JP6086046B2 - Silver powder manufacturing method and silver powder manufacturing apparatus - Google Patents

Description

  The present invention relates to a method for producing silver powder and a production apparatus used for the production, and more specifically, a resin-type silver paste or a fired-type silver paste used for forming a wiring layer or an electrode of an electronic device. The present invention relates to a method for producing silver powder as a component.

  Silver pastes such as resin-type silver paste and fired-type silver paste are frequently used to form wiring layers and electrodes in electronic devices. These silver pastes are applied or printed, and then heat-cured or heat-baked 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 silver paste, it is possible to form a wiring having a uniform thickness and an electrode having a uniform thickness.

  The properties required for silver powder for silver paste vary depending on the application and use conditions, but the general and important thing is that the particle size is uniform, there is little aggregation, and the dispersibility in the silver paste is high. It is. This is because when the particle size of the silver powder 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. Conversely, if the particle size is uneven and the dispersibility is poor, the silver particles are not uniformly present in the printed film, so the thickness and thickness of the wiring and electrodes are not uniform, and curing or firing is not uniform. Therefore, the resistance of the conductive film tends to increase, or 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 main component of the silver paste and therefore has a large proportion of the silver paste price. In order to reduce manufacturing costs, it is important not only to have high productivity and low unit cost of raw materials and materials to be used, but also to have low waste liquid and exhaust treatment costs.

  Silver powder used in the silver paste described above is often manufactured in a batch system in which a reducing agent solution is introduced into a tank containing a solution containing an ammine complex of silver salt such as silver nitrate. However, in the batch method, the reduction reaction starts locally at the position where the reducing agent is charged, and the nuclei of silver particles are generated from the start to the end of the charging of the reducing agent. It is difficult to obtain silver powder.

  For this reason, not a batch method but a reduction method in which a solution containing a silver salt ammine complex and a reducing agent solution are continuously mixed has been proposed.

Patent Document 1 provides a second flow path b in which the silver ammine complex aqueous solution S ₁ flows through a fixed first flow path a and joins in the middle of the first flow path a, and the organic reducing agent is passed through the second flow path b. and flowing the additive S ₂ as needed, a manufacturing method of the first flow path a and silver powder contacting mixture to be reduced and deposited at the merging point m between the second flow path b is disclosed.

However, silver powder obtained by this method, the average particle diameter D _IA of the primary particles obtained by image analysis of a scanning electron microscope image is 0.6μm or less, the crystallite diameter is not more 10nm or less, a fine particle. For this reason, it is unsuitable for the use of a general silver paste, and the use is limited. Further, it is difficult to say that the production method has a low silver concentration in the reaction solution and excellent productivity.

  Including the manufacturing method described above, the raw material used as the silver source is generally silver nitrate. However, silver nitrate generates toxic nitrous acid gas in the process of dissolution in aqueous ammonia and requires a device for recovering this. Further, since a large amount of nitrate nitrogen and ammonia nitrogen is contained in the wastewater, an apparatus for the treatment is also required. Furthermore, silver nitrate is a dangerous and deleterious substance, so it needs to 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. Silver chloride is not a dangerous or deleterious substance and has the 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 a sufficient purity for use in the electronics industry.

  For example, in Patent Document 2, after silver chloride is dissolved in ammonia water so that the silver concentration is 1 to 100 g / l, a reducing agent is added to this solution in the presence of a protective colloid and stirred, A method for obtaining silver ultrafine particles by liquid phase reduction of a silver ammine complex is disclosed. However, since the silver powder obtained by this method is as fine as 0.1 μm or less in particle size, its use is limited for the electronics industry.

  As described above, many proposals have been made for the production method of silver powder, but silver powder having an average particle diameter of 0.1 μm to several μm and a uniform primary particle diameter, that is, the particle size distribution of the primary particles is narrow, and the dispersibility However, it is not possible to obtain a silver powder with high productivity and to obtain silver powder at a low cost with excellent productivity.

JP 2005-048236 A JP-A-10-265812

  In view of such a conventional situation, the present invention can produce silver powder having an average particle size of 0.1 μm to several μm, a narrow primary particle size distribution, and high dispersibility at high productivity and low cost. It aims at providing the manufacturing method of silver powder, and the manufacturing apparatus of silver powder used for the manufacture.

  In order to achieve the above-mentioned object, the present inventor examined the particle size control of the obtained silver particles in a method for producing silver powder in which a reducing agent solution is continuously mixed and reduced to a solution containing a silver complex to obtain silver powder. As a result, the knowledge that the retention time in the slurry containing the formed silver particles greatly affects the particle size and aggregation state of the silver particles has been obtained, and the present invention has been achieved.

That is, the silver powder production method of the present invention supplies a silver solution containing a silver complex and a reducing agent solution quantitatively and continuously into the flow path, and mixes the silver solution and the reducing agent solution in the flow path. In a silver powder production method for obtaining silver powder by quantitatively and continuously reducing a silver complex in a reaction solution, the reaction solution obtained by mixing the silver solution and the reducing agent solution in the flow channel is removed from the flow channel. The reaction liquid is sent to a receiving tank, and the average residence time in the receiving tank is longer than the time from when the reaction liquid is discharged from the flow path until the oxidation-reduction potential of the reaction liquid reaches the minimum value. The reaction liquid retained in the receiving tank is sent to the outside of the tank and separated into solid and liquid while the reaction liquid is allowed to stay with stirring so that the reaction liquid is discharged from the flow path into the receiving tank. It is characterized by.

