Learning Objectives
- Understand the steps in metallurgical extraction
- Learn about concentration, roasting, calcination, and smelting
- Study thermodynamic principles (Ellingham diagrams)
- Understand electrochemical principles of extraction
- Learn refining methods for metals
Key Concepts
Occurrence of Metals
Metals occur as oxides, sulphides, carbonates, halides, silicates in ores. Ore: Mineral from which metal can be profitably extracted. Gangue: Unwanted material in ore.
Steps in Metallurgy
1. Concentration/Beneficiation:
- Hydraulic washing: Based on density difference. For oxide ores (tin).
- Magnetic separation: For magnetic ores (magnetite, chromite).
- Froth floatation: For sulphide ores (Cu, Pb, Zn). Ore particles attach to oil froth; gangue sinks. Collectors (xanthates) enhance hydrophobicity. Depressants (NaCN) selectively prevent floatation.
- Leaching: Dissolving ore in chemicals. Al₂O₃ in NaOH (Bayer's process): Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O. Au in NaCN (cyanide leaching): 4Au + 8NaCN + 2H₂O + O₂ → 4Na[Au(CN)₂] + 4NaOH.
2. Extraction (Reduction):
- Roasting: Heating sulphide ore in air to convert to oxide. 2ZnS + 3O₂ → 2ZnO + 2SO₂.
- Calcination: Heating carbonate/hydrated ore without air. CaCO₃ → CaO + CO₂.
- Smelting: Reduction with carbon/CO in a blast furnace. ZnO + C → Zn + CO. Fe₂O₃ + 3CO → 2Fe + 3CO₂.
- Self-reduction (auto-reduction): Cu₂S + 2Cu₂O → 6Cu + SO₂. Used for copper.
- Electrolytic reduction: For highly electropositive metals (Na, K, Ca, Al). Hall-Heroult process for Al: Al₂O₃ dissolved in cryolite (Na₃AlF₆) electrolysed.
Thermodynamic Principles (Ellingham Diagram)
Plots ΔG° of oxide formation vs temperature. Lower line indicates more stable oxide.
A metal can reduce the oxide of another metal that is higher on the Ellingham diagram.
C-CO line slopes downward steeply, so carbon becomes a better reducing agent at higher temperatures.
At ~1073 K: C line crosses Fe₂O₃ line → carbon can reduce iron oxide above this temperature.
3. Refining
- Distillation: For volatile metals (Zn, Hg).
- Liquation: For low MP metals (Sn, Bi, Pb).
- Electrolytic refining: Impure metal = anode, pure metal = cathode, salt solution = electrolyte. Used for Cu, Zn, Ni, Ag, Au.
- Zone refining: Based on impurities being more soluble in melt. For semiconductors (Si, Ge, Ga).
- Vapour phase refining: Mond process (Ni + 4CO → Ni(CO)₄ → Ni + 4CO). Van Arkel method (Ti + 2I₂ → TiI₄ → Ti + 2I₂).
- Chromatography: For separating elements with similar properties (Ru, Os, lanthanoids).
Summary
Metallurgy involves concentration, extraction, and refining of metals from ores. Concentration methods depend on ore type. Extraction uses roasting/calcination followed by reduction (carbon, electrolysis, or self-reduction). Ellingham diagrams guide the choice of reducing agent. Refining purifies crude metal through distillation, electrolysis, zone refining, or vapour phase methods.
Important Terms
- Gangue: Unwanted impurities in ore
- Flux: Substance that combines with gangue to form fusible slag
- Slag: Fusible product of flux and gangue
- Roasting: Heating sulphide ore in air
- Calcination: Heating carbonate ore without air
- Ellingham Diagram: ΔG° vs T plot for oxide formation
Quick Revision
- Froth floatation: sulphide ores; Magnetic separation: magnetic ores
- Roasting: sulphide → oxide; Calcination: carbonate → oxide
- Al: Hall-Heroult (electrolysis in cryolite); Fe: blast furnace
- Ellingham: lower line = more stable oxide, reduces the one above
- Electrolytic refining: impure anode, pure cathode
- Zone refining: Si, Ge; Mond process: Ni; Van Arkel: Ti, Zr