Gold:
Occurrence of gold
Gold is mainly found in the native state either like vein gold, placer gold or alluvial gold. It is present to a small extent in the combined state like sulphide as well, telluride and arsenosulphide. It is known as king of the metals.
Some well known ores of gold are given below:
a) Calaverite (AuTe2)
b) Sylvanite (AuAgTe2)
c) Bismuth aurite, (BiAu2)
(2) Extraction of gold (Au):
A. Mac-Arthur-Forest Cyanide process
The powdered gold ore, after concentration through the Froth-floatation method, is roasted to eliminate oxidisable impurities of arsenic, tellurium, and sulphur. After that the roasted ore can be treated along with the dilute solution of KCN in existence of atmospheric oxygen when gold dissolves due to the formation of an aurocyanide complex.
After that the metal is extracted via the addition of zinc dust.
B. chlorine process of Plattner: The roasted ore is moistened along with water and located in wooden vats with false perforated bottoms. Roasted ore can be saturated with the current of chlorine, gold chloride hence formed is leached with water (H2O) and the solution is treated with the reducing agent such as FeSO4 or H2S to precipitate gold.
Equation of the process is given below:
So the impure gold acquired consists of impurities of Ag (silver) and Cu (copper). Removal of Ag (silver) and Cu (copper) from the gold is known as parting. This can be done by heating impure gold with the concentrated H2SO4 or HNO3 (sulphuric acid / nitric acid) when Ag and Cu dissolve leaving behind Au.
Equation is described below:
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