Refining of metals

The metals prepared by different methods contain impurities. The methods used for the purification of metals are called refining. The refining method depends on the nature of metal and the nature of impurities. Some common methods are as follows
1. Distillation
Volatile metals like zinc and mercury are purified by boiling the impure metal to get vapors of the pure metal which is condensed and collected.
2. Liquation
Low melting metals like tin and lead are purified by this method. The impure metal is melted on the sloping floor of a furnace. The metal melts and flows down leaving behind the high melting impurities.
3. Poling
Impure metal is melted and stirred with green logs of wood. The impurities rise to the surfaces, get oxidised and removed as gases (CO2) or slag. The metal may get oxidised (eg:- Cu to Cu2O). The hydrocarbons in green wood reduces the metal oxide to the metal. Example:- Refining of impure Cu and Sn.
4. Cupellation
Impure silver and gold contain base metals like lead and bismuth as impurities. These are removed by heating the metal placed in a cupel (boat shaped crucible made of cement or bone ash) in a reverberatory furnace in a current of hot air. The impurities are oxidised and carried away by the current of air. The process is stopped when a shining bright surface appears.
5. Electrolytic refining
The method is based on the process of electrolysis. The crude metal is made the anode a thin sheet of pure metal the cathode. The electrolyte is the solution of a salt of the metal. On passing electricity the metal dissolves from the anode and an equal number of metal ions of the solution gets deposited at the cathode. The impurities settle down below the anode mud (eg:- refining of Cu using CuSO4 solution as electrolyte).
At anode Cu ------> Cu 2+ + 2electron
At cathode Cu 2+ + 2electron -------> Cu
6. Zone refining
Metals of very high purity can be obtained by this method. The impure metal rod is mounted horizontally and heated by a circular electric heater at one end in an atmosphere of inert gas to form a thin molten zone. By slowly moving the heater, the molten zone is moved from one end of the rod to the other end. Pure metal crystallises while impurities pass into the molten zone. By repeated the passes of the molten zone very high purity can be attained at one end. The other end is discarded. Ge, Si and Ga used in semi conductors are refined in this manner.
7. Vapour phase refining
By this chemical reaction the metal is converted to a compound, which forms a vapour, which is decomposed to get pure metal.
8. Monds process
Crude nickel is heated with carbon monoxide to form volatile nickel carbonyl. The impurities remain as solids. The vapours on further heating decomposes to give pure nickel.
Ni + 4Co -------> Ni(CO)4
Ni(CO)4 --------> Ni + 4CO
9. Van Arkel de Boer method
Crude titanium or zirconium is heated with iodine to get vapours of the tetraiodide. The vapours are then decomposed on a tungsten filament kept at high temperature on which the pure metal gets deposited. The pure metal is then peeled off from the filament after cooling.
Ti + 2I2 ---525k----> TiI4 ----1675k----> Ti + 2I2
Zr + 2I2 ----870k---> ZrI4 ----2075k----> Zr + 2I2

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