Chemistry of inorganic compounds – Oxygen or compound thereof – Metal containing
Patent
1984-06-13
1986-04-15
Doll, John
Chemistry of inorganic compounds
Oxygen or compound thereof
Metal containing
423127, 23301, 23305R, C01F 702, B01J 112
Patent
active
045826978
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a process for the production of aluminium trihydroxide having a median diameter which can be varied within the range of 2 to 100 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 an auxiliary seed which has itself been obtained by the decomposition of a supersaturated solution of sodium aluminate in the presence of ground aluminium trihydroxide having a specific surface area, measured according to the B.E.T. method, of at least one square meter 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 temperature and concentration of the treated medium.
For this reason, it is common practice in the Bayer process to favour the precipitation of the aluminium trihydroxide from supersaturated sodium aluminate solutions resulting from the alkaline attack of aluminous ores due to the recycling of a considerable fraction of the aluminium trihydroxide which is obtained 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 grains of aluminium trihydroxide of 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 particle size of which is controlled, i.e. whose granulometry deviates only slightly about the median size.
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 catalytic substrates.
By the number of publications issued in this field, the specialist 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 aluminium trihydroxide particles.
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 between 6 and 30 microns, by grinding a coarse aluminium trihydroxide which is obtained by the Bayer process. Such a process is described in French Pat. No. 2 298 510 which claims the production of an aluminium hydroxide intended for use in cosmetology, of which the mean diameter is less than 25 microns, by grinding a coarse hydroxide in the presence of an organic acid. Such a process can be used in the production of an aluminium hydroxide having a mean diameter greater 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 between 15 and 1 microns, the use of such a means is extremely costly because the mean diameter which is required demands a considerable energy consumption and the use of a very high grinding capacity.
The second group which uses chemical resources proposes processes for producing an aluminium trihydroxide havi
REFERENCES:
patent: 3545923 (1970-12-01), Mercier et al.
patent: 3649184 (1972-03-01), Featherston
patent: 3838980 (1974-10-01), Gnyra
patent: 4234559 (1980-11-01), Tschamper
patent: 4364919 (1982-12-01), Yamada et al.
Cristol Benoit
Mordini Jacques
Aluminum Pechiney
Doll John
Leeds Jackson
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