Formation Process and Crystal Growth of Zirconium Oxide Sulfate Crystals

Abstract

Zirconium sulfate solutions were hydrothermally treated at temperatures from 200° to 250°C for various periods of time. Crystalline phases, morphology and amount of solid products were investigated to study, especially, the formation process of zirconium oxide sulfate (ZOS), being submicron thin hexagonal crystals with a chemical composition of Zr3O5SO4⋅nH2O. Crystalline phases of the solid products varied with H2SO4 concentration of the solution; at 240°C for 3 days, m-ZrO2 and t-ZrO2, ZOS and/or a pseudo-ZOS phase (PZOS) and ZrOSO4⋅H2O at H2SO4 concentrations below 0.2mol/l, in the range of 0.2-2.0mol/l and above 2.5mol/l respectively, PZOS formed generally at lower temperature or at the early stage of the hydrothermal process, and its amount decreased with increasing H2SO4 concentration. Since the PZOS phase was metastable at higher temperature or higher H2SO4 concentration, it gradually changed to ZOS with a prolonged hydrothermal treatment time. The ZOS crystals were considered to be formed by a neucleation and crystal growth process. The crystal size of the ZOS increased with increasing H2SO4 concentration but was almost independent of Zr concentration. Effect of seeding and repeated treating on the crystal growth of ZOS was confirmed in a solution containing 1.6mol/l H2SO4 at 250°C.