Kinetics Of Dehydroxylation Research Articles

Effect of Surface Area on the Dehydroxylation Kinetics of Kaolinite Mineral

Effect of Surface Area on the Dehydroxylation Kinetics of Kaolinite Mineral

Kinetics of dehydroxylation, in nitrogen and water vapour, of kaolinite and smectite from Australian Tertiary oil shales

Dehydroxylation of kaolinite and smectite is significant in oil shale processing. The possibility of shifting these endothermic reactions from the retort to the combustor by increasing the water vapour pressure was confirmed and the kinetics of dehydroxylation was determined by thermogravimetry. For kaolinite, non-isothermal and isothermal dehydroxylation in dry N 2 was second-order, with activation energies of 163 and 208 kJ mol −1 and pre-exponential factors of 2.0 × 10 12 and 3.0 × 10 13 s −1 respectively. Increasing water vapour pressure markedly increased the dehydroxylation temperature. Isothermal data were fitted to a first-order model at water vapour pressures 5 kPa, but Arrhenius plots showed large changes in the kinetic parameters. Non-isothermal results increasingly deviated from second-order with increasing water vapour pressure, but this was successfully accounted for by the Altorfer model. For Rundle smectite, non-isothermal kinetic data for dehydroxylation in dry N 2 were best fitted by the Ginstling-Brounshtein diffusion model, with an activation energy of 83 kJ mol −1 and a pre-exponential factor of 1.0 × 10 7 min −1.

Flash calcines of kaolinite: kinetics of isothermal dehydroxylation of partially dehydroxylated flash calcines and of flash calcination itself

Kaolinite has been flash calcined to produce calcines kinetically frozen at different levels of dehydroxylation. The kinetics of subsequent isothermal dehydroxylation of the calcines were consistent with a three-dimensional diffusion model. Activation energies and rate constants generally decrease as dehydroxylation of the calcine increases. A decrease in reaction rate at high levels of dehydroxylation is consistent with collapse of the interlamellar space, restricting diffusion of water out of the particles. The kinetics of flash calcination were shown to obey the same principles as isothermal dehydroxylation.

Dehydroxylation kinetics of kaolinite and montmorillonite from Queensland Tertiary oil shale deposits

Water released during clay dehydroxylation was measured using a small reactor coupled to a mass spectrometer. Dehydroxylation of Rundle montmorillonite and Nagoorin kaolinite occurs over the temperature ranges 250–700 °C and 280–600 °C, respectively. The low temperature of montmorillonite decomposition suggests iron substitution for aluminium in the crystal lattice (a nontronite structure). The montmorillonite dehydroxylation kinetics best fit a diffusion-controlled reaction in a sphere (a D(3) mechanism) to about 80% reaction. Kaolinite dehydroxylation is best represented by second-order kinetics (to at least 90% of reaction). Activation energies for the dehydroxylation reactions were determined as 145 ± 15 and 99 ± 8 kJ mol −1 for kaolinite and montmorillonite, respectively.

The kinetics of dehydroxylation and mullitization of zettlitz kaolin in the presence of calcium(II) as an ingredient

The method of differential thermal analysis and Kissinger's approach were used in this study to investigate the kinetics of dehydroxylation and mullitization. The relatively high resistance to the breaking of structural bonds and the diffusion to water molecules through the lamellar gas layer of water vapour limits the activation energy to 178 kJ mol −1, as determined in the experimental work. It is assumed that the relatively moderate activation energy value of mullitization, 487 kJ mol −1, obtained during the exothermic reaction is the result of a moderately defective structure and of the influence of calcium(II) which decreases the activation energy.

FTIR spectra of hydroxyls and dehydroxylation kinetics mechanism in montmorillonite

The application of Fourier transform infrared (FTIR) spectroscopy to the analysis of the hydroxyl groups bands' intensities of montmorillonite from Texas shows four regions of intensity loss rate for thermally shocked samples at 290 673 K. However, what is surprising is the persistence of very weak water stretching (∼3470 cm−1) and bending (∼1628 cm−1) vibrations at 553<T<773 K. It is speculated that this water, formed because of dehydroxylation, is trapped in the hexagonal cavities of the dehydrated montmorillonite lattice. However, conclusive evidence will require surface-sensitive spectroscopic measurements as this water could also be adsorbed on the external surfaces of processed samples. In the range 773<T<823 K, the main dehydroxylation of the AlAl-OH group results, and this reaction induces structural transformations in the montmorillonite lattice. FTIR measurements at 803 K for 0<t< 25 h were used to determine the kinetics mechanism of dehydroxylation in montmorillonite from Texas. The experimental data was tested, using diffusion controlled as well as six decomposition models to ascertain the kinetics mechanism of the AlAl-OH group's dehydroxylation. It appears that the dehydroxylation process can be described by the contracting spherical movement model rather than by a diffusion controlled model, suggesting surface nucleation, growth over the surface, and then advancement of the dehydroxylated/hydroxylated interface toward the center of the montmorillonite particles.

