Why do we smelt iron?
Iron is never found as clean metal in rocks. It is joined with oxygen in iron ore, such as haematite (Fe2O3) and magnetite (Fe3O4). Smelting means heating the ore with a chemical that pulls the oxygen out, so we are left with the metal. Taking oxygen away from a compound is called reduction.
Smelting happens in two big jobs. First make iron (blast furnace). Then clean the iron and remove extra carbon (steelmaking).
The blast furnace: making pig iron
A blast furnace is a tall steel tower lined with heat-proof bricks. The charge (what we put in) has three things:
- Iron ore: the source of iron.
- Coke: baked coal. It is the fuel and also makes the gas that removes oxygen.
- Limestone (CaCO3): it joins the sand-like waste in the ore and makes slag.
Hot air (about 1000 °C or more) is blown in near the bottom. The reactions:
- Coke burns: C + O2 → CO2 (gives heat).
- CO2 meets more hot coke: CO2 + C → 2CO.
- CO takes oxygen from the ore: Fe2O3 + 3CO → 2Fe + 3CO2.
The iron melts and drips down. The furnace runs day and night for years. The heavy melted iron has about 4% carbon and is called pig iron (hot metal).
Slag: the useful waste
Iron ore has rocky waste, mainly silica (SiO2). Limestone breaks down by heat: CaCO3 → CaO + CO2. Then CaO + SiO2 → CaSiO3. This is slag. It is lighter than iron, so it floats on top and is drained out separately. Slag is not thrown away: it is used to make cement and road material.
Steelmaking: taking the carbon away
Pig iron with 4% carbon is hard but brittle: it cracks when hit. Steel has much less carbon (up to about 2%, most common steel less than 0.3%), so it is strong and tough. In a basic oxygen converter, a water-cooled lance blows pure oxygen into the melted iron:
- C + O2 → CO / CO2 (carbon leaves as gas)
- Silicon, phosphorus and sulfur also burn into oxides. Lime joins them and floats away as slag.
The burning gives heat, so no extra fuel is needed. In about 20 minutes the carbon is right. Scrap steel can be added to the converter to recycle it. Steel can also be made by melting scrap in an electric arc furnace. Then small amounts of other metals (like chromium or manganese) can be added to make alloy steels.
| Carbon | Kind | Use |
|---|---|---|
| over 2% | Cast / pig iron | Pipes, pans, engine blocks |
| 0.8 to 2% | High-carbon steel | Knives, tools, springs |
| 0.25 to 0.8% | Medium steel | Rails, axles |
| under 0.25% | Mild steel | Bars, sheets, car bodies |
Making ingots and continuous casting
Melted steel must be made into solid shapes. There are two ways.
Ingot casting (old way): pour steel into big moulds, let it set, take out the ingot, reheat it and roll it. It wastes time and metal.
Continuous casting (modern way): steel from the ladle goes into a small tank called the tundish, then into a water-cooled copper mould. The skin freezes first. Rollers pull the bar down while water sprays cool the inside. A torch cuts it into slabs, blooms or billets. Over 95% of the world's steel is cast this way because it saves energy and gives a more even product.
Try it
In the 3D: on the last step, drag the carbon slider from 4% down to 0.2%. Watch the black carbon dots disappear and read the name of the steel.
At home (with an adult): a steel nail and a thin cast iron piece (or an old iron pan bit) are different. Bend a thin steel wire: it bends. Cast iron cannot be bent; it snaps. Same iron, different carbon.
Key formulas and definitions
- Fe₂O₃ + 3CO → 2Fe + 3CO₂ (reduction of ore)
- C + O₂ → CO₂; CO₂ + C → 2CO
- CaCO₃ → CaO + CO₂; CaO + SiO₂ → CaSiO₃ (slag)
- Pig iron: about 4% C. Steel: below 2% C
- Continuous casting: ladle → tundish → mould → cut slabs
Worked examples
1. Write the reaction in which carbon monoxide reduces haematite in a blast furnace.
Fe₂O₃ + 3CO → 2Fe + 3CO₂. Iron(III) oxide loses oxygen, so it is reduced; CO gains oxygen, so it is oxidised.
2. Why is limestone added to the blast furnace?
Heat breaks it into CaO. CaO joins silica (SiO₂), the rocky waste, and makes calcium silicate slag, which floats on the iron and is drained off.
3. A 1000 kg batch of pig iron has 4% carbon. Steel needs 0.4% carbon. How many kg of carbon must be burned off?
Carbon at start = 4% of 1000 = 40 kg. Carbon needed = 0.4% of 1000 = 4 kg (ignoring small mass change). Burn off 40 − 4 = 36 kg.
4. Give two reasons continuous casting is better than ingot casting.
It saves energy (no reheating of ingots) and gives more steel from the same melt with a more even structure. It is also faster and can run non-stop.
Common mistakes
- Saying the furnace melts iron ore. It does not just melt it: a chemical reaction (reduction) removes oxygen.
- Thinking coke is only fuel. It also makes CO, the gas that actually takes the oxygen away.
- Believing steel is a different metal from iron. Steel is iron with a small, controlled amount of carbon.
- Mixing up the two stages: the blast furnace makes pig iron (lots of carbon); the converter makes steel (less carbon).