Kaolinite Dehydroxylation Kinetics

Johnson, Howard B.; Kessler, Frank

doi:10.1111/j.1151-2916.1969.tb13365.x

Cited by 50 publications

(19 citation statements)

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“…Indeed in such case k is inversely proportional to the square of the particle dimensions, and the factor of 100 in the ratio of the reaction rates between the value reported by Criado et al (1984) and the one presently found for the kaolinite KGa-1 can be attributed to the factor of 10 ratio between average particle sizes in the two samples. A similar dependence of reaction rates on particle dimensions is also reported by Johnson and Kessler (1969).…”

Section: Ln(bft)=ln(af(~) ) Ea Rtsupporting

confidence: 79%

“…It has been frequently observed that in isothermal experiments, whatever the choice of the kinetic model, seldom a good fit of the conversion vs. time curve is obtained for conversion levels (e) larger than 0.6 (Brindley et al 1967). This observation has been explained either as an effect of the particle-size distribution in the sample (Holt et al 1962;Johnson and Kessler 1969) or as due to a change in mechanism, occurring at high conversion factors (Criado et al 1984). The evidence that the kaolinite dehydroxylation proceeds through a series of steps has been further supported by controlled rate thermal analysis experiments (Ortega et al 1993), and by several studies of the structural modifications occurring during the reaction (Leonard 1977;MacKenzie et al 1985;Suitch 1986).…”

Section: Introductionmentioning

confidence: 87%

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Kinetic study of the kaolinite-mullite reaction sequence. Part I: Kaolinite dehydroxylation

et al. 1995

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When dehydroxylation of kaolinite powder is carried out in the usual way, the linear relations anticipated for first-order kinetics and for the Arrhenius plot of log k versus 1/T are satisfied only very approximately. Factors relating to the form of the specimen, (shape, size, compaction, container, etc.) are shown to be very important. A method is developed for obtaining data for a specimen in the form of an infinitely thin disc. The first-order kinetic relation and the Arrhenius relation are then linear, and the latter gives an activation energy of 65 K cal./mol. The dehydroxylation process is shown by x-ray analysis to proceed crystal by crystal and this leads to an interpretation of the first-order kinetics. The x-ray method is used to study the distribution of reacted and unreacted material throughout a disc of material. Although isothermal conditions are employed, large differences are found between the interior and exterior of a partially dehydroxylated disc. These effects ~e attributed to the influence of a water vapor atmosphere within the heated disc. (1) INTRODUCTION The loss of weight when kaolinite is fired to temperatures exceeding about 450~ under normal atmospheric conditions is commonly ascribed to "dehy-dration" and the water involved in the reaction is designated "structural water." Neither term is correct, as the crystal lattice loses hydroxyl groups. The process is better described as "dehydroxylation" and it can be represented chemically by the equation OH+OH=H2OI' +O The mechanism is most probably one of proton migration so that if two protons momentarily find themselves associated with the same oxygen ion, there is a probability that a water molecule will be formed and will detach itself from the lattice. The present work was undertaken in the hope that a kinetic study of the process accompanied by x-ray examination would provide more detailed information about the process than is currently available. The crystal structure of kaolinite (Brindley and Robinson, 1946; Brindley and Nakahira, 1956) is so wet| known that no detailed description is required here. It suffices to recall that the four (OH) groups of the structural unit AI2Si2Os(OH)4 are located wholly in the octahedral sheet of the structure, three of them occupying positions on the outside of the layer structure and one within the structure. A previous study of the dehydroxylation of kaolinite 1 Contribution no. 56-35 from the College

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Section: Ln(bft)=ln(af(~) ) Ea Rtsupporting

confidence: 79%

Section: Introductionmentioning

confidence: 87%

Kinetic study of the kaolinite-mullite reaction sequence. Part I: Kaolinite dehydroxylation

et al. 1995

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“…A Table 4. Activation energies of the nucleation process from the Arrhenius plot of ln (z0) Johnson and Kessler (1969). The evaluation of the induction (i.e.…”