The kinetics of dehydroxylation of kaolinite

AbstractThe dehydroxylation of kaolinite has been investigated by isothermal thermogravimetry. Kinetic analysis using the Avrami equation shows that a combination of atomic mechanisms operates throughout the temperature range 734 K to 890 K. An empirical activation energy of 222 kJ mol-1 was calculated from the Arrhenius relationship using rate constants based on diffusion and homogeneous models. The activation energy (Ea) was calculated for a series of degrees of dehydroxylation by the time to a given fraction method, showing an increase in Ea during the early stages of the reaction. The isothermal plots indicate that OH is retained in the final stages of the reaction. The observations are explained in terms of a reaction mechanism in which kaolinite grains dehydroxylate from the edges inwards, parallel to (001).

Dehydroxylation kinetics of some pure smectites

Thermogravimetric curves were recorded for six smectites; Manito (U.S.A.) and Sampor (U.S.S.R.) nontronites, and Polkville (U.S.A.), Askangel (U.S.S.R.), Jelsovy Potok (C.S̆.A.) and Bakony (Hungary) montmorillonites. Kinetic analysis of the dehydroxylation wave was performed by using the first order equations of Horowitz and Metzger (1963), Coats and Redfem (1964), Dave and Chopra (1966), as well as the method of Šatava and S̆kvara (1969). A unimolecular nucleation mechanism seems to prevail, according to the last method, independent of diffusional effects. Nontronite dehydroxylation is associated with a higher activation energy ( E a) and preexponential factor ( A), in comparison to the dehydroxylation of montmorillonite (mean E a = 39.1 and 21.8 kcal mol −1, respectively). The compensation effect, correlating E a and log A, was found to hold and a formula was established.

ChemInform Abstract: KINETICS OF DEHYDROXYLATION AND EVALUATION OF THE CRYSTALLINITY OF KAOLINITE

Chemischer InformationsdienstVolume 15, Issue 8 Physical Inorganic Chemistry ChemInform Abstract: KINETICS OF DEHYDROXYLATION AND EVALUATION OF THE CRYSTALLINITY OF KAOLINITE J. G. CABRERA, J. G. CABRERASearch for more papers by this authorM. EDDLESTON, M. EDDLESTONSearch for more papers by this author J. G. CABRERA, J. G. CABRERASearch for more papers by this authorM. EDDLESTON, M. EDDLESTONSearch for more papers by this author First published: February 21, 1984 https://doi.org/10.1002/chin.198408031Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume15, Issue8February 21, 1984 RelatedInformation

Kinetics of dehydroxylation and evaluation of the crystallinity of kaolinite

This study presents results on the kinetics of kaolinite dehydroxylation. The accuracy of various methods of determining the values for the kinetic parameters and their sensitivity in detecting the mechanism of reaction is investigated. In particular, the differential order of reaction method of Baker, the general method of Achar et al., the integral method of Boy and Bohme, and the method of Coats and Redfern as modified by Fong and Chen are considered. Kaolinites from well-known sources are used to study the influence of crystallinity on the values of kinetic parameters. The statistical significance of the various mathematical methods for the assessment of the data obtained from non-isothermal thermogravimetry is determined by comparison with experimental and theoretical data using a computer programme developed for this purpose. The study demonstrates that the kinetic parameters can be used to quantify the degree of crystallinity of kaolinite and also confirms other findings that the dehydroxylation of kaolinite is a second-order reaction.

Dehydroxylation of kaolinite group minerals

The kinetics of dickite dehydroxylation have been investigated by the method of thermal analysis. It has been found that this is a two-stage process. The higher temperature of dickitedehydroxylation than that of kaolinite is the result of the different arrangement of the layers in its structure. A disturbance in the order of the stacking of the layers in the dickite structure causes a lowering of the temperature of dehydroxylation and a reduction of the activation energy of this process. This causes in turn the appearance of an additional peak or bend at 580‡ in the DTA and DTG curves.