  The silver solution is preferably obtained by dissolving silver chloride in aqueous ammonia, the reducing agent is ascorbic acid, and the mixing ratio at the time of mixing the silver solution and the reducing agent solution is reduced with respect to 1 mol of silver. The agent is preferably 0.25 to 0.50 mol.

  Moreover, it is preferable to add at least one selected from polyvinyl alcohol, polyvinyl pyrrolidone, a modified silicone oil surfactant, and a polyether surfactant as a dispersant to the reducing agent solution.

  Furthermore, the silver concentration in the reaction solution is preferably 25 to 50 g / L.

  The silver powder production apparatus of the present invention is a production apparatus used in the above production method, wherein a silver solution containing a silver complex and a reducing agent solution are mixed in a flow path to quantitatively and continuously form a silver complex in a reaction solution. A flow path having a mechanism for automatically reducing, a mechanism for quantitatively and continuously supplying the silver solution and the reducing agent solution to the flow path, and a receiving tank for retaining the reaction liquid discharged from the flow path, And a solid-liquid separation mechanism that separates the retained reaction liquid from the receiving tank for solid-liquid separation, and the capacity of the receiving tank is equal to the sum of the flow rate of the silver solution and the flow rate of the reducing agent solution. More than the amount of liquid obtained by multiplying the time to finish, the discharge position to which the reaction liquid is discharged from the flow path and the reaction liquid in the tank are sent to the solid-liquid separation mechanism in the receiving tank It has a detour mechanism for detouring the flow of the reaction liquid between the liquid feed position and Is shall.

  According to the present invention, silver powder having an average particle size of 0.1 μm to several μm, a narrow primary particle size distribution, and high dispersibility can be obtained. In addition, since the present invention is a production method that continuously reduces, the productivity is extremely high, and the method can be easily carried out even on an industrial scale, and can be carried out at a low cost.

  Furthermore, the present invention is a silver powder that has an appropriate particle size and high dispersibility, and is suitable as a silver powder for pastes such as resin-type silver paste and fired-type silver paste used for forming wiring layers and electrodes of electronic devices. And its industrial value is extremely high.

It is a figure which shows typically an example of the manufacturing apparatus of the silver powder used for the manufacturing method of the silver powder which concerns on this invention. It is a figure which shows typically an example of the reaction tube used for the manufacturing method of the silver powder which concerns on this invention, (A) is a front view of a reaction tube, (B) is sectional drawing of a reaction tube. It is a figure which shows the measurement result which measured the time-dependent change of the oxidation-reduction potential of Example 1. It is a figure which shows the measurement result which measured the time-dependent change of the oxidation-reduction potential of Example 2.

  Hereinafter, specific embodiments of a silver powder production method and a silver powder production apparatus used in the production method 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 appropriately changed without changing the gist of the present invention.

  The silver powder obtained by the method for producing silver powder according to the present invention comprises silver particles. Here, the silver particle morphology is defined as follows. That is, silver particles are considered to be unit particles when judged from the apparent geometric form, and are called primary particles. In addition, a combination of two or more primary particles by necking and aggregation of the primary particles and the necked primary particles is called secondary particles. Accordingly, silver particles are composed of primary particles and secondary particles.

<Method for producing silver powder>
A method for producing silver powder to which the present invention is applied includes a method in which a silver solution containing a silver complex and a reducing agent solution are quantitatively and continuously supplied into a flow path, and the silver solution and the reducing agent solution are supplied into the flow path. The reduction reaction of the silver complex is caused quantitatively and continuously in the reaction solution mixed in step (1), and the silver particle slurry of the reduced solution after the reduction reaction is quantitatively and continuously discharged. As a result, in the silver powder production method, the concentration of the silver complex and the concentration of the reducing agent in the reduction reaction field are kept constant, the rate of nucleation and the concentration thereof are constant, and further constant grain growth can be achieved. . For this reason, in this silver powder production method, silver powder having a uniform particle size distribution, an average particle size of 0.1 μm to several μm, and a sharp primary particle size distribution can be obtained. Furthermore, in this silver powder manufacturing method, the reaction liquid obtained by mixing the silver solution and the reducing agent solution in the flow path is retained while stirring in the receiving tank in which the reaction liquid discharged from the flow path is retained. Thereafter, by performing solid-liquid separation, excessive aggregation can be suppressed, and a silver powder having a narrow particle size distribution and excellent dispersibility can be obtained. Moreover, in this silver powder manufacturing method, the silver powder can be continuously obtained by continuously supplying the silver solution and the reducing agent solution and discharging the silver particle slurry, and the silver powder can be manufactured with high productivity. Can do.

