Describe the following:
(i) Role of depressant in forth floatation process.
(ii) Role of silica in the metallurgy of copper.
(iii) Role of cryolite in the metallurgy of aluminium.

Explanation

(i) Role of depressant is to prevent one type of sulphide ore particles from forming the froth with air bubbles. For example : NaCl is used a depressant to separate PbS ore from ZnS ore.
(ii) To remove FeO, Silica is added during smelting which form ferrous silicate `(FeSiO_(3))` slag which floats over molten matte and can be easily removed.
(iii) Cryolite is added to alumina to make it a good conductor of electricity and to reduce the melting point of mix to around 1140K.