weight cut off may be further performed. However, when the molecular weight cut off is 10,000 or less, the filtration area becomes narrow and the filtration rate is reduced. Is not preferable because the decrease in the resistance becomes remarkable.

  The solid particles referred to here include all particles remaining in solid form in the silver solution, particularly particles having a particle diameter of 10 nm or more, in addition to the impurity particles contained in the silver solution.

  As the silver compound, silver chloride is preferably used as described above. By using silver chloride, there are few problems of gas recovery and environmental impact as when silver nitrate is used as a starting material. As such silver chloride, high-purity silver chloride is stably produced for industrial use. A silver solution can be obtained by dissolving this silver chloride in, for example, aqueous ammonia. Ammonia water that dissolves silver chloride may be a normal one that is used industrially, but is preferably as highly pure as possible in order to prevent contamination with impurities.

  The ammonia amount relative to the silver amount in the nucleation silver solution is preferably 20 to 100 in terms of a molar ratio of silver and ammonia. When the amount of ammonia with respect to the amount of silver is less than 20 in terms of molar ratio, when using silver chloride, silver chloride is difficult to dissolve in aqueous ammonia, so that a silver chloride dissolution residue is generated and acts as a non-uniform nucleus, The particle size of the resulting silver particles may be non-uniform. On the other hand, when the ammonia amount relative to the silver amount exceeds 100, the nucleation reaction rate is slow, and it takes a long time to complete the reduction, which is not preferable.

  The silver concentration in the nucleation silver solution is preferably 0.1 g / L to 6.0 g / L. When the silver concentration is less than 0.1 g / L, sufficient nuclei are not generated with respect to the amount of silver in the later-described particle growth silver solution, and the particle size of the silver powder may become too large. On the other hand, if the silver concentration exceeds 6.0 g / L, the grains grow with nucleation, and silver nuclei having a uniform particle diameter cannot be obtained. When it is desired to suppress the growth of nuclei and obtain a silver nucleus solution in which silver nuclei having a finer and uniform particle size are dispersed, the silver concentration is more preferably 1.0 g / L or less. From these, by making the silver concentration in the silver solution for nucleation preferably 0.1 g / L to 6.0 g / L, more preferably 0.1 g / L to 1.0 g / L, The nuclei generated per silver amount can be made fine and uniform in particle size, and the number thereof can be made almost constant. And thereby, the particle size of the silver particle to produce | generate can be controlled by the ratio of the silver amount in the silver solution for nucleation, and the silver amount in the silver solution for particle growth mentioned later. Details will be described later.

  Thus, in silver nucleus solution preparation process S1, the silver complex in a silver solution is reduced with a strong reducing agent by mixing the solution containing a strong reducing agent mentioned above, a dispersing agent, and the silver solution for nucleation. Then, silver particles serving as a nucleus of silver particle growth in a particle growth step S3 described later are generated.

  In the reduction reaction, the above-described strong reducing agent can be diluted with pure water or the like and used as an aqueous solution in order to control the uniformity or reaction rate of the reaction.

[Nuclear-containing reducing agent solution preparation process]
In the nucleus-containing reducing agent solution preparation step S2, the silver nucleus solution prepared in the silver nucleus solution preparation step S1 and the reducing agent are mixed to obtain a nucleus-containing reducing agent solution containing the nucleus. The reducing agent solution containing the nucleus acts as a reducing agent in a reduction reaction in a particle growth step S3 described later.

  The reducing agent mixed with the silver nucleus solution in the nucleus-containing reducing agent solution preparation step S2 is a weak reducing agent having a higher standard electrode potential and a weak reducing power than the strong reducing agent added in the above-described silver nucleus solution preparation step S1. is there. Specifically, the weak reducing agent to be added is preferably a reducing agent exceeding 0.056 V, particularly preferably ascorbic acid (0.058 V). This ascorbic acid is particularly preferable because of its slow reducing action and the uniform growth of particles from the nucleus.

  The difference in standard electrode potential between the strong reducing agent and the weak reducing agent is more preferably 1.0 V or more. If the difference in standard electrode potential is small, new nuclei may be generated during mixing with the particle growth silver solution described later, resulting in mixing of fine particles and non-uniform particle size. On the other hand, by combining a strong reducing agent having a standard electrode potential difference of 1.0 V or more and a weak reducing agent, nucleation during the grain growth phase can be suppressed, and silver particles having a uniform particle size can be obtained. Can be obtained.

  Moreover, it is preferable to set it as 1 equivalent-3 equivalent with respect to the silver amount in the silver solution for particle growth used for particle growth in the particle growth process S3 mentioned later as addition amount of a weak reducing agent. When the addition amount is less than 1 equivalent with respect to the silver amount in the silver solution for grain growth, unreduced silver remains, which is not preferable. On the other hand, when the addition amount is more than 3 equivalents, the cost increases, which is not preferable.

  In the reduction reaction in the particle growth step S3 to be described later, the above-described reducing agent solution can be diluted with pure water or the like in order to make the reaction uniform or control the reaction rate.

[Particle growth process]
In the particle growth step S3, the content of the nucleus-containing reducing agent solution and the solid particles obtained in the nucleus-containing reducing agent preparation step S2 is 20 mass ppm or less with respect to the amount of silver, and silver for particle growth containing a silver complex A silver particle slurry is obtained by growing silver particles by mixing the solution and reducing the silver complex.

  The silver solution for particle growth is a solution obtained by filtering a solution containing a silver complex obtained by dissolving a silver compound with a complexing agent in the same manner as the nucleation silver solution described above. This silver solution for particle growth is mixed with the prepared nucleus-containing reducing agent solution to reduce the silver complex in the silver solution, and the particles are grown based on the nucleus in the reducing agent solution to form a silver particle slurry. For the solution.

  Here, it is important to use a silver solution for particle growth in which the content of solid particles is 20 mass ppm or less with respect to the amount of silver in the silver solution. When solid particles are included in the silver solution for particle growth, the solid particles act as nuclei in the subsequent particle growth step S3, so that the number of nuclei is apparently increased, so the particle size is controlled to a desired value. Difficult to do. Therefore, the number of nuclei grown in the particle growth step S3 can be controlled by setting the content of solid particles contained in the silver solution for particle growth to 20 mass ppm or less with respect to the silver amount. The particle size can be stably controlled to a desired value. The content of solid particles is an ultrafilter having a fractional molecular weight capable of collecting particles having a particle size of 10 nm or more, such as a fractional molecular weight of 150,000 or less, preferably 100,000 or less. More preferably, after filtration with an ultrafilter of 10,000 or less, the solid particles collected by ultrafiltration are dissolved with nitric acid and analyzed.

  The particle growth silver solution may be a solid particle content of 20 ppm by mass or less based on the amount of silver in the silver solution. However, before mixing with the core-containing reducing agent solution, the particle growth silver solution is limited. It is preferable to perform external filtration. Thereby, the solid particles contained in the silver solution for particle growth can be reduced. Although it is necessary to change the filtration accuracy depending on the particle size of impurities contained in the silver solution, for example, when ultrafiltration with a molecular weight cut off of 150,000 is performed, impurity particles having a particle size of 10 nm or more can be removed. . Usually, the solid particles having a particle size of less than 10 nm have a small amount and have little influence on the particle size control. Therefore, by removing the solid particles having a particle size of 10 nm or more, the content is reduced with respect to the silver amount in the silver solution. It can be 20 mass ppm or less. If the removal of the solid particles is insufficient, ultrafiltration with a smaller molecular weight cut off may be performed. However, when a material with a molecular weight cut-off of 10,000 or less is used, the filtration area becomes narrower and filtration is performed. The speed is likely to decrease, and unlike a small amount of processing such as analysis, production on an industrial scale is unfavorable because productivity decreases.

