Thermal Decomposition Of Kaolinite Research Articles

Extraction of lithium from coal gangue by a roasting-leaching process

ABSTRACT Lithium is an important strategic metallic element, which plays a vital role in the production of batteries. Coal gangue has been regarded as a promising resource of lithium. In this study, the coal gangue collected from Jungar coalfield contains 437 ppm of lithium, which is significantly higher than the average content of coal (31.8 ppm) in China. Lithium extraction from the coal gangue was systematically investigated via the integrated process of roasting and acid leaching with the mechanism analysis. The majority (94%) of lithium was extracted when raw samples are roasted at 400°C for 20 min before 4 h leaching with 2 mol/L hydrochloric acid at 60°C, which represented the optimal conditions. X-Ray Diffraction patterns and surface microscopic analysis of the roasting samples showed that thermal decomposition of kaolinite, as the carrier minerals of lithium in the gangue, happened during the roasting, resulting in the easy release of lithium by acid leaching. It was found that diffusion is the dominant control factor of lithium leaching based on the kinetic analysis. In this paper, an integrated process of roasting and leaching with moderate conditions was developed and performed for lithium extraction from coal gangue.

Effect of alkylamine chain length on high-temperature phase transformation and thermal decomposition process of kaolinite intercalation compounds

The structural changes and thermal decomposition of kaolinite (Kaol) intercalation compounds during heating process limit their application in polymers which need to be heated during the preparation process. In this study, homologous compounds, namely, hexylamine (HEA) and octadecylamine (OCA), were used to intercalate Kaol to explore the effect of molecular chain length on the structural stability and thermal decomposition of intercalated compounds. After the intercalation by HEA and OCA, the expansion only occurred along the C-axis, and the basal spacing increased to 28.7 and 57.3 Å, respectively. The morphology of Kaol changed remarkably, and all appeared curly. Compared with Kaol/HEA intercalation compounds, the Kaol/OCA intercalation compounds had better curling effect due to their different molecular chain length. The thermal decomposition process of Kaol/alkylamine intercalation compounds can be divided into two distinct stages. The molecules that adhered to the surface of the nanoscrolls and existed in the lumen of nanoscrolls began to deintercalate in the first step, and the other molecules that intercalated Kaol layers began to decompose in the second step. Thus, Kaol can be used as a potential carrier material, a carrier of sustained-release drug, or a carrier material for the catalyst. The reaction is regulated by controlling the temperature of the reaction.

Thermal decomposition of selected coal gangue

The thermal behavior of two mineral-type coal gangues under different temperature conditions was investigated by heating treatment of kaolinite-type coal gangue and illite-type coal gangue based on thermal analysis, X-ray diffraction (XRD), infrared spectroscopy (IR) and scanning electron microscopy (SEM). Thermogravimetric, differential thermogravimetric and differential scanning calorimetry results show the thermal decomposition process of kaolinite-type coal gangue and illite-type coal gangue can be divided into three stages, thermal decomposition of kaolinite and illite clay minerals occurred in the first stage and the third stage, and thermal decomposition of carbon material in coal gangue samples occurred in the second stage. Compared with illite-type coal gangue, the volatile of kaolinite-type coal gangue is released more intensely and pyrolysis performance of kaolinite-type coal gangue was better; initial temperature of heat decomposition was lower. The XRD, IR and SEM results showed the mineral phase transformation of kaolinite-type coal gangue occurred at 500 °C, the microscopic kaolinite changed obviously and its flake and layered structure started to break, and the phase of kaolinite transformed into amorphous glassy state, which was metakaolinite. In contrast, the mineral phase transformation of illite-type coal gangue occurred at 900 °C. According to the results of various research methods, it is considered that the thermal stability of kaolinite-type coal gangue is lower than that of illite-type coal gangue. This study has some theoretical significance for the rational use of coal gangue to produce highly activity powder materials.

