Method:
1 add acid for precipitation.
Its principle is that thiosulfate ion will be decomposed into sulfur and sulfur dioxide gas under the action of acid, and silver ion will combine with sulfur to form silver sulfide precipitate; When silver sulfide is sent to a high-temperature furnace for combustion, it is decomposed into elemental silver and sulfur dioxide gas.
2 electrolysis
A stainless steel plate is used as a cathode, and direct current is applied to generate metallic silver on the cathode. Silver with purity of 90%-98% can be obtained through good control, and the recovery rate is about 90%-95%.
3 metal substitutes
Replacing it with active metal has the advantages of simple method, low purity and low recovery rate.
4 sodium borohydride reduction
Due to the strong reducibility of sodium borohydride, metallic silver can be substituted in its alkaline solution. The recovery rate of secondary recovery method is as high as 99. The purity of recovered silver is about 96%-98%.
Reducing sugar reduction method
In recent years, some people have developed a method to recover silver from organic reduction solution. It first uses tartaric acid and sucrose to prepare a reducing sugar solution, and then uses this reducing sugar solution to reduce the silver in the waste fixing solution. The main advantages of this method are carbon, silicon, iron, aluminum, copper and so on. Will not be mixed with silver powder, so that the purity of recovered silver is high (>: 99%) and the recovery rate of silver is above 95%.
6 sodium dithionite reduction method
Sodium bisulfite (Na2SO4) can directly reduce silver from waste fixing solution and oxidize it into Na2S2O3. This method can not only make the recovery rate of silver high, but also get high-grade silver, and at the same time improve the mass concentration of the main components of the waste fixing solution, so that the waste fixing solution can be regenerated.
My trick is to add excessive salt to the color fixing water, filter it and bake it at high temperature (1200℃).
With the development of large-scale pipeline projects such as west-to-east gas transmission, X-ray 100% photographic detection has been widely accepted in China. A large amount of waste developer and fixer will be produced in the process of developing X-ray film. After development, only about 20% silver remained in the emulsion layer to form a black-and-white image, and 80% silver was dissolved in the fixing solution. The discharge of "silver" will cause environmental pollution. It does not meet the National Hazardous Waste List issued by 19981October 4th, nor does it meet the HSE requirements of North Testing Company, and it also causes great waste. According to preliminary statistics, the West-to-East Gas Transmission Project will produce 33,200 liters of waste fixing solution, including about 390 kilograms of silver, which is equivalent to 1300 yuan/kg at the price of silver in 2002. How can we recycle a lot of precious silver?
This paper comprehensively introduces the related technical methods.
At present, there are three main technologies that can be applied to silver recovery, including electrolytic recovery, metal replacement and chemical precipitation. The recovery rate of electrolytic silver is 90 ~ 95%, and the recovery rate of metal replacement and chemical precipitation of silver can be greater than 99%.
