Reduction of phosphogypsum for SO2 production: Insights into the release characteristics of SO2 following the solid–solid reaction of CaS and CaSO4 under different atmospheres

Abstract Phosphogypsum (PG) severely pollutes the environment and is difficult to recycle. PG is primarily composed of CaSO 4 · 2H 2 O. In this study, the characteristics of SO 2 released from the solid–solid reaction between calcium sulphide (CaS) and CaSO 4 were thoroughly investigated using thermodynamic calculations. Experiments were performed by tuning the molar ratio of CaS to CaSO 4 , reaction atmosphere (S, CH 4 , N 2 , and air), and the heating rate. As shown by the phase diagram, high reaction temperatures favour CaO stability, and the corresponding maximum SO 2 equilibrium partial pressure increases. The total SO 2 production significantly increased with increasing molar ratio and slightly increased when the ratio exceeded 1:3. The SO 2 productions were ranked from highest to lowest as follows: S, CH 4 , N 2 , CO, and air. The total SO 2 production decreased with increasing heating rate. For the reaction between CaS and CaSO 4 , a higher molar ratio of CaS to CaSO 4 no less than 1:3, both S and CH 4 reductive atmospheres, and a lower heating rate (2°C/min) favour the total SO 2 emission.