CRYSTALLIZATION OF CALCIUM-SULFATE HEMIHYDRATE IN CONCENTRATED PHOSPHORIC-ACID SOLUTIONS

We aim to show the existence of agglomeration by measuring and modelli ng secondary nucleation and crystal growth rates of calcium sulphate h emihydrate, CaSO4-0.5 H2O, in concentrated phosphoric acid solutions, Using a batch crystallizer we measured the evolution of the population density as a function of supersaturation, H2SO4 excess and stirring r ates. All experiments were carried out at 90 degrees C in solutions at 40 wt.% of P2O5, simulating the usual conditions for crystallizing he mihydrate in the industrial processes of phosphoric acid production. N ucleation and growth rates were calculated from the population number densities, using the moments analysis method. A model is presented for describing the crystallization process of hemihydrate. It is shown th at secondary nucleation and growth rates are quadratic functions of su persaturation. H2SO4 concentrations affect supersaturation but at the same supersaturation the growth rates are not significantly different. Nucleation is independent of the stirring rate, whereas growth rates are slightly affected for stirring rates up to 500 rpm. Taking agglome ration into account, the moments method fits very well the experimenta l data. (C) 1997 Elsevier Science S.A.