Chemistry of inorganic compounds – Oxygen or compound thereof – Metal containing
Patent
1996-05-03
2000-02-29
Griffin, Steven P.
Chemistry of inorganic compounds
Oxygen or compound thereof
Metal containing
423626, 423628, 423629, 51309, 501153, C01F 736
Patent
active
060305991
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a process for producing water-dispersible, nanocrystalline alumina hydrates having a boehmite structure and to the use of said materials.
The physical and chemical properties of alumina hydrates primarily depend on the parameters of the manufacturing process. Manufacturing processes are known whereby special product properties required for specific applications are provided. The production of water-dispersible alumina hydrates, too, has already been described.
According to DE 2 408 233-C2, an acid-dispersible boehmite is used and gaseous acids, e g. hydrogen chloride or NO.sub.2, are passed over said powdery material predominantly in a fluidized bed. Said process uses commercially available boehmite powders as starting materials which have to be aftertreated to make them water-dispersible. As a result of the high intrinsic crystallinity of the powdery starting materials (>5 nm, determined by radiographic analysis on the 021 reflex of the boehmite), said aftertreated powders, after the addition of water, yield colloidal systems which are highly dispersible (D>97%), but have a milky, turbid appearance. Said colloidal systems have low translucence values ranging from 0 to 50%.
The translucence value is a measure of the size of the colloidal boehmite particles present in the suspension: the higher the translucence value, the smaller the colloids; the smaller the colloids in the suspension, the larger the specific surface of the suspension; the larger the specific surface of the suspension, the higher the binding strengths of the particles.
According to U.S. Pat. No. 4,676,928-A, there are added acids to an alumina hydrate suspension obtained from the aqueous phase of the Ziegler process. The treated alumina hydrate suspension having a pH value of between 5 and 9 is subjected to thermal aging and dried. The crystallinity of the alumina hydrate increases considerably during aging. The crystallites reach sizes of 8.5 to 25 nm. The boehmite powders produced by said process are also readily water-dispersible (D>98%), but said powders, too, yield milky to milky-turbid suspensions having translucence values of 0.5 to 50%. In this case, too, the reason for turbidity is the high crystallinity of the alumina hydrate suspension which is even enhanced by subsequent hydrothermal aging.
Either of said processes is used to provide high solids contents (up to 30%) in the ready-to-use alumina hydrate suspensions, but even by dilution of the milky-turbid suspensions to obtain comparably low solids contents (about 5 to 10%) the poor translucence values cannot be improved. The reason for low translucence is the degree of crystallinity of the starting materials. The boehmites produced according to said processes yield suspensions having particle diameters of>100 nm.
According to EP 0 505 896-A1, commercially available alumina hydrates of different crystalline structures (alumina monohydrates or alumina trihydrates or similar products) are used for producing water-dispersible boehmite. In a dissolving process, the highly crystalline aluminum component reacts with a large quantity of nitric acid at a pH value of less than 3.5 and elevated pressure and temperature. The resultant boehmite alumina hydrates are crystalline and dispersible in water. The materials produced by said process present crystallite sizes of greater than 6 nm (measured on the 021 reflex) and are primarily used for producing abrasives.
None of the manufacturing processes described hereinabove provides in a direct way a water-dispersible alumina hydrate powder having a boehmite structure. Said processes are expensive because commercially available standard products are used as starting materials which have to be made water-dispersible by expensive subsequent upgrading. The crystallinity of the products produced according to said processes is higher than 5 nm. Up to now, it has not been possible to provide water-dispersible materials having smaller crystallites than those known in the art. Said processes inevitably provide water
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Meyer Arnold
Noweck Klaus
Schimanski Jurgen
DiMauro Peter
Griffin Steven P.
RWE-DEA Aktiengesellschaft fur Mineraloel und Chemie
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