Conditions for the Formation of Fe<SUB>3</SUB>O<SUB>4</SUB> by the Air Oxidation of Fe(OH)<SUB>2</SUB> Suspensions.
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The optimum conditions were studied for the formation of Fe<SUB>3</SUB>O<SUB>4</SUB> by the air oxidation of Fe(OH)<SUB>2</SUB>. The suspensions obtained by mixing NaOH and FeSO<SUB>4</SUB> solutions in various values of <I>R</I>(=2NaOH/FeSO<SUB>4</SUB>) 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. Fe<SUB>3</SUB>O<SUB>4</SUB> is formed at higher temperatures than is FeOOH. The temperature of formation becomes low as <I>R</I> approaches 1.0. In neutral suspensions (<I>R</I><1), Fe<SUB>3</SUB>O<SUB>4</SUB> is formed <I>via</I> green rust II or a mixture of green rust II and Fe(OH)<SUB>2</SUB>. By further oxidation, the Fe<SUB>3</SUB>O<SUB>4</SUB> formed gradually changes to γ-Fe<SUB>2</SUB>O<SUB>3</SUB>. A mixture of α-FeOOH and either NaFe<SUB>3</SUB>(OH)<SUB>6</SUB>(SO<SUB>4</SUB>)<SUB>2</SUB> or α-Fe<SUB>2</SUB>O<SUB>3</SUB> is formed as the final oxidation product. In alkaline suspensions (<I>R</I>>1), Fe<SUB>3</SUB>O<SUB>4</SUB> is formed directly. The morphology and ferrous-ion content of Fe<SUB>3</SUB>O<SUB>4</SUB> powder change considerably with the presence of green rust II before the formation of Fe<SUB>3</SUB>O<SUB>4</SUB>. It is suggested that Fe<SUB>3</SUB>O<SUB>4</SUB> is formed near the surface of the particles of Fe(OH)<SUB>2</SUB> and green rust II by the coprecipitation of ferrous ions with ferric hydroxo-complexes.
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