Unraveling the Al/Si Separation Mechanism During Carbochlorination of Mullite: Kinetics and Phase Transformation Pathways

Carbochlorination is a promising route for valorizing high-alumina fly ash (HAFA). However, process optimization is hindered by an insufficient understanding of the reaction mechanisms governing its predominant and most refractory phase mullite. To bridge this knowledge gap, this study decouples the intrinsic behavior of mullite from the complex HAFA matrix, systematically investigating its carbochlorination through non-isothermal kinetic analysis combined with multimodal characterization techniques. Non-isothermal thermogravimetric analysis demonstrates that the carbochlorination process follows the Avrami-Erofeev nucleation-growth model (n = 2/3) with an activation energy of 70.76 kJ/mol and pre-exponential factor of 11.54 s−1. Multimodal characterization including XRD, SEM-EDS, and 27Al/29Si MAS-NMR reveals a carbon-mediated chlorination mechanism. The process initiates with the cleavage of Cl–Cl bonds, triggering the dissociation of Al-O-Si frameworks. Owing to differential chlorination affinities, Al is preferentially and completely converted into volatile AlCl3, whereas only a portion of Si forms SiCl4. The remaining Si reorganizes into SiO2, and the aluminosilicate structure subsequently transforms into defective mullite (Al4.8Si1.2O9.6) and an intermediate sillimanite phase (Al2SiO5). The deoxygenation of sillimanite facilitates its re-conversion to mullite, while the accumulated SiO2 undergoes high-temperature phase transitions, ultimately crystallizing as quartz and cristobalite. The significant mass loss observed is primarily attributed to the volatilization of AlCl3 and SiCl4, which drives the efficient separation of Al and Si. These findings provide a fundamental mechanistic framework that is essential for optimizing Al/Si separation and advancing carbochlorination processes for aluminosilicate-rich secondary resources.