Modified cation exchange membrane, method for preparing the same, and method for purifying silver electrolyte using the same

Abstract

This invention belongs to the field of non-ferrous metal smelting, specifically involving a modified cation exchange membrane, its preparation method, and a method for purifying silver electrolyte using it. It employs a customized special separation membrane combined with specific pretreatment and membrane separation processes to achieve efficient and precise separation of divalent copper ions and monovalent silver ions, controlling the copper ion concentration in the electrolyte within a reasonable range, reducing the generation of waste electrolyte, minimizing silver ion loss, ensuring the recyclability of the electrolyte, reducing production costs, and making the process green and environmentally friendly, suitable for large-scale industrial applications.

Description

Technical Field

[0001] This invention belongs to the field of non-ferrous metal smelting, specifically relating to modified cation exchange membranes, their preparation methods, and methods for purifying silver electrolytes using them. Background Technology

[0002] Silver, a precious metal with excellent electrical and thermal conductivity and ductility, is widely used in electronics, jewelry, chemical catalysis, aerospace, and other fields. Electrolytic refining is currently the mainstream process for producing high-purity silver (purity ≥99.99%) in industry. Its core principle is to utilize the difference in standard electrode potential between silver and impurity metals in the electrolyte. This causes the silver in the anode plate (gold-silver alloy) to preferentially dissolve into monovalent silver ions (Ag⁺) and enter the electrolyte, where it is then reduced and deposited on the cathode surface to obtain high-purity silver. The most abundant impurity in the gold-silver alloy is copper. Because its standard electrode potential is close to that of silver, copper easily undergoes electrochemical dissolution into divalent copper ions (Cu²⁺) and enters the electrolyte. However, copper ions are difficult to deposit at the cathode, leading to the continuous accumulation of copper ions in the electrolyte.

[0003] As the concentration of copper ions in the electrolyte increases, a series of problems arise: First, it reduces the conductivity of the electrolyte, increasing energy consumption in the electrolysis process; second, it causes defects such as burrs and dendrites on the surface of the silver products deposited at the cathode, affecting the product's appearance and physical properties; third, when the copper ion concentration exceeds a critical value, co-deposition occurs, significantly reducing the purity of the silver products and failing to meet the requirements of high-purity silver in high-end applications. Generally, the copper ion concentration in the electrolyte should not exceed 50 g / L. When the copper content reaches 50-60 g / L, a portion of the electrolyte must be discharged as waste electrolyte. Therefore, deep purification of copper ions in silver electrolytes to control their concentration at a reasonable level is a crucial step in ensuring the stable operation of the electrolytic refining process, reducing waste electrolyte generation, and improving product quality.

[0004] Currently, industrial methods for purifying copper ions in silver electrolytes mainly include chemical precipitation, displacement, and solvent extraction. Chemical precipitation involves adding precipitants such as sulfides and hydroxides to the electrolyte, causing copper ions to form a poorly soluble precipitate, which is then removed by filtration. This method is simple to operate and low in cost, but it suffers from problems such as difficulty in precisely controlling the amount of precipitant, the introduction of new impurities, and incomplete separation of the precipitate from the electrolyte. Furthermore, it is difficult to achieve deep removal of copper ions; the concentration of copper ions in the purified electrolyte can typically only be reduced to 500-1000 mg / L, which cannot meet the requirements for high-purity silver preparation.

[0005] The displacement method uses a metal more reactive than copper (such as iron or zinc) as a displacement agent to replace copper ions in the electrolyte with metallic copper, which is then deposited. While this method can reduce the copper ion concentration to some extent, the displacement agent easily reacts with silver ions in the electrolyte, leading to silver loss. Furthermore, the displacement products tend to form fine particles dispersed in the electrolyte, increasing the difficulty of subsequent separation and making it difficult to achieve deep purification.

[0006] Solvent extraction involves selecting a specific extractant to selectively introduce copper ions from the electrolyte into the organic phase, thus separating them from silver ions. This method offers advantages such as high separation efficiency and good selectivity. However, it also suffers from drawbacks, including high extractant costs, susceptibility to emulsification, complex subsequent back-extraction processes, and environmental pollution caused by the volatility of the organic phase. These issues limit its large-scale industrial application.

[0007] Membrane separation technology, as a highly efficient, energy-saving, and environmentally friendly separation technology, boasts advantages such as simple operation, mild separation conditions, and no secondary pollution, and has been widely applied in water treatment, chemical separation, and biomedicine. The core of membrane separation technology is utilizing the selective permeability of membranes to achieve separation based on differences in particle size, charge properties, and molecular structure of the substances being separated. In silver electrolyte systems, monovalent silver ions (Ag⁺) and divalent copper ions (Cu²⁺) differ in charge properties and ionic radii (Ag⁺ ionic radius is approximately 115 pm, Cu²⁺ ionic radius is approximately 73 pm), and may form different complexes in specific electrolyte systems, providing a possibility for separating these two ions using membrane separation technology.

[0008] However, the application of existing membrane separation technologies in the purification of copper ions from silver electrolytes faces numerous challenges. On the one hand, silver electrolytes are typically acidic systems (such as nitric acid systems) and contain high concentrations of silver ions and other impurity ions, placing extremely high demands on the corrosion resistance and stability of the membrane materials. On the other hand, conventional membrane materials exhibit poor selective permeation of Ag⁺ and Cu²⁺, making efficient separation difficult. Furthermore, the membranes are easily contaminated and clogged by impurities in the electrolyte, leading to a rapid decline in membrane flux and impacting separation efficiency and membrane lifespan. Therefore, developing a membrane separation technology that is adaptable to silver electrolyte systems, possesses high selectivity, and exhibits good stability to achieve deep purification of copper ions has become a pressing technical problem in current high-purity silver electrolytic refining processes. Summary of the Invention

[0009] The purpose of this invention is to provide a modified cation exchange membrane that uses polyperfluorosulfonic acid resin as the base material and aminophosphonic acid as the functional group, so that the membrane can selectively separate Ag⁺ and Cu²⁺ based on the ion size sieving effect and the specific complexation effect of the functional group.

[0010] To achieve the above objectives, the technical solution adopted by the present invention is as follows: a modified cation exchange membrane, characterized in that: polyperfluorosulfonic acid resin is used as the base material, and grafted with an element containing aminophosphonic acid groups to obtain the modified cation exchange membrane.

[0011] Another object of the present invention is to provide a method for preparing the above-mentioned modified cation exchange membrane, comprising the following steps:

[0012] (1) Preparation of casting solution: Polyperfluorosulfonic acid resin with a mass ratio of 1:8-1:12 is mixed with N-methylpyrrolidone solvent and stirred at 60-80℃ to obtain resin solution. Aminophosphonic acid monomer, N,N'-methylenebisacrylamide and ammonium persulfate are added to resin solution and stirred at 60-80℃ to obtain casting solution.