During electrolysis, two electrodes are inserted into the solution, and direct current is applied to make the cathode silver plated. Electrolysis can be divided into low current density equipment and high current density equipment. The low current density of fixing solution is less than 3 amps/square foot, while the high current density is greater than 10 amps/square foot. When high current density is used, the stirring rate must be increased on the cathode surface. Bleaching fixer must use ultra-high current density, that is, 60 ~ 90 amps/square. The phenomenon that bleaching agent hinders electrolysis is caused. The cathode is a rotating cylinder to improve the stirring speed. The voltage between the electrodes is very low, about 0.5 to 0.7 volts. All anode materials are made of carbon (because carbon can conduct electricity and resist corrosion at the same time), and cathode materials are made of stainless steel. Metallic silver can be obtained directly by electrolysis, but the selection of electrolysis equipment and the control of electrolysis conditions have great influence on the quality and recovery rate of silver. In fixing and bleaching/fixing waste liquid, silver ions exist as complexes of Ag(S2O3)2-3. When the current density is too high or the silver concentration in the recovery solution is too low, it is easy to produce black silver sulfide precipitation, which affects the quality of recovered silver. The only equipment needed is to use the carbon rod of dry battery as a simple anode (graphite is good, but it is not easy to obtain), then use stainless steel sheet as a cathode, adjust the electrode distance, and apply a voltage of 2 to 5 volts; It is best to be able to stir the solution. At first, you can get 90-98% pure silver at the cathode, and then you will get darker and dirtier silver. The end point of operation is that the silver concentration in the solution drops to 100 ppm, and there will be silver sulfate mud. The treatment of bleaching fixer requires high voltage and high final concentration. In the residual silver solution of about 500 ppm, this kind of wastewater cannot be discharged into the sewer. Chemical hazards include: hydrogen sulfide produced when the current is high, or ammonia gas produced when mixed with developer. General plate electrolysis equipment can recover silver to about 300 mg/L, and high-quality transfer electrolysis system (including rotating cathode and fluidized bed electrolysis system) can recover silver to below 1000 mg/L, in which the largest unit of fluidized bed electrolysis recovery system can provide 1000 ampere, and the daily recovery of silver by a single equipment can exceed 20 kg, and the stainless steel plate can be used as cathode to recover silver to/kloc-0. The residual silver ions (less than 100 mg/L) after electrolytic recovery can be recovered by precipitation with the reagent (code TMT) developed by Kodak Company in the United States, and the silver can be treated below 0.5 mg/L, which can meet the discharge standard.
The metal replacement method uses iron material, and because of the replacement, the waste liquid is put into it to precipitate silver. This method makes the fixing solution contain iron, so it must be discarded. However, as long as 20% of the waste liquid is discarded and the iron content is reduced, the bleaching fixer can still be reused.
In the chemical replacement method, silver can be removed from the waste liquid by sodium sulfide or sodium borohydride (NaBH4). Silver sulfide can be obtained by the reaction of sodium sulfide, and metallic silver can be obtained by sodium borohydride. The advantages of chemical treatment are high speed, the reaction rate can reach above 99%, and the purity of silver is above 95%. Common methods: adding saturated solution of sodium sulfide, the silver ions in wastewater become black silver sulfide powder and precipitate into "silver mud". This dark silver mud is heated and dissolved with nitric acid to obtain silver nitrate crystals, which are then reduced to silver in an electrolytic cell. This method is simple, but the precipitate must be purified to obtain pure metallic silver, and the added chemicals are expensive and have low economic benefits. If silver is to be recovered from waste black-and-white films or X-rays, it must be dissolved in the solution first. The unwashed leftover bits and pieces can be dissolved with fixing solution, while the cleaned leftover bits and pieces must be made into compounds with oxidant (such as potassium ferricyanide, ferric EDTA or copper chloride), and then silver compounds is dissolved with fixing solution. As described above, silver metal can be extracted from the obtained fixing solution by electrolysis.
Related new technologies and methods:
According to overseas media reports, CSRS Company of the United States introduced the equipment to recover "silver" from the fixing solution of the developing and printing machine. The electrolytic silver recovery machine system produced by CSRS company is one of the advanced recovery and treatment systems in the world at present. It uses intelligent micro-processing technology to recover the "silver" to be used in the liquid medicine at the first time, which not only has high recovery rate, but also can effectively prolong the service life of the fixing agent. The system operation panel adopts internationally recognized touch buttons. When the machine is running, a warning light of "Recycle Now" will appear to remind the operator that the machine will enter a "sleep" state when it is not running. The whole recycling machine adopts closed loop and closed design, which can protect operators from chemicals. At present, the product has obtained safety marks such as UL, FCC, TUV and CE. Scientists have been studying ways to recover silver from the waste liquid from developing photos, but most of the recovery processes are inefficient and sometimes cause more pollution. Now the situation may change: a scientist at Oak Ridge National Laboratory has developed a process to recover 99.999% silver from photographic waste liquid. A key problem in most silver recovery processes is the production of silver sulfate, which is a pollutant that is difficult to remove. The old procedure is to add a small amount of hypochlorous acid to a large amount of photographic waste liquid containing silver. The procedure of Oak Ridge National Laboratory is to pump the silver-containing waste liquid into a reaction tank and add excessive hypochlorous acid, so that thiosulfuric acid in the fixing solution is oxidized in the reaction tank. After fine adjustment of acidity, silver will be precipitated in the form of silver chloride. Secondly, sodium hydrosulfite was added as reducing agent to convert silver chloride into silver. According to the program test results of Oak Ridge National Laboratory, the silver content in the waste liquid can be reduced from 500mg per liter to below 1mg. Researchers can obtain almost pure silver by filtering waste liquid.
