Method for recovering silver from copper electrolyte

Abstract

To provide a method for efficiently collecting silver which is liquated to a copper electrolyte by copper electrolytic refining.SOLUTION: There is provided a method for collecting silver included in a copper electrolyte, the method comprising: an extraction step for bringing the copper electrolyte into contact with, an ion liquid phase including an ammonium salt type ion liquid, for extracting silver included in the copper electrolyte; and a reverse extraction step for bringing a solution including a reducer into contact with the ion liquid phase, for reducing the silver included in the ion liquid phase for crystallizing metal silver. In the extraction step, the copper electrolyte in which chloride ion density is 0.1 mmol / L or greater and 10 mmol / L or smaller is brought into contact with the ion liquid phase, for extracting chloro complex ion of the silver in the ion liquid phase. In the reverse extraction step, as the reducer, one kind or more kinds selected from a hydrazine monohydrate, sulfite salt, and sodium borohydride, are included in the solution, the same solution is brought into contact with the ion liquid phase.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for recovering silver contained as an impurity in a copper electrolyte by copper electrorefining. [Background technology]

[0002] Copper electrorefining is a process in which a sulfuric acidic copper sulfate solution is used as the electrolyte, and crude copper produced from copper concentrate through a dry processing process is used as the anode to electrolyze the anode, producing highly pure copper (electrolytic copper) on the surface of the cathode.The cathode can be a thin copper plate called a seed plate, which is made separately from electrolytic copper, or a reusable stainless steel plate.

[0003] The main anode reaction in copper electrorefining is the dissolution reaction of copper. The blister copper used as the anode contains impurities such as gold, silver, lead, nickel, iron, antimony, bismuth, selenium, tellurium, and arsenic. Most of the silver accumulates at the bottom of the electrolytic cell as a muddy solid substance called anode slime, but the soluble amount of silver chloride can be dissolved into the electrolyte, and the silver dissolved in the electrolyte will deposit on the cathode together with the copper.

[0004] Silver is a precious metal, and suppressing its deposition on the cathode and increasing its recovery rate can improve the added value of the entire copper electrorefining process. One method for improving the silver recovery rate is to optimize the electrolysis conditions, as disclosed in Patent Document 1, but recovering silver dissolved in the electrolyte has been difficult. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 8-176878 [Patent Document 2] Japanese Patent Application Publication No. 2017-043556 [Patent Document 3] Japanese Patent Publication No. 2021-143386 Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention has been proposed in view of the above circumstances, and has as its object to provide a method for efficiently recovering silver dissolved in a copper electrolyte by electrolytic refining of copper. [Means for solving the problem]

[0007] As a result of extensive research into solving the above-mentioned problems, the present inventors discovered that silver can be efficiently recovered by extracting a copper electrolyte solution containing chloride ions in a specific concentration range by bringing it into contact with an ammonium salt-type ionic liquid, and then performing back-extraction in which silver contained in the ionic liquid phase is reduced to precipitate metallic silver, thereby completing the present invention.

[0008] (1) A first aspect of the present invention is a method for recovering silver contained in a copper electrolyte solution, comprising: an extraction step of extracting silver contained in the copper electrolyte solution by bringing the copper electrolyte solution into contact with an ionic liquid phase containing an ammonium salt-type ionic liquid; and a stripping step of reducing the silver contained in the ionic liquid phase by bringing the ionic liquid phase into contact with a solution containing a reducing agent, thereby precipitating metallic silver. In the extraction step, the ionic liquid phase is contacted with a copper electrolyte having a chloride ion concentration of 0.1 mmol / L or more and 10 mmol / L or less, thereby extracting silver chloro complex ions into the ionic liquid phase.

[0009] (2) A second aspect of the present invention is a method for recovering silver according to the first aspect of the present invention, wherein in the stripping step, the ionic liquid phase is contacted with a solution containing one or more reducing agents selected from the group consisting of hydrazine monohydrate, sulfite, and sodium borohydride.

