Question

Outline the principles behind the refining of metals by the following methods:
(i) Zone refining method
(ii) Chromatographic method

Answer 1

The process or method of purifying an impure metal is known as refining of metal. After extraction from the raw materials, crude metals comprise 96-99% principal metal and rest is impurity so it is important to undergo metal refining as crude metals have inferior physical, chemical or mechanical properties and cannot be used by industries.

Complete answer
(i) Zone refining method: Method of Zone refining is also known as zone melting. In this technique, a circular form of mobile heater is usually fixed at one corner of a rod made from impure metal. In this method, the impurities are mainly concentrated at one end of the block of metal such that the rest of the block gets purified. This technique is utilised in the case where impurities dissolve in molten state in comparison to that in solid form of the metal. Silicon and germanium are generally purified by this technique.
(ii) Chromatographic method: This method follows the principle of chromatography technique in which components are made to move at different rates in a mixture or differential absorption of an absorbent. In this chromatographic method of metal refining, an impure metal is placed in a medium (i.e. liquid or gas). Then the medium is migrated through an absorbent. Different components of an impure metal will be absorbed at different levels. And thereafter, the absorbed components will be removed using a suitable solvent. Different forms of this method are available like Column Chromatography (using aluminium trioxide), Gas chromatography, Thin Layer Chromatography, etc.

Note:
Refining process differs from other processes like calcining or smelting which generally include a chemical change in the raw material. On the other hand, in the refining process, the final product is chemically identical to the raw material i.e. original one (only it is its purest form).