Metallurgy

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Metallurgy — Extraction and Properties of Metals

Metallurgy is the science and technology of extracting metals from their ores, refining them, and preparing them for use. The process involves several stages: concentration of ore, extraction of metal, refining, and shaping.

Occurrence of Metals: Metals occur in nature in combined form (ores) or rarely in native state (gold, silver, copper, platinum). The crustal abundance decreases: Al > Fe > Ca > Na > K > Mg.

Types of Ores:

• Oxides: Haematite (Fe₂O₃), Bauxite (Al₂O₃·xH₂O), Magnetite (Fe₃O₄), Zincite (ZnO), Cassiterite (SnO₂)
• Sulfides: Galena (PbS), Zinc blende (ZnS), Copper pyrite (CuFeS₂), Cinnabar (HgS), Argentite (Ag₂S)
• Carbonates: Limestone (CaCO₃), Magnesite (MgCO₃), Siderite (FeCO₃), Malachite (Cu₂CO₃(OH)₂), Azurite (Cu₃(CO₃)₂(OH)₂)
• Halides: Rock salt (NaCl), Fluorspar (CaF₂), Carnallite (KCl·MgCl₂·6H₂O)
• Sulfates: Gypsum (CaSO₄·2H₂O), Epsom salt (MgSO₄·7H₂O), Barite (BaSO₄)
• Silicates: Feldspar, Mica, Clay

⚡ Exam tip: Not all ores are minerals, but all minerals are not ores. An ore is a mineral from which metal can be extracted economically. For example, Al₂O₃ is a mineral found in bauxite, but not all Al₂O₃ deposits are economical to mine.

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Metallurgy — Chemistry Study Guide

Major Steps in Metallurgy:

1. Concentration of Ore: Removes gangue (undesirable impurities). Methods depend on physical/chemical properties:

a) Gravity Separation / Floatation:

• Used for sulfide ores (chalcopyrite, galena, etc.)
• Froth floatation: Ore is mixed with water + oil + frothing agent + collecting agent
• Galena (PbS) is floated with potassium ethyl xanthate as collector
• Malachite (Cu₂CO₃(OH)₂) — also uses collectors

b) Magnetic Separation:

• For magnetic ores like magnetite (Fe₃O₄) and pyrolusite (MnO₂)
• Crushed ore is passed through a magnetic separator

c) Leaching:

• Chemical method for low-grade ores
• Bauxite: Bayer’s process — NaOH dissolves Al₂O₃ as aluminate
• Silver/Gold: Cyanide process

2. Extraction of Metal:

A) Reduction of Oxides — Thermodynamic Principles: The Ellingham diagram plots ΔG° vs T for formation of oxides. Lower the ΔG° (more negative), more stable the oxide. A metal can reduce the oxide of another metal if its oxide has more negative ΔG° at that temperature.

Important relationships:

• At equilibrium: ΔG° = −RT ln Kₚ
• For reaction: 2M + O₂ → 2MO; ΔG° must be negative for reduction to occur spontaneously

For FeO reduction by CO: FeO + CO → Fe + CO₂ ΔG° = ΔG°(FeO) − ΔG°(CO₂) + ΔG°(CO) At 1000K, this becomes favorable because ΔG°(CO₂) is more negative than ΔG°(FeO).

B) Carbon as Reducing Agent: Used for metals in the middle of reactivity series (Fe, Zn, Sn, Pb, Cu).

• Fe₂O₃ + 3CO → 2Fe + 3CO₂ (Blast furnace, 1500°C)
• ZnO + C → Zn + CO (2000°C in horizontal retort)
• SnO₂ + 2C → Sn + 2CO
• PbO + C → Pb + CO

C) Self-Reduction (Auto-reduction): Used for metals like Cu, Pb, Hg, Ag that form unstable oxides:

• 2Cu₂S + 3O₂ → 2Cu₂O + 2SO₂
• Cu₂S + 2Cu₂O → 6Cu + SO₂ (in Bessemer converter)
• PbS + 2PbO → 3Pb + SO₂
• 2HgS + 3O₂ → 2HgO + 2SO₂; 2HgO → 2Hg + O₂

D) Electrochemical Extraction: Used for very reactive metals (Na, K, Ca, Mg, Al) — metals that cannot be economically reduced by carbon.

