Principles of Metallurgy
This domain tests the extraction of metals from ores: concentration, roasting, calcination, reduction, flux and slag, and refining, all decided by the activity series.
Ore to metal
A mineral is a naturally occurring compound of a metal; an ore is a mineral from which the metal can be extracted profitably. Gangue is the unwanted rocky matter. The activity series decides the method: highly reactive metals (K, Na, Ca, Al) are obtained by electrolysis; moderately reactive metals (Zn, Fe, Pb, Cu) by reduction with carbon; unreactive metals (Au, Ag) occur native.
The steps
- Concentration removes gangue: hydraulic washing for density difference, magnetic separation for magnetic ores, froth flotation for sulphide ores, leaching by dissolving the ore in a suitable solvent.
- Roasting heats a sulphide ore strongly in air to give the oxide: 2ZnS + 3O2 → 2ZnO + 2SO2.
- Calcination heats a carbonate ore in limited air: CaCO3 → CaO + CO2.
- Reduction: ZnO + C → Zn + CO, or the thermite reaction Fe2O3 + 2Al → 2Fe + Al2O3, which is so exothermic that it is used in welding rails.
- Flux combines with the gangue to form a fusible slag, which is removed.
- Refining: electrolytic refining for copper and zinc, liquation for low-melting metals such as tin, distillation for zinc and mercury.
Worked example
Extract zinc from zinc blende. First roast: 2ZnS + 3O2 → 2ZnO + 2SO2. Then reduce with coke: ZnO + C → Zn + CO. Zinc lies in the middle of the activity series, so carbon can displace it; electrolysis would be needlessly expensive.
Traps
Roasting needs a sulphide ore and plenty of air, calcination a carbonate ore and limited air - swapping them loses marks; slag is waste, not the metal; aluminium is extracted by electrolysis of alumina, not by carbon.