JP6368672B2 - Silver smelting method - Google Patents

Description

  The present invention is a method for obtaining silver by reducing silver chloride.

  In the smelting of non-ferrous metals, various precious metals are recovered as smelting by-products of general-purpose metals. Precious metals are concentrated in slime during the electrorefining of general purpose metals.

  In the recovery of the noble metal, the noble metal in the slime is further dissolved in the acid and separated for each element. Among such noble metals, it is known that silver is relatively high in content and forms an insoluble salt in halide ions and water, so that separation is easy.

  Silver is often industrially separated as silver chloride, and silver can be obtained by reducing it to metallic silver. Specifically, silver chloride is reduced, further melted and cast into a silver anode, and then metal silver is recovered through electrolytic purification using the silver anode. At this time, when reducing silver chloride, iron powder reduction or coke reduction is used (Patent Document 1).

  An industrially important point is a method for dissolving silver chloride safely and efficiently at low cost. In order to dissolve silver chloride, it is necessary to make silver a complex. In addition to halide ions, ammonia, cyanide ions, and thiocyan ions are known for complexing silver.

  If silver is complexed with ammonia and reduced, explosive lightning silver may be generated, which is not a preferable method for smelting silver. Cyanide ions are highly toxic and should not be used.

JP 2001-316736 A Special table 2014-501850 gazette

By the way, a thiosulfate ion can also be mentioned as a compound for complexing silver.
Patent Document 2 discloses a technique for electrodepositing silver dissolved with thiosulfuric acid without applying a voltage. Although there is an aspect that thiosulfuric acid is expensive, the technique of Patent Document 2 enables cost reduction by repeatedly using thiosulfuric acid. However, as described above, silver chloride that is industrially advantageous for silver recovery is not targeted as a silver compound for dissolving thiosulfuric acid.

  Further, even if the method of Patent Document 2 is applied to dissolution of silver chloride as it is, if a solution in which silver chloride is dissolved is electrolytically collected, decomposition of thiosulfuric acid occurs and cost merit is reduced.

  In addition, as a method of smelting silver chloride into metallic silver, there is a dry smelting method in which coke and smelting reduction are performed. At this time, silver chloride becomes silver and chlorine, chlorine is liberated as radicals or chlorine gas, and the furnace wall is removed. It may erode violently. For this reason, the amount of absolute chlorine that can be charged into the furnace is limited, so it cannot be said that the amount of silver chloride that can be processed at one time is large and is not efficient.

  Further, as a method for dissolving silver chloride, there is a method using iron powder as a reducing agent. In this method, iron chloride is charged after dissolving silver chloride with hydrochloric acid. Therefore, the silver after reduction also contains chlorine. Further, since silver chloride is dissolved in hydrochloric acid in advance, protons as counter ions of chloride ions are present in the liquid, and these protons react with iron powder. As a result, the amount of iron powder used is increased.

  In addition, when iron powder is used, unreacted iron and iron oxide generated as a result of reduction must be separated from silver again. Generally, a dry separation method is employed, and iron is distributed and removed to the slag. The problem when such a metal reducing agent is used is the same even when copper powder or zinc powder is used.

  Therefore, in order to improve productivity when recovering silver using a metal reducing agent, the silver used for dissolving silver chloride is previously recovered from the solution prior to the introduction of the reducing agent. A method is needed. Furthermore, it is still desired to construct a process that does not go through a dry smelting process such as a silver separation oxidation furnace by not reducing with metal components.

  As a result of intensive studies to solve the above problems, the present inventors have found that crude silver having a low chlorine content can be obtained by dissolving silver chloride with a sodium thiosulfate solution and reducing it with glucose. It was.

