Phase equilibria in the Fe2O3-SeO2-H2O system

The Fe2O3-SeO2-H2O system was investigated by allowing amorphous ferric selenite (Fe-2(SeO3)(3) center dot 4H(2)O) precipitate to react with a solution of selenious acid (H2SeO3) at 25 degrees C for periods up to 3 months. The graphical method based on the chemical analysis of the final wet residue and the associated solution in each experiment for Fe and Se indicated the existence of Fe2(SeO3)3 center dot 5H(2)O (Se(IV) <8.0 mol/L, hereafter "mol/L'' is expressed by "M'') and Fe(HSeO3)(3) (Se(IV) >8.0 M). The powder X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses showed a high degree of crystallinity of both compounds. The precipitates in equilibrium with Se(IV) solutions having different Se(lV) concentration at different temperatures up to 170 degrees C were investigated by differential thermal analysis (DTA) and thermogavimetric (TG) analysis as well as XRD and chemical analyses, for a better understanding of the solid phases involved in the Fe2O3-SeO2-H2O system in a wide temperature range. Fe-2(SeO3)(3) center dot 5H(2)O, Fe-2(SeO3)(3) center dot 3H(2)O and Fe2Se2O7 were detected depending upon the reaction conditions, Se(IV) concentration of the solution phase, and temperature. (c) 2005 Elsevier B.V. All rights reserved.