PROCESS FOR THE PRODUCTION OF AN ALUMINIUM TRIHYDROXIDE OF LARGE GRANULOMETRY

摘要

Process for the decomposition of an over-saturated alkaline aluminate solution according to the Bayer process of alkaline etching of bauxites in order to obtain simultaneously a high yield and a large size grading aluminium trihydroxide of which 10 % at the most of the resultant particles have their smallest size lower than 45 microns, by introducing a primer, said process comprising contacting the whole of the primer used with the whole of the alkaline aluminate solution to be decomposed, characterized in that: a) in the decomposition area of the Bayer process comprising "n" stages in cascade, there is created a suspension with a high content of dry material of at least 700 g/l of alkaline aluminate solution to be decomposed in at least one stage by introducing the primer comprised of aluminium trihydroxide crystals of non-selected grading, b) after a dwell time in the decomposition area at a maximum temperature selected in the range of 50-75oC until a weight ratio of dissolved Al2O3/caustic Na2O at the most equal to 0.7 is obtained, a fraction comprised of 50 % by volume at the most of the dry material high content suspension circulating in the decomposition area is taken away, c) then, after the taking, said fraction is introduced in the classification area of which c1, the separated grained portion, is removed and represents the production of Al(OH)3 of large size grading and, c2, the other separate portion forming a suspension is withdrawn from the classification area and admixed to the remaining fraction of the circulating suspension in the decomposition area which has not been subjected to classification, d) the resultant suspension of the operation effected at c2 is subjected to a solid-liquid separation, the separated solid phase forming the unselected size grading aluminium trihydroxyde primer, recycled in the decomposition area of the Bayer process.