Preparation of α-hemihydrate gypsum from phosphogypsum in recycling CaCl2 solution

Preparing α-hemihydrate gypsum (α-HH) from phosphogypsum (PG) in electrolyte solution and atmospheric pressure has been developed as an energy-efficient and profitable method to dispose PG, which can alleviate its environmental contamination and reduce the use of natural gypsum. However, the used electrolyte solution in this method can trigger secondary pollution if not disposed properly. In this study, the feasibility of recycling electrolyte solution (CaCl2 solution) was investigated. Laboratory results suggested that CaCl2 solution could be recycled for only once due to the significant mechanical strength decrease of products, if it was directly recycled under the same reaction conditions. In contrast, when modifier dosage in each recycle was controlled properly, the α-HH with favourable crystal morphology and relatively high mechanical strength could be prepared. In this way, CaCl2 solution could be recycled for 6 times. Pilot scale tests further demonstrated that α-HH product of each recycle was purer than PG and the harmful substances of the α-HH could meet the requirements of relevant standards. These results suggest that the recycling use of CaCl2 solution in the hydrothermal preparation from PG to α-HH is feasible.