That is, the present invention includes the following inventions.
(1) A method of recovering silver by dissolving silver chloride in a sodium thiosulfate solution and reducing it with an aldehyde.
(2) The method according to (1), wherein the aldehyde is obtained by bringing a sugar into contact with an alkali.
(3) The method according to (2), wherein the sugar is glucose.
(4) The method according to any one of (1) to (3), wherein the sodium thiosulfate solution for dissolving silver chloride has a pH of 7 to 10.
(5) The method according to any one of (1) to (4), wherein the sodium thiosulfate solution for dissolving silver chloride contains 150 to 200 g of thiosulfate ions with respect to 100 g of silver chloride. .
(6) The method according to any one of (1) to (5), wherein the reduction is performed at 40 to 60 ° C.
(7) Any one of (1) to (6) is characterized in that the solution after the silver is recovered by reduction with the aldehyde is used to adjust the thiosulfate ion concentration and dissolve silver chloride again. The method of crab.
(8) In the method according to any one of (1) to (7), sodium salt is added to the solution after silver is recovered by reduction with the aldehyde, and the resulting precipitate is recovered and recovered. A method in which a solution obtained by dissolving the deposited precipitate in water is added when silver chloride is dissolved in a sodium thiosulfate solution.
(9) In the method according to any one of (1) to (7), a sodium salt is added to the solution after the silver is recovered by reduction with the aldehyde, and the resulting precipitate is recovered and recovered. A method of recovering silver by reducing a solution obtained by dissolving the deposited precipitate in water with an aldehyde.
(10) In the method according to any one of (1) to (7), a sodium salt is added to the liquid after the silver is recovered by reduction with the aldehyde, and the resulting precipitate is removed. The resulting solution is reduced with aldehyde to recover silver.

  According to the present invention, metallic silver having a low chlorine content can be obtained from silver chloride, and silver can be smelted at low cost by repeatedly using thiosulfuric acid.

It is a figure which shows the operation flow of this invention. It is a plot which shows the relationship between the sodium concentration and silver concentration in a solution. In Example 3, it is a plot which shows Na density | concentration and Ag leaching rate in a liquid in each silver leaching time when a sodium thiosulfate solution is repeatedly used for leaching of silver chloride.

Hereinafter, the present invention will be described in detail.
As shown in FIG. 1, the present invention is characterized in that silver chloride is dissolved in a sodium thiosulfate solution to obtain metallic silver by aldehyde reduction.

  Hereinafter, as an embodiment of the present invention, an aspect applied to one step of a treatment method of slime generated in an electrolytic purification process in copper smelting will be described, but the present invention is not limited to this, and silver chloride is used. It can be applied to all methods for smelting metallic silver.

  In this embodiment, although not shown in FIG. 1, silver is separated as silver chloride in the copper smelting step. This silver chloride is insoluble in water, but if an appropriate ligand is selected, a complex is formed and dissolved in water. For example, ammonia, chloride ions, thiosulfate ions, and cyanide ions are known as useful ligands for silver complexation.

  Of these, thiosulfate ions are very useful from the standpoint of reagent toxicity and solubility, and typically a solution containing 150 to 200 g of thiosulfate ions per 100 g of silver chloride is used for dissolution of silver. . Such a solution is obtained as a solution obtained by dissolving 212 to 282 g of sodium thiosulfate or 332 to 443 g of sodium thiosulfate pentahydrate in water. If the thiosulfate ion concentration is too low, silver will not dissolve. If it is too high, the cost increases.

  Since thiosulfate ions are decomposed under acidic conditions, it is preferable to wash silver chloride with an alkaline solution in advance. Moreover, decomposition | disassembly of thiosulfuric acid can be prevented if a solution is maintained at pH 7-10.

The silver dissolved in the thiosulfuric acid solution forms an anionic complex of silver having a thiosulfate ion as a ligand (bis (thiosulfato) silver (I) acid ion, hereinafter referred to as “thiosulfate silver ion”). .
AgCl + 2Na ₂ S ₂ O ₃ → [Ag (S ₂ O ₃ ) ₂ ] ³⁻ + 4Na ⁺ + Cl ⁻
By reducing this complex, it can be recovered as metallic silver. As means for recovering silver by reduction, contact with base metal powders such as iron powder and copper powder, reduction by electrolysis, etc. are known, but reduction by base metal powder may reduce components other than the target silver Yes, it affects the purity of the resulting silver. In addition, reduction by electrolysis yields high-purity silver. However, as described above, decomposition of thiosulfuric acid occurs, and the sulfur produced during the decomposition is suspended during electrolysis, and this sulfur is entrained and electrodeposited. There is a problem that the installation cost is large. From the viewpoint that no special equipment is required and the reduction can be easily performed, a water-soluble reductant, for example, aldehyde, hydrazine and the like, preferably a reduction with an aldehyde, is preferable. For example, it can be easily removed by melting in a later step.

  It is well known that silver is reduced by aldehyde. This reaction is known as the silver mirror reaction. Formic acid, formalin, and the like are known as aldehydes, but considering their toxicity, it is preferable to use aldehydes obtained by hydrolyzing sugars.

