Nickel is refined by heating the impure metal in a stream of CO to form a volatile carbonyl which is then decomposed. This refining method is

1. Impure Ni + 4CO $\rightarrow$ $Ni(CO)_4$ (volatile) at ~350 K, leaving solid impurities behind. 2. Heating the vapour to ~460 K decomposes it: $Ni(CO)_4 \rightarrow Ni + 4CO$, giving pure nickel. 3. This vapour-phase route is the **Mond process**; Van Arkel uses iodine for Ti/Zr. _Source: Samacheer Kalvi Class 12 Chemistry Vol.1, Unit 1 'Metallurgy', §1.6.5 (Mond process)_