Question

When copper pyrites are roasted in excess of air, a mixture of CuO + FeO is formed as impurities. This can be removed as slag during reduction of CuO. The flux added to form slag is :
(A) $Si{O_2}$ which is an acid flux
(B) Limestone which is a basic flux
(C) $Si{O_2}$ which is a basic flux
(D) CaO which is a basic flux

Answer 1

Metal oxides are basic in nature and non-metal oxides are acidic in nature. If acid and base are present in the reaction mixture, then most probably they will react.

Complete answer:
The flux that is added to remove Iron oxides from its mixture with copper oxide need to be acidic in nature because acidic reactants will react with the basic reactant. Iron oxides are basic because metal oxides are always basic in nature.
The only acid flux given in the options is Silica which is an acidic flux.
Basic flux is not useful in removal of iron oxides from its mixture with copper oxide.
So, the true answer for this question is (A) $Si{O_2}$ which is an acid flux.

Additional Information:
When Copper pyrites are roasted , $Si{O_2}$ is added with it to remove iron oxide from the mixture. Let’s see how $Si{O_2}$ reacts.
$FeO + Si{O_2} \to \mathop {FeSi{O_3}}\limits_{slag}$
Thus, by addition of Silicon dioxide, we can improve the purity of the ore.

Note: Do not get confused between the nature of Silicon dioxide, oxides of non-metals are always basic. Do not consider that Iron oxide reacts with limestone because both are basic in nature.