Gold and / or silver recovery method

Abstract

To provide a method for recovering gold and / or silver at a high recovery rate from a material containing gold and / or silver.SOLUTION: A gold and / or silver recovery method includes bringing a material containing gold and / or silver into contact with a leaching agent A containing thiosulfate and amino acid. A gold and / or silver recovery method includes bringing a material containing gold and / silver into contact with a leaching agent B containing thiosulfate; and then adding amino acid to the leaching agent B in a state where the material containing metal and / or silver is contacted.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for recovering gold and / or silver from a material containing gold and / or silver. [Background technology]

[0002] Due to the depletion of reserves and the increase in the demand for precious metals, the extraction of precious metals from secondary resources is a timely solution. One of the promising secondary resources is printed circuit boards (PCBs). PCBs contain significant amounts of precious metals such as gold (Au), silver (Ag), palladium (Pd), platinum (Pt), copper (Cu), nickel (Ni), cobalt (Co), zinc (Zn), and aluminum (Al), but the amounts of these metals present in nature are limited. Therefore, recycling of e-waste generated from used electronic devices is of great importance to maximize the sustainable use of natural resources and minimize the impact on the environment.

[0003] The conventional gold leaching method, the cyanidation process, causes air and water pollution due to acid mine drainage and hazardous chemicals. Therefore, alternative liquefaction agents for gold leaching have attracted attention in recent years, among which chlorides, thioureas, thiocyanates, and thiosulfates have attracted the most attention. The development of chloride leaching methods has been hindered mainly by their dangerous working environment, poor reaction selectivity, high requirements for corrosion protection of equipment, thiourea is reported to be carcinogenic, and thiocyanate is very sensitive to temperature and pH.

[0004] Thiosulfate leaching, on the other hand, has been widely explored as the most promising alternative method due to its reduced environmental risk, high reaction selectivity, low corrosivity, relatively low cost, fast leaching rate, etc. However, the method using thiosulfate alone has several limitations, such as the need for high reaction conditions.

[0005] To overcome the limitations of the above techniques, various leaching methods using thiosulfate have been developed, among which copper-ammonia-thiosulfate has been widely studied.2+ The development of ammonia-thiosulfate salts containing Cu made it possible to leach gold at room temperature (see Non-Patent Document 1). 2+ The presence of Cu can accelerate the leaching of gold in ammonia-thiosulfate by the formation of gold(I) thiosulfate complexes. 2+ As a substitute for Ni 2+ , Co 3+ , Fe 3+ It has also been reported that it can be used.

[0006] However, at higher thiosulfate concentrations, thiosulfate forms a stable complex with Au, Au(S2O3)2. 3- However, a low thiosulfate concentration makes the leaching of Au very slow and reduces the stability of the Au-thiosulfate complex.

[0007] Amino acids such as glycine, alanine, and histidine have also been investigated as alternative gold leaching agents to cyanide, as they are non-toxic, non-volatile, and recyclable. In particular, glycine has attracted widespread attention because it is low cost, has the simplest structure, and has a high affinity for metals (see Non-Patent Documents 2 and 3). Glycine can be easily produced on an industrial scale and is obtained as a by-product from various microorganisms. It can form stable complexes with Au and Ag over a wide range of pH, Eh, and temperature. [Prior art documents] [Non-patent literature]

[0008] [Non-Patent Document 1] Xu, B.; Kong, W.; Li, Q.; Yang, Y.; Jiang, T.; Liu, X. A Review of Thiosulfate Leaching of Gold: Focus on Thiosulfate Consumption and Gold Recovery from Pregnant Solution. Metals (Basel) 2017, 7, 222, doi:10.3390 / met7060222. [Non-Patent Document 2] Oraby,EA; Eksteen,JJThe Selective Leaching of Copper from a Gold-Copper Concentrate in Glycine Solutions. Hydrometallurgy 2014, 150,14-19, doi:10.1016 / j.hydromet.2014.09.005. [Non-Patent Document 3] Oraby,EA; Eksteen,JJThe Leaching of Gold, Silver and Their Alloys in Alkaline Glycine-Peroxide Solutions and Their Adsorption on Carbon. Hydrometallurgy 2015,152,199-203, doi:10.1016 / j.hydromet.2014.12.015. Summary of the Invention [Problem to be solved by the invention]

[0009] As described above, the recovery of gold and / or silver from materials containing gold and / or silver has been studied, but the recovery rate has not always been sufficient. An object of the present invention is to provide a method for recovering gold and / or silver from a material containing gold and / or silver at a high recovery rate. [Means for solving the problem]

[0010] In order to achieve the above object, the present inventors have conceived of the present invention, which combines a thiosulfate with an amino acid.

