Leach Gold by Chlorination
Gold dissolves in aqueous chloride solution to form gold+ and gold3+ chloride complexes.
Au + 2Cl- = AuCl2 - +e
Au + 4Cl- = AuCl4 - + 3e
It’s possible that a solution of auric chloride can be decomposed by many reducing agents such organic compounds, metals, charcoal, and ether. The presence of hydrochloric acid trends to hinder the deposition of metal. Nowadays, the action of carbon can be expressed according to the following reaction:
AuCl4 - + 3e = Au + 4Cl- ORP = 1000 mV (1MCl-)
From pure solutions of AuCl4- in very dilute acid, it is possible to obtain gold loadings on activated carbon of about 60% by mass, but the gold could be lightly attached and fragments are easily dislodged.
Silver and lead form insoluble chlorides in chloride media. This is important because insoluble products can reduce the solubility of gold due to the formation of an insoluble layer. Nevertheless, passivation can occur to any important degree when the silver and lead content of the gold alloy is more than 12%.
Copper and zinc form relatively unstable chloro-complexes. All these compounds are less stable than auric chloride. It is important the large relatively differences in the stability constants of the copper cyanide chloride and gold complexes; it means that less copper can be dissolved in chloride media than in cyanide.
Gold telluride minerals are soluble in acid chloride media when there is an oxidant such as iron ferric or chlorine, and dissolve to form some complexed compounds of gold and tellurium. The behaviour of carbonates is interesting because the decomposition of these minerals help to recover gold because there is a better exposition of locked gold.
The process has been applied at Emperor (Fiji) that treats a gold-tellurium ore does not easily to cyanidation. Other plant that employed this process was Antimony Products, Pty. Ltd. (South Africa) for the recovery of gold from antimony rich slag that was stockpiled for a long time. The process consisted in getting a gold concentrate by flotation which was leached in chloride media. Antimony was separated by hydrolysis and the liquor was treated with activated carbon for recovering the precious metals. The gold content in the stockpile was 85 g/t Au with 9.9% Sb, an the flotation concentrate reported 935 g/t Au and 31.6% Sb. Gold recoveries were more than 80%. The treatment has a bottleneck in the filtration step because the rate was low and special care was considered during this stage.