  The solid particles include undissolved silver sulfide and the like in addition to the impurity particles contained in the silver solution for particle growth. That is, the solid particles referred to here include all particles remaining in solid form in the silver solution for particle growth, particularly particles having a particle size of 10 nm or more.

  As the silver compound in the silver solution for grain growth, it is preferable to use silver chloride from the viewpoint that there are few problems of gas recovery and environmental influence as in the case of using silver nitrate. Moreover, although a detailed reason is unknown, by using silver chloride, it becomes possible to achieve both high productivity and particle size uniformity by combination with a production method using a nucleus. A silver solution can be obtained by dissolving this silver chloride in, for example, aqueous ammonia. Ammonia water that dissolves silver chloride may be a normal one that is used industrially, but is preferably as highly pure as possible in order to prevent contamination with impurities.

  The silver concentration in the silver solution for grain growth is preferably 20 g / L to 90 g / L. Even if the silver concentration is low, particle growth can occur and silver particles can be obtained. However, if it is less than 20 g / L, the amount of waste water increases and the cost increases, and silver powder is produced with high productivity. I can't. On the other hand, if the silver concentration exceeds 90 g / L, it is close to the solubility of silver chloride in aqueous ammonia, and silver chloride may be reprecipitated. In the production method of the present invention, it is possible to control the particle growth by the nuclei within the above silver concentration range, and it is possible to produce a silver powder having a small particle size. In order to obtain silver particles having a more uniform particle size by uniformizing the rate of particle growth, the silver concentration is more preferably 50 g / L or less.

  In the method for producing silver powder according to the present embodiment, the amount of silver in the mixed core-containing reducing agent solution, that is, the ratio between the amount of silver in the nucleation silver solution and the amount of silver in the silver solution for particle growth. Thus, it is possible to control the particle diameter of the obtained silver powder, and it is possible to easily obtain a silver powder having a desired particle diameter. That is, by setting the silver concentration in the nucleation silver solution in the above-described range, the number of nuclei generated per silver amount can be made almost constant, so the amount of silver in the nucleation-containing reducing agent solution That is, the particle size of the silver powder can be controlled by the ratio between the number of silver nuclei and the amount of silver in the silver solution for particle growth. Moreover, in this silver powder production method, since the generation of nuclei and the growth of particles are separated, the range in which the number of nuclei in the reaction solution can be controlled is widened, and a wide range of particle size control can be easily performed. Silver powder can be obtained with high silver concentration and high productivity. Specifically, in order to obtain silver powder having an average particle diameter of 0.3 μm to 2.0 μm by observation with a scanning electron microscope, the amount of silver in the silver solution for particle growth is the amount of silver in the silver solution for nucleation The amount of silver is preferably 50 to 1500 times, more preferably 50 to 500 times. Calculate the number of silver nuclei from the amount of silver in the nucleation silver solution and the particle size of the silver nuclei, and obtain the amount of silver necessary to grow to the desired particle size. It is more preferable to calculate the amount of silver.

  Here, in the particle growth step S3, as described above, a reaction solution is prepared by quantitatively and continuously supplying and mixing the nucleus-containing reducing agent solution and the silver solution for particle growth containing the filtered silver complex, In the reaction solution, the silver complex is reduced to grow silver particles. Thus, by supplying and mixing each solution quantitatively and continuously, the concentration of the silver complex and the concentration of the reducing agent in the reduction reaction field can be kept constant, and constant particle growth can be achieved. Moreover, silver powder can be manufactured with high productivity. In the following description, the particle growth silver solution may be simply referred to as a silver solution, and the nucleus-containing reducing agent solution may be simply referred to as a reducing agent solution.

  As a reaction tube for continuously supplying and mixing the core-containing reducing agent solution and the particle growth silver solution to reduce the silver complex, a first supply tube for supplying the particle growth silver solution (silver solution supply) Tube), a second supply tube (reducing agent solution supply tube) for supplying the core-containing reducing agent solution, and a mixing tube for mixing the silver solution and the reducing agent solution can be used. In this way, each solution of the nucleus-containing reducing agent solution and the particle growth silver solution is individually supplied to the reaction tube and mixed in the mixing tube to cause a reduction reaction. Specifically, for example, a Y-shaped tube is given as a representative example. Further, in the reaction tube, the static mixer can be arranged from the position inside the mixing tube and immediately after the solutions supplied from the supply tubes merge.

  The shape and size of each supply tube and mixing tube are not particularly limited, but a cylindrical shape is preferable in terms of easy connection between the respective pipes. In particular, the mixing tube is preferably cylindrical because it is necessary to dispose a static mixer inside.

  As materials for the silver solution supply pipe and the reducing agent solution supply pipe, materials that do not react with the silver solution and the reducing agent solution may be selected, and may be selected from vinyl chloride, polypropylene, polyethylene, and the like. In addition, as a material of the mixing tube, it is important for selection that it does not react with the silver solution or the reducing agent solution and that silver after the reduction reaction does not adhere, and glass is preferable.

  As a material of the static mixer, glass is preferable like the mixing tube. Further, the number of elements of the static mixer is not particularly limited, but if the amount is too small, the reduction reaction does not proceed uniformly and fine particles are not preferable. On the other hand, if the amount is too large, the mixing tube needs to be lengthened unnecessarily. Therefore, it is not preferable. Therefore, it is preferable to appropriately determine the flow rate and flow rate of each solution.

  In the reaction tube, the reaction solution of the silver solution and the reducing agent solution may flow in the mixing tube until the reduction reaction in the reaction solution is completed 100% by being sufficiently stirred and mixed by the static mixer described above. desirable. In addition, for example, a reducing pipe may be connected to the downstream side of the static mixer to make the reaction field sufficiently long so that the reduction reaction is completed 100%. Thereby, it can prevent that an unreduced silver complex remains and a coarse silver particle is produced | generated.

  As a means for supplying the silver solution for particle growth and the nucleus-containing reducing agent solution to the reaction tube, a general metering pump can be used, but those having small pulsation are preferable. Moreover, it is preferable that the flow volume of the silver solution for particle growth and a nucleus containing reducing agent solution is 10 times or less of the other. If there is a difference of 10 times or more in the flow rate of each solution, there is a problem that uniform mixing is difficult. Moreover, it is preferable that the flow rate of each solution shall be 0.1 L / min or more and 10 L / min or less. When the flow rate is less than 0.1 L / min, productivity deteriorates, which is not preferable. On the other hand, when the flow rate is higher than 10 L / min, it is difficult to mix uniformly, which is not preferable.

