Extraction of Metals: Ores, Methods and Corrosion Care

Metals rarely occur pure in nature; they are locked inside rocks as ores — minerals from which a metal can be profitably extracted. Extraction of metals is the sequence of steps that frees the metal: concentrating the ore, converting it to an oxide, reducing the oxide to metal, and refining the metal to purity. Corrosion is the reverse tragedy — the metal slowly returning to its compounds, as when iron rusts.

This topic connects chemistry to the real economy. The method of extraction depends entirely on a metal's position in the reactivity series, which is why aluminium needs electricity while iron needs only coke. And corrosion destroys enormous value every year, so preventing it — by painting, galvanising or alloying — is applied chemistry at its most practical.

Key Concepts

• Mineral vs ore: a mineral is any naturally occurring compound of a metal; an ore is a mineral from which extraction is economically worthwhile.
• Gangue: the worthless sand, clay and rock mixed with the ore, removed during concentration.
• Calcination: heating a carbonate ore in limited air to get the oxide (CaCO3 → CaO + CO2). Roasting: heating a sulphide ore in excess air (2ZnS + 3O2 → 2ZnO + 2SO2).
• Reduction: removing oxygen from the metal oxide to get the metal — with carbon/coke for moderately reactive metals, or electrolysis for highly reactive ones.
• Refining: purifying the crude metal, commonly by electrolytic refining (pure copper cathodes) or distillation for volatile metals like zinc.
• Corrosion: gradual eating away of a metal by air and moisture; rusting is the corrosion of iron specifically (hydrated iron(III) oxide, Fe2O3·xH2O).

How It Works

The extraction route is chosen by reactivity:

1. Concentration: remove gangue — by gravity separation, froth flotation (sulphide ores) or magnetic separation.
2. Conversion to oxide: calcine carbonate ores, roast sulphide ores. Oxides reduce most easily.
3. Reduction — low reactivity metals (Au, Ag, Pt): found native or nearly so; simple heating or physical methods suffice.
4. Reduction — medium reactivity (Zn, Fe, Pb): reduce the oxide with coke (carbon) in a furnace. Example: Fe2O3 + 3CO → 2Fe + 3CO2 in a blast furnace.
5. Reduction — high reactivity (K, Na, Ca, Mg, Al): carbon cannot do the job; use electrolysis of the molten ore. Aluminium comes from electrolysing molten alumina dissolved in cryolite.
6. Refining: electrolytic refining gives 99.9%+ pure metal for electrical use.

Worked Example

Aluminium is extracted by electrolysing alumina: 2Al2O3 → 4Al + 3O2. What mass of aluminium is obtained from 204 g of pure alumina?

1. Molar mass of Al2O3 = (2 × 27) + (3 × 16) = 54 + 48 = 102 g/mol.
2. Moles of alumina = 204 ÷ 102 = 2 mol.
3. From the equation, 2 mol Al2O3 gives 4 mol Al.
4. Mass of aluminium = 4 × 27 = 108 g.
5. Reality check: industrial cells need enormous electricity for this step — which is why aluminium was once more precious than gold, and why recycling aluminium saves about 95% of that energy.

Common Misconceptions

• "Every metal is extracted by heating its ore with carbon." Carbon only reduces oxides of metals below it in reactivity. Highly reactive metals like aluminium, sodium and calcium hold their oxygen too tightly — only electrolysis can free them.
• "Corrosion and rusting are the same word." Rusting is iron's specific corrosion product. Saying "copper rusts" is wrong — copper corrodes to a green basic carbonate patina, a different substance entirely.
• "A coat of paint protects metal forever." Paint works only while the coating is intact. Once scratched, moisture reaches the metal and corrosion restarts underneath — which is why galvanising (a zinc coating that protects even when scratched, by sacrificial protection) is used for long-life structures.

Real-World Applications

• Blast furnaces: produce millions of tonnes of iron from haematite ore using coke reduction.
• Galvanised steel: zinc-coated buckets, roofs and bridges resist rust for decades through sacrificial protection.
• Stainless steel: iron alloyed with chromium and nickel forms a self-healing oxide skin, used in cutlery and surgical instruments.
• Aluminium recycling: remelting scrap uses a fraction of the energy of extraction from bauxite, making it one of the most recycled materials on Earth.

Extraction follows reactivity: concentrate the ore, convert to oxide, then reduce — by heat for noble metals, by carbon for the middle of the series, by electrolysis for the reactive top — and finally refine. Corrosion is extraction in reverse, and stopping it with coatings, galvanising or alloys is just as much real chemistry as the furnace work.