Experimental and numerical study on immobilization and leaching characteristics of fluoride from phosphogypsum based cemented paste backfill

Phosphogypsum (PG) is the typical by-product of phosphoric acid and phosphate fertilizers by acid digestion. The application of cemented paste backfill was feasibly investigated for the remediation of PG. The present study evaluated the fluorine immobilization mechanisms and attempted to construct a related thermodynamic and geochemical modeling to describe the stabilization performance. Physico-chemical and mineralogical analyses were performed on PG and hardened PCPB. The correlated macro and microstructural properties were obtained from analyzing the combination of unconfined compressive strength and SEM-EDS imaging. Acid/base dependent leaching tests were performed to ascertain the fluoride leachability. Additionally, GEMS and Phreeqc were applied in this study as tools to characterize the PCPB hydration and deduce its geochemical characteristics. The results proved that multiple factors are involved in fluorine stabilization, among which the C-S-H gel was found to be associated with retention. Besides, the concentration of acid/base highlights in regulating the leaching behavior. Although the quantitative comparison with the experimental data shows further modification should be introduced into the simulation before being used as a predictive implement to determine PG management options, the modeling enabled the identification of the impurity phases controlling the stability and leachability.