The present invention relates to a process for manufacturing crystalline alumina, .alpha.--Al.sub.2 O.sub.3, by calcination of aluminum hydroxide, Al(OH).sub.3, up to a temperature above that for transformation to .alpha.--Al.sub.2 O.sub.3 and including addition of boron and/or fluorine containing compounds as mineralizers.
Alumina is normally produced on a large scale via the Bayer process. In that process bauxite is digested in a caustic soda solution followed by a crystallization step from which aluminum hydroxide is obtained in the form of agglomerates up to 100.mu. in size. The aluminum hydroxide is transformed to alumina by calcination in rotary kilns or fluidized bed type furnaces. In that process every effort is made to achieve as complete as possible conversion to .alpha.-aluminum oxide, which takes place from about 1200.degree. C. and with certainty at higher temperatures. The transformation to .alpha.-aluminum oxide and crystal growth at the treatment temperature is accelerated and/or the transformation temperature lowered by the addition of small amounts of so-called calcining agents or mineralizers.
Particularly effective mineralizers in this connection are NaF, CaF.sub.2, AlF.sub.3, Na.sub.3 AlF.sub.6 and x(BF.sub.4).sub.n, where x stands for metallic elements, in particular elements with a valency of 1 or 2 and n is balancing factor taking into account the valency of x. According to the German patent DE-AS No. 11 59 418 a few tenths of a percent of hydrogen fluoride gas in the furnace atmosphere has the same effect.
The alumina produced this way is always such that the individual particles are plate-shaped with the largest dimension perpendicular to the c-axis.
Depending on the rate of throughput or rate of heating-up and the type and amount of the fluorine compound, the temperature of transformation to .alpha.-aluminum oxide and its crystal size and shape can be varied within a limited range. Up to now, however, it has not been possible to produce isometric .alpha.-alumina (corundum) crystals this way.
By isometric, and the frequently used synonyms equi-axed, cubic, spherical, polyhedral etc. Al.sub.2 O.sub.3 crystals is to be understood crystals which have a ratio of diameter D perpendicular to the crystallographic c-axis and height H parallel to the c-axis close to the value 1.
For many applications in the industries processing calcined aluminum oxide there is a need to overcome the disadvantages of this raw material, caused by the normally pronounced plate-shaped character of the .alpha.-aluminum oxide, by altering its properties such that they are closer to those of an isometric material.
In the preparation of surfaces, in particular when polishing soft or brittle materials such as polymers, non-ferrous and noble metals, glass and semi-conductor materials, every effort is frequently required to avoid any damage which penetrates below the surface, such as can be created by sharp-edged thin platelet-shaped crystals of the preparation material. Another disadvantage of such crystals with large diameter to height ratio D/H is that when employed as a rubbing compound, especially for lapping and polishing, the crystals break easily and form cutting edges of random geometry. The intended advantage of the single crystal grains with constant cutting geometry on all crystals due to the natural morphology and high incidence of specific cutting edges is thus to some extent lost.
For the above reasons alumina products made via the calcination of aluminum hydroxide have up to now been unable to find application in certain areas of surface treatment technology e.g. some applications in the field of optics.
In the U.S. Pat. No. 4,193,768 a process for manufacturing corundum crystals is proposed. That proposal is such that fine nuclei of corundum crystals are mixed with an initial aluminum oxide hydrate and, in order to precipitate corundum on the fine corundum nuclei, the resultant mixture is subjected to a hydrothermal treatment until the fine corundum particles have grown to the desired size. This process does indeed produce good crystals for the above mentioned purposes, but is very involved and therefore uneconomical.