JP5200588B2 - Method for producing high purity silver - Google Patents

Description

  The present invention relates to a method for producing high-purity silver. More specifically, when obtaining metallic silver from a refining intermediate containing a sparingly soluble silver compound and an impurity element, wet purification is performed without performing dry refining or electrorefining treatment. The present invention relates to a method for producing silver by which 99.999 mass% high-purity metallic silver can be obtained only by purification.

Conventionally, as one of the methods for smelting silver, recovery from anode slime generated in the electrolytic purification step in a smelting process of copper, lead or the like has been performed. For example, as a method for separating and purifying silver from the anode slime, a method is widely used in which the anode slime is dry-treated to obtain crude silver, which is electrolytically purified.
As a method for the dry treatment and electrolytic purification, it is known that, for example, a purity of 99.999% by mass (hereinafter sometimes referred to as 5N) can be obtained according to the Mevias silver electrolysis method. ing.

However, in the dry process, there are problems that dust and exhaust gas are generated on the environment side, and there is a risk of hot work and burns on the work side. Under such circumstances, an efficient method for separating and recovering silver by a wet method has attracted attention. For example, high-purity silver chloride is produced by a method comprising the following steps (1) to (3). There has been proposed a wet method (see, for example, Patent Document 1) in which the obtained silver chloride is treated with a reducing agent in an alkaline aqueous solution to obtain metallic silver powder.
(1) A leaching step of leaching the refining intermediate in a sulfite aqueous solution and extracting silver into the liquid to form a leachate containing silver and an insoluble residue.
(2) a step of neutralizing the leachate to make it acidic, precipitating silver chloride to form a mother liquor with the silver chloride, and (3) adding an oxidizing agent to the silver chloride in an acidic aqueous solution. The silver chloride refining process that forms a solution containing the purified silver chloride and impurity elements by oxidizing and separating the impurity elements to form a solution containing the purified silver chloride and impurity elements

  According to this method, compared to the conventional method in which dry treatment is performed and silver electrolytic purification is performed through a silver anode, not only environmental and work aspects are reduced, but also the number of days for commercialization is shortened. In addition, on the equipment side, there is no need for large-scale equipment such as melting furnaces and dry-type exhaust gas treatment equipment, and electrolyzers and rectifiers, and a combination of pumps and reaction tanks for transporting liquids. Since it is configured, there are significant advantages such as the possibility of automation using a centralized management system. For this reason, it can be said that it is an advantageous method also in terms of equipment cost and manufacturing cost with a small number of personnel. However, this method has a problem that a purity of 99.99% by mass (hereinafter sometimes referred to as 4N) is obtained, but a purity of 5N cannot be obtained.

  From the above situation, 99.999 mass% high-purity metallic silver can be obtained only by wet refining using a refining intermediate containing a sparingly soluble silver compound and an impurity element as a raw material. There is a need for a manufacturing method.

Republished patent WO2005 / 023716 (first page, second page)

  The object of the present invention is to solve the above-mentioned problems of the prior art, when obtaining metallic silver from a refining intermediate containing a sparingly soluble silver compound and an impurity element, without performing refining treatment by dry refining or electrolysis. An object of the present invention is to provide a silver production method capable of obtaining 99.999 mass% high-purity metallic silver only by purification.

  In order to achieve the above object, the present inventors have conducted extensive research on a method for producing high-purity silver from a refined intermediate containing a sparingly soluble silver compound and an impurity element. In the silver chloride production step of neutralizing the leachate to make it acidic and depositing silver chloride to form a mother liquor with the silver chloride, the pH is adjusted to a specific value when adding acid, and When the precipitate was separated from the mother liquor immediately after the silver chloride was precipitated, it was found that 99.999 mass% high-purity metallic silver could be obtained, and the present invention was completed.

That is, according to the first invention of the present invention, a method for producing silver from a refining intermediate containing a hardly soluble silver compound and an impurity element by the following steps (1) to (4),
In the step (2), before adding acid to the leaching solution containing silver obtained in the step (1), the leaching solution containing silver is microfiltered with a filter having an opening of 1 μm or less, and the acid is removed. During the addition, the pH is adjusted to 3.5 to 5.5, and after precipitation of silver chloride, the whole amount of the mother liquor and the precipitate is filtered within 60 minutes to separate the precipitate from the mother liquor. A method for producing silver is provided.
(1) The refining intermediate is leached in an aqueous sulfite solution, silver is extracted into the aqueous sulfite solution, and then the insoluble residue formed is separated to obtain a leachate containing silver.
(2) An acid is added to the leaching solution to precipitate silver chloride, and then the mother liquor is separated to obtain a precipitate containing silver chloride.
(3) The precipitate is oxidized in an acidic aqueous solution to elute the impurity element, and then the liquid containing the impurity element is separated to obtain purified silver chloride.
(4) The silver chloride is reduced in an alkaline aqueous solution to obtain silver powder.