[0013] (2) Coating and film formation: Select a polytetrafluoroethylene microporous membrane with a thickness of 50-80 μm as the support membrane, and uniformly coat the prepared casting solution on the surface of the polytetrafluoroethylene support membrane.

[0014] (3) Drying, curing and crosslinking: The support film coated with casting solution is sent into a drying oven and dried by gradient heating: dry at 40-60℃ for 2-3 h; then heat to 80-100℃ and dry for 4-6 h; finally heat to 120-150℃ and dry for 1-2 h; transfer the film to a high-temperature reactor, heat to 160-180℃ under nitrogen atmosphere and keep warm for 2-3 h to obtain the crosslinked film;

[0015] (4) Hydrolysis activation: The cross-linked membrane is placed in a 0.5-1.0 mol / L sodium hydroxide solution and soaked at 40-60℃ for 2-4 h. Then, the membrane is repeatedly washed with deionized water until the pH of the effluent is neutral. Then, it is placed in a 0.5-1.0 mol / L sulfuric acid solution and soaked for 1-2 h. Finally, it is rinsed with deionized water until neutral to obtain the modified cation exchange membrane.

[0016] The selective separation of Ag⁺ and Cu²⁺ by the special cation exchange membrane prepared in this invention is mainly based on the synergistic mechanism of ion size sieving effect and functional group specific complexation. The specific working principle is as follows:

[0017] (1) Ion size sieving effect: The sizes of hydrated ions formed by g⁺ and Cu²⁺ in aqueous solution in silver electrolyte are significantly different and in the opposite order to the bare ion radii. The bare ion radius of Ag⁺ is about 115 pm, and the hydrated ion radius is about 302 pm; the bare ion radius of Cu²⁺ is about 73 pm, and the hydrated ion radius is about 419 pm. This is because Cu²⁺ is a divalent ion with a much higher charge density than monovalent Ag⁺, resulting in a stronger electrostatic attraction to water molecules and a thicker, denser hydrated shell. The special cation exchange membrane prepared in this invention provides a basic spatial constraint for the selective permeation of ions by precisely controlling the pore size (pore size controlled within 0.5-1.0 nm). It should be noted that the hydrated radii of both ions are smaller than the membrane pore size, and simple size sieving cannot achieve effective separation. It needs to be combined with the complexation effect of functional groups to achieve the desired effect.

[0018] (2) Functional group-specific complexing effect: The aminophosphonic acid group (-NH-CH2-PO(OH)2) grafted onto the membrane matrix has a special chemical structure. The nitrogen atom, phosphorus atom and hydroxyl group in its molecule can all act as coordinating atoms to form complexes with metal ions. Among them, Cu²⁺ is a transition metal ion with empty d orbitals, which can easily form stable five- or six-membered chelate complexes with aminophosphonic acid groups, and the complexation stability constant is relatively high. When the complexation reaction occurs, some water molecules around Cu²⁺ will be stripped from the hydrated shell, the effective migration size is reduced, and the complexed Cu²⁺ can move in the membrane matrix through coordination migration. However, the complexation effect of Ag⁺ with aminophosphonic acid groups is weak, the complexation stability constant is much lower than that of Cu²⁺, and it cannot form a stable complex. Its hydrated shell is difficult to strip, and when it migrates in the membrane pores, the hydraulic resistance and steric hindrance effect between the hydrated layer and the membrane pore wall are significant, and the migration rate is extremely slow. Driven by pressure, Cu²⁺ is preferentially permeated through the membrane to form the permeate, while Ag⁺ is retained in the concentrate for highly efficient and selective separation.

[0019] (3) Acid Resistance Stabilization Effect: The substrate material of the membrane is poly(perfluorosulfonic acid) resin, which has a perfluorocarbon main chain structure, extremely high chemical stability, and resistance to strong acids, strong alkalis, and strong oxidants. Simultaneously, the stable cross-linked structure formed through the cross-linking reaction further enhances the structural stability of the membrane, preventing swelling, degradation, or structural damage in acidic silver electrolytes with a pH of 3-3.5. Furthermore, the fixed groups (-SO3) of the cation exchange membrane... - -PO(OH) - The negatively charged cations form the Donnan potential, which allows only cations to pass through while repelling anions. For different cations, the Donnan potential has a more significant hindering effect on Ag⁺, because it cannot reduce the effective size through complexation and is difficult to overcome the potential barrier, which further enhances the retention effect of Ag⁺ and ensures long-term stable separation performance.

[0020] A third objective of this invention is to provide a method for deep purification of copper ions in silver electrolyte based on the above-mentioned modified cation exchange membrane, comprising the following steps:

[0021] S1. Physical impurity removal: The silver electrolyte to be treated is sequentially filtered for impurity removal, activated carbon adsorption, and ultrafiltration to remove solid impurities, resulting in treated solution A.

[0022] S2. Membrane separation: The treatment liquid A is separated by the modified cation exchange membrane according to any one of claims 1-6 to obtain permeate and concentrate. The concentrate is purified silver electrolyte and is directly fed into the electrolyte storage tank in the silver electrolysis refining workshop. The permeate is subjected to copper recovery treatment.

[0023] Specifically, in step S1, the filtration and impurity removal involves passing the silver electrolyte to be treated through a precision filter. The precision filter includes a polypropylene pleated filter element, with a filtration accuracy of 0.45 μm, a filtration pressure controlled at 0.1-0.2 MPa, and a filtration flow rate of 10-15 m³ / (m²·h).

[0024] Activated carbon adsorption involves passing the filtered and impurity-removed silver electrolyte into an adsorption column filled with activated carbon modified by nitric acid oxidation. The specific surface area is 800-1000 m² / g, the adsorption temperature is controlled at 25-35℃, and the residence time of the silver electrolyte in the adsorption column is 30-60 min.

[0025] Ultrafiltration involves passing the silver electrolyte, after adsorption by activated carbon, through an ultrafiltration membrane module made of polyvinylidene fluoride for precision filtration. The ultrafiltration membrane has a molecular weight cutoff of 5000-10000 Da, an operating pressure of 0.2-0.3 MPa, and a filtration flow rate of 5-8 m³ / (m²·h).

[0026] In step S2, the copper recovery process using the permeate is as follows:

[0027] ① Silver recovery: Add sodium chloride solution to the permeate, stir and react for 30-60 min, and separate the solid and liquid to obtain silver chloride precipitate and silver-removed liquid; the amount of sodium chloride added is 1.2-1.5 times the theoretical reaction amount.

