Refining


When a metal is extracted from its ore,then it is usually contaminated impurities. For obtaining metals of high purity, there are many techniques that are used, depending upon the differences in properties of the metal and the impurity.

1. Distillation


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


2. 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


3. 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.


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



4. Zone refining

It is used when 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


5. Vapour phase refining

In Vapour phase refining, the metal is converted into its volatile compound and collected elsewhere. It is then decomposed to give pure metal.

The two basic nessecary conditions for this are :

  • The metal should form a volatile compound with an available reagent
  • The volatile compound should be easily decomposable, so that the recovery is easy


6. Chromatographic methods

This method is based on the principle 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. Later the adsorbed components are removed by using suitable solvents





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