Abstract: Process for the low-temperature treatment according to the Bayer cycle of gibbsite-bearing bauxite of low reative silica content, involving desilication of the said bauxite before leaching, by a liquor of low caustic soda concentration and characterized in that this sodium liquor consists of one or more washing waters originating from the Bayer cycle, mixed or otherwise with leaching liquor, wherein the weighted reactive silica content is preferably lower than 5% of the alumina content of the bauxite and the concentration of the sodium liquor used for desilication ranges from 50 to 120 g/liter of Na.sub.2 O caustic.

Abstract: A method of decomposing a super-saturated solution of alkali metal aluminate Ld from the Bayer process, by introducing primer Sa and forming a suspension with a high content of dry material, of at least 700 g/l of alkali metal aluminate solution to be decomposed, is characterized in that, for the purpose of obtaining Al(OH).sub.3 with a large, even particle size, a zone (Zs) for separating particularly fine solid particles of Al(OH).sub.3 is formed in decomposition zone (B) comprising a cascade of n stages of the Bayer process, the separating zone being fed with at least part L(n-3)1 of the flow of suspension Ln-3 circulating in said zone (B), from which a fraction LS.sub.1 is extracted, containing at least 5% of the total number of fine particles with a maximum diameter of 40 microns present in the flow Ln-3, while the residual suspension LS.sub.2 emerging from the separating zone Zs is recycled to zone (B), then the fraction Ls.sub.

Abstract: A process for the production of aluminum trihydroxide having controlled median diameter of 2 to 100 microns, with a unimodal distribution and minimum deviation, by decomposing a hot supersaturated sodium aluminate solution in the presence of seed aluminum trihydroxide, separating the resultant solid and liquid phases and recovering the solid phase constituted by precipitated aluminum trihydroxide. The process comprises grinding aluminum trihydroxide until the aluminum trihydroxide has a specific BET surface area of at least 1 m.sup.2 /g, introducing the ground aluminum trihydroxide into a hot fraction of the supersaturated sodium aluminate solution to precipitate trihydroxide seed and then decomposing the remaining fraction of the supersaturated sodium alimuniate solution in the presence of the seed. This decomposition leads to the precipitation of aluminum trihydroxide having the required median diameter.

Abstract: A process for the production of aluminum trihydroxide having a controlled median diameter of less than 4 microns, with a unimodal distribution and minimum deviation. The process comprises grinding aluminum trihydroxide until the aluminum trihydroxide has a specific BET surface area of at least 8 m.sup.2 /g, adding the ground aluminum trihydroxide to a sodium aluminate solution, and then decomposing the solution to precipitate aluminum trihydroxide having the required median diameter.