The thermal transformation behavior and products of pyrite during coal gangue combustion

•Pyrite would be transformed into nano-hematite via nano-magnetite and jarosite at high temperature in air.•Nano-magnetite as an intermediate was formed in a weak reducing–weak oxidizing environment.•The nano-hematite possessed the nanospheric pseudomorph of magnetite in morphology.•SO2 was released twice during the phase transformation reaction. Pyrite is a common and important mineral in coal and coal gangue, and its thermal transformation behavior and products during combustion have an essential impact on the environment, coal combustion technology, and recycling utilization of coal ash and coal gangue. Two samples of coal gangue containing high pyrite content were studied using XRD, TG-MS, SEM-EDS, and TEM to reveal pyrite's thermal transformation behavior and products during coal gangue combustion. The results showed that pyrite was transformed into hematite nanospheres during coal gangue combustion in air and released SO2 gas in the process. The transformation proceeds in the sequence of pyrite (0 ∼ 400 °C) – nano-microspheric magnetite and jarosite (400 ∼ 500 °C) – nano-microspheric hematite (500 ∼ 1000 °C). Nano-hematite possessed the morphology of a nanospheric pseudomorph of magnetite. Nano-magnetite and jarosite as intermediates were transitional and unstable and formed in a weakly reducing–weakly oxidizing environment. The formation and thermal decomposition of jarosite was associated with the release of structural water from kaolinite dehydroxylation. During the phase transition reaction, SO2 was released twice: the first release was related to the formation of magnetite and jarosite, while the second release was related to the thermal decomposition of jarosite. Temperature was the main factor affecting the nanocrystallization of hematite, whose crystallinity improved with an increase in temperature. This research portrays a new thermal phase transformation mechanism of pyrite in coal gangue, which can provide a theoretical basis for the recycling of coal gangue.