JP7804281B2 - Method for recovering silver from copper electrolyte - Google Patents

Description

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

Copper electrorefining involves electrolysis using a sulfuric acidic copper sulfate solution as the electrolyte and crude copper produced from copper concentrate through a dry processing process as the anode, producing highly pure copper (electrolytic copper) on the cathode surface. 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.

The main anodic 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. Of these, 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 into the electrolyte will precipitate on the cathode along with the copper.

Silver is a precious metal, and by suppressing silver deposition on the cathode and increasing the recovery rate, it is possible to 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, for example, but recovering silver that has dissolved into the electrolyte has been difficult.

Japanese Patent Application Publication No. 8-176878 Japanese Patent Application Laid-Open No. 2017-043556 Japanese Patent Application Laid-Open No. 2021-143386

The present invention was proposed in light of these circumstances, and aims to provide a method for efficiently recovering silver that has dissolved into a copper electrolyte during copper electrolytic refining.

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

(1) The first aspect of the present invention is a method for recovering silver contained in a copper electrolyte solution, comprising: an extraction step in which the copper electrolyte solution is brought into contact with an ionic liquid phase containing an ammonium salt-type ionic liquid to extract silver contained in the copper electrolyte solution; and a stripping step in which the ionic liquid phase is brought into contact with a solution containing a reducing agent to reduce the silver contained in the ionic liquid phase and precipitate metallic silver. In the extraction step, the ionic liquid phase is contacted with a copper electrolyte solution 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.

(2) A second aspect of the present invention is a method for recovering silver according to the first aspect, 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.

(3) A third aspect of the present invention is the first or second aspect, wherein the ammonium salt type ionic liquid is represented by the following formula (I):
(wherein R ¹ , R ² and R ³ represent a substituted or unsubstituted hydrocarbon group having 1 to 12 carbon atoms; R ¹ , R ² and R ³ may be the same or different from one another and may bond to each other together with the nitrogen atom to which they are bonded to form a cyclic amine; R ⁴ represents a substituted or unsubstituted hydrocarbon group having 1 to 4 carbon atoms; A ^{1 -} and B 1 ^- represent counter anions; and n represents an integer of 2 to 8.)

(4) A fourth aspect of the present invention is the method for recovering silver according to the third aspect of the present invention, wherein, in formula (I), A ⁻ is a halide ion, B ⁻ is N ⁻ (CF ₃ SO ₂ ) ₂ , and n is 3.

According to the present invention, silver dissolved in the copper electrolyte during copper electrolytic refining can be efficiently recovered.

FIG. 1 is a graph showing the accounting of the mole fraction of silver by form versus chloride ion concentration in solution.

Specific embodiments of the present invention are described in detail below. Note that the present invention is not limited to the following embodiments, and various modifications are possible without departing from the spirit of the present invention. Note that in this specification, the expression "X to Y" (X and Y are arbitrary numerical values) means "greater than or equal to X and less than or equal to Y."

The silver recovery method in this embodiment is a method for recovering silver that has dissolved into the electrolyte during copper electrorefining.

Specifically, the process 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.

In this silver recovery method, in the extraction step, an ionic liquid phase containing an ammonium salt-type ionic liquid is contacted with a copper electrolyte solution 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.

In addition, 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 back-extracting and recovering the silver extracted into the ionic liquid phase.

This method allows for efficient recovery of silver contained in the copper electrolyte. More specifically, while suppressing the extraction of copper, iron, and other elements from the copper electrolyte, silver is selectively extracted into an ionic liquid phase containing an ammonium salt-type ionic liquid, and can then be effectively recovered by back-extraction.

[Ammonia 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 for extraction. Specifically, the ammonium salt-type ionic liquid can be, for example, a compound represented by the following formula (I):

In the above formula (I), R ¹ , R ² and R ³ each represent a substituted or unsubstituted hydrocarbon group having 1 to 12 carbon atoms, and R ¹ , R ² and R ³ may be the same or different from one another. Furthermore, R ¹ , R ² and R ³ may be bonded to one another together with the nitrogen atom to which they are bonded to form a cyclic amine.

From the viewpoint of enhancing the property of two-phase separation from the aqueous phase, the number of carbon atoms in R ¹ , R ² and R ³ is preferably 2 to 12, more preferably 3 to 8, and particularly preferably 4 to 8. The hydrocarbon groups in R ¹ , R ² and R ³ may each be saturated or unsaturated, and examples thereof include aromatic groups, cyclic hydrocarbon groups, straight-chain hydrocarbon groups and branched-chain hydrocarbon groups.

Examples of the substituents in the substituted hydrocarbon groups of R ¹ , R ² , and R ³ 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 improving the properties of separation into two phases from the aqueous phase. These may be used alone or in combination of two or more.

