Process for precipitation of aluminum hydroxide from aluminate solution

Claim 1

A process for continuous precipitation of aluminum hydroxide from an aluminate solution produced by digestion of bauxite in concentrated sodium hydroxide solutions at elevated pressure and temperature with subsequent separation of undissolved components, where the precipitation is carried out in more than one stage and in the presence of seed crystals, and where the precipitated aluminum hydroxide after washing can be transformed to oxide by heating to temperatures over 700° C., while the mother liquor remaining after the precipitation is concentrated by evaporation and returned for reuse in the digestion of bauxite, said process comprising: passing the aluminate mother liquor counter-current to a feed of aluminum hydroxide crystals through at least two precipitation stages arranged in series; maintaining in each precipitation stage a lower turbulent zone with a high suspension density, and agitating said zone sufficiently to maintain crystals therein suspension; maintaining over each turbulent zone a non-turbulent, stagnant liquid layer from which the majority of the crystals settle back into the respective lower turbulent zone; maintaining the suspension density in the last as well as preceding precipitation stages at a constant level by sending crystal suspension from the lower turbulent zone in all precipitation stages but the first to the preceding precipitation stage; removing mother liquor from the upper stagnant zone of all but the last stages and sending such removed mother liquor to the following stage; and removing coarse crystal suspension from the turbulent zone in said first stage for separation of the crystals for further processing, while returning the remaining mother liquor to the system.