Mechanism of gas-phase passivation inhibition during FeS self-ignition based on pore structure and fractal characteristics

Reactive FeS that spontaneously combusts was prepared under field conditions and subjected to gas-phase passivation using various atmospheres and durations. The goal was to uncover how gas-phase passivation inhibits the spontaneous combustion of reactive FeS. The surface characteristics and elemental composition of FeS were analyzed at different gas-phase passivation levels using scanning electron microscopy (SEM) and X-ray energy-dispersive spectroscopy (EDS). The effect of gas-phase passivation on the evolution of the FeS pore structure was investigated using low-temperature liquid nitrogen adsorption, and the FHH fractal theory was employed to address changes in the fractal properties of the FeS surface. The results highlighted the intricate and densely dispersed pore structure of reactive FeS, along with its pronounced surface roughness. With increased levels of gas-phase passivation, the FeS surface became smoother and more regular, the number of oxidation-expanding pores increased, most sulfur elements were converted into sulfur dioxide gas, and iron trioxide passivation films formed on the FeS surface. Gas-phase passivation significantly reduced the specific surface area and cumulative pore volume of reactive FeS. Specifically, the cumulative pore volume decreased by approximately 20.9%, while the specific surface area decreased by 28.5% as gas-phase passivation deepened. The passivation process transformed more micropores into transition pores, with resulting iron-oxygen compounds adhering to the FeS surface. This reduced the number of reactive adsorption sites and substantially slowed down spontaneous combustion. As gas-phase passivation deepened, the volume proportion of FeS micropores decreased to 16.5%, and the micropore area decreased by around 6%. Gas-phase passivation had a significant impact on reducing the spatial fractal dimension and complexity of FeS, with a greater influence on spatial complexity than surface complexity. The passivation effect was initially enhanced by higher oxygen concentrations within the first 6-hours of passivation, but its effectiveness diminished thereafter.