Experimental study on the heat treatment reaction process of bentonite

Energy changes of bentonite during the heat treatment reaction process

The TG-DSC curves was shown in Fig. 3, TG and DTG curves showed that there were two obvious weight loss phases around 100 ℃ and 700 ℃, and the bentonite reached its maximum weight loss rates at 95.81 ℃ and 686.21 ℃, respectively. The DSC curves showed steep endothermic peaks near 100.76 ℃ and 685.31 ℃, and flat endothermic peaks near 898.91 ℃ and 1128.56 ℃. Based on the temperatures corresponding to the four endothermic peaks, the whole reaction process was divided into 4 phases: 30–400 ℃, 400–800 ℃, 800–1000 ℃, and 1000–1300 ℃.

TG-DSC curves of bentonite.

Phase I (30–400 ℃): The absorption water and interlayer water on the surface of montmorillonite escaped with increasing temperature²³, which led to the generation of the endothermic peak at 100.76 ℃ and the significant loss of thermogravimetry. The amount of thermogravimetric loss at this phase was 6.70%, reached the maximum loss rate of 2.36%/min at 95.81 ℃. The large amount of thermogravimetry loss and the high loss rate indicate that the absorption water and interlayer water content of montmorillonite was high and escaped rapidly.

Phase II (400–800 ℃): The constitution water of montmorillonite escaped with increasing temperature, and the two hydroxyl groups in the crystal structure were removed in the form of one water molecule, led to the generation of an endothermic peak and a significant loss of thermogravimetry at 685.31 ℃. The thermogravimetric loss in this phase was 4.70%, reached the maximum loss rate of 0.79%/min at 686.21 ℃. Compared with the first phase, the thermogravimetric loss and loss rate in this phase have decreased. The temperature at which montmorillonite undergoes dehydroxylation was an indicator for evaluating the heat resistance of montmorillonite and can reflect the thermal stability of bentonite²⁴. Although the constitution water of montmorillonite is removed at this phase and its original characteristics begin to be lost, the montmorillonite crystal still maintained its original structure, with only layered structure distortion and torsion, and no obvious amorphization.

Phase III (800–1000 ℃): The crystal structure of montmorillonite began to break down, disintegrate and produce amorphous phase²⁵, which led to an endothermic peak at 898.91 ℃, and the original layered structure of montmorillonite disappeared to produce anhydrous montmorillonite. The amount of thermogravimetric loss at this phase is 0.38%, and the thermogravimetry remains basically unchanged.

Phase IV (1000–1300 ℃): Quartz was transformed into cristobalite by high-temperature heat treatment²⁶, and quartz absorbed heat during the transformation process resulting in an endothermic peak at 1128.56 ℃. The exothermic process that followed resulted from the recrystallization of the amorphous phase of bentonite. The melting point of albite is typically around 1100 °C²⁷, and thus, the observed endothermic peak is attributed to the phase transition of quartz and the melting of albite. The subsequent exothermic process is due to the recrystallization of the amorphous phase of bentonite.

Variation in the XRD patterns of bentonite heat treated at different temperatures

To investigate the phase changes of bentonite during heat treatment, XRD analysis was conducted on the raw bentonite and its thermally treated products. Figures 4, 5, 6, 7, 8 display the XRD patterns of samples subjected to various heat treatment temperatures. The analysis reveals that the primary phases in the raw bentonite include montmorillonite, cristobalite, albite, and quartz. The diffraction peak for the (001) plane of montmorillonite in the raw bentonite appears at 2θ = 6.52° with a peak height of 298 and an interlayer spacing of 14.747 Å. After treatment at 100 °C, the diffraction peak shifts to 2θ = 9.08° with a reduced peak height of 140 and an interlayer spacing of 10.021 Å. This reduction in peak height by 158 and interlayer spacing by 4.726 Å indicates that the loss of adsorbed and interlayer water leads to the disappearance of the hydration layer, resulting in a decreased interlayer spacing and lower diffraction peak intensity²⁸. Following treatment at 700 °C, the diffraction peak for the (001) plane of montmorillonite appears at 2θ = 9.2° with a peak height of 128, further reduced due to the loss of constitution water. After treatment at 900 °C, the montmorillonite diffraction peak significantly diminishes, and a broad hump appears near 2θ = 22°, suggesting the decomposition of montmorillonite into an amorphous phase at this temperature²⁹. Following treatment at 1100 °C, the diffraction peaks for…