Process for precipitating alumina from Bayer process liquor

DETAILED DESCRIPTION It is a principal object of the invention to obtain high yields of alumina of large particle size from Bayer process liquor.
To this and other ends, the invention broadly contemplates the provision of improvements in a process for precipitating aluminum trihydroxide from Bayer process pregnant liquor including the steps of dividing an incoming stream of the pregnant liquor into a major portion and a minor portion; adding to only the minor portion a first particle size fraction of aluminum trihydroxide particles as seed for inducing precipitation of aluminum trihydroxide; passing only the minor portion, containing the first seed fraction, through a first reaction circuit comprising at least one vessel under conditions for inducing precipitation and agglomeration of aluminum trihydroxide particles from the liquor, the particles thus precipitated and agglomerated being carried from the first circuit in the minor portion of the stream; beyond the first circuit, reuniting the minor portion with the major portion to provide a recombined stream; adding to the recombined stream a second particle size fraction of aluminum trihydroxide particles as seed for inducing precipitation of aluminum trihydroxide, the second seed fraction being larger than the first seed fraction in median particle size; and passing the recombined stream, containing the second seed fraction, through a second reaction circuit comprising at least one vessel under conditions for inducing precipitation and growth of aluminum trihydroxide particles from the liquor, the particles thus precipitated and grown being carried from the second circuit in the recombined stream.
More particularly, in accordance with the invention, in a first aspect thereof, the aforementioned minor portion of the incoming stream is not more than about 25% by volume of the total incoming stream.
In a further important aspect of the invention, the process involves providing a reaction circuit having a first series of vessels for carrying out the agglomeration stage and a second series or set of vessels for carrying out the growth stage of precipitation of alumina from Bayer process liquors