Conditions for the Formation of Fe3O4 by the Air Oxidation of Fe(OH)2 Suspensions.

Abstract

Abstract The optimum conditions were studied for the formation of Fe3O4 by the air oxidation of Fe(OH)2. The suspensions obtained by mixing NaOH and FeSO4 solutions in various values of R(=2NaOH/FeSO4) were subjected to oxidation with air at various temperatures. The oxidation products were then examined by X-ray powder diffraction, chemical analysis, electron microscopic observation, and BET surface-area determination. Fe3O4 is formed at higher temperatures than is FeOOH. The temperature of formation becomes low as R approaches 1.0. In neutral suspensions (R<1), Fe3O4 is formed via green rust II or a mixture of green rust II and Fe(OH)2. By further oxidation, the Fe3O4 formed gradually changes to γ-Fe2O3. A mixture of α-FeOOH and either NaFe3(OH)6(SO4)2 or α-Fe2O3 is formed as the final oxidation product. In alkaline suspensions (R>1), Fe3O4 is formed directly. The morphology and ferrous-ion content of Fe3O4 powder change considerably with the presence of green rust II before the formation of Fe3O4. It is suggested that Fe3O4 is formed near the surface of the particles of Fe(OH)2 and green rust II by the coprecipitation of ferrous ions with ferric hydroxo-complexes.