Li Yungang used sodium hydrosulfite to extract silver from waste fixing solution [J]. Hydrometallurgy, 1999, (2): 26-30. Taking the mass concentration of residual roots, the mass (grade) of silver powder and the mass concentration of Na _ 2S _ 2O _ 3 in the waste fixer after reduction as investigation indexes, the effect of extracting silver from the waste fixer with sodium dithionite (Na _ 2S _ 2O _ 4) as reducing agent and the regeneration of the waste fixer were studied. The results show that this method can not only obtain high purity metallic silver and waste fixing solution with low silver concentration (ρ (Ag) < 0.05 g/L), but also improve the mass concentration of the main component Na2S2O4 in fixing solution, thus obtaining waste fixing solution.
The method and technological conditions of reducing silver in waste fixing solution with organic acid (Ar(OH)3COOH) for rivers are introduced. The experimental results show that the reduction rate of silver is 99.20%, the total recovery rate is 94.5%, and the mass fraction of silver in the recovered silver powder (tablet) is 97.43%. Using the latest reducing sugar method to extract silver has the characteristics of low cost, simple operation, good effect, high purity and easy popularization.
Four steps of electrolytic silver extraction: 1. Electrolysis II. Purification 3. Replace 4. Purification.
The existing state of silver in fixing solution is a complex of sulfur and sulfate, which cannot be directly replaced.
1. Find two carbon rods through electrolysis, clean them, and connect them to the positive and negative poles of the adjustable regulated power supply (DC power supply, the current should be above 10A). Insert two carbon rods as far as possible into the fixing solution. The wire connecting the carbon rod must not touch the liquid. Energize and adjust the voltage to make the carbon rod connected to the anode produce trace gas. Metallic silver will slowly deposit on the carbon rod of the negative electrode. When will it end? I forgot.
2. Purification: Put the negative carbon rod into excessive dilute nitric acid to completely melt the metallic silver deposited on the surface, and form a mixed solution of silver nitrate and nitric acid. Filter solid magazines with filter paper.
3. Replacement Add excessive iron powder to the mixed solution. After the reaction is completed, the remaining solid is a mixture of metallic silver and metallic iron. Filter out solids with filter paper and rinse with clean water.
4. Purification Add excessive dilute hydrochloric acid to the solid to dissolve the iron powder. The remaining solid is relatively pure metallic silver.
Related information: topic technology 1: method of recovering silver from silver-containing waste. Silver is a precious and rare metal with a wide range of uses. Silver has good conductivity, thermal conductivity and high chemical stability, and its halide is a good photosensitive material. Due to the limited geological reserves of silver, the process of producing silver is complicated and the cost is high, so it is particularly important to recover silver from various silver-containing wastes, which can not only reduce the pollution of heavy metals in silver-containing wastes to the environment, but also recover silver powder, and the quality of silver powder is relatively high.
Silver extraction from waste medicine: As we all know, an important component of photosensitive materials is silver halide, and a large amount of silver compounds will remain in the liquid medicine during washing. And many printing companies can't recycle it, so they have to take it for nothing. It wastes resources and pollutes the environment.
