The bayer process for production of alumina trihydrate by alkaline digestion of bauxite, the process comprising a predesilication step

A process for the production of alumina trihydrate by alkaline digestion of bauxite using the Bayer process, comprising:

• (a) grinding bauxite, (b) predesilication of the ground bauxite by bringing the ground bauxite into contact with an aqueous sodic solution, (c) digestion of the ground bauxite after predesilication by bringing it into contact with a sodium aluminate liquor, (defined as a “green liquor”), (wherein the digestion comprises forming a slurry that includes the ground bauxite, the aqueous sodic solution and the sodium aluminate liquor and treating the slurry to separate insoluble residues from the sodium aluminate liquor, and wherein the resulting portion of the slurry without insoluble residues is defined as a “pregnant liquor”), (d) crystallizing the pregnant liquor whereby alumina trihydrate is precipitated and wherein the sodium aluminate liquor without the precipitated alumina trihydrate is defined as a “spent liquor”, wherein the aqueous sodic solution that is used for the predesilication treatment is at least partially drawn off from the Bayer line and is treated in such a manner that it has a caustic concentration of between 60 g Na2O/liter and 220 g Na2O/liter and a weight content of carbonates, sulphates or sulphites, which, expressed as a percentage related to the caustic concentration, is less than half the corresponding impurities weight content of the spent liquor.