JPH0327602B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention uses silver sulfite as a raw material, and sodium sulfite as a reducing agent, generally an alkali metal sulfite salt (including ammonium sulfite salt).The alkali sulfite salt has a reducing effect. React in water at the indicated temperature to reduce silver sulfite to metallic silver,
This relates in particular to a method for producing metallic silver in the form of spherical ultrafine particles, in particular in extremely fine form. According to the method of the present invention, silver powder having spherical particles and ultrafine particles can be easily produced in a large quantity and efficiently in a short period of time. (Prior art and problems to be solved) In the conventionally known and practiced method for producing silver powder, silver ions in a silver nitrate aqueous solution are mixed with formalin,
It is reduced to metallic silver using a reducing agent such as hydrazine. However, the silver particles that can be produced using such reducing agents are called hexagonal crystalline particles or tooth-shaped dendritic particles, and all other silver particles that are produced are crystalline, and their particle size is 10 to 10. 40
They are coarse particles on the order of microns. However, the silver powder produced and provided in the silver powder manufacturing industry, which is a semiconductor-related industry, is primarily used as an electrically conductive agent. Therefore, in order to obtain good electrical conductivity, it is necessary to increase the electrical contact points (contact melting points) between adjacent silver particles as much as possible. As a means to achieve this, (1) the silver particles can be made as fine as possible to increase the number of silver particles per unit weight and the surface area to increase the number of contact points, or (2) the shape of the silver particles can be changed so that the number of contact points can be increased as much as possible. The process is to create something with a specific shape. For this reason, in conventional manufacturing methods, it is customary to produce silver powder in the form of fine particles by mechanically crushing the once-formed silver powder in the form of elementary particles for a long time using a ball mill, stamp mill, etc. . However, the silver powder produced by this pulverization has only fine particles, and the particle shape is scale-like or flaky, and the size of the particles varies widely. In other words, only silver powder with a wide particle size distribution and a coexistence of coarse particles and fine particles can be obtained. Therefore, when such silver powder is kneaded with a synthetic resin to prepare a silver powder paste or conductive ink and used for coating on ceramic capacitors or other applications, the shape of the silver particles causes problems in the coating layer. The arrangement of the particles inside became irregular and complex, making it impossible to achieve close packing of silver particles, and it was difficult to dramatically increase the contact points between silver particles and therefore the conductivity. . Furthermore, in the conventional technology, when producing silver powder, it is necessary to mechanically grind the silver powder for a long period of time, so that grinding costs are required to make the silver particles finer, resulting in high costs. In turn, this becomes a bottleneck for mass production of fine silver powder, and has the disadvantage of being an extremely complicated and impractical method that involves contamination with impurities, production loss, and noise. (Means for Solving the Problems) The present inventors have recently conducted research in order to eliminate the shortcomings of these conventional techniques. As a result, it was quite surprising that an inexpensive reducing agent, anhydrous sodium sulfite, which had not been considered for silver powder production until the present day of technological innovation, was added to an aqueous solution of silver nitrate. We have discovered that by generating silver sulfite as a body compound in water, and then applying the reducing power of sodium sulfite and heating, the silver ions of silver sulfite can be completely reduced to metallic silver. Furthermore, it has been found that the silver particles obtained by this reduction method are spherical and ultrafine particles with a particle size of 0.5 to 1 micron. It has also been found that the time required for reduction to produce silver sulfite and convert it to metallic silver is only about 12 minutes or less. Furthermore, as a result of continued research, the present inventor found that it is not always necessary to start from an aqueous solution of silver nitrate as a raw material, and even if silver sulfite is produced by some method, it is possible to use sodium sulfite in an aqueous solution to achieve the necessary reducing effect. We have discovered that fine silver powder with the desired properties can be produced by reducing silver sulfite at a temperature of . It has also been found that the reducing agent is not limited to sodium sulfite, and that not only common alkali metal sulfite salts such as potassium sulfite and lithium sulfite, but also ammonium sulfite salts can be used. Silver sulfite is sparingly soluble in water, so when it is dispersed in water as fine solid particles and subjected to the reducing action of sodium sulfite by stirring or other suitable means, the desired fine metal particles can be obtained. It was discovered that silver can be produced and precipitated. In addition, the reducing action of alkali metal sulfite salts (ammonium salts) that reduce silver sulfite to metallic silver is not very strong at low temperatures, so the reduction reaction should preferably be carried out at a reaction temperature of 70°C or higher, especially 74°C or higher. was found to be good. Therefore, according to the first aspect of the present invention, the amount of silver sulfite dispersed in water as fine particles is sufficient to reduce all or substantially all of the silver sulfite to metallic silver, or in a slightly excess amount. water-soluble alkali metal sulfite salt or ammonium sulfite salt in an amount of
