Q.Give the basic requirement for vapour phase refining.

Forming a volatile iodide and decomposing it back on a hot filament is the Van Arkel process.

The Van Arkel method purifies metals such as titanium and zirconium by converting them into a volatile, thermally unstable iodide and then decomposing that iodide on a hot filament:

Ti (impure) + 2 I2 --(773 K)--> TiI4 (volatile)

TiI4 --(1675 K, on hot W filament)--> Ti (pure) + 2 I2

…

In electrolytic refining, the impure metal is always made the anode (so it dissolves into solution), while a strip of pure metal serves as the cathode where the purified metal deposits — for copper, impure ('blister') copper is the anode.

In the electrolytic refining of copper, a large block of impure copper (blister copper, obtained from the smelting/converter stage, typically ~98-99% pure) is made the anode, and a thin sheet of pure copper is made the cathode, both dipped in an electrolyte of acidified copper sulphate solution. On passing current, copper from the impure anode dissolves as $Cu^{2+}$ ions ($Cu \rightarrow Cu^{2+}+2e^-$ at the anode) and migrates through solution to deposit as pure metallic copper at the cathode ($Cu^{2+}+2e^- \rightarrow Cu$). More reactive impurities (e.g. Fe, Zn) dissolve into solution but stay in solution (not deposited, since Cu is more easily reduced), while noble/less-reactive impurities (Ag, Au, …

The van-Arkel (iodide) method purifies metals by converting the impure metal to a volatile iodide, which decomposes on a hot filament to deposit pure metal; it is used for metals like zirconium and titanium.

In the van-Arkel method, the crude metal is heated with iodine in an evacuated vessel to form the volatile metal iodide (e.g. ZrI4, TiI4). This vapour is then decomposed on a red-hot tungsten filament (around 1800 K), depositing pure metal on the filament while iodine is released and can react with more crude metal, continuing the cycle.

…

Mond's process purifies nickel by converting it to a volatile carbonyl and then decomposing that carbonyl back to pure metal.

…

Distillation purifies metals that are volatile enough to vaporise at manageable temperatures, leaving non-volatile impurities behind. Zinc is the classic example — it boils at $907^\circ\text{C}$, well below the melting points of its common impurities like iron and lead.

Mond's process refines impure nickel by first converting it to volatile nickel tetracarbonyl, Ni(CO)₄, then thermally decomposing that vapour to deposit pure nickel.

Step 1 (formation of volatile complex, ~330–350 K):

Ni (impure) + 4CO → Ni(CO)₄ (volatile)

Step 2 (thermal decomposition at higher temperature, ~450–470 K):

Ni(CO)₄ → Ni (pure) + 4CO

…