Manufacture of gamma-iron(III) oxide

In a process for the manufacture of acicular .gamma.-iron(III) oxide wherein an aqueous solution of an iron(II) salt is reacted with an aqueous solution of an alkali metal hydroxide, the resulting suspensions of iron(II) hydroxide are oxidized with oxygen or oxygen-containing gases to goethite, the resulting goethite is reduced to magnetite and the magnetite is oxidized to acicular .gamma.-iron(III) oxide, the improvement comprising:

• carrying out the oxidation of the iron(II) hydroxide suspension in three stages, wherein in a first stage from 0.1 to 4% by weight of the iron(II) present in the suspension is oxidized over a period of from 0.1 to 4 hours;
• in a second stage from 10 to 25% by weight of the iron(II) hydroxide originally present in the suspension is oxidized over an additional period of 1.5 to 6 hours by introducing at the beginning of said second stage from 0.04 to 0.1 mole per hour of oxygen/gram atom of the iron contained in the suspension and gradually increasing the introduction of oxygen during said second stage up to 0.1 to 0.25 mole per hour of O.sub.2 /g atom of iron at the end thereof;
• and in a third stage oxidizing the remaining iron(II) by introducing 0.3 to 0.8 moles of oxygen per hour per gram atom of iron contained in the suspension.