Chemistry of inorganic compounds – Treating mixture to obtain metal containing compound – Group iiia metal or beryllium
Patent
1984-06-13
1986-03-04
Carter, Herbert T.
Chemistry of inorganic compounds
Treating mixture to obtain metal containing compound
Group iiia metal or beryllium
423127, 423629, 423121, 23301, 23305A, C01F 702
Patent
active
045740747
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a process for the production of aluminium trihydroxide having a median diameter which can be varied as required of less than 4 microns, having a unimodal distribution and minimum deviation. The aluminium trihydroxide is produced by the decomposition of a hot supersaturated solution of sodium aluminate in the presence of ground aluminium trihydroxide having a specific surface, measured according to the B.E.T. method, of at least equal to 8 square meters per gram.
It is well established in the art to carry out the precipitation of aluminium trihydroxide from a supersaturated solution of sodium aluminate by the addition of a primer consisting of previously crystallised aluminium trihydroxide. The spontaneous generation of seed crystals in a solution of this type has been found to be extremely slow and difficult to produce and even non-existent, depending on the conditions of termperature and concentration of the treated medium.
For this reasons it is common practice in the Bayer process to favour the precipitation of aluminium trihydroxide from supersaturated sodium aluminate solutions resulting from the alkaline attack of aluminous ores due to the recycling of a considerable in a previous cycle.
However, such as it is carried out, this priming process results not only in the recycling of a very considerable quantity of previously precipitated aluminium trihydroxide, but above all in the production of grains of aluminium trihydroxide of an eminently variable size. The median dimensions and the deviation about this value are difficult to control, due to the fact that the dimensions of the grains of aluminium trihydroxide increase during the consecutive cycles and cause the formation of new seeds in a periodic rhythm.
However, those skilled in the art would like to be able to produce, for particular uses, aluminium trihydroxide, the median diameter of the precipitated particles of which is less than 4 microns, and the granulometry of which deviates only slightly about the desired median dimensions.
In fact, certain uses of aluminium trihydroxide require a granulometry, which is specific to them. In particular for uses such as fireproofing charges for synthetic polymers, gentle abrasives in cosmetology and charges used in the paper industry.
By the number of publications issued in this field, the specialized literature reveals the importance and the complexity of the research carried out by those skilled in the art to try to provide industrially viable solutions to the above-mentioned problems, and to control the size of the particles of aluminium trihydroxide.
Among the numerous solutions which have been proposed, some lead to the use of mechanical means and others, which are more numerous, lead to processes which use the resources of chemistry.
The first group which uses mechanical means concerns the production of aluminium trihydroxide, the median diameter of which is generally from 1 to 30 microns, by grinding a coarse aluminium trihydroxide which is obtained according to the Bayer process. A process of this type is described in French Pat. No. 2,298,510 which claims the production of an aluminium hydroxide intended for use in cosmetology, the mean diameter of which is from 1 to 25 microns, by grinding a coarse hydroxide in the presence of an organic acid. Such process can be used in the production of an aluminium hydroxide having a mean diameter of more than 15 microns, because it remains reasonable in terms of energy consumption and technological investment. However, if an aluminium hydroxide is to be produced which has a much smaller mean diameter, such as less than 4 microns, the use of such a means is extremely costly, because the mean diameter which is desired demands a considerable energy consumption and the use of a very high grinding capacity which is unacceptable within the scope of an industrial production.
The second group which uses chemical resources proposes processes pursuing the aim of obtaining an aluminium trihydroxide having a regulated granulometry and comprisin
REFERENCES:
patent: 3838980 (1974-10-01), Gnyra
patent: 4150952 (1979-04-01), Lafleur et al.
Cristol Benoit
Mordini Jacques
Aluminum Pechiney
Carter Herbert T.
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