Dehydroxylation of kaolinite group minerals

The kinetics of dehydroxylation of kaolinite and halloysite have been tested by the methods of thermal analysis. They are determined by the particle sizes of these minerals and the order of stacking of the layers in their structures. The Arrhenius activation energyE and order of reactionn decrease with the increase in specific surface area of the minerals. An increase in the degree of disorder of the kaolinite lattice diminishes the value ofE. It also influences then value for fractions finer than 1Μm.

Effect of electric fields on the dehydroxylation kinetics of pseudoboehmite

The dehydroxylation kinetics of pseudoboehmite were studied both in the presence and absence of an electric field using an electrolysis cell in conjunction with an X-ray furnace. At higher field strengths, the onset temperature of the reaction is lowered and the decomposition rate significantly increased by the electric field, particularly at the negative electrode, in which region the crystallite size is also greater. The activation energy for the reaction is not significantly changed by electrolysis. The electrolysis mechanism is discussed in terms of cleavage of hydrogen bonds and the migration of the resulting protons to the cathode where they may be discharged as water or hydrogen, or may enter and stabilize the structure of the decomposition product.

Kinetics of talc dehydroxylation

The kinetics of the dehydroxylation of talc have been measured in the temperature interval 1100–1160 K by means of isothermal weight-change determinations. The reaction follows first-order kinetics. Over the indicated temperature range the enthalpy of activation was found to be 101±4 kcal mol −1, and the entropy of activation was found to be 16±4 cal mol −1 K −1. The error estimates correspond to one standard deviation. The enthalpy necessary to break the MgOH bond was estimated from the heat of reaction for MgOH(g) → Mg(g)+OH(g). This turns out to be 97 kcal mol −1 in reasonable agreement with the measured enthalpy of activation. These activation parameters are consistent with the mechanism proposed for dehydroxylation of talc consisting of MgOH bond scission and subsequent migration of magnesium. These results contradict a previous report on the kinetics of talc dehydroxylation in which a diffusion-controlled expression was claimed to represent the rate of talc weight loss. It is suggested that the presence of adsorbed water on the talc used in the previous investigation is responsible for the discrepancy.

Kaolinite Dehydroxylation Kinetics

The rates of dehydroxylation of three kaolinites which varied in particle thickness were studied from 417° to 480°C under very low water vapor partial pressures. The dehydroxylation rate was directly proportional to the surface area and had an activation enthalpy of 41.0 kcal/mol. The data did not fit either a diffusion‐controlled or a phase‐boundary‐controlled reaction in the radial direction, even when the particle diameter distribution was taken into account. The reaction appears to proceed through the kaolin particles by a pseudo‐phase‐boundary‐controlled mechanism, principally in the [001] direction.

The questionable kinetics of kaolin dehydroxylation

Georgia Kaolin, an accepted “standard” kaolin, evolves carbon dioxide and carbon monoxide when heated to 4–500 °C. While the total amount is small, it is evolved more readily than the water and with a different temperature dependence than the water. Weight loss data are not satisfactory for studying the early stages of the dehydroxylation. Separate measurement of the water is necessary.

Kinetics and mechanism of dehydroxylation of crocidolite

The kinetics of dehydroxylation of crocidolite have been studied in vacuo. The kinetic data may be interpreted in terms of a diffusion-controlled reaction based on radial diffusion out of a cylinder. The activation energy for the process is 49 kcal/mole.

Kinetics of Dehydroxylation of Kaolinite and Halloysite

Previous studies of the kinetics of dehydroxylation of kaolinite and halloysite point to first‐order reactions, in approximate conformity with the Arrhenius relation. Isothermal weight‐loss measurements have shown that the rate constants are markedly dependent on factors such as specimen size, shape, and compaction. A technique has been developed for determining the reaction kinetics of infinitely thin disk‐type specimens. The reactions are then strictly first order and the Arrhenius relation is obeyed. Activation energies of 65 and 55 kcal. per mole are obtained for kaolinite and halloysite, respectively. Comparison is made between the behavior of kaolinite and halloysite on the one hand and of macrocrystalline anauxite on the other. For anauxite, nucleation and growth of nuclei produce a sigmoid‐type reaction curve, but for the fine‐grained minerals it is believed that nucleation alone is the rate‐controlling process. The dependence of the reaction rate on geometrical factors is attributed to the retention of water vapor within the powder specimen. The influence of water vapor on these reactions is discussed generally.