A microscopic study of the precipitation of metallic iron in slag from iron-rich coal during high temperature gasification

The behavior of iron in molten slag from iron-rich coal during high temperature gasification conditions was studied. The proportion of crystalline phase in the slag was measured by X-ray diffraction (XRD) and the microstructure of the slag was quantitatively analyzed by CCSEM. Metal ions in acid-digested slag samples were analyzed by ICP-AES. It was confirmed that the precipitation and settling processes of metallic iron in slag were significantly affected by both the atmosphere and the reaction temperature. In a strongly reducing atmosphere such as 10/90 (vol%) mixtures of CO2/H2, the Fe content of bulk slag was reduced due to the precipitation of metallic iron in molten slag below 1550 °C. The profile of the iron content of the slag was related to the dissolved oxygen transfer in molten slag. At temperatures higher than that of the equilibrium reaction of FeO (slag) = Fe (l) + [O] (slag), metallic iron is no longer precipitated and the slag becomes a homogeneous phase. In addition, some heavy metals showed the same behavior as iron in the strongly reducing atmosphere were extracted by 1M/HCl, which results in higher extraction rates for Fe, Mg, Zn, and Pb.