[0011] That is, the present invention is as follows. [1] A method for recovering gold and / or silver, comprising contacting a material containing gold and / or silver with a leaching agent A containing thiosulfate and an amino acid.

[0012] [2] A method for recovering gold and / or silver, comprising contacting a material containing gold and / or silver with leaching agent B containing thiosulfate, and then adding an amino acid to leaching agent B in the state in which the material containing gold and / or silver has been in contact.

[0013] [3] The method for recovering gold and / or silver according to [1] or [2] above, wherein the amino acid is at least one selected from the group consisting of glycine, histidine, asparagine, alanine, cysteine, valine, and aspartic acid. [4] The method for recovering gold and / or silver according to any one of the above [1] to [3], characterized in that the pH of the leaching solution A containing thiosulfate and an amino acid or the leaching solution B containing thiosulfate is 8.0 or higher. [5] The method for recovering gold and / or silver according to any one of the above [1] to [4], characterized in that the leaching solution A containing thiosulfate and an amino acid or the leaching solution B containing thiosulfate contains at least one metal compound selected from a copper compound, a nickel compound, and a cobalt compound, and ammonia.

[0014] [6] The method for recovering gold and / or silver according to any one of the above [1] to [5], wherein the material containing gold and / or silver is a printed circuit board. [7] The method for recovering gold and / or silver according to any one of the above [1] to [6], wherein the material containing gold and / or silver is a crushed or pulverized material. Effect of the Invention

[0015] According to the method for recovering gold and / or silver of the present invention, gold and / or silver can be recovered at a high recovery rate from a material containing gold and / or silver. [Brief description of the drawings]

[0016] [Figure 1] FIG. 1 is a diagram showing a presumed mechanism for gold recovery according to the present invention. [Diagram 2] (a) and (b) show the gold leaching rate and pH change in the thiosulfate-glycine system (pH 9), respectively. [Diagram 3] FIG. 1 shows the gold leaching rate in a thiosulfate-glycine system (pH 11). [Figure 4] (a) and (b) show the gold leaching rate and pH change in the thiosulfate-histidine system (pH 9), respectively. [Diagram 5] (a) and (b) show the change in the leaching rate of gold and silver when the thiosulfate concentration in the thiosulfate-glycine system is changed. [Figure 6] (a) and (b) show the change in the leaching rate of gold and silver, respectively, when the glycine concentration in the thiosulfate-glycine system is changed. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] The first invention relates to a method for recovering gold and / or silver, which is characterized by contacting a material (hereinafter also referred to as material) containing gold and / or silver (hereinafter also referred to as gold, etc.) with a leaching agent A containing thiosulfate and an amino acid.

[0018] The second invention relates to a method for recovering gold and / or silver, which comprises contacting a material containing gold and / or silver with leaching agent B containing thiosulfate, and then adding an amino acid to leaching agent B in the state in which the material containing gold and / or silver has been in contact.

[0019] In the present invention, a liquid containing a reagent necessary for leaching gold or the like is referred to as a leaching agent (or extractant), and the liquid after the gold or the like has been leached is referred to as a leaching solution (or extract).

[0020] Furthermore, if the material contains a significant amount of metal (e.g., copper) that may adversely affect the recovery of gold and other metals in the present invention, it is preferable to remove this before contacting the material with leaching agent A or leaching agent B (hereinafter, collectively referred to as leaching agents).

[0021] The recovery method of the present invention is a novel technique that combines thiosulfate and amino acids, and the combination of thiosulfate and amino acids can synergistically improve the recovery rate of gold and / or silver. Furthermore, without using cyanide, the recovery rate of gold and / or silver can be stably increased compared to conventional techniques while taking environmental issues into consideration.

[0022] In the recovery method of the present invention, the stability of the complex between the gold and / or silver in the solution and the leaching agent is considered to be an important factor, and the present inventors believe that the complex is stabilized by the following mechanism: Since thiosulfate has a higher leaching kinetics than cyanide or glycine, in the first step, thiosulfate forms an Au(Ag)-thiosulfate complex, which triggers the leaching of Au(Ag), and in the second step, amino acids form Au(Ag)-amino acid complexes, stabilizing the leached Au(Ag).

