Phosphogypsum Decomposition by Char under a CO2 Atmosphere

Abstract

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.