Write the principles of the following methods :

(i) vapour phase refining

(ii) zone refining

(iii) chromatography

(i) Vapour phase refining

The principle of this method is based on the decomposition of the volatile compound formed by the metal to be recovered with a suitable reagent.

For ex- monds process for obtaining Ni.

In this process, nickel (metal to be refined) is heated in a stream of CO

(reagent) forming nickel tetracarbonyl- Ni(CO)₄ (volatile compound). This is

subjected to higher temperature so that it is decomposed to give pure Ni.

(ii) Zone refining

This method is based on the principle that the impurities are more soluble in the melt of metal than in the solid state of the metal.

This is used to get metal in ultrapure state. Here when impure molten metal is allowed to cool down , the pure metal is crystalline out first , while impurities present in the metal remain in liquid phase.

(iii) Chromatography

This method is based on the principle that different components of a mixture are differently adsorbed on an adsorbent. Later the adsorbed components are removed (eluted) by using suitable solvents (eluant).