[0028] ② Copper Recovery: Sodium hydroxide solution is added to the silver-removed liquid to adjust the pH to 8-9, and the mixture is stirred for 60-90 minutes. Solid-liquid separation yields copper hydroxide precipitate and the copper-removed liquid. Hydrochloric acid is added to the copper hydroxide precipitate to dissolve it, controlling the amount of hydrochloric acid added to maintain a pH of 1-2, resulting in a copper chloride solution. The copper chloride solution is then evaporated and concentrated, cooled and crystallized, centrifuged, and dried to obtain copper chloride crystals. Specifically, the copper chloride solution is evaporated and concentrated at 100-120℃ and cooled and crystallized at 0-10℃. Sulfuric acid is added to the copper-removed liquid to adjust the pH to 6-7, and residual organic matter is removed by activated carbon adsorption before being discharged in compliance with standards.

[0029] In step S2, the modified cation exchange membrane cleaning method includes the following steps:

[0030] a. Physical flushing: Stop the membrane separation operation and first back-flush the membrane module with modified cation exchange membrane installed with deionized water. The flushing pressure is 0.3-0.5 MPa, the flushing flow rate is 30-40 m³ / (m²·h), and the flushing time is 30-60 min.

[0031] b. Chemical cleaning: A mixed solution of hydrochloric acid with a concentration of 0.5-1.0 mol / L and thiourea with a concentration of 0.1-0.2 mol / L is introduced into the membrane module at a temperature of 40-50℃. The mixed solution circulates in the membrane module for 60-90 minutes, and the rinsing direction is switched every 15-20 minutes during the circulation process.

[0032] c. Alkali washing and neutralization: Rinse the membrane module with deionized water until the pH of the effluent is neutral, then pass in a 0.1-0.2 mol / L sodium hydroxide solution at a temperature of 30-40℃ for 30-40 min.

[0033] d. Activation and regeneration: After alkaline washing, rinse the membrane module with deionized water until the pH of the effluent is neutral. Then, pass a 0.5-1.0 mol / L sulfuric acid solution through the membrane module for activation treatment at a temperature of 30-40℃ for a circulation time of 30-40 min. Finally, rinse the membrane module with deionized water until the conductivity of the effluent is ≤10 μS / cm. The membrane module is then cleaned and put back into use.

[0034] The above-mentioned solutions address the problems of low purification efficiency, difficulty in achieving deep removal, easy introduction of impurities or silver loss, and poor environmental performance in existing silver electrolyte copper ion purification methods. Furthermore, considering the current situation where copper impurities in gold-silver alloys tend to accumulate in the electrolyte after electrochemical dissolution, requiring the discharge of some waste electrolyte when the copper concentration reaches 50-60 g / L, this invention provides a method for deep purification of copper ions in silver electrolyte. This method employs a customized special separation membrane combined with specific pretreatment and membrane separation processes to achieve efficient and precise separation of divalent copper ions and monovalent silver ions. This controls the copper ion concentration in the electrolyte within a reasonable range, reduces waste electrolyte generation, minimizes silver ion loss, ensures the recyclability of the electrolyte, reduces production costs, and is environmentally friendly, making it suitable for large-scale industrial applications. Detailed Implementation

[0035] The technical solution of the present invention will be further described in detail below with reference to the embodiments.

[0036] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows: A method for deep purification of copper ions in silver electrolyte, comprising four core steps: electrolyte pretreatment, membrane separation purification, membrane cleaning and regeneration, and reuse of the purified electrolyte, as detailed below:

[0037] 1 Electrolyte Pretreatment

[0038] The specific components of the silver electrolyte raw material include: Ag concentration of 80-150 g / L, HNO3 concentration of 2-8 g / L, and Cu < 50 g / L. Copper is the most abundant impurity in gold-silver alloys, and it accumulates in the electrolyte after electrochemical dissolution. The purpose of pretreatment is to remove suspended particulate matter, colloidal impurities, and some organic matter that easily causes membrane fouling from the electrolyte, thereby improving membrane separation efficiency and membrane lifespan. The specific steps are as follows:

[0039] (1) Filtration and impurity removal: The silver electrolyte raw material is passed into a precision filter for preliminary filtration. The precision filter uses a polypropylene pleated filter element with a filtration accuracy of 0.45 μm. The operating pressure is controlled at 0.1-0.2 MPa and the filtration flow rate is 10-15 m³ / (m²·h) to remove suspended particulate matter and large-diameter impurities in the electrolyte.

[0040] (2) Activated carbon adsorption: The electrolyte after preliminary filtration is passed into the adsorption column, which is filled with modified activated carbon adsorbent. The modified activated carbon is treated with nitric acid oxidation and has a specific surface area of ​​800-1000 m² / g. The adsorption temperature is controlled at 25-35℃ and the residence time of the electrolyte in the adsorption column is 30-60 min. The organic impurities and pigments in the electrolyte are removed by adsorption, avoiding the adsorption of organic matter on the membrane surface and causing membrane fouling.

[0041] (3) Ultrafiltration: The adsorbed and decolorized electrolyte is passed through an ultrafiltration membrane module for precision filtration. The ultrafiltration membrane is made of polyvinylidene fluoride (PVDF) with a molecular weight cutoff of 5000-10000 Da, an operating pressure of 0.2-0.3 MPa, and a filtration flow rate of 5-8 m³ / (m²·h). This further removes colloidal particles and residual fine impurities from the electrolyte, resulting in a pretreated electrolyte. Testing showed that the suspended particulate matter content in the pretreated electrolyte was ≤1 mg / L, and the organic matter content was ≤5 mg / L, meeting the feed requirements for subsequent membrane separation.

[0042] 2. Membrane separation purification

[0043] Membrane separation and purification is the core step of this invention. A customized special cation exchange membrane is used as the separation membrane, utilizing the difference in selective permeation of the membrane for Ag⁺ and Cu²⁺ to achieve efficient separation of the two. Specific process parameters and operating steps are as follows:

[0044] 2.1 Preparation process of special cation exchange membranes

[0045] The special cation exchange membrane used in this invention uses polyperfluorosulfonic acid resin as the substrate material. Aminophosphonic acid functional groups are introduced through grafting modification to improve the membrane's selective adsorption and permeation performance for Cu²⁺. The specific preparation process includes the following steps:

[0046] (1) Pretreatment of substrate material: Polyperfluorosulfonic acid resin with a molecular weight of 10,000-20,000 Da was selected as the substrate material and placed in a vacuum drying oven at 80-100℃ for 8-12 h to remove adsorbed moisture and volatile impurities from the resin, ensuring the uniformity of the subsequent dissolution process. After drying, the polyperfluorosulfonic acid resin was pulverized to a particle size of 100-200 mesh to facilitate rapid dissolution.