• Na: Downs cell — NaCl (molten) → Na + Cl₂
• Al: Hall-Héroult process — Al₂O₃ (molten) + Na₃AlF₆ → Al + CO₂
• Mg: Electrolysis of MgCl₂ (from magnesite)

E) Hydrometallurgy: Extraction using aqueous solutions:

• Ag: 4Ag + 8NaCN + O₂ + 2H₂O → 4Na[Ag(CN)₂] + 4NaOH; then Zn displaces Ag
• Au: 4Au + 8NaCN + O₂ + 2H₂O → 4Na[Au(CN)₂] + 4NaOH; then Zn displaces Au

3. Refining of Metals:

a) Electrolytic Refining:

• Anode: Impure metal
• Cathode: Pure metal strip
• Electrolyte: Aqueous solution of metal’s salt
• Cu refining: CuSO₄ solution
• At anode: M → M²⁺ + 2e⁻ (impurities fall as anode mud)
• At cathode: M²⁺ + 2e⁻ → M (pure)

b) Zone Refining:

• For ultra-pure metals (Si, Ge, Ga, In)
• A heated coil moves along a metal rod, melting and recrystallizing the pure metal
• Impurities concentrate in the molten zone and move to one end

c) Van Arkel Process:

• For very pure Ti, Zr, Si, etc.
• Metal reacts with I₂ to form volatile MI₄; MI₄ is decomposed on a hot wire
• Ti + 2I₂ → TiI₄; TiI₄ → Ti + 2I₂ (at 1400°C)

d) Liquation:

• For metals with low melting points (Pb, Sn, Zn, Hg)
• Impure metal is heated just above melting point; pure metal melts and flows away

e) Distillation:

• For volatile metals: Zn, Cd, Hg
• Distilled to separate from less volatile impurities

f) Cupellation:

• For noble metals (Ag, Au) — Pb is oxidised to PbO which is absorbed by bone ash

g) Pollarisation:

• For removal of dissolved gases from molten metals

⚡ Exam tip: In the Blast furnace, the temperature zones are:

• Lower part (combustion zone): 1500-2000°C — C + O₂ → CO₂
• Middle (reduction zone): 700-1200°C — CO₂ + C → 2CO; Fe₂O₃ + 3CO → 2Fe + 3CO₂
• Upper (preheating zone): 300-700°C — moisture removed, limestone decomposes
• CaCO₃ → CaO + CO₂; CaO + SiO₂ → CaSiO₃ (slag)

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Metallurgy — Comprehensive Chemistry Notes

The Blast Furnace — Complete Process:

The blast furnace is a refractory-lined shaft furnace. The overall process for iron extraction:

Raw materials: Iron ore (haematite: Fe₂O₃), coke (C), limestone (CaCO₃), hot air (blown in at 1000-1200°C).

Chemical Reactions:

1. Combustion zone (hearth): C + O₂ → CO₂ (exothermic, T = 2000°C) CO₂ + C → 2CO (endothermic, T = 1000-1100°C)

2. Reduction zone (middle): Fe₂O₃ + 3CO → 2Fe + 3CO₂ (indirect reduction) Fe₂O₃ + CO → 2FeO + CO₂ (intermediate) FeO + CO → Fe + CO₂ (indirect reduction)

3. Direct reduction: FeO + C → Fe + CO (some direct reduction occurs at high T)

4. Slag formation (bottom): CaCO₃ → CaO + CO₂ (decomposition of limestone, ~900°C) CaO + SiO₂ → CaSiO₃ (lance slag, fused at 1500°C) CaO + Al₂O₃·SiO₂ → CaAl₂Si₂O₈ (slag with clay impurities)