According to the second invention of the present invention, in the first invention, in the step (2), the mother liquor and the precipitate are removed using a reaction tank having a structure capable of extracting the whole amount of the mother liquor and the precipitate. When filtering, the silver manufacturing method characterized by extracting the whole quantity of mother liquid and a precipitate from this reaction tank is provided.

According to a third aspect of the present invention, there is provided the method for producing silver according to the first or second aspect , wherein the hardly soluble silver compound is silver chloride.

  The method for producing high-purity silver according to the present invention, when obtaining metallic silver from a refining intermediate containing a hardly soluble silver compound and an impurity element, only by wet refining without performing dry refining or electrorefining treatment, Since 99.999 mass% high-purity metallic silver can be obtained, its industrial value is extremely high.

Hereinafter, the manufacturing method of the high purity silver of this invention is demonstrated in detail.
The method for producing high-purity silver according to the present invention is a method for producing silver from a refined intermediate containing a hardly soluble silver compound and an impurity element by the following steps (1) to (4), In the step 2), when adding the acid, the pH is adjusted to 3.5 to 5.5, and immediately after the precipitation of silver chloride, the whole amount of the mother liquor and the precipitate is filtered, and the precipitate is removed from the mother liquor. It is characterized by separating.
(1) The refining intermediate is leached in a sulfite aqueous solution, silver is extracted into the sulfite aqueous solution, and the insoluble residue thus formed is separated to obtain a leachate containing silver.
(2) An acid is added to the leaching solution to precipitate silver chloride, and then the mother liquor is separated to obtain a precipitate containing silver chloride.
(3) The precipitate is oxidized in an acidic aqueous solution to elute the impurity element, and then the liquid containing the impurity element is separated to obtain purified silver chloride.
(4) The silver chloride is reduced in an alkaline aqueous solution to obtain silver powder.

In the present invention, in the step (2), an acid is added to the leachate to precipitate silver chloride, and then the mother liquor is separated to obtain a precipitate containing silver chloride. Significant importance to adjust the pH to 3.5 to 5.5 and to ensure that after the silver chloride is precipitated, the mother liquor and the total amount of the precipitate are immediately filtered and the precipitate is separated from the mother liquor. Have As a result, it is possible to produce silver having a purity of 5N, which is higher than that of the conventional method for producing high purity silver.
Here, the refining intermediate used for this invention and each process are demonstrated. In the description of each process, the action and its technical significance will also be described.

1. Refining intermediate The refining intermediate that is a raw material of the present invention contains silver chloride and an impurity element as a hardly soluble silver compound, and the process for producing it is not particularly limited. Intermediates generated in the processing step of silver-containing liquid such as plating solution and photographic developer, refining step of noble metal, etc. are included, including anode slime generated in the electrolytic refining step of refining processes such as copper, nickel and lead.
Here, as impurity elements, in addition to copper, nickel, lead, iron, cobalt, manganese, sulfur, zinc, cadmium, tin, group 15 elements such as arsenic, antimony, and bismuth, group 16 elements such as selenium and tellurium, And gold and platinum group elements.

2. Step (1) In the step (1), the refining intermediate is leached in an aqueous sulfite solution, silver is extracted into the aqueous sulfite solution, and then the insoluble residue produced is separated to remove silver. It is a step of obtaining a leachate containing. Here, the refining intermediate is suspended in a sulfite aqueous solution.
In the leaching reaction in the above step, when the sulfite is sodium sulfite and the hardly soluble silver compound is silver chloride, silver is selectively leached according to the following formula 1.

Formula 1: AgCl + Na ₂ SO ₃ → Na [AgSO ₃ ] + NaCl
Here, Formula 1 is a reaction in which silver chloride reacts with sodium sulfite to produce a stable silver sulfite complex salt: Na [AgSO ₃ ].

