Study on the solubility and morphology of calcium sulfate dihydrate in nitric acid and phosphoric acid aqueous medium

In order to investigate the solubility of CaSO₄·2H₂O at different contents of H₃PO₄ and HNO₃ aqueous medium, a series of tests was designed for determining the solubility of CaSO₄·2H₂O at 25°C and 65°C in different acidic concentration solvents, the crystal morphology of the deposition solid-phase after liquid-solid equilibrium was examined by microscope, and the crystalline of the deposition solid-phase was characterized by XRD. The results showed that the solubility of CaSO₄°2H₂O increases gradually with the increase of H₃PO₄ and HNO₃ content, the solubility at 65°C is always higher than at 25°C. In the mixed acid solvent system, when changing the content of HNO₃ at a fixed H₃PO₄ content of 20%, the solubility of CaSO₄·2H₂O increases with the increasing content of HNO₃; however when changing the content of H₃PO₄ at a fixed HNO₃ content of 10%, the solubility first increases and then decreases with the increase of H₃PO₄ content. The micrographs showed that the crystal morphologies of CaSO₄·2H₂O were also changed at the different experimental conditions, but basically were rod-like and flat-long flake. XRD analysis showed that the deposition solids were basically a stable form of CaSO₄·2H₂O, but when both the H₃PO₄ concentration and HNO₃ concentration is 20%, the characteristic peaks of CaSO₄·2H₂O all disappear, and only at 2θ = 25.41 ° a weak peak of CaSO₄·1/2H₂O comes forth, this means the crystal type of deposition solid-phase at this condition has been transformed and the crystal particles are also very small.