When ZnS and PbS minerals are present together, Then NaCN added to separate them in the froth floation process as a depressant, beacuse :-

Options

A. `Pb (CN)_(2)` is precipitated while no effect on ZnS
B. ZnS forms soluble complex `Na_(2)[Zn(CN)_(4)]`
C. PbS forms soluble complex `Na_(2)[Pb (CN_(4))]`
D. If decreases the floatation property of PbS by making it hydrophillic

Answer

To solve the question regarding the separation of ZnS and PbS minerals using NaCN in the froth flotation process, we can break it down into the following steps:

### Step-by-Step Solution:

1. **Understanding Froth Flotation Process**:
- The froth flotation process is a method used to separate hydrophobic materials from hydrophilic ones. In this process, the ore (containing valuable minerals) is mixed with water and chemicals (collectors and depressants) to create a froth. The valuable minerals attach to air bubbles and rise to the surface, while the unwanted materials remain submerged.

**Hint**: Remember that the key to froth flotation is the difference in hydrophobicity between the minerals.

2. **Identifying the Minerals**:
- In this case, we are dealing with two sulfide minerals: Zinc sulfide (ZnS) and Lead sulfide (PbS). Both of these minerals can be present together in a mixture.

**Hint**: Recognize that both ZnS and PbS are sulfides and may behave similarly in some processes.

3. **Role of NaCN**:
- Sodium cyanide (NaCN) is added to the mixture as a depressant. A depressant is a substance that selectively inhibits the flotation of certain minerals while allowing others to float.

**Hint**: Think about how the addition of a chemical can change the behavior of one mineral without affecting the other.

4. **Formation of Soluble Complex**:
- When NaCN is added, it reacts with ZnS to form a soluble complex known as sodium tetra cyanido zincate (Na2[Zn(CN)4]). This complex is soluble in water, which means ZnS can be separated from the mixture.

**Hint**: Focus on the chemical reaction that occurs between NaCN and ZnS, leading to the formation of a soluble complex.

5. **Separation of Minerals**:
- As ZnS forms a soluble complex and goes into solution, PbS remains unaffected and can be collected separately. This allows for the effective separation of the two minerals.

**Hint**: Consider the implications of one mineral becoming soluble while the other remains solid in the froth flotation process.

6. **Conclusion**:
- The correct reason for using NaCN in the froth flotation process to separate ZnS and PbS is that ZnS forms a soluble complex (Na2[Zn(CN)4) with NaCN, while PbS does not react similarly and remains in the froth.

**Hint**: Look for the option that accurately describes the formation of the soluble complex as the key to the separation process.

### Final Answer:
NaCN is used as a depressant in the froth flotation process because ZnS forms a soluble complex (Na2[Zn(CN)4]), allowing for the separation of PbS which does not form a similar complex.