  It is preferable that the reaction solution in which the silver solution and the reducing agent solution are mixed in the reaction tube to complete the reduction reaction is once received in a predetermined tank (hereinafter, this tank is referred to as “receiving tank”). In the receiving tank, it is necessary to stir so that silver particles produced by the reduction do not settle. When the silver particles settle, the silver particles form an aggregate and the dispersibility is deteriorated, which is not preferable. Stirring in the receiving tank may be performed with an ability not to allow silver particles to settle, and may be performed using a general stirrer. The reaction liquid entering the receiving tank is sent to a filter such as a filter press by a pump, and can be continuously flowed to the next step.

  When the silver particle slurry is generated as described above, the silver particle slurry is filtered, washed, and dried to generate silver powder.

  The cleaning method is not particularly limited. For example, a method in which silver particles are put into water, stirred using a stirrer or an ultrasonic cleaner, and then filtered and collected using a filter press or the like is used. It is done. In this washing method, it is preferable to repeat the operation consisting of charging into water, stirring washing and filtration several times. The water used for washing is water that does not contain an impurity element harmful to silver powder, and it is particularly preferable to use pure water.

  Next, the washed silver powder is dried to evaporate water. Although it does not specifically limit as a drying method, For example, the silver particle after washing | cleaning is set | placed on a stainless steel bat, and about 40-80 degreeC is used using commercially available drying apparatuses, such as an atmospheric oven or a vacuum dryer. This can be done by heating at a temperature.

  As described above in detail, according to the above-described method for producing silver powder, it is possible to produce silver powder that is controlled to have a uniform particle size that does not contain fine particles. Specifically, the silver powder produced by this production method has an average primary particle size of 0.3 μm to 3.0 μm, more preferably 0.3 μm to 2.0 μm, as observed by a scanning electron microscope. More preferably, it is 0.3 to 0.5 μm, and the relative standard deviation (standard deviation σ / average particle diameter d) of the particle diameter is 0.3 or less, preferably 0.25 or less. Here, the primary particle means what is considered as a unit particle, judging from the appearance.

  According to such a silver powder having a uniform and narrow particle size distribution, it can be suitably used as a silver powder for a paste such as a resin-type silver paste or a fired-type silver paste used for forming a wiring layer or an electrode of an electronic device.

  In addition, the method for producing silver powder according to the present embodiment causes a reduction reaction by quantitatively and continuously supplying and mixing the silver solution for particle growth and the nucleus-containing reducing agent solution. The silver concentration in the liquid is kept constant, constant particle growth can be achieved, and silver powder having a more uniform particle size can be produced with high productivity. Thus, the silver powder production method according to the present embodiment is easy to control the particle size of the silver powder and is excellent in mass productivity, and its industrial value is extremely large.

  EXAMPLES The present invention will be described in further detail below with reference to examples and comparative examples, but the present invention is not limited to these examples.

Example 1
2.88 g of silver chloride (manufactured by Sumitomo Metal Mining Co., Ltd.) was added to a mixed liquid of 66 mL of 25% by mass ammonia water maintained at 36 ° C. in a 38 ° C. bath and 1.22 L of pure water while stirring. Thus, a silver solution for nucleation (the silver concentration in the solution was 1.8 g / L, and the molar ratio of ammonia to the amount of silver was 44) was prepared. Next, 43 g of polyvinyl alcohol (PVA205, manufactured by Kuraray Co., Ltd.) as a dispersant was dissolved in 7.33 L of pure water at 36 ° C., and 0.91 mL of hydrazine monohydrate as a strong reducing agent (for nucleation) The reducing agent solution obtained by adding 3.6 equivalents to the amount of silver in the silver solution was kept at 36 ° C. in a warm bath. Then, a silver nucleus solution was added to the reducing agent solution at a flow rate of 64 mL / min to produce silver nuclei, thereby obtaining a silver nucleation solution.

  Next, 665 g of ascorbic acid as a weak reducing agent (1.4 equivalents with respect to the amount of silver in the silver solution for particle growth described below) was added to the obtained silver core solution to obtain a core-containing reducing agent solution. .

  On the other hand, 842 g of silver chloride (manufactured by Sumitomo Metal Mining Co., Ltd.) was added with stirring to 18 L of 25% by mass ammonia water maintained at a liquid temperature of 32 ° C. in a 33 ° C. bath to obtain a salt complex solution. . This solution was ultrafiltered (fractionated molecular weight: 150,000). Further, an antifoaming agent (manufactured by Adeka Co., Ltd., Adecanol LG-126) was diluted 100 times in volume ratio, and 8.3 mL of this antifoaming agent diluted solution was added to the salt complex solution for particle growth. A silver solution (silver concentration in the solution was 35 g / L) was kept at 32 ° C. in a warm bath. A part of the silver solution for particle growth was collected and subjected to ultrafiltration with a molecular weight cut-off of 10,000 to determine the content of solid particles, which is 20 ppm by mass or less with respect to the amount of silver in the solution. It was confirmed. The amount of polyvinyl alcohol added to the core-containing reducing agent solution is 3.8% by mass with respect to the amount of silver in the particle growth silver solution.

  Using a tube pump (manufactured by MASTERFLEX), the silver solution for particle growth and the nucleus-containing reducing agent solution were fed at 2.7 L / min and 0.90 L / min, respectively, and mixed to obtain a reaction solution. The silver complex was reduced in the reaction solution to obtain a silver particle slurry, which was stored in a receiving tank. After the feeding of the two liquids was completed, stirring in the receiving tank was continued for 30 minutes.

  The reaction liquid after completion of the stirring was filtered using a filter press, and the silver particles were separated into solid and liquid. Subsequently, the recovered silver particles were put into 23 L of 0.05 mol / L NaOH aqueous solution, and 17.8 g of stearic acid emulsion (manufactured by Chukyo Yushi Co., Ltd., Cellosol 920) was added thereto and stirred for 15 minutes. It was recovered by filtration with a filter press. After repeating the operation consisting of addition to 0.05 mol / L NaOH aqueous solution, stirring, and filtration two more times, the recovered silver particles were put into 23 L of pure water, washed by stirring for 15 minutes, and a filter press The operation consisting of filtration by was performed. Thereafter, the silver particles were transferred to a stainless bat and dried at 60 ° C. for 10 hours in a vacuum dryer to obtain silver powder.

  FIG. 2 shows a scanning electron microscope (SEM) image of the obtained silver nucleus, and FIG. 3 shows an SEM image of the silver powder. As apparent from these SEM images, both of the obtained silver nucleus and silver powder were composed of uniform particles. Moreover, the average particle diameters of silver nuclei and silver powder obtained by measuring the particle diameters of 300 or more primary particles from the SEM image and averaging them by the number of particles are 0.11 μm and 0.81 μm, respectively. The relative standard deviation (standard deviation σ / average particle diameter d) of the particle diameter of the silver powder obtained from the results was 0.18, and it was confirmed that the silver powder was uniform and free of fine particles. Moreover, the particle size calculated from the number of added nuclei was 0.8 μm, and it was confirmed that silver powder having the aimed particle size was obtained.

