TWI428451B - Valuable metal recovery method from lead-free waste solder - Google Patents

Description

Method for recovering precious metals from lead-free waste solder

The following discloses a method for recovering precious metals from Pb-free waste solder, in particular, a method for recovering tin or silver from Pb-free waste solder, wherein the Pb-free waste solder contains tin, silver, and a mixture thereof.

In view of environmental protection and resource recycling, it is important to recover tin and silver in Pb-free waste solder. Recently, the recovery of silver and tin from industrial waste (including Pb-free waste solder) is generally classified into an electrolysis method and a simple recovery method.

The simple recovery method is a method for recovering a Pb-free solder ball source, which comprises melting a Pb-free solder at a high temperature to initially separate the slag phase from the impurities, and then adding high-purity tin therein and regulating the quality of the tin. However, the solder balls formed according to the above method may be subject to deterioration in quality.

The electrolysis method is a separation and recovery method comprising: preparing a cathode plate composed of high-purity tin (Sn), and an anode plate having a composition of 90 to 98% by weight of Pb-free solder; containing 3 to 8 vol% Electrolysis is carried out in an electrolyte of H ₂ SiF ₆ , 2 to 10% by volume of H ₂ SO ₄ and 3 vol% of Sn; Sn is recovered from the cathode plate and silver (Ag) is recovered from the anode fine mud, thereby recovering high purity Sn. However, the above method has the disadvantage of producing a large amount of harmful waste water, which may cause environmental pollution and requires high initial investment cost.

In order to solve the above problems, the inventors have studied the technique of treating waste solder, which is more efficient and simpler than the known methods in the related art, and requires a lower initial cost. The inventors have found that the use of an electrolyte containing chloride ions (Cl ^- ) in the electrolytic refining process can mainly solve the problem of increasing the recovery rate of Sn or Ag, thereby completing the present invention. Accordingly, it is an object of the present invention to provide a method for recovering precious metals from Pb-free waste solder which has the advantage of overcoming the difficulties and/or reproducibility of existing methods for extracting, separating and/or refining precious metals from spent solder; And, high purity Sn or Ag can be efficiently and economically recovered.

An embodiment of the present invention provides a method of recovering precious metals from Pb-free waste solder, and more particularly, a method of recovering Sn or Ag from a Pb-free waste solder containing Sn, Ag, or a mixture thereof.

In a general aspect, a method for recovering Sn or Ag from a Pb-free waste solder, comprising: (1) preparing an anode from a Pb-free waste solder, wherein the Pb-free waste solder comprises Sn, Ag, or a mixture thereof; (2) Applying an electric current to an anode prepared by the operation (1) and a cathode in an electrolyte containing chloride ions (Cl ^- ); (3) a reaction initiated by applying a current, and concentrating Ag on an anode formed on the anode In the mud, simultaneously depositing Sn on the cathode; and (4) recovering the electrodeposited Sn or recovering the concentrated Ag from the anode slime.

In another general aspect, a method of recovering Sn or Ag from a Pb-free waste solder, comprising: (1) preparing an anode from a Pb-free waste solder, wherein the Pb-free waste solder comprises Sn, Ag, or a mixture thereof; Applying an electric current to the prepared anode and a cathode in an electrolyte containing chloride ions (Cl ^- ); (3) reacting by applying an electric current to concentrate Ag as an anode slime contained in the anode a material for simultaneously depositing Sn on the cathode; and (4) recovering an anode fine sludge containing Ag concentrated therein, chemically dissolving the recovered fine mud and performing solid-liquid separation thereof; (5) using solid-liquid separation Post-recovered Ag and Ag obtained by precipitation with a reduction of Ag compound produced by the filtrate to form a crude Ag anode, and electrolytic refining of Ag in a silver nitrate electrolyte; and (6) recovery operation (3) Electrodeposited Sn or electrodeposited Ag of operation (5).

According to the method of recovering Sn or Ag from the Pb-free waste solder according to the present invention, an electrolyte containing chloride ions (Cl ^- ) can be used.

More particularly, the present invention is recovered by a waste-free solder is Sn or Pb Ag may comprise a method comprising applying a current to Cl ^- in the electrolyte of the anode and the cathode, the anode and the cathode lines containing Sn, Ag or mixtures thereof It is formed without Pb waste solder to prevent precipitation of Sn oxide while improving the enrichment rate of silver in the anode slime and the current efficiency of Sn electrodeposition.

