Explain vapour phase refining.

(N/A) In this method,the metal is converted into its volatile compound,which is collected and decomposed to give pure metal. Two requirements for this process are:
$(i)$ The metal should form a volatile compound with an available reagent.
$(ii)$ The volatile compound should be easily decomposable,so that the recovery is easy.
Examples of this technique:
$(a)$ Mond's Process for Refining Nickel: Nickel is heated in a stream of carbon monoxide to form a volatile complex,nickel tetracarbonyl. This complex is decomposed at a higher temperature to obtain pure metal.
$Ni (\text{impure}) + 4 CO \xrightarrow{330-350 \ K} Ni(CO)_{4}$
$Ni(CO)_{4} \xrightarrow{450-470 \ K} Ni (\text{pure}) + 4 CO$
$(b)$ Van Arkel Method for Refining Zirconium or Titanium: This method is useful for removing oxygen and nitrogen impurities from metals like $Zr$ and $Ti$. The crude metal is heated in an evacuated vessel with iodine to form a volatile metal iodide.
$Zr + 2 I_{2} \rightarrow ZrI_{4} (\text{impure})$
The metal iodide is decomposed on a tungsten filament,electrically heated to about $1800 \ K$. The pure metal deposits on the filament.
$ZrI_{4} \rightarrow Zr (\text{pure}) + 2 I_{2}$