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.

Green production of a novel sorbent from kaolin for capturing gaseous PbCl2 in a furnace

“…To further confirm the water or hydroxyl species in the sorbents, FTIR analysis was conducted (these curves are shown in Figure S1) between 3000-4000 cm -1 where the parallel vibration frequency of hydrogen-oxygen bond occurs. All sorbents possessed the board flat peaks at 3420 cm -1 that represents hydrogen bonds indicating adsorbed or bound water, while raw kaolin had four noticeable hydroxyl peaks (connected with aluminium atoms) between 3600-3700 cm -1 (Cheng et al, 2019). The rehydroxylation reported in some studies was not observed in this study, which may be attributed to the lower pressure of hydrothermal in this study than that in the literature (Zemenová et al, 2014;Kloužková et al, 2013).…”

“…The sharp peak of RK at 93 ppm represents Si 3 (connecting with three bridge oxygen atoms), and peaks of other minerals at 112 ppm represent Si 4 (connecting with four bridge oxygen atoms, which means all the four bonds of SiO4 group connects with another four SiO4 groups). The conversion from Si 3 of RK to Si 4 of MK was due to the dehydroxylation after calcination (Cheng et al, 2019). When comparing the curves between MK and modified sorbents, the pressured hydrothermal treatment seemed to increase the proportion of Si 3 as the curves were lifted on the right side.…”

“…In fact, water played a role as mineraliser during the hydrothermal process, and reformed the layered structure of silicon and aluminium oxides (Sun et al, 2008). and broad peak between 15-30⁰ indicated the amorphous phase of aluminosilicate (Cheng et al, 2019). Though all the curves of the modified sorbents and metakaolin were very similar, a weak peak (marked with a "club") occurred at 12.3⁰ for HC that was almost the characteristic peak of kaolin (calculated by the layer distance of the kaolin lattice cell).…”

Phase formation, microstructure development, and mechanical properties of kaolin‐based mullite ceramics added with Fe2O3

“…Mullite has been synthesized using various methods, such as sol‐gel process, chemical syntheses, 13,14 co‐precipitation, 6,15 hydrothermal reaction, 16,17 flash combustion, 18–19 and chemical vapor deposition 20,21 . However, the production of high‐purity and high‐density mullite is still a challenge.…”

“…[7][8][9] Other applications include window materials under harsh chemical conditions and mechanical stress at high temperatures. [10][11][12] Mullite has been synthesized using various methods, such as sol-gel process, chemical syntheses, 13,14 co-precipitation, 6,15 hydrothermal reaction, 16,17 flash combustion, [18][19] and chemical vapor deposition. 20,21 However,…”