Digestion dissolves alumina by heating ore crushed below 5 mm in 25-30% caustic soda at 150-200 °C for 2-4 hours in a sealed vessel. That temperature puts the vessel at roughly 5 to 15 bar absolute from steam alone, several times the pressure of a village autoclave, so it needs a vessel built to a proven design with a relief valve, not an improvised one.
Part of Simplified Bayer Process
The pressure-vessel stage of the Bayer process: particle size, caustic strength, temperature, time and the pressure and caustic hazards.
The Vessel, Not The Chemistry, Is The Limit
The carrier gives the conditions: ore below 5 mm, caustic soda at 25-30% by weight, 150-200 °C, 2-4 hours, in a pressure vessel. Finer ore dissolves faster, as I recall, but the corpus gives no grinding spec beyond the carrier’s.
Pressure from steam alone, from steam tables as I recall them (not in the corpus): about 4.8 bar absolute at 150 °C and about 15.5 bar absolute at 200 °C. Strong caustic lowers the vapour pressure of the liquid, so these are upper bounds, but they are the right scale. Subtracting atmospheric pressure (about 1 bar) gives roughly 3.8 and 14.5 bar gauge. The wiki’s autoclave runs at 121 °C and 15 psi, about 1 bar gauge, with a relief valve at 110% of working pressure (Autoclave Construction). A digester at 200 °C therefore runs at more than ten times the autoclave’s gauge pressure. The autoclave article’s design figures do not carry over, and Vessel Construction says to copy a certified design and hydrostatically test it. Hot strong caustic also attacks some metals and can crack stressed steel, as I recall; the corpus is silent on suitable vessel materials.
What You Need
- A certified or code-derived pressure vessel rated above the figures above, with a relief valve and gauge.
- Crushed ore below 5 mm (carrier).
- 25-30% caustic soda (Caustic Soda).
- A heat source that can be controlled and turned off from outside the danger zone.
- A hot filter for the slurry after digestion.
Method: One Digestion
Step 1: Charge crushed ore and caustic. The ratio is not in the corpus.
Step 2: Seal and heat slowly to the target temperature. Watch the gauge, not the clock.
Step 3: Hold 2-4 hours at temperature.
Step 4: Cool and vent through a valve that discharges away from people, before opening.
Step 5: Filter the slurry to remove the red mud, and wash the mud to recover caustic.
Worked Example: Caustic Inventory
At the stoichiometric floor of 1.48 kg NaOH per kg of metal (Simplified Bayer Process), the weight of 30% solution holding it is 1.48 / 0.30 = 4.9 kg, and of 25% solution 1.48 / 0.25 = 5.9 kg. Per kilogram of aluminum to be made, the vessel must hold at least about 5-6 kg of liquor plus the ore, before any excess caustic. A day’s output at 1 kg per hour needs 24 times that circulating.
Common Mistakes
- Using an autoclave as a digester. It is built for a tenth of the pressure.
- Skipping the relief valve because the gauge reads low.
- Opening hot. Hot caustic liquor flashes to steam.
Safety
Pressure, steam and caustic together
Stops: a relief valve set at no more than 110% of working pressure and vented away from people (as the autoclave article requires), a fuel or heat shut-off reachable from outside the blast line, and a rule that nobody stands in line with the closure while the vessel is warm. The vessel stays pressurised for a long time after heat is removed and has no short coast-down. Caustic at this temperature blinds and burns on contact; wear a face shield and rubber gear. Never use a vessel of unknown history.
What’s Next
- Learn next: Precipitation and Calcination - recovering alumina from the liquor
- Combine with: Vessel Construction - building or choosing the vessel