Study on the Kinetics of Hydration Transformation from Hemihydrate Phosphogypsum to Dihydrate Phosphogypsum in Simulated Wet Process Phosphoric Acid

Abstract

The key technology of wet process phosphoric acid recrystallization is phosphogypsum phase transformation. In this study, the hydration of α-hemihydrate phosphogypsum (α-HH) to dihydrate phosphogypsum (DH) and the influence of process parameters on hydration kinetics are performed by modifying a dispersive kinetic model in the simulation of wet process phosphoric acid recrystallization. Results show that the modified dispersive kinetic model is very important in describing the entire kinetic process, indicating that α-HH–DH hydration includes induction of nucleation and growth restriction. The hydration rate of α-HH–DH substantially accelerates with the decrease of temperature and phosphoric acid concentration because the activation entropy of the reaction increases during the induction stage and the growth stage, which reduces the activation energy barrier. Moreover, the hydration rate of α-HH–DH considerably accelerates with the increase of SO42– ion concentration. Activation entropy increases in the induction stage, causing the activation energy barrier to decrease. Activation enthalpy increases in the growth stage, causing the activation energy barrier to decrease. The influence of process parameters on the rate of the α-HH–DH hydration reaction follows the order SO42– ion concentration > phosphoric acid concentration > temperature. Therefore, controlling the three parameters of temperature, phosphoric acid concentration, and SO42– ion concentration are important for improving the conversion rate of α-HH–DH and the purity of DH products in the production of wet process phosphoric acid.