Currently there are about three main techniques that can be applied to silver recovery, including: electrolytic recovery, metal replacement and chemical precipitation. The silver recovery rate of electrolytic recovery is 90-95%, and the silver recovery rate of metal replacement and chemical precipitation can be greater than 99%.
Electrolysis method to two electrodes inserted into the solution, connected to the direct current, silver will be plated on the cathode. Electrolysis can be divided into low current density equipment and high current density equipment. The low current density used in the fixing solution is less than 3 amperes/square foot, while the high current density is greater than 10 amperes/square foot. The cathode surface must be agitated at a higher rate when using high current densities. Bleach fixing solution because of the bleach has blocked electrolysis phenomenon, must be used in ultra-high current density, that is, 60 to 90 amperes / square foot. The cathodes are rotating cylinders to increase the stirring rate. The voltage between the electrodes is very low, between about 0.5 and 0.7 volts. Carbon is used for the anodes (because it conducts electricity and resists corrosion) and stainless steel for the cathodes. Electrolysis can be directly obtained by metallic silver, but the choice of electrolysis equipment and electrolysis conditions to control the quality of silver recovery and recovery rate has a great impact. Fixing and bleaching / fixing waste liquid, silver ions to Ag (S2O3) 2-3 mixtures exist, the current density is too high or the recovery of silver concentration in the liquid is too low, easy to produce black silver sulfide precipitation, affecting the quality of silver recovery.
The equipment needed is just a dry cell with a carbon rod as a simple anode (graphite is better, but not easy to get), and then stainless steel sheet to do the cathode, adjust the electrode distance, and apply 2 to 5 volts voltage; can stir the solution is better. At the beginning, you can get 90 to 98% purity of silver at the cathode, continue to get darker, dirtier silver; the end of the operation is the silver concentration in the solution down to 100 ppm, and there will be silver sulfate sludge. The treatment of bleach fixing solutions requires higher voltages and terminates in a higher concentration of silver, about 500 ppm, remaining in solution; this wastewater cannot be discharged to the sewer. Chemical hazards include the generation of hydrogen sulfide when the current is high, or ammonia when mixed with the developer solution. With the general plate electrolysis equipment can recover silver to about 300 mg / L, with high quality transfer electrolysis system (including rotating cathode and fluidized bed electrolysis system) can recover silver to less than 100 mg / L, of which the fluidized bed electrolysis recycling system can provide the largest unit up to 1,000 amps, the amount of silver recovered by a single unit of equipment can be more than 20 kilograms per day, and the stainless steel plate as the cathode, the recovery of silver to less than 100 mg / L, can still get very good results. With stainless steel plate as the cathode, silver recovery can be reduced to less than 100 mg/L. The silver metal can still be obtained with good metallicity and can be easily stripped from the stainless steel plate, which is the better silver recovery equipment at present. After electrolytic recovery of residual silver ions (less than 100 mg / L) can be used in the United States Kodak developed chemicals (code name TMT) precipitation recovery, silver can be processed to 0.5 mg / L or less, in line with the discharge standard.
Metal substitution method uses iron materials, put into the waste liquid so that silver precipitation due to substitution. This method makes the fixing solution contain iron, so it must be discarded. However, for bleached fixer, as long as 20% of the waste liquid is discarded to reduce the amount of iron, it can still be reused.
Chemical replacement method can be used sodium sulfide or sodium borohydride (sodium borohydride, NaBH4) to remove the silver in the waste solution, the reaction can be obtained from the sodium sulfide silver sulfide, sodium borohydride from the metal silver. The advantage of chemical treatment is fast, the reaction rate can reach more than 99%, and the purity of silver is more than 95%. General method: add sodium sulfide saturated solution, silver ions in the wastewater into a black silver sulfide powder, precipitation down into the "silver mud". This black paint silver mud after heating, plus nitric acid dissolution, silver nitrate crystals, and then in the electrolytic cell reduction for silver. This method is simple, but the resulting precipitate must be purified to obtain pure metallic silver, and the addition of chemicals are expensive, less economically efficient if you want to recover silver from the waste of black and white film or X-ray film, it is necessary to dissolve the silver into a solution. Undeveloped film can be used fixing solution to dissolve the silver halide, has been developed by the waste film must first use oxidizing agents (such as potassium ferricyanide, ferric EDTA or copper chloride) to make the silver compounds, and then dissolve the silver compounds with the fixing solution. The resulting fixing solution can be used to remove the silver metal by the electrolysis method described above.
