Preparation method of 5N-grade high-purity silver

Abstract

The invention discloses a preparation method of 5N-grade high-purity silver. The preparation method comprises the following steps that 1, a concentrated HNO3 solution with the molar concentration being 14-18 mol / L is prepared; 2, turning the coarse silver, wetting the coarse silver with pure water, and adding the coarse silver into the HNO3 solution obtained in the step 1; 3, heating the mixture obtained in the step 2, keeping the temperature, and carrying out chemical reaction; 4, adding a certain amount of pure water into the silver nitrate solution obtained in the step 3; 5, concentrated nitric acid is added into the silver nitrate solution diluted in the step 4; step 6, preparing an electrolytic bath, an anode plate, a cathode plate and a conductive rod; 7, preparing an electrolyte; 8, after the electrolytic cell is installed, the electrolyte is introduced into the electrolytic cell, and finally direct current is introduced; ninthly, after electrolysis is completed, the negative plate is taken out, and silver on the negative plate is stripped; and tenthly, the silver obtained in the ninth step is dried, and 5N high-purity silver is obtained. According to the preparation method, the high-purity silver preparation process can be shortened, and energy consumption is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of high-purity noble metal preparation, and particularly to a method for preparing 5N grade high-purity silver. Background Art

[0002] Due to its excellent electrical conductivity, thermal conductivity, and oxidation resistance, high-purity silver has important applications in various fields such as electronics, photovoltaics, medical, jewelry, chemical industry, photography, energy, scientific research, aerospace, and environmental protection. Especially in semiconductor manufacturing, high-purity silver is mainly used for interconnect materials, especially the interconnect wires inside the chip, to ensure efficient signal transmission; as electrode materials for electrodes of components such as transistors and capacitors to improve electrical conductivity; also used for welding chips to substrates to ensure low resistance and high reliability; as bonding wire materials to connect chips to external circuits. In the photovoltaic industry, high-purity silver is mainly used for electrode materials, conductive pastes, conductive films, etc.

[0003] The preparation of high-purity silver requires multiple steps of purification processes to remove impurities (such as copper, lead, iron, sulfur, etc.).

[0004] The electrolytic refining method is the mainstream industrial method. By electrolytically purifying crude silver (such as silver ingots or silver-containing waste materials), the purity can reach 4 - 5N. The general steps are to convert crude silver (such as silver anode plates) into silver nitrate solution through acid dissolution (nitric acid or sulfuric acid), and filter to remove insoluble impurities. In the electrolytic cell configuration, the anode uses a crude silver plate; the cathode uses a pure silver sheet or a stainless steel plate, and the electrolyte is a silver nitrate solution. Pass direct current (voltage 0.3 - 1.5V), the crude silver at the anode dissolves, and silver ions (Ag⁺) migrate to the cathode and deposit in the form of high-purity silver crystals. The product can stably reach above 4N. Purifying silver through chemical precipitation or reduction reactions is suitable for small-batch or laboratory preparation. First, dissolve crude silver in nitric acid to form a silver nitrate solution; add hydrochloric acid or sodium chloride to precipitate silver chloride (AgCl), filter to remove insoluble impurities, then use a reducing agent (such as ascorbic acid, formaldehyde, hydrazine hydrate) to reduce the silver nitrate solution, and finally collect the silver precipitate, wash repeatedly to remove residual reagents, and melt into silver ingots at high temperature after drying. The product purity is affected by raw materials and reducing agents, usually 3N - 4N.

[0005] The zone melting method makes the impurities in the metal ingot migrate directionally through local heating, suitable for preparing ultra-high-purity silver (purity greater than 5N). First, the silver raw material needs to be purified to above 4N through electrolysis or chemical methods, and then place the silver rod in an environment protected by an inert gas (such as argon), and locally heat it with a high-frequency induction coil to form a molten zone. Slowly move the molten zone (speed about 1 - 10 cm / h), and the impurities will accumulate at the end of the molten zone due to the segregation effect. After repeated melting several times, cut off the end with concentrated impurities and retain the high-purity main part. The zone melting method can prepare ultra-high-purity silver with a purity greater than 5N, but the equipment is complex, the cost is extremely high, the efficiency is low, and it is only suitable for small-batch production.

