To 1:12.5 dilute nitric acid demineralization of silver, the temperature rises to more than 50 degrees Celsius, the rate of dissolution increased dramatically. Add appropriate amount of coagulant in the demineralized silver solution, can accelerate the clarification rate.
Soluble silver mud by washing, drying, grinding, burning, melting and other processes, can be 99.99% Ag purity. film matrix can be recycled after washing.
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Recovery of silver in waste fixing solution
With the West-to-East Gas Pipeline Project and other large pipeline projects, the ray 100% photographic detection in the country has also begun to be widely accepted, and the ray film Processing process will produce a large number of waste film fixing solution, after the development process, remain in the emulsion layer constitutes a black and white image of silver only about 20%, 80% of the silver dissolved in the fixing solution, "silver" emissions 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 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 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 from the stainless steel plate, and it 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 bleaching 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%, 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 extracted silver from spent fixing solution with 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 Liandisulfite Silver Sodium (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 to restore silver method and process conditions. The test results show that the silver reduction with organic acid (Ar(OH)3COOH) from the waste fixing solution, the silver reduction rate of 99.20%, the total recovery rate of 94.5%, the recovered silver powder (film) in the silver mass fraction of 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 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 the purer metallic silver.
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Operate this way in the lab:
Supplies: beaker, crucible, iron stand (with iron ring), tripod, alcohol blowtorch.
Waste fixing solution (obtained from a photographic studio), zinc flakes, hydrochloric acid (or sulfuric acid), nitric acid.
Principle: when the fixing solution (sodium thiosulfate) is used to remove the silver halide (generally silver bromide) from the negative and the photos without the light-sensitive part of the silver halide, a soluble silver thiosulfate complex ion is formed. Under the action of acid, silver thiosulfate complex ion can be converted into insoluble silver sulfide. Silver sulfide can be reduced to metallic silver by calcination. If nitric acid is used to dissolve silver sulfide, silver nitrate is generated. There is still a small amount of silver halide in the solution, the silver can be reduced out of zinc. Their reactions are as follows:
1. The role of the fixing solution:
2Na2S2O3 + AgBr = Na3 [Ag (S2O3) 2] + NaBr
2. The role of the acid and fixing solution:
6HCl + 2Na3 [Ag (S2O3) 2] = 2H3 [Ag (S2O3) 2] + 6NaCl
2H3[Ag(S2O3)2] = 3H2S2O3 + Ag2S2O3
3H2S2O3 = 3H2O + 3SO2↑ + 3S↓
Ag2S2O3 + H2O = Ag2S↓ + H2SO4
The total reaction equation is:
6HCl + 2Na3[Ag( S2O3)2] = 6NaCl + Ag2S ↓ ten 3S ↓
+ 3SO2↑ + H2SO4 + 2H2O
3. Zinc and a small amount of silver halide in the waste fixing solution:
Zn + 2AgBr = ZnBr2 + 2Ag ↓
4. Calcination of silver sulfide is reduced to silver:
Ag2S ＋ O2 = SO2↑ + 2Ag ↓
Ag2S ＋ O2 = SO2↑ + Ag2S O2 = SO2↑ + 2Ag↓
5. Silver sulfide dissolved in nitric acid to produce silver nitrate:
3Ag2S + 8HNO3 (dilute) = 6AgNO3 + 2NO↑ + 3S↓ + 4H2O
6. Silver and nitric acid reaction to produce silver nitrate:
6Ag + 8HNO3 (dilute) = 6AgNO3 + 2NO↑ + 4H2O
6. 4H2O
Ag + 2HNO3 (concentrated) = AgNO3 + NO2 ↑ + H2O
Operation: placed in a large beaker of 500 ml of waste fixer (preferably with a photo studio in the waste fixer, generally containing silver up to 0.4% or so), add 6N of hydrochloric acid, until there is no gas and precipitation, stirring as it is added and put into the zinc sheet, heated to a boil, a few minutes after the bottom of the beaker, there is a dark brown color. After a few minutes, there is a black-brown precipitate at the bottom of the beaker, which is silver sulfide precipitate containing silver. After filtration, wash the precipitate with water, after drying, put in the crucible with an alcohol torch burning (start with a warm fire), a few minutes later, there is a molten liquid silver appeared, after cooling, that is, the small silver particles. If the filtered silver sulfide (generally known as "silver mud") without calcination, can add nitric acid, so that the "silver mud" completely dissolved; such as dilute nitric acid treatment, the beginning of a little heat that can be dissolved, the resulting solution is silver nitrate solution. If you want to make silver nitrate crystals, it must be heated, so that it is concentrated into a supersaturated solution. Then let it cool naturally, silver nitrate crystals precipitated, the rest of the solution can be reserved for the next time to continue to produce silver nitrate with.
Because the reaction has irritating gases (NO, NO2, SO2, etc.) produced. So the reaction should be carried out in a fume hood or in a well-ventilated environment.