Related new technologies and methods:
According to overseas media reports, the U.S. company CSRS launched the recovery of film processing machine fixing solution in the "silver" equipment. The electrolytic silver recycling machine system manufactured by CSRS is one of the advanced recycling systems in the world, which adopts intelligent microprocessing technology and recovers the "silver" that is about to be applied to the drug solution in the first time, which not only has a high recycling rate, but also can effectively prolong the service life of the fixing agent. The operation panel of the system adopts touch keys with international common marking, when the machine is running, the warning light of "Recycle Now" will appear to remind the operator, and the machine enters the "sleep" state when it is not running. The whole recycling machine adopts the closed circuit and airtight design, which can protect the operator from chemical agents. The product has already obtained UL, FCC, TUV, CE and other safety marks.
Scientists have been researching ways to recover silver from photofinishing waste, but most recovery processes have been inefficient and sometimes caused more contamination. That may be changing now: a scientist at Oak Ridge National Laboratory has developed a process that recovers 99.999 percent of the silver from photographic waste. A key problem in most silver recovery processes is the generation of silver sulfate - a difficult contaminant to remove - and the old procedure was to add a small amount of hypochlorite to a large volume of silver-containing photographic waste. The procedure at Oak Ridge National Laboratory was to pump the silver-containing waste solution into a reaction tank, add an excess of hypochlorite to oxidize the thiosulfate in the fixing solution in the reaction tank, and then precipitate the silver as silver chloride by fine adjustment of the acidity. Next, sodium dithionite is added as a reducing agent to convert silver chloride to silver. Using the results of the Oak Ridge National Laboratory program, the silver content in the waste stream can be reduced from 500 milligrams per liter to less than 1 milligram. The researchers were able to obtain nearly pure silver by filtering the waste stream.
Li Yun-Gang Extraction of silver from spent fixing solution by sodium bisulfite (Na2S2O4)[J]. Wet Metallurgy,1999,(2):26-30.Taking the mass concentration of residual roots in the waste fixing solution after reduction, the quality of silver powder (grade of silver powder) and the change of the mass concentration of Na2S2O3 in the waste fixing solution as the indexes of investigation, the effect of extracting the silver in the waste fixing solution by using sodium Liandisulfite(Na2S2O4) as a reductant and the regeneration of the waste fixing solution were investigated. The results show that this method of silver extraction can not only obtain higher purity of metallic silver and low silver mass concentration of the spent fixing solution (ρ (Ag) 〈0.05g/L〉, but also make the main component in the fixing solution of Na2S2O3 mass concentration rise, so that the spent fixing solution is regenerated.
Jiang Guohong organic acid (Ar(OH)3COOH) from the waste fixing solution in the reduction of silver method and process conditions. The test results showed that the silver reduction rate was 99.20% with organic acid (Ar(OH)3COOH) from waste fixing solution, the total recovery rate was 94.5%, and the mass fraction of silver in the recovered silver powder (flake) was 97.43%. The latest method of using reducing sugar to extract silver, this method has the characteristics of low cost, simple operation, good yield, high purity and easy to promote.
Methods in secondary school textbooks:
Electrolytic method of silver extraction in four steps: 1. electrolysis 2. purification 3. replacement 4. purification
Silver element in the state of existence in the fixing solution is sulfur with sulfate complexes, can not be used directly with the replacement reaction.