[0010] (3) A third aspect of the present invention is the first or second aspect of the present invention, wherein the ammonium salt-type ionic liquid is represented by the following formula (I): [ka] (In the formula, R 1 , R 2 and R 3 represents a substituted or unsubstituted hydrocarbon group having 1 to 12 carbon atoms, and R 1 and R 2 and R 3 R may be the same or different from each other and may be bonded together with the nitrogen atom to which they are attached to form a cyclic amine. 4 represents a substituted or unsubstituted hydrocarbon group having 1 to 4 carbon atoms. - and B - represents a counter anion, and n represents an integer of 2 to 8. The present invention provides a method for recovering silver, which is a compound represented by the formula:

[0011] (4) The fourth aspect of the present invention is the third aspect of the present invention, wherein in the formula (I), A - is a halide ion, and B - is N - (CF3SO2)2, and n is 3. [Effects of the Invention]

[0012] According to the present invention, silver dissolved in a copper electrolyte by electrolytic refining of copper can be efficiently recovered. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a graph showing the accounting of the mole fraction of silver by form versus chloride ion concentration in solution. DETAILED DESCRIPTION OF THE INVENTION

[0014] Specific embodiments of the present invention will be described in detail below. Note that the present invention is not limited to the following embodiments, and various modifications are possible within the scope of the present invention. Note that in this specification, the expression "X to Y" (X and Y are arbitrary numerical values) means "X or more and Y or less."

[0015] The silver recovery method according to this embodiment is a method for recovering silver that has dissolved into an electrolyte solution during electrolytic refining of copper.

[0016] Specifically, the method includes an extraction step in which silver contained in a copper electrolyte solution is extracted by bringing the copper electrolyte solution into contact with an ionic liquid phase containing an ammonium salt-type ionic liquid, and a back-extraction step in which silver contained in the ionic liquid phase is reduced by bringing the ionic liquid phase into contact with a solution containing a reducing agent, thereby precipitating metallic silver.

[0017] In this silver recovery method, in the extraction step, an ionic liquid phase containing an ammonium salt-type ionic liquid is brought into contact with a copper electrolyte having a chloride ion concentration of 0.1 mmol / L or more and 10 mmol / L or less, thereby extracting silver chloro complex ions into the ionic liquid phase.

[0018] Furthermore, in this silver recovery method, in the back-extraction step, a solution containing one or more reducing agents selected from hydrazine monohydrate, sulfite, and sodium borohydride is brought into contact with the ionic liquid phase, thereby recovering the silver extracted into the ionic liquid phase by back-extraction.

[0019] According to this method, silver contained in the copper electrolyte can be efficiently recovered. More specifically, silver can be selectively extracted into an ionic liquid phase containing an ammonium salt-type ionic liquid while suppressing the extraction of copper, iron, and the like from the copper electrolyte, and then effectively recovered by back-extraction.

[0020] [About ammonium salt-type ionic liquids] As described above, in the silver recovery method according to the present embodiment, a copper electrolyte containing silver is contacted with an ionic liquid phase containing an ammonium salt-type ionic liquid to extract silver. Specifically, the ammonium salt-type ionic liquid can be, for example, a compound represented by the following formula (I):

[0021] [ka]

[0022] Here, in the above formula (I), R 1 , R 2 and R 3 each represents a substituted or unsubstituted hydrocarbon group having 1 to 12 carbon atoms, and R 1 and R 2 and R 3 may be the same or different from each other. 1 , R 2 and R 3 may be linked together with the nitrogen atom to which they are attached to form a cyclic amine.

[0023] R 1 , R 2 and R 3 The carbon number of each of R is preferably 2 to 12, more preferably 3 to 8, and particularly preferably 4 to 8, from the viewpoint of enhancing the property of separating the water phase into two phases. 1 , R 2 and R 3 The hydrocarbon group may be saturated or unsaturated, and examples thereof include aromatic groups, cyclic hydrocarbon groups, straight-chain hydrocarbon groups, and branched-chain hydrocarbon groups.

[0024] R 1 , R 2 and R 3 Examples of the substituent in the substituted hydrocarbon group include hydrocarbon groups (which may be saturated or unsaturated, such as aromatic groups, cyclic hydrocarbon groups, straight-chain hydrocarbon groups, and branched-chain hydrocarbon groups), hydroxyl groups, ether groups, morpholino groups, ester groups, amide groups, perfluoroalkyl groups, nitrile groups (cyano groups), amino groups, imino groups, urea groups, carboxyl groups, carbonyl groups (aldehyde groups, ketone groups), sulfonic acid groups (sulfo groups), nitro groups, and halogens. Among these, straight-chain hydrocarbon groups and branched-chain hydrocarbon groups are preferred from the viewpoint of enhancing the property of separating into two phases from the aqueous phase. These may be used alone or in combination of two or more.

