Chapter 9: Metals — Part 3

01 General Principles

Most metals are extracted from ores found in the Earth's crust. The ease of extraction depends on reactivity: metals lower than carbon are reduced by carbon, while those above carbon (like aluminium) require electrolysis.

02 Extraction of Iron in the Blast Furnace

Iron is extracted from the ore hematite (Fe₂O₃) using coke, limestone, and hot air in a continuous process.

1. Heat Production: Coke burns in air to produce carbon dioxide (exothermic): C + O₂ ? CO₂.
2. Reducing Agent Formation: CO₂ reacts with more coke to form carbon monoxide: CO₂ + C ? 2CO.
3. Hematite Reduction: Carbon monoxide reduces the iron(III) oxide to molten iron: Fe₂O₃ + 3CO ? 2Fe + 3CO₂.
4. Slag Formation: Limestone decomposes to calcium oxide (CaCO₃ ? CaO + CO₂). The CaO reacts with sandy impurities (SiO₂) to form slag (CaO + SiO₂ ? CaSiO₃), which is used for road building.

03 Extraction of Aluminium

Aluminium is extracted from purified bauxite ore by electrolysis.

• Role of Cryolite: Aluminium oxide has a very high melting point; it is dissolved in molten cryolite to lower the melting point of the electrolyte, significantly reducing energy costs.
• Electrodes: Both electrodes are made of graphite (carbon).
• Anode Replacement: The carbon anodes must be regularly replaced because they react with the oxygen gas produced to form carbon dioxide, which erodes them.
• Ionic Half-Equations:
  • Cathode (Reduction): Al³⁺ + 3e^- ? Al.
  • Anode (Oxidation): 2O^2- ? O₂ + 4e^-.