When copper pyrites is roasted in excess of air, a mixture of `CuO + FeO` is formed. `FeO` is present as impurity. This can be removed as slag during reduction of `CuO`. The flux added to form slag is

Options

A. `SiO_(2)`, which is an acidic flux
B. Limestone, which is a basic flux
C. `SiO_(2)`, which is the basic flux
D. `CaO`, which is a basic flux

Explanation

To solve the question regarding the flux added to remove the impurity of FeO during the reduction of CuO when copper pyrites is roasted, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Reaction**: When copper pyrites (CuFeS2) is roasted in excess air, it undergoes oxidation to form copper oxide (CuO) and ferrous oxide (FeO). The reaction can be summarized as:
\[ 2CuFeS_2 + 5O_2 \rightarrow 2CuO + 2FeO + 4SO_2 \]
Here, FeO is the impurity that needs to be removed.

2. **Identifying the Nature of Impurity**: The impurity FeO is a metal oxide and is basic in nature. Basic oxides can react with acidic fluxes to form slag.

3. **Choosing the Appropriate Flux**: To remove the basic impurity (FeO), we need to add an acidic flux. The common acidic flux used in metallurgy is silicon dioxide (SiO2).

4. **Reaction of Flux with Impurity**: The reaction between the basic impurity (FeO) and the acidic flux (SiO2) can be represented as:
\[ FeO + SiO_2 \rightarrow FeSiO_3 \]
The product, FeSiO3, is known as slag, which can be removed from the mixture.

5. **Conclusion**: Therefore, the flux added to form slag during the reduction of CuO, which helps in removing the impurity FeO, is silicon dioxide (SiO2).

### Final Answer:
The flux added to form slag is **SiO2**.

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