How is electroplating wastewater (metal) decomposed?

Electrolytic recovery method, metal replacement method and chemical precipitation method. Among them, the silver recovery rate of electrolysis recovery is 90-95%, and the recovery rate of metal replacement and chemical precipitation silver can be greater than 99%.

In the electrolysis method, two electrodes are inserted into the solution and a direct current is connected, and silver is plated on the cathode. Electrolysis methods can be divided into two types: low current density equipment and high current density equipment. Fixers use low current densities of less than 3 amps/square foot, while high current densities use greater than 10 amps/square foot. When using high current densities, the agitation rate of the cathode surface must be increased. The bleach-fixing solution must use an ultra-high current density, that is, 60 to 90 amps/square foot, because bleach blocks electrolysis. The cathode is in the shape of a rotating cylinder to increase the stirring rate. The voltage between the electrodes is very low, about 0.5 to 0.7 volts. The anode material is made of carbon (because carbon can conduct electricity and resist corrosion), and the cathode is 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 a great impact on the quality and recovery rate of silver recovery. In fixing and bleaching/fixing waste liquids, silver ions exist as Ag(S2O3)2-3 complexes. When the current density is too high or the silver concentration in the recovery liquid is too low, black silver sulfide precipitation is likely to occur, affecting the quality of the recovered silver.

The required equipment is just to use a carbon rod from a dry battery as a simple anode (graphite is better, but not easy to obtain), and then use a stainless steel sheet as the cathode, adjust the distance between the electrodes, and apply 2 to 5 volts. Voltage; it is better to stir the solution. Initially, you can get 90 to 98% pure silver at the cathode, and continue to get darker, dirtier silver; the end point of the operation is that the silver concentration in the solution drops to 100 ppm, and there is silver sulfate sludge. The treatment of bleaching and fixing solutions requires higher voltage and has a higher end concentration. In the silver residual solution of about 500 ppm, this 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. Silver can be recovered to about 300 mg/L with general flat plate electrolysis equipment, and silver can be recovered to less than 100 mg/L with high-quality transfer electrolysis systems (including rotating cathodes and fluidized bed electrolysis systems). Among them, the fluidized bed electrolysis recovery system is the largest. The unit can provide up to 1,000 amps, and the daily silver recovery volume of a single device can exceed 20 kilograms. A stainless steel plate is used as the cathode, and the silver can be recovered to less than 100 mg/L. Silver metal with good metallicity can still be obtained, and it is easy to remove from the stainless steel plate. Stripping is currently the best silver recycling equipment. The remaining silver ions (less than 100 mg/L) after electrolysis recovery can be precipitated and recovered using the agent (codename TMT) developed by the American Kodak Company. The silver can be treated to less than 0.5 mg/L, which can meet the discharge water standards.

The metal substitution method uses iron materials and puts them into waste liquid to cause silver to precipitate due to substitution. This method contains iron in the fixer, which must be discarded. However, for bleaching and fixing solutions, as long as 20% of the waste solution is discarded and the iron content is reduced, it can still be reused.

Chemical replacement method can use sodium sulfide or sodium borohydride (NaBH4) to remove silver from the waste liquid. Silver sulfide can be obtained from the reaction of sodium sulfide, and metallic silver can be obtained from sodium borohydride. The advantage of chemical treatment is that it is fast, the reaction rate can reach more than 99%, and the purity of silver is more than 95%. The commonly used method is to add a saturated solution of sodium sulfide, and the silver ions in the wastewater turn into black silver sulfide powder, which precipitates 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 the electrolytic cell. This method is simple, but the precipitate produced must be purified before pure metallic silver can be obtained, and the added chemicals are expensive and have low economic benefits. If you want to recover silver from discarded black and white films or X-ray films, you must First dissolve the silver into a solution. Undeveloped waste films can be dissolved in silver halide with fixing solution. Developed waste films must first use an oxidizing agent (such as potassium ferricyanide, ferric EDTA or copper chloride) to make the silver into a compound, and then use a fixing solution to dissolve the silver compound. . The obtained fixing solution can be used to remove silver metal by the aforementioned electrolysis method.

Relevant new technologies and methods:

According to overseas media reports, the American CSRS company has launched equipment to recover the "silver" in the fixer solution of the film processor. The electrolytic silver recycling machine system manufactured by CSRS is one of the most advanced recycling systems in the world. It uses intelligent microprocessing technology to recover the "silver" that is about to be put into the liquid medicine in the first time. It not only has a high recovery rate, but also can effectively extend the service life of the fixer. The system's operation panel adopts internationally recognized touch buttons. When the machine is running, a "recycle now" warning light will appear to remind the operator. When not running, the machine enters the "sleep" state. The entire recycling machine adopts a closed circuit and closed design to protect the operator from chemicals. At present, this product has obtained UL, FCC, TUV, CE and other safety marks.

Scientists have been studying methods to recover silver from photo printing waste, but most recovery processes are inefficient and sometimes cause more pollution. Now that may be changing: A scientist at Oak Ridge National Laboratory has developed a process that can recover 99.999% of silver from photographic waste.

A key problem in most silver recovery processes is the production of silver sulfate, a difficult-to-remove contaminant. Older procedures involved adding small amounts of hypochlorous acid to large amounts of silver-containing photographic waste. The procedure of Oak Ridge National Laboratory is to pump the silver-containing waste liquid into a reaction tank, add excess hypochlorous acid, so that the thiosulfate in the fixer solution is oxidized in the reaction tank, and through fine adjustment of the acidity, the silver That is, silver chloride precipitates out. Next, sodium dithionite is added as a reducing agent to convert silver chloride into silver. Using the results of the Oak Ridge National Laboratory's program test, the silver content in the waste liquid can be reduced from 500 mg per liter to less than 1 mg per liter. Researchers filter the waste liquid to obtain nearly pure silver.