[0023] The present invention will be described in detail below. Materials that can be treated in the recovery method of the present invention are not particularly limited as long as they contain gold and / or silver (gold only, silver only, or both gold and silver), and examples thereof include printed circuit boards, accessories, medical materials, ores, catalysts, etc., but printed circuit boards (waste printed circuit boards) that contain a significant amount of gold and / or silver (high gold and silver content) and are discarded in large quantities are preferred.

[0024] A printed board is a type of substrate and may be either a printed wiring board (PWB) or a printed circuit board (PCB), or a mixture of both. In the present invention, however, a printed circuit board is preferred because it contains more gold and / or silver.

[0025] A printed wiring board is a board made of an insulator with conductor wiring (simply wired) on or inside it, and is in a state before electronic components are attached. This type of printed wiring board includes, for example, defective products that occurred during the manufacturing process. A printed circuit board is a printed wiring board to which electronic components are soldered so that it functions as an electronic circuit. Examples of such printed circuit boards include those separated and recovered from discarded electrical products and electronic devices.

[0026] These printed circuit boards may be treated as is, but it is preferable to treat them in a crushed form (e.g., in pieces on the order of centimeters or even in granules on the order of millimeters) or even in a pulverized form (e.g., in a powder form on the order of μm), as this produces a homogeneous treated material, increases the surface area, and improves the leaching efficiency.

[0027] The average particle size of the pulverized material is, for example, preferably 20 μm to 300 μm, more preferably 20 μm to 250 μm, and further preferably 30 μm to 200 μm. The average particle size can be measured, for example, using a laser scattering particle size distribution measuring device (LA-950, Horiba, Ltd.).

[0028] Leaching agent A of the present invention contains thiosulfate and an amino acid. Leaching agent B contains thiosulfate, but an amino acid is added later, resulting in a composition similar to that of leaching agent A. The amino acid can be added to leaching agent B taking into consideration the reaction time of thiosulfate, but it is preferable to add it within 10 hours, more preferably within 6 hours, and even more preferably within 3 hours after contacting leaching agent B with the material containing gold or the like.

[0029] As described above, the thiosulfate contained in the leaching agent forms a complex with gold or the like (gold-thiosulfate complex, silver-thiosulfate complex), thereby triggering the leaching (elution) of gold or the like from the material. Examples of thiosulfate include sodium thiosulfate (Na2S2O3), potassium thiosulfate (K2S2O3), calcium thiosulfate (CaS2O3), etc., with sodium thiosulfate and potassium thiosulfate being preferred, and sodium thiosulfate being particularly preferred.

[0030] The amount of thiosulfate added (amount of thiosulfate required for leaching per mg of metal) is preferably about 350 to 1000 mg, more preferably about 380 to 800 mg, in the case of leaching gold. In addition, in the case of leaching silver or leaching gold and silver, it is preferably 30 to 100 mg, more preferably 40 to 80 mg. Considering that the amounts of thiosulfate used in the past were 2288 mg for gold, 692 mg for silver, and 531 mg for gold and silver, in the present invention in which thiosulfate is combined with an amino acid, the amount of thiosulfate used can be significantly reduced compared to the past.

[0031] Here, in the leaching of gold and / or silver with thiosulfate, the leaching process is very complicated, and it is difficult to maintain reaction stability. Therefore, in the present invention, in order to maintain the reaction stability, the leaching agents (leaching agent A and leaching agent B before the addition of amino acid) are used to ionize copper (Cu 2+ ), which produces nickel ions (Ni 2+ ) or cobalt ions (Co 3+It is preferable to contain a cobalt compound which generates a copper compound, and a copper compound is particularly preferable. Specific examples include sulfates, nitrates, carbonates, chlorides, etc. of these metals, and copper sulfate is particularly preferable. It is also preferable to contain ammonia together with these metal compounds. This allows the formation of a copper-ammonia complex, etc., which causes the Cu in the solution to be dissolved. 2+ It is possible to stabilize the ammonia, promote the oxidation of gold, etc., and promote the leaching of gold, etc. by thiosulfate. In order to impart a buffering effect, it is preferable to add an ammonium salt such as ammonium chloride together with ammonia.

