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

Abstract

As a high-temperature sorbent, kaolinite undergoes the flash calcination process in the furnace resulting in the dehydroxylation and structural distortion, which are closely related to its heavy metal/alkali metal adsorption characteristics. We investigated the flash calcination of kaolinite by the experiments using a drop tube furnace and by the characterization of flash-calcined products using thermogravimetric-differential scanning calorimeter (TG-DSC), X-ray diffraction (XRD), Fourier Transform Infrared Spectrometer (FTIR)and nuclear magnetic resonance (NMR). There were three kinds of hydroxyl groups in kaolinite during flash calcination at 800–1300 °C, E-type (~50%, easy), D-type (~40%, difficult) and U-type (~10%, unable) according to the removal difficulty. The hydroxyl groups activation was believed to be the first step of the removal of E-type and D-type hydroxyl groups. The kinetics model of dehydroxylation groups at 900–1200 °C was established following Arrhenius equation with the activation energy of 140 kJ/mol and the pre-exponential factor of 1.32 × 106 s−1. At 800 °C, the removal of E-type hydroxyl groups resulted in the conversion of a part of VI-coordinated Al in kaolinite to V-coordinated Al and the production of meta-kaolinite. When the temperature rose up to 1200 °C, mullite was produced and a part of V-coordinated Al converted to IV-coordinated Al and VI-coordinated Al. Finally, the adsorption characteristics of kaolinite was discussed according to the results of dehydroxylation and structural distortion.