Write the chemical reactions involved in the process of extraction of Gold. Explain the role of dilute NaCN and Zn in this process.
Gold is very unreactive and is found as a native metal in the free state. So, it does not need to be chemically separated. Thus, gold is leached to get pure metal.
During leaching process, gold (Au) is first oxidized by oxygen of the air to Au+ cation which then combines with CN-ions to form the soluble complex, sodium dicyanoaurate(I).
Gold is then extracted from this complex by displacement method by using a more electropositive zinc metal. In this method, zinc (Zn) acts as a reducing agent and it reduces Au+ to Au. Zinc itself gets oxidized to Zn2+ ions which combine with CN- ions to form the soluble complex, sodium tetracyanozincate(II).
Role of NaCN in the extraction of gold is as follows:
NaCN is used in leaching of gold.
In this process, finely powdered gold is treated with the dilute solution of NaCN, a current of air is passed. As a result, gold passes into solution forming soluble complex cyanides while the impurities remain unaffected which are filtered off.
4 Au + 8NaCN + O2 + 2H2O → 4Na[Au(CN)2] + 4NaOH
Zinc ores are usually found together with gold and silver. Zinc concentrates are roasted and then leached with sulphuric acid to dissolve their zinc content, leaving a residue that contains lead, silver, and gold. The remaining lead-gold-silver residue is treated by a process in which the residue is heated to a high temperature (about 800°C ) under strongly oxidising conditions. The noble silver and gold remain in the elemental form, while the lead oxidises and is removed. The gold and silver alloy thus produced is refined.
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