Q.Describe a method for refining nickel.

$Ni(s) + 4CO(g) \xrightarrow[330-350\ K]{} Ni(CO)_4(g) \xrightarrow[450-470\ K]{} Ni(s) + 4CO(g)$ …

Step 1 – Choice of method

Nickel is refined by a vapour-phase refining technique known as the Mond process. This class of method requires that the metal can form a volatile compound with some available reagent, and that this compound can be readily decomposed to give back the pure metal — nickel satisfies this via its reaction with carbon monoxide.

Step 2 – Formation of the volatile compound

Impure nickel is heated in a stream of carbon monoxide gas at a relatively low temperature, about 330–350 K. Nickel reacts to form nickel tetracarbonyl, a volatile compound, while solid impurities remain unreacted:

Step 3 – Decomposition to pure metal

The gaseous $Ni(CO)_4$ is then passed into a separate, hotter chamber maintained at about 450–470 K, where it is thermally unstable and decomposes back into pure nickel metal (which deposits) and carbon monoxide gas (which is recycled back to the first stage):

Step 4 – Result …

Identify that nickel forms a volatile carbonyl with CO at low temperature (Mond process) and that this compound decomposes cleanly back to pure metal + CO a …

• Confusing the Mond process (nickel) with the van Arkel method (titanium/zirconium, using iodine) — both are vapour-phase refining but use different reagents. …