1. Electrolysis find two charcoal rods, cleaned, connected to the positive and negative poles of the adjustable voltage regulator (DC power supply, the current should be more than 10A). Insert the two charcoal sticks into the fixing solution, as far as possible to separate the distance. The wires connecting the charcoal rods should not touch the liquid, energize the power supply, adjust the voltage so that the charcoal rods connected to the positive pole produce a slight gas. Metallic silver will slowly deposit on the negative charcoal concentrate rod. By what time it ends I forget.
2. Purification Put the carbon concentrate rod of the negative electrode into an excess of dilute nitric acid to completely melt the metallic silver deposited on the surface, forming a liquid mixture of silver nitrate and nitric acid. Filter the solid magazines with filter paper.
3. Displacement An excess of iron powder is added to the miscible liquid, and when the reaction is complete, the remaining solid is a mixture of metallic silver and metallic iron. The solid is filtered out with filter paper and rinsed out with light water.
4. Purification Add an excess of dilute hydrochloric acid to the solid to dissolve the iron powder. The remaining solid is relatively pure metallic silver.
Waste liquid automatic silver extraction machine Address: 336000 Jiangxi Yichun District Institute of Practical Technology Tel: 0795-3265550 Portable gold and silver straight lifting machine This machine can be directly extracted from the gold and silver-containing liquid gold and silver, without adding any chemicals, 220V, 60W civilian electricity can be operated (equivalent to a household electrical appliance, do not need professional knowledge). It consists of portable password box, special power supply, liquid circulator, pole piece, and my mouth straight lifting room. Especially for the photo studio and hospital waste fixing liquid (containing silver), electroplating containing gold and silver liquid processing, can produce silver 400g or gold 100g of 90% grade precious metals, for processing liquid more occasions, can be more than one machine used in series, after treatment of waste liquid can be regenerated to use. Especially suitable for gold and silver containing waste liquid treatment of enterprises and laid-off workers and individual entrepreneurial wealth. This machine is easy to carry, safe, easy to operate, intuitive and efficient, no need for a fixed site can be mobile operation, the price is appropriate (about 1500 yuan).
Related information: thematic technology a method of silver recovery from silver-containing waste silver is a valuable rare metal, widely used, silver has a good electrical conductivity, thermal conductivity and high chemical stability, its halide to be a good photographic material, due to the limited geologic storage of silver and the complexity of the process of production of silver and the cost of high, so from a variety of silver-containing waste regeneration recovery of silver is particularly important, so that Not only can reduce the pollution of heavy metals in silver-containing waste to the environment, but also can be recycled to produce silver powder, with good environmental and economic benefits. The following information is 15 yuan per copy, the full set of 100 yuan.
SQ05501 Recovery of silver from precipitation solution SQ05502 Chemical method of silver extraction and regeneration of color bleaching precipitation solution SQ05503 Recovery of silver from photographic waste solution SQ05504 Process study of silver recovery from waste precipitation solution SQ05505 Recovery of silver from silver-containing waste solution SQ05506 Sulfurization method of recovery of silver from waste fixing solution SQ05507 Recovery of silver from waste by rapid precipitation method SQ05508 Recovery of silver from various silver-containing wastes SQ05509 Development of silver recovery and high purity silver from silver-containing wastes SQ05510 Extraction of silver from wastes by high-temperature iron reduction SQ05511 Recovery of silver from silver-containing wastes by using sodium borohydride SQ05512 Regeneration of silver in wastes SQ05512 Regeneration of silver from wastes SQ05512 Above information patents are 35 yuan each, and the other information is 15 yuan each. The center also on behalf of a variety of thematic information, Chinese and foreign standards, patents and so on.
Silver scrap medicine: as we all know, an important part of the photographic material is silver halide, and in the process of rinsing the liquid will leave a lot of silver compounds. And a lot of printing society can not be recycled had to white to fall. Resulting in a waste of resources, and pollution of the environment. Now the transfer of waste fixing solution to extract silver technology, the technology is very simple, the equipment only needs a few crucibles, a furnace, a small fan, a fifty-liter container can be.