Occurrence and principles of extraction of gold
Gold -
Atomic mass : 196.97 Valency : 1 and
3
Atomic number : 79 Symbol : Au
Position in the periodic table : Period Number -6, Group
Number -11.
Occurrence
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).
Ores
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
steps :
i) Crushing, grinding and concentration
The gold ore is crushed and powdered, and then
concentrated by froth- floatation process.
ii) Roasting
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 electro- refining.
iv) Electrolytic refining
This 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.
Properties
Physical properties
i) Gold is a lustrous yellow metal which melts at
1063 o C to a green liquid and boils
at 2610 o C.
ii) It is the most malleable and ductile of all
metals.
iii) It is a good conductor of heat and electricity.
Chemical properties
i) Gold is unaffected by dry or moist air or oxygen.
ii) Alkalies have no action on gold.
iii) With Aquaregia
Gold dissolves in aquaregia (3 parts of con.HCl + 1 part
of con.HNO3)
to
form auric chloride.
2Au + 9 HCl + 3HNO3 ® 2AuCl3 + 6H2O + 3NOCl
Auric chloride Nitrosyl chloride
iv) With chlorine
At 200 o C, gold is readily attacked by chlorine forming
auric chloride.
2Au + 3Cl2 ® 2AuCl3
Uses
i) It is used in coinage, jewellery and ornamental
vessels.
ii) Gold leaf is used for decorations, lettering and
in medicine as tonic.
iii) "Purple of cassius" which is a
combination of gold with colloidal stannic hydroxide is used for making ruby red glass and high class pottery.
Fineness of gold
Gold content of an alloy of gold is termed its fineness
and is generally expressed in
carats. Pure gold is 24 carats. Ornaments available these days are made of 18 carat gold, which is an alloy containing 18
parts by weight of gold alloyed with 6
parts by weight of another metal generally copper.
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