USA: +1-585-535-1023

UK: +44-208-133-5697

AUS: +61-280-07-5697

50.6 EXTRACTION OF GOLD

Mining of gold in Ghana began long before the arrival of the Europeans, mainly by panning by hand. The metal was extracted from alluvial sand and gravel deposited on the banks and beds of rivers.

The use of machines in the mining of the metal started in about 1880 in Tarkwa area and extended to Obuasi in 1898. Ghana’s pre-independence name of ‘Gold Coast’ shows the importance of the metal in the economy and history of the country.

 

OCCURRENCE

Since gold is very unreactive it occurs in free state in nature. The primary sources of gold are:

(i) Alluvial gravel. It is found in the beds of certain rivers. It is referred to as alluvial gold.

(ii) Auriferous quartz. Gold also occurs in the veins of quartz. This is called relf gold.

EXTRACTION

The various steps involved in the extraction of gold are:

1. The concentrated ore is roasted to remove all the oxidisable impurities.

2. Treatment with KCN

 

The roasted ore is then treated with a solution of sodium cyanide or potassium cyanide for some days. In the presence of atmospheric oxygen, gold dissolves in the form of a complex.

 

4Au + 8NaCN + 2H20 + O2      →        4Na[Au(CN)2] + 4NaOH

Sodium dicyanoaurate (1)

 

3.Precipitation of Gold. From the solution the metal is precipitated by adding zinc shavings.

2Na[Au(CN)2]+Zn                   →        Na2[Zn(CN)4] +2Au ↓

Sodium tetracyanozincate (IT)

 

The process is called Mac Arthur Forest Cyanide Process.

4. Purification. Gold obtained in the above process is impure and contains metals such as silver and copper as impurities, Removal of silver and copper impurities from impure gold is known as parting. It is achieved by boiling impure gold with cone. H2SO4 Silver and copper dissolve in hot cone. H2SO4 while the gold remains as such. Parting can also be done with cone. HNO3. Purification of gold can also

be done by electrolysis.

 

The anode is impure gold. The cathode is a strip of pure gold coated with a thin layer of graphite. The electrolyte is an aqueous solution of gold (III) chloride or gold (III) trixonitrate(V) and trioxonitrate(V) acid. The gold ions are the cathode on electrolysis:

Au+3 + 3e-       →        Au

Crude mining and extraction of gold and diamond is on a small scale in Ghana. It is popularly called ‘Galamsey’ Gold bearing ores or tailings, i.e., waste from previous ore treatment, is first concentrated by washing several times with water in a pan or other suitable container

to remove mud and other fifth.

The gold bearing product is then poured on a jute bag or any wide mesh cloth laid on an inclined table. It is then repeatedly washed with water to remove the lighter gangue. The heavier gold material, trapped in the. sack or cloth, is washed off into a pan with water and then decanted. The solid gold product is rubbed well with mercury to form an amalgam leaving the remaining gangue.

In the final stage, the gold amalgam is put in a white cloth and squeezed hard to let out as much mercury as possible. gold product thus obtained is white due to mercury contamination. This is washed with water in a pan. The heavier mercury falls to the bottom and the gold remains on top. The gold is harvested (collected) ready to be sold. The gold may also be recovered by distillation.

The gold produced by both Ashanti Goldfields Company Limited and ‘Galamsey’ operators is not very pure. Pure gold is obtained by refining their products outside Ghana.

USES OF GOLD

  1. For preparing ornaments.
  2. For electrogilding (i.e., gold plating by electrolysis).
  3. For filling teeth, and in switch contacts.
  4. For making compounds of gold.
  5. For preparing alloys.

Do You Know?

Pure gold is soft and is generally hardened by adding silver or copper to it for the purpose of
making ornaments. The weight of gold in gold ornaments is expressed in terms of ‘carats’. Pure
gold is taken as 24 carats. 18 carat gold means that it contains 18 parts by weight of gold in 24
parts by weight of the given alloy. The percentage of gold in this sample may be calculated as:

Percentage of gold in 18 carat gold

Most jewellery is made out of 22 carat gold