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Quick Summary
Metallurgy covers the extraction of metals from their ores. This chapter carries 8–10 marks in the TN HSE board exam. Questions on extraction of iron (blast furnace), aluminium (Hall-Héroult), and refining methods are frequently asked.
Key Concepts and Definitions
- Metallurgy: Science and technology of extracting metals from ores and preparing them for use.
- Ore: Mineral from which a metal can be extracted economically.
- Gangue: Unwanted rocky material associated with the ore.
- Flux: Substance added to remove gangue; forms slag.
- Slag: Fusible product formed by flux + gangue.
- Calcination: Heating ore in limited air to remove moisture and CO₂.
- Roasting: Heating ore in excess air to convert sulphides/carbonates to oxides.
- Smelting: Reduction of metal oxide with a reducing agent at high temperature.
Concentration of Ores
| Method | Principle | Used For |
|---|---|---|
| Gravity separation | Density difference | Cassiterite (SnO₂), gold |
| Froth flotation | Sulphide ores are wetted by oil | Galena (PbS), ZnS |
| Magnetic separation | Magnetic properties | Magnetite (Fe₃O₄) |
| Leaching | Chemical dissolving | Bauxite (Al), gold |
Froth Flotation: Ore + water + pine oil + collectors → froth carries sulphide ore; gangue sinks.
Reduction Methods
1. Carbon reduction: Used for Zn, Fe, Cu (moderately reactive metals) 2. Hydrogen reduction: Used for WO₃ → W (oxide reduced by H₂) 3. Alumino-thermite process: Al reduces metal oxides with high heat of reaction
4. Electrolytic reduction: Highly reactive metals (Na, Al, Mg) — cannot be reduced by carbon
- 8Al + 3Mn₃O₄ → 4Al₂O₃ + 9Mn + heat
Extraction of Iron (Blast Furnace)
Ore: Haematite (Fe₂O₃), Magnetite (Fe₃O₄)
Reactions in blast furnace: 1. C + O₂ → CO₂ (combustion zone, ~1800°C) 2. CO₂ + C → 2CO (reduction zone) 3. Fe₂O₃ + 3CO → 2Fe + 3CO₂ (at ~700°C) 4. CaCO₃ → CaO + CO₂ (limestone decomposition) 5. CaO + SiO₂ → CaSiO₃ (slag formation)
Products: Pig iron (90–95% Fe), slag
Extraction of Aluminium (Hall-Héroult Process)
Ore: Bauxite (Al₂O₃·2H₂O)
Purification (Bayer's Process):
- Bauxite + NaOH → NaAlO₂ → Al(OH)₃ → Al₂O₃ (alumina)
Electrolysis:
- Electrolyte: Al₂O₃ dissolved in molten cryolite (Na₃AlF₆) + CaF₂
- Cathode: Al deposits → 4Al³⁺ + 12e⁻ → 4Al
- Anode: O₂ released → 6O²⁻ → 3O₂ + 12e⁻ (carbon anode burns)
- Temperature: ~950°C
Extraction of Copper and Zinc
Copper:
- Ore: Copper pyrites (CuFeS₂)
- Roasting: 2CuFeS₂ + O₂ → Cu₂S + 2FeS + SO₂
- Smelting → Blister copper → Electrolytic refining
Zinc:
- Ore: Zinc blende (ZnS) or Zincite (ZnO)
- Roasting: 2ZnS + 3O₂ → 2ZnO + 2SO₂
- Reduction: ZnO + C → Zn + CO (at 1200°C)
Refining Methods
| Method | Used For |
|---|---|
| Distillation | Volatile metals: Zn, Hg |
| Liquation | Low melting metals: Sn, Pb |
| Electrolytic refining | Cu, Ni, Ag, Au (highest purity) |
| Zone refining | Semiconductors: Si, Ge |
| Vapour phase refining | Ni (Mond process), Ti |
Electrolytic Refining of Copper:
- Anode: Impure Cu
- Cathode: Pure Cu
- Electrolyte: CuSO₄ solution
- Anode mud: Au, Ag, Pt collect below anode
Important Questions
One Mark
1. Define gangue. 2. What is the role of cryolite in Hall-Héroult process? 3. Name the ore of aluminium. 4. What is zone refining used for? 5. Define roasting.
Short Answer (3 marks)
1. Explain the froth flotation method of concentration. 2. Describe the electrolytic refining of copper with a neat diagram. 3. Compare calcination and roasting with examples.
Long Answer (5 marks)
1. Explain the extraction of iron from haematite using the blast furnace with reactions and a labelled diagram. 2. Describe the extraction and refining of aluminium by Hall-Héroult process.
Board Exam Tips
- Draw labelled blast furnace and Hall-Héroult cell diagrams — they carry 2 marks each
- Know the zone where each blast furnace reaction occurs (temperature zones)
- Anode mud composition is a common one-mark question
- Distinguish between pig iron, cast iron, and wrought iron
Revision Checklist
- [ ] Concentration methods and their principles
- [ ] Blast furnace reactions and products
- [ ] Hall-Héroult process and reactions
- [ ] Extraction of Cu and Zn
- [ ] All refining methods with examples
- [ ] Bayer's process steps
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