Effect of H2S concentration in gasified gas on the microstructure and leaching properties of coal slag

•The effects of reducing atmospheres and H2S concentration on microstructure and leaching behavior of gasified slag were investigated.•Adding of H2S causes heavy metals enrichment in the iron phase under a strongly reducing condition.•Heavy metals are easy to co-precipitate with Fe/FeSx in iron phase.•H2S has no obvious effect on microstructure of slag under a mildly reducing condition. During coal gasification, the resulting slag varies in the mineral compositions and amount due to the ambient gas in gasifier. In this study, the microstructure and leaching behavior of typical slag were investigated under simulated mildly/strongly reducing gasification conditions with different H2S concentration. XRF, XRD and SEM-EDX were used to study the properties of slag. And ICP-AES was used to determine the metallic ions in acid-digested slag samples. The results indicate that iron phase was separated from molten slag phase in melting process under a strongly reducing condition. In the strongly reducing condition with H2S, FeSx(x