Montedison S.p.a.

Bruno Viviani

Giuseppe Sironi

Francesco Ferrero

Gian Lorenzo Marziano

Giuliano Fagherazzi

Agostino Geddo

Angelo Garberi

A process is disclosed for the preparation of black and brown thermostable pigments consisting essentially of mixed phases of iron and manganese oxides, characterized in that: (a). pyrolusite, in the form of particles having a mean diameter not exceeding 20 microns, is mixed and maintained in contact together with heptahydrated ferrous sulphate at a temperature between room temperature and 95.degree. C.; and (b). the mass thereby obtained is dried, crushed and screened, and then calcined in the presence of air in a fluidized bed reactor at a temperature between 800.degree. and 900.degree. C. The starting pyrolusite/heptahydrated ferrous sulphate mixture may be additioned with one or more compounds selected from the group consisting of FeCl.sub.2, SiO.sub.2, FeCl.sub.3, Na.sub.2 SiO.sub.3, Fe.sub.3 O.sub.4, NaAlO.sub.2, KAl(SO.sub.4).sub.2 and NaBO.sub.2. The total quantity of additives is between 0.5% and 5% by weight based on the final product. The starting pyrolusite/heptahydrated ferrous sulphate mixture may also be additioned with water in a quantity not exceeding 20% by weight. The residence time of the solids in the fluidized bed reactor is between 1 and 6 hours. The black and brown thermostable pigments obtained from the process are also claimed.

Process for the preparation of thermostable black and brown pigments and

A process is disclosed for preparing .alpha.-Fe.sub.2 O.sub.3 in the form of acicular particles substantially free from occluded boils or cavities, and containing from 0.2 to 2% by weight of SiO.sub.2, and having a mean axial ratio between 6 and 8, and an intrinsic coercive force of 320 - 370 Oe. An aqueous solution of FeSO.sub.4. 7H.sub.2 O is treated with alkali, in the presence of air, thus forming acicular .alpha.-FeOOH, which is then dehydrated, reduced at Fe.sub.3 O.sub.4 under fluidizing conditions, and oxidized to .gamma.-Fe.sub.2 O.sub.3 at 200.degree.-350.degree. C under fluidizing conditions. The formation of the acicular .alpha.-FeOOH is carried out at a yield of 0.5-2.5 kg of ferric Fe per hour per m.sup.3 of reactor; the reaction product is filtered and washed and the .alpha.-FeOOH particles are coated with 0.2-2% by weight of SiO.sub.2, from an aqueous suspension of .alpha.-FeOOH brought to a temperature ranging from 50.degree. to 90.degree. C, adding Na.sub.2 SiO.sub.3, and adjusting the final pH to 5.5-6.5; the .alpha.-FeOOH treated with SiO.sub.2 is dehydrated and simultaneously reduced to Fe.sub.3 O.sub.4 at a temperature between 480.degree. and 550.degree. C with a gaseous mixture selected from the class consisting of H.sub.2 + H.sub.2 O, CO + CO.sub.2, and H.sub.2 + CO + H.sub.2 O + CO.sub.2.

Process for preparing ferrimagnetic acicular ferric oxide

A method is disclosed for controlling the particle size of equiaxial magnetite obtained by a process for preparing same starting from ferrous sulphate solutions, characterized in that the reaction forming the magnetite is conducted in a suspension to which .gamma. FeO(OH) has been added in such an amount that Fe of the .gamma. FeO(OH) is 0.1-10% of the Fe.sup.+.sup.+ initially present in the solution.

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Novara, Italy

Bruno Viviani