`NaCN` used in the froth floatation method for the purification
of ore is

Options

A. `ZnS` which contain `PbS`
B. `Cu_(2)S `which contain `Fe_(2)S_(3)`
C. `PbS` which contain `ZnS`
D. `PbS` which contain `SiO_(2)`

Explanation

To solve the question regarding the use of NaCN in the froth flotation method for the purification of ores, we can break down the explanation into clear steps:

### Step-by-Step Solution:

1. **Understanding Froth Flotation**:
- Froth flotation is a method used to separate hydrophobic materials from hydrophilic ones. It is commonly used for the concentration of ores, particularly sulfide ores.

2. **Role of Sodium Cyanide (NaCN)**:
- Sodium cyanide (NaCN) is used as a depressant in the froth flotation process. A depressant is a substance that prevents certain minerals from floating while allowing others to do so.

3. **Mechanism of Action**:
- In the case of sulfide ores, NaCN selectively prevents the formation of froth from certain sulfide minerals. For example, it can prevent zinc sulfide (ZnS) from floating while allowing lead sulfide (PbS) to float.

4. **Chemical Reaction**:
- The interaction of NaCN with zinc sulfide can be represented by the following reaction:
\[ 4 \text{NaCN} + \text{ZnS} \rightarrow \text{Na}_2\text{Zn(CN)}_4 + \text{Na}_2\text{S} \]
- This reaction shows that sodium cyanide forms a complex with zinc sulfide, which prevents it from forming froth.

5. **Separation of Ores**:
- By using NaCN, the lead sulfide (PbS) can be separated from the zinc sulfide (ZnS) because PbS will float and can be collected, while ZnS will remain in the slurry.

6. **Conclusion**:
- Therefore, NaCN is used in the froth flotation method specifically for the purification of ores containing zinc and lead sulfides, allowing for the selective separation of these minerals.

### Final Answer:
NaCN is used in the froth flotation method as a depressant to separate lead sulfide (PbS) from zinc sulfide (ZnS).

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