  The sulfite ion concentration of the sulfite aqueous solution used in the above step is not particularly limited, but is preferably 70 to 160 g / L, and more preferably 95 to 130 g / L. That is, if it is less than 70 g / L, the amount of silver compound dissolved in the sulfite aqueous solution is small, and the equipment capacity is increased. The higher the sulfite ion concentration, the higher the amount of silver compound dissolved. However, the amount of sulfite dissolved in the aqueous solution depends on the amount of salt other than sulfite in the aqueous solution, but the sulfite ion concentration is 160 g / L or less as an industrially feasible range.

  The sulfite used in the above step is not particularly limited, and any water-soluble sulfite can be used. For example, potassium sulfite, sodium sulfite, calcium sulfite, ammonium sulfite, cesium sulfite, Rubidium sulfite, amine sulfite and the like are used, and among these, sodium sulfite is particularly preferable from the viewpoint of economy and availability.

  The method for producing the aqueous sulfite solution used in the above step is not particularly limited. In addition to the method for dissolving the above sulfite in water, alkali metal and / or earth metal hydroxides and / or carbonates. It can manufacture by making sulfur dioxide gas react with the aqueous solution or slurry of this. Among them, particularly in the sulfuric acid production process of metal smelting, it is possible to use an absorbent obtained by absorbing the sulfur dioxide gas generated from the smelting process into an aqueous solution of alkali metal hydroxide and / or alkali metal carbonate, This is preferable because it can be used industrially at low cost. In particular, when sulfuric acid is produced from a smelting gas, it is more economical to effectively utilize a sulfite-containing waste liquid obtained by absorbing sulfur dioxide remaining without being converted by a detoxification tower.

  8-12 are preferable and, as for the pH of the said process, 10-11 are more preferable. That is, when the pH is less than 8, sulfite begins to rapidly change to bisulfite and the silver compound is insufficiently dissolved. On the other hand, when the pH exceeds 12, metallic silver is precipitated from the silver sulfite complex, The apparent leaching rate is reduced.

  20-80 degreeC is preferable and the temperature of the said process has more preferable 30-60 degreeC. That is, when the temperature is lower than 20 ° C., the solubility of sulfite ions decreases, so that it is difficult to set the sulfite concentration of the sulfite aqueous solution to 70 g / L or more. On the other hand, when it exceeds 80 ° C., it is reduced to metallic silver by the decomposition reaction of the silver sulfite complex salt.

  In the leachate obtained in the above step, in addition to the silver sulfite complex salt, the impurity elements contained in the refining intermediate are also contained in a small amount, but are dissolved and contained with an alkali salt or sulfite.

3. Step (2) In the step (2), an acid is added to the leachate obtained in the step (1) to precipitate silver chloride, and then the mother liquor is separated to obtain a precipitate containing silver chloride. (Hereinafter referred to as crude silver chloride). Here, when adding the acid, the pH is adjusted to 3.5 to 5.5, and after the precipitation of silver chloride, the mother liquor and the total amount of the precipitate are immediately filtered to separate the precipitate from the mother liquor. is important.
Here, when the refining intermediate used as a raw material contains a hardly soluble silver compound other than chloride, a predetermined amount of chloride ion can be added for precipitation as silver chloride. As a supply source of the chloride ions, water-soluble chlorides such as hydrochloric acid and sodium chloride are preferably used.

  In the said process, when adding an acid, as pH, it adjusts to 3.5-5.5. That is, when the pH is less than 3.5, the impurity element quality in the precipitated silver chloride is high, and the impurity element is sufficient when the precipitate is oxidized in an acidic aqueous solution in the subsequent step (3). Therefore, silver powder having a purity of 5N or more cannot be obtained. On the other hand, when the pH exceeds 5.5, crude silver chloride having a smaller impurity element content is recovered, but the amount of silver deposited is reduced, which is not preferable from the standpoint of actual yield.

The behavior of the pH and impurity elements will be described in more detail.
First, when an acid is added to the above leachate under a conventional condition to lower the pH, generally the silver sulfite complex is decomposed and silver is precipitated as silver chloride. Here, the lower the pH, the lower the silver concentration in the mother liquor and the higher the silver recovery rate. At the same time, the dissolved impurity elements such as gold, selenium, tellurium, and lead coprecipitate with the silver. In particular, since gold is highly dependent on pH, if the pH is less than 5, decomposition of the sulfito complex salt rapidly proceeds and is mixed into the crude silver chloride. In addition, the sulfurous acid gas generated by the decomposition of sulfite by the addition of acid accelerates the reduction of impurity elements such as selenium and is mixed into the crude silver chloride. The extent to which such impurity elements are contained in the crude silver chloride varies depending on the raw material poorly soluble silver compound, but usually several tens to several thousand ppm are mixed.