[0032] As mentioned above, iron ions (Fe 3+ A combination of an iron compound and oxalic acid which gives rise to

[0033] The amino acids contained in leaching agent A or added to leaching solution B are intended to stabilize the leached gold and other metals by forming complexes with the gold and other metals (gold-amino acid complexes, silver-amino acid complexes), as described above.

[0034] Examples of such amino acids include glycine (Gly), histidine (His), asparagine (Asn), alanine (Ala), cysteine ​​(Cys), valine (Val), aspartic acid (Asp), etc. Glycine is preferred because it can be easily produced on an industrial scale, it can be obtained as a by-product from various microorganisms, and it can form stable complexes with Au and Ag over a wide range of pH and temperature.

[0035] Although the results for elements other than glycine and histidine are not shown in the examples, asparagine (Asn), alanine (Ala), cysteine ​​(Cys), valine (Val), and aspartic acid (Asp) have been shown to be effective in leaching gold and other metals on their own in the past, they are also considered to be effective in the method of the present invention.

[0036] The pH of the leaching agent A is preferably 8.0 or more, and in order to increase the leaching rate, it is more preferably 8.0 to 11.0, even more preferably 8.0 to 10.5, and particularly preferably 8.0 to 10.0. The pH can be adjusted by adding an alkaline agent such as ammonia or NaOH.

[0037] The pH of the leaching agent B containing thiosulfate is preferably 8.0 or more, more preferably 8.0 to 10.0 at which the thiosulfate is easily functional, and even more preferably 8.0 to 9.5. The pH of the leaching agent B to which an amino acid is added is preferably 8.0 or more, more preferably 8.0 to 11.5 at which the amino acid is easily functional, and even more preferably 8.5 to 11.0. The pH can be adjusted by adding an alkaline agent such as ammonia or NaOH.

[0038] The amount of amino acid added (amount of amino acid required for leaching per mg of metal) is preferably about 2000 to 5000 mg, more preferably about 3000 to 4000 mg, in the case of gold leaching, and is preferably 200 to 500 mg, more preferably 250 to 400 mg, in the case of silver leaching or gold and silver leaching.

[0039] The putative leaching mechanism associated with gold leaching at the gold surface in the material in a thiosulfate-glycine system using the leaching agents described above is shown in Figure 1.

[0040] The treatment temperature for the leaching treatment of the present invention is not particularly limited as long as it is a temperature at which gold and the like can be leached, and is, for example, 10 to 100°C, preferably 15 to 80°C, and more preferably 20 to 50°C.

[0041] The leaching process of the present invention can be carried out in a batch or continuous mode. Examples of batch-type leaching processes include a process in which a material is supplied to leaching agent A stored in a container, or a process in which leaching agent A is supplied to a material stored in a container and the material is immersed (contacted) in leaching agent A, thereby leaching gold and / or silver into leaching agent A. Another embodiment is to supply a material to leaching agent B stored in a container, or to supply leaching agent B to a material stored in a container, and immerse (contact) the material in leaching agent B to form a gold and / or silver-thiosulfate complex, and then supply an amino acid to leaching agent B in which the material is immersed, to form a gold and / or silver-amino acid complex.

[0042] An example of a continuous leaching treatment is one in which leaching agent A is continuously supplied to and discharged from a container filled with the material. Another embodiment is one in which leaching agent B is continuously supplied and discharged from a container filled with a material, and an amino acid is supplied to the discharged leaching agent B to form a gold and / or silver-amino acid complex. In addition, multiple containers filled with materials can be prepared, and the supply path of leaching agent A or leaching agent B can be switched for each container.At this time, for the container containing material for which leaching processing has been completed, the processed material can be replaced with unprocessed material.

[0043] Another continuous method is to continuously supply the material and leaching agent in predetermined amounts, and allow the leaching solution containing leached gold and other metals to overflow from the container. The amounts of material and leaching agent supplied to the container are set taking into consideration the processing time (residence time) of the material in the container. The leaching agent in the container is adjusted, for example, by measuring the concentrations of thiosulfate and amino acids, pH, etc., periodically (intermittently) or continuously. The material that has completed the leaching process is, for example, allowed to settle at the bottom of the container and removed periodically or continuously.

