Phase equilibria in the Fe 2O 3–SeO 2–H 2O system

Abstract

The Fe 2O 3–SeO 2–H 2O system was investigated by allowing amorphous ferric selenite (Fe 2(SeO 3) 3·4H 2O) precipitate to react with a solution of selenious acid (H 2SeO 3) 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 Fe 2(SeO 3) 3·5H 2O (Se(IV) < 8.0 mol/L, hereafter “mol/L” is expressed by “M”) and Fe(HSeO 3) 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(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 Fe 2O 3–SeO 2–H 2O system in a wide temperature range. Fe 2(SeO 3) 3·5H 2O, Fe 2(SeO 3) 3·3H 2O and Fe 2Se 2O 7 were detected depending upon the reaction conditions, Se(IV) concentration of the solution phase, and temperature.