Mechanism of rock magnetic susceptibility response in coal fire zone under high-temperature environment

Abstract

Coal fires lead to substantial depletion of coal reserves and the emission of harmful gases, thereby causing severe ecological pollution. The magnetic method offers means to monitor alterations in the magnetic properties of coal seams and surrounding rocks after exposure to coal fires, aiding in identifying the extent of such fires. This study focuses on borehole core samples from the Hongliulin coal mine area in Yulin, Shaanxi Province, China. The samples from the six drill holes exhibited diverse lithologies and underwent high-temperature (25–800 °C) heat treatment and mass magnetic susceptibility testing in both air and nitrogen environments. The results indicate that the mass magnetic susceptibility of rocks follows a three-stage pattern during high-temperature heat treatment: minimal change up to 400 °C; at 400–600 °C, the pronounced increase in mass susceptibility is presumably attributed to the conversion of weakly magnetic minerals, such as siderite and pyrite, into strongly magnetic minerals, including magnetite, maghemite and pyrrhotite; and at 600–800 °C, the decrease in rock susceptibility observed at this stage may be due to the high-temperature oxidation of magnetite and pyrrhotite to hematite. Moreover, the mass magnetic susceptibility of the samples treated in a nitrogen environment was greater than that of the samples treated in air, which was attributed to variations in the oxygen concentration influencing the formation of magnetite and hematite. In low oxygen environments, magnetite formation prevails over hematite, leading to higher mass magnetic susceptibility. Exploring the change mechanism of the magnetization rate of different rocks after high-temperature treatment under different heating environments can provide some guidance for magnetic detection.