[0047] (2) Preparation of casting solution: The pretreated poly(perfluorosulfonic acid) resin was added to N-methylpyrrolidone solvent at a mass ratio of 1:8 to 1:12. The solution was placed in a constant temperature water bath and stirred at 60-80℃ for 2-4 h at a stirring rate of 300-500 r / min to completely dissolve the poly(perfluorosulfonic acid) resin and form a uniform resin solution. Then, aminophosphonic acid monomer (10%-20% of the mass of poly(perfluorosulfonic acid) resin), crosslinking agent (N,N'-methylenebisacrylamide, 2%-5% of the mass of poly(perfluorosulfonic acid) resin), and initiator (ammonium persulfate, 1%-3% of the mass of poly(perfluorosulfonic acid) resin) were added to the resin solution. The solution was stirred at 60-80℃ for 1-2 h to obtain a uniform and stable casting solution. Nitrogen gas was introduced during the stirring process to prevent the initiator from decomposing and becoming ineffective at high temperatures and the monomer from being oxidized.

[0048] (3) Coating and film formation: Select a polytetrafluoroethylene microporous membrane with a thickness of 50-80 μm as a support membrane, fix it on the stage of the coating machine, adjust the coating gap to 150-200 μm, and uniformly coat the prepared casting liquid on the surface of the polytetrafluoroethylene support membrane. The coating speed is controlled at 5-10 cm / s to ensure that the coating thickness is uniform and consistent, without defects such as bubbles and cracks.

[0049] (4) Drying and curing: The support film coated with casting solution is sent into a drying oven and dried by gradient heating: first, it is dried at 40-60℃ for 2-3 hours to remove some of the solvent in the casting solution; then the temperature is raised to 80-100℃ and dried for 4-6 hours to allow the solvent to evaporate completely and promote the tight bonding between the resin and the support film; finally, the temperature is raised to 120-150℃ and dried for 1-2 hours to achieve preliminary curing and enhance the mechanical strength of the film.

[0050] (5) Crosslinking reaction: The dried and pre-cured membrane is sent into a high-temperature reactor and heated to 160-180℃ under a nitrogen atmosphere. The temperature is maintained for 2-3 hours to allow the polyfluorosulfonic acid resin and aminophosphonic acid monomer to undergo a grafting crosslinking reaction, forming a stable crosslinking structure. This ensures that the functional groups are firmly bonded to the membrane matrix and prevents them from falling off during use.

[0051] (6) Hydrolysis activation: The cross-linked membrane is placed in a 0.5-1.0 mol / L sodium hydroxide solution and soaked at 40-60℃ for 2-4 h for hydrolysis treatment, so that the sulfonic acid groups and grafted aminophosphonic acid groups in the membrane are fully dissociated to form groups with ion exchange activity; after hydrolysis, the membrane is repeatedly rinsed with deionized water until the pH of the effluent is neutral, and then soaked in a 0.5-1.0 mol / L sulfuric acid solution for 1-2 h for acidification activation to further enhance the ion exchange activity of the membrane; finally, it is rinsed with deionized water until neutral to obtain the finished special cation exchange membrane.

[0052] The special cation exchange membrane obtained through the above preparation process has an ion exchange capacity of 1.2-1.5 mmol / g, a thickness of 80-120 μm, a selectivity coefficient for Cu²⁺ (relative to Ag⁺) ≥100, and exhibits excellent chemical stability and mechanical strength in an acidic nitric acid system with a pH of 3-3.5, and can operate stably for a long time.

[0053] 2.2 Assembly and Operation of Membrane Separation Unit

[0054] (1) Membrane separation device assembly: The membrane separation device adopts spiral wound membrane modules with an effective membrane area of ​​5-10 m². The device is equipped with a feed inlet, permeate outlet, concentrate outlet, pressure monitoring point, and temperature monitoring point. The prepared special cation exchange membrane is installed into the membrane module, ensuring that the membrane is installed flat and well sealed. Sealing gaskets are used to separate the membranes to avoid leakage. After assembly, the membrane module is tested for air tightness to ensure that there is no leakage before it is put into use.

[0055] (2) Membrane separation operation: The pretreated electrolyte is pumped into the membrane separation device. The feed pressure is adjusted to 0.5-1.0 MPa by a frequency converter pump, and the operating temperature is controlled to 30-40℃ by a constant temperature heating device. The feed flow rate is adjusted to 20-30 m³ / (m²·h). Under pressure, Cu²⁺ in the electrolyte specifically complexes with the functional groups on the membrane surface and permeates through a special cation exchange membrane to form permeate, while Ag⁺ is retained in the concentrate. During the separation process, the recovery rate of the permeate is adjusted by a flow control valve (controlled at 20-30%) to ensure that the Ag concentration in the concentrate is stable at 80-150 g / L, while ensuring the enrichment concentration of Cu²⁺ in the permeate, so that the copper ion concentration in the concentrate is controlled below 50 g / L.

[0056] (3) Multi-stage membrane separation optimization: To achieve deep purification of copper ions, the concentration of copper ions in the electrolyte is controlled below 50 g / L, and a two-stage membrane separation process is adopted. The concentrate after the first-stage membrane separation is used as the feed liquid for the second-stage membrane separation. The operating parameters of the second-stage membrane separation are the same as those of the first stage (feed pressure 0.5-1.0 MPa, temperature 30-40℃, feed flow rate 20-30 m³ / (m²·h)). The concentrate after the second-stage membrane separation is the silver electrolyte after deep purification of copper ions. After testing, its Cu²⁺ concentration is ≤50 g / L, which meets the requirements for high-purity silver electrolytic refining and can avoid the discharge of waste electrolyte due to excessive copper ion concentration. The permeate after the second-stage membrane separation is combined with the permeate after the first-stage membrane separation and enters the subsequent copper recovery process.

[0057] 3. Membrane cleaning and regeneration

[0058] As membrane separation operations continue, some impurity ions (such as Pb²⁺, Pd²⁺, etc.) and organic matter will adsorb onto the membrane surface or clog the membrane pores. Simultaneously, some Cu²⁺ may be firmly adsorbed onto aminophosphonic acid groups, leading to a decrease in membrane flux and a deterioration in separation performance. Therefore, it is necessary to periodically clean and regenerate the special cation exchange membrane to restore its separation performance. The specific steps are as follows:

[0059] (1) Physical rinsing: Stop the membrane separation operation and first back-rinse the membrane module with deionized water. The rinsing pressure is 0.3-0.5 MPa, the rinsing flow rate is 30-40 m³ / (m²·h), and the rinsing time is 30-60 min. The loose impurities attached to the membrane surface and some of the ions that are not firmly adsorbed are removed by the shearing action of the water flow.

[0060] (2) Chemical cleaning: The prepared chemical cleaning solution is introduced into the membrane module. The chemical cleaning solution is a mixture of 0.5-1.0 mol / L hydrochloric acid solution and 0.1-0.2 mol / L thiourea solution. The hydrochloric acid can dissolve metal oxide impurities on the membrane surface and in the membrane pores, while the thiourea can form stable complexes with heavy metal ions such as Cu²⁺ and Ag⁺ adsorbed on the membrane, thereby desorbing them from the membrane surface. The cleaning temperature is controlled at 40-50℃, and the circulation time of the cleaning solution in the membrane module is 60-90 min. During the circulation process, the rinsing direction is switched every 15-20 min to improve the cleaning effect.

