Metallurgy
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Direct answer
Metallurgy is the craft of winning a metal from its ore: concentrate, convert to an oxide, reduce, then refine. Sulphide ores are concentrated by froth flotation and roasted; carbonates and hydroxides are calcined; the oxide is then reduced by carbon, a reactive metal or electrolysis — a choice governed by the Ellingham diagram, which plots oxide-formation free energy against temperature.
What you must remember
- Froth flotation suits sulphides: powdered ore with pine oil agitated so sulphide rides the froth; depressants like NaCN selectively hold back ZnS when it is mixed with galena.
- Bayer leaching: bauxite in hot NaOH dissolves as sodium aluminate, reprecipitated as Al(OH)3 and calcined to alumina.
- Roasting heats sulphides in excess air (2ZnS + 3O2 → 2ZnO + 2SO2); calcination heats carbonates and hydroxides in limited air (CaCO3 → CaO + CO2).
- Ellingham rules: the lower the line, the more stable the oxide; a metal reduces any oxide whose line sits above it; the downward C → CO line crosses most metal lines at high temperature, making carbon the universal reductant — Al2O3 and MgO lie too low, forcing electrolysis.
- Aluminothermite: Cr2O3 + 2Al → 2Cr + Al2O3; Fe2O3 + 2Al welds rails.
- Blast furnace: Fe2O3 + 3CO → 2Fe + 3CO2, with limestone flux; the product is carbon-rich pig iron.
- Copper: chalcopyrite smelted to matte (Cu2S-FeS), then bessemerisation — 2Cu2O + Cu2S → 6Cu + SO2 — giving blister copper.
- Zinc: ZnO reduced by coke; zinc boils near the reaction temperature, so the metal leaves as vapour and is condensed.
- Aluminium: alumina electrolysed in molten cryolite Na3AlF6 (lowering the melting point); carbon anodes burn away and need replacement.
- Refining: electrolytic (impure anode dissolves, pure metal deposits; silver and gold collect in the anode mud), Mond for nickel via volatile Ni(CO)4, van Arkel for titanium and zirconium, zone refining for semiconductor-grade silicon and germanium.
Common confusion
The classic muddle is roasting versus calcination: roasting suits sulphides with SO2 evolving, calcination suits carbonates and hydroxides with CO2 or water. Students misread Ellingham diagrams by comparing lines at room temperature — reduction is decided by delta G at the working temperature. They also forget why cryolite is added: molten alumina alone would demand too high a temperature.
Exam-focused takeaway
JEE Main frames metallurgy as direct NCERT facts: match ore with process, name the depressant, write roasting equations, pick the refining method. JEE Advanced builds multi-step extraction sequences, reads Ellingham diagrams for threshold temperatures, and probes electrode reactions and the anode mud. Draw the ore-to-refined-metal flowchart before answering; sequence questions reward students who have traced the path.
Frequently asked questions
What does a depressant do in froth flotation?
It selectively stops one sulphide from floating — NaCN added to a PbS-ZnS mixture depresses ZnS so only galena rises.
How is an Ellingham diagram used?
Compare free-energy lines at the reaction temperature: the lower-line metal reduces the one above it; carbon's falling C → CO line is why high temperature enables carbon reduction.
Why is aluminium won by electrolysis rather than carbon reduction?
Alumina's line lies below the carbon lines at practical temperatures, so carbon cannot reduce it; electrolysis in molten cryolite is required.
What is the thermite reaction?
Fe2O3 + 2Al → 2Fe + Al2O3, so exothermic that molten iron forms — used to weld rails; chromium is won likewise.
Why is zinc collected as a vapour?
Zinc boils near the reduction temperature, so it leaves the retort as vapour and condenses in cooled receivers, protected from reoxidation.
What is zone refining for?
Ultra-pure silicon and germanium: a molten zone sweeps the rod, carrying impurities that dissolve better in the liquid.