JPH10251714A - Production of silver powder for powder metallurgy - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain silver powder whose particle diameter and bulk are optimum for powder metallurgy by adding an alkali hydroxide soln. to a silver nitrate soln. to form silver oxide, adding a formaldehyde soln. and reducing the silver oxide at a specified temp. SOLUTION: A soln. prepd. by dissolving an alkali hydroxide such as sodium hydroxide in pure water is added to a silver nitrate soln. to form silver oxide. A formaldehyde soln. is added under cooling to <=60 deg.C, the amts. of the alkali hydroxide and formaldehyde are regulated so that a soln. after reduction is adjusted to pH0.1-6 and the objective silver powder for powder metallurgy is obtd. In this case, the amt. of the formaldehyde soln. added is preferably regulated to 1.1-1.5 equiv. based on the amt. of formed silver. The reason why the silver oxide is reduced at <=60 deg.C is that formaldehyde as a reducing agent is effectively utilized at <=60 deg.C and formic acid as a reactional product is used for controlling particle diameter. Fine silver powder vanishes by dissolution, nucleus-grown powder is formed and silver powder not uniform in particle diameter and fit for powder metallurgy is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a method for producing silver powder used in powder metallurgy.

[0002]

2. Description of the Related Art Conventionally, as a method for producing silver powder for powder metallurgy, a physical production method such as an atomizing method and a chemical production method by a reduction method are representative.

[0003] Silver powder obtained by the atomizing method is
Since large particles having a particle size of several tens of μm are obtained, there is a problem that shrinkage after compression and sintering is small, the apparent density after sintering does not increase, and affects subsequent steps.

[0004] On the other hand, silver powder produced by the chemical reduction method requires a lot of trouble in handling due to the fineness of the powder, and it is necessary to improve the working environment. In addition, when the bulk is large, there is a limit in the capacity of the mold to be formed during the compression forming, and there is a problem that it is difficult to obtain a product having a large weight. Further, there is a problem that the gas is adsorbed much due to the fine powder, and the gas is not released at the time of compression but swells in a subsequent step.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing a silver powder which solves the above-mentioned drawbacks and has an optimum particle size and bulk for powder metallurgy.

[0006]

In order to achieve the above object, the present invention provides a method for powder metallurgy by adding a caustic alkali solution to a silver nitrate solution to form silver oxide, and then adding a formalin solution to reduce silver oxide. In a method for producing silver powder, a method for producing a silver powder for powder metallurgy is characterized in that a reduction temperature at the time of reducing silver oxide is reduced to 60 ° C. or less to cause a reaction. In the method for producing silver powder of the present invention, the amount of formalin solution to be added is 1.1 to 1.5 equivalents to the amount of silver, and the pH after reduction is 0.1.
It is preferred to be 1-6.

In the present invention, the reaction needs to be performed at a reduction temperature of 60 ° C. or less. This is for the purpose of effectively utilizing formalin as a reducing agent and for the purpose of using formic acid as a reaction product for controlling the particle size. When the reaction temperature is higher than 60 ° C, formalin vaporizes and does not act effectively on the input amount. In addition, regarding the particle size control by formic acid, the reaction proceeds too rapidly at 60 ° C or higher due to the high activity of formic acid. This makes it difficult to control the particle size.

Further, the amount of the formalin solution to be added is set to 1.1 equivalent or more with respect to the amount of silver because it is necessary to add more than the stoichiometric amount in order to reduce the total amount of silver oxide.
The reason for setting the ratio to less than 1.5 is that not only is excess formalin not required, but also, on the other hand, there arises a problem that the particle size control becomes difficult due to an increase in the amount of formic acid produced.

The purpose of adjusting the pH after reduction to 0.1 to 6 is to effectively control the particle size with formic acid, and the dissolution and reduction reaction of silver with formic acid proceeds only in the acidic region. Although the clear reaction mechanism is unknown, it is presumed that a chemical battery is formed in a system in the presence of formic acid in an acid solution, and the generation and generation of the electromotive force drives the dissolution and reduction from the silver surface.

As a result of these reactions, the dissolution reaction proceeds as finer particles become, and those particles disappear or decrease, and silver crystal particles with further advanced crystal growth are generated. In the silver powder according to the present invention, the fine powder is not dissolved, while the powder that grows nuclei is produced, which is suitable for powder metallurgy in which large and small powders are mixed. Further, fine powder having a size of 0.3 μm or less is eliminated, which makes it easy to handle in handling.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Silver nitrate used in the present invention is generally obtained by dissolving granular or powdery silver ingot in nitric acid. To the obtained silver nitrate solution, a solution in which caustic alkali such as caustic soda and potassium hydroxide is dissolved in pure water is added to generate silver oxide, and then a formalin solution is added while cooling to 60 ° C or less, and the solution after the reduction reaction is added. By adjusting the amount of caustic alkali and formalin so that the pH of the solution becomes 0.1 to 6, a silver powder for powder metallurgy can be obtained.

