Write the chemical reaction involved in the extraction of gold by cyanide process. Also give the role of zinc in the extraction.

Explanation

Chemical reaction involved in the process of extraction of Gold are :
`4Au + 8NaCN + 2H_(2)O + O_(2) to underset("(Soluble complex)")underset("Sodium dicyanoaurate (I)")(4Na[Au(CN)_(2)] + 4 NaOH)`
Finely powdered native gold is treated with a dilute solution of NaCN while a current of air is continuously passed . As a result , gold get oxidised which then combine with `CN^(-)` forming their respective soluble complex cyanides while the impurities remain unaffected whcih are filtered off .
Zinc (Zn) is more electro positive metal which is used to recover the metal (i.e., gold ) from its cyanide complex by reduction or displacement method .
`2[Au underset((aq))((CN)_(2)^(-)) + Zn(s) to 2Au(s) + [underset((aq))(Zn)(CN)_(4)]^(2-)`