a) Distillation (b) Poling (c) Liquation (d) Electrolysis etc.
The process that has to be adopted for purification of a given metal
depends on the nature of the metal and its impurities.
a) Distillation:
This method is very useful for purification of low
boiling metals like zinc and mercury containing high boiling metals as
impurities. The extracted metal in the molten state is distilled to obtain
the pure metal as distillate.
b) Poling:
The molten metal is stirred with logs (poles) of green
wood. The impurities are removed either as gases or they get oxidized
and form scum (slag) over the surface of the molten metal. Blister copper
is purified by this method. The reducing gases, evolved from the wood,
prevent the oxidation of copper.
c) Liquation:
In this method a low melting metal like tin can be
made to flow on a slopy surface to separate it from high melting impurities.
d) Electrolytic refining:
In this method, the impure metal is made
to act as anode. A strip of the same metal in pure form is used as
cathode. They are put in a suitable electrolytic bath containing soluble
salt of the same metal. The required metal gets deposited on the cathode
in the pure form. The metal, constituting the impurity, goes as the anode
mud. The reactions are:
Anode:M----->Mn+n. Cathode: Mn++--->M
(M = pure metal )
Where n = 1,2,3, ...
We use this electrolytic method
to refine copper.
For this an impure copper is taken
as anode and pure copper strips are
taken as cathode. The electrolyte is an acidified solution of copper sulphate.
As a result of electrolysis copper in pure form is transferred from the
anode to the cathode.
Anode: Cu------> Cu2+ + 2e Cathode: C u2+ +2e------>Cu
The soluble impurities go into the solution, where as insoluble
impurities from the blister copper get deposited at the bottom of anode as
anode mud which contains metals like antimony selenium, tellurium, silver,
gold and platinum; recovery of these metals may meet the cost of refining.
Anode:M----->Mn+n.