$\mathrm{Ni}+4\mathrm{CO}\stackrel{330-350 \mathrm{K}}{⟶}\mathrm{Ni}(\mathrm{CO}{)}_4\stackrel{450-470\mathrm{k}}{⟶}\mathrm{Ni}+4\mathrm{CO}$ This sequence of reactions involved in

$\mathrm{Ni}+4\mathrm{CO}\stackrel{330-350 \mathrm{K}}{⟶}\mathrm{Ni}(\mathrm{CO}{)}_4\stackrel{450-470\mathrm{k}}{⟶}\mathrm{Ni}+4\mathrm{CO}$

Pure nickel is refined.

Similar things happen in Monds process.

In the Mond process of refining of Nickel : Nickel is heated in presence of carbon monoxide to form nickel tetracarbonyl and the obtained nickel tetracarbonyl is decomposed by subjecting it to a higher temperature to obtain a pure nickel metal.

Therefore, this sequence of reactions is involved in Mond's process for refining of nickel.