Explain the zone refining process or explain refining by fractional crystallization.

(N/A) Principle: This method is based on the difference in the solubility of impurities in the molten and solid state of the metal.
Process: As shown in the figure,a mobile heater surrounding the rod of impure metal is fixed at one end. The molten zone moves along with the heater as it is moved forward. As the heater moves forward,the pure metal crystallizes out of the melt left behind,and the impurities pass into the adjacent new molten zone created by the movement of the heater.
Repetition: The process is repeated several times,and the heater is moved in the same direction repeatedly. Impurities get concentrated at one end,and this end is cut off.
Applications: This method is very useful for producing semiconductors and other metals of very high purity,e.g.,germanium,silicon,boron,gallium,and indium.