General Principles and Processes of Isolation of Elements
This domain tests the logic of the metallurgical flow sheet: concentrate, convert to oxide, reduce, then refine.
Concentration and pre-treatment
- froth flotation for sulphide ores, using pine oil or xanthate collectors
- magnetic separation for magnetic gangue such as Fe3O4
- Bayer process for bauxite: hot NaOH dissolves alumina as Na[Al(OH)4]; Al(OH)3 is precipitated and calcined
- cyanide leaching for gold and silver, giving [Au(CN)2]⁻
- roasting: sulphide to oxide in excess air, releasing SO2
- calcination: carbonate or hydrated ore to oxide in limited air, releasing CO2
Reduction to the metal
- Ellingham diagrams plot ΔG° of oxide formation against temperature; a metal line lying below another oxide line can reduce it
- carbon reduces oxides above the temperature at which the C→CO line crosses the metal-oxide line; below that, CO is the better reductant
- thermite: Cr2O3 + 2Al → Al2O3 + 2Cr, strongly exothermic
- Hall–Héroult: Al2O3 in molten cryolite plus CaF2, carbon anodes; cryolite lowers the melting point and improves conductivity
- Mond process for nickel via Ni(CO)4; van Arkel for Ti and Zr via iodides
- zone refining for Si and Ge; liquation for Sn and Pb
Refining
- electrolytic refining of copper in acidified CuSO4 with pure copper cathodes and impure anodes; gold and silver collect as anode mud
Worked example
Zinc can be extracted from ZnO by carbon, but alumina cannot be reduced by carbon at practical temperatures, because the Al2O3 line in the Ellingham diagram lies far below the C→CO line, so aluminium oxide is far more stable and aluminium is won electrolytically instead.
Traps that cost marks
- Swapping roasting and calcination.
- Forgetting that CO, not carbon itself, is often the reductant in blast furnaces.
- Saying cryolite is a reducing agent; it is a flux/solvent.