Chapter 6 — General Principles and Processes of Isolation of Elements — covers the metallurgy of extracting metals from their ores. From mining to refining, this chapter explains every step of how we get pure metals. It carries 3-5 marks in Board exams and is mostly conceptual with important reactions to learn.
Key Concepts
Important Ores
| Metal | Ore | Formula |
|---|---|---|
| Aluminium | Bauxite | Al₂O₃·2H₂O |
| Iron | Haematite | Fe₂O₃ |
| Iron | Magnetite | Fe₃O₄ |
| Copper | Copper pyrite | CuFeS₂ |
| Copper | Malachite | Cu(OH)₂·CuCO₃ |
| Zinc | Zinc blende / Sphalerite | ZnS |
| Zinc | Calamine | ZnCO₃ |
| Tin | Cassiterite | SnO₂ |
Steps of Metallurgy
The overall process follows this flow:
Mining → Concentration of Ore → Extraction of Metal → Refining
Step 1: Concentration of Ore (Beneficiation)
| Method | Principle | Used For |
|---|---|---|
| Hydraulic Washing (Gravity Separation) | Difference in density — lighter gangue washed away | Oxide ores (tin, iron) |
| Magnetic Separation | Difference in magnetic properties | Magnetic ores (magnetite from non-magnetic gangue) |
| Froth Flotation | Difference in wetting properties | Sulphide ores (Cu, Pb, Zn) |
| Chemical Leaching | Selective dissolution in chemical | Bauxite (Bayer’s process), Ag, Au |
Leaching Reactions
Al₂O₃ + 2NaOH + 3H₂O → 2Na[Al(OH)₄] (aluminate — dissolves)
SiO₂, TiO₂, Fe₂O₃ — left as red mud (don’t dissolve)
Na[Al(OH)₄] + CO₂ → Al(OH)₃↓ + NaHCO₃
2Al(OH)₃ → Al₂O₃ + 3H₂O (calcination)
Gold/Silver Leaching (MacArthur-Forrest/Cyanide Process):
4Au + 8NaCN + 2H₂O + O₂ → 4Na[Au(CN)₂] + 4NaOH
2Na[Au(CN)₂] + Zn → Na₂[Zn(CN)₄] + 2Au↓
Step 2: Extraction of Metal
Method depends on the metal’s reactivity:
| Metal Type | Method | Examples |
|---|---|---|
| Highly reactive (K, Na, Ca, Mg, Al) | Electrolytic reduction | Hall-Héroult (Al), Downs process (Na) |
| Moderately reactive (Zn, Fe, Pb, Sn) | Carbon reduction (smelting) | Blast furnace (Fe), ZnO + C → Zn + CO |
| Less reactive (Cu, Hg, Ag) | Self-reduction / roasting | 2Cu₂S + 3O₂ → 2Cu₂O + 2SO₂ |
Thermodynamics of Metallurgy — Ellingham Diagram
Key principles from Ellingham diagram:
1. Metal whose oxide line is LOWER can reduce the oxide above it
2. A metal can reduce oxide of another metal that lies above it in the diagram
3. C line goes DOWN with temperature (entropy favours C + O₂ → CO₂ at high T), so carbon becomes a better reducing agent at high temperatures
4. Al line is always below Fe line → Al can reduce Fe₂O₃ (thermite reaction!)
Extraction of Iron — Blast Furnace
Zone of Reducing (middle, ~700-1200°C):
CO₂ + C → 2CO
Fe₂O₃ + 3CO → 2Fe + 3CO₂ (main reaction!)
Zone of Slag Formation: CaO + SiO₂ → CaSiO₃ (calcium silicate slag)
Zone of Charge (top, ~500°C): 3Fe₂O₃ + CO → 2Fe₃O₄ + CO₂
Extraction of Aluminium — Hall-Héroult Process
- Electrolysis of Al₂O₃ dissolved in molten cryolite (Na₃AlF₆)
- Cryolite lowers melting point from 2050°C to ~950°C
- CaF₂ (fluorspar) added to further lower melting point
- Cathode: Al³⁺ + 3e⁻ → Al (liquid, collected at bottom)
- Anode: C + O²⁻ → CO₂ (carbon anode consumed — needs replacement)
Extraction of Copper — From Copper Pyrite (CuFeS₂)
Step 2 — Smelting with SiO₂: FeS + SiO₂ → FeSiO₃ (slag) + SO₂
Step 3 — Self-reduction (Bessemer converter):
2Cu₂S + 3O₂ → 2Cu₂O + 2SO₂
2Cu₂O + Cu₂S → 6Cu + SO₂ ← Self-reduction!
