41. Electrolysis: Extracting Metals from Oxides

1. Why Use Electrolysis?

  • Reactivity: Electrolysis is required for metals that are more reactive than carbon (e.g., aluminium, magnesium, and calcium) because carbon cannot displace them from their oxides.

  • Cost: This method is much more expensive than reduction with carbon because it requires vast amounts of energy to melt the compounds and produce the electric current.

2. Preparing the Electrolyte

  • Ore Source: Aluminium is extracted from an ore called bauxite.

  • Purification: The bauxite is first purified to obtain pure aluminium oxide (Al₂O₃), which is a solid and cannot act as an electrolyte because its ions are fixed in place.

  • Melting Point: Aluminium oxide has a very high melting point (over 2000°C).

  • Cryolite: To save energy, the aluminium oxide is mixed with a mineral called cryolite, which lowers the melting point, allowing it to be melted at a lower temperature.

3. The Electrolysis Process

The molten mixture (electrolyte) contains free-moving aluminium ions (Al³⁺) and oxide ions (O²⁻).

  • The Electrodes: Both the anode (positive) and cathode (negative) are typically made of carbon (graphite).

  • At the Anode (+): Negative oxygen ions (O²⁻) are attracted here. They lose electrons (oxidation) to become oxygen atoms, which pair up to form oxygen gas (O₂).

  • At the Cathode (-): Positive aluminium ions (Al³⁺) are attracted here. They gain electrons (reduction) to become neutral aluminium atoms, which collect as molten metal at the bottom.

4. Chemical Equations

  • Cathode Half Equation (Reduction): Al³⁺ + 3e⁻ → Al

  • Anode Half Equation (Oxidation): 2O²⁻ → O₂ + 4e⁻

  • Overall Equation: 2Al₂O₃(l) → 4Al(l) + 3O₂(g)

5. Maintenance Note

  • Anode Replacement: In industrial setups, the carbon anodes must be replaced regularly because the oxygen gas produced reacts with the hot carbon to form carbon dioxide (CO₂), gradually burning the anodes away.