(Example 2)
1.11 g of silver chloride (manufactured by Sumitomo Metal Mining Co., Ltd.) was added to a mixed solution of 25 mL of 25% by mass ammonia water maintained at a liquid temperature of 36 ° C. in a 38 ° C. bath and 0.485 L of pure water while stirring. Thus, a silver solution for nucleation (the silver concentration in the solution was 1.5 g / L, and the molar ratio of ammonia to the silver amount was 44) was prepared. Next, 31 g of polyvinyl alcohol as a dispersant (manufactured by Kuraray Co., Ltd., PVA205) was dissolved in 1.0 L of pure water at 36 ° C., and 0.12 mL of hydrazine monohydrate as a strong reducing agent (for nucleation) The reducing agent solution obtained by adding 1.2 equivalents to the amount of silver in the silver solution was kept at 36 ° C. in a warm bath. Then, a silver nucleus solution was added to the reducing agent solution at a flow rate of 20 mL / min to produce silver nuclei, thereby obtaining a silver nucleation solution.

  Next, 103 g of ascorbic acid as a weak reducing agent (1.4 equivalents with respect to the amount of silver in the silver solution for particle growth described below) was added to the obtained silver core solution to obtain a core-containing reducing agent solution. .

  On the other hand, 175 g of silver chloride (manufactured by Sumitomo Metal Mining Co., Ltd.) was added to 3.29 L of 25 mass% ammonia water maintained at a liquid temperature of 36 ° C. in a 38 ° C. bath while stirring to dissolve the silver complex solution. Obtained. This solution was ultrafiltered (fractionated molecular weight: 150,000). Further, an antifoaming agent (manufactured by Adeka Co., Ltd., Adecanol LG-126) was diluted 100 times in volume ratio, and 1.7 mL of this antifoaming agent dilution was added to the silver complex solution for particle growth. A silver solution (silver concentration in the solution was 35 g / L) was kept at 36 ° C. in a warm bath. A part of the silver solution for particle growth was collected and subjected to ultrafiltration with a molecular weight cut-off of 10,000 to determine the content of solid particles, which is 20 ppm by mass or less with respect to the amount of silver in the solution. It was confirmed. In addition, the addition amount of the polyvinyl alcohol added to the nucleus-containing reducing agent solution is 18% by mass with respect to the silver amount of the particle growth silver solution.

  Using a tube pump (manufactured by MASTERFLEX), the silver solution for particle growth and the nucleus-containing reducing agent solution were fed at 2.4 L / min and 0.80 L / min, respectively, and mixed to obtain a reaction solution. The silver complex was reduced in the reaction solution to obtain a silver particle slurry, which was stored in a receiving tank. After the feeding of the two liquids was completed, stirring in the receiving tank was continued for 30 minutes.

  The reaction liquid after completion of stirring was filtered using a membrane filter having an opening diameter of 0.3 μm to separate the silver particles into solid and liquid. Subsequently, the collected silver particles are put into 2 L of 0.05 mol / L NaOH aqueous solution, and 3.6 g of stearic acid emulsion (manufactured by Chukyo Yushi Co., Ltd., Cellosol 920) is added thereto and stirred for 15 minutes. The solution was collected by filtration through a membrane filter having an opening diameter of 0.3 μm. After repeating the operation consisting of addition to 0.05 mol / L NaOH aqueous solution, stirring, and filtration two more times, the recovered silver particles were put into 2 L of pure water, washed by stirring for 15 minutes, and a filter press The operation consisting of filtration by was performed. Thereafter, the silver particles were transferred to a stainless bat and dried at 60 ° C. for 10 hours in a vacuum dryer to obtain silver powder.

  FIG. 4 shows an SEM image of the obtained silver nucleus, and FIG. 5 shows an SEM image of the silver powder. As apparent from these SEM images, both of the obtained silver nucleus and silver powder were composed of uniform particles. Moreover, the average particle diameter calculated | required by measuring the particle diameter of 300 or more primary particles from the SEM image and averaging with the number of particles was 0.13 micrometer and 0.64 micrometer, respectively, and was obtained from the measurement result. The relative standard deviation (standard deviation σ / average particle diameter d) of the particle diameter of the silver powder was 0.22, confirming that it was uniform and free of fine particles. Moreover, the particle size calculated from the number of added nuclei was 0.7 μm, and it was confirmed that a silver powder having a target particle size was obtained.

(Example 3)
2.21 g of silver chloride used for the nucleation silver solution, 50 mL of 25% aqueous ammonia (silver concentration in the solution is 3.0 g / L, 44 molar ratio of ammonia to the amount of silver), silver nucleation reducing agent Silver powder is obtained in the same manner as in Example 2, except that 0.23 mL of hydrazine monohydrate, a strong reducing agent used in the solution, is added (1.2 equivalents relative to the amount of silver in the nucleation silver solution). And evaluated. A part of the silver solution for particle growth was collected and subjected to ultrafiltration with a molecular weight cut-off of 10,000 to determine the content of solid particles, and it was 20 mass ppm or less based on the amount of silver in the solution. It was confirmed that.

  When SEM observation was performed, both of the obtained silver nucleus and silver powder were composed of uniform particles. The average particle diameters of silver nuclei and silver powder measured by SEM observation are 0.14 μm and 0.42 μm, respectively. The relative standard deviation of the particle diameter of the silver powder obtained from the measurement results (standard deviation σ / average particle diameter) d) was 0.25, which was confirmed to be uniform and free of fine particles. Moreover, the particle size calculated from the number of added nuclei was 0.4 μm, and it was confirmed that silver powder having the aimed particle size was obtained.

Example 4
45.0 g of silver chloride (manufactured by Sumitomo Metal Mining Co., Ltd.) was added to a mixed liquid of 25% by mass of 25% by weight ammonia water at 36 ° C. and 175 L of pure water with stirring, and dissolved. A nucleation silver solution obtained by adding 1350 g of polyvinyl alcohol (manufactured by Kuraray Co., Ltd., PVA205) dissolved in 50 L of pure water at 50 ° C. (the silver concentration in the solution is 0.15 g). / L, the molar ratio of ammonia to silver was 45) was maintained at 36 ° C. Next, a reducing agent solution obtained by adding 9.72 mL of hydrazine monohydrate, which is a strong reducing agent (2.5 equivalent to the amount of silver in the nucleation silver solution), to 37.6 L of pure water. Was maintained at 36 ° C. Then, a reducing agent solution was added to the nucleation silver solution at a flow rate of 630 mL / min to produce silver nuclei, thereby obtaining a silver nucleation solution.

  Next, 20.5 kg of ascorbic acid, which is a weak reducing agent (1.4 equivalents with respect to the amount of silver in the silver solution for particle growth described below) and 69 L of pure water, are added to the obtained silver nucleus solution to form the nucleus. It was set as the containing reducing agent solution.

  On the other hand, 12.6 kg of silver chloride (manufactured by Sumitomo Metal Mining Co., Ltd.) was added to 270 L of 25% by mass ammonia water maintained at a liquid temperature of 32 ° C. with stirring to obtain a silver complex solution. This solution was ultrafiltered (fractionated molecular weight: 150,000). Furthermore, a silver solution for particle growth obtained by diluting an antifoaming agent (manufactured by Adeka Co., Ltd., Adecanol LG-126) 100 times by volume and adding 124 mL of this antifoaming agent dilution to the silver complex solution. (The silver concentration in the solution was 35 g / L) was kept at 32 ° C. in a warm bath. A part of the silver solution for particle growth was collected and subjected to ultrafiltration with a molecular weight cut-off of 10,000 to determine the content of solid particles, which is 20 ppm by mass or less with respect to the amount of silver in the solution. It was confirmed. The amount of polyvinyl alcohol added to the core-containing reducing agent solution is 3.8% by mass with respect to the amount of silver in the particle growth silver solution.