  Sugar usually has a ring structure and a straight chain structure, but the equilibrium tends to a straight chain structure having an aldehyde group in an alkaline solution (the following formula 1). This linear aldehyde and silver thiosulfate ion react to obtain metallic silver. At this time, when the solution is heated, the reduction rate increases. However, when the heating temperature is high, decomposition of thiosulfate ions also occurs. Therefore, the reduction is preferably performed at 40 to 60 ° C.

  Here, any saccharide can be used as an aldehyde source as a reducing agent used in the present embodiment, but glucose is preferable in terms of cost. When using glucose, it is preferable to add 0.2 to 1.0 weight times with respect to silver. More preferably, 0.35 to 0.50 times by weight is added. If the amount of glucose is too small, it will be difficult to recover an effective amount of silver. On the other hand, if the amount of glucose is excessive, the merit from the cost side will be reduced and the rate of silver reduction will increase. There is a problem that COD rises.

  The reduced and precipitated silver is recovered by a suitable method, further melted and cast into a silver anode. Since the thiosulfate ions remain in the solution after the silver is removed, sodium thiosulfate can be added to adjust the thiosulfate ion concentration and used again to dissolve silver chloride. is there. If unreacted glucose remains in the solution, it can be used to reduce the solution in which silver chloride is dissolved again. Therefore, the amount of glucose added during the reduction is reduced from the amount added to the silver. It is possible.

  However, the concentration of ions other than thiosulfate ions, such as sodium ions and chloride ions, increases to some extent while repeatedly using a leachate containing thiosulfate, so that ions other than thiosulfate silver ions and thiosulfate ions For example, a salt with sodium ions may precipitate, and the silver recovery efficiency may be significantly reduced.

  Therefore, after repeating to some extent, a sodium salt such as sodium chloride is preferably added to the liquid after leaching silver chloride, preferably in a solid state, to precipitate the sodium salt of thiosulfate silver ion. This is preferably continued until no precipitate is formed. Below, the reaction formula assumed is shown.

3 [Ag (S ₂ O ₃ ) ₂ ] ^3- → [Ag ₃ (S ₂ O ₃ ) ₄ ] ^5- + 2S ₂ O ₃ ^2- (1)
[Ag ₃ (S ₂ O ₃ ) ₄ ] ^5- + 5Na ⁺ → Na ₅ [Ag ₃ (S ₂ O ₃ ) ₄ ] ↓ (2)

First, in the formula (1), it is considered that the form of thiosulfate silver ion changes depending on the content ratio of silver ion and thiosulfate ion in the solution. Next, by adding Na salt as shown in formula (2), sodium tetrakis (thiosulfato) trisilver (I) (hereinafter referred to as “Nathiothiosulfate”) is formed, and this complex salt is saturated and precipitated. Conceivable. Therefore, in order to precipitate Na thiosulfate silver salt, the form of the complex with [Ag (S ₂ O ₃ ) ₂ ] ³⁻ or [Ag ₃ (S ₂ O ₃ ) ₄ ] ⁵⁻ is one or both. Typically, the thiosulfate ion has a ratio of {(molar concentration of thiosulfate ion) / (molar concentration of silver ion)} to be 4/3 to 2 so that the content ratio is present. It is preferable that it is contained in.

  Further, in the formula (2), as the solution to which sodium chloride is added, sodium thiosulfate is saturated, and the solution immediately before precipitation is used to precipitate sodium thiosulfate. It is desirable from the viewpoint that the amount of the amount can be suppressed.

  Here, the state immediately before the silver thiosulfate Na is precipitated is specified by the sodium ion concentration relative to the silver ion concentration in the solution. In practice, silver is collected, and the sodium ion concentration is specified from the stoichiometric ratio according to the silver ion concentration in the operating conditions. From this point of view, it is preferable to use a solution having a sodium ion concentration of 100 g / L or more under the operating conditions as the solution immediately before the silver thiosulfate Na is precipitated.

