Chemistry of inorganic compounds – Oxygen or compound thereof – Metal containing
Patent
1990-01-31
1992-09-22
Langel, Wayne A.
Chemistry of inorganic compounds
Oxygen or compound thereof
Metal containing
423628, C01F 702
Patent
active
051495201
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to alpha alumina particles and platelets of small particle size and to processes for their production. As used herein, the term "platelets" refers to particles having a plate-like or tabular shape. Alpha alumina particles and platelets are useful as ceramic and abrasive materials.
BACKGROUND OF THE INVENTION
Alpha alumina particles having average particle sizes of less than about 1.5 microns are known in the prior art. It is also known that alpha alumina particles can be made with a plate-like or tabular shape, in particle sizes greater than about 2 microns. However, alpha alumina particles having both plate-like shape and particle sizes below about 1.5 microns are heretofore unknown. As used herein with reference to platelets, the term "particle size" refers to the size of a largest dimension.
It is a principal objective of the present invention to provide alpha alumina particles having a plate-like shape with an average particle size below about 1.5 microns.
A related objective of the present invention is to provide an economical process for production of alpha alumina particles of the type described, which process avoids formation of boehmite gel at an intermediate stage.
Additional objectives and advantages of the present invention will become apparent to persons skilled in the art from the following specification and claims.
SUMMARY OF THE INVENTION
The principal starting material for the process of the present invention is an aqueous aluminate solution, such as the solution obtained from the Bayer process. The aluminate solution contains about 50-400 g/l caustic soda (expressed as Na.sub.2 CO.sub.3) and about 0.50-0.80 grams Al.sub.2 O.sub.3 per gram of caustic soda (expressed as Na.sub.2 CO.sub.3) The term "caustic soda" as used herein refers to sodium hydroxide. The aluminate solution preferably has a caustic soda content of about 140-240 g/l (expressed as Na.sub.2 CO.sub.3) The solution also preferably contains about 0.60-0.70 grams Al.sub.2 O.sub.3 per gram of caustic soda (expressed as Na.sub.2 CO.sub.3). Initial temperature of the solution is about 20.degree.-100.degree. C.
The aluminate solution is treated with alumina hydrate seed material and particles of an alpha alumina promoter, thereby to precipitate alumina hydrate particles containing the promoter. Preferably, the solution is also cooled as the seed material and promoter are added. The promoter preferably comprises about 0.01-10 wt % of the Al.sub.2 O.sub.3 content in the precipitate, more preferably about 0.01-5 wt % and most preferably about 0.01-1 wt %. Average particle size of the promoter particles is generally less than about 1 micron.
The seed material preferably comprises particles of alumina trihydrate. The alumina trihydrate seed material may be gibbsite, bayerite or mixtures thereof. More preferably, the seed material comprises predominantly gibbsite.
Precipitated alumina hydrate particles containing the promoter are separated from the aluminate solution, dried, and calcined at an elevated temperature of about 900.degree.-1500.degree. C. to produce particles comprising predominantly alpha alumina. The alpha alumina particles produced preferably have an average crystallite size of less than about 1 micron. If the alpha alumina particles are agglomerated, they may be comminuted by grinding to an average particle size of less than about 1 micron.
As used herein, the term "alumina hydrate" refers to Al.sub.2 O.sub.3.xH.sub.2 O, where x varies from 1 to 3; in other words, the water of hydration varies between 15 and 34.6% by weight of the alumina hydrate as determined by calcination. The expression "alumina trihydrate" refers to Al.sub.2 O.sub.3.3H.sub.2 P or Al(OH).sub.3, also called "aluminum hydroxide". The terms "gibbsite" and "bayerite" refer to different crystalline forms of alumina trihydrate.
Alpha alumina particles produced by the claimed process are useful for making sintered ceramic shapes. The alpha alumina particles are pressed into a green shape and then the green
REFERENCES:
patent: 4487756 (1984-12-01), Mizrah et al.
patent: 4585645 (1986-04-01), Sucech
patent: 4806333 (1989-02-01), Egerton
patent: 4818515 (1989-04-01), Ceresa et al.
patent: 4822592 (1989-04-01), Misra
patent: 4946666 (1990-08-01), Brown
Marhanka John E.
Pearson Alan
Sucech Steven W.
Aluminum Company of America
Klepac Glenn E.
Langel Wayne A.
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