[0006] The vacuum distillation method utilizes the vapor pressure difference between silver and impurity metals to separate and purify under vacuum conditions. The crude silver is placed in a vacuum furnace (pressure ≤ 10⁻³ Pa) and heated to melting (the melting point of silver is 961.8 °C); low-boiling impurities (such as zinc and lead) evaporate preferentially, and the silver vapor condenses to form high-purity silver. The condenser collects the distilled silver, and the impurities remain in the furnace. This method can only remove low-boiling impurities, and the product purity reaches above 4N. Other auxiliary purification technologies include ion exchange method and solvent extraction method, etc.

[0007] Patent CN105297074 discloses a method for preparing high-purity silver. This invention relates to an electrolytic refining method for high-purity silver in the laboratory. The steps are that analytical pure silver nitrate is prepared into high-purity silver nitrate through chemical treatment and electrolysis coupling, and then high-purity elemental silver is prepared through stepwise electrolysis; ultrasonic waves and silver oxide are used to adjust the pH of the silver nitrate solution, ultraviolet radiation is used to reduce Pt precious metal ions, and alumina column filtration and concentration are used to obtain a high-purity silver nitrate solution, and electrolysis is carried out using Pt electrodes to obtain 99.999% high-purity silver. The defects of this method are long preparation time, high cost, and it is only limited to laboratory preparation.

[0008] Patent CN106480476 discloses a method for producing high-purity silver from alloyed silver. This invention includes five steps: casting the first silver anode plate, the first silver electrolysis, casting the second silver anode plate, the second silver electrolysis, and treating silver waste liquid, and it belongs to a treatment method for high-lead alloyed silver; the high lead in the electrolyte is precipitated by sulfuric acid under heating, and the electrolytic waste liquid is treated by chlorination, dried and then smelted. The 99.997 high-purity silver produced by this invention through the second silver electrolysis meets the requirements of No. 1 silver in the national standard "GB / T 4135-2002". The defects of this method are large dosage of chemicals, complex technological steps, high cost, and the purity of silver does not reach the 5N level.

[0009] The preparation of 5N high-purity silver by electrolytic refining method requires the raw material purity to be above 99.9%. For low-purity crude silver, it is impossible to directly electrolyze to prepare 5N high-purity silver by electrolytic refining method, and multiple electrolyses are required, which will greatly increase the production cost and extend the production cycle. Therefore, it is necessary to develop a new method for directly electrolyzing low-purity crude silver (<99.9%) to prepare 5N high-purity silver in one step. Summary of the Invention

[0010] The purpose of the present invention is to provide a method for preparing 5N high-purity silver in view of the above situation, which can shorten the preparation process of high-purity silver and reduce energy consumption.

[0011] The specific scheme of the present invention is: a method for preparing 5N high-purity silver, including the following steps: Step 1: Prepare a concentrated HNO3 solution with a molar concentration of 14 - 18 mol / L; Step 2: Chip the crude silver, moisten it with pure water, and add it to the HNO3 solution obtained in Step 1; Step 3: Heat the mixture obtained in Step 2, keep it warm and carry out a chemical reaction. After the reaction is completed, cool the reactant to room temperature, let it stand for 2 - 3 hours and then filter. After filtration, a silver nitrate solution is obtained; Step 4: Add a certain amount of pure water to the silver nitrate solution obtained in Step 3; Step 5: Add concentrated nitric acid to the silver nitrate solution diluted in Step 4, and control the HNO3 concentration in the solution to be greater than 8 g / L; Step 6: Prepare an electrolytic cell, an anode plate, a cathode plate and a conductive rod. The material of the electrolytic cell is polymethyl methacrylate, the anode plate is a ruthenium - iridium coated titanium - based anode, and the cathode plate and the conductive rod are 99.9% pure titanium cathodes; Step 7: Prepare the electrolyte. The silver electrolyte is the solution in Step 5, and the silver ion concentration is 80 - 120 g / L; Step 8: After installing the electrolytic cell, introduce the electrolyte into the electrolytic cell until it covers the cathode plate and the anode plate. Finally, pass a direct current, and the current density is 100 - 200 A / m 2 , during the electrolysis process, the solution in the electrolytic cell is continuously circulated to ensure uniform concentration; Step 9: After electrolysis is completed, take out the cathode plate, strip the silver on the cathode plate, rinse it thoroughly with deionized water at 90 - 100 °C, and then boil it in deionized water for 30 min, repeat 2 - 3 times; Step 10: Dry the silver obtained in Step 9 to obtain 5N high - purity silver.

[0012] Further, in the present invention, the molar ratio of nitric acid addition to crude silver in Step 2 is 0.6 - 0.9:1.

[0013] Further, in the present invention, the silver ion concentration in the silver solution in Step 5 is 100 - 120 g / L, and the HNO3 concentration is greater than 8 g / L.

