The Bayer process makes pure alumina by dissolving it from bauxite in hot concentrated caustic soda, filtering off the insoluble iron-and-silica residue called red mud, then cooling the clear liquor and seeding it so aluminum hydroxide crystallises out, which is calcined to alumina at 1,050 °C. The caustic is recycled, but the digestion needs a pressure vessel at 150-200 °C and the red mud is a strongly alkaline waste produced in quantity, which is why this stage is as hard as the electrolysis that follows.
Part of Aluminum Smelting
The five-step caustic route from bauxite to alumina powder, with the mass balance and the reasons it resists scaling down.
Caustic Dissolves Alumina But Leaves Iron And Silica Behind
The carrier’s steps are: crush the ore below 5 mm; digest it in 25-30% caustic soda at 150-200 °C for 2-4 hours in a pressure vessel; filter off the red mud; cool the clear solution slowly and seed with aluminum hydroxide crystals for 24-48 hours; calcine the hydroxide at 1,050 °C. The two hardest stages have their own articles: Crushing and Digestion and Precipitation and Calcination.
The carrier’s equations balance. Digestion: Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O. Precipitation: NaAlO₂ + 2H₂O → Al(OH)₃ + NaOH, which returns the caustic. Calcination: 2Al(OH)₃ → Al₂O₃ + 3H₂O.
Worked Example: The Mass Balance Per Kilogram Of Metal
Using atomic masses recalled from standard tables (Al 26.98, O 16.00, Na 22.99, H 1.008):
- Alumina needed: 1.89 kg per kg of aluminum (Ore Processing).
- Caustic at the stoichiometric floor: 2 × 40.0 = 80.0 g per 101.96 g alumina, or 0.785 kg per kg alumina; for 1.89 kg that is 1.48 kg of NaOH in circulation. Real digestion uses more than the floor, and I cannot source the excess.
- Hydroxide precipitated: 2 × 78.0 = 156.0 g per 101.96 g alumina, or 1.53 kg per kg; for 1.89 kg that is 2.89 kg of Al(OH)₃.
- Water driven off on calcining: 2.89 - 1.89 = 1.0 kg per kg of metal.
Caustic that is truly recycled is not consumed; what is lost is whatever stays in the red mud, which I cannot quantify from the corpus. Supplying even the initial charge needs a caustic soda route: Caustic Soda gives causticization of soda ash with lime, which makes 1.325 kg of soda ash per kilogram of NaOH (106.0 / 80.0, as 2 NaOH come from one Na₂CO₃).
Why It Does Not Scale Down Gracefully
- A pressure vessel at 150-200 °C, far above the autoclave’s 121 °C (Crushing and Digestion works out the pressure).
- Hot filtering of a caustic slurry.
- Two reagent circuits (caustic and seed hydroxide) to keep in balance.
- Red mud: a large, alkaline waste stream. Ore is at least 3.15 kg per kg of metal and everything in it that is not alumina ends in the heap.
Common Mistakes
- Letting the caustic run to waste. The process only works because it is recycled.
- Filtering cold. Alumina precipitates early and is lost with the mud.
- Treating it as a bench recipe. Pressure and caustic make it an industrial plant.
Safety
Hot caustic, steam and pressure
Digestion holds 25-30% caustic at 150-200 °C under pressure; a leak flashes to steam carrying caustic that blinds and burns skin at once. Wear a face shield, rubber gloves and apron, work in the open, and see Crushing and Digestion for the vessel stops. Never let caustic touch aluminum, zinc or tin vessels.
What’s Next
- Learn next: Crushing and Digestion - the pressure stage in detail
- Combine with: Hall-Héroult Electrolysis - what the alumina is for