Process
In the Bayer process, bauxite is digested by washing with a hot solution of sodium hydroxide, NaOH, at 175 °C. This converts the aluminium oxide in the ore to sodium aluminate, 2NaAl(OH)4, according to the chemical equation:
- Al2O3 + 2 NaOH + 3 H2O → 2 NaAl(OH)4
The other components of bauxite do not dissolve. The solution is clarified by filtering off the solid impurities. The mixture of solid impurities is called red mud, and presents a disposal problem. Next, the alkaline solution is cooled, and aluminium hydroxide precipitates as a white, fluffy solid:
- NaAl(OH)4 → Al(OH)3 + NaOH
Then, when heated to 980°C (calcined), the aluminium hydroxide decomposes to aluminium oxide, giving off water vapor in the process:
- 2 Al(OH)3 → Al2O3 + 3 H2O
A large amount of the aluminium oxide so produced is then subsequently smelted in the Hall–Héroult process in order to produce aluminium.
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