<p>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 (<i>n</i>&#xa0;=&#xa0;2/3) with an activation energy of 70.76 kJ/mol and pre-exponential factor of 11.54&#xa0;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 (Al_4.8Si_1.2O_9.6) 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.</p>