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Chemistry 3% exam weight

Metallurgy

Part of the JEE Main study roadmap. Chemistry topic chem-019 of Chemistry.

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Metallurgy

🟢 Lite — Quick Review (1h–1d)

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Metallurgy is the sequence of operations used to obtain a pure metal from its ore. The four mandatory stages are (i) concentration of ore, (ii) conversion to oxide (or other suitable form), (iii) reduction to crude metal, and (iv) refining. For JEE Main, concentrate on the Ellingham diagram (ΔG° vs T plot for oxide formation) — a metal/element whose oxide-formation line lies below can reduce the oxide of one whose line lies above. Remember the two key pyrometallurgical steps: calcination (limited air, drives off CO₂/H₂O from carbonates and hydroxides of Zn, Fe, Cu) and roasting (excess air, converts sulphides to oxides, e.g. 2ZnS + 3O₂ → 2ZnO + 2SO₂). High-yield facts: Mond process purifies Ni via Ni(CO)₄; van Arkel purifies Ti, Zr, Hf via the volatile iodide; froth flotation concentrates sulphide ores using pine oil collectors; the Bayer process digests bauxite with NaOH to give sodium aluminate. Feasibility formula: ΔG° = ΔH° − TΔS°, and ΔG° = −nFE° links free energy to cell potential.

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Ore Dressing (Concentration)

Removal of gangue (earthy impurities) is the first step. Hydraulic washing / levigation works for oxide and carbonate ores (e.g., haematite, SnO₂) — lighter sand particles wash away in a stream of water while denser ore settles. Magnetic separation pulls out magnetite (Fe₃O₄), chromite, and wolframite using electromagnets. Froth flotation is the technique for sulphide ores (ZnS, PbS, CuFeS₂); the ore is wetted by oil (pine oil/Eucalyptus oil) while gangue is wetted by water. Air is blown in, sulphide particles rise with the froth, and collectors like potassium ethyl xanthate stabilise the froth. Leaching is used when the ore is soluble in a reagent but gangue is not — bauxite (Al₂O₃·2H₂O) is digested in concentrated NaOH to form soluble NaAlO₂, leaving behind insoluble Fe₂O₃, SiO₂, TiO₂ (Bayer process).

Conversion to Oxide

Calcination = heating ore strongly below its melting point in limited/absence of air. It drives off volatile impurities as CO₂ or H₂O:

  • ZnCO₃ → ZnO + CO₂
  • CaCO₃ → CaO + CO₂
  • 2Fe(OH)₃ → Fe₂O₃ + 3H₂O

Roasting = heating ore strongly below its melting point in excess air, mainly applied to sulphide ores:

  • 2ZnS + 3O₂ → 2ZnO + 2SO₂
  • 2Cu₂S + 3O₂ → 2Cu₂O + 2SO₂ The released SO₂ is used to manufacture H₂SO₄.

Reduction and the Ellingham Diagram

The Ellingham diagram plots ΔG° of formation of oxides (per mole O₂) against temperature. A more negative ΔG° means a more stable oxide. A metal whose oxide line lies lower in the diagram can reduce the oxide of any metal whose line lies above it. For carbon, the C → CO line slopes downward above ~710 °C because ΔS° is large and positive (formation of gaseous CO from solid C increases entropy). This is why coke can reduce Fe₂O₃, ZnO, SnO₂ at blast-furnace temperatures but cannot reduce the very stable oxides Al₂O₃, MgO, CaO — these require electrolytic reduction in molten state (Hall–Héroult for Al, Down’s process for Na).

Refining Methods

  • Distillation: Zn, Hg, Sn boil and condense separately.
  • Liquation: low-melting metals like Sn, Pb, Bi are melted and tapped away from solid impurities.
  • Electrolytic refining: impure metal = anode, thin strip of pure metal = cathode, acidified salt solution of metal = electrolyte. At cathode: Mⁿ⁺ + ne⁻ → M. Used for Cu, Zn, Ni, Ag, Au.
  • Zone refining: a moving molten zone sweeps impurities to one end; used for semiconductors (Si, Ge, B, Ga, In).
  • Vapour phase refining — Mond process: Ni + 4CO → Ni(CO)₄ (400 K) → Ni + 4CO (450 K). Impure nickel forms the volatile tetracarbonyl that decomposes back to pure Ni.
  • Vapour phase refining — van Arkel: Ti + 2I₂ → TiI₄ → Ti + 2I₂ (on a hot tungsten filament). Used for Ti, Zr, Hf, V.

