Synthesis of High-Whiteness Ultrafine Anhydrite from Phosphogypsum: A New Method for Activating Persulfate in the Transformation Process of Sulfuric Acid–Persulfate Medium

Abstract Phosphogypsum (PG) is a solid waste generated during the wet-process production of phosphoric acid, which severely restricts the development of the phosphorus chemical industry. The conventional crystal transformation process of PG suffers from high temperature, high energy consumption, and limited improvement in product whiteness. In this study, an ultrasound-assisted crystal transformation and impurity removal process in the H2SO4–(NH4)2S2O8 system was developed for the preparation of anhydrite (AH). The effects of H2SO4 concentration, (NH4)2S2O8 concentration, ultrasonic temperature and time, liquid-to-solid ratio, and ultrasonic power on the leaching rates of P and F, the degradation rate of organic matter, and the whiteness of the product were investigated. The optimal conditions were determined as follows: 25% H2SO4, 6% (NH4)2S2O8, 75 °C, an ultrasonic power of 200 W, a liquid-to-solid ratio of 5:1, and an ultrasonic time of 6 h. Under these conditions, the leaching rates of P and F reached 99.76% and 95.12%, respectively; the degradation rate of organic matter was 83.07%; the purity of AH was 98.40%; and the whiteness was 91.50%. XRD, FT-IR, SEM, and kinetic analyses revealed that the leaching of P and F impurities occurred concurrently with the dissolution of dihydrate gypsum and the recrystallization of anhydrite. The improvement in whiteness is attributed to the synergistic effect of impurity leaching and organic matter degradation. This crystal transformation method provides a new approach for the preparation of high-value-added anhydrite from PG.