Process for producing coarse grains of aluminum hydroxide

Abstract

Coarse grains of aluminum hydroxide having high resistance to disintegration to powder in a calcination step are continuously produced from a supersaturated sodium aluminate solution of the Bayer process through (1) a first precipitation step of adding recycle seed aluminum hydroxide to a supersaturated sodium aluminate solution having a molar ratio of Na.sub.2 O as caustic soda to Al.sub.2 O.sub.3 in solution of less than 1.8 supplied to a precipitation tank of substantially complete mixing type in a ratio of 30-150 kg of the recycle seed to 1 m.sup.3 of the solution, thereby partially decomposing the seed-added solution until the molar ratio of the solution reaches 2.0-2.4 while keeping the solution at 65.degree.-80.degree. C.; (2) a second precipitation step of supplying the resulting sodium aluminate solution in a slurry state from the first precipitation step to at most three precipitation tanks of growth type arranged in series and decomposing the solution in the slurry state until the molar ratio reaches at least 2.6 while keeping the solution in the slurry state at a precipitated solid aluminum hydroxide concentration of 400-1,500 g/l and at a slurry temperature of 45.degree.-65.degree. C. in the precipitation tanks of growth type, and (3) a third step of supplying the decomposed solution in a slurry state from the second precipitation step to a classifier, thereby classifying the precipitated aluminum hydroxide grains into coarse grains of aluminum hydroxide as product and fine grains of aluminum hydroxide, and recycling the fine grains of aluminum hydroxide as seed.

Dependencies are inferred from the claim text.