To recover alumina from the filtered caustic liquor, cool it slowly, add seed crystals of aluminum hydroxide, and leave it 24-48 hours while hydroxide crystals grow and the caustic is released for reuse, then filter, wash and calcine the crystals at 1,050 °C to drive off the water. Each kilogram of aluminum needs 2.89 kg of hydroxide, and the alumina’s purity carries straight into the metal, so washing matters more than speed.
Part of Simplified Bayer Process
The back half of the Bayer process: seeded crystallisation of aluminum hydroxide from liquor, then calcination to dry alumina powder.
Seeding Makes The Hydroxide Come Out Of Solution
The carrier says: cool the clear solution slowly, add seed crystals of aluminum hydroxide, let crystals grow over 24-48 hours (NaAlO₂ + 2H₂O → Al(OH)₃ + NaOH), then calcine at 1,050 °C (2Al(OH)₃ → Al₂O₃ + 3H₂O). The released caustic goes back to digestion (Crushing and Digestion). The corpus gives no seed quantity, cooling rate or liquor strength for this step, so those must be found by trial on small batches.
What You Need
- Clear, filtered liquor from digestion.
- A tank that can cool slowly and be stirred gently, made of a caustic-compatible material.
- Seed crystals saved from an earlier batch.
- A filter and wash water.
- A kiln reaching 1,050 °C (Refractory Materials for the lining).
- A dry, closed store for the product.
Method: One Batch
Step 1: Cool slowly without agitating hard; sudden cooling gives fine crystals that filter badly (my recollection; the carrier only says slowly).
Step 2: Seed with a portion of saved hydroxide and wait 24-48 hours. You should see a white sludge settling.
Step 3: Filter and wash the crystals. Residual caustic left in the crystals ends up as sodium in the alumina, which I recall disturbs the electrolyte later; the corpus does not say.
Step 4: Return the filtrate to digestion.
Step 5: Calcine at 1,050 °C until the water is gone, then cool and store sealed.
Worked Example
Using 1.89 kg of alumina per kg of aluminum (from Ore Processing):
- Hydroxide mass ratio: 2 × 78.0 / 101.96 = 1.53 kg per kg alumina.
- Hydroxide needed: 1.89 × 1.53 = 2.89 kg per kg of metal.
- Water removed in the kiln: 2.89 - 1.89 = 1.0 kg.
The kiln energy for that is not sourced in the corpus.
Purity Passes Straight Through
Electrolysis reduces the alumina; it does not refine out iron and silicon that came with it, as I recall. A poorly washed, iron-bearing alumina makes a poorer metal. That matters more for conductors than for castings.
Common Mistakes
- Rushing the cooling. You get fine, unfilterable crystals.
- Calcining wet hydroxide in a closed crucible. Steam builds up.
- Storing alumina open. It takes up moisture, and damp alumina fed to a hot fluoride bath spits (Hall-Héroult Electrolysis).
Safety
Caustic liquor and a 1,050 °C kiln
Liquor at this stage is still strong caustic that burns skin and eyes; wear a face shield and rubber gloves. Calcining hydroxide releases steam; load the kiln dry and heat gradually. Kiln work at 1,050 °C: keep the area clear and use tongs and heat-proof gloves.
What’s Next
- Learn next: Hall-Héroult Electrolysis - where the alumina is used
- Combine with: Simplified Bayer Process - the whole mass balance