Process for producing ferrogmagnetic iron oxide powder comprising a pre-treatment with a reducing agent

Claim 1

A process for producing a ferromagnetic iron oxide which comprises: treating a water or water-miscible organic solvent suspension of a ferromagnetic iron oxide with a reducing agent present in said suspension for a period of 3 minutes to 5 hours, removing any unoxidized reducing agent, adding an aqueous solution containing Co +2 ions or Co +2 ions and one or more other metal ions and an alkali solution to the suspension and heating; wherein: said reducing agent is used in an amount of 0.01 to 30 weight % based on the iron oxide and said reducing agent has a standard electrode potential of not greater than -0.4 V; said ferromagnetic iron oxide is maghemite, magnetite, FeOx where 1.33 < x < 1.50, or partially oxidized metallic iron; said Co +2 ions or said Co +2 ions and the other metal ions are present in an amount of 0.5 to 10 atomic % based on the amount of Fe present in the iron oxides; said other metal ions are selected from the group consisting of Cr +3 , Cr +6 , Mn +2 , Fe +2 , Fe +3 , Ni +2 and Zn +2 , and when said other metal ions are present, the ratio of Co +2 ions to the other metal ions is at least 1/3; said alkali is present in an amount such that the hydroxyl ion concentration after neutralizing the Co +2 ions or Co +2 and said other metal ions is at least 0.5 N; said heating is at 8°-150° C and; wherein the ferromagnetic iron oxide so produced has a higher coercive force than that obtained from said process without said treatment with said reducing agent.