A reaction is carried out at a temperature at which an alkali metal sulfite salt or an ammonium salt exhibits a reducing action, thereby reducing silver sulfite to precipitate spherical ultrafine particles of metallic silver and/or aggregates of spherical ultrafine particles of metallic silver. A method for producing metallic silver in the form of spherical ultrafine particles is provided. Furthermore, according to the second invention, a water-soluble alkali metal sulfite salt is added to the aqueous silver nitrate solution in an amount sufficient or slightly in excess to convert all or substantially all of the silver nitrate to silver sulfite by metathesis. Alternatively, ammonium sulfite is added as a solid or an aqueous solution and reacted, and all or substantially all of the silver sulfite that is produced and precipitated and dispersed in water as fine particles is reduced. The temperature at which the alkali metal sulfite or ammonium sulfite exhibits a reducing effect when a sufficient amount or a slightly excessive amount of a water-soluble alkali metal sulfite or ammonium sulfite is added as a solid or an aqueous solution to convert it into metallic silver. A metal in the form of spherical ultrafine particles, which is characterized by reducing silver sulfite and precipitating spherical ultrafine metal silver particles and/or aggregates of spherical ultrafine metal silver particles. A method for producing silver is provided. The mechanism of the reducing action exhibited by the alkali metal sulfite salt (or ammonium salt) used in the first method of the present invention is chemically known. Since silver sulfite itself produces metallic silver, silver sulfate, and sulfur dioxide gas just by boiling it with water, it is best to avoid raising the reaction temperature to the boiling point of water. The raw material silver sulfite is preferably dispersed in water at a concentration of about 4% by weight or less. The alkali sulfite salt (ammonium salt) as a reducing agent may be reacted in a stoichiometrically necessary amount, and may be used in a slightly excess amount. This reducing agent may be added to the aqueous silver sulfite dispersion in a solid state, but it is preferable to dissolve it as an aqueous solution before adding it. Implementation of the method of the invention is significantly easier and more economical than conventional manufacturing methods. In the second method of the present invention, an alkali sulfite salt, preferably anhydrous sodium sulfite, is added in an amount sufficient to react with the total amount of silver ions in the silver nitrate aqueous solution and convert it to silver sulfite, and reduce this to metallic silver. When added uniformly to a silver nitrate solution all at once or continuously and heated, silver ions are easily reduced to metallic silver and precipitated as fine silver particles. Further, in a preferred embodiment of the present invention, the entire amount of anhydrous sodium sulfite to be reacted is continuously and evenly added during the first 10 minutes while stirring the silver nitrate aqueous solution that has been preheated to about 90°C. As the solution progresses, silver sulfite forms and precipitates in a colloid-like manner, and first, an aqueous dispersion of silver sulfite is formed which takes the form of a white curdled milk. Thereafter, while stirring is continued, this silver sulfite is subjected to a reducing action by the reducing power of the added sodium sulfite and the heat in the liquid, and is precipitated as fine silver particles. Continue stirring for another 2 minutes to complete the reduction of silver sulfite. During this time, the reaction solution changes color from white to grayish-white to gray. The final liquid temperature is the reducible temperature.
A temperature of 74°C or slightly higher, for example 76°C, can be produced without failure. Therefore, in this method, it is preferable to raise the silver nitrate aqueous solution to a predetermined temperature in advance before starting the reduction operation. In practice, it takes too much time to heat this together with the reaction vessel (tank) to raise the temperature of a large amount of solution. Therefore, by pouring boiling water that has been pre-boiled and heated to approximately 100°C into the silver nitrate stock solution in the reaction tank, a dilute silver nitrate aqueous solution with a predetermined concentration (a 4% solution is appropriate when calculating the amount of silver) is made, and at the same time, the predetermined solution is added. You can heat it up all at once. After adjusting the silver nitrate concentration and liquid temperature to predetermined values using this method, the reduction operation is started. In this case, after the addition of the reducing agent starts, the liquid temperature gradually decreases over time, but when the reaction time of 12 minutes has passed and the reduction has finished, the liquid temperature is the reducible temperature, preferably 74°C, or If it is more than that, all the silver ions in the silver nitrate aqueous solution become silver sulfite, and the silver sulfite is completely reduced to silver.