An example of the behavior of an impurity element when producing crude silver chloride in the above process will be described with reference to the drawings.
FIG. 1 is a graph showing the relationship between the pH during recovery of crude silver chloride and the silver concentration in the mother liquor. Moreover, FIG. 2 is a figure showing the relationship between pH at the time of rough | crude silver chloride collection | recovery, and the gold quality in the precipitated rough | crude silver chloride. 1 and 2, it can be seen that most of the silver in the mother liquor is precipitated by about pH 4, while the gold quality in the precipitated crude silver chloride increases rapidly at pH 5 or lower.

  In addition, when the pH is in the range of about 3.5 to 5.5, after silver chloride is precipitated, it is in an unstable region where sulfite gradually decomposes, and impurity elements may gradually precipitate over time. Observed. Fig. 3 shows the relationship between the retention time after adding acid to pH 5 and precipitating silver chloride (silver chloride recovery retention time) and the quality of impurity elements such as gold, lead, iron, selenium and tellurium in crude silver chloride. FIG. Here, it can be seen that the impurity elements increase with time, and in particular, lead and selenium increase significantly. That is, since the impurity element precipitates with time after the silver chloride is precipitated, it is desirable to separate the mother liquor and the precipitate in as short a time as possible, and within 60 minutes after the silver chloride is precipitated, the mother liquor is separated. preferable.

  From the above explanation, when adding acid by the method of the present invention, the pH is adjusted to 3.5 to 5.5, and immediately after the precipitation of silver chloride, the total amount of the mother liquor and the precipitate is filtered, It can be seen that the rapid separation of the precipitate from the mother liquor is an effective means for precipitating silver chloride with a low impurity element content.

At this time, if the mother liquor remains in the reaction vessel, it becomes silver chloride with a large amount of impurity elements, and the purity of newly precipitated silver chloride is lowered. Therefore, it is preferable to filter the whole amount of the mother liquor and the precipitate for each reaction to separate the mother liquor and the precipitate. For this reason, as a reaction form, it is preferable to carry out by a batch method, and as a shape of a reaction tank, it is preferable to set it as the structure which can draw out the whole quantity of mother liquid and a deposit from a reaction tank. That is, when the mother liquor and the precipitate are filtered using a reaction tank having a structure in which the whole amount of the mother liquor and the precipitate can be extracted, it is preferable to extract the entire amount of the mother liquor and the precipitate from the reaction tank.
For example, a reaction vessel having a cone-shaped or inclined bottom is used. Usually, a Werman pump or the like can be used for the slurry-like liquid containing the solid. However, a self-contained type hose pump or an air cushion pump that can draw out the mother liquor to the end is suitable here.

  In the above step, before adding the acid to the leachate, for example, it is preferable to perform microfiltration with a filter having an opening of 1 μm or less. Thereby, content of the impurity element to a precipitate can be reduced. In other words, industrially, solid-liquid separation devices such as filter presses and centrifuges are usually used for separation from insoluble residue after leaching. In these devices, filter cloth with a low air permeability is used. Even if it is used, leakage of a small amount of insoluble residue usually occurs. The main component of the insoluble residue is lead sulfate, which cannot be dissolved and removed even when the precipitate is oxidized in an acidic aqueous solution in the subsequent step (3). Further, when the solution leached with the aqueous sulfite solution is left, the impurity elements gradually precipitate.

  Therefore, it is effective to remove a solid content contained in a trace amount by adding an acid to the leachate and performing microfiltration using a filter having an opening of 1 μm or less before silver chloride is precipitated. Here, as a filter, it is preferable to use a cartridge filter which is usually about 10 to 50 mg / L of the obtained solid component, is inexpensive, and has a small space and a large filtration area.

The acid in the above step is not particularly limited, and mineral acids such as sulfuric acid and hydrochloric acid are used, but sulfuric acid is more preferable.
Although the reaction temperature of the said process is not specifically limited, 20-100 degreeC is preferable.

