Zone refining is based on the principle that ..........

1.  Impurities of low boiling metals can be separated by distillation.

2.  Impurities are more soluble in molten metal than in solid metal.

3.  Different components of a mixture are differently adsorbed on an adsorbent.

4.  Vapours of volatile compound can be decomposed in pure metal.

Zone refining is based on the principle that the impurities are more soluble in molten state than in solid state of metal. A circular mobile heater fixed at one end of impure metal rod. The molten zone moves along with heater which is moved forward.
 
As the heater moves forward, the pure metal crystallises out of the melt and the impurities pass on into adjacent molten zone. The process is repeated several times and the heater is moved in the same direction.
 
At one end, impurities get concentrated. This end is cut off. e.g., germanium, silicon, gallium etc., are refined by this method