Direct ammonium sulfate production from untreated industrial phosphogypsum under mild conditions: process evaluation

BACKGROUND Industrial phosphogypsum valorization typically requires energy‐intensive pretreatments. This study provides a baseline approach by evaluating direct, single‐step conversion of raw, untreated phosphogypsum into ammonium sulfate and calcium carbonate under mild laboratory conditions (55 °C, 3 h, atmospheric pressure, 1:4 solid‐to‐liquid ratio) using 12–14 wt% NH 4 OH solutions. RESULTS Untreated phosphogypsum demonstrated sufficient intrinsic reactivity to produce crystalline (NH 4 ) 2 SO 4 with a high purity of 96–98.29%, containing 20.31–20.54 wt% nitrogen and 23.03–23.54 wt% sulfur. The highest reaction efficiency and product purity were achieved using a 13 wt% NH 4 OH solution. Due to heterogeneous reaction constraints and mass transfer resistance, the overall conversion efficiency remained moderate at 23.13–29.31%, yielding 1.53–1.93 g of product against a theoretical yield of 6.61 g. CONCLUSION The consistent formation of high‐purity (NH 4 ) 2 SO 4 directly from raw, inactivated phosphogypsum validates the technical viability of this simplified waste‐to‐resource framework. While surface‐related and thermodynamic limitations present clear efficiency boundaries that require future process optimization, the elimination of energy‐intensive pretreatments represents a significant step toward cost‐effective industrial sustainability. This study lays a solid foundation for upcoming pilot‐scale validations, comprehensive mass–energy balances, and integrated CO 2 sequestration strategies.