Dehydration behaviour and impurity change of phosphogypsum during calcination

The preparation of β-hemihydrate gypsum is an important approach for large-scale utilization of phosphogypsum (PG) because of the simple production technology by calcination. However, the dehydration behaviour and impurity change during the transformation of PG to β-hemihydrate gypsum at low temperatures are unclear. In this study, the dehydration of PG at different temperatures was investigated by a calcination test and in-situ X-ray diffraction analysis. The variation in crystal morphology and structure of PG was clarified using scanning electron microscopy and Materials Studio software. Furthermore, the change of impurity was analyzed by energy-dispersive spectroscopy, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy measurements. The results show that the dehydration of PG to β-hemihydrate gypsum depends on calcination temperature and time. Moreover, an increase in the calcination temperature could accelerate the dehydration of PG and shorten the dehydration time; meanwhile increasing the calcination time or calcination temperature could make a part of β-hemihydrate gypsum further dehydrate to anhydrite. The morphology and elemental distribution of PG did not dramatically change during calcination, but the grain size decreased and the elemental distribution density increased. It should be noted that the calcination did not decrease the phosphorus and fluorine contents in PG at 120 °C, whereas the contents increased due to the removal of crystal water. The phosphorus impurity form transformed from CaHPO4·2H2O and Ca(H2PO4)2·H2O to CaHPO4 and Ca(H2PO4)2, respectively. Therefore, the calcination could not remove the phosphorus and fluorine from PG, and pretreatment is required to reduce the adverse effects of impurities in the industrial production of β-hemihydrate gypsum.