Phase equilibria in the Fe2O3–SeO2–H2O system

The Fe2O3–SeO2–H2O system was investigated by allowing amorphous ferric selenite (Fe2(SeO3)3·4H2O) precipitate to react with a solution of selenious acid (H2SeO3) at 25 °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)3·5H2O (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. ecipitates in equilibrium with Se(IV) solutions having different Se(IV) concentration at different temperatures up to 170 °C were investigated by differential thermal analysis (DTA) and thermogravimetric (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. Fe2(SeO3)3·5H2O, Fe2(SeO3)3·3H2O and Fe2Se2O7 were detected depending upon the reaction conditions, Se(IV) concentration of the solution phase, and temperature.