Method for preparing magnetic iron oxide

A method for preparing magnetic iron oxide Fe21.333 O32, comprising the following steps:

• (1) preparing a ferrous salt solution with solid soluble ferrous salt;
• (2) preparing a hydroxide solution;
• (3) mixing said hydroxide solution and said ferrous salt solution in a co-current manner for reaction, controlling said hydroxide solution and said ferrous salt solution at an alkali ratio of 0.6-0.8, controlling said ferrous salt solution at a feeding speed of 200-300 ml/min, and said hydroxide solution at a feeding speed of 50-60 ml/min, and controlling the reaction temperature not exceeding 30° C.;
• (4) after the reaction in step (3) is finished, yielding a first mixture, charging the first mixture with a gas containing oxygen for oxidation, and controlling the first mixture at a pH value of 6-8 until the oxidation is finished to yield a second mixture;
• (5) filtering, washing with water and drying said second mixture to yield a precursor;
• and
• (6) calcining the precursor obtained in step (5) at 250-400° C.