Extraction of Metals

• Occurrence — ores (oxides, sulphides, carbonates, halides).
• Gangue — unwanted earthy material in ore.
• Calcination — carbonate ore heated in limited air → oxide + CO₂.
• Roasting — sulphide ore heated in excess air → oxide + SO₂.
• Reduction — C, CO, or electrolysis to obtain metal; refining by electrolysis.

1. Ore — mineral from which metal extracted profitably.
2. Calcination — ZnCO₃ → ZnO + CO₂ (no O₂).
3. Roasting — 2ZnS + 3O₂ → 2ZnO + 2SO₂.
4. Reduction of oxide — ZnO + C → Zn + CO (smelting).
5. Electrolytic reduction — Na, Mg, Al from molten compounds (high reactivity).
6. Less reactive — Cu, Hg sulphides roasted; oxide reduced by heat alone sometimes.
7. Aluminium — Hall–Héroult process; electrolysis of Al₂O₃ in molten cryolite.
8. Gangue removal — froth flotation, magnetic separation (intro).
9. Thermit — extracts molten iron for welding.
10. Environmental — SO₂ from roasting causes acid rain; controlled in industry.

Difference between roasting and calcination for zinc ores

Zinc carbonate ore (calcinatio n):
ZnCO₃(s) --heat--> ZnO(s) + CO₂(g) (no air/O₂ needed)
Zinc sulphide ore (roasting):
2ZnS(s) + 3O₂(g) --heat--> 2ZnO(s) + 2SO₂(g) (excess air)
Both give ZnO → then reduced with carbon to Zn.
Conclusion: calcination for carbonate; roasting for sulphide.

• Using calcination for sulphide ore (needs roasting with O₂).
• Misconception: all metals extracted by carbon reduction (Al, Na need electrolysis).
• Confusing ore with pure metal.
• Forgetting SO₂ as roasting product (pollution concern).
• Mixing smelting and refining steps.

• Define calcination and roasting with one equation each.
• Why is electrolysis used for aluminium extraction?
• What is gangue?
• Write reduction of ZnO with carbon.

Open the Practice tab for graded questions on Extraction of Metals.