  Although most of the elements contained in the raw material are separated from the silver chloride obtained through the steps (1) and (2) of the present invention, some of them are dissolved in the sulfite aqueous solution. Part of impurity elements such as selenium, tellurium and lead are contained.

4). Step (3) In step (3), the precipitate obtained in the step was oxidized in an acidic aqueous solution to elute the impurity element, and then the liquid containing the impurity element was separated and purified. This is a step of obtaining silver chloride. In the above step, the precipitate containing silver chloride is suspended in an acidic aqueous solution, and an oxidizing agent is added while adjusting the redox potential.

The slurry concentration of the suspension in the above step is not particularly limited, but is preferably 100 to 500 g / L from the viewpoint of improving the dispersibility of the slurry and preventing resorption of the eluted impurity elements.
The acidic aqueous solution used in the above step is not particularly limited, and various mineral acids are used. Among them, in particular, chlorine is generated by an oxidizing agent to dissolve impurity elements present in a metal form. Hydrochloric acid is preferred because it is easy and the solubility of the assumed impurity element chloride in water is high.

  The oxidation-reduction potential (silver / silver chloride electrode standard) in the above step is not particularly limited, and is preferably adjusted to 800 to 1200 mV, more preferably 900 to 1000 mV. That is, by setting the oxidation-reduction potential (silver / silver chloride electrode standard) to 800 mV or higher, elements that exist as metals or intermetallic compounds and become soluble in an acidic aqueous solution by oxidation, such as selenium and tellurium, are dissolved. Can do.

The oxidizing agent used in the above step is not particularly limited, and chlorine gas, hydrogen peroxide solution, chlorate, etc. with little contamination to silver chloride are used.
The reaction temperature in the above step is not particularly limited, and the reaction rate can be accelerated as the temperature increases, but 40 to 80 ° C. is preferable. That is, at a temperature lower than 40 ° C., the impurity dissolution reaction is slow. On the other hand, when the temperature exceeds 80 ° C., when hydrogen peroxide solution or chlorate is used, the self-decomposition is promoted and the amount of chemicals used increases.

5. Step (4) The step (4) is a step in which the silver chloride is reduced in an alkaline aqueous solution to obtain 99.999 mass% high-purity silver powder. In the above step, high-purity silver chloride can be suspended in an alkaline aqueous solution.

  The initial slurry concentration of the high purity silver chloride in the above step is not particularly limited, and is preferably 100 to 500 g / L from the dispersibility of the slurry.

Although it does not specifically limit as aqueous alkali solution used at the said process, What is prepared using 1-5 equivalent alkali hydroxide and / or alkali carbonate with respect to silver is preferable. That is, addition of 1 equivalent or more prevents unreduced silver chloride from remaining and contaminating the resulting metallic silver. On the other hand, even if it exceeds 5 equivalents, it is not economical because no further effect can be obtained. In particular, it is more preferable to add an alkali component so that the pH of the aqueous alkali solution is 13 or more.
Further, the alkali hydroxide or alkali carbonate is not particularly limited, and water-soluble alkali metal salts, ammonium salts, and the like are used. Sodium is preferred.

  Although it does not specifically limit as a reducing agent used at the said process, Hydrazine, saccharides, formalin, etc. with little contamination to silver are used. The amount of reducing agent added in the above step can be adjusted by measuring the redox potential of the solution. That is, the end point is when the oxidation-reduction potential (silver / silver chloride electrode standard) is stable at −700 mV or less.

  The reduction temperature in the above step is not particularly limited, and is preferably 70 to 100 ° C, more preferably 90 to 100 ° C, which can accelerate the reaction rate. That is, when the temperature is less than 70 ° C., the residual silver chloride increases. On the other hand, when the temperature exceeds 100 ° C., it is necessary to use a pressurized container. In particular, in order to suppress the residual of silver chloride, it is more preferable to add an alkali component so that the pH of the aqueous alkali solution is 13 or more and to perform the reduction treatment by heating to 90 ° C or higher. In order to further reduce the residual silver chloride, it is effective to perform the reduction treatment again. In this case, since the amount of silver chloride to be reduced is very small, a very small amount of chemical is sufficient.

  Hereinafter, the present invention will be described in more detail by way of examples and comparative examples of the present invention, but the present invention is not limited to these examples. In addition, the analysis of the metal used by the Example and the comparative example was performed by Pd for the ICP-MS method, and other elements by the emission spectroscopy.