In another general aspect, a method for recovering Ag from a Pb-free waste solder includes: chemically dissolving an anode fine mud and performing solid-liquid separation; and recovering Ag as a residue after solid-liquid separation and by precipitation and The Ag obtained by the reduction of the Ag compound produced by the filtrate forms a crude Ag anode, and then electrolytic refining of Ag in a silver nitrate electrolyte, whereby high purity and high yield of Ag can be recovered.

The invention will be described in more detail below.

The present invention can provide a method for recovering Sn or Ag from Pb-free waste solder, comprising: (1) preparing an anode from Pb-free waste solder, wherein the Pb-free waste solder comprises Sn, Ag or a mixture thereof; (2) Applying an electric current to the prepared anode and a cathode in an electrolyte containing chloride ions (Cl ^- ); (3) reacting by applying an electric current to concentrate Ag as an anode slime contained in the anode a material that simultaneously deposits Sn on the cathode; and (4) recovers the electrodeposited Sn or recovers the concentrated Ag from the anode slime.

According to the present invention, the method for recovering Sn or Ag from the Pb-free waste solder may further include: after the operation (3), recovering the anode fine mud containing the Ag concentrated therein and sequentially performing its chemical dissolution and solid-liquid separation; And an undissolved Ag residue obtained after solid-liquid separation, and Ag obtained by dissolving Ag dissolved in the recovered filtrate by precipitation and reduction to form a crude Ag anode, and in a silver nitrate electrolyte Electrolytic refining of Ag is performed.

More specifically, the present invention can provide a method for recovering Sn or Ag from Pb-free waste solder, comprising: (1) preparing a working electrode (ie, an anode) from Pb-free waste solder, wherein the Pb-free waste solder Containing Sn, Ag or a mixture thereof; (2) an anode prepared by the operation (1) and a cathode applied by applying an electric current to an electrolyte containing chloride ions (Cl ^- ); (3) a reaction initiated by applying an electric current, Concentrating Ag as a material contained in the anode fine mud on the anode while electrodepositing Sn on the cathode; and (4) performing chemical dissolution and solid-liquid separation of the anode fine slurry containing Ag concentrated therein; (5) recovering Ag as a residue and Ag obtained from the filtrate by precipitation and reduction using solid-liquid separation to form a crude Ag anode, and performing electrolytic refining of Ag in a silver nitrate electrolyte; and (6) The electrodeposited Sn of operation (3) or the electrodeposited Ag of operation (5) is recovered (see Fig. 1).

According to the present invention, the operation (1) may comprise melting and casting the Pb-free waste solder at a temperature of 400 ° C to prepare an anode for preliminary electrolytic refining. The Pb-free solder is not particularly limited as long as it contains Sn and Ag.

According to the present invention, the purpose of preliminary electrolytic refining is to recover high-purity Sn from the cathode while concentrating Ag in the anode slime, and for this purpose, the present invention can use a sulfuric acid solution containing chloride ion (Cl ^- ) as an electrolyte. . The chloride ion (hereinafter referred to as Cl ^- ) prevents precipitation of Sn oxide by Sn ions when the concentration of Sn ions in the electrolyte increases, and at the same time increases the enrichment rate of Ag in the anode slime. Therefore, the above operations are of great significance to the present invention.

According to the present invention, the concentration of Cl ^- added may be in the range of 0.05 to 0.5 mol/liter, preferably 0.1 to 0.3 mol/liter. If the Cl ^- concentration is less than 0.05 mol / liter, the increase in the Ag concentration and the effect of preventing the precipitation of the Sn oxide may be lowered. On the other hand, even when the Cl ^- concentration is higher than 0.5 mol / liter, the enrichment rate of Ag is not significantly different from that at a Cl ^- concentration of 0.5 mol / liter, and further the problem of increasing corrosion potential of the electrolyte is required. .

The addition of Cl ^- to the electrolyte may be carried out using any one of Cl ^- containing acids or salts, preferably at least one selected from the group consisting of HCl, NaCl, KCl and NH ₄ Cl, and more preferably low-contamination due to cations. HCl.

The sulfuric acid solution used as the electrolyte may have a sulfuric acid concentration of 0.5 to 2 mol/liter. If the sulfuric acid concentration is outside the above range, the ionic conductivity of the electrolyte may be lowered, and the excess voltage may be increased. In addition, the excess voltage may be more pronounced due to the formation of anode fines. Therefore, the process maintenance time (i.e., hold time) may be greatly reduced, and therefore, the object of the present invention will not be attained.

