Transformation behaviour of pyrite during microwave desulfurization from coal: Phase and structural change of Fe-S compounds

The selective heating of pyrite (FeS2) in coal by microwave makes its removal more efficient, but the gas produced by high-sulfur coal during microwave irradiation may affect the conversion process of pyrite. In this article, the transformation mechanism of pyrite in coal during microwave desulfurization was investigated experimentally and computationally. X-ray diffraction (XRD) was used to analyze the phase change of pyrite in air and inert atmosphere. The thermodynamic transformation patterns of pyrite under different atmospheres were further determined by FactSage. Materials Studio was used to derive the changes of atomic coordination, density of states, band structure, and bond length of iron sulfides to elucidate the causes of pyrite transformation behavior. The results show that the transformation paths of pyrite in different atmospheres are similar, troilite (FeS) can be produced directly from FeS2 or indirectly from pyrrhotite (Fe7S8). The generation temperatures of FeS2 are different for the four atmospheric conditions, however, the theoretical generation temperature of Fe7S8 is 612.5 °C for all of them. From the calculated results, Fe7S8 is the least stable, while pyrite is the most stable. With the addition of an electric field equivalent to a microwave field, FeS becomes the final product of the pyrite sulfur conversion process due to a significant decrease in bond length.