Example 1
Using a refining intermediate (hereinafter referred to as chlorine leaching residue) containing a sparingly soluble silver compound obtained by chlorine leaching of anode slime produced in the copper electrolysis process as a raw material, the raw material was treated by the following method. The purity of the produced silver powder was analyzed. The composition of the chlorine leaching residue is shown in Table 1.

Step (1) First, as a sulfite aqueous solution, an absorption liquid obtained by absorbing a sulfur dioxide gas generated in a smelting process of metal smelting in an aqueous sodium hydroxide solution is received in a leaching tank having a capacity of 10 m ³ , and the leaching liquid It was. The concentration of this leached sodium sulfite was measured and found to be 210 g / L.
Next, 1.2 tons of a chlorine leaching residue having the composition shown in Table 1 was introduced into the leachate in a wet amount. At this time, since the pH of the aqueous sulfite solution was lowered by the acid adhering to the chlorine leaching residue, the pH of the leaching solution was adjusted to 9 with caustic soda having a concentration of 18% by mass. After stirring for 1 hour, the silver leachate and the insoluble residue were separated using a filter press.

Step (2) The leachate obtained in the above step was passed through a cartridge filter having an opening of 1 μm and then received in a silver chloride production tank having a capacity of 5 m ³ . In addition, the bottom of the silver chloride production tank has an inclined structure, and the deposit and the mother liquor can be fed from the bottom of the tank to the filter press with a hose pump.
Next, after adding thin sulfuric acid to the silver chloride production tank and adjusting the pH to 3.5 to 5.5, the whole amount of the precipitate and the mother liquor is immediately fed to the filter press, and the precipitate and the mother liquor are separated by filtration. The crude silver chloride was recovered. The filter press filtration time was about 60 minutes.
This operation was then performed again. Thereafter, the obtained crude silver chloride was recovered and analyzed. Here, for analysis of the crude silver chloride, the crude silver chloride was adjusted to pH 10 by adding water and sodium hydroxide, hydrazine having a concentration of 60% by mass was added until the oxidation-reduction potential became stable, and the obtained silver powder was analyzed. The results are shown in Table 2. The recovered crude silver chloride was 263 kg in terms of moisture, and the silver recovery rate was 95%.

Steps (3) and (4) 263 kg of the crude silver chloride obtained in the above step was suspended in 1 m ³ of hydrochloric acid solution having a concentration of 8 mol / L, and this slurry was heated to 60 ° C. An oxidizing treatment was performed by adding 60 L of hydrogen oxide water dropwise over about 1 hour so as to add the entire amount. Here, the oxidation-reduction potential (silver / silver chloride electrode standard) of the slurry was 1000 mV or more. After cooling to 40 ° C., solid-liquid separation was performed with a centrifuge, followed by washing with water to recover purified silver chloride.
Thereafter, 263 kg of purified silver chloride in the obtained wet amount was suspended in 1 m ³ of an aqueous solution of sodium hydroxide having a concentration of 8.6% by mass, heated to a temperature of 70 ° C., and then hydrazine having a concentration of 60% by mass was converted into a redox potential. Was added until it became stable at −700 mV or less, and silver chloride was reduced to silver powder. After the reduction, the mixture was cooled to 40 ° C. or lower, filtered with a centrifuge, the silver powder was washed in the centrifuge, and the obtained silver powder was analyzed. The results are shown in Table 3.

  From Table 3, it turns out that the silver powder obtained by processing the said chlorine leaching residue at the process of this invention has a purity of 99.999 mass%.

(Comparative Example 1)
Using the chlorine leaching residue used in Example 1 as a raw material, it was processed by the method shown below, and the purity of the silver powder produced was analyzed.
Step (1) First, as an aqueous sulfite solution, an absorbent obtained by absorbing a sulfur dioxide gas generated in a smelting step of metal smelting in an aqueous sodium hydroxide solution was used. Using this as a leaching solution, 45 g of chlorine leaching residue was suspended in a volume of 400 ml, and the pH was adjusted to 10 with sodium hydroxide. After the adjustment, the mixture was stirred for 1 hour and silver was leached, followed by filtration with a glass filter to recover the silver leaching solution and insoluble residue. The concentration of sodium sulfite in the leachate was 200 g / L.

