It is mainly refined by chemical method and electrolysis method, the former is suitable for small batch production, and the latter is large in scale.
1. Chemical refining Commonly used oxalic acid reduction method. The crude gold is dissolved in aqua regia, HCl/Cl 2 , HCl/H 2 O 2 , the solution is heated to 70 ° C, and the pH is adjusted to 1 - 1.5 with 20% NaOH solution. The reduction reaction is:
2HAuCl 4 +3H 2 C 2 O 4 ====2Au↓+8HCl+6CO 2
Add an excess of solid oxalic acid with stirring. After the reaction is stable, adjust with lye and keep pH=1-1.5, and add appropriate amount of oxalic acid until the reaction is complete. The spongy gold produced is washed with 1:1HN0 3 and pure water, and the purity of gold is 99.9%-99.99%, and the recovery rate is >99%.
2. Electrolytic refining of crude gold containing Au>90%, Ag 0.1%-0.5% is cast as anode, using pure gold flake or Qin plate as cathode, containing Au 250-300g/L, HCl 150-300 g/L Electrolysis in an acidic HAuCl 4 solution. When the anode is dissolved, impurity metals such as Cu, Pb, Pd, and Pt are simultaneously dissolved and accumulated in the electrolyte, which may contaminate the cathode product. When the electrolyte contains (g/L) Au90, Cul0, Pb1, Pt10, Pd1, it should be replaced and sent for purification. Adhesion of poorly soluble chlorides (PbCl 2 , AgCl) on the surface of the anode causes passivation of the anode, which often makes it difficult to carry out electrolysis of the cell voltage. For this reason, it is necessary to periodically input an alternating voltage during electrolysis to form an asymmetric pulsating current, so that AgCl loosens and falls off. The anode generally needs to be covered with a bag to collect the anode slime. The electrolyte temperature is 50-70 ° C, the current density is 300 A/m 2 , and the AC pulsating current electrolysis is 500-1000 A/m 2 . When the current density is too low, HAuCl is formed and the muddy gold powder is decomposed. The cell voltage is between 0.3 and 0.8 V depending on factors such as anode composition and pole pitch. Current efficiency is typically 95%. The residual rate is 15%-20%, and the residual pole is remelted after being carefully removed from the anode mud to return to the anode for electrolysis. After the main component of anode slime gold and silver, lead chloride, a small amount of platinum group metals, silver chloride was dissolved sodium thiosulphate scraps of cast powder back to the anode.
China's pure gold quality standards (GB4134-84) are listed.
 China Pure Gold Quality Standard / % | ||||||||
species | Au | Ag | Cu | Fe | Pb | Bi | Sb | sum |
No. 1 gold  Au-1 | 100 | 0.01 | 0 | 0.002 | 0.001 | 0.002 | 0.002 | 0.01 |
Gold No. 2 Â Au-2 | 100 | 0.03 | 0.02 | 0.003 | 0.003 | 0.002 | 0.002 | 0.05 |
No. 3 gold  Au-3 | 99.9 | 　 | 　 | 　 | 　 | 　 | 　 | 0.1 |
Internationally, the Kat purity unit is also used, and the pure Au is 24K. China's Immigrant Bank stipulates that the 1K gold content is 4.1666%. The corresponding criteria for the K value and the gold content are as follows:
K value | twenty four | twenty two | 20 | 18 | 14 | 12 | 10 | 9 | 8 |
Gold content /% | 99.9984 | 91.6652 | 83.332 | 74.9988 | 58.3324 | 49.9992 | 41.666 | 37.4994 | 33.3328 |
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