The preparation of magnetic iron oxide

Two methods for the preparation of magnetic iron oxide starting from FeSO₄·TH₂O are described. The traditional method involves preparation of Fe(OH)₂ seed material, conversion of the seed to acicular γ-FeOOH crystals, dehydration of the crystals to α-Fe₂O₃, hydrogen reduction of Fe₂O₃ to magnetite, and finally reoxidation of magnetite particles to γ-Fe₂O₃. Magnetic iron oxide particles thus obtained, after a total processing time of about 25 h, have a coercivity between 330 and 380 Oe. The other method, known as the direct method, involves fewer steps, a shorter overall processing time, and yields magnetic iron oxide having an equivalent or higher coercivity value. A sample from the seed preparation solution is recrystallized at 250°C, yielding a product, γ-Fe₂O₃, which has a coercivity value of 320 Oe. In another procedure the seed is prepared in the presence of H₂O₂ as the oxidizing agent and recrystallized at 300°C for 2 h to yield γ-Fe₂O₃ having 390 Oe coercivity.