Process for producing cobalt- and iron-containing ferromagnetic powder by heat-treatment in the presence of an oxidizing agent

Claim 1

A process for producing a Co- and Fe-containing ferromagnetic iron oxide powder which comprises adding to a suspension of an acicular ferromagnetic iron oxide an oxidizing agent, an aqueous alkali solution, an aqueous solution containing Co +2 ions and an aqueous solution of Fe ions and heat treating the resultant mixture, at a temperature of at least about 80° C said CO +2 ions being present in said aqueous solution containing Co +2 ions in an amount of 0.5 to 10 atomic percent based on the amount of iron present in the acicular ferromagnetic iron oxide, said Fe ions being added to the suspension in an amount of at least 0.1 equivalent but not more than 1 equivalent based on the Co +2 ions, said aqueous alkali solution containing an amount of alkali such that after neutralizing the Co +2 ions and the Fe ions, the hydroxyl ion concentration is at least 0.5 mol/liter; and the amount of oxidizing agent, when it is a gas, is at least 0.1 liter/min per liter of the suspension and when it is a liquid or solid, the amount is at least 0.1 equivalent based on the Co +2 ions.