[0012]

Embodiments of the present invention and a conventional example will be described below.

[0013]

Example 1 After dissolving 108 g (1 equivalent) of electrolytic silver powder in 130 ml of nitric acid (62%), 80 g (2 equivalents) of caustic soda was dissolved in pure water, and the silver nitrate solution was added to a solution of 200 ml while stirring. Minutes to produce silver oxide. While the silver oxide-containing solution was cooled to 60 ° C. or lower, 52 ml (1.4 equivalents) of a formalin solution (37%) was charged in 5 minutes. The pH after the reduction was 3.4. The reaction solution was removed, decanted twice with 200 ml of pure water, filtered and dried. As a result, the average particle size was 4.2 μm and the tap density was 3.0 by the air permeation method.

[0014]

Example 2 After dissolving 108 g (1 equivalent) of electrolytic silver powder in 130 ml of nitric acid (62%), 80 g (2 equivalents) of caustic soda was dissolved in pure water, and the silver nitrate solution was added to a solution of 200 ml with stirring. Minutes to produce silver oxide. While the silver oxide-containing solution was cooled to 60 ° C. or lower, 44 ml (1.2 equivalents) of a formalin solution (37%) was charged in 5 minutes. The pH after the reduction was 5.4. The reaction solution was removed, decanted twice with 200 ml of pure water, filtered and dried. As a result, the average particle size was 2.6 μm and the tap density was 2.4 by the air permeation method.

[0015]

Conventional Example 1 After dissolving 108 g (1 equivalent) of electrolytic silver powder in 130 ml of nitric acid (62%), 80 g (2 equivalents) of caustic soda was dissolved in pure water, and the silver nitrate solution was added to a solution of 200 ml while stirring. Minutes to produce silver oxide. While maintaining the silver oxide-containing solution at 90 ° C, a formalin solution (37
%) 74 ml (2 equivalents) were charged in 5 minutes. PH after reduction
Was 12.4. The reaction solution was removed, decanted twice with 200 ml of pure water, filtered and dried. As a result, the average particle size by the air permeation method was 2.2 μm, and the tap density was 1.6.

[0016]

Conventional Example 2 After dissolving 108 g (1 equivalent) of electrolytic silver powder in 130 ml of nitric acid (62%), 80 g (2 equivalents) of caustic soda was dissolved in pure water, and the silver nitrate solution was added to a solution of 200 ml while stirring. Minutes to produce silver oxide. While the silver oxide-containing solution was cooled to 60 ° C. or lower, 74 ml (2 equivalents) of a formalin solution (37%) was charged in 5 minutes. The pH after the reduction was 1.5. Remove the reaction solution and add 200 ml
After decanting twice, the mixture was filtered and dried. as a result,
Average particle size by air permeation method is 25μm, tap density is 4.5
Met.

(Evaluation) The silver powders of Examples 1 and 2 were compacted to a compressed density (apparent density) of 5.0 and sintered at 850 ° C. for 1 hour. Table 1 shows the state of compression formation, sintering density, and state during sintering.
Shown in For the silver powders of Conventional Examples 1 and 2,
2, and the results are shown in Table 1.

[0018]

[Table 1]

[0019]

As described above, in the method for producing a silver powder for powder metallurgy according to the present invention, a reduction reaction of silver oxide is carried out at a temperature of 60 ° C. or less, and a formalin solution is used in an amount of 1.1 to 1.1% based on the amount of silver.
By adjusting the pH to 0.1 to 6 after reduction by 1.5 equivalents,
Silver powder for powder metallurgy that could not be produced conventionally can be easily obtained, and excellent results can be obtained as a raw material for powder metallurgy.

Claims (2)

[Claims] 1. A method for producing silver powder for powder metallurgy by adding a caustic alkali solution to a silver nitrate solution to generate silver oxide, and then adding a formalin solution to reduce silver oxide,
A method for producing a silver powder for powder metallurgy, wherein the reduction temperature when reducing silver oxide is set to 60 ° C. or lower. 2. The method for producing silver powder for powder metallurgy according to claim 1, wherein the amount of the formalin solution to be added is 1.1 to 1.5 equivalents to the amount of silver, and the pH after reduction is 0.1 to 6. .