Thermal decomposition and oxidation of pyrite with different morphologies in the coal gangue of North China

Pyrite is common in coal gangue formed in a reduced environment and has a significant influence on the recycling utilization of coal gangue. Four types of pyrite with different structures and morphologies, namely octahedral pyrite framboid, pentagonal dodecahedral pyrite framboid, irregular grain pyrite, and euhedral octahedral crystalline pyrite, were selected from coal gangue to study their thermal behavior and phase transformation processes during heat treatments in different atmosphere via high-temperature XRD, TG-DSC, SEM-EDS, and XRF. The results show that pyrite in coal gangue can be decomposed into pyrrhotite when heated in an inert atmosphere. Pyrites with different morphologies and structures exhibit distinct thermal behavior. The higher S/Fe ratio promotes the better crystallinity of pyrite and thus lowers the phase transformation temperature from pyrite to pyrrhotite. The thermal decomposition temperature of the sulfur-rich and well crystalline pyrites was approximately 600 degrees C, and the transformation temperature of sulfur-deficit pyrites to pyrrhotite was approximately 640 degrees C. The S release rate from the former two pyrites was also slower than that from the latter two. The pyrites can be oxidized to nano-hematite when heated in an air atmosphere at above 400-500 degrees C, with the transformation proceeding in the sequence of pyrite-nano-microsphere magnetite-nano-microsphere hematite. The nano-hematite aggregates had a pseudomorph of the original pyrite crystals. This study has great significance for the high-level recycling utilization of coal gangue, such as iron removal and direct material.

Automated summary

Pyrite in coal gangue can be decomposed into pyrrhotite when heated in an inert atmosphere. Pyrites with different morphologies and structures exhibit distinct thermal behavior. When heated in an air atmosphere, pyrites can be oxidized to nano-hematite.