PCB Technical - ​About PCB circuit board plating waste liquid

In the long run, the investment of PCB plating waste liquid in comprehensive applications can waste money and reduce costs. PCB plating uses a variety of chemical products. The waste liquid generated by these chemical commodities is a useful material for chemical consumption after comprehensive application and disposal, and once it is discharged from the consumption process, it becomes the most harmless substance. The comprehensive application of PCB plating waste liquid is not only to increase the loss caused by discharge, but is also very meaningful from the perspective of avoiding purification and reducing the cost of sewage disposal.

Gold recovery

Gold is the most volatile metal chemically. It has good decoration, corrosion resistance, abrasion resistance, resistance to discoloration and low-temperature oxidation. It also has low contact resistance and excellent brazing properties. Since its output is very small, when applying it, we must think about how to use the least amount to maximize its properties.

Recycling process:

(one),

(1) Heat the PCB gold waste liquid to 80-90 degree Celsius, and slowly add the ferrous chloride solution with stirring from time to time, the response is as follows;

Au3++3Fe2+==3Fe3++Au↓

As the gold ions are restored from time to time, the color of the solution gradually changes from yellow to white, 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 sports layering solution, add two drops of 1% red blood salt, the color is blue, indicating that the gold has been fully recovered.

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.

(2) 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 no yellow appears. Rinse the gold powder repeatedly with distilled water until the pH value of the washed water is about 7 o'clock

(3) Drying: Put the washed gold powder in a drying box for drying to obtain orange sponge granular gold slag.

(4) Melting casting: Place the gold powder in a quartz crucible, heat it to about 1200°C in a low-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.

(two),

Under good ventilation conditions, pour the PCB waste gold plating solution into a porcelain dish, heat and evaporate to a thin form, concentrate with five times distilled water, and add ferrous sulfate acidified with hydrochloric acid under stirring from time to time until no more precipitation occurs. . The gold appeared as a black powder and settled 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.

(3) Under good ventilation, use hydrochloric acid to adjust the pH of the spent gold plating solution to about 1. Heat the solution to 70-80 degree Celsius and add zinc powder with stirring from time to time until the solution becomes bright yellow-white and a small amount of gold powder is deposited. In this process, stick to about PH=1. The subsequent disposal method is opposite to (2).

(4) Removal of unqualified gold plating. Unqualified gold plating should be re-plated as much as possible, and the removal should be stopped only when there is no rescue. Unqualified gold plating on PCB can be removed with alkaline gold stripping solution containing cyanide 50~60g/l. This kind of gold stripping liquid has a fast stripping speed and does not damage the nickel substrate. The amount of gold dissolved can reach 25g/l. The operating temperature of the nickel plating after the gold stripping is 30-50 degree Celsius.

Silver recycling

Silver is also one of the precious metals. The methods for recovering silver in waste liquid are as follows:

Use 20% sodium hydroxide to adjust the pH of the waste liquid to between 8-9, and then add the sodium sulfide solution to generate silver sulfide precipitation;

2Ag++Na2S==Ag2S↓+2Na+↓

(2) Wash the silver sulfide precipitate with water until there is no remaining sodium sulfide. After filtering the water, put the silver sulfide in the crucible, heat it to 800~900 degree Celsius to stop desulfurization, and take it out to cool.

(3) Take 100 parts (quantity) of silver slag, add 10 parts of borax and 5 parts of sodium chloride, stir and then burn with a crucible to obtain crude silver.

(4) Dissolve the crude silver with nitric acid to make it a silver nitrate solution, then decolorize with activated carbon and filter.

(5) When the filtrate is diluted to the precipitation of silver nitrate crystals, the suction filtration method is used to make all the silver nitrate crystals precipitate and absorb dry. Initially put the crystallized silver nitrate in a desiccator and then it can be used. If the purity of the silver nitrate obtained is not high, the purity can be improved by repeated crystallization.

Palladium recovery

Palladium is also one of the precious metals. The methods for recovering palladium in PCB waste liquid are as follows:

In the palladium-containing waste liquid, add concentrated ammonia water to make the palladium completely be ammonia chromium, and then add concentrated hydrochloric acid to generate dichlorodiamino palladium salt precipitation, and then stop filtering and cleaning, it can be sold as a chemical raw material.

Nickel recovery

In the nickel-containing waste liquid, add sulfuric acid, the pH value of the conditioning solution is 2 to 3, and the insoluble impurities are filtered out. Then it is heated and diluted, and crystallized below 30°C for more than 48 hours. To obtain crude nickel sulfate. Filtrate recycling. Crude nickel sulfate is recrystallized to remove impurities. The pH value was adjusted to 2 to 3 with chemically pure sulfuric acid at first. Use a titanium evaporator to evaporate the solution to 50~52Be°. After 48 hours of natural crystallization in a dust-free room below 30°C, refined nickel sulfate can be obtained. Place the refined crystalline nickel sulfate in a plastic drain basket with pitted holes. Drain in a ventilated and clean place for 5-10 days to inspect the packaging.

Copper recycling

(1) Recover copper from copper-containing waste liquid that does not contain chelate.

Add excessive iron filings in the PCB-containing copper waste liquid, control the temperature of the solution to 30--50 degree Celsius, and react for about 1 hour until the end of the response, remove the iron filings, filter, wash and dry the replaced copper to obtain metal copper powder, The purity is over 99%.

(2) Recover copper from copper waste liquid containing chelating agent.

In the copper-containing waste liquid, adjust the PH value to above 11, add calcium hydroxide that exceeds the amount of copper ions, and stir the solution at the same time to generate copper hydroxide precipitation, filtration, washing, and precipitation. Add excess sulfuric acid to the precipitation, and then evaporate and crystallize. Take out the crystals and obtain copper sulfate crystals after drying.