Sepiolite and bentonite have a wide range of industrial applications based on their physicochemical properties such as surface area, thermal behavior, chemical composition, and mineralogic composition The thermal behavior and kinetics of naturally occurring sepiolite and bentonite were determined in order to give an idea about the potential use of naturally occurring clay minerals in possible applications Naturally occurring sepiolite and bentonite samples were heated to the temperature that was achieved at the end of the dehydroxylation process Mineralogic and thermal characteristics of raw and heat treated samples were investigated by X-ray diffraction, Fourier transform infrared spectroscopy, and nitrogen adsorption/desorption analyses Changes in the structure following heat treatment were used for the evaluation of the dehydroxylation properties of the samples The dehydroxylation properties of the minerals are strongly affected by the crystal structure Kinetic analyses, which were related to the dehydroxylation of naturally occurring sepiolite and bentonite, were conducted using dynamic thermogravimetry/derivative thermogravimetry analysis under nitrogen atmosphere Flynn–Wall–Ozawa, Kissenger–Akahira–Sunose, and Friedman isoconversional methods were used to determine the activation energies of the dehydroxylation reactions of the samples The results indicate that the activation energy of naturally occurring sepiolite showed a little variation at a particular conversion rate (03–07), while the activation energy of naturally occurring bentonite showed a significant variation within the range of variation of the conversion rate The present study shows that the dehydroxylation reactions of naturally occurring sepiolite and bentonite were single mechanism reaction and complex mechanism reaction, respectively

Thermal behavior and dehydroxylation kinetics of naturally occurring sepiolite and bentonite

In this study, the mechanism and the kinetic parameters of the thermal decomposition of gibbsite Al(OH)3 were studied by differential thermogravimetry technique under non-isothermal conditions, between room temperature and 1200 K at heating rates of 5, 10, 15 and 20 ◦Cmin−1. The obtained differential thermogravimetry curves show clearly three distinct peaks. The first peak is due to the partial dehydroxylation of gibbsite. Among the 32 types of differential equations of non-isothermal kinetics, we have found that the most suitable mechanism is (A3/2: g(x) = [− ln(1 − x)]) also called Avrami-Erofeev equation of order 2/3. The values of the activation energy EA and of the pre-exponential factor K are 157 kJmol−1 and 7.58 × 10 s−1, respectively. The second peak corresponds to the decomposition of gibbsite to boehmite. Decomposition is controlled by the rate of second-order reaction (F2: g(x) = (1− x)−1 − 1), under the applied conditions. The activation energy EA and pre-exponential factor K correspond to 243 kJmol−1 and 3.73× 10 s−1, respectively. The third peak is due to transformation of boehmite to alumina. However the mechanism for such transformation is better described by the 3/2 rate order reaction (F3/2: g(x) = (1 − x)−1/2 − 1). In addition, the values of EA and K were determined to be around 296 kJmol−1 and 1.82× 10 s−1, respectively. The results of differential thermogravimetry were supplemented by the differential thermal analysis. X-ray powder diffraction analysis was carried out for samples of gibbsite treated at different temperatures between 200 and 1200 ◦C in 200 ◦C steps.

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Mechanism and Kinetic Parameters of the Thermal Decomposition of Gibbsite Al(OH)3 by Thermogravimetric Analysis

The products evolved during the thermal decomposition of kaolinite–urea intercalation complex were studied by using TG–FTIR–MS technique. The main gases and volatile products released during the thermal decomposition of kaolinite–urea intercalation complex are ammonia (NH3), water (H2O), cyanic acid (HNCO), carbon dioxide (CO2), nitric acid (HNO3), and biuret ((H2NCO)2NH). The results showed that the evolved products obtained were mainly divided into two processes: (1) the main evolved products CO2, H2O, NH3, HNCO are mainly released at the temperature between 200 and 450 °C with a maximum at 355 °C; (2) up to 600 °C, the main evolved products are H2O and CO2 with a maximum at 575 °C. It is concluded that the thermal decomposition of the kaolinite–urea intercalation complex includes two stages: (a) thermal decomposition of urea in the intercalation complex takes place in four steps up to 450 °C; (b) the dehydroxylation of kaolinite and thermal decomposition of residual urea occurs between 500 and 600 °C with a maximum at 575 °C. The mass spectrometric analysis results are in good agreement with the infrared spectroscopic analysis of the evolved gases. These results give the evidence on the thermal decomposition products and make all explanation have the sufficient evidence. Therefore, TG–MS–IR is a powerful tool for the investigation of gas evolution from the thermal decomposition of materials and its intercalation complexes.

TG–MS–FTIR (evolved gas analysis) of kaolinite–urea intercalation complex

TG-MS-FTIR (evolved gas analysis) of kaolinite-urea intercalation complex

Raw material kaolin Sedlec Imperial and four types of rehydroxylated samples were used to study the processes of the first and second dehydroxylation of kaolinite by thermal analysis and IR spectroscopy. Activation energy (E
 a) of these processes was calculated from DSC curves using five isoconversional methods. IR spectroscopy was used to compare structures of the original and rehydroxylated samples. It was proven that the structure of rehydroxylated metakaolinite can closely resemble that of the raw kaolinite under intensive hydrothermal treatment but does not reach the original structure. The E
 a values of the second dehydroxylation reach 87–92 % of E
 a values of the first dehydroxylation.

Investigation of the first and second dehydroxylation of kaolinite

Thermal analysis techniques remain important tools amongst the large variety of methods used for analysis of the dehydroxylation of kaolinite. In the present study, the kinetics of dehydroxylation of Algerian kaolinite, wet ball milled for 5 h followed by attrition milling for 1 h, was investigated using differential thermal analysis (DTA) and thermogravimetry (TG). Experiments were carried out between room temperature and 1350 °C at heating rates of 5, 10 and 20 °C min−1. The temperature of dehydroxylation was found to be around 509 °C. The activation energy and frequency parameter evaluated through isothermal DTA treatment were 174.69 kJ mol−1 and 2.68 × 109 s−1, respectively. The activation energies evaluated through non-isothermal DTA and TG treatments were 177.32 and 177.75 kJ mol−1, respectively. Growth morphology parameters n and m were found to be almost equal to 1.5.

Z. Takkouk

Papers

Thermal analysis of dehydroxylation of Algerian kaolinite