Kinetics of Kaolinite Dehydroxylation

When kaolinite is heated under pressures of self-generated H₂O vapor from 0.4 to 3.2 atm, the dehydroxylation is best described by a nucleation-and-growth equation with m varying from 1.10 to 2.16 as the pressure is increased. The temperature coefficient of the rate constant, expressed as an apparent activation energy, varies from 62.4 to 260 kcal/mol. The H₂O vapor evolved in a self-generated-atmosphere sample holder was automatically swept into a gas chromatograph at fixed intervals. The measurement is specific for H₂O.     

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.     

The Multistep Nature of the Kaolinite Dehydroxylation: Kinetics and Mechanism

The thermal dehydroxylation of kaolinite has been reexamined using small sample weights, a homogeneous particle size distribution and high-vacuum conditions in order to reduce the influences of heat and mass-transfer phenomena. The controlled rate thermal analysis (CRTA) technique, which was specially developed to minimize the pressure and temperature gradients through the sample, was employed to carry out meaningful kinetic experiments. Two advanced isoconversional methods, the Vyazovkin and the Galwey methods, were used complementarily to determine the dependence of the activation energy on the degree of conversion. For this purpose, the Vyazovkin method has been adapted to CRTA experiments. It was demonstrated that there are at least two different stages, revealing the multistep nature of this reaction. The activation energy for the first step, which is assigned to nucleation and the growth of nuclei, decreases from 100 to 75 kJ/mol. The second stage corresponds to a diffusion process and the activation energy rises to 120 kJ/mol because of the metakaolinite formation, which closes the interlamellar channels and leaves isolated patches of kaolinite from which the water escapes with difficulty.     

Kinetics of Thermal Dehydroxylation and Carbonation of Magnesium Hydroxide

The kinetics of simultaneous dehydroxylation and carbonation of precipitated Mg(OH)₂ were studied using isothermal and nonisothermal thermogravimetric analyses. Specimens were analyzed using X-ray diffraction, transmission electron microscopy, and through measurements of the volume of carbon dioxide evolved in a subsequent reaction with hydrochloric acid. From 275° to 475°C, the kinetics of isothermal dehydroxylation in helium were best fit to a contracting-sphere model, yielding an activation energy of 146 kJ/mol, which was greater than values reported in the literature for isothermal dehydroxylation under vacuum (53–126 kJ/mol). The carbonation kinetics were complicated by the fact that dehydroxylation occurred simultaneously. The overall kinetics also could be fit to a contracting-sphere model, yielding a net activation energy of 304 kJ/mol. The most rapid carbonation kinetics occurred near 375°C. At this temperature, Mg(OH)₂ underwent rapid dehydroxylation and subsequent phase transformation, whereas thermodynamics favored the formation of carbonate. During carbonation, MgCO₃ precipitated on the surface of disrupted Mg(OH)₂ crystals acting as a kinetic barrier to both the outward diffusion of H₂O and the inward diffusion of CO₂.     

Adsorption Kinetics and Equilibria of Strontium onto Kaolinite

Adsorption of Sr²⁺ onto kaolinite has been studied by means of a radiotracer technique using the ⁹⁰Sr isotope. Bangham’s and McKay models have been applied to kinetic results in Sr²⁺ concentrations between trace ?0.1 mol.L^?1. The magnitudes of film and intra-particle diffusion coefficients are 10?10 and 10?14 m²·s^?1, respectively. Concentration dependence of diffusion coefficients indicated that Sr²⁺ ions are adsorbed on two different adsorption sites by an exothermic and spontaneous process. The Freundlich isotherm parameters and exchange equilibrium constants derived from selectivity coefficients indicate that Sr adsorption are depressed by competing cations in the order of Na⁺< Mg²⁺< Al³⁺.     

Kinetics and Mechanism of High-Temperature Reactions of Kaolinite Minerals

The kinetics of mullite growth from kaolinite and halloysite at high temperatures were studied by quantitative X-ray diffraction analysis. The evidence in both cases is consistent with a nucleation-controlled model.     

Mechanism for the Dehydroxylation of Kaolinite, Dickite, and Nacrite from Room Temperature to 455°C

The mechanism for the dehydroxylation of Keokuk kaolinite, dickite, and nacrite (Al₂Si₂O₅(OH)₄) was investigated in detail with the aid of powder X-ray diffraction, the Rietveld method, and infrared spectroscopy. It was determined that these polymorphic clay minerals dehydroxylate via an inhomogeneous mechanism in the range room temperature to 455°C. This result implies that when these samples dehydroxylate (up to 40% dehydroxylated at 455°C) the individual crystallites spontaneously react either completely, to form the metaphase of these minerals, or not at all, retaining all the structural details of the unheated crystalline material. These results amplify previous nuclear magnetic resonance results concerning this mechanism and are used to explain the appearance of the differential thermal analysis curves for these minerals.     

Inhomogeneous Dehydroxylation of Talc

Dehydroxylation and enstatite development during the decomposition of talc were investigated by transmission electron microscopy using in situ heating. Bubbles and voids are produced during dehydroxylation, and enstatite then precipitates near the voids. Evidence of preferential bubble nucleation at dislocations was obtained. Enstatite crystallites of random orientation were observed initially but on further heating the product phase was formed topotactically.     

