In the purification of impure Ni by Mond's process, metal is purified by

Options

A. electrolytic reduction
B. Vapour phase thermal decomposition
C. Thermite reduction
D. Carbon reduction

Explanation

To solve the question regarding the purification of impure nickel by Mond's process, we can break it down into the following steps:

### Step-by-Step Solution:

1. **Understanding Mond's Process**:
- The Mond process is a method used to purify nickel. It involves the formation of a volatile compound, nickel carbonyl, which can be decomposed to yield pure nickel.

2. **Initial Reaction**:
- Impure nickel is reacted with excess carbon monoxide (CO) at a temperature range of 60°C to 80°C.
- This reaction forms nickel carbonyl (Ni(CO)4) gas, while the impurities remain as solid residues.

3. **Formation of Nickel Carbonyl**:
- The reaction can be represented as:
\[ \text{Ni (impure)} + 4 \text{CO} \rightarrow \text{Ni(CO)}_4 \text{ (gas)} + \text{impurities (solid)} \]
- This step is crucial as it separates the nickel from its impurities.

4. **Decomposition of Nickel Carbonyl**:
- The nickel carbonyl gas is then heated to a higher temperature of around 180°C.
- At this temperature, nickel tetra carbonyl decomposes to form pure nickel and releases carbon monoxide gas:
\[ \text{Ni(CO)}_4 \text{ (gas)} \rightarrow \text{Ni (pure)} + 4 \text{CO (gas)} \]

5. **Final Product**:
- The result of this process is the production of pure nickel, while carbon monoxide is released as a byproduct.

6. **Conclusion**:
- Therefore, in the purification of impure nickel by Mond's process, the metal is purified by **vapor phase thermal decomposition**.

### Final Answer:
The purification of impure nickel by Mond's process is achieved through **vapor phase thermal decomposition**.

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