According to the present invention, the temperature of the electrolyte can be from 25 to 60 ° C, which is strictly defined by the economic loss caused by maintaining a constant temperature to consume too much energy.

According to the invention, the electrolysis can be carried out at a current density of 5 to 25 mA/cm 2 .

After the operation (3), an anode fine mud layer containing Ag is formed on the surface of the anode, and the voltage is thereby increased. When the voltage increase is higher, the hold time is shorter. In addition, the rate of increase of the voltage rapidly increases as the concentration of sulfuric acid increases and the current density increases. Therefore, in order to appropriately increase the voltage and the electrolysis rate, the current density is preferably 10 mA/cm 2 (see Fig. 3).

According to the present invention, the operations (4) and (5) performed after the operation (3) are for obtaining a high-purity Ag powder from the anode fine mud in which Ag is concentrated. More specifically, the anode fine mud concentrated with Ag is successively chemically dissolved and solidified and separated. Then, the Ag as a residue is retained after the preliminary recovery of the solid-liquid separation, and the Ag portion partially dissolved and contained in the filtrate is recovered as Ag powder by precipitation and reduction. The above two Ag moieties may be melted, cast or cast, and sintered to form a coarse Ag anode. Finally, electrolytic refining of Ag is performed in a silver nitrate electrolyte to complete the above operation.

According to the present invention, after the operation (3), the crude Ag anode formed using the Ag-concentrated anode fine mud can be substantially produced by using Ag residue and Ag powder, which is chemically dissolved. It is obtained by concentrating an anode fine sludge of Ag and performing solid-liquid separation thereof, which is produced by chemical precipitation and reduction of the filtrate recovered after the solid-liquid separation.

The solid-liquid separation may be substantially included in the solid solution to dissolve the dissolved product after dissolving the anode fine mud concentrated with Ag in hydrochloric acid, nitric acid or aqua regia. Chemical precipitation is carried out by adding ions such as chloride ion (Cl ^- ), sulfate ion (SO4 ^2- ), phosphate ion (PO4 ^3- ) or the like to the filtrate recovered after the solid-liquid separation. An Ag precipitate is produced.

More specifically, the recovery of Sn or Ag from the electrodeposited Sn or the Ag-concentrated anode slime can be carried out in the following manner: in the case of electrodeposited Sn on the cathode, in ultrapure water (usually called The cathode is washed with ionized water (DI water) and dried to obtain a product in the form of a needle-like powder; and, in the case of Ag concentrated in the anode fine mud, the anode fine mud is recovered by post-treatment and recovered from the anode slime Separation of Ag.

Thereafter, the treating may include: taking out an anode fine mud layer formed on the surface of the anode at a predetermined interval; washing the collected anode fine mud layer with 5% hydrochloric acid solution and DI water and filtering the washed product to recover The anode fine mud; chemically dissolves the anode fine mud in concentrated hydrochloric acid or nitric acid and/or aqua regia to obtain an Ag residue, and further performs solid-liquid separation to obtain an Ag precipitate from the recovered filtrate. The Ag residue obtained above and the Ag precipitate were filtered and washed with DI water to give a final product.

According to the present invention, ions which react with Ag and form a precipitate, such as chloride ion (Cl ^- ), sulfate ion (SO ₄ ^2- ), phosphate ion (PO ₄ ^3- ) or the like, can be added thereto. This filtrate is used to form an Ag precipitate, whereby an Ag precipitate such as AgCl, AgSO ₄ , Ag ₃ PO ₄ or the like is produced. In this aspect, the Ag powder can be recovered from the Ag precipitate by chemical reduction, as shown in the following reaction scheme:

[Reaction process]

2AgCl+Na ₂ CO ₃ → 2NaCl+CO ₂ +1/2O ₂ +Ag

2AgCl+2NaOH → Ag ₂ O+2NaCl+H ₂ O

Ag ₂ O+HCOOH → +2Ag+CO ₂ +H ₂ O

The Ag powder and the Ag residue recovered according to the above chemical reaction may be combined and melted (or sintered), followed by casting to form a coarse Ag anode for Ag electrolytic refining. The melting can be performed at a temperature of 970 ° C or higher, and the sintering can be performed at a temperature of 700 ° C or higher. Further, Ag can be obtained with a purity of 99.99% or more by secondary electrolytic refining in a silver nitrate solution (AgNO ₃ ) containing nitric acid using the formed crude Ag anode.

Other features and aspects will be apparent from the detailed description, drawings and claims below.