[0061] (3) Alkaline washing and neutralization: After chemical cleaning, rinse the membrane module with deionized water until the pH of the effluent is neutral, and then pass in 0.1-0.2 mol / L sodium hydroxide solution for alkaline washing. The alkaline washing temperature is 30-40℃ and the circulation time is 30-40 min to neutralize the acidic substances remaining on the membrane surface. At the same time, sodium hydroxide can react with the organic matter remaining on the membrane surface to remove the residual organic impurities.

[0062] (4) Activation and regeneration: After alkaline washing, rinse the membrane module with deionized water until the effluent pH is neutral. Then, introduce 0.5-1.0 mol / L sulfuric acid solution for activation treatment at 30-40℃ for 30-40 min to fully dissociate the sulfonic acid and aminophosphonic acid groups on the membrane surface and restore the membrane's ion exchange activity. Finally, rinse the membrane module thoroughly with deionized water until the effluent conductivity is ≤10 μS / cm, at which point it can be put back into membrane separation operation. After cleaning and regeneration, the membrane flux can be restored to more than 90% of the initial flux, and the separation performance remains basically stable.

[0063] 4. Reuse of purified electrolyte and treatment of permeate

[0064] (1) Reuse of purified electrolyte: The concentrated solution after secondary membrane separation (purified electrolyte) is tested to have a Cu²⁺ concentration ≤50g / L, an Ag concentration of 80-150g / L, an HNO₃ concentration of 2-8g / L, and a pH value controlled at 3-3.5. All indicators meet the requirements of electrolyte for silver electrolytic refining. It is directly pumped into the electrolyte circulation system of the electrolytic refining workshop to realize the recycling of electrolyte, reduce the generation of waste electrolyte and the cost of replenishing new electrolyte.

[0065] (2) Permeate treatment: After the permeates from the primary and secondary membranes are combined, the Cu²⁺ concentration is enriched (usually 150-300 g / L), while containing a small amount of Ag⁺ (≤50 mg / L) and nitric acid. The permeate is further processed to recover copper resources. The specific steps are as follows: ① Silver recovery: Add excess sodium chloride solution to the permeate (the amount added is 1.2-1.5 times the theoretical reaction amount), stir for 30-60 min to form silver chloride precipitate of Ag⁺, filter to separate the silver chloride precipitate, and recover the silver resources after washing and drying; ② Copper recovery: Add sodium hydroxide solution to the permeate after silver removal, adjust the pH value to 8-9, stir for 60-90 min to form copper hydroxide precipitate of Cu²⁺, filter to separate the copper hydroxide precipitate; add hydrochloric acid to dissolve the copper hydroxide precipitate, control the amount of hydrochloric acid added to make the pH value of the solution 1-2, and obtain copper chloride solution; evaporate and concentrate the copper chloride solution (temperature 100-120℃), cool and crystallize (temperature 0-10℃), centrifuge and dry to obtain copper chloride crystal product; ③ Wastewater treatment: The filtrate after copper recovery contains a small amount of sodium chloride and sodium nitrate. Add sulfuric acid to adjust the pH value to 6-7, remove residual organic matter by activated carbon adsorption, and discharge in compliance with standards.

[0066] Beneficial effects

[0067] Compared with existing methods for purifying copper ions in silver electrolytes, this invention has the following significant advantages:

[0068] 1. High purification efficiency and precise control: This invention employs a customized special cation exchange membrane combined with a two-stage membrane separation process. Utilizing the high selectivity of the membrane for Ag⁺ and Cu²⁺, the Cu²⁺ concentration in the silver electrolyte can be controlled from a high level of nearly 50-60 g / L to below 50 g / L. This high purification control precision effectively avoids the discharge of waste electrolyte due to excessively high copper ion concentration, ensuring the circulation stability of the electrolyte. This is far superior to existing chemical precipitation and displacement methods, meeting the stringent purity requirements of high-purity silver (purity ≥99.99%) electrolytic refining.

[0069] 2. Low silver ion loss and high resource utilization: The special cation exchange membrane used in this invention has excellent Ag⁺ retention performance. The Ag⁺ retention rate during membrane separation is ≥99.9%, and the Ag concentration in the electrolyte after purification remains basically stable, avoiding silver loss caused by the reaction of silver ions with the replacement agent in existing processes such as replacement methods. At the same time, the small amount of Ag⁺ in the membrane separation permeate is specifically recovered, further improving the utilization rate of silver resources. The total silver recovery rate is ≥99.95%, reducing production costs.

[0070] 3. Green and environmentally friendly process with no secondary pollution: This invention uses membrane separation technology, and the entire purification process does not require the addition of any chemical precipitants, displacement agents or extractants, thus avoiding secondary pollution caused by the use of chemical reagents; at the same time, copper resources can be recovered after the permeate is treated to obtain copper chloride crystal by-products, and the filtrate is neutralized and discharged in compliance with standards, realizing the resource utilization of waste and conforming to the development concept of green metallurgy.

[0071] 4. Simple operation, good stability, and suitable for industrial-scale application: The process steps of this invention are clear, and operations such as pretreatment, membrane separation, and membrane cleaning and regeneration are easy to automate. The special cation exchange membrane used is custom-prepared and has excellent chemical stability and mechanical strength in an acidic nitric acid system with a pH of 3-3.5. It can be reused after cleaning and regeneration, and the membrane has a service life of ≥1 year. The entire process operates under mild conditions (temperature 30-40℃, pressure 0.5-1.0MPa) and has low energy consumption (energy consumption for purifying 1 ton of electrolyte ≤50 kWh), making it easy to promote and apply on an industrial scale.

[0072] 5. The electrolyte can be recycled, reducing production costs: After purification, all indicators of the electrolyte meet the requirements for use in silver electrolytic refining and can be directly reused in the electrolytic recycling system, reducing the amount of new electrolyte replenishment (replenishment amount reduced by 60-80%), and lowering the cost of electrolyte preparation and raw material consumption; at the same time, the recovered copper resources can be sold as by-products, further improving the economic benefits of the process.

[0073] Example 1

[0074] This embodiment uses the circulating electrolyte from a silver electrolytic refining workshop as raw material. It consists of an aqueous solution of AgNO3 and HNO3, with the following specific components: Ag concentration 120 g / L, Cu²⁺ concentration 55 g / L, HNO3 concentration 5 g / L, Pb concentration 0.4 g / L, sulfate 0.08 g / L, Pd concentration 8 mg / L, HNO2 concentration 7 mg / L, and pH value 3.2. The method of this invention is used for deep purification of copper ions, and the specific steps are as follows:

[0075] (1) Pretreatment: The electrolyte raw material was passed through a precision filter (polypropylene pleated filter element, filtration accuracy 0.45 μm), with an operating pressure of 0.15 MPa and a filtration flow rate of 12 m³ / (m²·h) to remove suspended particulate matter; then it was passed through an adsorption column filled with modified activated carbon (specific surface area 900 m² / g), with an adsorption temperature of 30℃ and a residence time of 45 min; then it was passed through an ultrafiltration membrane module (PVDF material, molecular weight cutoff 8000 Da), with an operating pressure of 0.25 MPa and a filtration flow rate of 6 m³ / (m²·h) to obtain the pretreated electrolyte, and its suspended particulate matter content was 0.8 mg / L and organic matter content was 3 mg / L.