  Furthermore, in this silver powder production method, it is possible to use silver chloride as a raw material, without the need for a recovery device for nitrous acid gas when silver nitrate is used as a starting material or a processing device for nitrate nitrogen in waste water, It is a process that has little impact on the environment, and the manufacturing cost can be reduced.

  In such a method for producing silver powder, the reaction liquid obtained by mixing the silver solution and the reducing agent solution is discharged from the flow path, and the residence time in the receiving tank in which the reaction liquid discharged from the flow path is retained. However, it is important to retain the mixture with stirring so that it is longer than the time for the reduction reaction to be completed, and to perform solid-liquid separation. As a result, in this silver powder production method, the reduction reaction is completed in the receiving tank, and the unreduced silver complex remaining in the reaction solution is almost consumed, so the silver complex is reduced during the solid-liquid separation. Aggregation of silver particles due to silver produced in this manner can be suppressed.

  Here, the time at which the reduction reaction ends (hereinafter referred to as the end time) is the time from when the reaction solution is discharged from the flow path until the reduction reaction ends in the receiving tank. That is, since the end time is a reference for the residence time of the reaction solution in the receiving tank, the start point is the time when the reaction solution is discharged from the flow path, and the end point is when the reduction reaction of the silver complex is completed. is there. The end time can also be determined by the silver ion concentration in the reaction solution. However, since the oxidation-reduction potential (ORP) decreases as the reduction reaction proceeds, the time until the ORP reaches the minimum value is determined. It is easy and quick to determine by measurement, which is preferable. Therefore, the end time is preferably a time until the oxidation-reduction potential (ORP) of the reaction solution reaches the minimum value. On the other hand, since the time until the ORP reaches the minimum value is the time required for the silver complex to be reduced, the measurement is difficult in a receiving tank to which a new silver complex is supplied. Therefore, it is determined by separating the reaction solution at the outlet of the flow path and measuring the ORP change of the collected reaction solution. Since the time until the ORP reaches the minimum value is stable if the reduction reaction is under the same conditions, in the production method in which the reduction reaction is continuously performed under the same conditions as in the present invention, The residence time in the receiving tank can be determined from the measurement result.

  Furthermore, in this method for producing silver powder, it is preferable that the reaction liquid retained in the receiving tank is sent out of the tank and solid-liquid separated while discharging the reaction liquid from the outlet of the flow path into the receiving tank. The total amount of each of the silver solution and the reducing agent solution may be mixed and retained in the receiving tank, followed by solid-liquid separation. However, in order to retain the entire amount, it is necessary to increase the capacity of the receiving tank, and the solid-liquid separation Since the time to do is required separately from the time for mixing the silver solution and the reducing agent solution, the productivity is lowered. It is possible to reduce the size of the receiving tank by mixing the silver solution and the reducing agent solution and discharging the reaction liquid from the flow path to the receiving tank while sending the reaction liquid in the receiving tank to the outside of the tank for solid-liquid separation. At the same time, it is possible to overlap the retention in the receiving tank and the solid-liquid separation, and the productivity can be improved.

  The liquid feeding from the receiving tank to the solid-liquid separation step may be intermittent, but is preferably continuous and more preferably a constant amount. Moreover, what is necessary is just to make it the average liquid feeding speed to a solid-liquid separation process correspond with the discharge speed from a flow path. In addition, if the reaction liquid is discharged from the flow path to the receiving tank and the solid-liquid separation is performed by sending the liquid outside the tank, the silver complex that has not undergone the reduction reaction is also sent to the solid-liquid separation process. If the average residence time is equal to or longer than the end time, the amount of the silver complex to be sent is small, and aggregation of silver particles during solid-liquid separation can be suppressed.

  Below, the manufacturing method of the silver powder which concerns on this Embodiment is demonstrated in detail.

  The silver solution is a solution containing a silver complex that is reduced to silver, and various silver salts can be used as a raw material for silver, but it is preferably obtained by dissolving silver chloride in aqueous ammonia. . Thus, in the method for producing silver powder, by using silver chloride as a raw material, a recovery device for nitrous acid gas and a treatment device for nitrate nitrogen in wastewater required for the method using silver nitrate as a starting material are installed. This is a process that is not necessary and has little impact on the environment, and can reduce manufacturing costs. Silver chloride having high purity is preferably used, and high purity silver chloride is stably produced for industrial use. 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.

  As the reducing agent, general hydrazine, formalin and the like can be used, but it is particularly preferable to use ascorbic acid. Ascorbic acid has a slow reducing action, so the crystal grains in the silver particles are easy to grow, the nucleation rate is slow when the silver solution and the reducing agent solution are mixed, and the primary particle size is uniformized and aggregated. Is easy and preferable. Moreover, in order to control the uniformity of reaction or reaction rate, it can also be used as an aqueous solution whose concentration is adjusted by dissolving or diluting a reducing agent with pure water or the like.