It is convenient to set the reduction completion temperature at 74°C to 76°C. The silver particles obtained by the method of the present invention are ultrafine particles having a spherical shape and a particle size of 0.5 to 1 micron, simply by the above-mentioned reduction reaction without being mechanically pulverized.
Therefore, the surface area per unit weight of silver is significantly large, and spherical particles have a round shape called a sphere.
A so-called close-packed arrangement of spheres is possible, that is, the particles are easily arranged regularly, uniformly, and evenly.
In an ideal state, one particle and other adjacent particles
There will be 12 points of contact. Therefore, the number of electrical contacts between spherical silver particles increases dramatically in combination with the fineness of the particles, so that the inherent electrical conductivity of metallic silver can be utilized to the fullest. Conventionally, it was thought that it was extremely difficult or almost impossible to produce spherical particles as silver powder particles, but the method of the present invention has made it possible to produce them easily. The metallic silver produced by the reduction reaction in the method of the present invention is initially silver particles precipitated in the form of a sponge, and is primarily aggregated. If a chemical treatment is further applied to reduce the cohesive force between silver particles by changing it, the dispersion of the silver particles can be improved. Originally, the finer the particles, the more easily they aggregate. For this reason, in the pigment and paint manufacturing industry, the problem of particle dispersion has been studied for a long time in order to produce paints with good pigment particle dispersion.
For example, particle dispersion can be improved by treating titanium oxide with naphthenic acid or phthalic acid to form a film of titanium naphthenate or titanium phthalate on the particle surface. Furthermore, even if the pigment precipitated in water has fine particles, when it is dried, the primary particles coagulate secondary, forming large lumps. No matter how many times this is crushed using a crusher or other methods,
It is difficult to disperse it back to its original state of primary particles. Therefore, in order to obtain printing inks and paints with good pigment dispersion, a method has been developed in which the water phase is replaced with an oil phase by mixing the wet pigment with oil-based synthetic resins and kneading the mixture. This is called flushing. This method does not involve drying or pulverizing steps, does not contain secondary aggregates, and has good dispersion. In this way, various studies have been conducted on the problem of particle dispersion, and it has become an independent dispersion technology in industries that handle pigments, paints, and powders. Therefore, in the method for producing ultrafine spherical silver particles of the present invention, it is preferable to carry out an additional chemical treatment step for improving particle dispersion, that is, for disintegration. The present inventor conducted research on a method for dispersing silver particles precipitated and precipitated by this method by chemical treatment. Discard the supernatant liquid of the spongy metal silver precipitate that was reduced and formed in the reaction tank, and add new water to this precipitate.
When the solution was made weakly acidic by adding a small amount of dilute hydrochloric acid (10%) while stirring, it was observed that the particles became more agglomerated.
However, on the other hand, if the material is made alkaline by adding alkali, the sponge-like aggregated silver particle precipitate will be dispersed, resulting in a silver powder precipitate that has a mud-like appearance as a whole. Having discovered this phenomenon, we considered a chemical treatment method using an alkali as the main dispersant, and dispersed spongy precipitated silver particles using the treatment method below.
I agree that good results can be obtained. In other words, such a chemical treatment method involves discarding the supernatant liquid of the spongy silver particles precipitated in the reaction tank, and when the amount of silver is, for example, 1 kg,
Add 15% fresh water and while stirring, first add 10% NaOH as the chemicals shown in Table 1 below.
After adding the aqueous solution, continue stirring for a predetermined period of time. When finished, hold the precipitate and discard the supernatant liquid. Next, add 15 ml of water and stir. It is preferable to repeat this process until the end according to the procedure shown in Table 1 below.

[Table] After completing the chemical treatment as described above, the silver particles were collected by filtration, dried, and pulverized by light beating, etc., to obtain fine silver powder with the desired properties. Therefore, in the method of the present invention, the precipitated ultrafine metallic silver or its aggregates are separated from the mother liquor liquid, and then sequentially treated with a dilute aqueous solution of an alkali metal hydroxide, preferably sodium hydroxide. An additional chemical treatment step can be performed in which the metal silver particles are treated with a dilute aqueous solution of ammonium hydroxide under stirring, thereby reducing the agglomeration between the metal silver particles and improving the dispersibility of the metal silver particles. preferable. Next, the present invention will be further explained using examples. Example 1 Silver nitrate aqueous solution consists of 1000 g of pure silver, 1250 c.c. of nitric acid (Be′36°), and 1875 c.c. of pure water (amount of nitric acid x 1.5).
