Describe the role of the following :

(i) SiO₂ in the extraction of copper from copper matte

(ii) NaCN in froth floatation process

(i) SiO₂ is used as a (acidic) flux in the process of extraction of copper from copper pyrites (CuFeS₂). This flux reacts with the (basic) gangue (impurity), FeO to form iron silicate, FeSiO₃, which is known as the slag.

The reactions involved in this process are as follows:

• Production of copper matte (mixture of Cu₂S and FeS) from copper iron pyrites (CuFeS₂)

• Oxidisation of FeS to FeO

• Production of slag

(ii) Sulphide ores are preferentially wetted by oils while the gangue is preferentially wetted by water .The ore particles get adsorbed on oil droplets and come to the surface where they can be collected as froth gangue is wetted by water and gets settled down. Eg. It is done for PbS ores.

Depressants help to separate two sulphide ores by selective prevention of froth formation by one ore and allowing the other to come into froth. NaCN is used as a depressant in order to separate two sulphide ores (ZnS and PbS).