Sintering of Mullite by Different Methods

In this study, we have investigated the sintering of mullite by three different methods: (i) reaction sintering by thermal decomposition of kaolinite, followed by reaction with alumina; (ii) reaction sintering of mullite by solid state reaction of pure silica and alumina; (iii) sintering of commercial electrofused mullite powder. In all the cases, the mixtures of powders were compacted by dry pressing and sintered at temperatures between 1000°C and 1600°C using a resistive furnace. The phase identification was performed by X-ray difractometry, and the microstructure was studied by scanning electron microscopy. Density and porosity were obtained using the Archimedes method. The flexural strength and the Vickers hardness were measured to evaluate the mechanical properties. Sintered ceramic bodies with a high content of mullite were produced following the three investigated routes. In both the second and third methods a residual phase of α-alumina was observed on the sintered bodies. The higher values of density and bending strength were achieved by specimens produced by the first method and sintered at 1600°C.

Metakaolinite as a catalyst for biodiesel production from waste cooking oil

The use of metakaolinite as a catalyst in the transesterification reaction of waste cooking oil with methanol to obtain fatty acid methyl esters (biodiesel) was studied. Kaolinite was thermally activated by dehydroxylation to obtain the metakaolinite phase. Metakaolinite samples were characterized using X-ray diffraction, N2 adsorption-desorption, simultaneous thermogravimetric analyse/differential scanning calorimetry (TGA/DSC) experiments on the thermal decomposition of kaolinite and Fourier-transform infrared spectrometer (FTIR) analysis. Parameters related to the transesterification reaction, including temperature, time, the amount of catalyst and the molar ratio of waste cooking oil to methanol, were also investigated. The transesterification reaction produced biodiesel in a maximum yield of 95% under the following conditions: metakaolinite, 5 wt-% (relative to oil); molar ratio of oil to methanol, 1:23; reaction temperature, 160°C; reaction time, 4 h. After eight consecutive reaction cycles, the metakaolinite can be recovered and reused after being washed and dried. The biodiesel thus obtained exhibited a viscosity of 5.4 mm2·s−1 and a density of 900.1 kg·m−3. The results showed that metakaolinite is a prominent, inexpensive, reusable and thermally stable catalyst for the transesterification of waste cooking oil.

Evidence for two stage mullite formation during thermal decomposition of kaolinite

Structural changes on heat treatment of English kaolinite have been studied by a standard X-ray technique. Changes in the lattice parameters, strain values, and crystal sizes of mullite and cristobalite have been calculated using a computer based Rietveld program. The results show two separate stages of mullite formation during the reaction of kaolinite. Changes in the composition of mullite during the course of its formation are discussed.

Kinetics and compensation effect in kaolinite dehydroxylation

Activation parameters (E, A, ΔH ≠, and ΔS ≠) and the effect of various factors involved in kaolinite dehydroxylation reaction have been studied using isothermal TG measurements. Thermal decomposition of kaolinite is found to be diffusion controlled within 0 < α < 0.65–0.70. The rate of the water release in dehydroxylation has obeyed the expression for diffusion in a spherical particle and/or the rate equation of the second order reaction, depending upon the kaolinite nature. The values of activation energy (140–300 kJ/mol) increased with increasing of partial water pressure in the reaction environment and increasing of structural disorder (natural, or induced by grinding). Activation parameters have shown a compensation behavior, following the equations; {E} = 51.1 + 12.961log {A} and {ΔH ≠}= 222.0 + 0.680{ΔS ≠} Isokinetic temperature β = (679 ± 2)K derived from these relations is in good agreement with the temperature range of the isothermal measurements performed.

New Data on Thermal Decomposition of Kaolinite and Halloysite

Using single‐crystal electron diffraction data it has been possible to demonstrate more directly the existence of considerable order in dehydroxylated kaolinite and halloysite. The two‐dimensional (a‐b) plane structure of dehydroxylated kaolinite is very similar to that of kaolinite. This phase, appropriately described by the term “metakaolinite,” decomposes first at about 825°C., γ‐Al2O3 being the chief crystalline product. In the case of halloysite the γ‐Al2O3 appears first at about 650°C. The products observed on the dehydroxylation of kaolinite depend to a large extent on the particle size of the sample used. Electron microscope studies of dehydroxylated material and the same material treated under very mild hydrothermal conditions show that the main phenomena accompanying dehydroxylation apparently are disalignment along the c‐axis, the appearance of discontinuities in the sheets themselves (not visible directly in the crude image of the electron microscope), and the reduction of the effective particle size. The cause of the exothermic peak cannot be the formation of γ‐Al2O3 but is assigned to the formation of mullite which is not necessarily detected in an X‐ray powder pattern.