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The kinetics, the mechanism and the thermodynamics of activated state of formation of primary strontian feldspar via sinter-crystallization of non-equilibrium melt during the thermal treatment of ceramic body was investigated in this work via differential thermal analysis using isoconversional Kissinger kinetic equation. The process of formation of non-equilibrium melt and subsequent crystallization of primary strontian feldspar requires the activation energy of 631±3 and 664±2 kJ mol −1 , respectively. The investigation of mechanism of formation of primary strontian feldspar reveals that the process is driven by the surface nucleation and diffusion controlled growth of the new phase. The nucleation rate decreases with the time of process and non-equilibrium melt can be formed only in metastable equilibrium with activated state of strontian feldspar. Deep consideration of kinetic data leads to the deduction of new kinetic approach that enables single calculation of activation energy and frequency factor of heterogeneous processes as well as the dependence of thermodynamic parameters of activated state on temperature. Further consideration of kinetic data reveals that the activation energy is directly proportional to the function of csch ( z )+1. For z = e , this term enables to derive the value for the parameter B ( x ) in empirical equation for Arrhenius temperature integral p ( x ) proposed by Doyle to be 1.0642. [ABSTRACT FROM AUTHOR]

The kinetics and mechanism of delamination including the destruction of kaolinite layered structure of kaolinite from termite nest were investigated by non-isothermal high-temperature X-ray diffraction analysis. The process is controlled by the rate of the 2nd order chemical equation described by the kinetic function F2 : (g( α )=(1−y)−1 −1). The apparent activation energy was assessed to be 417±9kJ·mol−1. The delamination is considered as the first step of the thermal transformation of kaolinite into metakaolinite but the temperature at which the maximum rate of dehydroxylation was reached is lower than the temperature for the maximum rate of delamination if a heating rate of ≤5°C·min−1 was applied and higher for heating rates ≥15°C·min−1. The superposition of both processes within the heating rates of 5 to 15°C·min−1 led to a significant decrease of activation energy. This behavior provides an option to modify the degree of metakaolinite delamination during the thermal treatment. [ABSTRACT FROM AUTHOR]

The kinetics and mechanism of formation of gehlenite, Al–Si spinel phase, wollastonite and anorthite from the mixture of kaolinite and calcite was investigated by differential thermal analysis under the heating rate from 283 to 293 K min−1 using Kissinger equation. The changes in the phase composition of the sample during the thermal treatment were investigated via simultaneous TG-DTA, in situ high-temperature x-ray diffraction analysis and high-temperature heating-microscopy. The crystallizations of gehlenite and Al–Si spinel phase show apparent activation energy of (411 ± 5) kJ mol−1 and (550 ± 9) kJ mol−1, respectively. The value of kinetic exponent corresponds to the process limited by the decreasing nucleation rate for gehlenite while constant nucleation rate is determined for Al–Si spinel phase. Anorthite crystallizes from the eutectic melt and the process shows the apparent activation energy of (1140 ± 25) kJ mol−1. The process is limited by the constant nucleation rate of a new phase. [ABSTRACT FROM AUTHOR]

The structural order of kaolinite is an important factor that shows a substantial effect on the processes which take place during the thermal treatment of kaolin. The influence of structural order on the dehydroxylation process was investigated by simultaneous thermogravimetry and differential thermal analysis (TG-DTA). The thermal analysis was performed on the samples with gradually decreasing structural order prepared by milling procedure. The apparent activation energy of dehydroxylation process decreases with decreasing structural order according to the exponential function. The extrapolation of experimental data leads to the estimation of apparent activation energy of 76.6kJmol−1 and of frequency factor of 0.12×104 s−1 related to completely disordered form of kaolinite, while the ordered form shows the apparent activation energy of 216.17kJmol−1 and the frequency factor of 9.26×104 s−1. The relationships between features such as the infrared pattern of treated material, the degree of structural order and the apparent activation energy were established. [ABSTRACT FROM AUTHOR]

This work describes the non-isothermal kinetics and mechanism of gas-solid (dry) carbonation of lime obtained by thermal decomposition of dolomite. The course of the process was studied by constant cooling rate derivative thermogravimetric analysis in the flow of carbon dioxide. The activation energy of thermal decomposition during heating of dolomite is lower than that for the process of gas-solid carbonation of lime during cooling. From a thermodynamic point of view, this difference in activation energy is equal to the positive isobaric work. In the mechanism of both processes, diffusion is a rate-determining reaction step. However, the different geometry of the growing nuclei causes a difference in their mechanism. [Display omitted] • Dry carbonation of lime shows a higher E a than thermal decomposition of CaCO 3. • The difference in activation energy is equal to the positive isobaric work. • The effective mass of the activated complex is higher for dry carbonation. • Both processes are controlled by the rate of CO 2 diffusion. • The difference in the mechanism lies in the geometry of growing nuclei. [ABSTRACT FROM AUTHOR]

