METALLURGY OF ALUMINIUM, COPPER AND ZINC – EXTRACTION PROCESSES

ALUMINIUM (Al) – Bayer + Hall-Héroult Process

• Ore: Bauxite (Al₂O₃·2H₂O)
• Step 1: Bayer Process (Purification)
  • Bauxite + hot conc. NaOH → sodium aluminate solution
  • Impurities → red mud (Fe₂O₃ etc.) (removed by filtration)
  • Cooling + seeding → Al(OH)₃ precipitate
  • Calcination → pure Al₂O₃ (alumina)
• Step 2: Hall-Héroult Process (Electrolysis)
  • Alumina dissolved in molten cryolite (Na₃AlF₆) → lowers melting point
  • Electrolysis in carbon-lined cell
• Electrode Reactions:
  • Cathode: Al³⁺ + 3e⁻ → Al (metal)
  • Anode: O₂ reacts with carbon → CO/CO₂
• Important Points:
  • Cryolite increases conductivity & reduces temperature (~950°C)
  • Carbon anode gets consumed → needs replacement
  • Aluminium collected as molten metal at bottom

COPPER (Cu) – Roasting + Smelting + Electrorefining

• Ore: Chalcopyrite (CuFeS₂)
• Step 1: Concentration
  • Froth flotation → removes gangue
• Step 2: Roasting
  • Sulfide → oxide + SO₂ gas
  • Partial oxidation of FeS → FeO
• Step 3: Smelting
  • FeO + SiO₂ → slag (FeSiO₃)
  • Remaining → copper matte (Cu₂S + FeS)
• Step 4: Conversion to Blister Copper
  • Cu₂S oxidized → Cu (98–99% pure)
• Step 5: Electrorefining
  • Anode: impure Cu | Cathode: pure Cu sheet
  • Electrolyte: CuSO₄ + H₂SO₄
  • Result → 99.9% pure copper
• Important Points:
  • Blister copper has SO₂ bubbles → rough surface
  • Precious metals (Ag, Au) settle as anode mud
  • Slag formation is key impurity removal step

ZINC (Zn) – Roasting + Reduction

• Ore: Zinc blende (ZnS)
• Step 1: Concentration
  • Froth flotation
• Step 2: Roasting
  • 2ZnS + 3O₂ → 2ZnO + 2SO₂
• Step 3: Reduction
  • ZnO + C → Zn (vapour) + CO
• Step 4: Condensation
  • Zn vapour cooled → liquid zinc metal
• Refining:
  • By distillation or electrolysis
• Important Points:
  • Zinc has low boiling point → forms vapour during extraction
  • Condensation step is essential
  • SO₂ formed → used in sulfuric acid manufacture