Experimental study of the impact of phase transitions in coal on magnetic susceptibility under high-temperature conditions

Coal fire detection has always been the foundation and focus of coal fire prevention and control. Magnetic detection is valued for its high accuracy and effectiveness among various detection methods. The magnetic susceptibility of coal in natural environments is influenced by complex factors, with phase parameters-representing specific physical and chemical states-directly impacting overall susceptibility. Therefore, to study the abnormal response characteristics of the magnetic susceptibility of coal caused by phase transformation at high temperatures, this study analyzes the magnetic susceptibility characteristics of three different types of metamorphic coals using thermogravimetric analysis (TG-DTG), variable temperature magnetic susceptibility analysis, and X-ray diffraction experiments. The experimental results show that the magnetic susceptibility characteristics of coal are closely related to the degree of coal metamorphism and the phase evolution of the coal phases at different pyrolysis stages. At room temperature, the magnetic susceptibility of coal positively correlates with the degree of metamorphism. When the temperature rises to the inverse Curie point, the magnetic susceptibility of coal increases due to the pyrolysis of ferromagnetic minerals in the coal. The downward trend in the DTG curve confirms that the rearrangement of the carbon structure and the chemical changes in the minerals during pyrolysis lead to the formation of magnetic substances, which in turn affect the magnetic susceptibility. After this rapidly decreases due to continuous heating reducing the stability of magnetic compounds and causing their decomposition.