Thermal Decomposition of Vermiculites: Kinetics of Dehydration and Dehydroxylation Processes

Thermal behavior study of pristine and modified halloysite nanotubes

Abstract: Pristine halloysite nanotubes (HNTs) were studied by thermogravimetry (TG) up to 800 °C. Etching of alumina from inside the tube (causing a significant increase in tube lumen) was realized by treating the material with an acidic H2SO4 solution at 50 °C. Both materials were characterized by TG-FTIR techniques and their thermal behaviors were compared with that of kaolinite. The coupling of TG with FTIR enables to detect the gases evolved during the TG experiments, thus confirming that only pristine HNTs undergo dehydration with the loss of interlayer water molecules at around 245 °C, while dehydroxylation occurs in all these materials in close temperature ranges around 500 °C. TG runs at five different heating rates (2, 5, 10, 15 and 20 °C min−1), was carried out in the same experimental conditions used for the thermal analysis study with the aim to investigate dehydration and dehydroxylation kinetics using some isoconversional methods recommended by the ICTAC kinetic committee, and thermogravimetric data under a modulated rising temperature program. Finally, the results of the kinetic analysis were discussed and explained in terms of the strengths of the hydrogen bonds broken during these processes.

Thermal study of vermiculites and mica-vermiculite interstratifications

Abstract: Simultaneous DTA-TG has been carried out on a set of natural vermiculite samples. Based on their dehydration behaviour the samples can be divided in two groups: (a) those with DTA endothermic peak temperatures at 140°–150°C and 240°–270°C (pure vermiculties) and (b) those with peak temperatures at 95°–115°C (vermiculite with mica or mica-vermiculite interstratifications). The low temperature at which the endothermic effect in group (b) appears is discussed on the basis of dilution due to the inert layers of mica, differences in chemical composition, and lowering of interlamellar water bond energy.

Rate of Thermal Dehydration of Kaolinite in Vacuum

Abstract: The dehydration of kaolinite was carried out in a vacuum to evaluate the kinetics without the marked effect of water vapor. Samples of carefully fractionated particles were used and the effect of surface area was determined. At pressures in the order of 1μ of Hg the reaction was found to be diffusion-controlled, with an activation energy of 43.5 kcal. The rate of dehydration was found to decrease linearly with increasing water vapor at moderate pressures. A change from parabolic to first-order kinetics was observed with increasing pressures.

Homogeneous and Inhomogeneous Mechanisms in the Dehydroxylation of Minerals

Abstract: In attempts to postulate dehydroxylation mechanisms for the clay minerals, three assumptions have generally been made: that the elements of water are lost more or less uniformly from all over the crystal, that the tetrahedral parts of the structure are less likely to be disturbed than are the octahedral parts, and that silica-rich material often presumed to occur as a secondary product is in fact pure SiO2. It is shown that all of these assumptions are doubtful. Dehydroxylation mechanisms in which the first assumption holds good are described as homogeneous, and it is shown that certain calcium minerals undergo dehydroxylation in this way. In contrast, where magnesium or other cations of similar size are predominant, dehydroxylation occurs by an in_homogeneous mechanism, in which no oxygen is lost from those parts of the crystal where oriented conversion to a crystalline product occurs; the oxygen for the expelled water comes entirely from other regions of the crystal which are wholly or partly converted into pores, while various migrations of cations occur. The dehydroxylation behaviour of serpentine, talc, kaolinite and. pyrophyllite is briefly reviewed. A new approach to the dehydroxylation of clay minerals is suggested, based on the concept of inhomogeneous mechanisms.

The Consequences for Differential Thermal Analysis of Assuming a Reaction to be First-Order

Abstract: AKt;TRACT From their isothermal experiments, Murray and White drew the conclusion that the dehydration reaOions of clays are first-order with a rate factor of Arrhenius form. The object of this paper is to discuss whether differential thermal analysis can be used to confirm or refute this hypothesis. The problem, previously considered by Murray and White, of a sample heated at a constant rate throughout is recalled. Particular attention is drawn to the effect of heating rate on the temperature at which the maximum rate of reaction occurs. To apply to differential thermal analysis, the treatment must be extended to take account of thermal gradients; as the mathematics involved is cumbersome, only results are considered. The most important result concerns the effects of sample size and dilution on the position of the turning-point. A disagreement with the results of experiments on kaolinite is pointed out; it is argued that only a small part of this disagreement is due to simplifications made in developing the theory, and it is concluded that the hypothesis that the clay reactions are first-order, with a rate factor of Arrhenius form, is only approximate.