Explain the role of each of the following:
(i) NaCN in the extraction of silver
(ii) SiO2 in the extraction of copper
(i) The roasted ore of gold is leached with a solution of sodium cyanide in the presence of oxygen for many days. The role of NaCN in this process is to dissolve the gold to form an aurocyanide complex, from which the metal is obtained by displacement.
4Au + 8NaCN + 2H2O + O2 ---> 4 Na [Au(CN)2] + 4KOH
2Na[Au(CN)2] + Zn ---> Na2[Zn(CN)4] + 2Au
(ii) Copper matte contains Cu2S and FeS. In the blast furnace, copper matte is added with powdered coke and silica. The oxidation of ore takes place in this process. As a result, cuprous oxide and ferrous oxide are produced. The role of silica in this process is to remove the iron oxide obtained as ‘slag’. FeO combines with silica (flux) to form iron silicate, FeSiO3(slag).
FeO + SiO2 
FeSiO2
Flux slag
