General Principles of Metallurgy

Metallurgy is the science of extracting metals from their ores and refining them for use. The process follows a general sequence: mining → concentration of ore → extraction (reduction) → refining → alloy making.

Occurrence of Metals

Metals occur in nature as: (1) Native/free state — noble metals like Au, Ag, Pt (low reactivity). (2) Combined state — most metals as oxides (haematite Fe₂O₃, bauxite Al₂O₃.2H₂O), sulphides (galena PbS, zinc blende ZnS, copper pyrites CuFeS₂), carbonates (limestone CaCO₃, siderite FeCO₃), halides (fluorspar CaF₂, rock salt NaCl), silicates.

Minerals vs Ores: A mineral is any naturally occurring compound of a metal. An ore is a mineral from which the metal can be extracted profitably. All ores are minerals but not all minerals are ores.

Concentration (Enrichment) of Ore

The process of removing gangue (earthy impurities) from the ore:

1. Gravity separation (hydraulic washing): Based on density difference. Lighter gangue is washed away by water. Used for oxide ores (tin stone SnO₂, haematite Fe₂O₃).

2. Magnetic separation: Ore or gangue is magnetic. Magnetic material attracted to electromagnetic belt roller. Used for: chromite (magnetic) from siliceous gangue; tin stone (non-magnetic) from wolframite (magnetic) impurity.

3. Froth flotation: For sulphide ores. Powdered ore + water + pine oil + frother (cresol). Sulphide particles (hydrophobic) attach to oil-coated air bubbles and float as froth; gangue (hydrophilic) sinks. Selective flotation of mixed ores using depressants (NaCN depresses ZnS by forming Na₂[Zn(CN)₄] surface layer, allowing PbS to float first).

4. Leaching (chemical): Dissolving ore selectively. Bauxite: Bayer's process — Al₂O₃.2H₂O + 2NaOH → 2NaAlO₂ + 3H₂O (dissolves Al₂O₃, rejects Fe₂O₃/SiO₂). NaAlO₂ + CO₂ + 2H₂O → Al(OH)₃ + NaHCO₃. Al(OH)₃ heated → Al₂O₃. Gold: cyanide process — 4Au + 8NaCN + 2H₂O + O₂ → 4Na[Au(CN)₂] + 4NaOH. Gold recovered by Zn displacement: 2Na[Au(CN)₂] + Zn → Na₂[Zn(CN)₄] + 2Au.

Extraction of Crude Metal

Thermodynamic Principles: Metal oxide reduction depends on Gibbs free energy change ($\Delta$ G = $\Delta$ H - T.$\Delta$ S). A reaction is feasible when $\Delta$ G is negative.

Ellingham Diagram: Plot of $\Delta$ G of oxide formation vs temperature. Key features:

• Lines slope upward (positive slope) because oxidation produces fewer gas molecules (S decreases).
• At the intersection point, two metals have equal $\Delta$ G — above this temperature, the lower metal can reduce the upper metal's oxide.
• Carbon line has negative slope (C + O₂ → CO₂, $\Delta$ S ≈ 0; but 2C + O₂ → 2CO, $\Delta$ S positive → steeper negative slope). Carbon becomes a better reducing agent at higher temperatures.
• Al line is very low (Al₂O₃ very stable) → aluminium can reduce almost any metal oxide (thermite reaction).

Reduction Methods:

1. Carbon reduction (smelting): For oxides of Zn, Fe, Sn, Pb. ZnO + C → Zn + CO (at ~1673 K). Fe₂O₃ is reduced in blast furnace with coke.
2. Aluminium reduction (thermite/aluminothermy): For Cr₂O₃, Mn₃O₄, Fe₂O₃. Cr₂O₃ + 2Al → Al₂O₃ + 2Cr. Very exothermic.
3. Electrolytic reduction: For highly electropositive metals (Na, K, Ca, Mg, Al). These cannot be reduced by C or H₂. Al: Hall-Heroult process (electrolysis of Al₂O₃ dissolved in cryolite Na₃AlF₆).
4. Self-reduction (auto-reduction): For Cu and Pb from sulphide ores. 2Cu₂S + 3O₂ → 2Cu₂O + 2SO₂; Cu₂S + 2Cu₂O → 6Cu + SO₂. The metal sulphide and oxide react to give metal.

Blast Furnace — Iron Extraction

Charge: Fe₂O₃ (haematite) + coke (C) + limestone (CaCO₃, flux). Temperature zones from bottom to top:

Products: Pig iron (4% C, brittle) → cast iron (3% C) → wrought iron (<0.2% C, malleable). Steel (0.2-2% C) is intermediate.

Refining of Crude Metal

1. Distillation: For volatile metals (Zn, Hg). Crude metal heated; metal vaporises, condenses separately.

2. Liquation: For low-melting metals (Sn, Pb, Bi). Crude metal heated gently; pure metal melts and flows away from higher-melting impurities.

3. Electrolytic refining: Most widely used. Impure metal = anode, pure metal = cathode, soluble salt of metal = electrolyte. Anode dissolves: Cu → $Cu^{2}$+ + 2e^-. Cathode deposits: $Cu^{2}$+ + 2e^- → Cu (pure). Impurities settle as anode mud. Used for Cu, Ag, Au, Al, Zn.

4. Zone refining: For ultra-pure metals (Ge, Si, Ga, In). Based on principle that impurities are more soluble in molten metal than in solid. A narrow molten zone is moved along a metal rod; impurities concentrate at one end.

5. Vapour phase refining: (a) Mond process (Ni): Ni + 4CO → Ni(CO)₄ (volatile, 330 K) → Ni + 4CO (decomposed at 450 K). (b) Van Arkel process (Zr, Ti): Zr + 2I₂ → ZrI₄ (volatile, 870 K) → Zr + 2I₂ (decomposed at 1800 K on hot W filament).

Specific Metal Extractions

Copper: From CuFeS₂ (copper pyrites). Concentrated by froth flotation → roasted (2CuFeS₂ + O₂ → Cu₂S + 2FeS + SO₂) → smelted with SiO₂ flux (FeS + SiO₂ → FeSiO₃ slag) → self-reduction (2Cu₂S + 3O₂ → 2Cu₂O + 2SO₂; Cu₂S + 2Cu₂O → 6Cu + SO₂) → blister copper (98.5%) → electrolytic refining → 99.99% Cu.

Aluminium: From bauxite (Al₂O₃.2H₂O). Bayer's process → pure Al₂O₃ → Hall-Heroult electrolysis (Al₂O₃ dissolved in cryolite Na₃AlF₆, electrolysed at ~1240 K. Cathode: $Al^{3}$+ + 3e^- → Al. Anode: C + $O^{2}$- → CO/CO₂, graphite anode consumed).

Zinc: From ZnS (zinc blende). Froth flotation → roasting (2ZnS + 3O₂ → 2ZnO + 2SO₂) → reduction with coke (ZnO + C → Zn + CO at 1673 K) → distillation or electrolytic refining.

The key problem-solving concept is applying the Ellingham diagram to predict which reducing agent can reduce a given metal oxide at a specific temperature — the metal whose oxide-formation line lies BELOW can reduce the oxide whose line lies ABOVE.

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