  When ascorbic acid is used as the reducing agent, stoichiometrically, 1 mol of silver can be reduced with 0.25 mol of ascorbic acid. The mixing ratio at the time of mixing the silver solution and the reducing agent solution is preferably larger than the mixing ratio based on the stoichiometry, specifically, the reducing agent is 0.25 to 0.50 mol per 1 mol of silver. It is preferable to use 0.30 to 0.40 mol. When the amount is less than 0.25 mol, unreduced silver complex remains in the waste liquid, and the yield of silver powder decreases. On the other hand, if it exceeds 0.50 mol, a large amount of ascorbic acid that is not used for reduction remains, which is disadvantageous in terms of cost.

  In the method for producing silver powder, it is preferable to adjust the silver concentration in the reaction solution mixed with the silver solution and the reducing agent solution in the range of 25 to 50 g / L. This makes it possible to produce silver powder with a narrower particle size distribution with high productivity. That is, by adjusting the silver concentration in the reaction solution in the range of 25 to 50 g / L, the particle size and particle size distribution of the silver particles produced by the reduction can be more strictly controlled.

  As described above, in this silver powder production method, since the silver solution and the reducing agent solution are continuously mixed quantitatively, the concentration of the silver complex and the concentration of the reducing agent in the reduction reaction field after mixing are kept constant. Be drunk. Therefore, since the rate of nucleation and its concentration are constant, abnormal grain growth due to concentration fluctuation is suppressed even at high silver concentration, and the growth rate of particles as a whole can be kept constant. Generation can be suppressed.

  Here, when the silver concentration is low, the growth rate of the particles is kept constant, but the particle growth is not sufficient, and the obtained primary particles may be fine. Such fine primary particles tend to cause excessive aggregation between silver particles in the drying process after washing. On the other hand, when the silver concentration is high, too many nucleations are generated even if the concentration of nucleation is kept constant, so that particle aggregation may occur and coarse particles may be generated. Therefore, by adjusting the silver concentration in the reaction solution after mixing the silver solution and the reducing agent solution in the range of 25 to 50 g / L, silver powder having a narrower particle size distribution can be obtained with higher productivity.

  More specifically, the particle size of the primary particles tends to decrease when the silver complex in the reaction solution is at a low concentration and increases when the concentration is high, and the particle size can be adjusted by adjusting the silver concentration in the reaction solution. Can be controlled. However, if the silver concentration is less than 25 g / L, the particle size may be too small and sufficient productivity cannot be obtained. Moreover, the tap density of the obtained silver powder may become too low. On the other hand, when the silver concentration exceeds 50 g / L, coarse particles due to aggregation of primary particles are generated, and thus the particle size distribution of the silver particles becomes wide.

  As a means for supplying the silver solution and the reducing agent solution to the reaction tube, a general metering pump can be used, but one with small pulsation is desirable. Further, the flow rate of the pump is desirably variable by inverter control, and the mixing ratio is controlled by adjusting the flow rates of the silver solution and the reducing agent solution to be constant.

  Moreover, it is preferable that the temperature of the reaction liquid at the time of silver reduction reaction shall be 25-40 degreeC. If it is less than 25 degreeC, when silver chloride is used as a raw material, the solubility with respect to the aqueous ammonia of silver chloride will become small, and a desired particle size may not be obtained because the silver concentration in a reaction liquid cannot be raised. On the other hand, when the temperature exceeds 40 ° C., the volatilization of ammonia becomes violent, the solubility decreases, the nucleation rate increases and the particle size may fluctuate, and further silver chloride precipitation may occur.

  Furthermore, in the receiving tank, it is necessary to sufficiently stir so that the silver particles generated by the reduction do not settle. When settled, silver particles form aggregates and dispersibility deteriorates, which is not preferable. Stirring may be performed with such a force that silver particles do not settle, and a general stirrer can be used. The reaction liquid that has entered the receiving tank and the excess reducing agent has been deactivated can be continuously transferred to the next step by sending the reaction liquid to a filter such as a filter press with a pump.

  In the method for producing silver powder, it is preferable to contain a dispersant in a reaction liquid in which a silver solution and a reducing agent solution are mixed. If the dispersant is not contained, silver particles generated by the reduction may aggregate to generate coarse particles or have poor dispersibility. The dispersant is preferably at least one selected from polyvinyl alcohol, polyvinyl pyrrolidone, modified silicone oil surfactants, polyether surfactants, or a combination of two or more of these. More preferred.

  It is preferable to add the dispersant to the reaction solution by adding it to the reducing agent solution in advance. Mixing the dispersant in the silver solution may be an option, but it has been experimentally confirmed that a silver powder having better dispersibility can be obtained by mixing in the reducing agent solution. This is presumably because by adding a dispersant to the reducing agent solution, the dispersant is present in the silver particle production field, and the aggregation of the silver particles can be efficiently suppressed. In addition, since polyvinyl alcohol and polyvinylpyrrolidone used as a dispersant may foam during the reduction reaction, an antifoaming agent may be added to the reducing agent solution or the silver solution.

  The content of the dispersant may be appropriately determined depending on the type of the dispersant and the silver powder particle size to be obtained, but is preferably 0.3 to 20% by mass with respect to the silver contained in the silver solution. 0.3 to 10% by mass is more preferable. If the content of the dispersant is less than 0.3% by mass, the silver particle aggregation suppressing effect may not be sufficiently obtained. On the other hand, even if the content exceeds 20% by mass, it is further aggregated. There is no improvement in the suppression effect, and only the load such as wastewater treatment increases.