[0076] (2) Membrane separation and purification: ① Preparation of special cation exchange membrane: Polyperfluorosulfonic acid resin with a molecular weight of 15000 Da was selected, vacuum dried at 80℃ for 10 h, and then pulverized to 150 mesh; the resin and N-methylpyrrolidone were mixed at a mass ratio of 1:10, stirred at 70℃ for 3 h to dissolve, and 15% aminophosphonic acid monomer, 3% N,N'-methylenebisacrylamide and 2% ammonium persulfate were added. The mixture was stirred at 70℃ for 1.5 h under nitrogen protection to obtain the casting solution; the casting solution was coated on a 60 μm thick polytetrafluoroethylene support membrane at a coating speed of 8 cm / s and a coating gap of 180 μm; gradient temperature drying: 50℃ for 2.5 h → 90℃ for 5 h → 130℃ for 1.5 h; crosslinking at 170℃ under nitrogen atmosphere for 2.5 h; hydrolysis by soaking in 0.8 mol / L sodium hydroxide solution at 50℃ for 3 h, rinsing with deionized water until neutral, 0.8 The electrolyte was activated by soaking in mol / L sulfuric acid solution for 1.5 h, and then rinsed with deionized water until neutral to obtain a special cation exchange membrane (ion exchange capacity 1.3 mmol / g, thickness 100 μm). ② Membrane separation operation: The pretreated electrolyte was pumped into a spiral wound membrane module (effective membrane area 8 m²), with the feed pressure controlled at 0.8 MPa, operating temperature at 35℃, feed flow rate at 25 m³ / (m²·h), and permeate recovery rate at 25%, for primary membrane separation. The primary membrane separation concentrate was used as the feed liquid for secondary membrane separation, with the same operating parameters as the primary separation. The secondary membrane separation concentrate was the purified electrolyte, with a Cu²⁺ concentration of 38 g / L, an Ag concentration of 118 g / L, an Ag⁺ rejection rate of 99.92%, and a pH value of 3.3. All indicators met the requirements for electrolytic refining.

[0077] (3) Membrane cleaning and regeneration: After 24 h of continuous membrane separation operation, the membrane flux decreased from the initial 8 m³ / (m²·h) to 5 m³ / (m²·h). Cleaning and regeneration were then performed: First, deionized water was used for backwashing at a pressure of 0.4 MPa, a flow rate of 35 m³ / (m²·h), and a time of 45 min; then, a mixed cleaning solution of 0.8 mol / L hydrochloric acid and 0.15 mol / L thiourea was introduced at a temperature of 45℃ for a circulation time of 75 min; subsequently, deionized water was used for rinsing until neutral, followed by alkaline washing with 0.15 mol / L sodium hydroxide solution at a temperature of 35℃ for a circulation time of 35 min; finally, 0.8 mol / L sulfuric acid solution was used for activation at a temperature of 35℃ for a circulation time of 35 min. After rinsing with deionized water, the membrane flux recovered to 7.8 m³ / (m²·h).

[0078] (4) Reuse of purified electrolyte and treatment of permeate: The purified electrolyte is directly pumped into the electrolysis circulation system without the need to discharge waste electrolyte; the primary and secondary permeates are combined (Cu²⁺ concentration enriched to 180 g / L, Ag⁺ concentration 45 mg / L), an excess sodium chloride solution (1.3 times the theoretical amount) is added first, the reaction is stirred for 45 min, and the silver chloride precipitate is separated by filtration to recover silver; then sodium hydroxide solution is added to adjust the pH value to 8.5, the reaction is stirred for 75 min, and the copper hydroxide precipitate is obtained by filtration; the precipitate is dissolved in hydrochloric acid to pH value 1.5 to obtain copper chloride solution; the copper chloride solution is evaporated and concentrated at 110℃, cooled and crystallized at 5℃, centrifuged and dried to obtain copper chloride crystal product (purity ≥99.5%); the filtrate is neutralized to pH value 6.5 and discharged in compliance with standards.

[0079] The copper ion purification effect of this embodiment is significant, reducing the copper ion concentration in the electrolyte from 55 g / L to 38 g / L, and controlling it below 50 g / L, thus avoiding the generation of waste electrolyte. The silver ion loss rate is only 0.08%, and the purified electrolyte meets the requirements of high-purity silver electrolytic refining. The process is stable and reliable.

[0080] Example 2

[0081] The silver electrolyte used in this embodiment consists of an aqueous solution of AgNO3 and HNO3, with the following specific components: Ag concentration 90 g / L, Cu²⁺ concentration 42 g / L, HNO3 concentration 3 g / L, Pb concentration 0.2 g / L, sulfate concentration 0.06 g / L, Pd concentration 6 mg / L, HNO2 concentration 6 mg / L, and pH value 3.1. The purification steps are as follows:

[0082] (1) Electrolyte pretreatment: filtration to remove impurities (pressure 0.1 MPa, flow rate 10 m³ / (m²·h)) → activated carbon adsorption (temperature 25℃, residence time 30 min) → ultrafiltration (pressure 0.2 MPa, flow rate 5 m³ / (m²·h)). After pretreatment, the electrolyte has a suspended particulate matter content of 0.6 mg / L and an organic matter content of 2 mg / L.

[0083] (2) Membrane separation and purification: The special cation exchange membrane (ion exchange capacity 1.2 mmol / g, thickness 80 μm) prepared in Example 1 was used, and the effective membrane area of ​​the membrane module was 5 m². The primary membrane separation parameters were: pressure 0.5 MPa, temperature 30℃, flow rate 20 m³ / (m²·h), and recovery rate 20%. The secondary membrane separation parameters were the same. After purification, the Cu²⁺ concentration of the electrolyte was 29 g / L, the Ag concentration was 88 g / L, the Ag⁺ rejection rate was 99.93%, and the pH value was 3.2, which met the requirements of electrolytic refining.

[0084] (3) Membrane cleaning and regeneration: After 30 hours of operation, the membrane flux decreased. After physical flushing, chemical cleaning, alkaline washing and activation, the membrane flux recovered to 92% of the initial value.

[0085] (4) Permeate treatment: Silver chloride and copper chloride products are recovered according to the method in Example 1 (copper chloride purity ≥ 99.4%), and the filtrate is discharged in compliance with the standards.

