Q.Describe a method for refining Nickel.

Mond's process purifies nickel by first converting it into a volatile compound (nickel tetracarbonyl) at a relatively low temperature, then decomposing that vapour on hot pure-nickel pellets at a higher temperature to leave behind pure metal — a two-step vapour-phase refining method.

Mond's process (vapour-phase refining of nickel):

Step 1 — formation of the volatile complex: impure nickel is heated in a stream of carbon monoxide gas at a relatively low temperature, $330$–$350\ K$. The nickel reacts with CO to form the volatile compound nickel tetracarbonyl, while non-volatile impurities are left behind: $Ni + 4CO \xrightarrow{330\text{-}350\ K} Ni(CO)_4\ (\text{volatile})$

Step 2 — decomposition to pure metal: the volatile $Ni(CO)_4$ vapour is then passed to a different, hotter chamber and heated to a higher temperature, $450$–$470\ K$, where it decomposes to deposit pure nickel metal (typically onto small pellets of pure nickel), and the liberated CO gas is recycled back to the first step: $Ni(CO)_4 \xrightarrow{450\text{-}470\ K} Ni\ (\text{pure}) + 4CO$

…