The extraction of gold from the sulphide or telluride ore involves the following steps :
i) Crushing, grinding and concentration
The gold ore is crushed and powdered, and then concentrated by frothfloatation process.
The concentrated ore is roasted in excess of air or oxygen when impurities of sulphur, arsenic and tellurium are oxidised and escape as their volatile oxides.
iii) Treatment with KCN
The finely powdered roasted ore is taken in large vats made up of cement or iron and having false wooden bottoms with holes. It is then treated with a dilute (0.5%) solution of KCN in presence of excess of air for 24 hours. As a result, gold dissolves in KCN to form a soluble complex called potassium aurocyanide.
4Au + 8KCN + 2H2O + O2 → 4K[Au(CN)2] + 4KOH (Pot. aurocyanide)
iv) Precipitation of gold
The above solution containing potassium aurocyanide is filtered to remove insoluble impurities and then treated with zinc dust. Gold being less electropositive than zinc, it gets precipitated.
2K[Au(CN2] + Zn →K2[Zn(CN)4] + 2 Au↓
The precipitated gold is recovered by filtration .It is further purified by electrorefining.