[0014] Further, in the present invention, the electrolysis current density in Step 8 is 100 - 150 A / m 2 , and the electrode spacing is 20 - 50 mm.

[0015] Further, in the present invention, the electrolyte is circulated by a peristaltic pump at a fixed flow rate in Step 8 to maintain the uniformity of the silver ion concentration in the electrolyte during the electrolysis process.

[0016] Further, the pure water used in Step 2 of the present invention controls the pH to be greater than 5.

[0017] Compared with the existing electrolytic refining technology, this technology can dissolve and filter silver scraps with a purity of 99%, and then electrolytically prepare high-purity silver with a purity of 99.999% in one step. This method can shorten the preparation process of high-purity silver, reduce energy consumption and costs. The prepared 5N high-purity silver can be used in cutting-edge technology fields such as integrated circuits, aerospace, and semiconductor chips. Detailed implementation mode

[0018] The technical solution of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of them. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative work belong to the protection scope of the present invention.

[0019] The present invention is a method for preparing 5N high-purity silver, which includes the following steps: Step 1: Prepare a concentrated HNO3 solution with a molar concentration of 14-18 mol / L. Step 2: After turning the coarse silver into shavings, moisten it with pure water and add it to the HNO3 solution obtained in Step 1. Further, in the present invention, the molar ratio of nitric acid addition to coarse silver is 0.6-0.9:1. Further, the pH of the pure water used in the present invention is controlled to be greater than 5. Step 3: Heat the mixture obtained in Step 2, keep it warm and carry out a chemical reaction. After the reaction is completed, cool the reactant to room temperature and let it stand for 2-3 hours, then filter. After filtration, a silver nitrate solution is obtained. Step 4: Add a certain amount of pure water to the silver nitrate solution obtained in Step 3. Step 5: Add concentrated nitric acid to the diluted silver nitrate solution in Step 4 to control the HNO3 concentration in the solution to be greater than 8 g / L. Further, in the present invention, the silver ion concentration in the silver solution is 100-120 g / L, and the HNO3 concentration is greater than 8 g / L. Step 6: Prepare an electrolytic cell, an anode plate, a cathode plate and a conducting rod. The material of the electrolytic cell is polymethyl methacrylate, the anode plate is a ruthenium-iridium coated titanium-based anode, and the cathode plate and the conducting rod are 99.9% pure titanium cathodes. Step 7: Prepare the electrolyte. The silver electrolyte is the solution in Step 5, and the silver ion concentration is 80-120 g / L. Step 8: After installing the electrolytic cell, introduce the electrolyte into the electrolytic cell until it covers the cathode plate and the anode plate, and finally apply direct current with a current density of 100-200 A / m 2 , during the electrolysis process, the solution in the electrolytic cell continues to circulate to ensure uniform concentration. Further, in the present invention, the electrolysis current density is 100-150 A / m 2, the electrode spacing is 20 - 50 mm; further, in the present invention, the electrolyte is circulated by a peristaltic pump at a fixed flow rate to maintain the uniformity of silver ion concentration in the electrolyte during the electrolysis process; Step Nine: After the electrolysis is completed, take out the cathode plate, strip the silver on the cathode plate, rinse it thoroughly with deionized water at 90 - 100 °C, and then boil it in deionized water for 30 min, repeat 2 - 3 times; Step Ten: Dry the silver obtained in Step Nine to obtain 5N high-purity silver.

[0020] The embodiments of the present invention will be specifically described below by setting specific parameters.

[0021] Example One: The 99% pure coarse silver powder is wetted with a small amount of high-purity water and then dissolved in concentrated nitric acid. The molar ratio of coarse silver powder to nitric acid is 1:0.6; after the reaction is complete, sediment for 3 - 5 hours and then filter. Add an appropriate amount of high-purity water to the filtrate to control pH = 5; let it stand for 2 - 3 hours and then filter. Add concentrated nitric acid to the filtrate to control the silver content in the solution to be 100 g / L; nitric acid 8 g / L. Prepare a polymethyl methacrylate (PMMA) electrolytic cell (400 mm × 600 mm × 600 mm), a ruthenium-iridium coated titanium-based anode plate (300 mm × 300 mm × 10 mm), and a pure titanium cathode plate (purity 99.9%, 500 mm × 500 mm × 10 mm). Fix the anode plate and the cathode plate on the anode rod and the cathode rod respectively. The electrode spacing is 30 mm, and the materials of the anode and cathode rods are pure titanium; pass direct current through the anode and cathode to maintain the cathode current density at 100 A / m 2 , the electrolysis duration is 72 hours; during the electrolysis process, the circulating electrolyte is introduced by a peristaltic pump. After the electrolysis is completed, take out the cathode plate, strip the silver on the cathode plate, rinse it thoroughly with deionized water at 90 - 100 °C, repeat 2 - 3 times, then boil it in deionized water for 30 min, dry it to obtain 5N high-purity silver.

