PROCESS FOR PRODUCING ALUMINIUM TRIHYDRATE CONTAINING GRAINS SMALLER THAN 45 MICROMETERS UP TO 10 MASS PER CENT

摘要

The invention concerns a process for the precipitation of aluminium hydroxide by seeding in two phases, which is intended to produce at a high level of productivity alumina with large grains, referred to as 'sandy coarse', by precipitation in a succession of tanks in cascade relationship of a supersaturated solution of sodium aluminate coming from alkaline attack on bauxite in accordance with the BAYER process. The process comprises three stages: in the first agglomeration stage, the supersaturated aluminate liquor is introduce into the tank, with an equivalent amount of terms of Na2O of between 110 and 175 g/liter, at a temperature of between 65 DEG and 80 DEG C., and seeding is effected with an amount of washed seed of between 20 and 120 g/l of aluminate, in the second stage, selective retention of the largest crystals of aluminium trihydrate formed is effected in the tanks, so as to produce a content of dry matter of between 300 and 800 g/liter of suspension, the whole of the aluminium trihydrate which is intended for the production of alumina is extracted in the course of or at the end of the second stage, in a third stage, referred to as a liquor depletion stage, an amount of seed of between 400 and 800 g/l of suspension, that is to say between 480 and 1200 g/liter of liquor, is introduced, at the end of the third stage, separation is effected on the one hand of a depleted aluminate liquor which is recycled in known fashion and on the other hand aluminium trihydrate with very fine grains, a small portion of which is recycled after washing as a seed for the agglomeration stage while the remainder is recycled as a seed to the liquor depletion stage.