Furthermore, ^R4 represents a substituted or unsubstituted hydrocarbon group having 1 to 4 carbon atoms. Examples of the hydrocarbon group for ^R4 include a linear hydrocarbon group and a branched hydrocarbon group. Specific examples include a methyl group, an ethyl group, a propyl group, and a butyl group.

Furthermore, A ^- and B ^- represent counter anions. Specifically, A ^- is preferably a halide ion from the viewpoint of ease of synthesis. Furthermore, examples of B ^- include N ^- (CF ₃ SO ₂ ) ₂ (hereinafter also referred to as "NTf ₂ ^- "), P ^- F ₆ , B ^- F ₄ , Cl ^- , and CF ₃ SO ₃ ^- . Among these, as B ^- , N ^- (CF ₃ SO ₂ ) ₂ is preferred from the viewpoint of increasing the hydrophobicity of the compound, and Cl ^- is also preferred from the viewpoint of reducing the production cost of the ionic liquid. These may be used alone or in combination of two or more.

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

[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.

Specifically, as another ionic liquid compound, for example, an ionic liquid whose cation is 1-butyl-3-methylimidazolium (hereinafter also referred to as "BMIM ⁺ ") and whose anion is N ^- (CF ₃ SO ₂ ) ₂ can be used. By using an ionic liquid phase formed by mixing such an ionic liquid with an ammonium salt type ionic liquid, the liquid becomes stable as a liquid at room temperature.

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

[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.

In the silver recovery method according to this embodiment, a copper electrolyte solution 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 solution brought into contact with the ionic liquid phase is important; specifically, a copper electrolyte solution with a chloride ion concentration of 0.1 mmol/L or more and 10 mmol/L or less is used for contact.

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, if the chloride ion concentration in the copper electrolyte is less than 0.1 mmol (0.0001 mol)/L, the molar fraction of silver chloro complex ions [AgCl ₂ ^- ] becomes almost zero, making extraction into the ionic liquid phase difficult. Furthermore, if the chloride ion concentration in the copper electrolyte exceeds 10 mmol (0.01 mol)/L, there is a problem in that copper electrodeposited on the cathode during copper electrolysis begins to deposit in a dendritic form.

In contrast, by contacting a copper electrolyte solution 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.

The chloride ion concentration in the copper electrolyte can be adjusted, without any particular limitation, by, for example, sampling a portion of the copper electrolyte, confirming the chloride ion concentration through analysis, and adding hydrochloric acid as appropriate. For example, if it is determined 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.

Here, the extraction rate of metal elements in an extraction operation in which a copper electrolyte containing silver is brought into contact with an ionic liquid phase containing an ammonium salt-type ionic liquid can be calculated as follows:

(extraction rate)
When the concentration of the metal element in the copper electrolyte before the extraction operation is C ₀ and the concentration of the metal element in the copper electrolyte after the extraction operation is C ₁ , the extraction rate E (%) of the metal element can be calculated by the following formula.
E=(C ₀ - _{C 1} )/C ₀ ×100

[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.

To reduce the extracted silver, the ionic liquid phase can be brought into contact with an aqueous solution containing a reducing agent.

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.

The precipitate containing metallic silver produced by reduction can be recovered by centrifugal separation, filtration, or other procedures. The recovered precipitate can then be dissolved in a nitric acid solution or similar, and the resulting solution analyzed to confirm the stripped metal components and their strip extraction rates. The strip extraction rate can be calculated as follows:

(reverse extraction rate)
When the amount of metal element in the ionic liquid phase before the stripping operation is W _IL and the metal concentration in the solution obtained by dissolving the precipitate recovered after the stripping operation in a nitric acid solution with a volume of V _HNO3 is C _HNO3 , the stripping rate E _back (%) of the metal element can be calculated using the following formula.
E _back = (C _HNO3 × V _HNO3 )/W _IL ×100

The present invention will be explained in more detail below using examples, but the present invention is not limited to these examples in any way.

In the examples, metal element analysis in the copper electrolyte and nitric acid solution was performed using ICP emission spectrometry and atomic absorption spectrometry. Chloride ion concentration analysis was performed using X-ray fluorescence spectrometry with silver chloride separation.

[Synthesis of ammonium salt-type ionic liquids and 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 (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.)
1-Methylimidazole (Tokyo Chemical Industry Co., Ltd.)
- Lithium bis(trifluoromethanesulfonyl)imide (manufactured by Aldrich)
- Dimethylamine (Tokyo Chemical Industry Co., Ltd.)
- Iodomethane (Tokyo Chemical Industry Co., Ltd.)
- Sodium sulfate (Fujifilm Wako Pure Chemical Industries, Ltd.)

