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From a long-term perspective, the investment in comprehensive utilization of PCB plating waste liquid can save money and reduce costs. Various chemical products are used for PCB plating. The waste liquid produced by these chemical products is a useful material for chemical production after comprehensive utilization treatment, and once it is discharged from the production process, it becomes the most harmful substance. The comprehensive utilization of PCB plating waste liquid is not only to reduce the loss caused by discharge, but also very meaningful from the perspective of preventing pollution and reducing the cost of sewage treatment.
Recycling technology of precious metals in PCB board electroplating waste liquid
Gold recovery
Gold is the most chemically stable metal. It has good decoration, corrosion resistance, abrasion resistance, resistance to discoloration and high temperature oxidation. It also has low contact resistance and excellent brazing properties. Because its output is very small, when using it, we must consider how to maximize its properties with the least amount.
1. Recycling process:
(1) Pickling and water washing: add 1:2.5 hydrochloric acid solution to the gold powder and boil, stir for 5 minutes, pour off the upper solution, repeat this 3 to 5 times, until it does not appear yellow. Rinse the gold powder several times with distilled water until the pH value of the washed water is about 7 o'clock
(2) The gold-containing waste liquid is heated to 80 to 90 degree Celsius, and ferrous chloride solution is slowly added under constant stirring. The reaction is as follows;
Au3++3Fe2+==3Fe3++Au↓
As the gold ions are continuously reduced, the color of the solution gradually changes from yellow to red, and the gold powder sinks to the bottom. Continue to add excess ferrous oxide solution and let it stand for several hours. Take two drops of the static stratified liquid, add two drops of 1% red blood salt, the color is blue, indicating that the gold has been fully reduced. Pour the clear liquid from the upper part, and filter by suction under reduced pressure, and the lower part is the precipitation of ocher gold powder.
(3) Melting casting: place the gold powder in a quartz crucible, heat it to about 1200°C in a high-temperature tubular electric furnace, melt it and inject it into a graphite mold to cast it into a gold ingot. If the gold powder is not blunt, borax can be added when melting, but it is not suitable to use a quartz crucible. The gold powder can be cast in a graphite crucible.
(4) Drying: Put the washed gold powder in a drying box for drying to obtain orange sponge granular gold slag.
2. Under good ventilation, adjust the pH of the waste gold plating solution to about 1 with hydrochloric acid. Heat the solution to 70-80 degree Celsius and add zinc powder under constant stirring until the solution becomes transparent yellow-white and a large amount of gold powder is precipitated. During this process, keep PH=1 or so. The subsequent processing method is the same as (2).
3. Under good ventilation, pour the waste gold plating solution into a porcelain dish, heat and evaporate until it becomes viscous, dilute it with five times distilled water, and add ferrous sulfate acidified with hydrochloric acid under constant stirring until no more precipitation Until precipitation. The gold is black powdered and deposited on the bottom of the porcelain dish. The precipitate is first boiled with hydrochloric acid and then with nitric acid, and then washed and dried. It is better if it can be fired at 700~800 degree Celsius for 30 minutes.
4. Removal of unqualified gold plating. Unqualified gold plating should be re-plated as much as possible, and only removed when it cannot be rescued. The unqualified gold plating of PCB circuit board can be removed with alkaline gold stripping solution containing cyanide 50~60g/l. This gold stripping solution has a fast stripping speed and does not damage the nickel substrate. The amount of gold dissolved can reach 25g/l. The operating temperature is 30-50 degree Celsius.
