Refining

➡️A metal extracted by any method is usually contaminated with some

impurity. For obtaining metals of high purity, several techniques are

used depending upon the differences in properties of the metal and the

impurity. Some of them are listed below.

(a) Distillation

 (b) Liquation

(c) Electrolysis 

(d) Zone refining

(e) Vapour phase refining 

(f ) Chromatographic methods

These are described in detail here.

Related Question: Outline the principles of refining of metals by the following methods:
(i) Zone refining
(ii) Electrolytic refining
(iii) Vapour phase refining

Distillation

➡️This is very useful for low boiling metals like zinc and mercury.
➡️The impure metal is evaporated to obtain the pure metal as distillate.

Liquation

➡️In this method a low melting metal like tin can be made to flow on

a sloping surface.

➡️In this way it is separated from higher melting

impurities.

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 more basic metal remains in the solution and the less basic ones

go to the anode mud. This process is also explained using the concept

of electrode potential, over potential, and Gibbs energy

➡️Copper is refined using an electrolytic method. Anodes are of impure copper and pure copper strips are taken as cathode. The electrolyte is acidified solution of copper sulphate and the net result of electrolysis is the transfer of copper in pure form from the anode

to the cathode.

Impurities from the blister copper deposit as anode mud which

contains antimony, selenium, tellurium, silver, gold and platinum;

recovery of these elements may meet the cost of refining. Zinc may

also be refined this way.

Zone refining

➡️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 mobile heater

surrounding the rod of impure metal is fixed at its one end

. The molten zone moves along with the heater which is

moved forward. 

➡️As the heater moves forward, the pure metal crystallises

out of the melt left behind and the impurities

pass on into the adjacent new molten zone

created by movement of heaters.

➡️The process

is repeated several times and the heater is

moved in the same direction again and again.

Impurities get concentrated at one end. This

end is cut off. This method is very useful for

producing semiconductor and other metals of

very high purity, e.g., germanium, silicon,

boron, gallium and indium.

Vapour phase refining

➡️In this method, the metal is converted into its volatile compound

which is collected and decomposed to give pure metal. So, the two

requirements are:

(i) the metal should form a volatile compound with an available

reagent,

(ii) the volatile compound should be easily decomposable, so that

the recovery is easy.

➡️Following examples will illustrate this technique

➡️Mond Process for Refining Nickel: In this process, nickel is heated

in a stream of carbon monoxide forming a volatile complex named as

nickel tetracarbonyl. This compex is decomposed at higher temperature

to obtain pure metal.

➡️van Arkel Method for Refining Zirconium or Titanium: This method is

very useful for removing all the oxygen and nitrogen present in the

form of impurity in certain metals like Zr and Ti. The crude metal is

heated in an evacuated vessel with iodine. The metal iodide being

more covalent, volatilises:

➡️The metal iodide is decomposed on a tungsten filament, electrically

heated to about 1800K. The pure metal deposits on the filament

Chromatographic methods

➡️Column chromatography is very useful for purification of the

elements which are available in minute quantities and the impurities

are not very different in chemical properties from the element to be

purified.