General Principles and Processes of Isolation of Elements
General Principles and Processes of Isolation of Elements
The exam tests the metallurgical sequence: concentration, roasting or calcination, reduction and refining, with Ellingham diagrams as the guiding principle.
Core rules
- Concentration: froth flotation for sulphide ores, magnetic separation for magnetic gangue, leaching for bauxite (Bayer process) and gold (cyanide process).
- Roasting: heating in air, usually sulphides → oxides + SO₂. Calcination: heating in limited air, carbonates or hydrates → oxides + CO₂ or H₂O.
- Ellingham diagram: ΔG° of oxide formation vs T. A metal whose line lies below another can reduce that oxide.
- Smelting uses coke; aluminium is obtained by Hall–Héroult electrolysis of Al₂O₃ in molten cryolite.
- Refining: electrolytic (Cu, Al), zone refining (Si, Ge), vapour phase (Mond for Ni, van Arkel for Ti).
Worked example Why does carbon reduce Fe₂O₃ but not Al₂O₃? The C→CO line crosses the Fe₂O₃ line near 1000 K, so above that temperature ΔG for 2Fe₂O₃+3C→4Fe+3CO₂ is negative. The Al→Al₂O₃ line lies far below the carbon lines at all practical temperatures, so Al₂O₃ cannot be reduced by carbon; aluminium is won by electrolysis.
Traps
- Mixing roasting and calcination.
- Forgetting cryolite's role: it dissolves alumina and lowers the melting point; it is not the source of aluminium.
- In copper matte, FeS is oxidised and removed as slag; the matte is Cu₂S.