Hydrated alumina is utilizable as a precursor for preparing the various types of crystalline alumina consisting essentially of spherical particles which retain the same morphological and granulometric characteristics of the precursor. The preparation of alpha-alumina is described in particular in a preceding patent application filed by the Applicant hereof.
In its various crystalline forms, alumina has a wide range of uses, for example as material for electronics, in the preparation of substrates for semiconductors and in packing of integrated circuits, as ceramic material for manufacturing cutting tools, or as catalyst or catalyst support in heterogeneous catalysis.
From the technical and patent literature various methods are known for preparing hydrated alumina in the form of spherical, monodispersed or polydispersed with a low polydispersion index, and non-aggregated particles.
In Journal Inorg. Nucl. Chem. 1973, vol. 35, pages 3691-3705 (Roger Brace and Egon Matijevic) there is described a method for preparing hydrated, spherical, monodispersed alumina, having sizes below one micron prepared from aluminum sulphate solutions through forced hydrolysis at temperatures higher than 90.degree. C., of the order of 100.degree. C., during a time ranging from a few hours to a few days.
According to this article, it is of substantial importance to operate with aluminum sulphate solutions having Al.sup.3+ concentration ranging from 2.10.sup.-4 to 5.10.sup.-3 moles/l in order to obtain products in the spherical and monodispersed form.
By operating with the concentrations indicated to be essential for obtaining alumina in the form of spherical and monodispersed particles, the precipitated hydrated alumina reaches the maximum value of 33% by weight after 4 days starting from Al.sup.3+ solutions at concentrations of 1.10.sup.-3 moles/l.
By raising the Al.sup.3+ concentration of the starting solution--the forced hydrolysis time being the same--the percentage of precipitated hydrated alumina tends to decrease. If hydrolysis is carried out at the same initial Al-salt concentrations during longer times, a re-dissolution of the precipitated alumina takes place, with further lowering of the precipitation yield.
The method described hereinabove is of no practical use from an industrial viewpoint, as the hydrated alumina productivity is very low, on the order of 1 mg/l. hour.
It is known too that if the forced hydrolysis of aluminum sec-butoxide is carried out in the presence of sulphate ions, it is possible to increase the reaction yield till reaching the theoretical yield, however it is necessary to operate always at very low Al.sup.3+ concentrations, generally on the order of 10.sup.-3 moles/l, which lead to a productivity of the order of 50 mg/l. hour.
Processes of the type cited hereinbefore are described for example in "Aluminum Hydrous Oxide Sols. II. Preparation of Uniform Spherical Particles by Hydrolysis of Al-sec-Butoxide" P. L. Catone and E. Matijevic, Journal of Colloid Interface Science, Vol. 48, No 2, August 1974, page 291. By this process it is possible to obtain quantitative yields, but always with very low productivities, which require very complicated processes and utilize aluminum compounds having much higher costs than those of aluminum sulphate.
Also known are other processes for preparing hydrated alumina obtained starting from aluminum salts, such as sulphate, at concentrations of about 0.6 moles/l in the presence of a 10% ammonia aqueous solution.
Processes like this one are indicated in South African Pat. No. 555,052. However, the alumina obtained by this process is not in the form of monodispersed particles, but of polydispersed particles and, above all, aggregates are present because the precipitation does not occur homogeneously.
Other processes are known in which alumina is prepared starting from aluminum sulphate, in the presence of substances capable of releasing OH.sup.- ions under the action of heat so as to allow homogeneous precipitation in shorter times.
The term "homogeneous precipitation" means that any local over saturation phenomenon caused by a rapid contact of the aluminum salt solution with the alkaline solution does not occur.
Also with this method, however, the alumina, although obtained by a quantitative reaction, is not in the form of spherical particles and, mostly, it appears in the form of agglomerates if it is operated with starting Al.sup.3+ concentrations higher than 5.10.sup.-3 moles/l.
There are known further methods for preparing alumina starting from aluminum nitrate or aluminum sulphate in the presence of succinic acid, of NH.sub.4 Cl and urea, which provide a homogeneous precipitation. Neither in these cases, however, it is possible to obtain alumina in the form of spherical particles with a low polydispersion index, as aggregates are always present.
With a view to obtaining hydrated alumina with a controlled morphology and free from aggregates, with high and industrially interesting yields, processes like the ones described hereinabove, which comprise precipitation from solutions, are not utilized, but different techniques are used.
Preparation methods of this type are described e.g. in published patent application EP 117,755, according to which techniques are utilized that involve the preparation of an aerosol of liquid particles of a hydrolizable aluminum compound, which is successively reacted with water vapour to obtain the hydrated alumina in the form of solid particles.