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.

Phosphogypsum (PG) severely pollutes the environment and is difficult to recycle. PG is primarily composed of CaSO4 · 2H2O. In this study, the characteristics of SO2 released from the solid–solid reaction between calcium sulphide (CaS) and CaSO4 were thoroughly investigated using thermodynamic calculations. Experiments were performed by tuning the molar ratio of CaS to CaSO4, reaction atmosphere (S, CH4, N2, and air), and the heating rate. As shown by the phase diagram, high reaction temperatures favour CaO stability, and the corresponding maximum SO2 equilibrium partial pressure increases. The total SO2 production significantly increased with increasing molar ratio and slightly increased when the ratio exceeded 1:3. The SO2 productions were ranked from highest to lowest as follows: S, CH4, N2, CO, and air. The total SO2 production decreased with increasing heating rate. For the reaction between CaS and CaSO4, a higher molar ratio of CaS to CaSO4 no less than 1:3, both S and CH4 reductive atmospheres, and a lower heating rate (2°C/min) favour the total SO2 emission. [ABSTRACT FROM AUTHOR]