Metallurgy
Introduction
Metallurgy is the science of extracting metals from their ores and purifying them for use. In ICSE Class 10 Chemistry, you study the extraction of aluminium and the properties of important alloys.
Occurrence of Metals
| Type | Definition | Examples |
|---|---|---|
| Native state | Found as free metal (less reactive) | Gold, silver, platinum, copper |
| Combined state | Found as compounds (more reactive) | Iron, aluminium, zinc, lead |
Common Ores
| Metal | Ores | Chemical formula |
|---|---|---|
| Aluminium | Bauxite, Cryolite | Al₂O₃·2H₂O, Na₃AlF₆ |
| Iron | Haematite, Magnetite | Fe₂O₃, Fe₃O₄ |
| Zinc | Zinc blende, Calamine | ZnS, ZnCO₃ |
| Copper | Copper pyrites, Cuprite | CuFeS₂, Cu₂O |
Extraction of Aluminium
Aluminium is extracted from its ore bauxite (Al₂O₃·2H₂O) through two main stages.
Stage 1: Purification of Bauxite — Baeyer's Process
Bauxite contains impurities of Fe₂O₃ and SiO₂. Baeyer's process removes these.
Steps:
- Crushed bauxite is treated with hot NaOH solution.
- Al₂O₃ dissolves (amphoteric nature): Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O
- Impurities (Fe₂O₃) do not dissolve and are filtered off.
- The filtrate (NaAlO₂) is diluted and CO₂ is passed through: 2NaAlO₂ + CO₂ + 3H₂O → 2Al(OH)₃↓ + Na₂CO₃
- Al(OH)₃ is calcined (heated strongly) to get pure Al₂O₃: 2Al(OH)₃ → Al₂O₃ + 3H₂O
Stage 2: Electrolytic Reduction — Hall-Heroult Process
Pure Al₂O₃ is dissolved in molten cryolite (Na₃AlF₆) and electrolysed.
- Cathode: Carbon lining of the tank.
- Anode: Carbon rods.
- Electrolyte: Al₂O₃ in molten cryolite + fluorspar (CaF₂).
Reactions:
- At cathode: Al³⁺ + 3e⁻ → Al (Reduction)
- At anode: 2O²⁻ → O₂ + 4e⁻ (Oxidation)
- Overall: 2Al₂O₃ → 4Al + 3O₂
Cryolite serves to: lower the melting point of Al₂O₃ and increase conductivity.
Alloys
An alloy is a homogeneous mixture of two or more metals (or a metal with a non-metal).
Purpose of Alloying
- Increased hardness.
- Lower melting point.
- Resistance to corrosion.
- Improved appearance.
Important Alloys
| Alloy | Composition | Properties | Uses |
|---|---|---|---|
| Stainless steel | Fe + Cr (12−18%) + Ni (8%) | Hard, rust-resistant | Cutlery, utensils, surgical instruments |
| Duralumin | Al + Cu (4%) + Mg (0.5%) + Mn (0.5%) | Light, strong | Aircraft bodies, shipbuilding |
| Brass | Cu + Zn (up to 40%) | Golden colour, malleable | Decorative items, musical instruments |
| Bronze | Cu + Sn (up to 20%) | Hard, resistant to corrosion | Statues, medals, coins |
| Solder | Pb + Sn (50−70%) | Low melting point | Electrical connections, welding |
Comparison: Aluminium vs Iron
| Property | Aluminium | Iron |
|---|---|---|
| Density | 2.7 g/cm³ (light) | 7.87 g/cm³ (heavy) |
| Corrosion resistance | Good (forms Al₂O₃ layer) | Poor (rusts easily) |
| Conductivity | Good (used in power lines) | Lower than Al |
| Cost | More expensive to extract | Cheaper |
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| Confusing Baeyer's and Hall-Heroult processes | Baeyer's = purification; Hall-Heroult = electrolysis |
| Thinking cryolite is the ore | Cryolite (Na₃AlF₆) is added as a flux — NOT the ore |
| Forgetting to include carbon in stainless steel | Stainless steel is primarily iron + chromium + nickel |
| Mixing brass and bronze compositions | Brass = Cu + Zn; Bronze = Cu + Sn |
ICSE Exam Focus
This chapter carries 6–8 marks. Key topics: extraction of Al (Baeyer's + Hall-Heroult), composition and uses of alloys.
Marks Blueprint: Extraction of Al — 4 marks, Alloys — 2 marks, Ores — 2 marks.
Self-Test Questions
-
Name the ore of aluminium. Describe Baeyer's process for purifying bauxite.
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Explain the Hall-Heroult process for the extraction of aluminium. Write the electrode reactions.
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What is the role of cryolite in the extraction of aluminium?
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Give the composition and uses of (a) stainless steel, (b) duralumin, (c) brass, (d) bronze, (e) solder.
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What is an alloy? Why are alloys harder than their constituent metals?
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Why is aluminium used for overhead power cables despite copper being a better conductor?
