The metal is converted into its volatile compound and collected. It is then decomposed to give pure metal.
The two requirements are:
1. The metal should form a volatile compound with an available reagent,
2. The volatile compound should be easily decomposable, so that the recovery is easy.
eg. i. Mond Process for Refining Nickel
• Nickel is heated in a stream of carbon monoxide forming a volatile complex, nickel tetracarbonyl.
• It is decomposed at high temperature giving the pure metal.
ii. Van Arkel Method for Refining Zirconium or Titanium
• Useful for removing all the oxygen and nitrogen present in the form of impurity in certain metals like Zr and Ti.
• The crude metal is heated in an evacuated vessel with iodine. The metal iodide being more covalent, volatilises:
Zr + 2I2 → ZrI4
• The metal iodide is decomposed on a tungsten filament, electrically heated to about 1800 K. The pure metal is thus deposited on the filament.
ZrI4 → Zr + 2I2