Li Yungang used sodium dithionite (Na2S2O4) to extract silver from waste fixer solution [J]. Hydrometallurgy, 1999, (2): 26-30. Based on the residual root content of the waste fixer solution after reduction The changes in mass concentration, silver powder quality (silver powder grade) and Na2S2O3 mass concentration in the waste fixer were used as inspection indicators. The effect of using sodium silver dithionite (Na2S2O4) as a reducing agent to extract silver from the waste fixer was studied. As well as the regeneration of waste fixer solution, the results show that this silver extraction method can not only obtain higher purity metallic silver and waste fixer solution with very low silver mass concentration (ρ (Ag) <0.05g/L>, but also obtain The mass concentration of the main component Na2S2O3 in the fixer can be increased, so that the waste fixer can be regenerated.

Jiang Guohong's method and process for reducing silver from waste fixer using organic acid (Ar(OH)3COOH). Conditions. The test results show that using organic acid (Ar(OH)3COOH) to reduce silver from waste fixer solution, the silver reduction rate is 99.20%, the total recovery rate is 94.5%, and the mass fraction of silver in the recovered silver powder (flakes) is 97.43 %. The latest method of reducing sugar is used to extract silver. This method has the characteristics of low cost, simple operation, good results, high purity and easy promotion.

Method in middle school textbooks:

< p>Four steps to extract silver by electrolysis: 1. Electrolysis 2. Purification 3. Replacement 4. Purification

The existence state of silver element in the fixer is a complex of sulfur with sulfate, which cannot be directly Use displacement reaction.

1. Find two carbon rods for electrolysis, clean them, and connect them to the positive and negative poles of the adjustable voltage regulator (DC power supply, the current should be more than 10A). Insert it into the fixer and keep it as far apart as possible. The wire connected to the carbon rod should not be in contact with the liquid. Apply electricity and adjust the voltage so that the carbon rod connected to the positive electrode will produce slight gas. The metal silver will slowly deposit on the carbon rod at the negative electrode. Up. I forgot when it ended.

2. Purify the carbon rod of the negative electrode into excess dilute nitric acid to completely melt the metallic silver deposited on the surface to form a mixture of silver nitrate and nitric acid. Liquid. Use filter paper to filter the solid magazine.

3. Add excess iron powder to the mixture. After the reaction is completed, the remaining solid is a mixture of metallic silver and metallic iron. Use filter paper to filter out the solid. , rinse with light water.

4. Purify and add excess dilute hydrochloric acid to the solid to dissolve the iron powder.

The remaining solid is relatively pure metallic silver. Liquid automatic silver extraction machine address: 336000 Jiangxi Yichun Regional Practical Technology Research Institute Tel: 0795-3265550 Portable gold and silver direct extraction machine This machine can directly extract gold and silver from gold and silver-containing liquid without adding any chemicals. 220V, 60W civilian use It can be operated with electricity (equivalent to household appliances, no professional knowledge is required). It consists of a portable password box, a dedicated power supply, a liquid circulation device, a pole piece, and a portable lifting chamber. Especially for the treatment of waste fixing liquid (containing silver) and electroplating gold-containing silver liquid from photo studios and hospitals, it can produce 400g of silver or 100g of gold of 90% grade precious metals per day. For situations where there is a lot of processing liquid, multiple machines can be used in series to process The waste liquid can be reused. It is especially suitable for enterprises that deal with waste liquid containing gold and silver, as well as laid-off workers and individual entrepreneurs to make money. This machine is easy to carry, safe, easy to operate, intuitive and efficient, can be operated mobilely without a fixed location, and the price is suitable (about 1,500 yuan).

Related information: Special topic technology - Method for recovering silver from silver-containing waste. Silver is a precious rare metal with a wide range of uses. Silver has good electrical conductivity, thermal conductivity and high chemical stability. Halide is a good photosensitive material. Due to the limited geological reserves of silver and the complicated and expensive process of producing silver, it is particularly important to recycle silver from various silver-containing waste materials. This can reduce the pollution of silver-containing waste materials. Heavy metals pollute the environment and can be recycled to produce silver powder, which has good environmental and economic benefits. The following information is RMB 15 each, and the full set is RMB 100.

SQ05501 Recovery of silver from precipitation liquid SQ05502 Chemical extraction and regeneration of silver from color precipitation liquid SQ05503 Recovery of silver from photographic waste liquid SQ05504 Research on the process of recovery of silver from waste precipitation liquid SQ05505 Recovery of silver from waste precipitation liquid SQ05505 Recovering silver from waste liquid SQ05506 Recovering silver from waste fixing liquid by vulcanization method SQ05507 Recovering silver from waste fixing liquid by rapid precipitation method SQ05508 Regenerating and recovering silver from various silver-containing waste materials SQ05509 Recovering silver and high-purity silver from waste liquid containing silver Developed SQ05510 High-temperature iron reduction method to extract silver from waste liquid SQ05511 Use sodium borohydride to recover silver from silver-containing waste liquid SQ05512 Regeneration of silver from waste fixer liquid The patent for the above information is 35 yuan per copy, and other materials are 15 yuan per copy. The center also searches various special information, Chinese and foreign standards, patents, etc.

Extracting silver from waste medicine: As we all know, an important component of photosensitive materials is silver halide, and during the washing process, the chemical solution will leave a lot of silver compounds. However, many printing companies cannot recycle them and have to throw them away in vain. It causes a waste of resources and pollutes the environment. The technology for extracting silver from waste fixer solution is now being transferred to the outside world. This technology is very simple. The equipment only requires a few crucibles, a stove, a small fan, and a large container of 50 liters.

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