Method for processing bauxite rich in alumina monohydrate

Abstract

Process for treating alumina monohydrate-based bauxite by high-temperature caustic digestion characterized in the following series of operations: a) digestion of the bauxite at a temperature in excess of 200° C. with a liquor concentrated to more than 160 g Na2O / liter and having a A / C weight ratio between 0.45 and 0.75, for a contact time which is sufficient to extract the aluminate monohydrate and to attain a A / C ratio between 1.10 and 1.20; b) cooling of the slurry and introducing alumina trihydrate in said slurry, at a temperature of less than 200° C., in order to obtain a A / C ratio of the aluminate liquor between 1.20 and 1.30; c) first diluting to a concentration between 160 and 175 g Na2O / liter; d) settling to separate the insoluble residue from a sodium aluminate liquor with an A / C ratio between 1.20 and 1.30; e) filtering the aluminate liquor; f) lowering the aluminate liquor with a complementary dilution before or during crystallization; g) after separation of the alumina trihydrate by filtration, recycling the spent liquor which, after concentration, is reused as digestion liquor with a caustic concentration above or equal to 160 g Na2O / liter.

Description

[0001] This invention relates to the treatment, by alkaline digestion according to the Bayer process, of alumina monohydrate bauxite or boehmite, including the addition of alumina trihydrate or gibbsite.STATE OF THE ART[0002] The Bayer process, which is widely described in the literature, constitutes the essential production technique of alumina to be transformed by igneous electrolysis into aluminum, or to be used in the hydrate state or transition, calcinated, sintered or melted alumina states, in numerous applications in the technical alumina field.[0003] According to this process, the bauxite mineral is digested when hot by means of an aqueous liquor of sodium hydroxide at the appropriate concentration, thereby making the alumina soluble and obtaining a slurry made up of particles of non-digested residue (mud) in a sodium aluminate liquor referred to as "aluminate liquor".[0004] This slurry is then usually diluted so as to separate the mud from the aluminate liquor by settling. ...

AI Technical Summary

Benefits of technology

[0032] Contrary to all expectations, the applicant observed that it was able to delay, at least partially, the dilution of the aluminate liquor until the start of its crystallization. This operating method, however, runs counter to the usual practice in which the specialist dilutes the slurry by adding water so as to facilitate the settling of the mud. This dilution has the twin advantage of reducing its viscosity and allowing the addition of numerous flocculants which are unstable in liquors which are too concentrated. On the other hand, it increases alumina supersaturation which favors reversion. But the precipitation kinetics are slow when the A / C does not exceed 1.10-1.15 or even 1.20, so that this risk has always been considered as minimal provided that settling times are not too long. If settling is too lengthy, the fine particles of the non-digested residue present in the slurry may in fact initiate the precipitation and an intense reversion may still develop 5 or 6 hours after the start of settling. Dilution before separation of the mud has consequently always been considered as obligatory by the specialist because it shortens settling time and preserves the stability of the flocculants.

[0033] It unexpectedly occurred to the applicant that, given the flocculants currently used-these are usually anionic polyelectrolytes, for example synthetic polyacrylates which may advantageously be combined with polysaccharide-based additive improving clarity-, it was possible to settle the insolubles in a concentrated aluminate liquor in an acceptable lapse of time, despite a very high A / C, higher than 1.20, without reversion occurring. It emerges that the delaying effect of the reversion due to the high caustic level prevails over the accelerating effect due to a slight increase in the settling time. Moreover, the strong aluminate concentration appreciably increases its boiling temperature, thereby making it possible to settle at a higher temperature and to reduce still further the risk of reversion.

Problems solved by technology

It has occurred that such a mode of liquor supersaturation, developed during the 1950s and referred to as "sweetening", is difficult to exploit on an industrial scale.

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