Extraction of metallic silver from waste fixing solution and preparation of silver nitrate 2007-02-221:15 supplies: beaker, crucible, iron frame (with iron ring), tripod, alcohol blowtorch.
Waste fixing solution or bleaching solution, zinc flake, hydrochloric acid (or sulfuric acid), nitric acid. Principle: When fixing solution (sodium thiosulfate) is used to remove silver halide (usually silver bromide) from negative film and photo without photosensitive part, soluble silver thiosulfate complex ions are formed. The complex ions of silver thiosulfate can be converted into insoluble silver sulfide under the action of acid. Silver sulfide can be reduced to metallic silver by calcination. If silver sulfide is dissolved with nitric acid, silver nitrate will be produced. There is still a little silver halide in the solution, which can be reduced by zinc. their
The reaction is as follows:
1. The role of the fixer:
2 na 2 S2 o 3+AgBr = Na3[Ag(s2o 3)2]+NaBr
2. The role of acid and fixer:
6 HCl+2na 3[Ag(s2o 3)2]= 2 H3[Ag(s2o 3)2]+6 NaCl
2H3[Ag(S2O3)2]=3H2S2O3+Ag2S2O3
3H2S2O3=3H2O+3SO2↑+3S↓
Ag2S2O3+H2O=Ag2S↓+H2SO4
The total reaction formula is:
6h cl+2na 3[Ag(s2o 3)2]= 6 NaCl+Ag2S↓ 13s↓
+3SO2↑+H2SO4+2H2O
3. The role of zinc and a small amount of silver halide in waste fixing solution:
Zn+2AgBr=ZnBr2+2Ag↓
4. Silver sulfide is calcined and reduced to silver:
Ag2S+O2=SO2↑+2Ag↓
5. Dissolve silver sulfide in nitric acid to generate silver nitrate;
3Ag2S+8HNO3 (diluted) = 6agno3+2no =+3s ↓+4h2o
6. Silver reacts with nitric acid to generate silver nitrate:
6ag+8HNO3 (diluted) = 6agno3+2no =+4h2o.
Silver+nitric acid (concentrated) = silver nitrate+nitrogen dioxide =+H2O
Operation: put 500ml of waste fixer into a big beaker (it is best to use waste fixer in a photo studio, which generally contains about 0.4% silver), add 6N hydrochloric acid until no gas and precipitate are generated, stir while adding, add zinc sheet and heat to boiling. After a few minutes, there was a dark brown precipitate at the bottom of the beaker, which was silver sulfide precipitate containing silver. After filtration, the precipitate is washed with clear water, dried, put into a crucible, and burned with an alcohol blowtorch (first with warm fire). After a few minutes, molten liquid silver appeared, and after cooling, fine silver particles were obtained. If the filtered silver sulfide (commonly known as "silver mud") is not calcined, nitric acid can be added to completely dissolve the "silver mud"; If treated with dilute nitric acid, it can be dissolved with a little heating at the beginning, and the solution obtained is silver nitrate solution. If silver nitrate crystal is to be prepared, it must be heated and concentrated into supersaturated solution. Then let it cool naturally, and after silver nitrate crystals are precipitated, the remaining solution can be reserved for the next time to continue preparing silver nitrate.
Irritating gases (nitric oxide, nitrogen dioxide, sulfur dioxide, etc. ) is formed in the reaction. So the reaction should be carried out in a fume hood or a well-ventilated environment.
Peripheral photo studios, hospital radiology departments, printing houses, electroplating factories, wireless power plants, waste recycling centers, boiler factories, electronic board factories, alloy cutter head factories, waste photos, films, CT films, X-rays, printing paper, glass mirror factories, thermos bottles factories, mining factories, original electronics factories, semiconductor component factories, film studios and photosensitive materials factories have acquired chemical plants (petrochemical plants and fertilizer plants) at high prices all the year round.