At present, there are about three main technologies that can be applied to silver recovery, including: electrolytic recovery method, metal replacement method and chemical precipitation method. Electrolytic recovery of silver recovery rate of 90 ~ 95%, metal replacement and chemical precipitation silver recovery rate can be greater than 99%. Electrolysis method with 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 high current density is used. 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 carbon rod with a dry cell as a simple anode (graphite is better, but not easy to get), and then stainless steel sheet as 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 about 500 ppm silver 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 flat 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 stainless steel flat plate as the cathode, the recovery of silver to less than 100 mg / L, can still get a very good silver recovery. With stainless steel plate as cathode, the silver recovery can reach 100 mg/L or less, and the silver metal can still be obtained with good metallicity, which can be easily stripped off from the stainless steel plate, and it is the better silver recovery equipment nowadays. Electrolytic recovery of residual silver ions (less than 100 mg / L) can be used to develop the United States Kodak agent (code name TMT) precipitation recovery, silver can be processed to 0.5 mg / L or less, in line with the discharge standard. The metal substitution method uses ferrous materials and puts them into the waste liquid so that silver is precipitated by substitution. This method makes the fixing solution contain iron, so it must be discarded. However, for bleaching fixing solution as long as 20% of the waste liquid is discarded to reduce the amount of iron, 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 liquid, 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 mentioned above. Relevant new technology and new methods: According to overseas media reports, the United States CSRS company launched the recovery of film developing machine fixing solution in the "silver" equipment. CSRS company manufactured electrolytic silver recovery machine system, is currently one of the world's advanced recycling system, which uses intelligent microprocessor technology, in the first time will be about to be applied to the liquid in the "silver" recovery, not only the recovery rate is high, but also can effectively extend 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 photo developing waste streams, but most recovery processes have been inefficient and sometimes caused more contamination. Now the situation may change: the United States Oak Ridge National Laboratory has a scientist has developed a process to recover 99.999% 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 involves pumping the silver-containing waste solution to a reaction tank
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