  The reaction solution after the reduction reaction is separated into solid and liquid, then washed and dried. For solid-liquid separation, filtration or a centrifuge can be used. However, when solid-liquid separation is performed by continuously feeding liquid, it is preferable to use a filter press. The washing method is not particularly limited. For example, silver particles are put into water, stirred using a stirrer or an ultrasonic cleaner, and then solid-liquid separated to collect silver powder. The method can be used. In this method, it is preferable to repeat the operations of adding silver particles into water, stirring and washing, and solid-liquid separation 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.

  Then, after washing with water, the moisture of the silver particles is evaporated and dried. As a drying method, for example, the silver particles after washing with water are placed on a stainless steel pad and heated at a temperature of about 40 to 80 ° C. using a general drying device such as an atmospheric oven or a vacuum dryer. It can be carried out.

  The dried silver powder is processed by a crusher to break dry agglomeration. The crusher is preferably an apparatus having a weak crushing force such as a vacuum decompression atmosphere rolling stirrer. This is because if the crushing ability is too strong, the aggregated form controlled in the reduction process is broken. Only the weak agglomeration agglomerated in the drying process needs to be solved.

<2. Silver powder manufacturing equipment>
As an example of the silver powder production apparatus used in the above-described silver powder production method, the silver powder production apparatus shown in FIGS. 1 and 2 will be described.

  The silver powder production apparatus 1 shown in FIG. 1 mixes a silver solution containing a silver complex and a reducing agent solution to form a flow path 15 having a mechanism for quantitatively and continuously reducing the silver complex in the reaction solution. A pipe 10, supply mechanisms 20 and 30 for supplying a silver solution and a reducing agent solution to the flow path 15 quantitatively and continuously, a receiving tank 40 for retaining the reaction liquid discharged from the flow path 15, and a post-retention A solid-liquid separation mechanism 50 is provided that feeds the reaction liquid from the receiving tank 40 and separates the reaction liquid into solid and liquid. In the silver powder production apparatus 1, the capacity of the receiving tank 40 is equal to or larger than the liquid amount obtained by multiplying the sum of the flow rate of the silver solution and the flow rate of the reducing agent solution by the time for the completion of the reduction reaction. Between the discharge position 40 a where the reaction liquid is discharged from the reduction pipe 14 that forms the flow path 15 together with the reaction tube 10, and the liquid supply position 40 b where the reaction liquid in the tank is sent to the solid-liquid separation mechanism 50, A detour mechanism 42 for detouring the flow of the reaction liquid is provided in the receiving tank 40.

  The flow path 15 is constituted by the reaction tube 10 whose specific example is shown in FIG. FIG. 2A is a view showing the front of the reaction tube 10, and FIG. 2B is a view schematically showing the X-X ′ cross section of the reaction tube 10. As shown in FIG. 2, this reaction tube 10 includes a silver solution supply tube 11 that is a pipe for supplying a silver solution containing a silver complex, a reducing agent solution supply pipe 12 that is a pipe for supplying a reducing agent solution, The solution supply pipe 11 and the reducing agent solution supply pipe 12 are joined to each other, and the mixing pipe 13 is a pipe for mixing the silver solution and the reducing agent solution. The silver solution supply pipe 11 is formed in a straight line, and the reducing agent solution supply pipe 12 is formed in a substantially L shape.

  As shown in FIG. 2, the reaction tube 10 has a linear portion of the reducing agent solution supply pipe 12 inside the silver solution supply pipe 11 so that both solutions are supplied in the same direction at the outlet of the reducing agent solution supply pipe 12. 12A is arranged on the same axis, and the silver solution supply pipe 11 and the reducing agent solution supply pipe 12 are combined. Thereby, in the reaction tube 10, the silver solution and the reducing agent solution can be supplied in the same direction, and the silver solution uniformly surrounds the reducing agent solution supplied from the outlet of the reducing agent solution supply tube 12. Mixed. In the reaction tube 10, such a mixing form suppresses local fluctuation of the mixing ratio and retention of the reaction liquid, and both liquids are mixed with a uniform reducing equivalent, so that the absorption amount is stably controlled. Is possible. Further, it is possible to prevent the reduced silver from being deposited near the outlet of the reducing agent solution supply pipe 12.

  In the reaction tube 10, it is preferable to arrange the reducing agent solution supply tube 12 so that the supply direction of the reducing agent solution with respect to the supply direction of the silver solution is 0 °. It should be noted that an angle may be provided to the extent that an error in manufacturing the reaction tube 10 is included, and the supply direction of the reducing agent solution with respect to the supply direction of the silver solution may be an angle within 5 °. That is, within the range in which the above-described mixing form is possible, the supply direction includes that the supply direction of the reducing agent solution with respect to the supply direction of the silver solution is within 5 °, and is coaxial with the reducing agent solution supply pipe 12. Arrangement means that the feed direction includes an angle that is within 5 °.