[0022] Example Two: The 99% pure coarse silver chips are wetted with a small amount of high-purity water and then dissolved in concentrated nitric acid. The molar ratio of coarse silver chips to nitric acid is 1:0.8; after the reaction is complete, sediment for 3 - 5 hours and then filter. Add an appropriate amount of high-purity water to the filtrate to control pH = 5.3; let it stand for 2 - 3 hours and then filter. Add concentrated nitric acid to the filtrate to control the silver content in the solution to be 120 g / L; nitric acid 8 g / L. Prepare a PMMA electrolytic cell (400 mm × 600 mm × 600 mm), a ruthenium-iridium coated titanium-based anode plate (300 mm × 300 mm × 10 mm), and a pure titanium cathode plate (purity 99.9%, 500 mm × 500 mm × 10 mm). Fix the anode plate and the cathode plate on the anode rod and the cathode rod respectively. The electrode spacing is 30 mm, and the materials of the anode and cathode rods are pure titanium; pass direct current through the anode and cathode to maintain the cathode current density at 180 A / m 2, the electrolysis duration is 72 hours; during the electrolysis process, the circulating electrolyte is introduced by a peristaltic pump. After the electrolysis is completed, the cathode plate is taken out, and the silver on the cathode plate is stripped, rinsed clean with deionized water at 90 - 100 °C, repeated 2 - 3 times, then boiled in deionized water for 30 min, and dried to obtain 5N high-purity silver.

[0023] Example 3: The 99% pure coarse silver flakes are wetted with a small amount of high-purity water and then dissolved in concentrated nitric acid. The molar ratio of coarse silver flakes to nitric acid is 1:0.8; after the reaction is complete, it is settled for 3 - 5 hours and then filtered. An appropriate amount of high-purity water is added to the filtrate to control the pH = 5.5; after standing for 2 - 3 hours, it is filtered. Concentrated nitric acid is added to the filtrate to control the silver content in the solution to be 80 g / L; nitric acid is 8.5 g / L. Prepare a PMMA electrolytic cell (400 mm × 600 mm × 600 mm), a ruthenium-iridium coated titanium-based anode plate (300 mm × 300 mm × 10 mm), and a pure titanium cathode plate (purity 99.9%, 500 mm × 500 mm × 10 mm). Fix the anode plate and the cathode plate on the anode rod and the cathode rod respectively, with the electrode spacing of 50 mm. The anode and cathode rod materials are pure titanium; direct current is passed through the anode and cathode to maintain the cathode current density at 100 A / m 2 , the electrolysis duration is 72 hours; during the electrolysis process, the circulating electrolyte is introduced by a peristaltic pump. After the electrolysis is completed, the cathode plate is taken out, and the silver on the cathode plate is stripped, rinsed clean with deionized water at 90 - 100 °C, repeated 2 - 3 times, then boiled in deionized water for 30 min, and dried to obtain 5N high-purity silver.

[0024] Comparative Example 1: An anode plate is cast with 99.95% silver powder, and a titanium plate is used as the cathode. During the electrolysis process, the electrolyte temperature is 30 - 35 °C, and the current density is 400 - 450 A / m 2 , the Ag+ concentration is 20 - 250 g / L, the acidity is 40 - 60 g / L, the electrode spacing is 145 - 175 mm, the cell voltage is 1 - 3 V, the electrolysis cycle is 2 - 3 d, and the grade of electrolytic silver powder is above 99.99%.

[0025] Comparative Example 2: The surface of the crude silver is scrubbed clean with deionized water, and nitric acid (1 / 1) is added according to the theoretical amount and heated to 80 °C. When the crude silver block no longer dissolves, the obtained solution is filtered, and then an excessive amount of saturated sodium hydroxide solution is added with stirring to obtain silver oxide precipitate. Then, through the processes of nitric acid dissolution, sodium hydroxide precipitation, and washing, this process can be repeated multiple times to achieve the purpose of purification. The brownish-black silver oxide solid is gradually added to concentrated ammonia water with stirring until all the silver oxide dissolves, and then filtered. The filtrate is added dropwise to the mixed solution of formaldehyde and PVA with stirring, and the solution is heated to 50 °C. High-purity silver powder precipitate is obtained. After washing the silver powder with deionized water and ethanol, the silver powder is placed in a vacuum drying oven for drying.

