Pyrolysis of 6PPD-rich organic industrial waste: product distribution, transformation pathways

Rapid industrialization and urban expansion have accelerated the generation of organic industrial waste (OIW), creating substantial environmental and processing challenges. This study investigates the pyrolysis of hazardous OIW, which was sampled and pretreated for particle size reduction for making it usable for pyrolysis. OIW was found to contain tire-additive-derived N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (6PPD) presents both valorization potential and product-stewardship concerns. In this study, a hazardous 6PPD-rich OIW from a specialty-chemical process was size-reduced to a mean particle size of approximately 425 µm and pyrolyzed under nitrogen in a fixed-bed reactor. Feedstock characterization by proximate/ultimate analysis, field-emission scanning electron microscopy-energy-dispersive spectroscopy (FESEM-EDS), and gas chromatography-mass spectrometry (GC-MS) confirmed a heterogeneous, ash-containing matrix dominated by 6PPD in both methanol and hexane extracts. Thermogravimetric analysis/derivative thermogravimetry (TGA/DTG) at 20–50 °C min−1 identified moisture and light-volatiles release below about 200 °C, primary devolatilization at about 200 °C–350 °C with DTG maxima of 233 °C–289 °C, and slower high-temperature conversion above 350 °C. At the fixed particle size, the highest liquid yield, 58 wt.%, was obtained at 425 °C and 2 h. GC-MS of the oil showed 2,7-dimethyloctan-4-one, 55.8% area; residual 6PPD, 11.8%; 2-methyl-2-pentene, 7.7%; aniline, 6.8%; and N-phenyl-p-phenylenediamine, 4.1%, consistent with N-alkyl C-N bond cleavage, β-scission, radical recombination, and oxygenation reactions. Fuel-property screening showed diesel-like cetane quality, 53, and a high flash point, 121 °C, but water, sulfur, viscosity, and nitrogenous aromatic carryover exceeded or challenged relevant distillate-fuel limits. Therefore, the oil should be treated as an intermediate requiring drying, filtration, sulfur control, blending, distillation, or hydrotreating before use; targeted monitoring of 6PPD, 6PPD-quinone, and related transformation products is also recommended.