Pure iron is extracted from haematite ore through a series of processes involving concentration, calcination/roasting, smelting, and refining. Here's a breakdown of the process:
1. Concentration of the Ore
- Purpose: To remove impurities (gangue) like sand, clay, etc., and increase the proportion of iron oxide.
- Method: Gravity separation or froth flotation.
- Gravity separation is used when the ore particles and the gangue have different densities. The heavier haematite settles down while lighter impurities are washed away.
- Froth flotation involves mixing the crushed ore with water, oil, and a frothing agent. Air is blown through the mixture, and the ore particles selectively adhere to the froth, which is then skimmed off.
2. Calcination or Roasting
- Purpose: To remove volatile impurities and moisture, and to convert iron carbonates and hydroxides to iron oxide (Fe₂O₃).
- Process: The concentrated ore is heated strongly in a limited supply of air.
- Reactions during calcination/roasting:
- Removal of water: Fe₂O₃.xH₂O (s) → Fe₂O₃ (s) + xH₂O (g)
- Oxidation of sulphides (if present): 2FeS₂ (s) + 5O₂ (g) → Fe₂O₃ (s) + 4SO₂ (g)
- Decomposition of carbonates: FeCO₃ (s) → FeO (s) + CO₂ (g) (FeO further oxidizes to Fe₂O₃)
- Reactions during calcination/roasting:
3. Smelting in a Blast Furnace
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Purpose: To reduce the iron oxide to metallic iron.
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Process: The calcined ore is mixed with coke (source of carbon) and limestone (flux) and fed into a blast furnace. Hot air is blown through the bottom of the furnace, causing the coke to burn and generate heat and carbon monoxide (CO).
Reactions inside the Blast Furnace:
- Combustion Zone:
- C (s) + O₂ (g) → CO₂ (g) + Heat
- CO₂ (g) + C (s) → 2CO (g)
- Reduction Zone (Various Temperatures):
- 3Fe₂O₃ (s) + CO (g) → 2Fe₃O₄ (s) + CO₂ (g) (at lower temperatures)
- Fe₃O₄ (s) + 4CO (g) → 3Fe (s) + 4CO₂ (g)
- Fe₂O₃ (s) + CO (g) → 2FeO (s) + CO₂ (g)
- FeO (s) + CO (g) → Fe (s) + CO₂ (g)
- At higher temperatures, carbon can directly reduce the iron oxide: FeO (s) + C (s) → Fe (s) + CO (g)
- Slag Formation Zone: Limestone (CaCO₃) decomposes to calcium oxide (CaO), which reacts with silica (SiO₂) and other impurities to form slag (calcium silicate, CaSiO₃):
- CaCO₃ (s) → CaO (s) + CO₂ (g)
- CaO (s) + SiO₂ (s) → CaSiO₃ (l) (slag)
The molten iron collects at the bottom of the furnace, and the molten slag floats on top of it. They are tapped off separately. The iron obtained at this stage is called pig iron, which contains about 4% carbon and other impurities like silicon, manganese, phosphorus, and sulphur.
- Combustion Zone:
4. Refining of Pig Iron
- Purpose: To reduce the carbon content and remove other impurities from pig iron to produce wrought iron or steel.
- Methods: Several methods exist, including:
- Puddling Process: Pig iron is melted in a special furnace, and the carbon is oxidized by haematite (Fe₂O₃) lining the furnace.
- Bessemer Process: Molten pig iron is poured into a Bessemer converter, and hot air is blown through it to oxidize impurities.
- Open Hearth Process: A slow oxidation process that allows for better control of the composition and quality of the steel.
- Electric Arc Furnace: Uses electric arcs to melt and refine the iron.
The specific refining method used depends on the desired properties of the final iron or steel product. The refining process reduces the carbon content and removes other undesirable elements, resulting in purer iron or steel with improved mechanical properties. Electrolytic refining can be further used to obtain high purity iron, but that's generally not done on an industrial scale from pig iron.
In summary, extracting pure iron from haematite ore involves concentrating the ore, calcining or roasting it to remove volatile impurities, smelting the calcined ore in a blast furnace using coke and limestone to reduce the iron oxide to pig iron, and finally, refining the pig iron to reduce the carbon content and remove other impurities, thus obtaining purer forms of iron like wrought iron or steel.
