Describe the principle controlling each of the following processes :
Zone refining of metals.

Explanation

This method is based on the principle that the impurities are more soluble in the melt than in the solid state of the metal. A circular mobile heater is fixed at one end of a rod of the impure metal. The molten zone moves along with the heater. As the heater moves forward, the pure metal crystallises out of the melt and impurities pass on into the adjacent molten zone. The process is repeated several times. The impurities get concentrated at one end. This end is cut off.