Process for the preparation of magnetite particles and their use

Process for the preparation of a low-Si magnetite having a silicon content of less than 0.025% by weight, a particle size, as determined from electron micrographs, of 0.1 to 0.5 μm, a BET surface area of 3 to 13 m2/g, a DIN pH (measured on the suspended powder) of 4 to 9 and a coercive force of 3 979 to 7 162 kA/m (50 to 90 Oe), comprising:

• a) introducing an alkaline component in the form of an aqueous solution under an inert gas,
• b) heating the alkaline component to a temperature of 30 to 90° C.,
• c) adding an iron component in the form of an aqueous solution in an amount sufficient to present a molar ratio of Fe component to one equivalent of alkaline component of 0.48 to 0.6 , the temperature being kept at 30 to 90° C., and the Fe(II) content of the Fe component being 0.2 to 1.5 mol % of Fe(III),
• d) optionally heating the suspension obtained under c) to a temperature of 60 to 100° C., it being necessary for this temperature to be higher than the temperature mentioned under c),
• e) treating the suspension obtained under c) with an oxidizing agent until an Fe(III) content of more than 65 mol % is present in the iron compound,
• f) adding, after the oxidation under e), an alkaline component in the form of an aqueous solution, optionally under inert gas, to the suspension obtained in e), in an amount sufficient to present a molar ratio of Fe used under c) to one equivalent of total alkaline component used of 0.42 to 0.47,
• g) heating the suspension obtained after f) to 50 to 100° C.,
• h) adding an Fe component in the form of an aqueous solution in an amount sufficient to establish a pH, measured in the suspension, of 6to8,
• i) oxidizing the resulting suspension of h) with an oxidizing agent to an Fe(III) content in the iron compound of more than 65 mol %, based on iron content, and then
• j) filtering the reaction product of I) and washing, drying and milling the residue.