Chemistry of inorganic compounds – Oxygen or compound thereof – Metal containing
Reexamination Certificate
2002-06-04
2004-12-07
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Oxygen or compound thereof
Metal containing
C423S127000
Reexamination Certificate
active
06827923
ABSTRACT:
The invention relates to a method for the production of aluminium hydroxide of improved whiteness, especially for application as a filler in paper and plastics.
Aluminium hydroxide (Al(OH)
3
, ATH) is usually produced by the Bayer process, starting from bauxite. The formation of aluminium hydroxide takes place by seeded crystallization from sodium aluminate solution using previously crystallized aluminium hydroxide as seed crystals. The aluminium hydroxide crystals produced are generally very coarse (average size ca. 100 &mgr;m) and most of the aluminium hydroxide produced is subsequently calcined to aluminium oxide (Al
2
O
3
, alumina) at temperatures in excess of 1000° C.
Most bauxites contain impurities, namely different minerals and organic matter from extraneous soil and vegetation. These may detract from the properties and discolour the aluminium hydroxide produced. The colour of such aluminium hydroxide is generally of little significance if it is caused by carbonaceous material derived from the organic carbon present in the starting bauxite, as any carbonaceous material is eliminated by oxidation during the calcination step.
The intermediate aluminiumn hydroxide however is also used as an industrial product in its own right, e.g., as pigment and/or fire retardant filler in paper, plastics and rubber compounds. Particularly in paper and plastics it is desirable to have the aluminium hydroxide crystals in a high whiteness form which does not impart any unwanted colouration to the end products. To produce white aluminium hydroxide crystals, special approaches are necessary. These generally involve the prior removal of contaminating organic compounds from the Bayer process liquor to render it virtually ‘water clear’. Some of these approaches are as follows:
High pressure/high temperature oxidation of the organic compounds using elemental oxygen. This can eliminate coloured organics but increases the carbonate content of the liquor which then requires an additional purification step.
Destruction of the organic contaminants by high temperature (>1000° C.) liquor calcination and subsequent redissolution of the calcinate in water to give a colourless solution of sodium aluminate.
Crystallizing aluminium hydroxide from a water-clear sodium aluminate results in a product having not only a very high whiteness (reflectivity), for example in paper coating applications, but also no tendency to impart unwanted colour to synthetic resins, especially the unsaturated polyester type used in the production of synthetic marble products
However, the currently available methods of achieving a water-clear sodium aluminate liquor are both energy intensive and expensive. Therefore, it was an object of the present invention to provide a more cost effective method for producing aluminium hydroxide of the required high whiteness and virtually free of coloured organics.
It has been found that by heating standard aluminium hydroxide obtained by the Bayer process having an average particle size of 20 to 200 microns to a temperature of 300 to 700° C., preferably 350 to 450° C., most of the organic contaminants (typically 60 to 70 wt % of the organic carbon content) are destroyed and that the resulting ‘activated’ product, which is largely X-ray amorphous and contains variable amounts of crystalline boehmite, has a high solubility in aqueous sodium hydroxide even at relatively low temperatures. This results in sodium aluminate liquor having a molar ratio of Na
2
O:Al
2
O
3
of 1.40 to 2.40, preferably 1.45 to 2.25 being obtainable without using hydrothermal conditions, i.e., at 85 to 105° C. Though not preferred, dissolution can also take place in an autoclave under hydrothermal conditions, the temperature thereby ranging up to 275° C. The boehmite content remains largely undissolved in form of very fine particles having a specific surface area of at least 10 m
2
/g, typically in excess of 50 m
2
/g. These superfine boehmite particles act as an adsorbent, thus eliminating most of the remaining impurities from the liquor by adsorption prior to and during the filtration step. Subsequently, the recovered boehmite particles may be utilized as starting material in alumina production or in other applications where the adsorbed impurities are not detrimental.
After filtration, the resulting water-clear liquor, which is supersaturated with respect to dissolved alumina, is cooled to 50 to 80° C. and seeded with previously crystallized aluminium hydroxide to induce crystallization of high whiteness product which is separated from the spent liquor by filtration or. The spent liquor does not accumulate organics so that it may be recycled without requiring a liquor purification step.
Thus, high whiteness aluminium hydroxide virtually free of organics can be produced from an inexpensive standard aluminium hydroxide by a relatively inexpensive process. Moreover, the seed crystals used in the process do not necessarily have to be superwhite themselves if further crystallization is essentially confined to crystal growth which will ‘cover’ the off-white seed crystals with aluminium hydroxide of high whiteness. In the last process steps, the produced aluminium hydroxide crystals are washed and dried according to methods known in the art.
The heating step is preferably conducted at a temperature of 350 to 450° C. for a time sufficient to obtain an activated aluminium (hydr)oxide having a weight loss on ignition of 5 to 15%.
The invention is further illustrated by the following non-limiting examples.
REFERENCES:
patent: 4225639 (1980-09-01), Matyasi et al.
patent: 4670231 (1987-06-01), Garcia-Clavel et al.
patent: 5342485 (1994-08-01), Armbrust, Jr.
patent: 6162413 (2000-12-01), Fujiwara et al.
Albemarle Corporation
Bos Steven
Manelli Denison & Selter PLLC
Melcher Jeffrey S.
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