Spontaneous Combustion Characteristic Stages of Active FeS and Its Inhibition Effect by Gas-Phase Passivation

To explore the spontaneous combustion characteristic stages of active ferrous sulfide (FeS) and its inhibition effect by gas-phase passivation. Active FeS samples were prepared and then the spontaneous combustion characteristics of active FeS and commercial FeS samples were compared. The active FeS spontaneous combustion process was divided into several characteristic stages from the perspective of spontaneous combustion kinetics. Furthermore, the inhibition effect of gas-phase passivation on the characteristic stages and the kinetics and thermal effects of active FeS spontaneous combustion process were investigated to further reveal the inhibition mechanism. The results indicate that spontaneous combustion process of active FeS is divided into the room temperature (RT) spontaneous combustion, oxygen-adsorption-induced weight gain, high-temperature combustion and burnout stages, while the spontaneous combustion process of commercial FeS excludes the RT spontaneous combustion stage. Active FeS presents greatly advanced characteristic stages, stronger spontaneous combustion reactivity and more intense reaction in the RT spontaneous combustion and high-temperature combustion stages, which agrees more with the sulfur corrosion products under actual on-site working conditions. The spontaneous combustion tendency of passivated FeS weakens gradually with the increase of passivation time until it loses its RT spontaneous combustion reactivity after 6 h passivation. After the oxide film formed on the surface of passivated FeS is destroyed, its high-temperature combustion reaction becomes relatively intense. The activation energy of passivated FeS in the spontaneous combustion reaction is much higher than that of active FeS, and their activation energies both soar with the deepening of passivation, indicating that the gas-phase passivation technology can effectively inhibit the FeS spontaneous combustion process. The gas-phase passivation time must be maintained for over 6 h despite the weakening inhibitory effect of passivation treatment on the FeS spontaneous combustion process.