Extraction of iron: ores, stages and methods
Metals are found in the Earth as ores: rocks that contain enough of a metal compound to make extraction worthwhile. The method of extraction depends on how reactive the metal is. Very reactive metals (potassium, sodium, calcium, magnesium, aluminium) are extracted by electrolysis. Moderately reactive metals (zinc, iron, lead) are extracted by reduction with carbon (coke). Unreactive metals (gold, silver) are found as the metal itself.
| Metal | Main ore | Method |
|---|---|---|
| Aluminium | Bauxite, Al₂O₃ | Electrolysis of molten oxide |
| Zinc | Zinc blende, ZnS | Roast to oxide, reduce with carbon |
| Iron | Haematite, Fe₂O₃; magnetite, Fe₃O₄ | Reduction with carbon monoxide in a blast furnace |
| Copper | Chalcopyrite, CuFeS₂ | Roasting then reduction; purified by electrolysis |
| Gold | Native metal | Separated from rock |
In the blast furnace, iron ore, coke (carbon) and limestone are fed in at the top and hot air is blown in near the bottom.
| Product | Use |
|---|---|
| Cast iron (about 4% carbon) | Hard but brittle: cooking pots, engine blocks, drain covers |
| Steel (0.1–1.5% carbon) | Strong: bridges, vehicles, tools, reinforcing bars. Stainless steel (with chromium and nickel) does not rust |
| Slag | Road building and cement |
Choose the mass of iron(III) oxide fed into the furnace. Find the iron that can be produced. (Fe = 56, O = 16, so Fe₂O₃ = 160)
Rust and protecting iron
Iron rusts when it is in contact with oxygen and water. Rust is hydrated iron(III) oxide. Prevent rusting by keeping air and water away (paint, oil, plastic), by galvanising (coating with zinc, which also protects by sacrificial action) or by making alloys such as stainless steel.
Key terms
- Ore
- A rock that contains enough of a metal compound to be worth extracting.
- Reduction
- Removal of oxygen from a compound, or gain of electrons.
- Slag
- Waste calcium silicate that floats on molten iron.
- Galvanising
- Coating iron with zinc to protect it from rusting.
Summary
- The extraction method depends on the reactivity of the metal: electrolysis, carbon reduction or none.
- In the blast furnace carbon monoxide reduces iron(III) oxide to iron; limestone removes sand as slag.
- Steel is iron with a small amount of carbon; stainless steel contains chromium and nickel.
- Rusting needs oxygen and water and can be prevented by coating or alloying.