Q.Give the basic requirement for vapour phase refining.

Refining (Purification) of Metals

After a crude metal is extracted it must be refined to high purity; the method chosen depends on the metal.

• Distillation — for low-boiling metals such as zinc and mercury.
• Liquation — for easily fusible metals such as tin.
• Electrolytic refining — the impure metal is the anode and a pure strip the cathode in a solution of the metal salt; used for Cu, Ag, Au, Al (impurities collect as anode mud).
• Vapour-phase refining, which relies on forming a volatile compound that is later decomposed:
  • Mond process for nickel — Ni + $\mathrm{4CO} \xrightarrow{330\text{–}350\,K}$ $\mathrm{Ni(CO)_4}$, which decomposes at higher temperature to pure Ni.
  • van Arkel method for titanium and zirconium — the metal is converted to its volatile iodide ($\mathrm{ZrI_4}$) and decomposed on a hot tungsten filament to give the ultra-pure metal. …