(Copper obtained is 99% pure — “blister copper”)
Step 3: Refining
| Method | Principle | Used For |
|---|---|---|
| Distillation | Difference in boiling points | Zn, Hg |
| Liquation | Low melting point metal flows out | Sn, Pb, Bi |
| Electrolytic Refining | Impure metal = anode, pure = cathode | Cu, Ag, Zn, Ni |
| Zone Refining | Impurities more soluble in melt | Si, Ge, Ga (semiconductors) |
| Van Arkel Method | Decomposition of volatile compound | Zr, Ti, Hf |
| Mond Process | Formation/decomposition of Ni(CO)₄ | Ni |
Van Arkel: Zr + 2I₂ →(870 K) ZrI₄ →(1800 K) Zr + 2I₂
Important Definitions
| Term | Definition |
|---|---|
| Ore | Mineral from which metal can be economically extracted |
| Gangue | Unwanted rocky/earthy material associated with ore |
| Flux | Substance added to remove gangue as fusible slag |
| Slag | Fusible product of flux + gangue |
| Roasting | Heating ore strongly in air (sulphide → oxide) |
| Calcination | Heating ore in absence/limited air (carbonate → oxide) |
| Smelting | Reduction of oxide to metal using carbon/CO at high temperature |
Solved Examples — NCERT Based
Example 1: Why Carbon Cannot Reduce Al₂O₃
Q: Why is carbon reduction not used for extracting aluminium?
Solution: In the Ellingham diagram, the line for Al₂O₃ lies below the line for CO at all temperatures. This means ΔG for Al₂O₃ formation is more negative than for CO/CO₂. Therefore, carbon cannot reduce Al₂O₃ — the reverse reaction is thermodynamically unfavourable. That’s why electrolytic reduction (Hall-Héroult process) is used instead.
Example 2: Choosing Concentration Method
Q: Which concentration method would you use for: (a) ZnS (b) Fe₃O₄ mixed with SiO₂ (c) Al₂O₃·2H₂O
Solution:
(a) ZnS is a sulphide ore → Froth flotation
(b) Fe₃O₄ is magnetic, SiO₂ is not → Magnetic separation
(c) Al₂O₃·2H₂O (bauxite) → Chemical leaching (Bayer’s process)
Example 3: Depressants in Froth Flotation
Q: How can you separate a mixture of ZnS and PbS using froth flotation?
Solution: Add NaCN as depressant. NaCN forms a layer of zinc cyanide [Na₂[Zn(CN)₄]] on ZnS, making it hydrophilic (sinks). PbS remains hydrophobic (floats with froth). Thus PbS is collected first, then ZnS is collected separately without the depressant.
Example 4: Electrolytic Refining
Q: In electrolytic refining of copper, what happens at each electrode? What is anode mud?
Solution:
Anode (impure Cu): Cu → Cu²⁺ + 2e⁻ (copper dissolves)
Cathode (pure Cu): Cu²⁺ + 2e⁻ → Cu (pure copper deposits)
Electrolyte: Acidified CuSO₄ solution
Anode mud: Insoluble impurities (Ag, Au, Pt) that settle below the anode — these are valuable and recovered separately!
Important Questions for Board Exams
1 Mark Questions
- What is the role of cryolite in Hall-Héroult process?
- Name the reducing agent used in thermite reaction.
- What is the difference between roasting and calcination?
- What is anode mud?
2 Mark Questions
- Explain froth flotation process for concentration of sulphide ores.
- Describe the role of depressant in froth flotation.
- What is zone refining? Name the metals refined by this method.
- How is copper extracted from low-grade ores?
3 Mark Questions
- Describe the extraction of aluminium from bauxite by Hall-Héroult process.
- Explain the significance of Ellingham diagram in metallurgy.
- Describe the extraction of copper from copper pyrite with equations.
- Explain Mond process and Van Arkel method with equations.
5 Mark Questions
- Draw a neat Ellingham diagram. Explain how it helps predict the choice of reducing agent. Why can carbon reduce FeO but not Al₂O₃?
- Describe the extraction of iron from haematite in a blast furnace. Write reactions occurring in different zones.
Quick Revision Points
- Concentration methods: gravity, magnetic, froth flotation (sulphides), leaching (bauxite/gold)
- Froth flotation: sulphide ore + water + pine oil + air; collectors enhance, depressants separate
- Bayer’s process: bauxite + NaOH → aluminate → Al(OH)₃ → Al₂O₃
- Ellingham diagram: lower line reduces upper; C becomes better reducer at high T
- Highly reactive metals → electrolysis; moderate → carbon reduction; low → self-reduction
- Iron: blast furnace, CO is the main reducing agent
- Al: Hall-Héroult, electrolysis of Al₂O₃ in cryolite
- Cu: roasting → smelting → self-reduction → blister copper → electrolytic refining
- Refining: electrolytic (Cu), zone (Si/Ge), Mond (Ni), Van Arkel (Zr/Ti)
- Thermite: Fe₂O₃ + 2Al → 2Fe + Al₂O₃ (welding railway tracks)
Chapter Navigation
Previous: Surface Chemistry Class 12 Notes
Next: The p-Block Elements Class 12 Notes
Related Chapters in Class 12 Chemistry
- The p-Block Elements Class 12 Notes
- The d and f Block Elements Class 12 Notes
- Coordination Compounds Class 12 Notes
Practice What You Learned
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