Metallurgy

Metallurgy: The science and technology of extracting metals from their ores and refining them for use.

Mineral: Naturally occurring substance containing metals or their compounds.

Ore: A mineral from which metal can be extracted profitably. All ores are minerals, but not all minerals are ores.

Gangue/Matrix: Unwanted impurities (sand, clay, rocks) present in an ore.

Native Metals: Metals found in free state in nature. Examples: Gold (Au), Silver (Ag), Platinum (Pt).

Steps in Metallurgy: (1) Concentration/Enrichment of ore, (2) Reduction to metal, (3) Refining/Purification.

Purpose: To remove gangue and increase percentage of metal in ore.

Hydraulic Washing/Gravity Separation: Based on density difference. Ore is heavier, gangue lighter. Used for oxide ores.

Magnetic Separation: Used when ore is magnetic. Example: Haematite (Fe₂O₃) separated by electromagnets.

Froth Flotation: For sulphide ores. Ore particles stick to froth (pine oil), gangue sinks. Example: Copper pyrites (CuFeS₂).

Leaching: Chemical method using acids/bases to dissolve ore. Example: Bauxite (Al₂O₃) treated with NaOH to remove SiO₂.

Calcination: Heating ore in limited air to remove moisture and volatile impurities. For carbonate ores. ZnCO₃ → ZnO + CO₂

Roasting: Heating ore in excess air to convert to oxide. For sulphide ores. 2ZnS + 3O₂ → 2ZnO + 2SO₂

Reduction by Carbon: For less reactive metals (Zn, Fe, Pb, Cu). ZnO + C → Zn + CO. Fe₂O₃ + 3C → 2Fe + 3CO

Reduction by Aluminium (Thermite Process): For highly reactive metals. Fe₂O₃ + 2Al → 2Fe + Al₂O₃ + Heat (very exothermic)

Electrolytic Reduction: For highly reactive metals (Na, K, Ca, Al). Cannot be reduced by carbon. Example: Al extraction from Al₂O₃.

Self Reduction: Some sulphide ores. 2Cu₂S + 3O₂ → 2Cu₂O + 2SO₂, then 2Cu₂O + Cu₂S → 6Cu + SO₂

Iron from Haematite (Fe₂O₃): Blast furnace method. Fe₂O₃ + 3CO → 2Fe + 3CO₂ (CO from coke acts as reducing agent)

Zinc from Zinc Blende (ZnS): Roasting: 2ZnS + 3O₂ → 2ZnO + 2SO₂. Reduction: ZnO + C → Zn + CO

Aluminum from Bauxite (Al₂O₃): Hall-Heroult process. Electrolysis of pure Al₂O₃ dissolved in molten cryolite (Na₃AlF₆)

Copper from Copper Pyrites (CuFeS₂): Roasting, then self-reduction. Final purification by electrolysis.

Lead from Galena (PbS): Roasting: 2PbS + 3O₂ → 2PbO + 2SO₂. Reduction: PbO + C → Pb + CO

Refining: Purification of extracted metal to remove impurities.

Electrolytic Refining: Used for Cu, Ag, Au. Impure metal as anode, pure metal deposited at cathode.

Distillation: For metals with low boiling points (Zn, Hg). Metal vaporized and condensed.

Liquation: For metals with low melting points (Sn, Pb). Heated on sloping hearth, pure metal flows down.

Alloy: Homogeneous mixture of two or more metals (or metal + non-metal). Examples: Brass (Cu + Zn), Bronze (Cu + Sn), Steel (Fe + C).

Properties of Alloys: Harder, stronger, more corrosion-resistant, lower melting point than pure metals.

Reactivity Series: K > Na > Ca > Mg > Al > Zn > Fe > Pb > (H) > Cu > Hg > Ag > Au > Pt

More Reactive Metals: Extracted by electrolysis (K, Na, Ca, Mg, Al). Cannot be reduced by carbon.

Moderately Reactive: Extracted by carbon reduction (Zn, Fe, Pb, Cu). Form stable oxides.

Less Reactive: Occur free in nature or easily reduced by heating alone (Ag, Au, Pt, Hg).

Displacement Reactions: More reactive metal displaces less reactive metal from its salt. Zn + CuSO₄ → ZnSO₄ + Cu

Corrosion: Iron rusts (forms Fe₂O₃·xH₂O) in presence of moisture and air. Prevented by galvanization (Zn coating), painting, oiling.