Board exam focus — Isolation of Elements (CBSE & HBSE)

General principles of metallurgy: concentration of ores (froth flotation, leaching), thermodynamic principles of extraction (Ellingham diagram), electrochemical reduction, and refining methods.

Concentration of Ores

Isolation of Elements

Metallurgy

Process of extracting metals from their ores. Steps:

1. Mining and crushing of ore
2. Concentration (enrichment/beneficiation)
3. Extraction (reduction to metal)
4. Refining (purification)

Ores

Concentration Methods

1. Gravity Separation (Hydraulic Washing): Based on difference in density between ore and gangue. Ore washed with stream of water; lighter gangue carried away. Used for: Heavy metal ores (SnO2/cassiterite, iron ores, galena)

2. Magnetic Separation: Based on magnetic properties of ore vs gangue. Ore carried over a magnetic roller; magnetic particles (ore) separate from non-magnetic gangue. Used for: Magnetite (Fe3O4), chromite (FeCr2O4), wolframite (FeWO4)

3. Froth Flotation: Based on preferential wetting by pine oil. Finely ground ore + water + pine oil + air → sulfide ore particles attach to froth (hydrophobic); gangue sinks (hydrophilic). Collectors: Pine oil, cresols (increase hydrophobicity of ore) Depressants: ZnSO4 prevents ZnS from floating when PbS is to be concentrated Used for: Sulfide ores (ZnS, PbS, Cu2S)

4. Leaching (Chemical Method): Selective dissolution of ore in a reagent.

Bayer's Process for Al: Al2O3 + 2NaOH(aq) → 2NaAlO2 + H2O NaAlO2 + 2H2O + CO2 → Al(OH)3 + NaHCO3 2Al(OH)3 → Al2O3 + 3H2O (calcination) Fe2O3 (impurity) doesn't dissolve in NaOH → filtered as red mud

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

Calcination and Roasting

Calcination: Heating ore in limited air to remove volatile substances. CaCO3 → CaO + CO2; Al2O3.2H2O → Al2O3 + 2H2O

Roasting: Heating ore in excess air to convert sulfides to oxides. 2ZnS + 3O2 → 2ZnO + 2SO2 2PbS + 3O2 → 2PbO + 2SO2 SO2 byproduct used to make H2SO4.

Thermodynamic Principles and Ellingham Diagram

Thermodynamic Principles of Extraction

Gibbs Energy and Metallurgy

A metal can be extracted from its oxide if: DeltaG(reduction) < 0 DeltaG = DeltaH - T DeltaS

For: MO + C → M + CO: DeltaG = DeltaG(CO) - DeltaG(MO) If DeltaG < 0, reaction is spontaneous → carbon can reduce the oxide.

Ellingham Diagram

Graph of DeltaG0 of formation vs temperature for metal oxides.

Key features:

1. Lines generally slope upward (DeltaS < 0 for M + O2 → MO; DeltaS > 0 for CO formation)
2. Lower on diagram = more stable oxide (more negative DeltaG = stronger tendency to form)
3. A metal can reduce the oxide of another metal if its oxide line is BELOW the other
4. C→CO2 line: nearly horizontal (both C and CO2 are solids/gases, DeltaS ~0)
5. C→CO line: slopes DOWNWARD (as T↑, CO entropy increases, DeltaG more negative)
6. The two C lines cross at ~710 C

Temperature ranges:

• Below 710 C: C + O2 → CO2 is preferred
• Above 710 C: 2C + O2 → 2CO is preferred (more negative DeltaG)

Reduction Methods Based on Ellingham:

Carbon reduction (for Fe, Zn, Sn, Cu): Fe2O3 + 3C → 2Fe + 3CO (blast furnace, >1000 C) ZnO + C → Zn + CO (>900 C)

Self-reduction (for Cu, Pb): Cu2S + O2 → 2Cu + SO2

Thermite Reaction (for Cr, Mn): Cr2O3 + 2Al → Al2O3 + 2Cr (DeltaG very negative; Al line below Cr line) Fe2O3 + 2Al → Al2O3 + 2Fe (used in welding, thermite process)

Electrochemical Reduction (for Na, Mg, Al): Lines for Na, Mg, Al are at the BOTTOM of Ellingham diagram (very stable oxides). Carbon or aluminum cannot reduce these oxides economically. Electrolysis is used.

Hall-Heroult Process (Al): Al2O3 dissolved in molten cryolite (Na3AlF6) as solvent. Electrolysis at 950 C: Cathode: Al3+ + 3e- → Al (liquid) Anode: C + O2^2- → CO2 + CO (carbon anodes consumed) Overall: 2Al2O3 + 3C → 4Al + 3CO2

Refining of Metals

Refining of Metals

Introduction

Crude metal obtained after extraction contains impurities. Refining removes impurities to get pure metal.

Refining Methods

1. Distillation: For low-boiling metals. Metal vaporized and condensed separately from impurities. Used for: Zinc (bp 907 C), mercury (bp 357 C).

2. Liquation: For easily fusible metals. Crude metal heated on inclined surface; pure metal (lower mp) flows away; impurities remain. Used for: Bismuth (mp 271 C), lead (mp 327 C), tin (mp 232 C).

3. Electrolytic Refining: Most important industrial method for high-purity metals.

Setup: Crude metal = anode, pure thin metal sheet = cathode, electrolyte = solution of metal salt.

Process:

• Anode dissolves (impure metal oxidizes): M → M^n+ + ne-
• Cathode deposits pure metal (M^n+ reduced): M^n+ + ne- → M

Anode mud: Less reactive impurities (Au, Ag, Pt in crude Cu) don't dissolve; collect below anode as anode mud. These precious metals are recovered profitably!

Used for: Cu (CuSO4 bath), Ag (AgNO3 bath), Au, Ni, Pb, Sn.

4. Zone Refining: For semiconductors requiring extremely high purity (Si, Ge, Ga).

Principle: Impurities have different solubility in molten vs solid zone. Process: A heating coil slowly moves along a rod of crude metal. Molten zone moves → impurities concentrated in molten zone → move to end of rod. Pure metal crystallizes behind the moving zone. Repeat multiple times → ultrapure metal.

Used for: Ge, Si (for semiconductors), Ga, In.

5. Vapour Phase Refining:

Mond Process (Ni): Ni + 4CO (50-60 C) → Ni(CO)4 (volatile nickel carbonyl) Ni(CO)4 (250 C) → Ni + 4CO (nickel deposited, CO recycled)

Van Arkel Method (Ti, Zr): Ti + 2I2 (500 C) → TiI4 (volatile) TiI4 (1400 C) → Ti + 2I2 (pure Ti deposited on hot tungsten wire)

6. Chromatography (rare): For elements present in trace amounts. Stationary phase retards impurities differently.

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