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Recover gold from gold-plated scrap
Gold can be used for stripping, or chemical fire on a gold-plated scrap. The fire method deplating has a lead melting gold removal method in which molten metal is melted by molten lead , or a thermal expansion deplating method in which a plating layer and a substrate are different in thermal expansion coefficient. The chemical method is a chemical deplating method in which a reagent is dissolved. The deplating process is now introduced.
1. Lead melting method
Lead-melting method is to remove the gold-plated parts to be treated in a molten electrolytic lead solution (lead melting point is 327 ° C), so that gold penetrates into the lead. The waste material after the withdrawal of the gold is taken out, the gold-containing lead liquid is cast into a precious lead plate, and the gold is further recovered from the precious lead by the ash blowing method or the electrolytic method. When ash is blown, silver may be added to the precious lead, gold and silver alloy may be blown by ash, water and gold may be quenched into gold and silver particles, and then gold nitrate may be used to obtain gold powder, and crude gold may be obtained after smelting the ingot.
Second, thermal expansion method gold plating
The method is based on the difference of the expansion coefficient of the gold and the tube alloy, and the gap between the gold plating layer and the tube body is generated by the thermal expansion method, and then boiled in the dilute sulfuric acid to completely fall off the gold layer. Finally, dissolution and purification are carried out. For example, a 1kg transistor is taken and heated in a muffle furnace at 800 ° C for 1 h to cool. At this time, a very thin gold plating layer is broken, air penetrates into the tube body and oxidizes the tube body, and a gap is formed between the gold plating layer and the tube body. Put it into the pickling tank with the resistance wire heater, add 6L of 25% sulfuric acid solution, boil for 1h, the gold plating layer will fall off easily due to the oxidation of the oxide layer and the boiling solution of the dilute sulfuric acid dissolved tube. . At the same time, there is sulfate precipitation. Remove the gold-removed transistor after a little cold. The solution in the tank is clarified and the upper acid is withdrawn for reuse. The precipitate contains gold powder and sulfate, and is diluted with water until all the sulfate is dissolved. After clarification, the solid solution is separated by decantation. In the solid precipitate, the powder further containing in addition to silicon and other impurities, then dissolved in aqua regia was concentrated after, dilution, filtration step, gold was replaced with zinc powder (or reduction with sodium sulfite) acid, A crude gold having a purity of 98% can be obtained.
Third, the chemical method of gold plating
Chemical deplating is to put the gold-plated waste into the deplating solution heated to 90 °C. After 1~2 min, gold enters the solution. When preparing the deplating solution, 75 g of sodium cyanide and sodium m-nitrobenzenesulfonate are weighed. 75 g was dissolved in 1 L of water until completely dissolved. If the amount of deplating is too much or the gold in the deplating solution is saturated, so that the gold plating layer cannot be removed, the deplating solution should be reconstituted. The waste after the withdrawal of gold is washed three times with distilled water, leaving the rinse water for the next rinse. Each liter of the gold-containing stripping solution was diluted with 5 L of distilled water, stirred well, and the pH was adjusted to 1-2 with hydrochloric acid. Adjust the pH value in the fume hood to avoid hydrogen cyanide poisoning. The gold in the stripping solution is then replaced with zinc or zinc wire until the solution is yellow. The supernatant is removed, and the gold powder is washed with water for 1 or 2 times, and then boiled with sulfuric acid to remove impurities such as zinc, and then the gold powder is washed with water, dried, and cast to obtain a crude gold ingot.
The gold solution deplated by chemical method can also be recovered from the deplating solution by electrolysis, and the electrolytic tail liquid can be used as a deplating solution after adding sodium cyanide and sodium m-nitrobenzenesulfonate. The biggest advantage of the electrolysis method is that the cyanide discharge is small or not discharged, and the cyanide solution can continue to be recycled in production, which is also beneficial to environmental protection.
Fourth, electrolysis method gold plating
Electrolytic gold withdrawal is carried out by using thiourea and sodium sulfite as electrolyte, graphite as cathode, and gold-plated waste as anode. By electrolysis, the gold on the plating layer is anodized to Au(I), and Au(I) then forms a complex cation Au[SC(NH 2 ) 2 ] 2 + into the solution with thiourea adsorbed on the gold surface. The Au(I) entering the solution is reduced to gold by sodium sulfite in the solution, precipitated at the bottom of the electrolytic cell, and the gold-containing precipitate is separated and purified to obtain pure gold.
