Experimental and theoretical investigation on the mechanism of cationic surfactants-assisted phosphogypsum crystal transformation

The massive discharge and stockpiling of phosphogypsum became an increasingly severe issue on environment. The transformation of phosphogypsum into fibrous alpha-type hemihydrate gypsum (α-HH) with high L/D(ratio of length-to-diameter) is one of the effective routes to achieve the high-valued utilization. In this work, six types of cationic surfactants, including CTAB, STAC, CHEC, CGG, CPAM and PDDA were selected to investigate their different effects on shape of α-HH crystals through hydrothermal method under the condition of 160 °C(0.6 MPa saturated vapour pressure) for 3 h. The results showed that the introduction of all selected surfactants can increase the length-to-diameter (L/D) ratio of α-HH crystals. As the concentration of surfactants increases, the L/D ratio first increases and then decreases, and the highest L/D ratio of 171.3 was obtained by using CTAB at 0.6 %. The mechanism of regulated growth of α-HH crystals into fibrous structure following c-axis in the presence of six cationic surfactants was also demonstrated. Moreover, the adsorption mechanism of cationic surfactants on the α-HH surface was investigated by molecular dynamics simulation and theoretical analysis. It is found that the order of binding energy (Eb) between the addition and α-HH crystal faces is Eb(11¯0) > Eb(110) > Eb(200) > Eb(002). This work provides insightful guidance from both experimental and theoretical perspectives for controlling the crystal form of α-HH and preparing high-strength gypsum.