Products:

• Cast iron (pig iron): 2-4% C, 1-3% Si, small amounts of Mn, P, S; hard, brittle, not malleable
• Slag: CaSiO₃ (used in road building, cement)

Steel Making: Steel is iron with 0.1-2.1% carbon and small amounts of Mn, Si, Cr, Ni.

a) Bessemer Process:

• Converter lined with silica (acidic) — cannot remove P, S
• Air blown through molten iron oxidizes C to CO₂ and Si to SiO₂
• Cannot control carbon content precisely; deprecated

b) Open Hearth Process:

• Better control over composition
• Regenerative furnaces recover waste heat
• Can remove P and S with basic lining

c) Linz-Donawitz (LD) Process / Basic Oxygen Process:

• Most common modern method
• Oxygen jet blown through molten iron in basic-lined vessel
• Basic lining (MgO, CaO) removes P as Ca₃(PO₄)₂ (Thomas slag)
• Lower N₂ content than Bessemer (no air blown through)

d) Electric Arc Furnace:

• For alloy steels and high-quality steel
• Graphite electrodes create arc at 3000°C
• Precise control of temperature and composition

Types of Steel:

• Low carbon steel: < 0.25% C — structural, pipes, sheets
• Medium carbon steel: 0.25-0.7% C — axles, gears, rails
• High carbon steel: 0.7-1.5% C — tools, springs, cutlery
• Stainless steel: Fe + Cr + Ni — Cr forms passive oxide layer preventing corrosion
• Manganese steel: Fe + Mn (12-14%) — wear-resistant
• Tungsten steel: Fe + W — retains hardness at high temperature (tool steel)

Copper Extraction — Pyrometallurgy:

Copper ore: Copper pyrite (CuFeS₂)

1. Concentration: Froth floatation
2. Roasting: 2CuFeS₂ + O₂ → Cu₂S + 2FeS + SO₂ (partial roasting)
3. Smelting in Bessemer-like converter:
   • Matte (Cu₂S + FeS) is formed
   • Air blown: 2FeS + 3O₂ + SiO₂ → 2FeO·SiO₂ + 2SO₂
   • Cu₂S + O₂ → 2Cu + SO₂ (self-reduction)
4. Blister copper: 98-99% pure, has blister surface due to SO₂ gas evolution
5. Electrolytic refining: Pure copper at cathode (99.99%)

Aluminium Extraction — Hall-Héroult Process:

Bauxite (Al₂O₃·xH₂O) contains SiO₂, Fe₂O₃ as impurities. Bayer’s Process for purification:

1. Al₂O₃·xH₂O + NaOH → Na[Al(OH)₄] (sodium aluminate solution)
2. Seeded with Al(OH)₃ at 50-60°C → Al(OH)₃ precipitates
3. 2Al(OH)₃ → Al₂O₃ + 3H₂O (at 1000°C, gives anhydrous Al₂O₃)

Electrolysis in Hall cell:

• Electrolyte: Al₂O₃ + Na₃AlF₆ (cryolite lowers MP from 2050°C to 950°C)
• Carbon anodes (consumed in process)
• Cathode: Iron lined with carbon
• Cell reaction: 2Al₂O₃ + 3C → 4Al + 3CO₂
• Anode reaction: C + 2O²⁻ → CO₂ + 4e⁻
• Energy consumption is very high (~15 MWh/tonne)

⚡ Exam tip: The cryolite is not just a solvent but also lowers the melting point significantly. Without it, the process would be economically unviable. The carbon anode is oxidized to CO₂, so it needs periodic replacement.

