Extraction of Metals

Metals are found in rocks called ores. To get the pure metal, we must remove other substances. The method we use depends on how reactive the metal is.

Reactivity and Method

• Very unreactive (e.g. gold): found native (as the metal) and just cleaned.
• Below carbon (e.g. iron): extracted by heating the ore with carbon or carbon monoxide (reduction).
• Above carbon (e.g. aluminium): extracted by electrolysis of the molten compound.

Extraction of Iron from Hematite (Fe2O3) in the Blast Furnace

Raw materials: hematite (iron(III) oxide), coke (carbon), and limestone (calcium carbonate). Hot air is blown in.

• Coke burns to heat the furnace and makes carbon dioxide: $$\mathrm{C + O_2 \rightarrow CO_2}$$
• Carbon dioxide reacts with more carbon to make carbon monoxide (a reducing agent): $$\mathrm{CO_2 + C \rightarrow 2CO}$$
• Carbon monoxide reduces iron(III) oxide to iron: $$\mathrm{Fe_2O_3 + 3CO \rightarrow 2Fe + 3CO_2}$$
• Limestone breaks down (thermal decomposition) to remove acidic impurities (sand, SiO2): $$\mathrm{CaCO_3 \rightarrow CaO + CO_2}$$ $$\mathrm{CaO + SiO_2 \rightarrow CaSiO_3\ (slag)}$$

The liquid iron sinks to the bottom; slag floats on top and is removed.

Extraction of Aluminium from Bauxite (Al2O3)

• Main ore: bauxite. Aluminium is extracted by electrolysis of molten aluminium oxide.
• Role of cryolite: dissolves Al2O3 and lowers its melting point so electrolysis uses less energy and conducts better.
• Electrodes: graphite (carbon). Cathode makes aluminium; anode makes oxygen and is used up.

Half-equations

Cathode (reduction): $\mathrm{Al^{3+} + 3e^- \rightarrow Al}$

Anode (oxidation): $\mathrm{2O^{2-} \rightarrow O_2 + 4e^-}$. Oxygen then reacts with hot carbon anodes: $\mathrm{C + O_2 \rightarrow CO_2}$. Anodes must be replaced regularly.

Common misconceptions

• The blast furnace is not electrolysis; it is a series of redox reactions using carbon/CO.
• Cryolite lowers melting point; it is not used to add aluminium.
• In molten electrolysis of Al2O3, oxygen gas forms at the anode, not hydrogen.