Thermal decomposition kinetics and magnetic transformation of pyrite during inert atmosphere roasting

Pyrite (FeS 2 ) is a typical refractory impurity in molybdenite concentrate, severely limiting the purity of molybdenum disulfide (MoS 2 ). Magnetization roasting followed by magnetic separation offers a clean physical pre-removal route for iron, yet the underlying kinetics and property evolution require systematic clarification. We investigates the thermal decomposition and magnetic transformation of relatively pure pyrite in argon, the results reveal a three-stage decomposition process. The primary reaction occurs at ∼ 600–700 °C, exhibiting an apparent activation energy of 339.91 kJ·mol −1 under the present single-heating-rate condition, fitting the three-dimensional diffusion model (D3) and the Avrami-Erofeev (n = 3) nucleation-growth model, indicating control by solid-state diffusion and nucleation. Optimal roasting occurs at 650 °C for 120 min, where pyrite completely converts to highly magnetic pyrrhotite (Fe 1-x S), yielding a peak saturation magnetization of 16.57 emu/g. This conversion is accompanied by a honeycomb-like porous structure and optimal particle size uniformity. Magnetic separation tests show that under 0.5 T, wet magnetic separation yields 77.50 % magnetic concentrate after three passe. Notably, the high local sulfur partial pressure ( P S 2 ) generated by bulk pyrite explains the necessity of 650 °C. Theoretically, trace pyrite impurities in complex matrices like molybdenite, which generate a much lower P S 2 , are expected to undergo this transformation at reduced temperatures; however, this hypothesis requires further validation. This study establishes the fundamental physicochemical mechanism of pyrite magnetization roasting and provides a theoretical basis for future exploration of low-temperature selective conversion of iron impurities in molybdenum purification. However, the selective conversion of pyrite and the thermal stability of MoS 2 under the same conditions require further experimental validation.