Zinc Extraction:

Roasting: 2ZnS + 3O₂ → 2ZnO + 2SO₂

Reduction: ZnO + C → Zn + CO (at 1200°C; Zn is volatile, BP = 907°C)

Purification: Electrolytic refining using ZnSO₄ electrolyte

Lead Extraction:

Roasting: 2PbS + 3O₂ → 2PbO + 2SO₂ Reduction: PbO + C → Pb + CO

Self-reduction: PbS + 2PbO → 3Pb + SO₂ (in reverberatory furnace)

Magnesium Extraction:

Electrolytic process:

• From magnesite: MgCO₃ → MgO + CO₂
• Or from carnallite: KCl·MgCl₂·6H₂O
• Electrolysis of molten MgCl₂ + NaCl mixture (lowers MP)
• At cathode: Mg²⁺ + 2e⁻ → Mg
• At anode: 2Cl⁻ → Cl₂ + 2e⁻
• Dow process (hydrometallurgical): Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O; then electrolysis

Sodium Extraction — Downs Process:

Electrolysis of molten NaCl (with CaCl₂ to lower MP from 801°C to 600°C):

• NaCl → Na⁺ + Cl⁻
• At iron cathode: Na⁺ + e⁻ → Na (metal is less dense than molten salt, floats)
• At graphite anode: 2Cl⁻ → Cl₂ + 2e⁻

Gold Extraction — Cyanide Process:

Leaching: 4Au + 8NaCN + O₂ + 2H₂O → 4Na[Au(CN)₂] + 4NaOH Precipitation: 2Na[Au(CN)₂] + Zn → Na₂[Zn(CN)₄] + 2Au

The gold cyanide complex is very stable (E° for Au/CN⁻ is negative).

Silver Extraction:

From Argentite (Ag₂S):

• NaCN leaching: Ag₂S + 4NaCN → 2Na[Ag(CN)₂] + Na₂S
• Precipitation: 2Na[Ag(CN)₂] + Zn → 2Ag + Na₂[Zn(CN)₄]

Electrometallurgical Series (Reactivity Series — key for extraction):

K > Na > Ca > Mg > Al > Zn > Fe > Ni > Sn > Pb > (H) > Cu > Hg > Ag > Au > Pt

More reactive metals are extracted by electrolysis. Less reactive metals can be extracted by reduction with carbon or by roasting alone.

Thermodynamic Aspects — Ellingham Diagrams:

The Ellingham diagram helps predict:

1. Which metal will reduce another
2. Temperature above which reduction by carbon becomes possible
3. Whether a metal can reduce its oxide at a given temperature

Key features:

• ΔG° becomes more positive (less favorable) for metal oxide formation as T increases
• Below the line: the oxide is stable
• Above the line: the metal is stable
• Where two lines intersect: the equilibrium temperature for that reduction

For C/CO reduction: The C → CO₂ line has a positive slope; the metal oxide lines have negative slopes. So at high enough T, C can reduce most metal oxides.

For FeO: Reduction by CO is possible below 700°C approximately (in the blast furnace, conditions favor reduction).

Passivation: Metals like Fe, Al, Cr become passive (resistant to corrosion) when a thin oxide layer forms on their surface.

• Fe: Passive in conc. HNO₃ (HNO₃ forms Fe₂O₃ layer)
• Al: Natural Al₂O₃ layer (6 nm thick) protects from further oxidation
• Cr: Forms Cr₂O₃ passive layer (stainless steel contains Cr)

⚡ Exam tip: In JEE, a common question type is matching columns: ore → method of concentration → reducing agent. Also, calculate the amount of metal obtained from a given amount of ore with % purity, or theoretical yield in metallurgical processes.

Thermite Process: For welding railway tracks,Cr, Mn metals: Cr₂O₃ + 2Al → 2Cr + Al₂O₃ (Goldschmidt process) Fe₂O₃ + 2Al → 2Fe + Al₂O₃ (thermite welding)

The thermite uses Al powder as reducing agent; reaction is highly exothermic (Fe produced in molten state).

⚡ Exam tip: Remember the铭言: “More negative ΔG° means more stable oxide.” The metal with more negative ΔG° for its oxide can reduce the other metal’s oxide if its line is below the other’s on the Ellingham diagram.

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