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Magnetization roasting and magnetic separation is an effective method of recovering iron from low-grade ores. The phase transformation mechanism of minerals is an important basis for optimizing technological parameters. In this study, biomass was used as a reducing agent to roast Baotou low-grade limonite in a high-temperature vacuum furnace. The phase transformation law and its influence on magnetic properties were studied via X-ray diffraction, scanning electron microscopy, and vibrating sample magnetometry. The results show that roasting temperature is related to the magnetism of iron ore. With an increase in temperature, iron content increases gradually, and limonite magnetism also increases, at 700 °C reaching a maximum saturation magnetization of iron concentrate of 43.06 emu/g. It was also found that during vacuum roasting, S elements migrated and accumulated on the surface of limonite and distributed evenly on the surface of mineral particles in the form of FeS, forming ash shells.