Question

The addition of silica during the extraction of copper from its sulphide ore

• A. converts copper sulphide into copper silicate
• B. reduces copper sulphide into metallic copper
• C. converts iron oxide into iron silicate
• D. reduces the melting point of the reaction mixture

Hint:

In metallurgy, remember the key principle: an acidic flux (like SiO$_2$) is used to remove a basic impurity (like FeO, CaO), and a basic flux (like CaO) is used to remove an acidic impurity (like SiO$_2$, P$_4$O$_{10}$).

Answer 1

The Correct Option is C

The primary ore for copper extraction is often copper pyrite (CuFeS$_2$).
During the smelting process in a reverberatory furnace, the ore is heated with silica (SiO$_2$).
One of the main impurities in the ore is iron. During roasting, iron sulfide (FeS) is oxidized to iron(II) oxide (FeO).
$2\text{FeS} + 3\text{O}_2 \rightarrow 2\text{FeO} + 2\text{SO}_2$
This iron oxide (FeO) is a basic impurity. Silica (SiO$_2$) is added as an acidic flux.
The flux reacts with the impurity to form a fusible slag, which is less dense and can be easily removed.
$\text{FeO} (\text{basic impurity}) + \text{SiO}_2 (\text{acidic flux}) \rightarrow \text{FeSiO}_3 (\text{slag})$
The product, iron(II) silicate (FeSiO$_3$), is the slag.
Therefore, the purpose of adding silica is to convert the iron oxide impurity into iron silicate slag.
While fluxes also lower the melting point (Option D), their primary chemical function in this context is the removal of impurities as slag, making Option (C) the most accurate and specific answer.