[0025] Also, R 4represents a substituted or unsubstituted hydrocarbon group having 1 to 4 carbon atoms. 4 Examples of the hydrocarbon group include a straight-chain hydrocarbon group and a branched-chain hydrocarbon group, and specific examples thereof include a methyl group, an ethyl group, a propyl group, and a butyl group.

[0026] Also, A - and B - represents a counter anion. - From the viewpoint of ease of synthesis, halide ions are preferred. - is N - (CF3SO2)2 (hereinafter referred to as “NTf2 - "), P - F6, B - F4, Cl - , CF3SO3 - Among these, B - From the viewpoint of increasing the hydrophobicity of the compound, N - (CF3SO2)2 is preferred, and from the viewpoint of reducing the production cost of the ionic liquid, Cl - These may be used alone or in combination of two or more.

[0027] Furthermore, n is an integer of 2 to 8. From the viewpoint of making the viscosity and hydrophobicity of the compound suitable for use as an extractant, n is preferably 2 to 6, and n is more preferably 3.

[0028] [Ionic liquid phase] In the silver recovery method according to the present embodiment, an ionic liquid phase containing the above-described ammonia salt-type ionic liquid is used, and silver is extracted by contacting this ionic liquid phase with a copper electrolyte containing silver. As the ionic liquid phase, it is preferable to use a compound of the above-described ammonia salt-type ionic liquid as an extractant, mixed with a compound composed of another ionic liquid that is not involved in the extraction.

[0029] Specifically, other ionic liquid compounds include, for example, compounds having a cation of 1-butyl-3-methylimidazolium (hereinafter referred to as "BMIM"). +"), the anion is N - An ionic liquid composed of (CF3SO2)2 can be used. By using an ionic liquid phase composed of a mixture of such an ionic liquid and an ammonium salt type ionic liquid, the liquid becomes stable at room temperature.

[0030] In the ionic liquid phase, the content of the ammonium salt ionic liquid is preferably 1% by mass to 50% by mass, and more preferably 1% by mass to 10% by mass, relative to 100% by mass of the ionic liquid.

[0031] [About the extraction process] In the extraction step, silver contained in the copper electrolyte is extracted into an ionic liquid phase containing an ammonium salt-type ionic liquid. The copper electrolyte to be treated is an electrolyte into which some of the impurities contained in the crude copper have been eluted by electrolytic refining of copper using crude copper as the anode, and in particular, silver has been eluted as an impurity. The copper electrolyte also contains chloride ions added as an additive from the viewpoint of smoothing the electrolytic copper electrodeposited on the cathode and achieving uniform electrodeposition.

[0032] In the silver recovery method according to the present embodiment, a copper electrolyte containing silver is brought into contact with an ionic liquid phase containing an ammonium salt-type ionic liquid, thereby extracting silver into the ionic liquid phase. The chloride ion concentration in the copper electrolyte brought into contact with the ionic liquid phase is important; specifically, a copper electrolyte having a chloride ion concentration of 0.1 mmol / L to 10 mmol / L is used for contact.

[0033] Figure 1 is a graph showing the relationship between the chloride ion concentration in the solution and the molar fraction of silver in each form. As shown in Figure 1, when the chloride ion concentration in the copper electrolyte is less than 0.1 mmol (0.0001 mol) / L, the silver chloro complex ion [AgCl2 - The mole fraction of chloride ions in the copper electrolyte is nearly zero, making extraction into the ionic liquid difficult. Furthermore, if the chloride ion concentration in the copper electrolyte exceeds 10 mmol (0.01 mol) / L, copper electrolytically deposits on the cathode in the form of dendrites.

[0034] In contrast, by contacting a copper electrolyte with a chloride ion concentration in the range of 0.1 mmol / L to 10 mmol / L with an ionic liquid phase containing an ammonium salt-type ionic liquid, silver chloro complex ions can be effectively extracted into the ionic liquid phase.

[0035] The chloride ion concentration in the copper electrolyte can be adjusted, for example, by collecting a portion of the copper electrolyte, analyzing it to determine the chloride ion concentration, and adding hydrochloric acid as needed. For example, if it is found that the chloride ion concentration has decreased due to extraction into the ionic liquid phase containing the ammonium salt-type ionic liquid, hydrochloric acid can be added to adjust the chloride ion concentration to within the above range.