  For example, when sodium ions and / or chloride ions are contained as impurities in the solution after leaching of silver chloride, the sodium concentration in the solution is adjusted to be in the range of 100 g / L to 130 g / L by adding the sodium salt. It is preferable to do this. When the sodium concentration is 100 g / L or more, precipitates are generated. As the knowledge obtained by the present inventors, the relationship between sodium concentration and silver concentration in the solution is shown in Table 1 below, and the plot is shown in FIG. According to Table 1 and FIG. 2, for example, when the silver concentration is 70 g / L or more and when the sodium concentration is 100 g / L or more, it is considered that the silver complex salt of thiosulfuric acid is precipitated. As will be described later, when sodium concentration exceeds 130 g / L, when sodium chloride is added, sodium chloride starts to precipitate this time.

Subsequently, the precipitate (Na ₅ [Ag ₃ (S ₂ O ₃ ) ₄ ]) obtained by the reaction of the formula (2) is dissolved in water to obtain a solution in which Na thiosulfate is dissolved.
Na ₅ [Ag ₃ (S ₂ O ₃ ) ₄ ] → [Ag ₃ (S ₂ O ₃ ) ₄ ] ^5- + 5Na ⁺

This aqueous solution is a solution of silver thiosulfate ions, and can recover silver under appropriate reducing conditions as described above. The solution after silver recovery is also used as thiosulfuric acid for leaching silver chloride again. You can also.
Utilizing this, the thiosulfuric acid can be regenerated by returning to the silver chloride leaching step and performing glucose reduction together with the newly leached silver.

According to the present inventors, when the amount of silver ions is too low relative to the thiosulfate ions in the solution, even if the sodium ion concentration is high, the silver complex salt dissolves even if the temperature of the solution is low. Has been found to affect the regeneration or recovery of thiosulfate. In order to improve the regeneration rate or recovery rate of this thiosulfuric acid, silver may be added, for example, the silver concentration may be 20 g / L or more, and this is expected to improve the regeneration rate or recovery rate of thiosulfuric acid. The In addition, when adding as silver chloride, it is preferable to carry out, before or before adding sodium chloride from the viewpoint of removing chloride ions as much as possible from the regenerated sodium thiosulfate solution.
In addition, regarding the amount of silver ions to be added, thiosulfate ions are {(molar concentration of thiosulfate ions) / ( It is preferable to add silver ions so that the molar ratio of silver ions)} is 4/3 to 2.

  As mentioned above, the sodium thiosulfate solution which has few impurities by removing sodium chloride which becomes the cause from the leaching solution where the leaching effect of silver is lowered, and recovering it as Na thiosulfate silver salt and dissolving it in water Is obtained. By reducing this solution to precipitate silver, the sodium thiosulfate solution can be regenerated as a silver chloride leachate.

  The silver thus obtained by reduction contains some impurities. However, if this is cast and electrolytically purified, high purity silver can be obtained. At this time, since a dry separation step such as a silver separation oxidation furnace is not required, the cost can be suppressed. In addition, since thiosulfuric acid reduces chlorine gas, the amount of chlorine entering during a dry process in a subsequent process is small, and the cost can be reduced because a large amount can be processed.

Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to these.
Example 1
Electrolytic slime discharged by copper smelting was dissolved with hydrochloric acid and hydrogen peroxide. Chloride leaching residue containing undissolved silver chloride was collected and washed with sodium hydroxide solution at pH 10. Table 2 shows the components of the chloride leaching residue. A solution of sodium thiosulfate (reagent grade, manufactured by Wako Pure Chemical Industries, Ltd.) (concentration was 155 g / L as thiosulfate ion) was prepared, and the pH was set to 7. The recovered chloride leaching residue 400 dry-g (including silver chloride 198 dry-g) was dissolved in 2 L of the prepared sodium thiosulfate solution. The undissolved part here was separated by filtration to obtain 2 L of silver thiosulfate solution (silver concentration 74 g / L).
To the silver thiosulfate solution, 125 g of glucose (reagent grade, manufactured by Wako Pure Chemical Industries, Ltd.) was added and stirred at 60 ° C. for 3 hours. The precipitated silver was collected and weighed to find 151 dry-g. The results are shown in Table 3.
For quantitative analysis, the noble metal component was concentrated by a dry assay method, then decomposed with nitric acid, and the metal component was quantified by ICP-AES. The composition of chlorine was confirmed by combustion absorption ion chromatography. Highly pure silver can be obtained by electrolytic purification of crude silver obtained by anode casting using such reduced precipitated silver.