The electrolyte composition was 2.5% of SC(NH 2 ) 2 and 2.5% of Na 2 SO 3 .
A graphite rod (Ф30 mm, length 500 mm) for the anode was placed on the central axis of the plastic drum.
The cathode graphite rod (Ф50 mm, length 400 mm) was placed side by side on the electrolytic cell.
Electrolytic tank and withdrawal drum: The electrolyzer is welded with PVC hard plastic and has a volume of 164L. The gold-removing roller is welded into a hexahedron with a polyvinyl chloride hard plastic plate, and each side has a boring hole of 3 mm to allow the drum to lift the liquid and circulate the electrolyte during electrolysis.
Electrolytic conditions: current density 200A/m 2 , cell voltage 4.1V, electrolysis time according to the thickness of the gold plating layer and the area of ​​the anode and cathode are equivalent, if equivalent, at a suitable current density, the gold dissolution rate is very large, the time can be short One point, a general time of 20 to 25 minutes is appropriate. The electroplating process has been widely used.
5. Gold plating with iodine -potassium iodide solution
This method is the same as the principle of leaching gold by iodination (Au+I=AuI, AuI+KI=KAuI 2 ). KAuI 2 can be leached more reducing agents, such as scrap iron, zinc, sulfur dioxide, oxalic acid, formic acid and hydrazine hydrate reduction, it can also be activated carbon adsorption, ion exchange exchange resin, a method to extract gold from the solution KAuI 2. For the regeneration of the solvent used for leaching, by comparison, it is considered that the process of reducing with sodium sulfite is reasonable, and the solution can be oxidized to elemental iodine by using the oxidizing agent sodium chlorate under acidic conditions to regenerate the solvent iodine, and also prevent Increased reduction costs and ecological environment pollution caused by the discharge of waste iodine solution. The method is simple, convenient to operate, and carefully operated, and the substrate to be plated can be regenerated. The specific process conditions are:
Leachate composition: iodine 50 ~ 80 / L. Potassium iodide 200 ~ 250g / L.
Deplating time: depending on the thickness of the coating, about 3 to 7 minutes each time, 3 to 8 times.
Extraction of precious liquid: reduction with sodium sulfite.
Regeneration conditions after reduction: the amount of sulfuric acid is 15% of the post-liquid, and the amount of sodium chlorate is about 20 g/L.
In the process of recovering gold with iodine-potassium iodide, the precious liquid is reduced with sodium sulfite to remove the gold, and some impurities should be hydrolyzed to oxidize and regenerate iodine. The produced crystalline iodine must be purified by sulfuric acid and then returned to use.
In order to de-gold plating and recover gold from the resulting solution, an electrolytic cell in which the anode and cathode regions are separated by a porous separator is used. The preferred material for the separator is a porous ceramic having a pore size of 0.0001 to 0.001 mm and a membrane wall thickness of 8 mm. When electrolyzing a polyiodide solution, the purpose of using a porous separator is to separate the electrode reaction product:
Au + +e - →Au; I 2 +2e→2I -
2H + +2e→H 2 ; 2I ― ―2e→I 2
2OH - 2e → 1/2O 2 + H 2 O
It can prevent the sponge gold deposited on the cathode from being dissolved in the acid washing solution. The electrode for electrochemically separating gold is made of glassy carbon. The process of electrolytic separation of gold is carried out under the conditions of voltage of 4 to 5 V and current density of 20 to 30 A/dm 2 for 2 to 3 hours, and the energy consumption per g of gold is 0.2 to 0.4 kW·h, and the gold recovery rate can be reached. 99% to 99.8%.
The gold scrap is sent to the non-ferrous metal. After the gold mud is filtered, it is sent to the smelting, and the cathode clarification liquid is returned to the process.
6. Nitric acid method gold plating
The method of deplating gold by the nitric acid method is as follows.
(1) The gold-plated waste parts are placed in a 30% to 40% nitric acid solution, and stirred while heating, so that the metal inside the plating layer of the component reacts with nitric acid, and the surface gold is loosened and peeled off, and the waste parts are taken out.
(2) The nitric acid solution containing the waste gold skin is allowed to stand, and the upper layer solution is poured out. The gold chips are washed with distilled water, dissolved in aqua regia, and then reduced to gold powder by using ferrous sulfate. After repeated washing and drying, it is sintered into a block to obtain crude gold.