Dissolution-Recrystallization Approach to Synthesize Type-II Anhydrite from Phosphogypsum in the H3PO4-H2SO4-H2O System

Type-II anhydrite was successfully fabricated in this work using phosphoric acid and sulfuric acid through a dissolution-recrystallization process using phosphogypsum (PG) as the raw material. The effects of different experimental influencing factors on the molar fraction of anhydrite were discussed. Controlling the optimal conversion conditions such as the initial phosphoric acid concentration of 32% P2O5, the H2SO4/H3PO4 (mass ratio) of 20%, the reaction temperature of 90 degrees C, and the H3PO4/PG (mass ratio) of 5, the molar fraction of the anhydrite reached above 99%, and the morphologies of the anhydrite product were rectangular flakes with the average particle size of 13.45 mu m. The purity and whiteness of the anhydrite were 98.45 and 81.8%. The 28-day compressive strength of the anhydrite was 37.2 MPa. The dehydration conversion of PG into anhydrite in the mixture of phosphoric-sulfuric acid follows the dispersive kinetic model. After impurities (Fe3+, Al3+, Mg2+, and SiF62- ions) were introduced into the system, the activation enthalpy increased, and the activation entropy decreased causing the nucleation barrier of anhydrite crystals to increase, thus inhibiting the dehydration conversion of PG into anhydrite. As the concentration of impurities increases in the mixture of phosphoric-sulfuric acid, the chemical shifts of Ca, S, and O elements in anhydrite are to the left, resulting in a weaker binding energy between Ca2+ and SO42- ions, thereby causing energy of dehydration conversion of PG into anhydrite to increase. The suggested type-II anhydrite synthesized from PG in the H3PO4-H2SO4-H2O system shows promising prospects for applications.