Kinetic pathway for thermal exfoliation of pyrophyllite

• First report on mechanistic pathway for thermal exfoliation of pyrophyllite
• A realistic kinetic model is established, supported with microscopic observation.
• Correlation between dehydroxylation and exfoliation explained

The thermal exfoliation of pyrophyllite on heating above 600 °C was studied to discern its kinetic pathway and correlate it with dehydroxylation. The irreversible thermal expansion data of pyrophyllite was collected from a thermomechanical analyzer at different heating rates (β). Detailed kinetic analysis was performed on the data using model-free and regression analysis (both linear and non-linear) methods. Multivariate non-isothermal kinetic analysis was used to fit the expansion data at all heating rates simultaneously using eighteen kinetic models. It is found that, the exfoliation of pyrophyllite can be successfully fitted with three kinetic models, viz., n-dimensional Avrami (An), two-dimensional diffusion (D2) and three-dimensional diffusion (D3) model with acceptable kinetic parameters. D2 model (Ea, activation energy = 392 kJ·mol− 1, regression coefficient = 0.998) provides the most realistic mechanistic pathway by considering the planar geometric constraint of nucleation and growth in the layered structure of pyrophyllite. Electron microscopic images of exfoliated pyrophyllite shows distinct exfoliated layers indicating escape route for heated gas and supports the choice of D2 model. Although this exfoliation process is a consequence of dehydroxylation reaction, its complexity due to diffusional and geometric constraints is reflected by the high energy barrier (Ea) reported consistently by every kinetic model compared to that of dehydroxylation.