The Multistep Nature of the Kaolinite Dehydroxylation: Kinetics and Mechanism

Abstract

The thermal dehydroxylation of kaolinite has been reexamined using small sample weights, a homogeneous particle size distribution and high-vacuum conditions in order to reduce the influences of heat and mass-transfer phenomena. The controlled rate thermal analysis (CRTA) technique, which was specially developed to minimize the pressure and temperature gradients through the sample, was employed to carry out meaningful kinetic experiments. Two advanced isoconversional methods, the Vyazovkin and the Galwey methods, were used complementarily to determine the dependence of the activation energy on the degree of conversion. For this purpose, the Vyazovkin method has been adapted to CRTA experiments. It was demonstrated that there are at least two different stages, revealing the multistep nature of this reaction. The activation energy for the first step, which is assigned to nucleation and the growth of nuclei, decreases from 100 to 75 kJ/mol. The second stage corresponds to a diffusion process and the activation energy rises to 120 kJ/mol because of the metakaolinite formation, which closes the interlamellar channels and leaves isolated patches of kaolinite from which the water escapes with difficulty.

Dehydroxylation and Structural Distortion of Kaolinite as a High-Temperature Sorbent in the Furnace

“…The difference of three kinds of hydroxyl groups could be understood from the dehydroxylation process. The dehydroxylation of kaolinite is generally considered to consist of two steps: (a) formation of water molecules by chemical reaction of OH groups and (b) physical diffusion of water molecules towards the surface of kaolinite [29][30][31]. There are different kinetics mechanisms to describe the dehydroxylation of kaolinite, such as first-order kinetics, diffusion model and both [29,32].…”

“…When the flash calcination temperature rose up to 1000 • C and 1200 • C, the resonances representing VI-coordinated Al and V-coordinated Al shifted to the positions with higher absolute values, accompanying the formation of new resonances. It indicated the existence of various forms of VI-coordinated Al and V-coordinated Al.Minerals 2019, 9, x FOR PEER REVIEW 10 of27 Al nuclear magnetic resonance (NMR) spectra (left) and29 Si NMR spectra (right) of 3 s flash-calcined products. For27 Al NMR spectra, gray areas, red areas and blue areas indicate the fit picks of VI-coordinated Al, V-coordinated Al and IV-coordinated Al, respectively.…”

“…27 Al nuclear magnetic resonance (NMR) spectra (left) and29 Si NMR spectra (right) of 3 s flash-calcined products. For27 Al NMR spectra, gray areas, red areas and blue areas indicate the fit picks of VI-coordinated Al, V-coordinated Al and IV-coordinated Al, respectively.…”

Kinetics of the chrysotile and brucite dehydroxylation reaction: a combined non-isothermal/isothermal thermogravimetric analysis and high-temperature X-ray powder diffraction study

“…Ortega et al (2010) mentioned that the formation of metakaolinite during the dehydroxylation of kaolinite closes the interlamellar space initially used for the outward diffusion of H 2 O molecules. They subsequently conclude that the change in diffusion paths after the closure is responsible for the change in the rate-determining step and the increase in E a .…”

A General Approach for Kinetic Modeling of Solid-Gas Reactions at Reactor Scale: Application to Kaolinite Dehydroxylation

“…Some improvements should be done in order to get better simulations. For example, several points could be improved: [24] have pointed out the fact that PSD has an impact on the shape of the kinetic curves and should be considered to determine properly kinetic constants in the case of dehydroxylation of kaolinite. This importance of PSD on kinetic modeling has also been underlined by several authors [25,26] or from a theoretical point of view [27][28][29][30][31][32][33][34]; -at the reactor scale, the change in the physical properties of the granular medium (including the porosity) due to the transformation of kaolinite into metakaolinite should be considered.…”