Today the Bayer process dominates the production of alumina as it is the lowest cost production route. The process uses a recirculating volume of concentrated caustic solution to dissolve the alumina minerals, separate them from still solid impurities, and then re-precipitate alumina tri-hydrate. Calcining completes transforms the tri-hydrate to alumina.
- Comminution: Incoming bauxite is ground to a smaller particle size to enable faster digestion and mixed with highly caustic liquor.
- Pre-desilication: The slurry is sent to pre-desilication stage (~100°C, atmospheric pressure). This helps to pre react any clays or other highly reactive silica containing minerals in the bauxite and start the formation of de-silication product (DSP). Pre-desilication minimises the residence time of the slurry in the more expensive hot digestion stages.
- Digestion: The pre-desilicated slurry is sent to the digestion process. Here the slurry is heated to between ~140°C and 260°C (depending on the type of bauxite – see the “Bauxite 101” section of this website). The alumina and silica minerals dissolve and then the silica re-precipitates as ‘desilication product’ (DSP). The DSP contains caustic, alumina and silica roughly in the mass ratio of: 1:1.25:1.5. Thus the removal of silica also causes valuable caustic and alumina to report to the waste stream.
- Flash down: The heated slurry is cooled to 105°C (and the pressure reduced back to atmospheric) by allowing steam evaporation or flashing in a cascade of vessels. DSP formation should be completed in this stage, however the bulk of the alumina minerals remained dissolved in the liquor.
- Separation & clarification: The solid and liquid components of the slurry are separated using cascade of settlers or deep cone separators to recover as much of the entrained liquor as possible. The high solids content mud is sent to permanent disposal site. The pregnant liquor and washings are sent to filtration or polishing to remove virtually every trace of solids content – helping avoid impurity contamination of the final product and preventing nuclei carryover to the precipitation process.
- Precipitation: The pregnant liquor is gradually cooled from approximately 80°C to 65°C in a cascade of large vessels. The dissolved alumina precipitates as alumina tri-hydrate (Al2O3.3H2O). The alumina is hot washed to remove oxalates which also precipitate out of the liquor. The spent liquor.
- Calcination: The alumina tri-hydrate (Al2O3.3H2O) is calcined at ~1000°C to drive off all chemically bound water, to leave alumina typically >99.5% Al2O3.
- Other: Streams of the spent liquor are now further treated to remove other impurities, partially evaporated to remove excess water, and used to preheat incoming slurry (to save on process energy costs). Additional caustic is added to make up for losses to the red mud stream.
In Bayer refining of high temperature bauxites (typically those bauxites requiring a digestion temperature of 250 °C or greater), there is a process variant know as sweetening. It involves injecting an extra ~25% of low temperature (gibbsitic) bauxite slurry into the flash down (Stage 4 ). The alumina in the gibbsitic bauxite slurry dissolves rapidly in the high temperature stream, allowing the concentration of dissolved alumina in the liquor to be significantly higher than could otherwise be achieved by processing high temperature bauxite alone. In this way “sweetening” allows extra alumina to be produced from an existing high temperature plant for only a very small capital and operating cost increase (pro rata, far less than the alumina produced).