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

In the metallurgy of copper, the chief ore is copper pyrite, written as $$\mathrm{CuFeS_2}$$. During the process of extraction, the concentrated ore is first subjected to roasting. We have

$$\mathrm{2\,CuFeS_2 + O_2 \;\longrightarrow\; Cu_2S + 2\,FeS + SO_2}$$

The iron present in $$\mathrm{FeS}$$ is then partially oxidised further when more air is blown through the roaster:

$$\mathrm{2\,FeS + 3\,O_2 \;\longrightarrow\; 2\,FeO + 2\,SO_2}$$

Now the roasted mass contains mainly $$\mathrm{Cu_2S}$$, $$\mathrm{FeO}$$ and some unchanged $$\mathrm{SiO_2}$$ which was originally present as an earthy impurity in the ore. If no extra silica were added, the $$\mathrm{FeO}$$ would stay in the mixture and contaminate the molten copper.

To remove this $$\mathrm{FeO}$$ impurity, we deliberately add more silica (sand) as a flux. A flux is a substance that reacts with an impurity to form an easily fusible slag. Stating the general idea first:

Flux principle: “Acidic flux removes basic impurity, and basic flux removes acidic impurity.”

Because $$\mathrm{FeO}$$ is basic in nature, we choose an acidic flux, namely silica $$\mathrm{SiO_2}$$. The two react according to the following chemical equation:

$$\mathrm{FeO + SiO_2 \;\longrightarrow\; FeSiO_3}$$

The product $$\mathrm{FeSiO_3}$$ is called ferrous silicate. It is a molten slag, lighter than the molten copper matte, so it floats on top and can be skimmed off easily. In this way the iron impurity is removed from the system.

Observe that silica does not react with copper sulphide, nor does it directly reduce anything. Its sole purpose here is to convert the basic oxide of iron into a fusible silicate slag. Therefore, among the given options, only the statement “converts iron oxide into iron silicate” correctly describes the role of silica.

Hence, the correct answer is Option B.