A coupled leaching-settling process driven by phase control for impurity removal and simultaneous valorization of phosphogypsum

Phosphogypsum (PG), the bulk solid waste of wet-process phosphoric acid production, remains underutilized because phosphorus (P) and fluorine (F) impurities restrict its use in building materials, while black impurities lower its whiteness. Conventional purification requires multi-step treatments—yet still fails to remove encapsulated impurities and yields low-value products. Here, a salt-free, one-reaction-stage route was reported in which phase-controlled leaching and settling separation were coupled in a pressurized dilute H 2 SO 4 solution, simultaneously removing all three classes of impurities and converting PG into high-value α-hemihydrate gypsum (HH). During the dihydrate-to-hemihydrate transformation at 120 °C, encapsulated P and F were released and dissolved by 0.4 M H 2 SO 4 . Induction with a short-columnar HH seed further steered crystallization toward large, short-columnar crystals (average length × diameter of 43.47 × 21.70 μm; aspect ratio of 2.0), thereby reducing surface adsorption of impurities and increasing the leaching efficiencies of P and F to 99.20% and 89.39%, respectively. The same morphology control widened the settling-velocity gap between HH and the black impurities to a maximum of over 2.5 mm/s, enabling their separation by simple sedimentation and raising the product whiteness from 49.2% to 84.4%. The resulting HH exhibited a dry compressive strength of 34.8 MPa, meeting the α30 grade of JC/T 2038–2010, with an estimated processing cost of ∼68 USD·t⁻ 1 and an added value of ∼77 USD·t⁻ 1 . This work offers a salt-free, single-reaction-stage route for the deep purification and valorization of PG.