Montedison S.p.a.

Giuliano Fagherazzi

Giuseppe Sironi

Francesco Ferrero

Bruno Viviani

Ariello Rolando Corradi

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 process is disclosed for preparing metallic powders based on iron and suitable for magnetic recording, starting from acicular particles of an iron compound selected from the class consisting of .alpha.-Fe.sub.2 O.sub.3, .alpha.-FeOOH and Fe.sub.3 O.sub.4, by reduction with a gas containing more than 50% by volume of hydrogen, characterized in that the particles of the iron compound are treated with titanium, in the form of TiO(OH).sub.2 or TiO.sub.2, and with tin, in the form of Sn(OH).sub.2 or SnO(OH).sub.2 ; after which they are subjected to at least a heat treatment in air at 400.degree.-550.degree. C; and finally to a reducing treatment at 340.degree.-420.degree. C. The titanium is employed in an amount of 0.5-1% by weight, calculated as TiO.sub.2, and the tin in an amount of 3-6% by weight, based on pure iron. Prior to reduction, the particles of the iron compound may be treated not only with titanium and tin, but also with cobalt and/or silica, the cobalt in an amount not exceeding 6% by weight with respect to pure iron and the silica in an amount not exceeding 5% by weight with respect to pure iron. The metallic powders based on iron suitable for magnetic recording, and containing 0.5-1% by weight, with respect to pure iron, of titanium calculated as TiO.sub.2, and 3-6% by weight of Sn, with respect to pure iron, and having an average particle length ranging from 0.3 to 1.mu. and an average particle diameter ranging from 0.1 to 0.4 .mu.; with or without a maximum of not more than 5% by weight of SiO.sub.2 with respect to pure iron and not more than 6% by weight of Co with respect to pure iron, are also disclosed.

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Novara, Italy

Giuliano Fagherazzi