Study on the Crystalline Morphology of Calcium Sulfate in H<sub>3</sub>PO<sub>4</sub> and HNO<sub>3</sub> Aqueous Medium

This study aims to investigate the morphologies and the existing phases of calcium sulfate crystal by the reaction of H2SO4 with Ca(NO3)2·4H2O in acid medium using microscopic method and XRD powder diffraction. A series of experiments were designed for studying the crystal morphology and the hydrated form under coexisitence of phosphoric acid and nitric acid at 338K. The micrograph results show that in the H3PO4-HNO3-H2O system, the crystal shapes are changed from thick-long rod to thin-short rod, then needle-like even tiny needle gradually with the increase of acid content in solution. The solubility of calcium sulfate increases gradually with the increase of phosphoric acid or nitric acid concentration in solution. The XRD results show that with the increasing of acid concentration, the form of calcium sulfate is transferred from dihydrate to hemihydrate, that is, at higher acid concentration condition calcium sulfate hemihydrate is more stable in solution with the particles to be tiny and needle-like; while at lower acid concentration condition the crystals are mainly calcium sulfate dihydrate with larger rod particles which indicate a good filtration properties.