Vapour Phase Method

In the vapour phase method of refining, the impure metal is first treated with a suitable reagent chosen specifically because it reacts with the metal to form a VOLATILE compound; this volatile compound is then physically transported elsewhere and thermally DECOMPOSED to release the pure metal, while any solid impurities -- which do not form the same volatile compound with the reagent -- are simply left behind at the first stage and never carried forward at all. Two named industrial processes illustrate this principle.

Mond process (refining nickel). Impure nickel is heated in a stream of carbon monoxide gas at around 350 K. The nickel reacts with CO to form a highly volatile compound, nickel tetracarbonyl: Ni(s) + 4CO(g) → Ni(CO)4. Because this compound is volatile, it is carried away from the solid impurities (which do not react with CO in this way) and transported to a separate, hotter chamber. There, on heating the nickel tetracarbonyl to around 460 K, the complex decomposes back into pure nickel metal and carbon monoxide gas -- effectively the reverse of the formation reaction -- and the released CO can be recycled back to treat more impure nickel. …