How to extract silver from x-ray film please 3Q

Looked for some information, I hope you can help the recovery of silver in the waste fixing solution With the West-to-East Gas Pipeline Project and other large pipeline projects, shooting 100% photographic detection in the country has also begun to be widely accepted, the process of ray film will produce a large number of spent showing the fixing solution, the development of the process, to stay in the emulsion layer constitutes a black-and-white image of the silver accounted for only 20% or so, 80% of the silver dissolved in the fixing solution, "silver" emissions will cause environmental pollution. The discharge of "silver" will cause environmental pollution. Neither in line with the January 4, 1998 promulgation of the "National Hazardous Waste List", nor in line with the HSE requirements of the North Detection Company, but also caused a great waste, preliminary statistics, in the West-to-East Gas Pipeline Project will produce 33,200 liters of waste fixing solution, which contains about 390 kg of silver, according to the price of silver in 2002, calculated as 1,300 yuan / kg, which is equivalent to 500,000 yuan was wasted. How can a large number of valuable silver to be recovered? This article comprehensively describes the relevant technical methods. At present, there are about three main technologies can be applied to silver recovery, including: electrolytic recovery method, metal replacement method and chemical precipitation method. Among them, the silver recovery rate of electrolytic recovery is 90-95%, and the silver recovery rate of metal replacement and chemical precipitation can be more 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 solution, silver ions to Ag (S2O3) 2-3 mismatch exists, the current density is too high or the recovery of silver concentration in the solution is too low, easy to produce black silver sulfide precipitation, affecting the recovery of silver quality equipment needed is just a carbon rod with a dry-cell battery for a simple anode (graphite is better, but not easy to obtain), and then stainless steel sheet to do the cathode, to adjust the distance between the electrodes, and apply 2 to 5 volts; it is better to be able to stir the solution. At the beginning, you can get 90 to 98% purity of silver in the cathode, continue to get blacker, 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 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. 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 precipitate produced by the purification of pure metallic silver can be obtained, and the addition of chemicals are expensive, less economically efficient if you want to recover silver from the waste of black-and-white films or X-rays, 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. Secondary school textbooks in the method: electrolytic method of silver four steps: 1. electrolysis 2. purification 3. replacement 4. purification of silver in the state of existence of the silver element in the fixing solution is a sulfur with sulfate complexes and can not be directly with the replacement reaction. 1. Electrolysis Find two charcoal rods, clean, 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. The metallic silver will be slowly deposited on the negative charcoal concentrate rod. To what time end I forget 2. purification Put the negative electrode of the charcoal concentrate rod into the excess of dilute nitric acid, the surface deposition of the metal silver completely melted, the formation of silver nitrate and nitric acid mixing liquid with filter paper to filter out the solids Magazine 3. replacement Add an excess of iron powder to the mixing liquid, the reaction is complete, the remaining solid is a mixture of metallic silver and metallic iron. Filter out the solid with filter paper and rinse it well with light water.4. Purification Add an excess of dilute hydrochloric acid to the solid to dissolve the iron powder. The remaining solid is the purer metallic silver