In this stage, the alumina is recovered by crystallisation from the pregnant liquor, which is supersaturated in sodium aluminate.
The crystalisation process is driven by progressive cooling of the pregnant liquor, resulting in the formation of small crystals of aluminium trihydroxite (Al(OH)3, commonly known as “hydrate”), which then grow and agglomerate to form larger crystals. The precipitation reaction is the reverse of the gibbsite dissolution reaction in the digestion stage:
Al(OH)4- + Na+ → Al(OH)3 + Na+ + OH-
The gibbsite crystals formed in precipitation are classified into size ranges. This is normally done using cyclones or gravity classification tanks (a series of thickeners utilising the same principles as settlers / washers on the clarification stage). The coarse size crystals are destined for calcination after being separated from spent liquor utilising vacuum filtration, where the solids are washed with hot water.
The fine crystals, after being washed to remove organic impurities, are returned to the precipitation stage as fine seed to be agglomerated.