The present invention will be described in detail with reference to the accompanying Examples, however, these examples are only intended to illustrate the invention, and the scope of the invention is not particularly limited.

Unless otherwise defined herein, the meaning of technical and/or scientific terms used in the specification is to be understood by those of ordinary skill in the art. Detailed descriptions of the technical configurations and/or functions that are known in the art are omitted for the sake of brevity in the following description and drawings, without obscuring the invention.

[Comparative example]

A Pb-free waste solder sample contained the following major precious metal components, namely 93% Sn, 4% Ag, and 0.9% Cu, which were melted at 400 ° C and cast to form an anode. The formed anode was processed to have an exposed area of 4 square centimeters. 260 ml of a sulfuric acid electrolyte having a concentration of 1 mol/liter was placed in an electrolytic bath equipped with a water jacket connected to a water bath to control the temperature, and a platinum plate having an exposed area of 25 cm 2 was used as a cathode. . Electrolytic refining was carried out for 25 hours under the conditions of a current density of 10 mA/cm 2 and a temperature of 40 °C.

[Example 1]

A Pb-free waste solder sample contained the following major precious metal components, namely 93% Sn, 4% Ag, and 0.9% Cu, which were melted at 400 ° C and cast to form an anode. The formed anode was processed to have an exposed area of 4 square centimeters. 260 ml of a sulfuric acid electrolyte having a concentration of 1 mol/liter and containing hydrochloric acid at a concentration of 0.1 mol/liter is placed in an electrolytic bath equipped with a water jacket connected to the water bath to control the temperature, and A platinum plate having an exposed area of 25 square centimeters was used as a cathode. Electrolytic refining was carried out for 25 hours under the conditions of a current density of 10 mA/cm 2 and a temperature of 40 °C.

[Embodiment 2]

A Pb-free waste solder sample contained the following major precious metal components, namely 93% Sn, 4% Ag, and 0.9% Cu, which were melted at 400 ° C and cast to form an anode. The formed anode was processed to have an exposed area of 4 square centimeters. 260 ml of a sulfuric acid electrolyte having a concentration of 1 mol/liter and containing hydrochloric acid at a concentration of 0.2 mol/liter is placed in an electrolytic bath equipped with a water jacket connected to the water bath to control the temperature, and A platinum plate having an exposed area of 25 square centimeters was used as a cathode. Electrolytic refining was carried out for 25 hours under the conditions of a current density of 10 mA/cm 2 and a temperature of 40 °C.

[Example 3]

A Pb-free waste solder sample contained the following major precious metal components, namely 93% Sn, 4% Ag, and 0.9% Cu, which were melted at 400 ° C and cast to form an anode. The formed anode was processed to have an exposed area of 4 square centimeters. 260 ml of a sulfuric acid electrolyte having a concentration of 1 mol/liter and containing a concentration of 0.3 mol/liter of hydrochloric acid is placed in an electrolytic bath equipped with a water jacket connected to the water bath to control the temperature, and A platinum plate having an exposed area of 25 square centimeters was used as a cathode. Electrolytic refining was carried out for 25 hours under the conditions of a current density of 10 mA/cm 2 and a temperature of 40 °C.

[Example 4]

A Pb-free waste solder sample contained the following major precious metal components, namely 93% Sn, 4% Ag, and 0.9% Cu, which were melted at 400 ° C and cast to form an anode. The resulting anode was processed to have an exposed area of 56 square centimeters. 4000 ml of a sulfuric acid electrolyte having a concentration of 1 mol/liter and containing hydrochloric acid at a concentration of 0.2 mol/liter is placed in an electrolytic bath equipped with a water jacket connected to the water bath to control the temperature, and A Sn-coated titanium plate having an exposed area of 255 square centimeters was used as a cathode. Electrolytic refining was carried out for 25 hours under the conditions of a current density of 10 mA/cm 2 and a temperature of 40 °C.

[Example 5]

0.2033 g of the anode slime concentrated in Ag prepared in the above Example 4 was placed in 40 ml of 35% hydrochloric acid, dissolved at the boiling temperature for 30 minutes, filtered and washed to obtain 0.0428 g of dissolved Ag residue.

[Embodiment 6]

The Ag dissolved residue powder (Ag 99.5%) obtained in Example 5 and the AgCl precipitate and the Ag powder additionally formed by reacting sodium hydroxide (NaOH) with formic acid (HCOOH) were mixed together to form a diameter of 50 mm. The disk shape was then sintered at a temperature of 750 ° C for 1 hour to form a coarse Ag anode for Ag electrolytic refining.

