Extraction from Gold Ores - Mac Arthur Forrest Cyanide Process:
The extraction of gold from sulphide or telluride ore includes the steps shown below:
1) Crushing, grinding and concentration:
The gold ore is at first crushed and powdered, and concentrated by froth-floatation procedure.2) Roasting:
The concentrated ore is roasted in surplus of air or oxygen whenever impurities of arsenic, sulphur, and tellurium are oxidised and escape as their volatile oxides.3) Treatment with KCN:
The finely powdered roasted ore is carried in large vats made up of cement or iron and containing false wooden bottoms with holes. It is then treated with a dilute 0.5 percent solution of KCN in the presence of excess of air for 24 hours. As an outcome, gold dissolves in KCN to provide a soluble complex termed as potassium aurocyanide.
4Au + 8KCN + 2H2O + O2 → 4K[Au(CN)2] + 4KOH Pot. aurocyanide4) Precipitation of gold:
The above solution having potassium aurocyanide is filtered to eliminate insoluble impurities and then treated with zinc dust. Gold being low electropositive than zinc, it acquires precipitated.
2K [Au (CN2] + Zn →K2 [Zn (CN)4] + 2 Au↓
The precipitated gold is recovered by filtration. It is further purified by electro-refining.5) Electrolytic refining:
This technique is used whenever very pure gold is needed. The impure gold made the anode whereas a thin sheet of pure gold performs as the cathode. The electrolyte contains a solution of auric chloride (AuCl3) acidified with 10-12 percent HCl. On passing current, the pure gold gets acquired on the cathode.
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