Q.Write the role of the following:

Concept: Roles of reagents in metallurgy, and leaching of bauxite (Bayer process).

Part (a)

(i) $NaAlF_4$ (with cryolite) in extraction of Al: it is added to molten alumina to lower its melting point (from ~2050 °C to ~950 °C) and to increase the electrical conductivity of the electrolyte in the Hall–Héroult cell.

(ii) CO in the refining of Ni (Mond process): CO combines with impure nickel to form the volatile complex nickel tetracarbonyl, which is then decomposed to give pure Ni: …

Part (a)

(i) Role of $NaAlF_4$ in aluminium extraction.

Pure alumina ($\ce{Al2O3}$) melts near 2050 °C — too high for electrolysis. Adding $NaAlF_4$ together with cryolite ($\ce{Na3AlF6}$) dissolves the alumina and forms a melt that:

1. melts at a much lower temperature (~950 °C), saving energy, and
2. conducts electricity well (it dissociates into ions), allowing reduction at the cathode. The fused mixture is electrolysed ($\ce{2Al2O3 + 3C -> 4Al + 3CO2}$); the additive acts as a flux/solvent and is not consumed.

(ii) Role of CO in refining of Ni (Mond process).

Impure nickel is warmed in a stream of CO. Only nickel reacts, forming volatile nickel tetracarbonyl, which is separated from solid impurities and then decomposed at higher temperature to deposit pure nickel:

$\ce{Ni + 4CO ->[330-350 K] Ni(CO)4 ->[450-470 K] Ni + 4CO}$ …