[0026] Comparative Example 3: Prepare the electrolyte with deionized water, analytical pure concentrated nitric acid and analytical pure silver nitrate. Use 99.99% pure metallic silver with a double-layer anode bag as the anode, and use a titanium mesh or silver mesh as the cathode for electrolysis. By controlling the electrolysis conditions, high-purity metallic silver with a purity of over 99.9999% can be obtained at the cathode.

[0027] By comparing the preparation methods, experimental phenomena and experimental results of the above Examples 1-3 and Comparative Examples 1-3, it can be seen that in Comparative Example 1, 99.95% silver needs to be melted and cast into an anode plate and electrolyzed once, and only high-purity products above 4N level can be obtained; in Comparative Example 2, the silver powder is subjected to multiple oxidation-dissolution cycles for purification-formaldehyde reduction process, which is cumbersome and requires delicate operation, and the quality of 5N high-purity silver products is unstable and the cost is high; in Comparative Example 3, 4N silver is used as the anode for refining, and the electrolyte is prepared with high-purity analytical pure test solution, and high-purity silver is prepared by electrolytic refining, the process is short but the cost is high; in Examples 1-3, by controlling the dosage of nitric acid and pure water, a pure silver nitrate electrolyte is prepared, and 5N high-purity gold can be prepared through one-step electrolysis, the process is short, the efficiency is high, and the product quality is stable.

Claims

1. A method for preparing 5N grade high-purity silver, characterized in that: The steps include: Step 1: Prepare a concentrated HNO3 solution with a molar concentration of 14-18 mol / L; Step 2: Wet the crude silver turnings with pure water and add them to the HNO3 solution obtained in step 1; Step 3: heating the mixture obtained in step 2, keeping it warm and performing a chemical reaction. After the reaction is completed, cooling the reactant to room temperature, standing it for 2 to 3 hours, and filtering it to obtain a silver nitrate solution; Step 4: Add a certain amount of pure water to the silver nitrate solution obtained in step 3; Step 5: Add concentrated nitric acid to the diluted silver nitrate solution in step 4 to control the HNO3 concentration in the solution to be greater than 8 g / L; Step 6: Prepare an electrolytic cell, an anode plate, a cathode plate and a conductive rod, wherein the electrolytic cell is made of polymethyl methacrylate, the anode plate is a ruthenium-iridium coated titanium-based anode, and the cathode plate and the conductive rod are 99.9% pure titanium cathodes; Step 7: Prepare electrolyte, the silver electrolyte is the solution in step 5, and the silver ion concentration is 80-120 g / L; Step 8: After the electrolytic cell is installed, the electrolyte is passed into the electrolytic cell until it covers the cathode plate and the anode plate, and finally a direct current is passed with a current density of 100-200 A / m 2 , as the electrolysis process proceeds, the solution in the electrolytic cell circulates continuously to ensure uniform concentration; Step 9: After the electrolysis is completed, take out the cathode plate, peel off the silver on the cathode plate, rinse it with 90-100℃ deionized water, and then soak it in deionized water for 30 minutes, repeat 2-3 times; Step 10: Dry the silver obtained in step 9 to obtain 5N high-purity silver.

2. The method for preparing 5N grade high-purity silver according to claim 1, characterized in that: In the step 2, the molar ratio of the amount of nitric acid added to the crude silver is 0.6 to 0.9:

1.

3. The method for preparing 5N grade high-purity silver according to claim 1, characterized in that: In the step 5, the silver ion concentration in the silver solution is 100-120 g / L, and the HNO3 concentration is greater than 8 g / L.

4. The method for preparing 5N grade high-purity silver according to claim 1, characterized in that: The electrolysis current density in step eight is 100-150 A / m 2 , the inter-pole distance is 20 to 50 mm.

5. The method for preparing 5N grade high-purity silver according to claim 1, characterized in that: In step eight, the electrolyte is circulated by adding the electrolyte at a fixed flow rate by a peristaltic pump to maintain the uniformity of the silver ion concentration in the electrolyte during the electrolysis process.

6. The method for preparing 5N grade high-purity silver according to claim 1, characterized in that: The pH of the pure water used in step 2 is controlled to be greater than 5.