Abstract: A method of forming and classifying a precipitate containing large, coarse and fine particles from a super-saturated precipitating mother slurry, e.g. sodium aluminate from the Bayer process. The method comprises separating the precipitating mother slurry into a plurality of separate streams, introducing each stream into a vessel provided with an agitated lower zone and a quiescent upper zone to form the precipitate, with the large and coarse particles remaining as a slurry in the agitated lower zone and the fine particles migrating to the quiescent zone, removing product slurry from the lower agitated zone, and removing spent liquor containing the fine particles from the quiescent upper zone. The invention also provides apparatus for carrying out the method. The combined precipitation and partial classification (separation of fine particles) makes it possible to deal with high solids charges increasingly encountered in industrial processes.

Abstract: A method and apparatus are disclosed for continuously drying, preferably with agglomeration and/or coating and sizing, and separating a solid product from a liquid feed material, especially bauxite slurry which passes through a very sticky phase during drying, without significant encrustation of the equipment used.

Abstract: An improved process for precipitating alumina from Bayer process liquors is disclosed. The process involves dividing an incoming pregnant liquor stream into a minor portion of about 10 to about 25% of the total incoming pregnant liquor stream, and directing that stream into a series of small tanks where the minor stream is permitted to agglomerate. Fine aluminum hydroxide seed having a median particle size of between about 30 and about 60 microns is added to this stream. The slurry resulting from the agglomeration stage is then directed to a set of larger vessels in which the growth stage will take place. At this point, the remaining portion of the pregnant liquor is combined with the slurry resulting from the agglomeration stage. A coarse seed charge of about 150 to about 600 grams/liter of seed having a median particle size of between about 80 and about 100 microns is charged into the second set of vessels. This improved process helps to increase the yield of a strong coarse aluminum trihydroxide product.