Explain vapour phase refining.

1.	Explain vapour phase refining.
Answer» Solution :In this method, the metal is converted into its volatile compound which is collected and decomposed to give pure metal. Thus, two requirements 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. Following examples will illustrate this technique : (a) Mond.s Process for Refining NICKEL : In this process, nickel is heated in a stream of carbon monoxide forming a volatile complex NAMED as nickel tetracarbonyl. This complex is decomposed at higher temperature to obtain pure metal. `underset(("Impure"))(N i) + 4CO overset(330 - 350 K)(rarr) N i(CO)_(4)` `N i (CO)_(4) overset(450 - 470K)(rarr) underset("(Pure)")(N i) + 4CO` (b) Van Arkel Method for Refining Zirconium or Titanium : This method is very 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: `underset("(impure)")(Zr) + 2I_2 to ZrI_4` The metal iodide is decomposed on a TUNGSTEN filament, electrically heated to about 1800 K. The pure metal deposits on the filament. `underset("(pure)")(ZrI_4 to Zr + 2I_2). `

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