Principles of metal extraction
The method used to extract a metal from its ore depends on the position of the metal in the reactivity series and on cost. The more reactive the metal, the more energy is needed to reduce its ions, so the more expensive the process. Most processes involve three stages: concentrating the ore, reducing the metal compound and refining the crude metal.
| Metal reactivity | Method | Example |
|---|---|---|
| Very reactive (K, Na, Ca, Mg, Al) | Electrolysis of molten compounds | Aluminium from bauxite |
| Reactive (Zn, Fe, Pb, Sn) | Reduction with carbon or carbon monoxide | Iron, zinc, tin |
| Unreactive (Cu, Ag, Au) | Roasting the sulphide, or found native | Copper from copper pyrites |
Aluminium
Aluminium is extracted from bauxite (impure Al₂O₃). The Bayer process purifies the ore: hot concentrated sodium hydroxide dissolves the amphoteric aluminium oxide but not the iron(III) oxide impurity. Pure aluminium hydroxide is precipitated, then heated to give pure aluminium oxide. This is dissolved in molten cryolite (Na₃AlF₆), which lowers the melting point from 2 070 °C to about 950 °C, and electrolysed.
| Electrode | Reaction |
|---|---|
| Cathode (carbon lining) | Al³⁺ + 3e⁻ → Al (molten aluminium collects at the bottom) |
| Anode (carbon blocks) | 2O²⁻ → O₂ + 4e⁻; the oxygen burns the anodes to CO₂, so they must be replaced |
Choose the current and the time. Find the mass of aluminium produced. (Al = 27, three electrons per ion, F = 96 500 C/mol)
Copper, zinc and titanium
| Metal | Ore | Process |
|---|---|---|
| Copper | Chalcopyrite, CuFeS₂ | Froth flotation concentrates the ore; roasting with air gives copper(I) sulphide; blowing air gives impure copper; electrolysis in CuSO₄ solution with impure copper as the anode gives 99.99% pure copper |
| Zinc | Zinc blende, ZnS | Roast to ZnO, then reduce with carbon at about 1 200 °C (the zinc distils off) or leach with acid and electrolyse |
| Titanium | Rutile, TiO₂ | Convert to TiCl₄, then reduce with molten magnesium in argon (Kroll process): expensive but gives a strong, light and corrosion-resistant metal |
Key terms
- Bauxite
- The main ore of aluminium, impure aluminium oxide.
- Cryolite
- Na₃AlF₆, used to lower the melting point of aluminium oxide.
- Froth flotation
- A method of concentrating an ore using bubbles.
- Refining
- Purifying a crude metal.
Summary
- The method of extraction depends on the reactivity of the metal.
- Aluminium: Bayer process, then electrolysis of alumina in molten cryolite at 950 °C.
- Copper is refined by electrolysis; titanium by the Kroll process.
- Recycling saves energy and conserves ores.