Iron and Copper Extraction

In the blast furnace, Fe₂O₃ + 3CO → 2Fe + 3CO₂; limestone (CaCO₃) decomposes to CaO which acts as the flux, combining with silica gangue to give CaSiO₃ slag (CaO + SiO₂ → CaSiO₃). Copper is extracted from copper pyrites CuFeS₂ by partial roasting followed by smelting in a Bessemer converter (self-reduction: Cu₂S + 2Cu₂O → 6Cu + SO₂) and finally electrolytic refining.

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Thermodynamic Basis of Reduction

Reduction of a metal oxide MₓOᵧ by a reductant R is feasible only when ΔG° (combined reaction) < 0. Using ΔG° = ΔH° − TΔS° and the Ellingham plot:

  • For metal oxide formation, ΔS° is negative (gas moles decrease: 2M(s) + O₂(g) → 2MO(s)), so the line slopes upward with T.
  • For C + ½O₂ → CO, ΔS° is positive (gas moles increase), so the line slopes downward.
  • The two lines therefore cross; above the intersection (~710 °C for Fe, ~1000 °C for Zn), carbon becomes a stronger reducing agent than the metal itself.

When ΔG° is converted to a corresponding cell potential via ΔG° = −nFE°, a reaction is spontaneous only if E°cell > 0. This is the same feasibility criterion as in electrochemistry, and explains why fused-salt electrolysis is the only viable route for Al, Mg, Na.

Edge Cases and Frequently-Missed Facts

  • Calcination is for carbonates/hydroxides; roasting is for sulphides. A common JEE trap asks for the product of “roasting limestone” — limestone is calcined, not roasted.
  • Coke cannot reduce Al₂O₃, MgO, CaO, because the C/CO line never crosses below their very stable oxide lines at practical furnace temperatures.
  • Bessemer converter uses self-reduction, not an external reductant. Cu₂S and Cu₂O produced during roasting exchange oxygen: 2Cu₂O + Cu₂S → 6Cu + SO₂.
  • Zone refining works because most impurities are more soluble in the molten phase than the solid; they are dragged along with the moving heater.
  • In the Mond process the carbonyl must decompose at a different temperature than it forms — that is why heating to 450 K is essential after the 400 K volatilisation step.
  • Froth flotation uses depressants (e.g., NaCN for ZnS) to selectively float one sulphide while suppressing another from a mixed ore (PbS–ZnS separation).
  • Chromatographic / ion-exchange methods are not metallurgical refining routes for metals, though they appear as distractors.

Worked Example

A sample of cuprite ore (Cu₂O) weighs 2.40 g. After complete reduction it yields 1.92 g of copper. Molar mass of Cu₂O = 143 g mol⁻¹ (2 × 63.5 + 16); mass of Cu in 2.40 g Cu₂O = (127/143) × 2.40 = 2.13 g. Percentage purity = (1.92 / 2.13) × 100 = 90.1 %.

Common Mistakes

  • Mixing up flux (added reagent, e.g., CaO, SiO₂, borax) with gangue (unwanted earthy material to be removed).
  • Forgetting that in electrolytic refining the anode mud contains noble metals (Ag, Au, Pt) — these do not oxidise and settle at the bottom.
  • Assuming all refining methods work for all metals — Mond process is specific to Ni, van Arkel is specific to Ti/Zr/Hf.

Practice Prompts

  1. Justify using coke as a reducing agent for ZnO above 1000 °C but not for Al₂O₃, with reference to the Ellingham diagram and the signs of ΔH°, ΔS°.
  2. Explain how electrolytic refining of blister copper yields pure copper at the cathode and accumulates silver/gold in the anode mud. Write the cathode half-reaction and identify the impurities that remain in solution versus those that plate out.

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