Hall- Heroult process

—> alumina is dissolved in molten cryolite (Na3AIF6)
—> electrolysis is used to extract Al

Why not use it in solution / molten (like NaCI)?

• aqueous AI2O3 is NOT used because the AI will readily oxidise by H ions in the solution (water)
• can’t use molten as AI2O3 has a melting point of 2000°C - not practical

—> when AI2O3 is dissolved in molten cryolite the melting point decreases to 1000°C-

- AIF3 can also be added to reduce the melting point further

—> AI sinks to the bottom of the cell and is syphoned off

—> O2, CO2 and HF are produced at the anode

Anode :

(positive electrode) : C (s) + 2O 2- (l) -> CO2 (g) + 4e
oxidation at anode

Cathode :

(negative electrode) : AI 3+ (l) + 3e -> AI (l)
reduction at cathode

Overall reaction : 2AI2O3 (l) + 3C (s) -> 4AI (l) + 3CO2 (g)

Where does the carbon come from?

• oxygen reacts with the carbon in the graphite to form carbon-dioxide
• thus slowly burning away the anodes
• thus the anodes have to be replaced periodically

—> due to the high melting point of AI2O3, it is dissolved at 950°C in the cryolite

—> this makes it very expensive

—> large amounts of energy required to maintain a high temperature to melt the alumina

—> large amounts of electricity required for electrolysis

—> graphite anodes need to be replaced regularly