  For example, in the reaction tube 10, the straight portion 12 </ b> A of the reducing agent solution supply pipe 12 disposed coaxially with the silver solution supply pipe 11 inside the silver solution supply pipe 11 is five times the inner diameter of the reducing agent solution supply pipe 12. It is provided in the above length. Thereby, the reducing agent solution that exits from the outlet of the reducing agent solution supply pipe 12 can be made into a laminar flow, and both solutions are uniformly mixed due to the difference in flow rate of each solution.

  The arrangement of the supply pipes 11 and 12 can be appropriately changed depending on the supply amount and flow rate of each solution, and the linear portion 12A of the reducing agent solution supply pipe 12 and the outside of the silver solution supply pipe 11 can be changed. The connecting portion may be curved, or the angle formed between the supply direction of the silver solution and the supply direction of the reducing agent solution supplied from outside the reaction tube may be reduced so that the reducing agent solution becomes more easily laminar. Further, the dimensions and the like of the supply pipes 11 and 12 are not particularly limited, and can be set as appropriate based on a desired flow rate, flow state, and the like when supplying each solution.

  It is important for the material of the reaction tube 10 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 or quartz is preferable. The silver solution supply pipe 11 and the reducing agent solution supply pipe 12 may be selected from materials that do not react with the silver solution or the reducing agent solution, and can be selected from vinyl chloride, polypropylene, polyethylene, and the like.

  Here, the silver solution and the reducing agent solution are mixed in the flow path 15 in order to control the particle size by sufficiently proceeding the reduction reaction of the silver complex in the flow path 15 using the silver powder production apparatus 1. It is preferable to configure the flow path 15 so that the time (flow time) from when the reaction solution flows down through the flow path 15 to the outlet (flow time) is 15 seconds to 60 seconds.

  If the flow time is less than 15 seconds, the reduction reaction becomes insufficient, a large amount of unreduced silver complex remains in the reaction solution, and the silver particles are connected to become coarse particles, or aggregated and dispersed. May get worse. On the other hand, in the time exceeding 60 seconds, the manufacturing apparatus is merely enlarged unnecessarily.

  In the silver powder production apparatus 1, when the flow-down time of 15 seconds or more cannot be achieved only by the reaction tube 10, as shown in FIG. 1, the reducing tube formed of a tubular material such as a soft tube in the mixing tube 13. 14 may be connected to adjust the length of the flow path 15. When the reducing tube 14 is formed of a soft tube, the soft tube may be spirally wound around the mixing tube 13. Thereby, the length of the flow path 15 can be adjusted without requiring a space.

  As the mechanisms 20 and 30 for supplying the silver solution and the reducing agent solution to the flow path 15 quantitatively and continuously, a general metering pump can be used, and one having small pulsation is preferable.

  The receiving tank 40 retains the reaction liquid discharged from the flow path 15 while stirring. The receiving tank 40 needs to be equal to or larger than the liquid amount obtained by multiplying the sum of the flow rate of the silver solution and the flow rate of the reducing agent solution by the time for the completion of the reduction reaction. In order to stir the reaction solution, the receiving tank 40 includes, for example, a stirrer or an ultrasonic cleaner. In addition, the receiving tank 40 is provided with a valve 44 on the bottom surface for sending the reaction liquid to the solid-liquid separation mechanism 50 after the discharge from the flow path 15 is completed. Here, for example, “L / minute” is used as the flow rate, and “minute” is used as the end time, for example.

  In the solid-liquid separation mechanism 50, the reaction liquid after staying in the receiving tank 40 is fed from the receiving tank 40, and the reaction liquid is solid-liquid separated. The solid-liquid separation mechanism 50 is, for example, a filter press.

  In the silver powder production apparatus 1, when the entire amount of the reaction liquid cannot be fed at once even if a filter press is used, the second liquid can be stored while further stirring the liquid fed from the receiving tank 40 (first receiving tank 40). The receiving tank 41 is preferably provided between the first receiving tank 40 and the solid-liquid separation mechanism 50. As a result, even when the liquid supply to the filter press is temporarily stopped due to discharge of the solid-liquid separated silver particles, a fixed amount is continuously sent from the first receiving tank 40 to the second receiving tank 41. The residence time can be further constant. The second receiving tank 41 may have the same capacity as or less than the first receiving tank 40, and is equipped with a stirrer or an ultrasonic cleaner as in the first receiving tank 40, and has a solid-liquid separation on the bottom surface. A valve 45 for feeding the mechanism 50 is provided.

  In the first receiving tank 40, there is a reaction liquid between a discharge position 40 a where the reaction liquid is discharged from the flow path 15 and a liquid supply position 40 b where the reaction liquid in the tank is sent to the solid-liquid separation mechanism 50. A bypass mechanism 42 for bypassing the flow is provided. The bypass mechanism 42 prevents liquid from being sent from the discharge position 40a via a short-circuit path. Thereby, it is suppressed that the reaction liquid discharged | emitted from the flow path 15 is sent out of a tank for a short time, and aggregation of the silver particle in the case of solid-liquid separation can be suppressed. As the bypass mechanism 42, the discharge of the reaction liquid from the liquid feeding inlet position (liquid feeding position 40 b) may be blocked by the weir. For example, as shown in FIG. 2, when the discharge of the reaction liquid from the flow path 15 and the suction for sending the liquid to the second receiving tank 41 are performed on the upper surface side of the first receiving tank 40, the reaction liquid is discharged. What is necessary is just to provide the weir 42 in the position 40a so that a reaction liquid may escape in the 1st receiving tank 40 from the lower part.

