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

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

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.

`=>` 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.

`=>` The reactions are :

Anode : `M → M^(n+) + n e^(-)`

Cathode : `M^(n+) + n e^(-) → M` ......(52)

`=>` Copper is refined using an electrolytic method.

● Anodes are of impure copper

● Pure copper strips are taken as cathode

● The electrolyte is acidified solution of copper sulphate

● The net result of electrolysis is the transfer of copper in pure form from the anode to the cathode.

Anode : `Cu → Cu^(2+) +2 e^(-)`

Cathode : `Cu^(2+) +2 e^(-) → Cu` .........(53)

● 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.

`text(Principle :)` 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 a 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 crystallises out of the melt and the impurities pass on into the adjacent molten zone.

● The process is repeated several times and the heater is moved in the same direction.

● At one end, impurities get concentrated. 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.

`=>` In this method, the metal is converted into its volatile compound and collected elsewhere. It is then 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.

(a) `text(Mond Process for Refining Nickel :)`

● In this process, nickel is heated in a stream of carbon monoxide forming a volatile complex, nickel tetracarbonyl.

`Ni +4CO overset(330-350K)→ Ni (CO)_4` .........(54)

● The carbonyl is subjected to higher temperature so that it is decomposed giving the pure metal.

`Ni (CO)_4 overset(450-470K)→ Ni +4CO` ...........(55)

(b) `text(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.

`Zr +2I_2 → ZrI_4` ........(56)

● The metal iodide is decomposed on a tungsten filament, electrically heated to about `1800K`. The pure metal is thus deposited on the filament.

`ZrI_4 → Zr +2I_2` ........(57)

`text(Principle :)` Different components of a mixture are differently adsorbed on an adsorbent.

● The mixture is put in a liquid or gaseous medium which is moved through the adsorbent.

● Different components are adsorbed at different levels on the column.

● after that, the adsorbed components are removed (eluted) by using suitable solvents (eluant).

● Depending upon the physical state of the moving medium and the adsorbent material and also on the process of passage of the moving medium, the chromatographic method is given the name.

● In one such method the column of `Al_2O_3` is prepared in a glass tube and the moving medium containing a solution of the components is in liquid form. This is an example of column chromatography.

`->` This 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.

● There are several chromatographic techniques such as paper chromatography, column chromatography, gas chromatography, etc. Procedures followed in column chromatography have been depicted in Fig.

`text(Uses of Aluminium :)`

● Aluminium foils are used as wrappers for chocolates.

● The fine dust of the metal is used in paints and lacquers.

● Aluminium, being highly reactive, is also used in the extraction of chromium and manganese from their oxides.

● Wires of aluminium are used as electricity conductors.

● Alloys containing aluminium, being light, are very useful.

`text(Uses of Copper :)`

● Copper is used for making wires used in electrical industry and for water and steam pipes.

● It is also used in several alloys that are rather tougher than the metal itself, e.g., brass (with zinc), bronze (with tin) and coinage alloy (with nickel).

`text(Uses of Zinc :)`

● Zinc is used for galvanising iron.

● It is also used in large quantities in batteries, as a constituent of many alloys, e.g., brass, (Cu 60%, Zn 40%) and german silver (Cu 25-30%, Zn 25-30%, Ni 40–50%).

● Zinc dust is used as a reducing agent in the manufacture of dye-stuffs, paints, etc.

`text(Uses of Iron :)`

● Cast iron, which is the most important form of iron, is used for casting stoves, railway sleepers, gutter pipes , toys, etc.

● It is used in the manufacture of wrought iron and steel.

● Wrought iron is used in making anchors, wires, bolts, chains and agricultural implements.

● Steel finds a number of uses. Alloy steel is obtained when other metals are added to it.

`->` Nickel steel is used for making cables, automobiles and aeroplane parts, pendulum, measuring tapes, chrome steel for cutting tools and crushing machines, and stainless steel for cycles, automobiles, utensils, pens, etc.