Alumina: the Bayer digest–wash–crystallise–calcine family

The Bayer process recovers alumina from bauxite, and it is the flowsheet the counter-current decantation module was built on. It composes from mechanisms taught earlier, and this page assembles them, naming each contribution rather than re-teaching it.

The flowsheet in one line

Digest the bauxite in hot caustic to dissolve the alumina, clarify and wash the red-mud residue out, crystallise alumina trihydrate from the cooled liquor onto seed, and calcine it to alumina: digest–clarify/wash–crystallise–calcine, with the caustic liquor recycled around the loop.

What each stage is doing

Digestion is caustic leaching — the tank- and pressure-leaching mechanisms of topics 4.3 and 4.6 — dissolving the alumina from the bauxite into a hot sodium-aluminate liquor while the iron and silica stay solid. Clarification and CCD washing is the solid–liquid-separation module: the insoluble red-mud residue is settled out and washed counter-current in a thickener train (topic 5.2) to recover the valuable caustic liquor clinging to it, which is the soluble-loss economics of topic 5.5 at industrial scale. Crystallisation is topic 7.3: the clarified liquor is cooled and seeded so alumina trihydrate crystallises out by the fall in solubility. Calcination is the thermal step of topic 3.2: the trihydrate is calcined in a kiln to drive off its water and leave alumina.

The defining numbers

The red-mud wash is sized on the CCD wash-water calculator below — the wash water a target wash ratio needs, the defining number of the washing circuit — and on the thickener water recovery calculator for the process water the thickeners return. The calciner is sized by residence time on the kiln residence-time calculator. The caustic liquor itself reads on the sodium hydroxide hub for the property data behind the digestion medium.

Where it differs from the others

Alumina is the family where solid–liquid separation and washing — not solvent extraction or adsorption — carries the flowsheet: recovering the caustic from the red mud by counter-current decantation is the heart of its economics. It recovers its product by crystallisation and calcination rather than electrowinning, and it runs a closed caustic loop, which makes wash efficiency a direct lever on reagent cost.