Gold is one of the important noble metals. It is widely used by all people in India. In pure form it is yellow in color. Gold is mainly found in the native state either mixed with quartz in ancient rocks (vein gold) or scattered in sand and gravel (placer gold) or in the alluvial sand (alluvial gold).
The important ore of gold is Alluvial sand or gravel Nearly half of the total world production of gold comes from the mines in South Africa.
Extraction from Gold Ores - Mac Arthur Forrest Cyanide Process.
The extraction of gold from the sulphide or telluride ore involves the following steps :
Firstprocess is concentration of the ore since it is a sulfide ore the ore is concentrated by froth floatation process. Where in the ore is wetted preferentially by froth and impurities by water.
The concentrated ore is roasted in excess of air or oxygen when impurities of sulphur, arsenic and tellurium are oxidized and escape as their volatile oxides.
Thefinely powdered roasted ore is taken in large vats made up of cement oriron 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 24hours. Gold dissolves in KCN to form a soluble complex called potassiumaurocyanide whereas the impurities are left out in water.
The reaction which is involved is
4Au + 8KCN + 2H2O + O2 →4K[Au(CN)2] + 4KOH
Thepotassium aurocyanide 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 electro refining.