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Question
How is gold extracted from its ore?
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Solution
The important ores of gold is Alluvial sand (or) Gravel.
Mac Arthur Forrest Cyanide process:
The extraction of gold from telluride (or) sulphide ore involves the following steps:
(i) Crushing grinding and concentration: The gold ore is crushed and powdered well and then concentrated by forth floatation process.
(ii) Roasting: The concentrated ore is roasted in excess of air when impurities of S, arsenic and tellurium are oxidised and escape as their volatile oxides.
(iii) Treatment with KCN: The 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 the 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 is filtered to remove the insoluble impurities and then treated with zinc dust. Gold being less electropositive than zinc, it gets precipitated.
2K[Au(CN)2]+Zn→K2[Zn(CN)4]Potassiumtetracyanozincate(II)+2AuGold↓
The precipitated gold is recovered by filtration. It is further purified by electrorefining.
(v) Electrolytic refining: In electrolytic refining, Anode is impure gold.
Cathode is a thin sheet of pure gold.
Electrolyte is a solution of Auric chloride(AuCl3)+10−12% HCl
On passing electric current, pure gold gets deposited on the cathode.
Mac Arthur Forrest Cyanide process:
The extraction of gold from telluride (or) sulphide ore involves the following steps:
(i) Crushing grinding and concentration: The gold ore is crushed and powdered well and then concentrated by forth floatation process.
(ii) Roasting: The concentrated ore is roasted in excess of air when impurities of S, arsenic and tellurium are oxidised and escape as their volatile oxides.
(iii) Treatment with KCN: The 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 the 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 is filtered to remove the insoluble impurities and then treated with zinc dust. Gold being less electropositive than zinc, it gets precipitated.
2K[Au(CN)2]+Zn→K2[Zn(CN)4]Potassiumtetracyanozincate(II)+2AuGold↓
The precipitated gold is recovered by filtration. It is further purified by electrorefining.
(v) Electrolytic refining: In electrolytic refining, Anode is impure gold.
Cathode is a thin sheet of pure gold.
Electrolyte is a solution of Auric chloride(AuCl3)+10−12% HCl
On passing electric current, pure gold gets deposited on the cathode.
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