  It is known from experience in actual operation that the above-described method of dissolving silver chloride with concentrated hydrochloric acid and reducing it with iron powder generally has a chlorine quality of around 1.5%. Compared with the results in Table 2, it can be seen that the chlorine content of the silver recovered by this method is low. The silver recovery rate was 97%, indicating a good yield.

(Example 2)
The thiosulfate ion concentration of the sodium thiosulfate solution after use in Example 1 was measured by the iodine oxidation titration method. As a result, it was found that it was 147 g / L, so sodium thiosulfate was added to 155 g / L, and then the sodium thiosulfate solution was again used for dissolution of the chloride leaching residue. Similarly, after reducing with glucose and collecting silver, the thiosulfuric acid concentration was measured again and found to be 134 g / L. The results are shown as the number of trials 1 in Table 3. Furthermore, even if this operation was repeatedly used while maintaining the thiosulfate ion concentration at 155 g / L or more, no problem occurred from the viewpoint of silver leaching rate. Table 4 shows the transition of Ag leaching rate with respect to repeated use of the thiosulfuric acid solution.

  In this example, the Ag leaching rate was evaluated instead of the Ag recovery rate, but the high leaching rate means that the silver quality contained in the crude silver recovered after leaching is high, that is, the amount of silver chloride is low. Of course, it is expected that the Ag recovery rate is also increased.

(Example 3) Regeneration of sodium thiosulfate with reduced leaching ability After leaching silver chloride with sodium thiosulfate and recovering silver, a sodium thiosulfate solution is repeatedly used for silver chloride leaching to produce precipitates. The number of repetitions was determined.
The operating conditions for leaching and recovery, and the leaching effect of each round are as follows.
The crude salt halide (Ag: 37.4 wt%, Cl: 12.0 wt%, Pb: 7.4 wt%, SiO _2: 20.4 wt%) 200dry-g the (silver chloride containing 99dry-g) , A solution of sodium thiosulfate solution (sodium thiosulfate (reagent grade, manufactured by Wako Pure Chemical Industries, Ltd.) (concentration was adjusted to 78 g / L as thiosulfate ion, pH was set to 7) dissolved in 2 L . The undissolved part here was separated by filtration to obtain a silver thiosulfate solution.
To the silver thiosulfate solution, 28 g of glucose (reagent grade, manufactured by Wako Pure Chemical Industries, Ltd.) was added and stirred at 60 ° C. for 5 hours, and the precipitated silver was collected by filtration. The composition of the obtained silver was Ag: 97% by weight, Cl: 0.06% by weight, Pb: 0.17% by weight, and SiO ₂ : 0.26% by weight. The Na concentration in the filtrate after silver recovery was 36 g / L.

Next, in order to reuse the filtrate after silver recovery as a crude silver chloride leachate, the thiosulfate ion concentration in the filtrate was measured by the iodine oxidation titration method, and then the amount of sodium thiosulfate in the filtrate was 2.93 g / g- It adjusted by adding sodium thiosulfate so that it might become Ag. Crude silver chloride was added to this adjustment solution so as to have a pulp concentration of 100 g / L, and silver was leached. FIG. 3 shows the Na concentration in the solution and the Ag leaching rate at each silver leaching cycle. According to FIG. 3, when the amount of sodium thiosulfate was adjusted so as to be constant with respect to the silver concentration and the operation of silver leaching was repeated, the concentration of Na in the liquid continued to increase gradually, The leach rate remained around 98%. The leaching rate of silver (%) is (1− (silver content in residue after leaching (g)) ÷ (silver content of crude silver chloride added before leaching (g))) × 100 ( %).
When leaching was repeated 14 times, precipitates were formed in the leaching solution and solidified, making it difficult to leach silver. From this, it was found that the number of repetitions of precipitation was 14 times. Concentrations of main components of the leaching solution after 14 times of leaching were 127 g / L for Na concentration, 37 g / L for Ag concentration, and 76 g / L for thiosulfate ion concentration. As will be described later, this precipitate is considered to be a silver thiosulfate complex from the Na concentration in the leachate.

