Production of cobalt-doped ferrimagnetic iron oxides

Claim 1

A process for the production of a cobalt-doped iron oxide acicular pigment protected at least partly against sintering comprising forming a suspension of FeOOH seeds in water containing zinc, phosphate and ferrous ions, introducing cobalt ions, adding alkali and air to precipitate ferric and cobalt ions onto the seeds, adding a phosphate solution in quantity of from about 0.2 to 1% by weight of P 2 O 5 into the water containing the seeds which are now pigment, separating the pigment from the suspension, reducing the pigment to Fe 3 O 4 , oxidizing the Fe 3 O 4 pigment in an oxygen-containing gas to bring the FeO content in the pigment to about 16 to 24% by weight, and heat treating the oxidized pigment under inert conditions at a temperature of about 600° to 800° C., the amount of zinc, cobalt and phosphate ions present being such as to impart to the end product a content by weight of about 0.7 to 2.4% of cobalt, about 0.1 to 2% of zinc, and about 0.1 to 2% of phosphorus expressed as P 2 O 5 , the coercive force of the resulting pigment amounting to at least about 425 Oe+175 Oe/%, by weight, of Co.