Chemistry of inorganic compounds – Treating mixture to obtain metal containing compound – Group iiia metal or beryllium
Patent
1994-10-04
1996-06-25
Bos, Steven
Chemistry of inorganic compounds
Treating mixture to obtain metal containing compound
Group iiia metal or beryllium
423127, 423629, C01F 702, C22B 2100
Patent
active
055297617
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to improvements in alumina plants, and particularly plants operating according to the Bayer process to extract alumina from bauxite.
Background of the Invention
The Bayer process, and numerous improvements and modifications thereof, is widely used in the processing of bauxite to produce metal grade alumina. Most alumina plants producing metal grade alumina currently use a precipitation process which is a combination of agglomeration and growth to meet the requirements for crystal size end morphology. The precipitation process is normally carried out at the lowest possible temperature to maximize the process yield.
Modern downstream alumina users now require that the alumina be provided with a low soda content, while maintaining the desired crystal size and morphology. To achieve a lower soda content, the precipitator start temperatures are usually increased, and this results in considerable loss of yield, and thus plant efficiency.
Summary of the Invention and Object
It is the object of the present invention to provide alumina plant improvements which result in a high yield of low soda alumina. In the present specification, the term "low soda" should be understood to include soda contents of about 0.25 to 0.35 per cent Na.sub.2 O.
In one aspect, the invention provides a process for the production of alumina using the Bayer process including a precipitation process having an agglomeration stage and a growth stage, characterized by controlling selected process parameter(s) during the agglomeration stage to limit the bound soda content of the slurry while maintaining conditions necessary for high yield, and controlling selected operating parameter(s) during the growth stage to control the soda pick-up during the growth stage so that a continuous decrease of soda content is obtained.
The growth stage is preferably divided into a multiplicity of separate stages and the pick-up of soda is controlled by limiting the supersaturation of the liquor in each growth stage.
In one preferred form of the invention, the supersaturation of the liquor is limited by controlling the temperature of the liquor in each growth stage such that each successive downstream growth stage has a small reduction in temperature when compared to an adjacent upstream stage. It has been found that acceptable results are produced when each successive downstream growth stage is operated such that the liquor temperature is 1.degree. to 3.degree. C., and preferably about 2.degree. C., cooler than the liquor temperature in the adjacent upstream stage. The temperature of each stage is preferably controlled within this profile so as to achieve in the last growth stage a temperature necessary for a high yield, for example, about 55.degree. C.
The temperature of the liquor in each growth stage may be controlled by use of forced cooling in each growth stage. For example, heat exchange means may be associated with each growth stage to control the liquor temperature to the desired temperature.
In a particularly preferred form of the invention, the bound soda content is limited during the agglomeration stage by controlling the temperature in the agglomeration stage so that it substantially falls within the range 75.degree. to 85.degree. C.
To further improve the results achieved, hydrate seed is added to both stages of the precipitation process. In a preferred form, a seed slurry is added to the agglomeration precipitator, or to the first of the agglomeration precipitators where more than one is used, the slurry having a solids content of about 10 to 20 percent of particles less than about 44 microns with a d.sub.50 of about 60 to 70 .mu.m.
In the case of the growth stage or stages, a seed slurry is preferably added to the feed to the first growth stage, with the slurry preferably containing hydrate solids typically with about 5 to 8 percent of particles less than about 44 microns and with a d.sub.50 of about 75 to 85 microns.
In another aspect, the invention provides an alumina plant operating according
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Brown Gregory P.
Wood David G.
Bos Steven
Comalco Aluminum Limited
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