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. India occupies 8th position among the gold producing countries of the world and account for about 2% of the total world production. The Kolar mines in Karnataka produce about 99.97% of our gold output.
Extraction from Gold Ores - Mac Arthur Forrest Cyanide Process.
The extraction of gold from the sulphide or telluride ore involves the following
i) Crushing, grinding and concentration
The gold ore is crushed and powdered, and then concentrated by froth- floatation 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
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 electro- refining.
v) Electrolytic refiningThis method is used when very pure gold is required. The impure gold is made the anode while a thin sheet of pure gold acts as the cathode. The electrolyte consists of a solution of auric chloride (AuCl3) acidified with 10-12% HCl. On passing current pure gold gets deposited on the cathode.
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