Effects of phosphorus impurities on the preparation of α-calcium sulfate hemihydrate from waste phosphogypsum with the salt solution method under atmospheric pressure

Abstract

Impurity is an important factor that determines the crystallization process of α-calcium sulfate hemihydrate (α-CSH), but the effects of phosphorus impurities in waste phosphogypsum (PG, >75% CaSO4·2H2O) on the α-CSH prepared using the salt solution method are still not well known. In this context, the phase transformation of PG to α-CSH was conducted in a concentrated Ca(NO3)2 solution at 95 °C under atmospheric pressure. The effects of analytical reagents Ca(H2PO4)2·H2O, CaHPO4·2H2O, Ca3(PO4)2 and H3PO4 used as impurities on the transformation kinetics, crystal morphology and particle fineness of the products were investigated in detail. The results show that the presence of Ca(H2PO4)2·H2O, CaHPO4·2H2O and Ca3(PO4)2 impurities retards the transformation process, and these phosphates have a more significant retarding effect on the induction period than the growth period of α-CSH. The low content of H3PO4 (i.e. 0.21 wt%) accelerates the transformation process, but with high concentrations of H3PO4 (i.e. 0.41 and 0.62 wt%), the phase transformation is inhibited. Except for Ca3(PO4)2, all the impurities studied are found to result in large diameters of the formed α-CSH crystals as well as big particle fineness. Moreover, at high concentration of Ca(H2PO4)2·H2O or CaHPO4·2H2O (e.g., 0.53 wt% Ca(H2PO4)2·H2O), partial transformation of dihydrate to anhydrite is induced. This research can provide useful information for selecting an appropriate PG pretreatment method to make it a suitable material for α-CSH production.