  Using a tube pump (manufactured by MASTERFLEX), the silver solution for particle growth and the nucleus-containing reducing agent solution were fed at 2.7 L / min and 0.90 L / min, respectively, and mixed to obtain a reaction solution. The silver complex was reduced in the reaction solution to obtain a silver particle slurry, which was stored in a receiving tank. After the feeding of the two liquids was completed, stirring in the receiving tank was continued for 30 minutes.

  The reaction liquid after completion of the stirring was filtered using a filter press, and the silver particles were separated into solid and liquid. Subsequently, the recovered silver particles were put into 114 L of 0.05 mol / L NaOH aqueous solution, and 162 g of stearic acid emulsion (manufactured by Chukyo Yushi Co., Ltd., Cellosol 920) was added thereto and stirred for 15 minutes. It was recovered by filtration with a press. After repeating the operation consisting of addition to 0.05 mol / L NaOH aqueous solution, stirring, and filtration two more times, the recovered silver particles were put into 114 L of pure water, washed by stirring for 15 minutes, and a filter press The operation consisting of filtration by was performed. Thereafter, the silver particles were transferred to a stainless bat and dried at 60 ° C. for 10 hours in a vacuum dryer to obtain silver powder.

  FIG. 6 shows an SEM image of the obtained silver nucleus, and FIG. 7 shows an SEM image of the silver powder. As apparent from these SEM images, both of the obtained silver nucleus and silver powder were composed of uniform particles. In addition, the average particle diameters of silver nuclei and silver powder determined by measuring the particle diameters of 300 or more primary particles from the SEM image and averaging them by the number of particles are 0.068 μm and 0.68 μm, respectively. The relative standard deviation (standard deviation σ / average particle diameter d) of the particle diameter of the silver powder obtained from the results was 0.20, and it was confirmed that the silver powder was uniform and free of fine particles. Moreover, the particle size calculated from the number of added nuclei was 0.7 μm, and it was confirmed that a silver powder having a target particle size was obtained.

(Example 5)
2.92 g of silver chloride (manufactured by Sumitomo Metal Mining Co., Ltd.) was dissolved in a mixed liquid of 60 mL of 25% by mass ammonia water maintained at 36 ° C. and 0.5 L of pure water with stirring. A nucleation silver solution (silver concentration in the solution) obtained by adding 43.6 g of polyvinyl alcohol (PVA205, manufactured by Kuraray Co., Ltd.), a dispersant dissolved in 6.76 L of pure water at 50 ° C. Was 0.30 g / L, and the molar ratio of ammonia to silver was 40), and was maintained at 36 ° C. Next, a reducing agent solution obtained by adding 0.63 mL of hydrazine monohydrate, which is a strong reducing agent (2.5 equivalents relative to the amount of silver in the silver solution for nucleation), to 1.22 L of pure water. Was maintained at 36 ° C. Then, a reducing agent solution was added to the nucleation silver solution at a flow rate of 60 mL / min to produce silver nuclei, thereby obtaining a silver nucleation solution.

  Next, 1261 g of ascorbic acid as a weak reducing agent (1.4 equivalents with respect to the amount of silver in the silver solution for particle growth described below) and 2.21 L of pure water are added to the obtained silver nucleus solution to form the nucleus. It was set as the containing reducing agent solution.

  On the other hand, 1587 g of silver chloride (manufactured by Sumitomo Metal Mining Co., Ltd.) was added to 18 L of 25% by mass ammonia water maintained at a liquid temperature of 32 ° C. with stirring to obtain a silver complex solution. This solution was ultrafiltered (fractionated molecular weight: 150,000). Further, an antifoaming agent (manufactured by Adeka Co., Ltd., Adecanol LG-126) was diluted 100 times in volume ratio, and 15.6 mL of this antifoaming agent dilution was added to the silver complex solution for particle growth. A silver solution (silver concentration in the solution was 67 g / L) was kept at 32 ° C. in a warm bath. A part of the silver solution for particle growth was collected and subjected to ultrafiltration with a molecular weight cut-off of 10,000 to determine the content of solid particles, which is 20 ppm by mass or less with respect to the amount of silver in the solution. It was confirmed. The amount of polyvinyl alcohol added to the core-containing reducing agent solution is 2.0% by mass with respect to the amount of silver in the particle growth silver solution.

  Using a tube pump (manufactured by MASTERFLEX), the silver solution for particle growth and the nucleus-containing reducing agent solution were fed at 2.7 L / min and 0.90 L / min, respectively, and mixed to obtain a reaction solution. The silver complex was reduced in the reaction solution to obtain a silver particle slurry, which was stored in a receiving tank. After the feeding of the two liquids was completed, stirring in the receiving tank was continued for 30 minutes.

  The reaction liquid after completion of the stirring was filtered using a filter press, and the silver particles were separated into solid and liquid. Subsequently, the collected silver particles were put into 17 L of 0.05 mol / L NaOH aqueous solution, and 20.4 g of stearic acid emulsion (manufactured by Chukyo Yushi Co., Ltd., Cellosol 920) was added thereto and stirred for 15 minutes. It was recovered by filtration with a filter press. After repeating the operation consisting of addition to 0.05 mol / L NaOH aqueous solution, stirring, and filtration two more times, the recovered silver particles were put into 17 L of pure water, washed by stirring for 15 minutes, and a filter press The operation consisting of filtration by was performed. Thereafter, the silver particles were transferred to a stainless bat and dried at 60 ° C. for 10 hours in a vacuum dryer to obtain silver powder.

  FIG. 8 shows an SEM image of the obtained silver nucleus, and FIG. 9 shows an SEM image of the silver powder. As apparent from these SEM images, both of the obtained silver nucleus and silver powder were composed of uniform particles. Further, the average particle diameters of silver nuclei and silver powder obtained by measuring the particle diameters of 300 or more primary particles from the SEM image and averaging them by the number of particles are 0.072 μm and 0.68 μm, respectively. The relative standard deviation (standard deviation σ / average particle diameter d) of the particle diameter of the silver powder obtained from the results was 0.19, and it was confirmed that the silver powder was uniform and free of fine particles. Moreover, the particle size calculated from the number of added nuclei was 0.7 μm, and it was confirmed that a silver powder having a target particle size was obtained.

(Example 6)
45 mL of 25% by mass ammonia water used for the nucleation silver solution, 1513 g of ascorbic acid used for the nucleus-containing reducing agent solution, 1904 g of silver chloride used for the silver solution for grain growth, 20 L of NaOH aqueous solution, 20 L of stearic acid emulsion A silver powder was obtained and evaluated in the same manner as in Example 2 except that the amount was 24.4 g. In this example, the silver concentration in the nucleation silver solution is 0.30 g / L, the molar ratio of the ammonia amount to the silver amount is 30, and the silver concentration in the particle growth silver solution is 80 g / L. The amount of polyvinyl alcohol added is 1.7% by mass relative to the amount of silver in the silver solution. Moreover, when a part of silver solution for particle growth was extract | collected and ultrafiltration of the molecular weight cut-off 10,000 was performed and content of a solid particle was calculated | required, it was 20 mass ppm or less with respect to the silver amount in a solution. It was confirmed that.

