Metallurgy

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Metallurgy is the science and technology of extracting metals from their ores, refining them, and preparing them for use. The three major steps are: (1) enrichment/concentration of ore, (2) extraction of metal (reduction), and (3) refining. The position of a metal in the reactivity series determines its extraction method.

The Reactivity Series (most reactive → least reactive):

K Na Ca Mg Al Zn Fe Pb Cu Hg Ag Au Pt C

(Learn: King Nick Came My Aunt Zara’s Zoo Fought In London Here Sat On A Cat)

Metals above hydrogen displace H₂ from dilute acids. Metals above carbon can be extracted by reduction of their oxides with carbon (coke). Metals below carbon are extracted by heating their oxides alone or with reducing agents. Gold and platinum are found native (uncombined).

Ores and Their Metal Content:

Metal	Common Ores	Composition
Iron	Haematite	Fe₂O₃
Iron	Magnetite	Fe₃O₄
Aluminium	Bauxite	Al₂O₃·2H₂O
Copper	Copper pyrite	CuFeS₂
Copper	Cuprite	Cu₂O
Zinc	Zinc blende	ZnS
Zinc	Calamine	ZnCO₃
Lead	Galena	PbS
Magnesium	Dolomite	CaCO₃·MgCO₃

⚡ Exam Tips:

• Aluminium is extracted by electrolysis of Al₂O₃ (Hall-Héroult process) — not by carbon reduction (Al₂O₃ + C cannot happen because Al is more reactive than carbon)
• Iron is extracted from haematite in a blast furnace — know all zones and reactions
• Copper extraction uses roasting, then reduction with silica, then electrolytic refining

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Steps in Metallurgy:

1. Enrichment/Concentration of Ore: Before extraction, the ore must be concentrated to increase metal content and remove gangue (worthless earthy material).

• Hydraulic washing: Used when ore particles are heavy and gangue is lighter (e.g., oxide ores). Ore is washed with water — gangue is carried away.

• Froth floatation: Used for sulphide ores (e.g., CuFeS₂, ZnS, PbS). Ore is powdered + water + collectors (pine oil, xanthates) + frothers (terpenes). Air is bubbled through — ore particles attach to bubbles and rise as froth; gangue sinks.

  • Collectors: Make ore particles hydrophobic
  • Frothers: Stabilise the foam
  • Activators: NaCN activates ZnS (depresses it so it doesn’t float with Cu ore)
  • Depressants: NaCN in excess prevents ZnS from floating (used in differential floatation of Cu-Pb-Zn ore)

• Magnetic separation: For magnetic ores (magnetite, chromite). Ore is passed over a moving belt with a magnet — magnetic ore sticks; gangue falls away.

2. Extraction of Metal:

A. Reduction with Carbon (Smelting) — for Fe, Zn, Pb, Sn: The metal oxide is heated with coke (carbon) in a furnace. The metal oxide loses oxygen (is reduced).

Example — Blast furnace for iron:

Haematite:  2Fe₂O₃ + 3C → 4Fe + 3CO₂
Alternatively: Fe₂O₃ + 3CO → 2Fe + 3CO₂  (CO is the actual reducing agent)

Blast Furnace Zones (top to bottom):

1. Top zone (descending ore + coke): Preheating zone (200–700°C) — ore and coke preheated; no reaction
2. Middle zone (800–1000°C): Reduction zone — CO reduces Fe₂O₃ to FeO; limestone decomposes
3. Fusion zone (1500–1800°C): Iron melts; coke burns; slag (CaSiO₃) formed
4. Hearth: Molten iron + slag collect; tapped separately

Limestone (CaCO₃) is added as flux — it combines with silica gangue (SiO₂) to form calcium silicate slag (CaSiO₃): CaCO₃ → CaO + CO₂; CaO + SiO₂ → CaSiO₃

B. Reduction with Aluminium (Gold Schmidt Thermite Process): Used for metals with very high melting points (Mn, Cr, V) — impossible to reduce with carbon (they form carbides).

Cr₂O₃ + 2Al → 2Cr + Al₂O₃ (ΔH highly exothermic; reaction is self-sustaining once ignited)

Also used to weld railway tracks (the molten iron produced is used as filler): Fe₂O₃ + 2Al → 2Fe + Al₂O₃ + heat (molten iron fills track gaps)

C. Electrolytic Extraction — for very reactive metals (Al, Na, K, Ca, Mg): These metals cannot be extracted by carbon because they react with carbon to form carbides, or the temperature required is impractical.

