Effective recovery of calcium and sulfur resources in FGD gypsum: Insights from the mechanism of reduction roasting and the conversion process of sulfur element

Flue gas desulfurized (FGD) gypsum is produced during the removal of SO2 from flue gas. The reductive decomposition of FGD gypsum to calcium sulfide (CaS) is a crucial method for resource utilization. This study investigated the reduction roasting behavior and mechanism to enrich and recycle the calcium and sulfur resources in FGD gypsum effectively. Firstly, thermodynamics calculation revealed that the complete reduction of CaSO4 to CaS is feasible. Experimental results showed that more than 95 % of the FGD gypsum can be reduced to CaS. Subsequently, the kinetic parameters of the reduction process were calculated. The results indicate that the reaction is controlled by the phase boundary, which is a typical shrinking sphere reaction process. Based on the kinetic study, the micromorphological transformation during the reduction process were studied. The results show that the reduction reaction initially starts from the boundary and gradually occurs towards the core. Finally, the reduction roasting mechanism is proposed. The intermediate processes of calcium sulfate reduction behavior were studied through XRD and XPS. The phase conversion and behavior of the main elements S in the process were explored. The results indicate that the process is a step-by-step reduction, and the transformation of S is SO42−-SO32−-S22−-S2−. The reduction product forms a hollowed-out spheroid structure that cannot only effectively enrich Ca and S resources, but also provide advantageous conditions for their subsequent reuse.