Verfahren zur Herstellung von feinteiligem mikrokristallinem alpha-Aluminiumoxyd

摘要

A microcrystalline a -Al2O3 powder having crystallites of less than 160 mm , a surface area of greater than 10m2/gm., and a packed density of at least 2 gm/c.c. at 10 tons/inch2, is obtained by ballmilling coarse a -Al2O3, then, in either order, (a) extracting all materials other than a -Al2O3 with aqueous HF, and contacting with a base, and (b) peptizing the particles of the required size, and separating off the larger particles. The final a -Al2O3 may contain 0.01-3% of metal oxides, e.g. Cr2O3, Ti2O3/t and Fe2O3, which form solid solutions with Al2O3. The coarse a -Al2O3 used may be macrocrystalline a -Al2O3 which is first crushed to particles of size 1-100m , or it may be obtained by heating another form of alumina in the presence of SiO2 at 1150-1250 DEG C. Other forms are colloidal boehemite, fine g -Al2O3, e.g. from burning AlCl3, Al(OH)3 precipitated from Al(NO3)3 with NH3, and Al2O3 from pyrolysis of Al acetate; this alumina is mixed with 1-2.5% SiO2, e.g. by impregnating with ethyl orthosilicate, which is hydrolysed in situ, or by mixing a colloidal suspension with a 1% SiO2 sol, gelling with NH4OH and heating to remove NH3. Milling of the coarse a -Al2O3 may be carried out in rotary or vibrating mills using steel balls in water or ethylene glycol. The Al2O3 may then be purified with 10-20% HCl. The HF treatment, with 5-50% acid, is continued until less than 1% of soluble material remains and the Al2O3 is then contacted with the base, e.g. NH4OH or tetramethyl ammonium hydroxide, and washed. Peptization may be effected with (a) HCl to a pH of 3, (b) Na pyrophosphate, polymetaphosphate, silicate or lignosulphonate or the Na salt of EDTA, or (c) proteins or fish glue. Larger particles are separated by sedimentation or fractional centrifugation. The dilute Al2O3 sol may be concentrated by flocculating with NH3 and acidifying the slurry with HCl to pH3, or by centrifuging or water evaporation. It may be purified by ion exchange. Powdered Al2O3 is obtained by flocculating with ammonium carbonate or bicarbonate and drying, or by evaporation. Dense bodies of Al2O3 can be obtained by moulding the powder at 4000 p.s.i. and 1500 DEG C. An organosol may be obtained by mixing a 5% aqueous sol with chloroform in the presence of ammonium myristate or orthotoluic acid, then adding a hydrocarbon, e.g. xylene and distilling off the chloroform.