  In the silver powder production apparatus 1 as described above, the unreduced silver complex in the reaction solution discharged from the flow path 15 is reduced by the retention in the receiving tank 40 (first receiving tank 40), and the reduction reaction is completed. As a result, aggregation of silver particles is suppressed. Therefore, in the silver powder manufacturing apparatus 1, the capacity of the receiving tank 40 (first receiving tank 40) is set to be equal to or larger than the liquid amount obtained by multiplying the sum of the flow rate of the silver solution and the flow rate of the reducing agent solution by the time for completing the reduction reaction. Thus, the average residence time can be made longer than the time when the reduction reaction is completed. That is, the average residence time is obtained by dividing the capacity of the receiving tank 40 (first receiving tank 40) by the sum of the inflowing flow rates. By setting the above-described capacity, the average residence time is equal to or longer than the time at which the reduction reaction ends. It can be.

  In the silver powder production apparatus 1, a silver powder having an average particle size of 0.1 μm to several μm, a narrow primary particle size distribution, and high dispersibility can be produced with high productivity and low cost.

  Specific examples of the present invention will be described below. However, the present invention is not limited to the following examples.

[Example 1]
In Example 1, 45230 g of silver chloride (Sumitomo Metal Mining Co., Ltd., purity 99.9999%, moisture content) was added to 540 L of 25% by mass ammonia water maintained at a liquid temperature of 36 ° C. in a tank heated with a hot water jacket at 38 ° C. 15.44%) was added with stirring to prepare a silver solution. An antifoaming agent (manufactured by Adeka Co., Ltd., Adecanol LG-126) was diluted 100 times in volume ratio, 374 ml of this antifoaming agent dilution was added to the silver solution, and the resulting silver solution was added in a warm bath to 36. Held at 0C.

  Next, 20100 g of ascorbic acid as a reducing agent (manufactured by Kanto Chemical Co., Inc., reagent) was dissolved in 5.35 L of pure water at 30 ° C. Further, 1760 g of polyvinyl alcohol as a dispersant (PVA205 manufactured by Kuraray Co., Ltd.) was dissolved in 10 L of pure water at 50 ° C. These two liquids were mixed to form a reducing agent solution, and the temperature was adjusted to 36 ° C.

  Next, the silver solution and the reducing agent solution are supplied to the reaction tube at a rate of 2.7 L / min and 0.9 L / min, respectively, using a smooth flow pump (APL-5 and BPL-2 manufactured by Takumina Co., Ltd.). The reaction solution discharged from the reaction tube was held in a receiving tank while stirring. As the reaction tube, a glass concentric tube (silver solution supply tube: inner diameter 10.0 mm, reducing agent solution supply tube) in which both solutions are mixed and stirred with the supply direction of the reducing agent solution relative to the supply direction of the silver solution being 0 °. : Inner diameter 3.6 mm, mixing tube length: 100 mm). Further, a soft vinyl chloride resin tube having an inner diameter of 12 mm and a length of 10 m was connected to the reaction tube outlet side as a reducing tube, and the reaction solution was fed to the receiving tank. The reduction rate at this time is 144 g / min in terms of silver, and the silver concentration in the reaction solution is 40 g / L. Further, the mixing ratio of ascorbic acid to 1 mol of silver determined from the supply rate is 0.33 mol.

  And what has a 1st receiving tank and a 2nd receiving tank as shown in FIG. 1 was used for the receiving tank. The first receiving tank having a capacity of about 200 L was used, and the liquid was supplied to the second receiving tank from the position of the liquid level 150 L to the second receiving tank. A second receiving tank having a capacity of about 60 L was used, and the liquid was fed to the filter press by level control using electrodes. The average residence time in the first receiving tank is 41.7 minutes.

  In addition, 100 mL of the reaction solution flowing down from the reduction tube during the reduction is collected, and the results of measuring the change with time of the oxidation-reduction potential (ORP) are shown in FIG. The ORP became minimum after 27 minutes and then gradually increased. The time until the end of the reaction was about 30 minutes, and it was confirmed that the average residence time of the first receiving tank was sufficient.

  Further, after the supply of the silver solution and the reducing agent solution is completed, stirring in the first and second receiving tanks is continued for 40 minutes, and then the remaining from the bottom valve of each of the first receiving tank and the second receiving tank. The whole amount was sent to the filter press.

  The silver solution after completion of 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 18 L of 0.01 mol / L NaOH aqueous solution, stirred for 15 minutes, and then recovered by filtration with a filter press. The operations consisting of pouring into an aqueous NaOH solution, stirring, and filtration were repeated twice more, and then operations comprising pouring into 18 L of pure water, stirring, and filtration were performed. After filtration, the silver particles were transferred to a stainless steel pad and dried in a vacuum dryer at 60 ° C. for 15 hours to obtain silver powder.