The exposed area of the coarse Ag anode made of Ag was adjusted to be 9 square centimeters. Electrolytic refining was carried out using a 500 ml polytetrafluoroethylene (PTFE) electrolytic bath. The crude Ag anode made of Ag was placed in a filter cloth made of polypropylene (PP) and having a mesh of 500 to prevent electrolyte contamination.

The cathode for electrolytic refining of a coarse Ag anode made of Ag is made of a titanium material having a purity of 99.9% or higher and has an exposed area of 9 square centimeters. The electrolyte used herein was a HNO ₃ solution containing 0.5 mol/liter of AgNO ₃ and having a concentration of 0.5 mol/liter.

Electrolytic refining was carried out for 2 hours under the conditions of a current density of 30 mA/cm 2 . As a result, it was confirmed that the amount of Ag electrodeposition was 2.172 gram and the current efficiency was 99.9% or more. Analysis of the Ag component under electrolytic refining revealed that the purity of Ag was 99.99% or more.

[Experimental Example 1]

Regarding the electrolytic refining in the comparative example and the examples 1 to 3, the relationship between the electrochemical dissolution/purification procedure and the Cl ⁻ concentration in the sulfuric acid electrolyte was confirmed.

As shown in Table 1, in the case of the comparative example, the amount of anodic dissolution was 2.3987 g, the amount of Sn electrodeposited on the platinum cathode plate was 1.8772 g, and the amount of anode fine mud formed was 0.1818 g. The electrodeposited Sn exhibits a needle shape and has a purity of 99.9% or higher. The Ag content in the anode slime was 43.1% and the Ag enrichment ratio in the anode slime was 81.7%. Here, in the case of Sn ²⁺ electrodeposition, the current efficiency was calculated to be 84.7%, and after standing for a while, a white Sn oxide precipitate was formed from the electrolyte.

In the case of Example 1, the amount of anodic dissolution was 2.4048 g, the amount of Sn electrodeposited on the platinum cathode plate was 1.8733 g, and the amount of anode fine mud formed was 0.1872 g. The electrodeposited Sn exhibits a needle shape and has a purity of 99.9% or higher. The Ag content in the anode fine mud was 46.23% and the Ag enrichment ratio in the anode fine mud was 89.9%. This, in terms of Sn ²⁺ to electrodeposition, the calculated current efficiency was 84.5%, and even upon standing for some time, the electrolytic solution does not occur Sn oxide precipitate.

In the case of Example 2, the amount of anodic dissolution was 2.4022 g, the amount of Sn electrodeposited on the platinum cathode plate was 1.9630 g, and the amount of anode fine mud formed was 0.1837 g. The electrodeposited Sn exhibits a needle shape and has a purity of 99.9% or higher. The Ag content in the anode slime was 48.6% and the Ag enrichment rate in the anode slime was 92.9%. Here, in the case of Sn ²⁺ electrodeposition, the current efficiency was calculated to be 88.6%, and precipitation of the Sn oxide did not occur in the electrolyte even after being left for a while.

In the case of Example 3, the amount of anodic dissolution was 2.4017 g, the amount of Sn electrodeposited on the platinum cathode plate was 2.0449 g, and the amount of anode fine mud formed was 0.1792 g. The electrodeposited Sn exhibits a needle shape and has a purity of 99.9% or higher. The Ag content in the anode fine mud was 50.5% and the Ag enrichment ratio in the anode fine mud was 94.2%. Here, in the case of Sn ²⁺ electrodeposition, the current efficiency was calculated to be 92.2%, and precipitation of the Sn oxide did not occur in the electrolyte even after being left for a while.

As can be seen from the results of the above Examples 1 to 3 and 2, by adding Cl ^- to the electrolytic solution, precipitation of Sn oxide in the electrolytic solution can be prevented, and Ag enrichment rate in the anode fine mud is improved.

The results of Examples 1 to 3 confirmed that the difficulty and/or repeatability of the existing method for extracting, separating, and purifying precious metals from waste solder can be overcome, and Sn or Ag can be efficiently and economically recovered.

[Experimental Example 2]

The relationship between the results of the electrochemical dissolution/purification procedure and the current density was analyzed under the conditions of temperature and electrolyte composition in Example 3.

