Phosphogypsum Decomposition by Char under a CO₂ Atmosphere

Reductive decomposition of phosphogypsum for CaO production is proposed to proceed under a CO₂ atmosphere to avoid the presence of CaS in the product. In combination with thermodynamic analysis, thermogravimetric tests are conducted with the monitoring of gas composition to explore the reaction mechanism between C/CaSO₄ and CO₂. Three reaction stages are identified, i.e., CaSO₄ reduction, reaction of CaS with CaSO₄, and CaS oxidation by CO₂. The reaction path depends on the C/CaSO₄ molar ratio and temperature. Further, phosphogypsum decomposition in CO₂ is further conducted in a fluidized bed reactor, and data show that over 96.2% of CaSO₄ in phosphogypsum is decomposed into CaO at 1000 °C and a C/CaSO₄ molar ratio of 1.0 within 83.7 min. Phosphogypsum decomposition in air or nitrogen has lower CaO production. Further, kinetic analyses of CaS oxidation with CO₂ in the presence of CaSO₄ were performed to illustrate the effect of the CaS–CaSO₄ reaction on the CaS–CO₂ reaction.