(i) Froth floatation method :

The method has been in use for removing gangue from sulphide ores. In this process, a suspension of the powered ore is made with water. To it, collectors and froth stabilisers are added. Collectors (e.g., pine oils, fatty acid, xanthates, etc.) enhance non-wettability of the mineral particles and froth stabilisers (e.g., Cresols, aniline) stabilise the froth.

The mineral particles become wet by oils while the gangue particles by water. A rotating paddle agitates the mixture and draws air in it. As a result, froth is formed which carries the mineral particles. The froth is light and is skimmed off. It is then dried for recovery of the ore particles.

(ii) Electrolytic refining of metals: In this method, the impure metal in pure form is used as Cathode. They are put in a suitable electrolyte both containing soluble salt of the same metal. The more basic metal remains in the solution and the less basic ones go to the anode mud. The reactions are

Anode :  $M\to M^{n+}+n{e}^{-}$

Cathode :  $M^{n+}+n{e}^{-}\to M$

Copper and zinc are refined by this method.

(iii) Zone refining of metals: 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 the rod of the impure metal. The molten zone moves along with the heater which is moved forward. As the heater moves forward, the pure metal crystalises out of melt and the impurities pass on into the adjacent molten zone. This process is repeated several times and the heater is moved in the same direction. At one end, impurities get concentrated.

This method is very useful for producing semiconductor and other metals of very high purity like germanium, silicon, boron etc.