Experimental study on the mechanism of typical flame retardants in suppressing the explosion of phthalic anhydride dust

Phthalic anhydride (PA), a critical chemical intermediate in polymer production, presents significant dust explosion hazards due to its fine particle size and high-temperature processing requirements. This study investigates the explosion characteristics of PA dust through experiments performed in the 20-L explosion device, 1.2-L Hartmann tube, and G-G furnace. The suppression mechanisms of three inert dusts, ammonium polyphosphate (APP), melamine polyphosphate (MPP), and melamine cyanurate (MCA) are also analyzed. At dust concentration of 1400 g/m³, PA exhibited the most severe explosion hazards: maximum explosion pressure (P_max) of 7.83 bar, deflagration index (K_St) of 164.19 bar·m/s, minimum ignition energy (MIE) of 100 mJ, and minimum ignition temperature (MIT) of 670 °C. The suppression tests revealed that all three inhibitors suppressed PA's explosion intensity with increasing mass fractions. Notably, APP and MPP outperformed MCA, achieving complete suppression at 40 wt% and 50 wt%, respectively, due to their dual-phase action: (1) physical suppression via heat absorption and formation of phosphorus-rich char layers isolating unburned particles, and (2) chemical suppression through radical scavenging (H•/OH•) and oxygen dilution by non-flammable gases (NH₃, H₂O). In contrast, MCA primarily suppressed explosions via gas-phase mechanisms by releasing NH₃/HNCO but lacked effective solid-phase char formation. Thermogravimetric and FTIR analyses further demonstrated that APP/MPP delay PA pyrolysis and enhance thermal stability, while residual char layers mitigate further combustion. This work provides critical insights into explosion inhibitor selection for PA dust explosion prevention, emphasizing the superiority of phosphorus-based inhibitors in industrial applications.