Polymorph and Morphology Formation of Cerium Carbonate from Reactive Crystallization

Semibatch reactive crystallization and hydrothermal reaction of cerium carbonate were investigated with cerium chloride and sodium carbonate as the reactants in order to determine the key factor and the foremost step that affect the polymorph and morphology. Pure Ce2(CO3)3·8H2O, pure CeCO3OH, and the concomitant polymorphism of the two were obtained at different temperatures and the concentrations of cerium chloride solution. The relative stability of Ce2(CO3)3·8H2O and CeCO3OH, the solubilities of the two forms in low concentration of sodium chloride solution and the dehydration and decomposition processes with heating were first discussed in this study. Rodlike forms, short rodlike forms, platelike aggregations or blocks, regular intercrescence, and regular spindle morphology of cerium carbonate crystals were obtained from semibatch and hydrothermal reactive crystallization, respectively. Raman spectroscopy and a focused beam reflection measuring (FBRM) instrument were applied in situ to monitor the reaction, nucleation, and growth processes. It was found that CeCO3OH is more stable than Ce2(CO3)3·8H2O. The thermodynamics driving force of supersaturation of cerium carbonate generated from the reaction between cerium chloride and sodium carbonate at different temperatures contributed mainly to the nucleation, which, in turn, controls the crystal phase formation and the morphology of cerium carbonate. Finally, the routes for cerium chloride and sodium carbonate to transform into cerium oxide were summarized.