  The obtained silver powder has an average particle diameter of 0.88 μm by observation with a scanning electron microscope (SEM), a value obtained by dividing the standard deviation of the particle diameter by the average particle diameter is 0.21, has high dispersibility, and is a paste. It was confirmed that the silver powder for use was good.

(Example 2)
In Example 2, the amount of ascorbic acid dissolved was adjusted so that the mixing ratio was 0.35 mol, and the same conditions and production apparatus as in Example 1 were used.

  The receiving tank has the configuration shown in FIG. 1, and the first receiving tank has a capacity of about 200 L. The second receiving tank was pumped from the position of the liquid level 110 L to the second receiving tank. A second receiving tank having a capacity of about 60 L was used, and the liquid was fed to the filter press by level control using electrodes. The average residence time in the first receiving tank is 30.6 minutes.

  In addition, 100 mL of the reaction liquid flowing down from the reduction pipe during the reduction was collected, and the results of measuring the temporal change of ORP are shown in FIG. The ORP became minimum after 20 minutes and then gradually increased. The time until the end of the reaction was about 20 minutes, and it was confirmed that the average residence time of the first receiving tank was sufficient.

  And after supply of a silver solution and a reducing agent solution was complete | finished, stirring in the 1st and 2nd receiving tank was continued for 30 minutes. The subsequent steps were performed in the same manner as in Example 1.

  The obtained silver powder has an average particle size of 0.93 μm as observed by a scanning electron microscope (SEM), a value obtained by dividing the standard deviation of the particle size by the average particle size is 0.19, has high dispersibility, and is a paste. It was confirmed that the silver powder for use was good.

(Comparative Example 1)
In Comparative Example 1, reduction was performed under the same conditions and apparatus as in Example 1, the liquid level position of the first receiving tank was adjusted, and the average residence time was 5 minutes. As a result, the silver powder recovered from the filter press became a hard aggregate and could not be easily crushed.

  DESCRIPTION OF SYMBOLS 1 Silver powder manufacturing apparatus, 10 Reaction tube, 11 Silver solution supply tube, 12 Reducing agent solution supply tube, 12A Straight part, 13 Mixing tube, 14 Reduction tube, 20,30 Supply mechanism, 40 1st receiving tank, 41 2nd Receiving tank, 42 bypass mechanism, 50 filter press

Claims (6)

A silver solution containing a silver complex and a reducing agent solution are quantitatively and continuously supplied into the channel, and the silver solution and the reducing agent solution are mixed in the channel to quantitatively and continuously form the silver complex in the reaction solution. In the manufacturing method of the silver powder which reduces automatically and obtains silver powder,
A reaction liquid obtained by mixing the silver solution and the reducing agent solution in the flow path is sent from the flow path to a receiving tank in which the reaction liquid is retained, and an average residence time in the receiving tank is After being discharged from the flow path, the reaction liquid is retained with stirring so that the time until the oxidation-reduction potential of the reaction liquid reaches the minimum value is reached, and the reaction liquid is discharged from the flow path into the receiving tank. However, the method for producing silver powder is characterized in that the reaction liquid retained in the receiving tank is sent to the outside of the tank for solid-liquid separation. The method for producing silver powder according to claim 1 , wherein the silver solution is obtained by dissolving silver chloride in aqueous ammonia. The reducing agent is ascorbic acid;
According to claim 1 or claim 2, characterized in that a 0.25 to 0.50 molar the reducing agent the mixing ratio at the time of mixing of the reducing agent solution and the silver solution per mol of silver A method for producing silver powder. The at least 1 sort (s) selected from polyvinyl alcohol, polyvinyl pyrrolidone, a modified silicone oil type surfactant, and a polyether type surfactant as a dispersing agent is added to the reducing agent solution. 4. The method for producing silver powder according to any one of 3 above. The method for producing a silver powder according to any one of claims 1 to 4 , wherein the silver concentration in the reaction solution is 25 to 50 g / L. An apparatus for producing silver powder used in the method for producing silver powder according to any one of claims 1 to 5 ,
A channel having a mechanism for mixing a silver solution containing a silver complex and a reducing agent solution in the channel to quantitatively and continuously reduce the silver complex in the reaction solution;
A mechanism for quantitatively and continuously supplying the silver solution and the reducing agent solution respectively to the flow path;
A receiving tank for retaining the reaction solution discharged from the flow path;
Provided with a solid-liquid separation mechanism for separating the solid after the staying reaction liquid is sent from the receiving tank,
The volume of the receiving tank is equal to or greater than the amount of liquid obtained by multiplying the sum of the flow rate of the silver solution and the flow rate of the reducing agent solution by the time at which the reduction reaction ends,
In the receiving tank, the flow of the reaction liquid is bypassed between a discharge position where the reaction liquid is discharged from the flow path and a liquid supply position where the reaction liquid in the tank is sent to the solid-liquid separation mechanism. An apparatus for producing silver powder, characterized by having a bypass mechanism.

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