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Refining: Refining of metal depends upon the nature of the metal and nature of impurities. Commonly used methods are given below.
1. Liquation: Impure metal that has low melting point e.g., Pb, Sn etc., are heated in a furnace having sloping base. The low melting metal flows down leaving high melting impurities in the furnace.
2. Distillation: Impure metal that has low boiling point e.g., Zn, Cd, Hg etc., are purified by distillation.
3. Oxidative refining: If the impurities can be easily oxidised, oxidative refining is used for the purification of metal. For example, pig iron and cast are converted to wrought iron by oxidative refining using Bessemer converter.
4. Poling: This method is used to purify a metal that has its oxide as impurity, e.g., Cu and Sn. The molten metal is stirred with green logs of wood (best is bamboo) the hydrocarbon (mainly CH4) produced by logs of wood reduce the metal oxide to metal.
3Cu2O + CH4 -----> CO + 2H2O +6Cu
5. Electro refining: In this method, blocks of impure metal are taken as anode and strips of pure metal at cathode. Electrolyte is aqueous solution of some suitable salt of the same metal. On passing current, impure metal from anode gives metal ions into the solution and from the solution the metal ions produce metal at cathode. Sometimes the waste under the anode (anode mud) may contain precious metals, as in case of copper, the anode mud may contain gold and silver.
At anode Cu --------> Cu++ + 2e-
(Impure)
At cathode Cu++ + 2e- -------> Cu
(Pure)
6. Vapour phase refining: In this process, metal is converted to a stable volatile compound to a stable volatile compound which on strong heating produces pure metal.
Van Arkel method: This method is used to collect ultra pure titanium and zirconium by using I2 as specific reagent.
Ti + 2I2 ---------> TiI4
(Impure) 523K (Volatile)
TiI4 -------> Ti + 2I2
(Pure)
Zr(s) + 2I2 (g) --------> ZrI4 (g)
870K (Volatile)
ZrI4 (g) ---------> Zr(s) + 2I2 (g)
2070K
Mond’s process: This method is used to purify nickel. The reagent used is carbon monoxide.
Ni + 4CO -----------> Ni(CO)4
(Impure) 330-350K (Volatile)
Ni(CO)4 --------> Ni + 4CO
(Pure)
7. Zone refining: Semiconductors, like Si, Ge, Ga etc., are purified by this method.
The underlying principle is that, impurities are highly soluble in molten metal and pure metal solidifies easily. The method is basically fractional crystallization.
A circular heater surrounds a rod of impure metal. The heater is moved forward. The pure metal crystallises out and impurities pass into the adjacent molten metal. The method is repeated to shift the impurities to one end which is cut off.
8. Chromatographic method: The underlying principle is the differential adsorption of different components on an adsorbent. Different types are column chromatography, paper chromatography, thin layer chromatography, etc.
In column chromatography a glass tube is packed with a suitable adsorbent such as Al2O3, silica, etc. The mixture to be separated is either liquid or is taken as solution in suitable solvent.
It is put at the top of the column. More adsorb able component moves down slowly. After some time, when the separation of components is visible in the column, the eluent (solvent) is poured from the top. The component which is least adsorbed is collected at the bottom, the first, followed by the other components.