[0036] Here, the extraction rate of metal elements by the extraction operation of bringing a copper electrolyte containing silver into contact with an ionic liquid phase containing an ammonium salt-type ionic liquid can be calculated as follows.

[0037] (extraction rate) When the concentration of the metal element in the copper electrolyte before the extraction operation is C0 and the concentration of the metal element in the copper electrolyte after the extraction operation is C1, the extraction rate E (%) of the metal element can be calculated using the following formula. E = (C0-C1) / C0 × 100

[0038] [About the back-extraction process] In the back-extraction step, the silver extracted as silver chloro complex ions into the ionic liquid phase in the extraction step described above is reduced and precipitated as solid metallic silver, which is back-extracted and separated from the ionic liquid phase for recovery.

[0039] Here, the extracted silver can be reduced by bringing the ionic liquid phase into contact with an aqueous solution containing a reducing agent.

[0040] The reducing agent is not particularly limited, but it is preferable to use one or more selected from hydrazine monohydrate, sulfite, and sodium borohydride. A solution containing these reducing agents is brought into contact with the ionic liquid phase in which silver has been extracted, thereby performing strip extraction. As mentioned above, silver is extracted in the form of a chloro complex ion, which is more stable than aquo ions. Therefore, by reducing silver using these reducing agents, which have relatively strong reducing power, silver can be stripped from the ionic liquid phase more efficiently and effectively.

[0041] The precipitate containing metallic silver produced by reduction can be recovered by centrifugal separation, filtration, or other procedures. The recovered precipitate is then dissolved in a nitric acid solution or the like, and the resulting solution is analyzed to determine the stripped metal components and the strip-extraction rate of those components. The strip-extraction rate can be calculated as follows:

[0042] (reverse extraction rate) The amount of metal elements in the ionic liquid phase before the back-extraction operation was calculated as W IL The precipitate recovered after the back-extraction operation is volume V HNO3 The metal concentration in the solution after dissolving in nitric acid solution C HNO3 When the back extraction rate of the metal element is back (%) can be calculated using the following formula: E back =(C HNO3 ×V HNO3 ) / W IL ×100 [Example]

[0043] The present invention will be explained in more detail below by showing examples, but the present invention is not limited to the following examples in any way.

[0044] In the examples, metal elements in the copper electrolyte and nitric acid solution were analyzed by ICP emission spectrometry and atomic absorption spectrometry, and chloride ion concentration was analyzed by X-ray fluorescence spectrometry with silver chloride separation.

[0045] [Synthesis of ammonium salt-type ionic liquids, preparation of ionic liquid phases] (Raw material for ammonium salt-type ionic liquid) The following raw materials were used to synthesize ammonium salt-type ionic liquids, which were used without further purification unless otherwise specified. 1,3-Dibromopropane (Fujifilm Wako Pure Chemical Industries, Ltd.) 1-Methylimidazole (Tokyo Chemical Industry Co., Ltd.) Lithium bis(trifluoromethanesulfonyl)imide (Aldrich) Dimethylamine (Tokyo Chemical Industry Co., Ltd.) Iodomethane (Tokyo Chemical Industry Co., Ltd.) Sodium sulfate (Fujifilm Wako Pure Chemical Industries, Ltd.)

[0046] Specifically, the compound (ammonium salt type ionic liquid) was synthesized according to the synthesis scheme shown in the following formula (II). 1 H-NMR was analyzed using the following analytical equipment. · 1 H-NMR: AV400M digital NMR (trade name), manufactured by Bruker Daltonics ICP-AES: Hitachi High-Tech Science Corporation, (product name) SPECTRO ARCOS

[0047] [ka]

