Deep removal of phosphate impurities in phosphogypsum by two-step crystal transformation for use as Portland cement retarder

The phosphate impurities in phosphogypsum (PG) were removed efficiently by a novel two-step crystal transformation approach, and the mechanism of the two-step transformation method for removing phosphate and metal impurities from PG was described. The effects of modified PG, natural gypsum, and PG as Portland cement retarders on the cement properties were compared and analyzed by XRD, SEM, and hydration heat analyses. Results indicated an optimal sulfuric acid concentration of 30% H2SO4 during the first crystal transformation and a liquid-to-solid ratio of 7:1 during the second crystal transformation. The content of total P2O5 in the modified PG was 0.02%, and part of heavy metal elements was also removed. The first-step crystal transformation removed the intercrystalline and insoluble phosphate impurities by breaking the crystal lattice of gypsum. The second-step crystal transformation rebuilt the crystal lattice of gypsum. The effect of phosphate impurities in the modified PG on cement hydration was significantly reduced, the setting time of the cement prepared with the modified PG was normal, and the strength was increased. The effect of the modified PG was better than that of PG as a cement retarder. The two-step crystal transformation approach provided a cost-effective method for PG purification.