- 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.
- Following are the methods used to refine the metals:-
- Zone refining
- Vapour phase refining
- 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.
(c) 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.
- The reactions are:
- Anode: M → Mn+ + ne–
- Cathode: Mn+ + ne– → M
- This method is used to refine Copper, Zinc etc.
- In case of copper refining-
- 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:
- Anode: Cu → Cu2+ + 2 e–
- Cathode: Cu2+ + 2e– → Cu
- 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.
(d) Zone refining
- This method is based on the principle that the impurities are more soluble in the molten form as compared to 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.
- Vapour Phase Refining
- The metal is converted into its volatile compound and collected elsewhere. It is then decomposed to give pure metal.
- There are 2 requirements for this method:-
- The metal should form a volatile compound with an available reagent,
- The volatile compound should be easily decomposable, so that the recovery is easy.
- For example: - Mond Process for Refining Nickel and van Arkel Method for Refining Zirconium or Titanium.
- Chromatographic method
- The principle of this method is that 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.
- The adsorbed components are removed or 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.
- The Al2O3 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.
- There are several chromatographic techniques such as paper chromatography, column chromatography, gas chromatography, etc.
Diagram showing column chromatography