Outline The Principle Of Refining Of Metals In The Following Methods
(A) Electrolytic Refining
(B) Vapour Phase Refining
(C) Zone Refining
(i) Zone refining: Zone refining is based on the principle that impurities are more soluble in the melt than in the solid state of the metal.
The impure metal bar is heated with the help of circular coils fitted around this bar and slowly moved longitudinally from one end to another of impure metal.
At the heated zone the bar melts and as the heater mover on pure metal crystallizes while the impurities pass into the adjacent molten portion. In this way the impurities are swept from one end of the bar to another.
As the molten bar is removed from the heating coil, it cools Pure metal crystallizes out, leaving the impurities in the molten metal portion.
When the molten metal portion carrying impurities with increased concentration reaches the end of the rod, it is allowed to cool and is cut off.
The process is repeated several times to get pure metal of 99.99% purity.
(ii) Electrolytic refining: Electrolytic refining is the process of refining impure metals by using electricity. In this process, impure metal is made the anode and a strip of pure metal is made the cathode. A solution of a soluble salt of the same metal is taken as the electrolyte. When an electric current is passed, metal ions from the electrolyte are deposited at the cathode as pure metal and the impure metal from the anode dissolves into the electrolyte in the form of ions. The impurities present in the impure metal gets collected below the anode. This is known as anode mud (or sludge).
Electrolytic refining of copper
(iii) Vapour phase refining: Vapour phase refining is the process of refining metal by converting it into its volatile compound and then, decomposing it to obtain a pure metal. To carry out this process, (i) the metal should form a volatile compound with an available reagent, and (ii) the volatile compound should be easily decomposable so that the metal can be easily recovered. Nickel, zirconium, and titanium are refined using this method.