Hall-Héroult Process (Aluminium):

• Electrolyte: Al₂O₃ dissolved in molten cryolite (Na₃AlF₆) — lowers melting point from 2074°C to ~950°C
• Cathode: Carbon lining of the cell
• Anode: Carbon rods (consumed during electrolysis)
• Electrolysis: 2Al₂O₃ → 4Al (l cathode) + 3O₂ (g anode)
• Anode effect: When alumina is low, CO₂/F₂ produced (undesirable)
• Energy cost: Very high — recycling aluminium uses only 5% of energy vs primary production

D. Auto-reduction (for Hg and Cu): Mercury and copper from their sulphides can be obtained by heating in air — no separate reducing agent needed.

2HgS + 3O₂ → 2HgO + 2SO₂; 2HgO + heat → 2Hg + O₂ Cu₂S + O₂ → 2Cu + SO₂

E. Biological/Bioleaching: Microorganisms (Thiobacillus ferrooxidans) oxidise low-grade sulphide ores to soluble sulphates. Used for copper extraction from low-grade ore — the copper dissolves in water and is then precipitated by scrap iron.

3. Refining of Metals:

Electrolytic refining (most common): Impure metal is made the anode; pure metal is the cathode; electrolyte is a solution of the metal’s salt (e.g., CuSO₄ for copper).

At anode: M → M²⁺ + 2e⁻ (impurities with lower reduction potential dissolve) At cathode: M²⁺ + 2e⁻ → M (pure metal deposits) Impurities: Less reactive metals (Ag, Au, Pt) fall to bottom as “anode mud”; more reactive metals (Fe, Zn) remain in solution.

Other methods:

• Distillation: For metals with low boiling points (Zn, Cd, Hg)
• Liquation: For metals with low melting points than impurities (Sn, Pb)
• Zone refining: For ultra-pure silicon, germanium (used in semiconductors); a heated coil moves along the rod — impurities concentrate in the melt and are carried to the end

⚡ Common Mistakes:

• Students confuse “flux” and “gangue” — flux is added to react with gangue and remove it; gangue is the worthless material in the ore
• In the blast furnace, the reducing agent is CO (not directly carbon); carbon first burns to CO, which then reduces iron oxide
• Aluminium cannot be extracted by carbon reduction because aluminium carbide (Al₄C₃) forms at high temperatures
• NaCN acts as a depressant for ZnS in Cu ore floatation — important in differential floatation

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🔴 Extended — Deep Dive (exam-level mastery)

For students preparing for top-rank selection.

Thermodynamics of Metal Extraction:

The Ellingham diagram plots ΔG°(T) = ΔH° – TΔS° for metal oxide formation reactions: 2M + O₂ → 2MO. Key insights:

1. Lower ΔG° = more stable oxide: More negative ΔG° means the oxide is more stable (metal has higher affinity for oxygen)
2. Lines cross = change in extraction method: Where two lines cross, the two metals compete for oxygen
3. Below the line = can reduce: A metal M can reduce the oxide of another metal N if ΔG°(MO₂) is more negative than ΔG°(NO₂)

For carbon (C + O₂ → CO₂): The CO₂ line has a negative slope (ΔS° positive because 1 mol CO₂ from 1 mol C increases gas moles). Below ~710°C, C can reduce most metal oxides. Above this temperature, carbon becomes a better reducing agent than many metals.

For Al (4Al + 3O₂ → 2Al₂O₃): The Al₂O₃ line is very low (highly negative ΔG°) — Al can reduce oxides of Fe, Mn, Cr, V, Si, etc. This is why thermite uses Al.

Kroll Process (Titanium): Ti is extracted by the Kroll process (not by electrolysis because TiCl₄ is covalent and the metal reacts with all crucible materials):

1. TiO₂ + 2Cl₂ + 2C → TiCl₄ + 2CO (in chlorine atmosphere)
2. TiCl₄ + 2Mg → Ti + 2MgCl₂ (at 800°C in argon atmosphere; TiCl₄ is reduced by Mg in a sealed retort)
3. MgCl₂ is removed by vacuum distillation
4. Titanium sponge is collected and arc-melted into ingots

This is expensive (Mg is costly; Ar is needed; multi-step process). Only ~100,000 tonnes/year produced vs millions of tonnes of steel.

