Step 1: Bayer's Process (Concentration/Purification)

Input: Bauxite (Al2O3·2H2O) — contains Fe2O3, SiO2, TiO2 impurities

Reactions: $Al_2O_3 + 2NaOH \rightarrow 2NaAlO_2 + H_2O \quad \text{(leaching)}$ $Fe_2O_3 \text{ remains undissolved (filtered off)}$ $NaAlO_2 + 2H_2O \rightarrow Al(OH)_3 \downarrow + NaOH \quad \text{(precipitation by dilution)}$ $2Al(OH)_3 \xrightarrow{\Delta} Al_2O_3 + 3H_2O \quad \text{(calcination to pure alumina)}$

Why selective: Al2O3 is amphoteric → dissolves in NaOH. Fe2O3 is basic → does not dissolve.

Step 2: Hall-Heroult Process (Extraction)

Setup: Steel cell lined with carbon (cathode). Carbon blocks (anode). Molten cryolite bath.

Electrolyte composition:

• Al2O3 (10–15%) dissolved in
• Molten Na3AlF6 (cryolite) — lowers mp from 2072°C to ~950°C
• CaF2 (5–6%) — increases electrical conductivity

Reactions:

• Cathode (reduction): $$$Al^{3+}$ + 3e^- \rightarrow Al$$ (liquid Al sinks to bottom, tapped off)
• Anode (oxidation): C + 2$O^{2-}$ \rightarrow CO_2 + 4e^- (carbon anode consumed)

Why carbon anodes are consumed: $O^{2-}$ from Al2O3 dissociation is oxidized at the carbon anode → CO2 is produced → carbon erodes.

Crystal Structure of Al2O3 (Corundum)

Corundum structure of Al2O3 — the stability of this close-packed oxide explains why it requires electrolysis.