Swiss Aluminium Ltd.

Bernhard Schepers

Tiberiu Mizrah

Werner J Borer

Volker Nobbe

Manfred Kullack

Bernd Schroder

Bernd Schr/o/ Der

The invention relates to a process for manufacturing crystalline alumina from aluminum hydroxide which after further conventional treatment is advantageously suitable for ceramic and refractory purposes or as a lapping and polishing agent. The process is such that aluminum hydroxide with a Na.sub.2 O content of .ltoreq.0.1 wt. % with respect to Al.sub.2 O.sub.3 is subjected either directly or after prior partial dehydration to a thermal treatment above the temperature for transformation to .alpha.--Al.sub.2 O.sub.3. The aluminum hydroxide has an addition of an ammonium containing mineralizer made to it, simultaneously in the presence of boron and/or fluorine, in particular an addition of NH.sub.4 BF.sub.4. The product is characterized by way of the essentially isometric shape of the single crystals.

Process for manufacturing crystalline alumina

The invention describes a process for the production of coarse crystalline alumina from aluminum hydroxide, which after preparation by known methods is advantageously useful for abrasives, lapping and polishing purposes or for refractory materials. The process is characterized by aluminum hydroxide, either directly or after previous partial dehydration in the range 200.degree.-550.degree. C., being added with a mineralizer of compound type x(BF.sub.4).sub.n, wherein x stands for NH.sub.4 and metallic elements and n is for the formal valence of x, and subsequently is subjected to an elevated temperature treatment between 750.degree. C. and a value higher than the phase change temperature to .alpha.-Al.sub.2 O.sub.3.

Process for the production of coarse crystalline alumina

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Bergheim, Germany

Bernhard Schepers