`NaCN` is sometimes added in the froth flotation process as a depressant when `ZnS` and `PbS` minerals are expected because `:`

Options

A. `Pb(CN)_(2)` is precipitated while no effect on ZnS
B. ZnS forms soluble complex `Na_(2)[Zn(CN)_(4)]` while PbS forms froth
C. PbS forms soluble complex `Na_(2)[Pb(CN)_(4)]` while ZnS forms froth
D. NaCN is never added in froth floatation process

Explanation

To answer the question regarding why `NaCN` is added in the froth flotation process as a depressant when `ZnS` and `PbS` minerals are expected, we can follow these 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, ore is mixed with water and chemicals, and air is introduced to create bubbles. The desired minerals attach to the bubbles and rise to form a froth, while the unwanted materials remain in the solution.

**Hint**: Think about how different minerals behave in the presence of water and air.

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

**Hint**: Recall the properties of ZnS and PbS and how they interact with flotation reagents.

3. **Role of NaCN as a Depressant**:
- `NaCN` is added to the flotation process as a depressant. Its role is to selectively inhibit the flotation of one mineral while allowing the other to float. Specifically, `NaCN` prevents the flotation of `ZnS` by forming a soluble complex, while allowing `PbS` to form froth.

**Hint**: Consider how depressants selectively affect the flotation of different minerals.

4. **Formation of Soluble Complex**:
- When `NaCN` is introduced, it reacts with `ZnS` to form a soluble complex, `Na2Zn(CN)4`. This complex dissolves in the solution, preventing `ZnS` from attaching to the air bubbles and rising to the froth.

**Hint**: Think about the chemical interactions that occur when depressants are added to the mixture.

5. **Froth Formation**:
- As a result, `PbS` remains unaffected by `NaCN` and forms the froth, while `ZnS` is dissolved in the solution. This separation allows for the collection of `PbS` from the froth while `ZnS` remains in the liquid phase.

**Hint**: Visualize the separation process and how the different minerals behave in the presence of the depressant.

6. **Conclusion**:
- Therefore, `NaCN` is used in the froth flotation process to selectively depress `ZnS`, allowing for the efficient separation of `PbS` from the mixture.

**Hint**: Summarize the entire process to reinforce your understanding of how NaCN functions in this scenario.

### Final Answer:
`NaCN` is added in the froth flotation process as a depressant when `ZnS` and `PbS` minerals are expected because it forms a soluble complex with `ZnS`, preventing it from floating, while allowing `PbS` to form the froth and be collected.