Process for producing iron oxide products from waste liquids containing ferrous salts

A process for producing a spinel-type iron oxide powder comprising the steps of:

• preparing a starting solution having a total Fe.sup.2.sup.+ concentration of 1 to 2 mols/l from a waste liquid containing ferrous salts and manganese compounds as impurities;
• adding an alkali to the solution to suspend 5 to 30 mole % of Fe.sup.2.sup.+ in the solution as ferrous precipitate and to thereby obtain a suspension;
• introducing an oxidative gas into the suspension while maintaining it at a temperature of 60.degree. to 90.degree.C and fully agitating it while continuously adding an alkali to the suspension so as to always maintain the pH of the suspension at a value in the range of 5 to 6 with a tolerance of .+-. 0.05, to thereby form particles of spinel-type iron oxide (FeO.sub.x.Fe.sub. 2 O.sub.3 wherein O<x.ltoreq.1) having a uniform quality;
• and
• suspending the particles of spinel-type iron oxide in an aqueous solution having a pH of 3.5 to 5.0 as adjusted with sulfuric acid or nonoxidative inorganic acid to solubilize polyvalent manganese contained in the particles of spinel-type iron oxide, subsequently separating the particles of spinel-type iron oxide from the suspension by filtration, washing the particles with water, and drying the same to obtain particles of spinel-type iron oxide having a manganese content of less than 0.2% by weight calculated as MnO.