[0048] (Synthesis of [1-(3-bromopropyl)-3-methylimidazolium bromide] (Compound [1])) A 250 mL acetone solution of 200 g (1.08 mol) of 1,3-dibromopropane was added to a four-neck round-bottom flask (1000 mL), and the atmosphere in the flask was replaced with argon. 8.21 g (100 mmol) of 1-methylimidazole was dissolved in 50 mL of acetone, and this solution was added dropwise over 30 minutes. The reaction solution was then heated to reflux in an oil bath set at 45°C for 20 hours to carry out the reaction. The white solid formed during the reaction and the reaction solution were separated by decantation. The white solid was dissolved in a small amount of ethanol, and then a large amount of acetone was added to reprecipitate the white solid. The product in the white solid was dissolved in acetone. The remaining solid and solution were again separated by decantation. The reaction solution and this solution were combined and then concentrated under reduced pressure using a rotary evaporator and a vacuum dryer. The reaction solution and water were then separated, and the upper aqueous phase was removed. Next, the mixture was separated into water (HO) and hexane, and the lower aqueous phase was removed. The resulting aqueous solution was concentrated under reduced pressure using a rotary evaporator and a vacuum dryer. Thus, a colorless liquid, Compound 1 (CH 12 Br2N2) was obtained in 81% yield (23.3 g, 81.7 mmol).

[0049] The obtained compound 1 The results of H-NMR were as follows: 1 H-NMR(400MHz,CDCl3)δ2.58(quin,J=6.6Hz,2H),3.50(t,J=6.2Hz,2H),4.13(s,3H ),4.61(t,J=6.8Hz,2H),7.58(t,J=1.8Hz,1H),7.64(t,J=1.8Hz,1H),10.35(s,1H)

[0050] (Synthesis of [1-(3-bromopropyl)-3-methylimidazolium bis(trifluoromethanesulfonyl)imide] ("Compound [2]") A 300 mL recovery flask was charged with 150 mL of water (HO), and 23.20 g (81.7 mmol) of compound [1] and 25.8 g (89.9 mmol) of lithium bis(trifluoromethanesulfonyl)imide were dissolved in the water. The reaction was then carried out at room temperature with stirring for 1 hour, and the reaction solution was extracted with chloroform (CHCl). The extracted solution was dehydrated using anhydrous magnesium sulfate (MgSO), filtered through Celite, and concentrated under reduced pressure using a rotary evaporator and a vacuum dryer. Thus, a colorless liquid, compound [2] (CH 12 BrF6N3O4S2) was obtained in 91% yield (35.91 g, 74.2 mmol).

[0051] The obtained compound 1 The results of H-NMR were as follows: 1 H-NMR(400MHz,CDCl3)δ2.44(quin,J=6.5Hz,2H),3.50(t,J=6.0Hz,2H),3.97(s,3 H),4.42(t,J=7.0Hz,2H),7.29(t,J=1.8Hz,1H),7.35(t,J=1.8Hz,1H),8.85(s,1H)

[0052] (Synthesis of [1-3-(dimethylaminopropyl)-3-methylimidazolium bis(trifluoromethanesulfonyl)imide] ("Compound [3]") Compound [2] and dimethylamine were added to a three-necked round-bottom flask (500 mL), and the atmosphere in the flask was replaced with argon. The reaction solution was then refluxed at room temperature for 24 hours. The reaction solution was concentrated under reduced pressure using a rotary evaporator. The concentrated reaction solution was separated with CHCl3 / 5% by mass NH3 aq., and the upper 5% by mass NH3 aq. phase was removed. The solution was then separated with CHCl3 / H2O, and the lower CHCl3 phase was washed. After dehydration with anhydrous MgSO4, the organic phase was filtered through Celite and concentrated under reduced pressure using a rotary evaporator. The resulting liquid was further separated with hexane, and the ionic liquid phase was washed. Compound [3] was obtained as a pale yellow liquid in 62% yield (11.5 g, 25.6 mmol) by concentrating under reduced pressure using a rotary evaporator and a vacuum dryer.

[0053] The obtained compound 1 The results of H-NMR were as follows: 1 H-NMR(400MHz,CDCl3)δ=2.00(tt,J=6.8Hz,6.6Hz,2H),2.19(s,6H),2.25(t,J=6. 4Hz,2H),3.96(s,3H),4.28(t,J=6.8Hz,2H),7.27(t,1H),7.33(t,1H),8.80(s,1H)

[0054] (Synthesis of 1-(3-trimethylammoniumpropyl)-3-methylimidazolium bis(trifluoromethanesulfonyl)imide iodide ("Compound [4]") Compound [3], iodomethane, and methanol were added to a three-necked round-bottom flask (300 mL), and the reaction mixture was stirred at room temperature for 24 hours. The reaction mixture was concentrated under reduced pressure using a rotary evaporator and a vacuum dryer to give compound [4] as a pale yellow solid in 100% yield (1.41 g, 2.38 mmol).

