The influence of impurities on phosphoric acid hemihydrate crystallization

The influence of four typical impurities on phosphoric acid hemihydrate (H3PO4 center dot 0.5H(2)O) 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 Fe (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 center dot 0.5H(2)O 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. (C) 2007 Elsevier B.V. All rights reserved.