1. Introduction

Very few elements (like gold, silver, platinum) occur in a free (native) state in nature. Most metals occur in combined form as compounds — oxides, sulphides, carbonates, etc. This chapter deals with the general principles of extracting metals from their ores and refining them to obtain pure metals — a branch of chemistry called metallurgy.

2. Basic Terminology

3. Steps Involved in Extraction of Metals

1. Concentration (dressing/enrichment) of the ore — removal of gangue
2. Conversion of ore into metal oxide — by calcination or roasting
3. Reduction of metal oxide to free metal
4. Purification/refining of the metal

4. Concentration of Ores

The removal of gangue from powdered ore is called concentration or ore-dressing. Method used depends on the physical/chemical difference between ore and gangue particles.

Method	Principle / Use
Hydraulic washing (gravity separation)	Based on difference in density of ore and gangue; a stream of water washes away lighter gangue particles. Used for oxide ores (e.g. haematite, tin stone).
Magnetic separation	Based on magnetic property of ore or gangue; used when one of them is magnetic (e.g. chromite, magnetite ores using magnetic rollers).
Froth flotation	Used mainly for sulphide ores. Ore particles are wetted by pine oil/collectors and made to form froth with water and air bubbles, while gangue is wetted by water and sinks. Depressants (e.g. NaCN) can selectively separate two sulphide ores.
Leaching (chemical method)	Ore is treated with a suitable chemical reagent that dissolves the ore but not the impurities. Example: Bayer's process — bauxite (Al₂O₃) is leached with hot concentrated NaOH solution to form soluble sodium aluminate, leaving impurities behind; the aluminate is then hydrolysed to get pure Al(OH)₃.

5. Conversion of Ore to Oxide

6. Extraction (Reduction) of Crude Metal

(a) Smelting (Reduction with carbon)

For moderately reactive metals (like Fe, Zn, Sn). The oxide is heated with a reducing agent (coke) at high temperature. Example — extraction of iron in a blast furnace:

Fe₂O₃ + 3CO → 2Fe + 3CO₂

(b) Self-reduction (Auto-reduction)

Used for metals like Cu, Hg, Pb where sulphide ore is partially roasted to oxide, and the remaining sulphide reacts with the oxide itself:

Cu₂S + 2Cu₂O → 6Cu + SO₂

(c) Electrolytic Reduction

Used for highly reactive metals (Na, Mg, Ca, Al) that cannot be reduced by carbon. Molten (fused) metal oxide/chloride is electrolysed:

• Cathode: Metal ion gains electrons and is deposited as metal (reduction)
• Anode: Oxidation occurs, releasing gas (e.g. O₂ or Cl₂)

Example: Extraction of aluminium by Hall-Héroult process — purified Al₂O₃ is dissolved in molten cryolite (Na₃AlF₆) to lower the melting point and increase conductivity, then electrolysed using carbon electrodes.

(d) Thermodynamic Principles (Ellingham Diagram)

The Gibbs energy of formation (ΔG°) of metal oxides varies with temperature, plotted in an Ellingham diagram. A reducing agent can reduce a metal oxide only if its own oxide formation line lies below that of the metal oxide at a given temperature (i.e., its ΔG° is more negative).

7. Refining of Metals

Methods to purify the crude metal obtained after reduction:

Method	Principle / Example
Distillation	For low boiling point metals like Zn, Hg — metal is vaporised and condensed to get pure metal.
Liquation	Low melting metals (like Sn) are made to flow on a sloping surface, separating from higher-melting impurities.
Electrolytic refining	Impure metal used as anode, pure metal strip as cathode, in a solution of metal salt. Pure metal deposits at cathode; impurities settle as "anode mud". Used for Cu, Ag, Au, Al, etc.
Zone refining	Based on principle that impurities are more soluble in molten metal than solid metal. A mobile heater melts a narrow zone that moves along a rod, sweeping impurities to one end. Used for ultra-pure semiconductors (Si, Ge, Ga).
Vapour phase refining (Mond process)	Impure metal converted to a volatile compound, decomposed to give pure metal. E.g. impure Ni + CO → Ni(CO)₄ (volatile) → decomposed by heating to pure Ni + CO.
Chromatographic methods	Based on differential adsorption of components on an adsorbent (e.g. column chromatography); used for separating elements present in trace amounts.

8. Extraction Examples: Case Studies

9. Important Concept Summary

Concept	Key Point
Calcination	Heating ore in limited/no air — used for carbonate/hydroxide ores
Roasting	Heating ore in excess air — used for sulphide ores
Froth flotation	Used for sulphide ores; based on wettability difference
Electrolytic reduction	Used for highly reactive metals (Na, Mg, Al)
Zone refining	Used for ultra-pure semiconductors
Mond process	Vapour phase refining of nickel
Van Arkel method	Refining of Ti, Zr via volatile iodides

10. Quick Revision Points

• Ore is the mineral from which metal extraction is commercially viable; every ore is a mineral, but not every mineral is an ore.
• Calcination = heating without/limited air; Roasting = heating with excess air. Don't mix them up.
• Froth flotation works only for sulphide ores due to differential wettability by water/oil.
• Ellingham diagrams help predict which reducing agent can reduce a given metal oxide at a given temperature.
• Highly reactive metals (Na, Mg, Ca, Al) are extracted only by electrolytic reduction, never by carbon reduction.
• Zone refining and Van Arkel method give ultra-high purity metals, mainly for semiconductors.
• Cryolite is added in Al extraction to lower the melting point of alumina and improve conductivity, not as a reducing agent.