Occurrence and principles of extraction of 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.
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. 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.
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.
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.
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
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.