(i) Zone refining: This method is used for production of semiconductors and other metals of very high purity, e.g., Ge, Si, B, Ca and In.
It is based on the principle that the impurities are more soluble in the molten state (melt) than in the solid state of the metal.
The impure metal in the form of bar is heated at one end with a moving circular heater. As the heater is slowly moved along the length of the rod, the pure metal crystallises out of the melt whereas the impurities pass into the adjacent molten zone. This process is repeated several times till the impurities are completely driven to one end of the rod which is then cut off and discarded.
(ii) Electrolytic refining: Many metals, such as \(\mathrm{Cu}, \mathrm{Ag}\), \(\mathrm{Au}, \mathrm{Al}, \mathrm{Pb}\), etc., are purified by this method. The impure metals is made anode while a thin sheet of pure metal acts as a cathode. The electrolytic solution consists of a salt or a complex salt solution of the metal. On passing the current, the pure metal is deposited on the cathode while the impurities fall down as anode mud.
(iii) Vapour-phase refining: The crude metal is freed from impurities by first converting it into a suitable volatile compound by heating it with a specific reagent at a lower temperature and then decomposing the volatile compound at some higher temperature to give the pure metal.
(a) Mond's process: When impure nickel is heated in a current of \(\mathrm{O}\) at \(330-350 \mathrm{~K}\), it forms volatile nickel tetracarbonyl complex leaving behind the impurities. The complex then heated to a higher temperature \((450-470 \mathrm{~K})\) when it undergoes thermal decomposition giving pure nickel.
\(\underset{\text { (Impure) }}{\mathrm{Ni}}+4 \mathrm{CO} \stackrel{330-350 \mathrm{~K}}{\longrightarrow} \mathrm{Ni}(\mathrm{CO})_4\)
\(\mathrm{Ni}(\mathrm{CO})_4 \stackrel{450-470 \mathrm{~K}}{\longrightarrow} \underset{\text { (Pure) }}{\mathrm{Ni}}+4 \mathrm{CO}\)
(b) Van Arkel method: This method is used for preparing ultra-pure metals by removing all the oxygen and nitrogen present as impurities in metals like zirconium and titanium (which are used in space technology).
Crude \(7 r\) is heated in an evacuated vessel with iodine at \(870 \mathrm{~K}\). Zirconium tetraiodide thus formed is separated. It is then decomposed by heating over a tungsten filament at \(1800-2075 \mathrm{~K}\) to give pure \(\mathrm{Zr}\).
\(\underset{\text { (Impure) }}{\mathrm{Zr}(s)}+2 \mathrm{I}_2(g) \longrightarrow \mathrm{ZrI}_4\)