  FIG. 10 shows an SEM image of the obtained silver nucleus, and FIG. 11 shows an SEM image of the silver powder. As apparent from these SEM images, both of the obtained silver nucleus and silver powder were composed of uniform particles. In addition, the average particle diameters of silver nuclei and silver powder determined by measuring the particle diameters of 300 or more primary particles from the SEM image and averaging them by the number of particles are 0.065 μm and 0.65 μm, respectively. The relative standard deviation (standard deviation σ / average particle diameter d) of the particle diameter of the silver powder obtained from the results was 0.20, and it was confirmed that the silver powder was uniform and free of fine particles. Moreover, the particle size calculated from the number of added nuclei was 0.7 μm, and it was confirmed that a silver powder having a target particle size was obtained.

(Example 7)
2.50 g of silver chloride (manufactured by Sumitomo Metal Mining Co., Ltd.) was poured into a mixed liquid of 34 mL of 25 mass% ammonia water at 36 ° C. and 0.5 L of pure water with stirring to dissolve. And this solution was ultrafiltered (fraction molecular weight 150,000). A nucleation silver solution obtained by adding 218 g of a dispersant polyvinyl alcohol (manufactured by Kuraray Co., Ltd., PVA205) dissolved in 6.8 L of pure water at 50 ° C. (the silver concentration in the solution was 0). .23 g / L, 30) in molar ratio of ammonia to silver was kept at 36 ° C.

  Next, a reducing agent solution obtained by adding 0.57 mL of hydrazine monohydrate, which is a strong reducing agent (3.0 equivalent to the amount of silver in the silver solution for nucleation), to 1.22 L of pure water. Was maintained at 36 ° C. Then, a reducing agent solution was added to the nucleation silver solution at a flow rate of 61 mL / min to produce silver nuclei, thereby obtaining a silver nucleation solution. A part of the silver solution containing the silver complex was collected and subjected to ultrafiltration with a molecular weight cut-off of 10,000 to determine the content of solid particles, which was 20 ppm by mass or less with respect to the amount of silver in the solution. It was confirmed that there was.

  Next, 1009 g of ascorbic acid as a weak reducing agent (1.4 equivalents with respect to the amount of silver in the silver solution for particle growth described below) was added to the obtained silver core solution to obtain a core-containing reducing agent solution. .

  On the other hand, 1270 g of silver chloride (manufactured by Sumitomo Metal Mining Co., Ltd.) was added to 18 L of 25% by mass ammonia water maintained at a liquid temperature of 32 ° C. in a 33 ° C. bath and dissolved to obtain a salt complex solution. It was. And this solution was ultrafiltered (fraction molecular weight 150,000). Further, an antifoaming agent (manufactured by Adeka Co., Ltd., Adecanol LG-126) was diluted 100 times in volume ratio, and 8.3 mL of this antifoaming agent diluted solution was added to the salt complex solution for particle growth. A silver solution (the silver concentration in the solution was 53 g / L) was kept at 32 ° C. in a warm bath.

  A part of the silver solution for particle growth was collected and subjected to ultrafiltration with a molecular weight cut-off of 10,000 to determine the content of solid particles, which is 20 ppm by mass or less with respect to the amount of silver in the solution. It was confirmed. In addition, the addition amount of polyvinyl alcohol added to the nucleus-containing reducing agent solution is 12.6% by mass with respect to the silver amount in the silver solution for particle growth.

  Using a tube pump (manufactured by MASTERFLEX), the silver solution for particle growth and the nucleus-containing reducing agent solution were fed at 2.7 L / min and 0.90 L / min, respectively, and mixed to obtain a reaction solution. The silver complex was reduced in the reaction solution to obtain a silver particle slurry, which was stored in a receiving tank. After the feeding of the two liquids was completed, stirring in the receiving tank was continued for 30 minutes.

  The reaction liquid after completion of the stirring was filtered using a filter press, and the silver particles were separated into solid and liquid. Subsequently, the recovered silver particles were put into 23 L of 0.05 mol / L NaOH aqueous solution, and 17.8 g of stearic acid emulsion (manufactured by Chukyo Yushi Co., Ltd., Cellosol 920) was added thereto and stirred for 15 minutes. It was recovered by filtration with a filter press. After repeating the operation consisting of addition to 0.05 mol / L NaOH aqueous solution, stirring, and filtration two more times, the recovered silver particles were put into 23 L of pure water, washed by stirring for 15 minutes, and a filter press The operation consisting of filtration by was performed. Thereafter, the silver particles were transferred to a stainless bat and dried at 60 ° C. for 10 hours in a vacuum dryer to obtain silver powder.

  FIG. 12 shows a scanning electron microscope (SEM) image of the obtained silver nucleus, and FIG. 13 shows an SEM image of the silver powder. As apparent from these SEM images, both of the obtained silver nucleus and silver powder were composed of uniform particles. In addition, the average particle diameters of silver nuclei and silver powder obtained by measuring the particle diameters of 300 or more primary particles from the SEM image and averaging them by the number of particles are 0.037 μm and 0.40 μm, respectively. The relative standard deviation (standard deviation σ / average particle diameter d) of the particle diameter of the silver powder obtained from the results was 0.21, confirming that it was uniform and free of fine particles. Moreover, the particle size calculated from the number of added nuclei was 0.37 μm, and it was confirmed that a silver powder having a target particle size was obtained.

(Example 8)
50.1 g of silver chloride (manufactured by Sumitomo Metal Mining Co., Ltd.) was added to 1360 mL of 25 mass% ammonia water at 36 ° C. with stirring and dissolved. This solution was subjected to ultrafiltration (fraction molecular weight 150,000), and then poured into 175 L of pure water and mixed. A nucleation silver solution obtained by adding 1350 g of polyvinyl alcohol (manufactured by Kuraray Co., Ltd., PVA205) dissolved in 50 L of pure water at 50 ° C. (the silver concentration in the solution is 0.23 g). / L, the molar ratio of ammonia to silver was 60) was maintained at 36 ° C.

  Next, a reducing agent solution obtained by adding 11.39 mL of hydrazine monohydrate, which is a strong reducing agent (3.0 equivalents relative to the amount of silver in the silver solution for nucleation), to 24.5 L of pure water. Was maintained at 36 ° C. Then, a reducing agent solution was added to the nucleation silver solution at a flow rate of 408 mL / min to produce silver nuclei to obtain a silver nucleation solution. A part of the silver solution containing the silver complex was collected and subjected to ultrafiltration with a molecular weight cut-off of 10,000 to determine the content of solid particles, which was 20 ppm by mass or less with respect to the amount of silver in the solution. It was confirmed that there was.

  Next, 20.5 kg of ascorbic acid as a weak reducing agent (1.4 equivalents with respect to the amount of silver in the silver solution for particle growth described below) and 42.3 L of pure water are added to the obtained silver nucleus solution. Thus, a nucleus-containing reducing agent solution was obtained.