Step (2) Thin sulfuric acid was added to the leachate obtained in the above step, the pH was adjusted to 1, and then held for 1 hour. Here, during the reaction, sodium sulfite was decomposed to generate sulfur dioxide gas. The precipitated crude silver chloride and the mother liquor were filtered with a glass filter, and the crude silver chloride was recovered and analyzed. Here, for analysis of the crude silver chloride, the crude silver chloride was adjusted to pH 10 by adding water and sodium hydroxide, hydrazine having a concentration of 60% by mass was added until the oxidation-reduction potential became stable, and the obtained silver powder was analyzed. The results are shown in Table 4.

  From Table 4, the pH when adding the acid is 1 and is maintained for 1 hour after the precipitation of silver chloride, and the pH when adding the acid and the holding time in the mother liquor after the precipitation of silver chloride is the present invention. Since these conditions are not met, it can be seen that the obtained silver chloride contains more impurity elements than Example 1.

Steps (3) and (4) Silver chloride obtained in the above step: 250 g in a wet amount is suspended in 500 ml of hydrochloric acid solution having a concentration of 8 mol / L, and the slurry is heated to 60 ° C. Then, 50 ml of hydrogen peroxide solution was added dropwise over about 1 hour so as to add the whole amount, and oxidation treatment was performed. Here, the oxidation-reduction potential (silver / silver chloride electrode standard) of the slurry was 1000 mV or more. After cooling, it was filtered with a glass filter to recover purified silver chloride.
The obtained wet amount of 250 g of purified silver chloride was suspended in 1000 ml of an aqueous solution of sodium hydroxide having a concentration of 8.6% by mass and heated to a temperature of 70 ° C. Silver chloride was reduced to silver powder by adding until stable below. After cooling, it was filtered and washed with a glass filter, and the resulting silver powder was analyzed. The results are shown in Table 5.

  From Table 5, in the comparative example 1, the silver powder of the purity exceeding 99.999 mass% cannot be obtained. That is, since the content of the impurity element of silver chloride obtained in the step (2) is high, there is a limit to the removal of the impurity element in the subsequent steps (3) and (4).

  As is clear from the above, the high purity silver production method of the present invention performs dry refining or electrorefining when obtaining metallic silver from a refined intermediate containing a sparingly soluble silver compound and an impurity element. As a method for producing high-purity silver economically from refining intermediates, particularly refining intermediates containing silver chloride, 99.999 mass% high-purity metallic silver can be obtained only by wet refining. Is preferable.

It is a figure showing the relationship between pH at the time of rough | crude silver chloride collection | recovery, and the silver concentration in a mother liquid. It is a figure showing the relationship between pH at the time of rough | crude silver chloride collection | recovery, and the gold quality in the precipitated rough | crude silver chloride. The figure which shows the relationship between the retention time after adding an acid to pH 5 and precipitating silver chloride (silver chloride recovery retention time) and the quality of impurity elements such as gold, lead, iron, selenium and tellurium in crude silver chloride is there.

Claims (3)

A method for producing silver from a refining intermediate containing a hardly soluble silver compound and an impurity element by the following steps (1) to (4),
In the step (2), before adding acid to the leaching solution containing silver obtained in the step (1), the leaching solution containing silver is microfiltered with a filter having an opening of 1 μm or less, and the acid is removed. During the addition, the pH is adjusted to 3.5 to 5.5, and after precipitation of silver chloride, the whole amount of the mother liquor and the precipitate is filtered within 60 minutes to separate the precipitate from the mother liquor. A method for producing silver.
(1) The refining intermediate is leached in a sulfite aqueous solution, silver is extracted into the sulfite aqueous solution, and the insoluble residue thus formed is separated to obtain a leachate containing silver.
(2) An acid is added to the leaching solution to precipitate silver chloride, and then the mother liquor is separated to obtain a precipitate containing silver chloride.
(3) The precipitate is oxidized in an acidic aqueous solution to elute the impurity element, and then the liquid containing the impurity element is separated to obtain purified silver chloride.
(4) The silver chloride is reduced in an alkaline aqueous solution to obtain silver powder.   In the step (2), when the mother liquor and the precipitate are filtered using a reaction tank having a structure capable of extracting the whole amount of the mother liquor and the precipitate, the whole amount of the mother liquor and the precipitate is extracted from the reaction tank. The method for producing silver according to claim 1, characterized in that: The method for producing silver according to claim 1 or 2 , wherein the hardly soluble silver compound is silver chloride.

Images