Creep During Dehydroxylation of Magnesium Hydroxide

The creep deformation of cold compacts of Mg(OH)₂ during the dehydroxylation reaction was studied. The overall creep behavior can be divided into three stages. The initial stage is initiated by the dehydroxylation reaction. During the second or steady state creep stage the highest creep rate was obtained. The steady state creep rate was determined as a function of temperature, pressure, and relative density of the green compact. The results are represented by ė=Aσⁿ⁼¹ρ^-1 exp(-17,500/RT)s^-1. Particle sliding was considered to be the mechanism for creep in the second stage. During the last stage, the creep rate asymptotically approached zero. The reduction of the creep rate and the surface area at this stage may be due to coalescence or beck-growth between the freshly formed MgO particles.     

Dehydroxylation of Kaolinite, Dickite, and Halloysite: DTA Curves Under PH2O= 15 to 10,000 psi

The dehydration temperature of the kandites, observed by high-pressure DTA, increases with increasing water pressure to about 400 psi. Above this pressure, dehydration occurs at lower temperatures. Under experimental conditions, the dehydroxylation of kaolinite, dickite, and halloysite is metastable, with dickite persisting in the stability field of kandite + water to temperatures as much as 265°C above its upper stability limit.     

Hydroxylation and Dehydroxylation Behavior of Silica Glass Fracture Surfaces

The hydroxylation and dehydroxylation behavior of amorphous silica fracture surfaces was studied using temperature-programmed static SIMS. The results show that vacuum heat treatments result in more extensive condensation of silanol groups on the silica glass fracture surface as compared to fumed silica (Cabosil). This is attributed to differences in the distribution of silanol groups on the two silica surfaces. The rehydration kinetics of the dehydroxylated silica fracture surfaces showed two distinct reaction rates—an initial rapid increase in the silanol concentration, followed by a slower rehydration for longer dosing times. The slower rehydration reaction was shown to follow first-order reaction kinetics with the reaction rate constant, suggesting hydrolysis of strained siloxane bonds on three-membered silicate ring structures. The much faster initial rehydration is attributed to the hydrolysis of extremely strained siloxane bonds in two-membered, edge-shared tetrahedral rings. The effect of the dehydration time and temperature (i.e., thermal history of the surface) on the rehydration kinetics is also discussed.     

Phase-Pure Mullite from Kaolinite

Sintering of kaolinite in the presence of certain carbonate mineralizers, viz., CaCO₃, Na₂CO₃, and K₂CO₃, has been conducted at 950°–1350°C. The influence of these mineralizers at the above temperatures is evaluated using XRD and SEM techniques. A comparative study of phase formation of the above compositions shows that the sodium- and calcium-fluxed samples give rise to multiphase systems, while K₂CO₃-incorporated samples give phase-pure mullite. The observation that kaolinite in the presence of K₂CO₃ can act as a precursor material for phase-pure mullite is of great industrial significance.     

Dehydroxylation of Porous Glass by Means of Chlorine

Water in reconstructed 96 % SiO₂ glass is effectively removed by exposing porous glass to chlorine at high temperatures prior to final firing in vacuum. The annealing point of the final glass is markedly increased by such treatment, which results in elimination of the OH band due to replacement of OH⁻ by Cl⁻ ions.     

萍乡南坑高岭土增白实验研究

本文研究了提纯剂(柠檬酸钠和NaOH按一定比例配置)、还原剂(Na2S2O4)和稳定剂(草酸)用量对高岭土增白效果的影响.实验结果表明:当提纯剂用量为0.4%、还原剂用量0.2%、稳定剂用量0.3%时试样增白效果较好,由原来的62.3°增加到了86.50°.     

河源高岭土增白技术的研究

本文研究了提纯剂(柠檬酸钠和NaOH按一定比例配置)、还原剂(Na2S2O4)和稳定剂(草酸)用量对高岭土增白效果的影响。实验结果表明:当提纯剂用量为0.5%、提纯时间25min、还原剂用量0.5%、稳定剂用量0.3%时试样的白度最高,由原来的74.75°增加到了86.50°。     

芦溪高岭土增白实验研究

研究了提纯剂（柠檬酸钠和NaOH按一定比例配制）、还原剂（Na2S2O4）和稳定剂（草酸）用量对高岭土增自效果的影响。实验结果表明：当提纯剂用量为0．4％、还原剂用量为0．2％、稳定剂用量为0．3％时,试样的白度达到最高,由原来的74．75。提高到了86．50°。     

芦溪高岭土增白实验研究

研究了提纯剂(柠檬酸钠和NaOH按一定比例配制)、还原剂(Na2S2O4)和稳定剂(草酸)用量对高岭土增白效果的影响。实验结果表明:当提纯剂用量为0.4、还原剂用量为0.2、稳定剂用量为0.3时,试样的白度达到最高,由原来的74.75°提高到了86.50°。     

Kaolinite Refractory Bricks for Blast Furnaces

This standard is suitable to the fireclay bricks for blast furnace.