Question

Explain the zone refining process with an example.

Answer 1

1. Zone Refining method is based on the principles of fractional crystallisation. When an impure metal is melted and allowed to solidify, the impurities will prefer to be in the molten region, i.e. impurities are more soluble in the melt than in the solid-state metal.
2. In this process, the impure metal is taken in the form of a rod. One end of the rod is heated using a mobile induction heater which results in the melting of the metal on that portion of the rod.
3. When the heater is slowly moved to the other end the pure metal crystallises while the impurities will move on to the adjacent molten zone formed due to the movement of the heater. As the heater moves further away, the molten zone containing impurities also moves along with it.

4. The process is repeated several times by moving the heater in the same direction again and again to achieve the desired purity level.

5. This process is carried out in an inert gas atmosphere to prevent the oxidation of metals.
6. Elements such as germanium (Ge), silicon (Si) and gallium (Ga) that are used as semiconductors are refined using this process.