Extraction of Aluminium

Aluminium is a highly reactive metal and does not occur in a free state. It is the third most abundant element in the Earth's crust, mainly found in bauxite (Al₂O₃·nH₂O).

The extraction of aluminium involves two major steps:

1. Concentration of Bauxite Ore

Bauxite contains impurities like silica (SiO₂), ferric oxide (Fe₂O₃), and titanium oxide (TiO₂), which are removed using Bayer’s process or Hall’s process.

A. Bayer’s Process (Using NaOH)

Bauxite is ground and heated with concentrated NaOH at 140–150°C under high pressure. Aluminium oxide reacts with NaOH, forming sodium aluminate (soluble). 

• Al₂O₃⋅2H₂O + 2NaOH→2NaAlO₂ + 3H₂O

Iron oxide remains undissolved and is filtered out. Silica dissolves, forming sodium silicate. Sodium aluminate solution is cooled and diluted, causing Al(OH)₃ precipitation. 

• NaAlO₂ + 2H₂O → NaOH + Al(OH)₃↓

Al(OH)₃ is heated at 1000°C, producing pure alumina (Al₂O₃).

• 2Al(OH)₃→Al₂O₃ + 3H₂O₂

B. Hall’s Process (Using Na₂CO₃)

Bauxite is leached with aqueous Na₂CO₃, forming sodium aluminate.

• Al₂O₃⋅2H₂O + Na₂CO₃ → 2NaAlO₂ + CO₂ + 2H₂

CO₂ is bubbled through the filtrate, precipitating Al(OH)₃.

• 2NaAlO₂ + 3H₂O + CO₂ → 2Al(OH)₃↓ + Na₂CO₃​

Al(OH)₃ is heated at 1000°C, yielding Al₂O₃ (same as Bayer’s process).

2. Electrolytic Reduction of Alumina

Electrolysis of molten alumina (Al₂O₃) mixed with cryolite (Na₃AlF₆) and fluorspar (CaF₂) is carried out in a steel tank.

• The tank’s graphite lining acts as the cathode.
• Graphite rods act as the anode.
• Purpose of Cryolite & Fluorspar: These lower alumina’s melting point from >2000°C to ~1000°C, making the process more energy-efficient.

Extraction of aluminium

Electrode Reactions:

a. At Cathode (Reduction)

• Al³⁺ + 3e⁻ → Al(l). 
• Aluminium metal is deposited at the cathode and sinks to the bottom of the tank.

b. At Anode (Oxidation)

• 2O₂⁻→O₂+4e⁻
• Oxygen gas is released, which reacts with graphite anodes to form CO₂, requiring periodic replacement of the anodes.

Collection of Aluminium: Since molten aluminium is denser than the electrolyte, it settles at the bottom. It is periodically extracted through an outlet. This method efficiently extracts pure aluminium from its oxide (Al₂O₃) using electrolysis.