Comparative Mineralogical Characterization and Metal Leaching Behavior of Phosphogypsum Produced by Hemihydrate–Dihydrate and Dihydrate Processes

Phosphogypsum (PG) produced by different wet-process phosphoric acid routes exhibits distinct mineralogical characteristics and metal ion release behaviors, which may influence its environmental risk and resource utilization potential. In this study, PG generated from the hemihydrate–dihydrate (HH-DH) and dihydrate (DH) processes was systematically investigated to elucidate the occurrence states, distribution characteristics, and leaching behavior of metal ions. Particle size distribution analysis, water-leaching experiments, Advanced Mineral Identification and Characterization System (AMICS), and Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) were employed to characterize the particle size distribution, mineral composition, elemental distribution, and mineral liberation behavior before and after leaching. The results showed that gypsum was the dominant mineral phase, while quartz and phosphorus-containing gypsum (PCG) were the major impurity minerals. K and Na mainly occurred in soluble mineral phases and exhibited high leaching rates, whereas Al, Fe, and Mg were primarily associated with relatively stable insoluble minerals and showed limited leaching behavior. After water leaching, the particle size distribution shifted toward finer particles, accompanied by decreases in gypsum content and mineral liberation degree, especially in HH-DH phosphogypsum. Significant differences in mineral association, metal occurrence states, and ion release behavior were observed between HH-DH and DH phosphogypsum. These findings provide a mineralogical basis for impurity removal, water washing pretreatment, environmental risk control, and the resource utilization of phosphogypsum.