Section: Resultsmentioning

confidence: 99%

Kinetic study of the kaolinite-mullite reaction sequence. Part II: Mullite formation

et al. 1995

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Abstract. The present work is a follow-up of the investigation on the decomposition reaction of kaolinite as a function of the defectivity of the starting material and the temperature of reaction. In the present work we study the high temperature reaction of mullite synthesis from kaolinite, from the starting point of the results obtained in the first part.Time resolved energy-dispersive powder diffraction patterns have been measured using synchrotron radiation in isothermal conditions. The apparent activation energy for mullite nucleation and growth is found to be related to the defective structure of the starting kaolinite, which thus must have an influence on the chemical homogeneity of the amorphous intermediate phase.The analysis of the kinetic data indicate that the initial reaction mechanism is controlled by mullite nucleation, while as the reaction proceeds it shifts towards a grain growth-limited process which is intermediate between phase boundary and diffusion controlled. The order of the reaction obtained from standard analysis of the isothermal kinetic data is lower in the case of the ordered kaolinite KGa-1, in agreement with a rate limiting process more strongly limited by diffusion.For each sample there is a small but significant decrease in the order of the reaction at higher temperature: we interpret the change as related to the variation of the diffusion process in the amorphous phase due to the growing grains of mullite and cristobalite.The values of the activation energies and induction times are comparable neither to a model of mullite formation from a monophasic gel, nor mullite formation from a diphasic gel, being intermediate between the two. We can infer that the amorphous precursors from natural kaolinites can be considered pseudo-monophasic geMike phases, approaching the monophasic gel-like behaviour as the defectivity of the initial kaolinite increases.

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“…It is established that the dehydroxylation kinetics is first-order with a dehydroxylation rate directly proportional to the kaolinite surface area (27,28). Moreover, the dehydroxylation of kaolinite is an endothermic process accompanied by a rearrangement of the aluminium ions, which changes from octahedral coordination in kaolinite, to mainly tetrahedral coordination in metakaolinite (27,28,(29)(30)(31). Subsequently, amorphous silica and spinel is released and at higher temperature they react resulting in mullite formation (32).…”

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Relation between the microstructure and technological properties of porcelain stoneware. A review

Romero¹,

Perez²

2015

Mater. construcc.

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Porcelain stoneware is a strongly sintered ceramic material fabricated from ball clays-quartzfeldspar mixtures. Porcelain stoneware is characterized by its excellent technical and functional properties (low water absorption, high mechanical properties, resistant to chemical substances and cleaning agents, aesthetic possibilities …). These characteristic and technical features make that among the different types of ceramic tile, porcelain stoneware is the ceramic product that in the last years has best withstood the economic crisis in the construction sector. These properties are related to the microstructure of porcelain stoneware, which is a grain and bond type with large particles of filler (quartz), mullite crystals, a silica-rich amorphous phase and porosity. The understanding of the relationship between the microstructure and the properties of porcelain stoneware is hardly important for the development and design of these materials whose tendency is the manufacture of thinner tiles with higher dimensions but must continue to comply the specific technical requirements. RESUMEN:Relación entre la microestructura y las propiedades tecnológicas en gres porcelánico. Revisión bibliográfica. El gres porcelánico es un material sinterizado fabricado a partir de una mezcla de arcillas, cuarzo y feldespato. Tecnológicamente, el gres porcelánico se caracteriza por sus excelentes características técnicas y funcionales (baja absorción de agua, buenas propiedades mecánicas, resistente a substancias químicas, posibilidades estéticas…). Estas cualidades han propiciado que, entre los diferentes tipos de baldosa cerámica, es el gres porcelánico el producto que mejor ha resistido la crisis económica sufrida por el Sector de la Construcción. Las propiedades tecnológicas están relacionadas con su microestructura, compuesta de partículas de cuarzo, cristales de mullita, fase amorfa y porosidad. El conocimiento de la relación entre la microestructura y las propiedades tecnológicas del gres porcelánico es de gran importancia para avanzar en el diseño y desarrollo de estos materiales, cuya tendencia es la fabricación de piezas de mayor formato y menor espesor pero que deben seguir cumpliendo con los requerimientos especificados para su aplicación.

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Kaolinite Dehydroxylation Kinetics

Cited by 50 publications

References 9 publications

Kinetic study of the kaolinite-mullite reaction sequence. Part I: Kaolinite dehydroxylation

Kinetic study of the kaolinite-mullite reaction sequence. Part I: Kaolinite dehydroxylation

Kinetic study of the kaolinite-mullite reaction sequence. Part II: Mullite formation

Relation between the microstructure and technological properties of porcelain stoneware. A review

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