  On the other hand, 35.2 kg of silver chloride (manufactured by Sumitomo Metal Mining Co., Ltd.) was added to 540 L of 25% by mass ammonia water maintained at a liquid temperature of 32 ° C. with stirring to obtain a silver complex solution. This solution was ultrafiltered (fractionated molecular weight: 150,000). Furthermore, a silver solution for particle growth obtained by diluting an antifoaming agent (manufactured by Adeka Co., Ltd., Adecanol LG-126) 100 times by volume and adding 374 mL of this antifoaming agent dilution to the silver complex solution. (The silver concentration in the solution was 53 g / L) was kept at 32 ° C. in a warm bath. A part of the silver solution for particle growth was collected and subjected to ultrafiltration with a molecular weight cut-off of 10,000 to determine the content of solid particles, which is 20 ppm by mass or less with respect to the amount of silver in the solution. It was confirmed. In addition, the addition amount of the polyvinyl alcohol added to the nucleus-containing reducing agent solution is 5.0% by mass with respect to the silver amount in the silver solution for particle growth.

  Using a tube pump (manufactured by MASTERFLEX), the silver solution for particle growth and the nucleus-containing reducing agent solution were fed at 2.7 L / min and 0.90 L / min, respectively, and mixed to obtain a reaction solution. The silver complex was reduced in the reaction solution to obtain a silver particle slurry, which was stored in a receiving tank. After the feeding of the two liquids was completed, stirring in the receiving tank was continued for 30 minutes.

  The reaction liquid after completion of the stirring was filtered using a filter press, and the silver particles were separated into solid and liquid. Subsequently, the collected silver particles are put into 343 L of 0.05 mol / L NaOH aqueous solution, and 490 g of stearic acid emulsion (manufactured by Chukyo Yushi Co., Ltd., Cellosol 920) is added thereto and stirred for 15 minutes. It was recovered by filtration with a press. After repeating the operations of adding 0.05 mol / L NaOH aqueous solution, stirring and filtration two more times, the recovered silver particles were put into 343 L of pure water, washed by stirring for 15 minutes, and filter press The operation consisting of filtration by was performed. Thereafter, the silver particles were transferred to a stainless bat and dried at 60 ° C. for 10 hours in a vacuum dryer to obtain silver powder.

  FIG. 14 shows a scanning electron microscope (SEM) image of the obtained silver nucleus, and FIG. 15 shows an SEM image of the silver powder. As apparent from these SEM images, both of the obtained silver nucleus and silver powder were composed of uniform particles. In addition, the average particle diameters of silver nuclei and silver powder determined by measuring the particle diameters of 300 or more primary particles from the SEM image and averaging them by the number of particles are 0.040 μm and 0.46 μm, respectively. The relative standard deviation (standard deviation σ / average particle diameter d) of the particle diameter of the silver powder obtained from the results was 0.22, confirming that it was uniform and free of fine particles. Moreover, the particle size calculated from the number of added nuclei was 0.40 μm, and it was confirmed that a silver powder having a target particle size was obtained.

Example 9
Silver powder was obtained in the same manner as in Example 8 except that 25% ammonia water used for the silver solution for nucleation was changed to 2260 mL (molar ratio of ammonia amount to silver amount was 50).

  FIG. 16 shows a scanning electron microscope (SEM) image of the obtained silver nucleus, and FIG. 17 shows a SEM image of the silver powder. As apparent from these SEM images, both of the obtained silver nucleus and silver powder were composed of uniform particles. Moreover, the average particle diameters of silver nuclei and silver powder obtained by measuring the particle diameters of 300 or more primary particles from the SEM image and averaging them by the number of particles are 0.035 μm and 0.43 μm, respectively. The relative standard deviation (standard deviation σ / average particle diameter d) of the particle diameter of the silver powder obtained from the results was 0.21, confirming that it was uniform and free of fine particles. Moreover, the particle size calculated from the number of added nuclei was 0.36 μm, and it was confirmed that silver powder having the aimed particle size was obtained.

(Comparative Example 1)
A reducing agent solution in which 31 g of polyvinyl alcohol (PVA205, manufactured by Kuraray Co., Ltd.) as a dispersant is dissolved in 1.0 L of pure water at 36 ° C. and 103 g of ascorbic acid as a weak reducing agent is added, and a silver solution for particle growth A silver powder was obtained in the same manner as in Example 1 except that each was fed to give a reaction solution. That is, in Comparative Example 1, the silver nucleus solution was not added to the reducing agent solution, and silver particles were not generated by the reduction reaction using the nucleus.

  The obtained silver powder was evaluated in the same manner as in Example 1. FIG. 18 shows an SEM image of the obtained silver powder. As is clear from the SEM image, it can be seen that fine silver particles have been generated. Moreover, the average particle diameter of the obtained silver powder was 0.34 μm, and the relative standard deviation (standard deviation σ / average particle diameter d) of the particle diameter of the silver powder obtained from the measurement results was 1.29. Thus, many fine particles were produced, and the particle size distribution was broad and not uniform.

(Comparative Example 2)
14.6 g of silver chloride used for the nucleation silver solution, 150 mL of 25% by mass ammonia water (the silver concentration in the nucleation silver solution is 1.5 g / L, and the ammonia amount relative to the silver amount is 20 in molar ratio), silver A silver nucleus solution was obtained in the same manner as in Example 5 except that the amount of hydrazine used for nucleation was changed to 6.33 mL.

  The obtained silver nuclei are precipitated, and in order to function as nuclei, the silver nuclei need to be uniformly redispersed in the nucleus-containing reducing agent solution. Therefore, the silver concentration in the nucleation silver solution is preferably 1.0 g / L or less.

(Comparative Example 3)
90.2 g of silver chloride used in the silver solution for nucleation, 5600 mL of 25% by mass ammonia water (the silver concentration in the silver solution for nucleation is 0.3 g / L, the amount of ammonia relative to the amount of silver is 120 in molar ratio), polyvinyl A silver nucleus solution was obtained in the same manner as in Example 4 except that 2700 g of alcohol and 19.44 mL of hydrazine used for silver nucleus generation were used.

  Even if it was held for 1 hour after the addition of hydrazine, the reaction was not completed, and when ascorbic acid was added thereto, it was confirmed that the silver nuclei were linked as shown in the SEM image in FIG. . Thus, when the amount of ammonia increases, nucleation takes a long time and productivity decreases, whereas when a weak reducing agent is added before the completion of the reaction, the uniformity of the nuclei decreases. Moreover, when the silver nuclei are connected and the particle size is not uniform, the uniformity of the particle size of the finally obtained silver powder may be affected. Therefore, the ammonia amount relative to the silver amount in the nucleation silver solution is preferably 100 or less in terms of molar ratio.

(Comparative Example 4)
2.25 g of silver chloride (manufactured by Sumitomo Metal Mining Co., Ltd.) was added to a mixed liquid of 50 mL of 25% by mass ammonia water maintained at a liquid temperature of 36 ° C. in a 38 ° C. bath and 0.46 L of pure water while stirring. Thus, a nucleation silver solution (silver concentration in the solution was 3.0 g / L, 44 in terms of a molar ratio of ammonia to the amount of silver) was prepared.

  Next, 31 g of polyvinyl alcohol as a dispersant (manufactured by Kuraray Co., Ltd., PVA205) was dissolved in 1.0 L of pure water at 36 ° C., and 0.23 mL of hydrazine monohydrate as a strong reducing agent (for nucleation) The reducing agent solution obtained by adding 1.2 equivalents to the amount of silver in the silver solution was kept at 36 ° C. in a warm bath. Then, a silver nucleus solution was added to the reducing agent solution at a flow rate of 20 mL / min to produce silver nuclei, thereby obtaining a silver nucleation solution. A part of the silver solution containing the silver complex was collected and subjected to ultrafiltration with a molecular weight cut-off of 10,000 to determine the content of solid particles, which was 55 ppm by mass with respect to the amount of silver in the solution. It was.

