Extraction and Uses of Metals

Choosing an extraction method from the reactivity series

• Metals above carbon in the reactivity series (K, Na, Li, Ca, Mg, Al) are too reactive to be reduced by carbon
  • They must be extracted by electrolysis of their molten compound
• Metals below carbon but above the unreactive group (Zn, Fe, Sn, Pb, Cu) can be reduced by heating their oxide with carbon (or carbon monoxide)
• Metals below hydrogen (Cu, Ag, Au) are unreactive and the least reactive of these are simply mined as the free element

Extraction of iron in the blast furnace

• Iron is extracted from haematite using a blast furnace — a continuous, high-temperature reactor charged at the top and tapped at the bottom
• Three raw materials are loaded in:
  • Iron ore (haematite, Fe₂O₃) — the source of iron
  • Coke (an impure form of carbon) — the reducing agent and the source of heat
  • Limestone (calcium carbonate, CaCO₃) — to remove acidic impurities from the ore
• Hot air is blasted in near the bottom

The chemistry inside the furnace

• Combustion zone (hottest, near the air blast) — the coke ignites in the incoming blast of preheated air, supplying the heat that drives the rest of the furnace
  • C(s) + O₂(g) → CO₂(g) (exothermic)
• Carbon-monoxide-forming zone — the very hot carbon dioxide reacts with more coke to make carbon monoxide, the true reducing agent
  • CO₂(g) + C(s) → 2 CO(g)
• Iron-forming zone — carbon monoxide rises and reacts with iron(III) oxide in the haematite, stripping its oxygen
  • Fe₂O₃(s) + 3 CO(g) → 2 Fe(l) + 3 CO₂(g)
  • The molten iron drips to the bottom and is tapped off
• Slag formation — limestone removes the sandy SiO₂ impurity:
  • First, the limestone is thermally decomposed: CaCO₃(s) → CaO(s) + CO₂(g)
  • Then the calcium oxide neutralises the acidic silica: CaO(s) + SiO₂(s) → CaSiO₃(l)
  • Calcium silicate (slag) floats on top of the denser molten iron and is tapped off separately and used for road foundations

Extraction of aluminium by electrolysis

• Aluminium sits above carbon in the reactivity series, so carbon cannot reduce its oxide — electrolysis is needed
• The ore (bauxite) is purified first to give pure aluminium oxide, Al₂O₃
• Al₂O₃ melts at over 2000 °C, far too hot to be practical on its own
  • It is therefore dissolved in molten cryolite (Na₃AlF₆)
  • The mixture melts around 950 °C, drastically cutting the energy bill, and cryolite is chemically inert so the electrolysis chemistry is unchanged
• The cell is a steel tank lined with carbon (graphite). The lining is the cathode; large carbon blocks dipping into the melt from above are the anodes

What happens at each electrode

• At the cathode (the carbon lining): aluminium ions gain electrons and are reduced to liquid aluminium
  • Al³⁺(l) + 3 e⁻ → Al(l)
  • Molten aluminium is denser than the electrolyte and sinks to the floor of the tank, from where it is siphoned off
• At the anode (the carbon blocks): oxide ions lose electrons and are oxidised to oxygen
  • 2 O²⁻(l) → O₂(g) + 4 e⁻
• The anodes are made of carbon, so the oxygen produced burns the carbon away:
  • C(s) + O₂(g) → CO₂(g)
  • The anodes wear out and have to be replaced regularly, which is a continuing cost
• Aluminium extraction uses a great deal of electricity, which is the main reason it is expensive compared with iron extraction