Extraction of Metals

1. Ease of obtaining metals from their ores – relationship to reactivity series

• Ores: Naturally occurring rocks that contain enough of a metal or its compound to make extraction economically viable.
• Ease of extraction depends on the position of the metal in the reactivity series:
  • Very reactive metals (potassium, sodium, calcium, magnesium, aluminium) are found combined in nature, usually as oxides or other stable compounds.
    • Cannot be extracted by simple chemical reduction with carbon because they form stable bonds with oxygen.
    • Require electrolysis of molten compounds for extraction (e.g., aluminium).
  • Moderately reactive metals (zinc, iron, tin, lead) are usually extracted by chemical reduction using carbon or carbon monoxide.
    • Found as oxides or sulfides which can be reduced.
  • Low reactivity metals (copper, silver, gold, platinum) may be found uncombined in nature as native elements.
    • Can be extracted by physical methods such as mining, followed by purification, or by heating sulfide ores in air.
• General rule: The higher the metal in the reactivity series, the more energy required for extraction.

2. Extraction of iron from hematite in the blast furnace

Main raw materials:

• Hematite (Fe₂O₃) – iron ore.
• Coke – form of carbon, acts as a fuel and reducing agent.
• Limestone (CaCO₃) – removes impurities like silica (SiO₂).
• Hot air – provides oxygen for combustion.

Main steps in the blast furnace:

(a) Burning of carbon (coke) to provide heat and produce carbon dioxide

• Coke reacts with oxygen from the hot air blast at the base of the furnace.
• Reaction is exothermic, releasing heat for the furnace operation.
• Equation:
  C + O₂ → CO₂

(b) Reduction of carbon dioxide to carbon monoxide

• As CO₂ rises through the hot coke, it is reduced to CO.
• Reaction:
  CO₂ + C → 2CO

(c) Reduction of iron(III) oxide by carbon monoxide

• Carbon monoxide reduces Fe₂O₃ to molten iron.
• This is the main extraction reaction.
• Equation:
  Fe₂O₃ + 3CO → 2Fe + 3CO₂

(d) Thermal decomposition of calcium carbonate to produce calcium oxide

• Limestone decomposes on heating:
  CaCO₃ → CaO + CO₂

(e) Formation of slag

• Calcium oxide reacts with silica impurities to form calcium silicate (slag):
  CaO + SiO₂ → CaSiO₃
• Slag floats on molten iron and can be removed.
• Slag uses: road building, cement manufacture.

Final products:

• Molten iron: tapped off at the base.
• Slag: tapped off above the molten iron layer.

3. Extraction of aluminium from purified bauxite / aluminium oxide

Main raw material:

• Bauxite – contains aluminium oxide (Al₂O₃) mixed with impurities.
• Purification (details not required for syllabus) gives pure Al₂O₃.

Key points in extraction process:

(a) Role of cryolite

• Aluminium oxide has a very high melting point (~2000°C).
• Cryolite (Na₃AlF₆) is added to:
  • Lower melting point to ~900°C, reducing energy cost.
  • Increase electrical conductivity of molten mixture.

(b) Why carbon anodes need to be regularly replaced

• Oxygen is produced at the anode and reacts with the carbon electrode to form CO₂:
  C + O₂ → CO₂
• This consumes the carbon anode, so it must be replaced regularly.

(c) Reactions at the electrodes (electrolysis of molten Al₂O₃ + cryolite)

• Cathode (negative electrode) – aluminium ions are reduced to aluminium metal:
  Al³⁺ + 3e⁻ → Al
• Anode (positive electrode) – oxide ions are oxidised to oxygen gas:
  2O²⁻ → O₂ + 4e⁻

Overall reaction:
2Al₂O₃ → 4Al + 3O₂

Products:

• Aluminium metal: collected at the cathode, dense and sinks to the bottom.
• Oxygen gas: reacts with carbon anodes to form CO₂.