Gas-solid migration and stabilization of sulfur during thermal treatment of sulfur-rich coal gangue: A multi-waste management strategy

Sulfur-rich coal gangue can release sulfur-containing gases during thermal utilization and mine backfilling; however, the coupling relationship between sulfur transformation and the evolution of Fe-bearing minerals remains poorly understood. In this study, ion chromatography, thermogravimetric analysis, X-ray diffraction, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, and gas analysis were systematically integrated to elucidate sulfur migration behavior during thermal treatment and to evaluate waste-derived sulfur-fixing agents prepared from steel slag and shell powder through water and alkali activation. The results revealed that sulfur occurred predominantly in pyrite and underwent a stepwise transformation of S 2 2− → S n 2− (n > 2)/S 0 → S(VI), mainly as SO 4 2− , accompanied by the oxidation of iron from Fe(II)-S to Fe(II)-O and Fe(III)-O. SO 2 release was most pronounced at approximately 500 °C, identifying the critical temperature range for sulfur-emission control. The alkali-activated agent maintained an SO 2 inhibition efficiency above 96.67% at 300-500 °C and increased sulfur retention from 41% to 70% compared with the water-activated agent. Although alkali activation did not substantially alter the CaCO 3 -dominated crystalline composition, it promoted reactive sulfur capture and sulfate formation in the solid residue. This study establishes a coupled Fe–S transformation mechanism and demonstrates a sustainable waste-derived approach for converting gaseous sulfur emissions into stable solid phases, providing new insights into the mitigation of sulfur-related environmental risks during sulfur-rich coal gangue utilization.