[0086] In this embodiment, the copper ion purification effect is excellent, reducing the copper ion concentration in the electrolyte from 42 g / L to 29 g / L and controlling it below 50 g / L. The silver loss rate is 0.07%, and there is no need to discharge waste electrolyte, which significantly improves the electrolyte recycling efficiency.

[0087] Example 3

[0088] The silver electrolyte used in this embodiment consists of an aqueous solution of AgNO3 and HNO3, with the following specific components: Ag concentration 140 g / L, Cu²⁺ concentration 58 g / L, HNO3 concentration 7 g / L, Pb concentration 0.5 g / L, sulfate concentration 0.09 g / L, Pd concentration 9 mg / L, HNO2 concentration 9 mg / L, and pH value 3.4. The purification steps are as follows:

[0089] (1) Electrolyte pretreatment: filtration to remove impurities (pressure 0.2 MPa, flow rate 15 m³ / (m²·h)) → activated carbon adsorption (temperature 35℃, residence time 60 min) → ultrafiltration (pressure 0.3 MPa, flow rate 8 m³ / (m²·h)). After pretreatment, the electrolyte has a suspended particulate matter content of 0.9 mg / L and an organic matter content of 4 mg / L.

[0090] (2) Membrane separation and purification: The special cation exchange membrane (ion exchange capacity 1.5 mmol / g, thickness 120 μm) prepared in Example 1 was used, and the effective membrane area of ​​the membrane module was 10 m². The primary membrane separation parameters were: pressure 1.0 MPa, temperature 40℃, flow rate 30 m³ / (m²·h), and recovery rate 30%. The secondary membrane separation parameters were the same. After purification, the Cu²⁺ concentration of the electrolyte was 41 g / L, the Ag concentration was 138 g / L, the Ag⁺ rejection rate was 99.91%, and the pH value was 3.4, which met the requirements of electrolytic refining.

[0091] (3) Membrane cleaning and regeneration: After 20 hours of operation, the flux of the membrane decreased. After cleaning and regeneration, the flux was restored to 91% of the initial value.

[0092] (4) Permeate treatment: Silver chloride and copper chloride products are recovered according to the method in Example 1 (copper chloride purity ≥ 99.6%), and the filtrate is discharged in compliance with the standards.

[0093] This embodiment successfully reduced the copper ion concentration in the electrolyte from 58 g / L to 41 g / L, controlling it below 50 g / L, thus avoiding the generation of waste electrolyte. The copper ion purification and control effect was excellent, with a silver loss rate of 0.09%, achieving stable purification and recycling of high-concentration copper ion electrolyte.

[0094] Compared with the prior art, the advantages of the present invention are:

[0095] 1. Design and fabrication of customized specialty cation exchange membranes: Targeting the ionic characteristics of silver electrolyte (AgNO3, HNO3 aqueous system, pH 3-3.5) and Ag⁺ and Cu²⁺, a specialty cation exchange membrane based on polyperfluorosulfonic acid resin and grafted with aminophosphonic acid groups was designed and fabricated. By precisely controlling the fabrication process parameters, the membrane possesses both excellent ion size sieving performance and specific complexing performance of functional groups. It can efficiently control the copper ion concentration below 50 g / L, while ensuring the stability of the membrane in acidic environments, thus solving the problems of poor selectivity and insufficient corrosion resistance of conventional membrane materials.

[0096] 2. Construction of a Synergistic Separation Mechanism: A synergistic separation mechanism based on the specific complexation of functional groups was employed, overcoming the limitations of single sieving or single complexation separation. Cu²⁺, due to its high charge density, forms a larger hydration radius, but through stable complexation with aminophosphonic acid groups, it achieves hydration layer stripping, reduces its effective size, and migrates rapidly. Although Ag⁺ has a smaller hydration radius, its weak complexation prevents hydration layer stripping, and it is retained due to the dual obstacles of Donnan potential and membrane steric hindrance. This significantly improves the separation selectivity of Ag⁺ and Cu²⁺, achieving an Ag⁺ retention rate ≥99.9% and a Cu²⁺ permeation rate ≥85%, realizing deep purification and precise control of high-concentration copper ion electrolytes.

[0097] 3. Multi-stage membrane separation and pretreatment synergistic process: The three-stage pretreatment process of "precision filtration + activated carbon adsorption + ultrafiltration" effectively removes suspended particulate matter, colloids and organic impurities in the electrolyte, avoiding membrane fouling and ensuring membrane separation efficiency. Combined with a two-stage membrane separation series process, the copper ion concentration can be precisely controlled from a high level of nearly 50~60g / L to below 50g / L, avoiding the generation of waste electrolyte and solving the problem that it is difficult to achieve precise control of copper ions with a single membrane separation process.

[0098] 4. Optimization of efficient membrane cleaning and regeneration process: Targeting the main causes of membrane fouling, a combined cleaning process of "physical flushing + acidic complexing cleaning + alkaline washing and neutralization + activation regeneration" was designed. This process can efficiently remove metal impurities and organic matter from the membrane surface and membrane pores, with a membrane flux recovery rate of ≥90%, extending the service life of the membrane (≥1 year) and reducing the replacement cost of membrane materials.

[0099] In summary, the deep purification method for copper ions in silver electrolyte provided by this invention, through the synergistic effect of customized membrane materials and optimized processes, can accurately control the copper ion concentration to below 50 g / L, avoiding the generation of waste electrolyte. It has the advantages of good purification control effect, low silver loss, green and environmentally friendly process, and economic feasibility. It effectively solves the problems existing in the prior art and the industry pain point of excessive accumulation of copper ions in electrolyte requiring the discharge of waste electrolyte. It provides technical guarantee for the stable operation of high-purity silver electrolytic refining process and has broad industrial application prospects.

Claims

1. A modified cation exchange membrane, characterized in that: Using polyperfluorosulfonic acid resin as the base material, graft modification with an element containing aminophosphonic acid groups was carried out to obtain a modified cation exchange membrane.

2. A method for preparing the modified cation exchange membrane according to claim 1, comprising the following steps: (1) Preparation of casting solution: Polyperfluorosulfonic acid resin with a mass ratio of 1:8-1:12 is mixed with N-methylpyrrolidone solvent and stirred at 60-80℃ to obtain resin solution. Aminophosphonic acid monomer, N,N'-methylenebisacrylamide and ammonium persulfate are added to resin solution and stirred at 60-80℃ to obtain casting solution. (2) Coating and film formation: Select a polytetrafluoroethylene microporous membrane with a thickness of 50-80 μm as the support membrane, and uniformly coat the prepared casting solution on the surface of the polytetrafluoroethylene support membrane. (3) Drying, curing and crosslinking: The support film coated with casting solution is sent into a drying oven and dried by gradient heating: dry at 40-60℃ for 2-3 h; then heat to 80-100℃ and dry for 4-6 h; finally heat to 120-150℃ and dry for 1-2 h; transfer the film to a high-temperature reactor, heat to 160-180℃ under nitrogen atmosphere and keep warm for 2-3 h to obtain the crosslinked film; (4) Hydrolysis activation: The cross-linked membrane is placed in a 0.5-1.0 mol / L sodium hydroxide solution and soaked at 40-60℃ for 2-4 h. Then, the membrane is repeatedly washed with deionized water until the pH of the effluent is neutral. Then, it is placed in a 0.5-1.0 mol / L sulfuric acid solution and soaked for 1-2 h. Finally, it is rinsed with deionized water until neutral to obtain the modified cation exchange membrane.

