AGGLOMERATION KINETICS OF CALCIUM-SULFATE HEMIHYDRATE CRYSTALS IN SULFO-PHOSPHORIC SOLUTIONS

In the present study we aim at describing the agglomeration kinetics o f calcium sulphate hemihydrate crystals, growing in concentrated sulph o-phosphoric solutions simulating an industrial process of phosphoric acid production. The experiments were carried out at 90 degrees C, wit h 40 wt% P2O5 and different excesses in H2SO4. population densities an d supersaturation were determined by periodically removing suspension samples. Agglomeration kinetics were deduced from the volume growth ra tes and agglomeration kernel calculated from the population density mo ments of order 0, I and 2. Agglomeration rates are linear functions of supersaturation if the excess in H2SO4 is less than 2%. At 2% and 4%, agglomeration is maximum for relative supersaturations of 10% and 25% , respectively. Beyond an excess of 4% in H2SO4, agglomeration is low. At very high supersaturations, the aggregates are brittle due to crys tal habit elongation. At last, it is shown that P2O5 loss is due rathe r to agglomeration than to incorporation of phosphate in the calcium s ulphate hemihydrate crystal lattice.