Insight into the effect of maleic acid on the preparation of α-hemihydrate gypsum from phosphogypsum in Na2SO4 solution

Abstract

The control of crystal morphology is crucial to preparation of α-hemihydrate gypsum (α-HH). α-HH was prepared from phosphogypsum (PG) in Na2SO4 solution at different temperatures. The effect of maleic acid (C4H4O4) on the dehydration rate and growth behavior of α-HH was investigated by X-ray diffraction (XRD) and scanning electron microscope (SEM), and the interactions of C4H4O4 molecule with the α-HH surface were studied using density functional theory (DFT) calculations. The results show that the increase in Na2SO4 concentration and solution temperature can shorten the dehydration time of PG. Maleic acid inhibits the conversion of PG to α-HH by prolonging the crystal growth period. α-HH crystals prefer to grow along the c-axis and present a long columnar crystal. With an increase in the maleic acid concentration from 0.00 to 5.76 mM, the morphology of α-HH evolves from a needle-like whisker to a short equiaxial crystal, and the corresponding aspect ratio decreases from 14.19 to 0.91. The calculation results indicate that the strong interactions between the C4H4O4 molecule and the (1 1 1) crystal plane of α-HH are largely attributable to the bonding of both the single-bond oxygen (O1) and double-bond oxygen (O2) in C4H4O4 with the Ca atoms of the (1 1 1) crystal plane and that the chemisorption of C4H4O4 can occur on the (1 1 1) crystal plane with the lowest adsorption energy of −201.65 kJ/mol. C4H4O4 adsorption on the (1 1 0) crystal plane is through a single O2Ca bond with an adsorption energy of −95.32 kJ/mol. The interaction between the (0 1 0) crystal plane and C4H4O4 is weak. It can be concluded that maleic acid can preferentially adsorb on the (1 1 1) crystal plane and retard crystal growth along the c-axis during the preparation of α-HH from PG.