[0044] The leachate from which the gold and other metals have been leached can be further treated in a predetermined manner to separate and recover the gold and other metals. The solution from which the gold and other metals have been separated can also be reused as an immersion agent by adjusting, for example, the concentration of thiosulfate. EXAMPLES

[0045] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples. <Characteristic Evaluation of Printed Circuit Boards (PCBs)> PCBs contain various metals such as Cu, Fe, Al, etc., which are attached to, coated on, and mixed with various types of plastics and ceramics. Therefore, it is difficult to make smaller particles with conventional grinding methods such as the planetary ball milling method. Thus, grinding was performed by the following operations.

[0046] Liquid nitrogen was added to the crushed PCBs of mm size, which are the objects of magnetic separation, obtained from Japanese recycling operators, and they were ground using a cryogenic grinder (JFC - 2000; Japan Analytical Industry Co., Ltd.) to increase the surface area for the Au / Ag leaching test. The ground samples were sieved to have a particle size of 75 μm to 500 μm.

[0047] The chemical composition of the ground PCBs was measured by microwave - assisted acid digestion (Ethos Plus, Milestone), ICP - OES analysis (ICP - OES; PerkinElmer, Optima 8300), and X - ray fluorescence analysis (XRF, ZSX Primus II, Rigaku, Japan). The elemental compositions obtained by both analyses are shown in Table 1.

[0048]

Table 1

[0049] As shown in Table 1, it was revealed that Cu (8.10%) is the most abundant, and Al (3.69%) and Fe (1.49%) are contained in large amounts. Au and Ag were detected at 0.026% and 0.28% respectively.

[0050] <Pretreatment of PCBs for Cu Removal> Since a large amount of thiosulfate is consumed when PCBs contain a large amount of Cu, it is beneficial to remove Cu from PCBs. Furthermore, it is also considered that a high Cu content in PCBs may have an adverse effect on the leaching of Au. Therefore, as a pretreatment, acid leaching was performed before leaching Au and Ag.

[0051] Specifically, prior to the leaching test, the PCBs were treated with 1M H2SO4 at 70°C for 7 days to leach out the Cu. The solid residue was then collected by filtration (0.45 μm), washed thoroughly with distilled water, and dried overnight in an oven at 55°C. This treatment removed 99.3% of the Cu from the samples.

[0052] <Au leaching using thiosulfate-amino acid leaching> The present embodiment combines thiosulfate-based (copper-ammonia-thiosulfate) Au leaching with amino acid Au leaching. As a comparison, thiosulfate-based and glycine-based Au leaching alone were examined. The details are as follows.

[0053] Gold leaching from PCBs using copper-ammonia-thiosulfate (60 mM sodium thiosulfate, 0.2 M NH4Cl, 0.26 M NH3, 10 mM CuSO4) in combination with amino acids (0.5 M) was performed at 2% material concentration, pH 9, 40°C, 160 rpm, and glycine and histidine were used as amino acids.

[0054] As controls, leaching tests were conducted with thiosulfate alone and amino acids alone. With thiosulfate alone (60 mM thiosulfate, 0.2 M NH4Cl, 0.26 M NH3, 10 mM CuSO4), leaching was performed at 2% material concentration, pH 9, 40°C, and 160 rpm. Similarly, with amino acids alone, leaching was performed with 0.5 M glycine or 0.5 M histidine, at 2% material concentration, pH 9, 40°C, and 160 rpm. During leaching, the pH was monitored and samples were taken for ICP-OES analysis. Leaching tests were performed in triplicate and the average metal contents were calculated.

[0055] The gold leaching rate when glycine was used as the amino acid is shown in Figure 2(a), and the change in pH is shown in Figure 2(b).

[0056] As shown in Figure 2(a), in the thiosulfate system alone, a significant amount of leaching (46.0%) was observed in the early stages, but the leaching rate decreased significantly over time. This is thought to be due to the instability of the Au-thiosulfate complex, which led to the formation of a precipitate. On the other hand, in the case of glycine alone, no leaching of Au was observed.

[0057] In contrast, in the thiosulfate-glycine system of the present invention, a synergistic effect of Au leaching by thiosulfate and glycine was observed. The Au leaching rate was very high, reaching 93.6% within 48 hours. The combination of the thiosulfate system with glycine increased the stability of Au in the leaching solution because Au first formed an Au-thiosulfate complex and then dissociated into an Au-glycine complex ([Au(Gly)2]). - ) was formed. - The stability constant of Au(SCN)2 is - , AuBr2 - , AuCl2 - This is much higher than other gold complexes such as

[0058] According to the results in Figure 2(b), no significant change in pH was observed in any of the test subjects. In the thiosulfate system, NH3 and NH4 + Similarly, in the case of glycine, a buffering effect was observed due to the zwitterion of glycine itself. Therefore, by combining thiosulfate with glycine, NH3 / NH4 + and a buffering effect is provided by glycine.

