Reductive pyrolysis of heavy metal Gypsum: Impurity phase evolution and transformation pathways

This study aims to clarify the non-linear reaction pathways and competitive interactions in the reductive pyrolysis of Acidic Water Treatment Gypsum (AWTG), which are currently impeded by complex impurity compositions. Representative AWTG was synthesized using log-normal distribution analysis of industrial data, and its pyrolysis mechanisms were systematically investigated via TG-MS, XRD, and SEM-EDS under varying carbon-to-sulfur (C/S) ratios. The results demonstrate that the final speciation of iron and arsenic is governed by the interplay between initial speciation and the reducing atmosphere. Based on the raw material formation process, the evolution of three distinct impurity forms was elucidated: (1) species in the Ca-S-O matrix, (2) As-Fe co-precipitates, and (3) independent mineral phases. Specifically, the in-situ generation of CaO and CaFe₂O₄ acts as a reduction barrier, constituting the predominant mechanism for arsenic immobilization in carbon-deficient environments. These findings provide a theoretical foundation for raw material selection and targeted impurity control in the industrial valorization of heavy metal gypsum.