A sample of galena is contaminated with zinc blende. Name one chemical which can be used to concentrate galena selectively by froth floatation method.

Answer

To solve the problem of selectively concentrating galena (lead sulfide, PbS) from a sample contaminated with zinc blende (zinc sulfide, ZnS) using the froth flotation method, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Components**:
- Galena (PbS) is the ore of lead.
- Zinc blende (ZnS) is the ore of zinc.
- The goal is to separate PbS from ZnS.

2. **Froth Flotation Method**:
- This method is used to separate hydrophobic materials from hydrophilic ones. In this case, we want to make PbS hydrophobic while keeping ZnS hydrophilic.

3. **Use of a Depressant**:
- To achieve selective separation, we need to add a chemical called a depressant. A depressant is a substance that prevents certain minerals from floating while allowing others to do so.

4. **Choosing the Depressant**:
- A common depressant used for this purpose is **sodium cyanide (NaCN)**.
- Sodium cyanide selectively forms a complex with zinc sulfide, making it hydrophilic and preventing it from floating.

5. **Formation of Complex**:
- When sodium cyanide is added to the mixture, it reacts with zinc sulfide to form a soluble complex, such as **Na2Zn(CN)4**.
- This complex prevents ZnS from forming froth, allowing only PbS to be collected in the froth.

6. **Separation**:
- The froth that forms will contain primarily galena (PbS), which can then be collected and further processed.

### Final Answer:
The chemical that can be used to concentrate galena selectively by the froth flotation method is **sodium cyanide (NaCN)**.

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