3. The method for preparing the modified cation exchange membrane according to claim 2, characterized in that: In step (1), the polyfluorosulfonic acid resin is pretreated as follows before preparing the casting solution: a polyfluorosulfonic acid resin with a molecular weight of 10,000-20,000 Da is selected as the base material, placed in a vacuum drying oven, dried at 80-100℃ for 8-12 h, and then pulverized to a particle size of 100-200 mesh to obtain the pretreated polyfluorosulfonic acid resin.

4. The method for preparing the modified cation exchange membrane according to claim 2, characterized in that: In step (1), the amount of aminophosphonic acid monomer added is 10%-20% of the mass of polyfluorosulfonic acid resin; the amount of N,N'-methylenebisacrylamide added is 2%-5% of the mass of polyfluorosulfonic acid resin; and the amount of ammonium persulfate added is 1%-3% of the mass of polyfluorosulfonic acid resin.

5. The method for preparing the modified cation exchange membrane according to claim 2, characterized in that: In step (1), nitrogen gas is introduced for protection during the stirring and dissolution process at a temperature of 60-80℃.

6. The method for preparing the modified cation exchange membrane according to claim 2, characterized in that: In step (2), a polytetrafluoroethylene microporous membrane with a thickness of 50-80 μm is selected as the support membrane, and it is fixed on the stage of the coating machine. The coating gap is adjusted to 150-200 μm, and the casting liquid is uniformly coated on the surface of the polytetrafluoroethylene support membrane at a coating speed of 5-10 cm / s.

7. A method for deep purification of copper ions in silver electrolyte, comprising the following steps: S1. Physical impurity removal: The silver electrolyte to be treated is sequentially filtered for impurity removal, activated carbon adsorption, and ultrafiltration to remove solid impurities, resulting in treated solution A. S2. Membrane separation: The treatment liquid A is separated by the modified cation exchange membrane according to any one of claims 1-6 to obtain permeate and concentrate. The concentrate is purified silver electrolyte and is directly fed into the electrolyte storage tank in the silver electrolysis refining workshop. The permeate is subjected to copper recovery treatment.

8. The method for deep purification of copper ions in silver electrolyte according to claim 7, characterized in that: In step S1, the filtration and impurity removal involves passing the silver electrolyte to be treated through a precision filter. The precision filter includes a polypropylene pleated filter element, with a filtration accuracy of 0.45 μm, a filtration pressure controlled at 0.1-0.2 MPa, and a filtration flow rate of 10-15 m³ / (m²·h).

9. The method for deep purification of copper ions in silver electrolyte according to claim 7, characterized in that: In step S1, activated carbon adsorption involves passing the filtered and impurity-removed silver electrolyte into an adsorption column filled with activated carbon modified by nitric acid oxidation. The specific surface area is 800-1000 m² / g, the adsorption temperature is controlled at 25-35℃, and the residence time of the silver electrolyte in the adsorption column is 30-60 min.

10. The method for deep purification of copper ions in silver electrolyte according to claim 7, characterized in that: In step S1, ultrafiltration involves passing the silver electrolyte adsorbed by activated carbon through an ultrafiltration membrane module made of polyvinylidene fluoride for precision filtration. The ultrafiltration membrane has a molecular weight cutoff of 5000-10000 Da, an operating pressure of 0.2-0.3 MPa, and a filtration flow rate of 5-8 m³ / (m²·h).

11. The method for deep purification of copper ions in silver electrolyte according to claim 7, characterized in that: In step S2, the copper recovery process using the permeate is as follows: ① Silver recovery: Add sodium chloride solution to the permeate, stir and react for 30-60 min, and separate the solid and liquid to obtain silver chloride precipitate and silver-removed liquid; ② Copper recovery: Add sodium hydroxide solution to the silver-removed solution to adjust the pH value to 8-9, stir the reaction for 60-90 min, and separate the solid and liquid to obtain copper hydroxide precipitate and copper-removed solution; add hydrochloric acid to the copper hydroxide precipitate to dissolve it, and control the amount of hydrochloric acid added to keep the pH value of the solution at 1-2 to obtain copper chloride solution; The copper chloride solution is concentrated by evaporation, cooled and crystallized, separated by centrifugation and dried to obtain copper chloride crystal product.

12. The method for deep purification of copper ions in silver electrolyte according to claim 11, characterized in that: In step ①, the amount of sodium chloride added is 1.2-1.5 times the theoretical reaction amount.

13. The method for deep purification of copper ions in silver electrolyte according to claim 11, characterized in that: In step ②, the copper chloride solution is evaporated and concentrated at 100-120°C and then cooled and crystallized at 0-10°C.

14. The method for deep purification of copper ions in silver electrolyte according to claim 11, characterized in that: In step ②, sulfuric acid is added to the copper-removed liquid to adjust the pH value to 6-7. After the residual organic matter is removed by activated carbon adsorption, the liquid meets the emission standards.

15. The method for deep purification of copper ions in silver electrolyte according to claim 7, characterized in that: In step S2, the modified cation exchange membrane cleaning method includes the following steps: a. Physical flushing: Stop the membrane separation operation and first back-flush the membrane module with modified cation exchange membrane installed with deionized water. The flushing pressure is 0.3-0.5 MPa, the flushing flow rate is 30-40 m³ / (m²·h), and the flushing time is 30-60 min. b. Chemical cleaning: A mixed solution of hydrochloric acid with a concentration of 0.5-1.0 mol / L and thiourea with a concentration of 0.1-0.2 mol / L is introduced into the membrane module at a temperature of 40-50℃. The mixed solution circulates in the membrane module for 60-90 minutes, and the rinsing direction is switched every 15-20 minutes during the circulation process. c. Alkali washing and neutralization: Rinse the membrane module with deionized water until the pH of the effluent is neutral, then pass in a 0.1-0.2 mol / L sodium hydroxide solution at a temperature of 30-40℃ for 30-40 min. d. Activation and regeneration: After alkaline washing, rinse the membrane module with deionized water until the pH of the effluent is neutral. Then, pass a 0.5-1.0 mol / L sulfuric acid solution through the membrane module for activation treatment at a temperature of 30-40℃ for a circulation time of 30-40 min. Finally, rinse the membrane module with deionized water until the conductivity of the effluent is ≤10 μS / cm. The membrane module is then cleaned and put back into use.