PHASE-FORMATION IN THE SYSTEM CAO-SEO2-H2O

The chemistry of the system CaO-SeO2-H2O has been investigated at 23 d egrees and 80 degrees C. The existence of four major phases in this sy stem-CaSeO3 . H2O, Ca(HSeO3)(2) . H2O, Ca-2(SeO3)(Se2O5), and Ca-2(HSe O3)(2)(Se2O5)-has been confirmed, and the equilibria among them determ ined. At 23 degrees C, CaSeO3 . H2O and Ca(HSeO3)(2) . H2O are the sta ble calcium selenites, CaSeO3 . H2O exists over the pH range 12.5-3.7, making it the most important phase for natural systems as well as mos t technological applications. Ca(HSeO3)(2) . H2O is the more acidic ph ase; it exists over the pH range 3.7-0.4. Incongruent dissolution of C a(HSeO3)(2) . H2O forms CaSeO3 . H2O, These results indicate Ca-2(HSeO 3)(2)(Se2O5) is not a stable phase at 23 degrees C. At 80 degrees C, C aSeO3 . H2O, Ca-2(SeO3)(Se2O5), and Ca-2(HSeO3)(2)(Se2O5) are the stab le calcium selenite phases, Ca(HSeO3)(2) . H2O has not been observed, Again, CaSeO3H2O exists over the broadest pH range, 11.0-3.9. Ca-2(SeO 3)(Se2O5) and Ca-2(HSeO3)(2)(Se2O5) dissolve incongruently, The format ion of Ca-2(SeO3)(Se2O5) and Ca-2(HSeO3)(2)(Se2O5) at 80 degrees C dem onstrates the increasing stability of the Se2O52- ion and the decreasi ng stability of the HSeO3- ion with increasing temperature.