[0059] In addition, the results when the pH was changed from 9 to 11 in the thiosulfate-glycine system, which is an example of the present invention, are shown in Figure 3. As shown in Figure 3, Au leaching was achieved even at pH 11, although it was inferior to the pH of 9.

[0060] Furthermore, the gold leaching rate when histidine was used instead of glycine as the amino acid is shown in Figure 4(a), and the change in pH is shown in Figure 4(b). As shown in Figure 4(a), no Au leaching was observed with histidine alone, whereas in the thiosulfate-histidine system, a synergistic effect of thiosulfate and histidine on Au leaching was observed, although not as strong as with glycine. Moreover, as shown in FIG. 4(b), no significant change in pH was observed in any of the test subjects.

[0061] <Study on thiosulfate concentration in thiosulfate-glycine system> Under the optimized conditions (0.5M glycine, 0.2M NH4Cl, 0.26M NH3, 10mM CuSO4), the leaching of Au and Ag was investigated by changing only the thiosulfate concentration of the thiosulfate-glycine system to 60, 40, 20, and 10mM. The pH was stable under all test conditions.

[0062] Figure 5(a) shows the leaching rate of Au, and Figure 5(b) shows the leaching rate of Ag. As shown in Figure 5(a), it was suggested that the higher the thiosulfate concentration, the greater the amount of Au leached. The higher the thiosulfate concentration, the greater the Au leaching rate in the initial stage. As shown in Figure 5(b), a similar trend was observed for Ag leaching, which was 95.3% in the system with high thiosulfate concentration.

[0063] The amount of thiosulfate required for leaching per 1 mg of metal was 393 mg to 789 mg for gold, 40 mg to 80 mg for silver, and 36 mg to 72 mg for gold and silver.

[0064] <Study on glycine concentration in thiosulfate-glycine system> Under the optimized thiosulfate concentration (60 mM thiosulfate, 0.2 M NH4Cl, 0.26 M NH3, 10 mM CuSO4), the leaching of Au and Ag was investigated by changing only the glycine concentration of the thiosulfate-glycine system to 0.50, 0.40, 0.20, and 0.05 M. The pH was stable under all test conditions.

[0065] Figure 6(a) shows the leaching rate of Au, and Figure 6(b) shows the leaching rate of Ag. As shown in Figure 6, similar to the thiosulfate concentration mentioned above, it was suggested that the higher the glycine concentration, the higher the leaching rate of Au and Ag. [Industrial Applicability]

[0066] INDUSTRIAL APPLICABILITY The present invention is industrially useful since it is possible to leach gold and / or silver from materials containing gold and / or silver while taking environmental issues into consideration.

Claims

1. A method for recovering gold and / or silver, comprising contacting a material containing gold and / or silver with a leaching agent A containing thiosulfate and an amino acid.

2. A method for recovering gold and / or silver, comprising contacting a material containing gold and / or silver with a leaching agent B containing a thiosulfate, and then adding an amino acid to the leaching agent B in the state in which the material containing gold and / or silver has been in contact.

3. 3. The method for recovering gold and / or silver according to claim 1 or 2, characterized in that the amino acid is at least one selected from the group consisting of glycine, histidine, asparagine, alanine, cysteine, valine, and aspartic acid.

4. 3. The method for recovering gold and / or silver according to claim 1 or 2, characterized in that the pH of the leaching solution A containing thiosulfate and an amino acid or the leaching solution B containing thiosulfate is 8.0 or higher.

5. 3. The method for recovering gold and / or silver according to claim 1 or 2, characterized in that the leaching solution A containing thiosulfate and an amino acid or the leaching solution B containing thiosulfate contains ammonia and at least one metal compound selected from a copper compound, a nickel compound, and a cobalt compound.

6. 3. The method for recovering gold and / or silver according to claim 1 or 2, characterized in that the material containing gold and / or silver is a printed circuit board.

7. 3. The method for recovering gold and / or silver according to claim 1 or 2, wherein the material containing gold and / or silver is a crushed or pulverized material.

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