Mond process for refining nickel metal:
Nickel is heated in a stream of CO to form nickel tetra carbonyl complex which is highly volatile.
Ni+4CO330−350K−−−−−−→Ni(CO)4
Nickel tetra carbonyl is heated at high temperature. It decomposes to give pure nickel.
Ni(CO)4450−470K−−−−−−→Ni+4CO