Sodium chloride was added as a solid to the solution obtained based on the conditions obtained here just before the precipitation of Na thiosulfate. The addition of sodium chloride was continued until no new precipitate was observed. Further, the generated precipitate was recovered by solid-liquid separation.
From both the results of calculating the molar ratio from the concentration of each ionic component obtained by dissolving the collected precipitate in water (Table 5) and conducting the electron beam microanalyzer (EPMA) analysis (Table 6), The composition of this precipitate was estimated as Na ₅ [Ag ₃ (S ₂ O ₃ ) ₄ ] as follows. In the table, the molar ratio of each ion component in Na [Ag (S ₂ O ₃ )], Na ₃ [Ag (S ₂ O ₃ ) ₂ ], Na ₅ [Ag ₃ (S ₂ O ₃ ) ₄ ], The atomic weight concentration ratio was calculated by theoretical calculation.

The leaching filtrate obtained after 14 times of leaching of silver chloride was stored, and the amount of Ag and Na ₂ S ₂ O ₃ with respect to the Na concentration in the solution was increased while increasing the amount of sodium chloride added thereto. The recovery rate was determined. The calculation method of the recovery rate is shown below using Ag as an example.
Ag recovery rate (%) = (1−Ag amount dissolved in liquid [g] ÷ Ag amount dissolved in liquid after 14 times of leaching [g]) × 100
In addition, since the silver thiosulfate sodium salt had already precipitated during storage in the filtrate before the addition of sodium chloride, the recovery rate obtained above from the Ag concentration and Na ₂ S ₂ O ₃ concentration before the addition of sodium chloride in the test results. Also added. The reason for the precipitation of silver sodium thiosulfate during storage is thought to be because the silver thiosulfate ion immediately after leaching changed to a form in which sodium salt was likely to precipitate during storage, that is, the change in the above formula (1) occurred. . The results of the addition test are shown in Table 7. According to Table 7, as sodium chloride was added, the Na concentration of the liquid increased and the recovery rate of Na ₂ S ₂ O ₃ and Ag also increased. However, if sodium chloride is added when the Na concentration in the liquid exceeds 136 g / L, the recovery rate of Ag and Na ₂ S ₂ O ₃ does not increase, and Na corresponding to the dissolved amount of sodium chloride further added in the liquid. There was no increase in concentration. From this, it is considered that sodium chloride began to precipitate when the Na concentration in the liquid exceeded 136 g / L.

Claims (9)

Dissolving silver chloride in sodium thiosulfate solution and reducing with aldehyde to recover silver ;
Sodium salt is added to the solution after the silver is recovered by reduction with the aldehyde, the resulting precipitate is recovered, and the solution obtained by dissolving the recovered precipitate in water is added to the silver chloride solution. Throw in when dissolving in sodium sulfate solution
Including a method. Dissolving silver chloride in sodium thiosulfate solution and reducing with aldehyde to recover silver;
Sodium salt is added to the liquid after the silver is recovered by reduction with the aldehyde, the resulting precipitate is recovered, and the solution obtained by dissolving the recovered precipitate in water is reduced with aldehyde. Recovering silver
Including a method. Dissolving silver chloride in sodium thiosulfate solution and reducing with aldehyde to recover silver;
Adjusting the thiosulfate ion concentration of the liquid after reducing with the aldehyde to recover the silver and repeatedly using it for dissolving the silver chloride;
Sodium salt is added to the solution after the silver is recovered by reduction with the aldehyde, the resulting precipitate is recovered, and the solution obtained by dissolving the recovered precipitate in water is added to the silver chloride solution. Throw in when dissolving in sodium sulfate solution
Including a method. Dissolving silver chloride in sodium thiosulfate solution and reducing with aldehyde to recover silver;
Adjusting the thiosulfate ion concentration of the liquid after reducing with the aldehyde to recover the silver and repeatedly using it for dissolving the silver chloride;
Sodium salt is added to the liquid after the silver is recovered by reduction with the aldehyde, the resulting precipitate is recovered, and the solution obtained by dissolving the recovered precipitate in water is reduced with aldehyde. Recovering silver
Including a method. The aldehyde is characterized by obtained by contacting the sugar in an alkaline method according to any one of claims 1 to 4. 6. The method of claim 5 , wherein the sugar is glucose. The method according to any one of claims 1 to 6 , wherein the sodium thiosulfate solution for dissolving silver chloride has a pH of 7 to 10. The method according to any one of claims 1 to 7 , wherein the sodium thiosulfate solution in which silver chloride is dissolved contains 150 to 200 g of thiosulfate ions with respect to 100 g of silver chloride. The method according to any one of claims 1 to 8 , wherein the reduction is performed at 40 to 60 ° C.