  Next, 103 g of ascorbic acid as a weak reducing agent (1.4 equivalents with respect to the amount of silver in the silver solution for particle growth described below) was added to the obtained silver core solution to obtain a core-containing reducing agent solution. .

  On the other hand, 175 g of silver chloride (manufactured by Sumitomo Metal Mining Co., Ltd.) was added to 3.29 L of 25 mass% ammonia water maintained at a liquid temperature of 36 ° C. in a 38 ° C. bath while stirring to dissolve the silver complex solution. Obtained. This solution was filtered using a membrane filter having a pore size of 0.1 μm. Further, an antifoaming agent (manufactured by Adeka Co., Ltd., Adecanol LG-126) was diluted 100 times in volume ratio, and 1.7 mL of this antifoaming agent dilution was added to the silver complex solution for particle growth. A silver solution (silver concentration in the solution was 35 g / L) was kept at 36 ° C. in a warm bath. A part of the silver solution for grain growth was collected and subjected to ultrafiltration with a molecular weight cut-off of 10,000 to determine the content of solid particles, which was 53 mass ppm with respect to the amount of silver in the solution. . In addition, the addition amount of the polyvinyl alcohol added to the nucleus-containing reducing agent solution is 18% by mass with respect to the silver amount of the particle growth silver solution.

  Using a tube pump (manufactured by MASTERFLEX), the silver solution for particle growth and the nucleus-containing reducing agent solution were fed at 2.4 L / min and 0.80 L / min, respectively, and mixed to obtain a reaction solution. The silver complex was reduced in the reaction solution to obtain a silver particle slurry, which was stored in a receiving tank. After the feeding of the two liquids was completed, stirring in the receiving tank was continued for 30 minutes.

  The reaction liquid after completion of stirring was filtered using a membrane filter having an opening diameter of 0.3 μm to separate the silver particles into solid and liquid. Subsequently, the collected silver particles are put into 2 L of 0.05 mol / L NaOH aqueous solution, and 3.6 g of stearic acid emulsion (manufactured by Chukyo Yushi Co., Ltd., Cellosol 920) is added thereto and stirred for 15 minutes. The solution was collected by filtration through a membrane filter having an opening diameter of 0.3 μm. After repeating the operation consisting of addition to 0.05 mol / L NaOH aqueous solution, stirring, and filtration two more times, the recovered silver particles were put into 2 L of pure water, washed by stirring for 15 minutes, and a filter press The operation consisting of filtration by was performed. Thereafter, the silver particles were transferred to a stainless bat and dried at 60 ° C. for 10 hours in a vacuum dryer to obtain silver powder.

  The obtained silver powder was evaluated in the same manner as in Example 1. FIG. 20 shows an SEM image of the obtained silver nucleus, and FIG. 21 shows an SEM image of the silver powder. The average particle diameter of the obtained silver powder was 0.42 μm, and the relative standard deviation (standard deviation σ / average particle diameter d) of the particle diameter of the silver powder obtained from the measurement results was 0.25. The particle size calculated from the number of added nuclei is 0.7 μm, and it is presumed that the impurity particles contained in the silver complex functioned as nuclei and influenced the particle size of the silver powder. Thus, when the nucleation silver solution is not filtered, the particle size of the silver nuclei is increased, and silver powder having a high uniformity in particle size can be obtained, but it is difficult to control the silver powder to a desired particle size. .

Claims (13)

A silver solution containing a silver complex and a reducing agent solution are continuously mixed to obtain a reaction solution. After the silver complex in the reaction solution is reduced to obtain a silver particle slurry, each step of filtration, washing, and drying is performed. A silver powder production method for producing silver powder,
A silver nucleus solution preparation step of obtaining a silver nucleus solution by mixing a silver solution for nucleation containing a silver complex, a solution containing a strong reducing agent, and a dispersant;
A core containing reducing agent solution preparing step for obtaining a core containing reducing agent solution by mixing a silver core solution obtained by the silver core solution preparing step and a weak reducing agent having a higher standard electrode potential than the strong reducing agent;
A core-containing reducing agent solution obtained by the above-described core-containing reducing agent solution preparation step and a silver solution for particle growth containing a silver complex, wherein the content of solid particles is 20 mass ppm or less with respect to the amount of silver To a reaction liquid, and a particle growth step of growing silver particles by reducing the silver complex in the reaction liquid;
Have
The equivalent of the strong reducing agent with respect to the amount of silver in the silver solution for nucleation is 1.0 equivalent or more and less than 4.0 equivalent, the standard electrode potential of the strong reducing agent is 0.056 V or less, and the nucleus A method for producing a silver powder, wherein the silver concentration in the production silver solution is 0.1 g / L to 6.0 g / L.   The method for producing silver powder according to claim 1, wherein the silver solution for nucleation has a solid particle content of 20 mass ppm or less with respect to the silver content.   The method for producing silver powder according to claim 2, further comprising a filtration step of ultrafiltration of the silver solution for nucleation before mixing with the solution containing the strong reducing agent.   The method for producing silver powder according to any one of claims 1 to 3, further comprising a filtration step of ultrafiltration of the silver solution for particle growth before mixing with the core-containing reducing agent solution. .   The method for producing silver powder according to claim 3 or 4, wherein the ultrafiltration has a molecular weight cut-off of 150,000 or less.   The method for producing silver powder according to any one of claims 1 to 5, wherein a difference in standard electrode potential between the strong reducing agent and the weak reducing agent is 1.0 V or more.   The method for producing silver powder according to any one of claims 1 to 6, wherein the strong reducing agent is hydrazine monohydrate, and the weak reducing agent is ascorbic acid.   The silver concentration in the silver solution for nucleation is 0.1 g / L to 1.0 g / L, and the silver concentration in the silver solution for particle growth is 20 g / L to 90 g / L. The manufacturing method of the silver powder of any one of Claims 1-7.   The method for producing silver powder according to any one of claims 1 to 8, wherein the silver complex is a silver ammine complex obtained by dissolving silver chloride in aqueous ammonia.   The method for producing silver powder according to any one of claims 1 to 9, wherein the ammonia amount with respect to the silver amount in the nucleation silver solution is 20 to 100 in terms of molar ratio.   The mixing amount of the dispersant is 1% by mass to 30% by mass with respect to the amount of silver in the particle growth silver solution after mixing the nucleus-containing reducing agent solution and the particle growth silver solution. The manufacturing method of the silver powder of any one of Claims 1-10.   The said dispersing agent is at least 1 sort (s) selected from polyvinyl alcohol, polyvinylpyrrolidone, a modified silicone oil type surfactant, and a polyether type surfactant, The any one of Claims 1-11 characterized by the above-mentioned. Silver powder production method.   13. The mixing of the nucleus-containing reducing agent solution and the silver solution for particle growth is performed by supplying each solution individually to a reaction tube and mixing with a static mixer disposed in the reaction tube. The manufacturing method of the silver powder of any one of these.