The effect on the crystallization of calcium sulfate hemihydrate with the addition of magnesium in an MSMPR crystallizer

The effect of Mg2+ ions on the purity of calcium sulfate hemihydrate (HH) in concentrated phosphoric acid solution (wt% P2O5 39) was studied, using a continuously mixed suspension mixed product removal (MSMPR) crystallizer. Impurity distribution into crystals in the MSMPR crystallizer was considered by the population balance equation. The presence of Mg2+ ions make the crystals needle-like and agglomerated in large quantities, leading to more three-dimensional defects formation. This effect on crystals increases the uptake ratio (U) of phosphate and reduces HH purity and phosphorus yield. Through the scanning electron microscopy (SEM) and the focused beam reflectance measurement (FBRM), the effect of Mg2+ ions in inhibiting the radial growth of crystals and increasing the critical nuclear radius (rcrit) of crystals in solution was discovered. The influence of Mg2+ ions is crucial for the preparation of HH in the phosphoric acid hemihydrate process.