.
were determined in this ratio, mixed, and further heated to dissolve the silver. This concentrated aqueous solution of silver nitrate is slightly acidic, but there is no need to make it neutral. Put this concentrated silver nitrate aqueous solution (liquid volume approximately 2) into a reaction tank (capacity approximately 40), add 23 cm of boiling water at 100℃ to this, and make the total volume 25. Calculating the amount of silver, you will obtain a 4% dilute silver nitrate aqueous solution. Ta. The liquid temperature was 90°C. Immediately after this, anhydrous sodium sulfite was continuously added to the silver nitrate aqueous solution in powder form by dropping it in an hourglass manner through the tube while rotating the stirrer. At this time, adjust the diameter of the tube in advance and add the entire amount of anhydrous sodium sulfite in 10 minutes.
1450g could be added to the reaction tank. After the addition was completed under stirring for 10 minutes, a white silver sulfite aqueous dispersion was obtained. Stirring was continued for 2 minutes to complete the reduction of silver sulfite by sodium sulfite. The liquid temperature at this time was 74°C. After the reduction step was completed, the supernatant was removed. One drop of dilute hydrochloric acid was added to this supernatant, but no white precipitate of silver chloride was observed. Thereafter, a dispersion process using the chemical treatment method shown in Table 1 in the previous period was started in the same reaction tank. 995g of fine spherical silver powder with a particle size of 0.5-1 micron
was gotten.

[Brief explanation of drawings]

The drawing is an electron micrograph of spherical ultrafine particle silver powder produced by the method of the present invention at a magnification of 10,000 times.

Claims (1)

[Claims] 1. Silver sulfite dispersed in water as fine particles is dissolved in water in an amount sufficient or slightly in excess to reduce all or substantially all of the silver sulfite to metallic silver. The alkali metal sulfite or ammonium sulfite salt is reacted at a temperature at which the alkali metal sulfite or ammonium salt exhibits a reducing action, thereby reducing the silver sulfite to form spherical ultrafine particles of metallic silver and/or A method for producing metallic silver in the form of spherical ultrafine particles, characterized by precipitating aggregates of metallic silver in the form of spherical ultrafine particles. 2. Silver sulfite is dispersed in water under stirring at a concentration of about 4% by weight or less.
The method described in section. 3. The method according to claim 1, wherein sodium sulfite is used as the alkali metal sulfite salt. 4. The precipitated ultrafine metallic silver or its aggregates are separated from the mother liquid and further stirred with a dilute aqueous solution of an alkali metal hydroxide, preferably sodium hydroxide, and a dilute aqueous solution of ammonium hydroxide. 2. The method of claim 1, further comprising an additional chemical treatment step for dispersing the metallic silver particles by reducing agglomeration between them and improving dispersibility of the metallic silver particles. 5 Adding a water-soluble alkali metal sulfite or ammonium sulfite salt in solid form or in an aqueous solution to an aqueous silver nitrate solution in an amount sufficient to convert all or substantially all of the silver nitrate to silver sulfite by double decomposition, or in an amount slightly in excess thereof. This is sufficient to reduce all or substantially all of the silver sulfite that is produced, precipitated, and dispersed in water as fine particles and convert it into metallic silver. Adding an amount or a slightly excessive amount of a water-soluble alkali metal sulfite salt or ammonium sulfite salt in a solid state or as an aqueous solution and allowing the reaction to occur at a temperature at which the alkali metal sulfite salt or ammonium sulfite exhibits a reducing action; A method for producing metallic silver in the form of spherical ultrafine particles, which comprises reducing silver sulfite to precipitate spherical ultrafine particles of metallic silver and/or an aggregate of spherical ultrafine particles of metallic silver. 6. Silver sulfite is dispersed in water under stirring at a concentration of about 4% by weight or less.
The method described in section. 7. The method according to claim 5, in which sodium sulfite is used as the alkali metal sulfite salt. 8. The precipitated ultrafine metallic silver or its aggregates are separated from the mother liquid and further stirred with a dilute aqueous solution of an alkali metal hydroxide, preferably sodium hydroxide and a dilute aqueous solution of ammonium hydroxide. 6. The method of claim 5, comprising an additional chemical treatment step of treating and thereby reducing agglomeration between metal particles and improving the dispersibility of the metal silver particles.