Specifically, compounds (ammonium salt-type ionic liquids) were synthesized according to the synthesis scheme shown in formula (II) below. ^1H -NMR of the compounds obtained in each process 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

(Synthesis of [1-(3-bromopropyl)-3-methylimidazolium bromide] ("Compound [1]")
A four-necked round-bottom flask (1000 mL) was charged with 250 mL of an acetone solution of 200 g (1.08 mol) of 1,3-dibromopropane, 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 produced during the reaction and the reaction solution were separated by decantation. Furthermore, 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, and the product in the white solid was dissolved in acetone. The remaining solid and solution were again separated by decantation. This solution and the reaction solution were combined and then concentrated under reduced pressure using a rotary evaporator and a vacuum dryer. The reaction solution/water was then separated, and the upper aqueous phase was removed. The mixture was then separated using water (H ₂ O)/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. Compound 1 (C ₇ H ₁₂ Br ₂ N ₂ ) was thus obtained as a colorless liquid in a yield of 81% (23.3 g, 81.7 mmol).

^{The 1} H-NMR results of the obtained compound were as follows:
¹ H-NMR (400 MHz, CDCl ₃ ) 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)

(Synthesis of [1-(3-bromopropyl)-3-methylimidazolium bis(trifluoromethanesulfonyl)imide] (“Compound [2]”))
150 mL of water (H ₂ O) was added to a recovery flask (300 mL), 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 water. The reaction was then carried out at room temperature with stirring for 1 hour, and the reaction solution was extracted with chloroform (CHCl ₃ ). The extracted solution was dehydrated using anhydrous magnesium sulfate (MgSO ₄ ), filtered through Celite, and concentrated under reduced pressure using a rotary evaporator and a vacuum dryer. Thus, colorless liquid compound [2] (C ₉ H ₁₂ BrF ₆ N ₃ O ₄ S ₂ ) was obtained in a yield of 91% (35.91 g, 74.2 mmol).

^{The 1} H-NMR results of the obtained compound were as follows:
¹ H-NMR (400 MHz, CDCl ₃ ) δ2.44 (quin, J=6.5Hz, 2H), 3.50 (t, J=6.0Hz, 2H), 3.97 (s, 3H), 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)

(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 to allow the reaction to proceed. The reaction solution was concentrated under reduced pressure using a rotary evaporator. The concentrated reaction solution was separated using CHCl ₃ /5% by mass NH ₃ aq., and the upper 5% by mass NH ₃ aq. was removed. Additionally, separation was performed using CHCl ₃ /H ₂ O, and the lower CHCl ₃ phase was washed. After dehydration using anhydrous MgSO ₄ , the organic phase was filtered through Celite and concentrated under reduced pressure using a rotary evaporator. Further separation was performed using the resulting liquid/hexane, and the ionic liquid phase was washed. Compound [3] was obtained as a pale yellow liquid in a 62% yield (11.5 g, 25.6 mmol) by concentrating under reduced pressure using a rotary evaporator and a vacuum dryer.

^{The 1} H-NMR results of the obtained compound were as follows:
¹ H-NMR (400 MHz, CDCl3) δ = 2.00 (tt, J = 6.8 Hz, 6.6 Hz, 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)

(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 solution was stirred at room temperature for 24 hours. The reaction solution 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).

^{The 1} H-NMR results of the obtained compound were as follows:
¹ H-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)

(Preparation of ionic liquid phase)
An ionic liquid phase was prepared by mixing 1.0 g (1.7 mmol) of the compound [4] with "BMIM ⁺ " and "NTf ₂ ^- " so as to give a concentration of 5% by mass.

[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 and reciprocally 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.

[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 having 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.

Table 2 below shows the analytical results of the ionic liquid phase obtained by each extraction procedure. 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.

[Back extraction of silver from the ionic liquid phase]
0.4 g of the ionic liquid that had been extracted five times was mixed with 2 mL of an aqueous hydrazine 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 aqueous hydrazine solution. Subsequently, the mixture was centrifuged at 25°C and 6000 rpm.

[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, and the metal components were analyzed to calculate the stripping rate. 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%.

Claims (4)

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. 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. The ammonium salt type ionic liquid is represented by the following formula (I):
(In the formula, R ¹ , R ² and R ³ represent a substituted or unsubstituted hydrocarbon group having 1 to 12 carbon atoms, and R ¹ , R ² and R ³ may be the same or different from one another and may bond to each other together with the nitrogen atom to which they are bonded to form a cyclic amine. R ⁴ represents a substituted or unsubstituted hydrocarbon group having 1 to 4 carbon atoms. ^{A 1 -} and B 1 ^- represent counter anions. n represents an integer of 2 to 8.)
is a compound represented by
The method for recovering silver according to claim 1 or 2. In the formula (I),
A ^- is a halide ion, B ^- is N ^- (CF ₃ SO ₂ ) ₂ ,
Further, n is 3;
The method for recovering silver according to claim 3.