[0055] The obtained compound 1 The results of H-NMR were as follows: 1H-NMR(400MHz,MeOD)δ=2.46(m,2H),3.19(s,9H),3.50(tt,2H),3.95(s,3H),4.36(t,J=7.2Hz,2H),7.62(t,1H),7.70(t,1H),8.99(s,1H)

[0056] (Preparation of ionic liquid phase) Compound [4] 1.0 g (1.7 mmol) was added to "BMIM" to give a concentration of 5 mass%. + "," NTf2 - " to prepare an ionic liquid phase.

[0057] [Extraction of silver from copper electrolyte] (extraction process) 2 mL of copper electrolyte solution having the composition shown in Table 1 below was mixed with 0.4 g of the ionic liquid phase prepared as described above, and the mixture was shaken horizontally at 25°C and 1500 rpm to bring the copper electrolyte into contact with the ionic liquid phase containing the ammonium salt-type ionic liquid. The chloride ion concentration in the copper electrolyte was 1.1 mmol / L.

[0058] [Table 1]

[0059] [Repeated extraction of silver from copper electrolyte] 0.4 g of the ionic liquid phase was mixed with 2 mL of a newly prepared copper electrolyte solution of the composition shown in Table 1, and the same extraction procedure was carried out. This procedure was repeated four times, for a total of five extraction procedures. In each case, 100% of the silver in the copper electrolyte solution was successfully extracted.

[0060] The analytical results of the ionic liquid phase obtained by each extraction procedure are shown in Table 2. As can be seen from the extraction rates of each metal element shown in Table 2, 100% of silver was extracted with almost no copper being extracted.

[0061] [Table 2]

[0062] [Back extraction of silver from ionic liquid phase] 0.4 g of the ionic liquid extracted five times was mixed with 2 mL of hydrazine aqueous solution containing 1% hydrazine monohydrate, and the mixture was shaken horizontally at 25°C and 1500 rpm to bring the ionic liquid phase into contact with the hydrazine aqueous solution. Subsequently, the mixture was centrifuged at 25°C and 6000 rpm.

[0063] [Sediment Analysis] 2 mL of the hydrazine solution was washed with ethanol, and the precipitate was recovered. This precipitate was dissolved in 2 mL of 1 mol / L nitric acid solution, after which the metal components were analyzed and the stripping rate was calculated. Table 3 below shows the results of the stripping rate. As shown in Table 3, 100% of silver was stripped from the ionic liquid phase, enabling effective recovery. Note that the stripping rates of other metal components were less than 20%.

[0064] [Table 3]

Claims

1. A method for recovering silver contained in a copper electrolyte, comprising the steps of: an extraction step of extracting silver contained in the copper electrolyte by bringing the copper electrolyte into contact with an ionic liquid phase containing an ammonium salt-type ionic liquid; a stripping step of contacting the ionic liquid phase with a solution containing a reducing agent to reduce silver contained in the ionic liquid phase and precipitate metallic silver; In the extraction step, the ionic liquid phase is brought into contact with a copper electrolyte having a chloride ion concentration of 0.1 mmol / L or more and 10 mmol / L or less, thereby extracting silver chloro complex ions into the ionic liquid phase. How to recover silver.

2. In the stripping step, a solution containing one or more reducing agents selected from the group consisting of hydrazine monohydrate, sulfite, and sodium borohydride is brought into contact with the ionic liquid phase. The method for recovering silver according to claim 1.

3. The ammonium salt type ionic liquid is represented by the following formula (I): 【Chemistry 1】 (In the formula, R １ , R ２ and R ３ represents a substituted or unsubstituted hydrocarbon group having 1 to 12 carbon atoms, R １ and R ２ and R ３ may be the same or different from each other, and may be bonded together with the nitrogen atom to which they are attached to form a cyclic amine. ４ represents a substituted or unsubstituted hydrocarbon group having 1 to 4 carbon atoms. － and B － represents a counter anion, and n represents an integer of 2 to 8. is a compound represented by The method for recovering silver according to claim 1 or 2.

4. In the formula (I), A － is a halide ion, and B － is N － (CF ３ SO ２ ) ２ and Further, n is 3; The method for recovering silver according to claim 3.

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