The influence of impurities on phosphoric acid hemihydrate crystallization

The influence of four typical impurities on phosphoric acid hemihydrate (H3PO4·0.5H2O) crystallization in terms of kinetics and morphology is studied quantitatively. A new method for estimating growth rate is developed by taking the slopes of linearlized lines of transient chord length distribution (CLD) of crystals during the process using an in-line device of Focused Beam Reflectance Monitoring (FBRM). The presence of cations can reduce the nucleation rate of phosphoric acid while anions have less effect. Impurities of Fe3+, Al3+, and F− (>50 ppm) can widen the metastable zone to some extent. Adding small amounts of H2SO4 (<150 ppm) can enlarge metastable zone, but such effect becomes less after adding more H2SO4. The presence of the SO42− tends to encourage H3PO4·0.5H2O crystal growth in a wide concentration range with the maximum growth rate at 5000 ppm. Addition of Fe3+ and Al3+ at low level can increase crystal growth rate until it reaches a maximum at 500, 50, and 100 ppm for F−, Al3+, and Fe3+, respectively. Impurities of cations appear to have significant effect on crystal morphology; anions, however, appear not to change crystal shapes much.