Efficient precipitation of soluble phosphorus impurities in the recycling of phosphogypsum to produce hemihydrate gypsum

Phosphogypsum (PG), a waste product of the phosphate fertilizer industry, is a source of environmental pollution. PG can be calcined to form the hemihydrate gypsum (β-CaSO4•0.5H2O, HPG), making it a potential substitute to natural gypsum. However, PG contains large amounts of soluble phosphorus(P) impurities, which pose a serious risk on the soil and groundwater, especially when they are abundantly accumulated and leached. This study systematically investigated that the pretreatment of PG by calcium hydroxide (CH) solution. Inductively coupled plasma optical emission spectroscopy (ICP) and X-ray diffraction (XRD) were used to study the precipitation of soluble P impurities in PG, differential thermal analysis (DTA) and BET (Brunauer-Emmett-Teller) were used to study the effect of CH solution on the physicochemical property of PG. In addition, the hardening characters and hydration process of HPG after stabilization with CH solution were studied in detail. The results show that pretreatment with CH solution can stabilize the soluble P impurities. However, the addition of CH solution can slightly reduce the mechanical properties of HPG and prolong the setting time. This was attributed to the production of excess CaHPO4·2H2O, which encapsulates the HPG. It is concluded that pretreatment of PG with 0.05% CH solution reduce environmental pollution without affecting the properties of the HPG produced.