Van Arkel Process (Ultra-Pure Metals): For semiconductors (Si, Ge) and refractory metals (Ti, Zr, Hf), the van Arkel method produces ultra-pure metals:

1. Impure metal reacts with iodine at high T: M + 2I₂ → MI₄ (volatile)
2. MI₄ is decomposed on a heated filament: MI₄ → M + 2I₂
3. Iodine is recycled

Cupellation and Bessemerisation:

• Cupellation: Lead containing gold and silver is oxidised — Pb forms PbO (litharge) which is absorbed by the cupel (porous container); gold and silver remain
• Bessemerisation: Impure copper (containing Fe, Zn, Ni) is oxidised by air blown through molten metal — base metals form oxides (slagged off); copper remains

Physical Metallurgy — Alloys:

Steel vs Iron:

• Cast iron: 2–4.3% C; hard, brittle; cannot be rolled/forged
• Wrought iron: <0.25% C; soft, ductile; fibrous slag inclusions
• Steel: 0.25–2% C; hard, strong, ductile; the most important alloy

Heat Treatment of Steel:

• Quenching: Steel heated to austenite region (above A₃ line) then rapidly cooled in water/oil → martensite (very hard, brittle)
• Tempering: Quenched steel reheated to 150–550°C → relieves internal stress; reduces brittleness while retaining hardness
• Annealing: Heated and slowly cooled → coarse pearlite (soft); relieves internal stress
• Normalising: Heated above A₃ then air-cooled → fine pearlite (stronger than annealed)
• Case hardening: Low-carbon steel heated in carbon-rich atmosphere (carburising) or nitrogen atmosphere (nitriding) → surface hardens

Iron-Carbon Phase Diagram:

• Eutectoid point (0.76% C, 727°C): Austenite transforms to pearlite
• Eutectic point (4.3% C, 1147°C): Liquid transforms to ledeburite (in cast irons)
• A₁ line: Pearlite ↔ austenite boundary
• A₃ line: Ferrite ↔ austenite boundary (for low-C steels)
• ACM line: Cementite ↔ austenite boundary

Pearlite: Lamellar mixture of ferrite (α-Fe, BCC, soft) and cementite (Fe₃C, very hard) — forms by eutectoid transformation. The alternating layers give pearlite its strength.

Common Alloy Steels:

Steel	Composition	Properties	Uses
Stainless	>10.5% Cr, <1.2% C	Corrosion-resistant	Cutlery, surgical instruments
Manganese (Hadfield)	12–14% Mn	Work-hardens; very tough	Crusher jaws, railway crossings
Nickel steel	3–5% Ni	Low coefficient of expansion	Watch springs, precision instruments
Tungsten steel	15–20% W	Retains hardness at high T	Cutting tools (high-speed steel)
Duralumin	4% Cu, 0.5% Mg, 0.5% Mn	Strong, light	Aircraft parts

Corrosion — Electrochemical Mechanism:

Rust is an electrochemical process:

1. Anode (Fe → Fe²⁺ + 2e⁻): Surface defects, stress points
2. Electrons flow through metal to cathodic sites
3. Cathode (in acidic): 2H⁺ + 2e⁻ → H₂; (in neutral/alkaline): O₂ + 2H₂O + 4e⁻ → 4OH⁻
4. Rust formation: Fe²⁺ + 2OH⁻ → Fe(OH)₂ → Fe₂O₃·H₂O (rust, brownish)

Conditions accelerating corrosion: Salts (NaCl), acids, galvanic contact with more noble metal, stress, porosity.

Prevention of Corrosion:

• Barrier protection: Paint, lacquer, plating (galvanisation — Zn coating; tinning — Sn coating)
• Sacrificial (cathodic) protection: More reactive metal (Mg, Zn) acts as anode; corrodes instead of steel (underground pipelines use Mg or Zn blocks)
• Electrical protection: Impressed current from DC source; forces steel to be cathode
• Alloying: Stainless steel (Cr forms Cr₂O₃ protective layer); weathering steels (Cu, P — forms protective rust layer)

NEET High-Yield Pattern:

• Blast furnace: haematite + coke + limestone → pig iron; products: pig iron (Fe + C), slag (CaSiO₃), waste gases (CO₂, N₂)
• Hall-Héroult: Al₂O₃ + cryolite → electrolysis → Al at cathode, O₂ at anode
• Thermite: Cr₂O₃ + Al → Cr + Al₂O₃
• Froth floatation: collectors (pine oil/xanthates) make ore hydrophobic; depressants (NaCN) prevent unwanted minerals from floating
• Electrolytic refining: impure metal anode; pure metal cathode; more reactive metals dissolve, less reactive fall as anode mud
• rust = Fe₂O₃·H₂O; requires Fe + H₂O + O₂