The results of Tables 2 and 3 above confirm the relationship between the composition change of the anode fine mud and the increase in current density. In addition, it can be seen that the voltage increases as the fine mud layer formed on the anode increases. Furthermore, it has been found that a preferred current density of 10 mA/cm 2 can be extended by prolonging the process holding time (i.e., holding time) by maximizing the voltage increase.

[Experimental Example 3]

The composition of the anode fines obtained in Examples 4 and 5 and the composition of the Ag residue obtained from the anode fines after solid-liquid separation were analyzed.

As shown in Fig. 4, the electrodeposited Sn exhibits a needle shape and has a purity of 99.9% or higher. The Ag content in the anode fine mud was 47.4% and the Ag enrichment rate was 95%.

Further, as shown in Fig. 5, the purity of the dissolved Ag residue of the anode fine mud was 99.5%. The filtrate was reacted with a 0.5 molar/liter NaCl solution to give 0.0702 grams of AgCl precipitate with an Ag content of 75.3%. Since the total amount of Ag is 0.0957 g, 99.5% of Ag can be recovered.

As described above, the method for recovering Sn or Ag from the Pb-free waste solder of the present invention overcomes the difficulty and reproducibility of the method of extracting, separating and purifying precious metals from the existing waste solder, and can efficiently and economically recover high purity. Sn or Ag.

A method for recovering Sn or Ag from a non-Pb waste solder according to the present invention, which uses an electrolyte containing chloride ions (Cl ^- ) in electrolytic refining to prevent precipitation of Sn oxide, which is an increase in Sn in the related art. A problem with Ag's recovery rate. In addition, since the Ag enrichment rate and the Sn electrodeposition rate in the anode fine mud are improved, the recovery ratio of Sn or Ag is improved.

1 is a schematic view showing a method of recovering Sn or Ag from a non-Pb waste solder according to an embodiment of the present invention;

Figure 2 illustrates the results of the relationship between the electrochemical dissolution/purification procedure and the Cl ^- concentration in the sulfuric acid electrolyte according to one embodiment of the present invention;

Figure 3 is a graph showing the results of the relationship between the electrochemical dissolution/purification procedure and the current density under the conditions of the predetermined temperature and composition of the electrolytic solution of Example 3 of the present invention;

Figure 4 is a view showing an analysis result of the composition of the anode slime obtained in Example 4 of the present invention;

Fig. 5 is a view showing the analysis results of the composition of the Ag residue obtained in Example 5 of the present invention.

Claims (7)

A method for recovering tin (Sn) or silver (Ag) from lead-free (Pb) waste solder, comprising: (1) preparing an anode from Pb-free waste solder, wherein the Pb-free waste solder comprises Sn, Ag or a mixture thereof (2) an anode and a cathode prepared by the operation (1) of applying an electric current to an electrolyte containing chloride ions (Cl ^- ); (3) a reaction initiated by applying an electric current to form a solution containing Ag concentrated therein Anodal fines on the surface of the anode while simultaneously depositing Sn on the cathode; and (4) recovering the electrodeposited Sn or recovering the concentrated Ag from the anode fines, wherein the method further comprises, after the operation (3) , chemically dissolving the anode slime concentrated with Ag and performing solid-liquid separation thereof; and using Ag residue after solid-liquid separation and Ag powder extracted from the filtrate to form a crude Ag anode, and then performing Ag in a silver nitrate electrolyte Electrolytic refining. The method of claim 1, wherein the Ag residue is used to form a coarse Ag anode with Ag powder by chemically dissolving an anode fine sludge concentrated with Ag in hydrochloric acid, nitric acid or aqua regia and performing the same Obtained by solid-liquid separation, which is produced by chemical precipitation and reduction of the filtrate recovered after the solid-liquid separation. The method of claim 2, wherein the chemical precipitation is obtained by adding chloride ion (Cl ^- ), sulfate ion (SO ₄ ^2- ), and phosphate ion (PO ₄ ^3- ) to the solid-liquid separation. In the filtrate, an Ag precipitate is produced. The method of claim 1, wherein the electrolyte of the operation (2) contains Cl ^{- at} a concentration of 0.05 to 0.5 mol/liter. The method of claim 1, wherein the electrolyte of the operation (2) is a sulfuric acid solution. The method of claim 5, wherein the sulfuric acid solution contains sulfuric acid at a concentration of 0.5 to 2 mol/liter. The method of any one of claims 1 to 6, wherein the temperature of the electrolyte is 20 to 60 ° C, and the electrolysis is carried out at a current density of 5 to 25 mA / cm 2 .