Roasting of copper pyrites is done:

Options

A. to remove moisture and volatile impurities
B. to oxidise free sulphur
C. to decompose pyrites into `Cu_(2)S and FeS`
D. to decompose `Cu_(2)S` into blister copper

Answer

To solve the question regarding the roasting of copper pyrites, we will analyze each of the provided options step by step.

### Step 1: Understanding the Process of Roasting
Roasting is a metallurgical process involving heating of ores in the presence of oxygen. In the case of copper pyrites (CuFeS₂), roasting is primarily done to convert the sulfide minerals into oxides.

### Step 2: Analyzing the Options
1. **Remove moisture and volatile impurities**:
- During roasting, moisture and any volatile impurities present in the ore are indeed removed. This is a necessary step to prepare the ore for further processing.

2. **Oxidize free sulfur**:
- The roasting process also involves the oxidation of sulfur present in the copper pyrites. Sulfur is converted to sulfur dioxide (SO₂) during this process.

3. **Decompose pyrite into Cu₂S and FeS**:
- Roasting leads to the decomposition of copper pyrites into a mixture of copper(I) sulfide (Cu₂S) and iron(II) sulfide (FeS). This is a key reaction that occurs during roasting.

4. **Decompose Cu₂S into blister copper**:
- This option is incorrect in the context of roasting. The decomposition of Cu₂S into blister copper occurs during smelting, not roasting.

### Step 3: Conclusion
Based on the analysis of the options:
- **Correct Options**: 1 (remove moisture and volatile impurities), 2 (oxidize free sulfur), and 3 (decompose pyrite into Cu₂S and FeS) are correct.
- **Incorrect Option**: 4 (decompose Cu₂S into blister copper) is incorrect.

### Final Answer
The roasting of copper pyrites is done primarily to:
1. Remove moisture and volatile impurities.
2. Oxidize free sulfur.
3. Decompose pyrite into Cu₂S and FeS.