The reaction mechanism, the equilibrium composition, the temperature range of stability of formed intermediates as well as the kinetics and thermodynamics of activated state during the formation of monoclinic strontium-aluminum-silicate feldspar stroncian (SrAl 2 Si 2 O 8 ) via the ceramic route from the mixture of SrCO 3 , Al 2 O 3 and SiO 2 is described in this work. Strontian does not appear up to the temperature of 1150 °C and is the only stable phase at the temperature ≥1600 °C. Three independent reactions lead to two parallel reaction pathways, i.e. the formation of strontian from single or binary oxides (1) and with Sr-gehlenite as the intermediate (2). Since the reaction rate constants ratio is higher than one ( k 1 / k 2 >1), the first reaction route is favored according to the Wegscheider principle. The kinetics of chemical reaction of 1.5 order corresponding to the kinetic function F 2/3 ((1− α ) −1/2 −1) was determined as the rate determining the mechanism of formation of strontian. The integral and differential methods show that the process requires average apparent activation energy of 229.3 kJ mol −1 . The determined average value of frequency factor is 2.1×10 5 s −1 . [ABSTRACT FROM AUTHOR]

Abstract: The synthesis of strontium aluminate cement, the course of hydration and the thermal stability of hydrates are described in this work. The activation energy of the process of SrAl2O4 formation was calculated using the model-free Kissinger kinetic equation. The synthesis of the main clinker phase required the activation energy of 218kJmol−1. The value of kinetic exponent was corresponded to the process controlled by increasing nucleation rate of a new phase. For the hydration of strontium aluminate cement an immediate and intensive evolution of heat without any measurable induction period is typical. The cubic tri-strontium aluminate hexahydrate (Sr3AH6) and alumina gel (AH3) phases were formed as the first and also the main hydration products. The formations of strontium aluminate heptahydrate (SrAH7) and gibbsite (γ-AH3) were recognized after 7th day of hydration process. [Copyright &y& Elsevier]

The mechanism and kinetics of the process of thermal decomposition of low-manganese ferroan dolomite were investigated by non-isothermal derivative thermogravimetric analysis in this work. The study of the effect of the composition of gaseous phase on the mechanism and kinetics of thermal decomposition process includes the kinetics experiments performed in nitrogen, carbon dioxide, air and oxygen. While the process of endothermic decomposition takes place in one-step in a carbon dioxide-free atmosphere or in air, three separate reactions take place in carbon dioxide. The activation energy of these three reactions is also significantly higher than that of the thermal decomposition of ferroan dolomite in nitrogen, oxygen or air. [Display omitted] • The influence of the kiln atmosphere on the thermal decomposition of dolomite was described. • In O 2 , air and N 2 , the E a of thermal decomposition are 182, 232 and 234 kJ mol−1, respectively. • In CO 2 , the decomposition of dolomite is divided into three separate endothermic steps. • These reactions show higher E a than the thermal decomposition of dolomite in air, N 2 , or O 2. • The content of Mn and Fe in the dolomite leads to the formation of Mn-srebrodolskite. [ABSTRACT FROM AUTHOR]

The preparation and properties of hexagonal strontium-yttrate-silicate apatite (SrY 4 [SiO 4 ] 3 O, space group P6 3 /m) as the main product of sinter-crystallization process, in which the non-equilibrium melt was formed in the temperature interval from 1300 to 1550 °C in the SrO-Y 2 O 3 -SiO 2 system is described in this work. The formation of non-equilibrium melt is facilitated by borate fluxes, alkaline fluxes and talc. The apparent activation energy and the frequency factor of the sinter-crystallization process were determined to be 1525 kJ·mol −1 and 1.04·10 45 s −1 , respectively. The material shows low value of linear thermal expansion coefficient of (1.1±0.1)·10 −6 °C −1 in the temperature range from 25 to 850 °C. The important feature of this compound is the formation of colored center after the exposition to X-ray radiation hence prepared material is an important candidate for optical applications, sensors and dosimeters. [ABSTRACT FROM AUTHOR]

The α → β transformation between orthorhombic ( α ) and hexagonal ( β ) polymorphs of strontium carbonate has significant effect on the behavior of SrCO 3 during thermal treatment. The kinetics, mechanism and thermodynamics of activated complex in the process of thermal decomposition were investigated by non-isothermal thermogravimetric analysis (TGA) using mechanism-free (Kissinger equation) as well as kinetic function fitting methods. Both techniques as well as the calculation of theoretical activation energy (third law method) show that the transformation of α -SrCO 3 to β -SrCO 3 reduces the activation energy and changes the mechanism of the process. While thermal decomposition of rhombohedral SrCO 3 is limited by the rate of process to the reaction interface ( R 3 : g ( α )=1−(1– α ) 1/3 ), the process limited by the rate of chemical reaction of 1/3 order ( F 1/3 : 1−(1– α ) 2/3 ) was determined for the hexagonal polymorph. The samples before and after thermal treatment were investigated by infrared spectroscopy, scanning electron microscopy and X-ray diffraction analysis. [ABSTRACT FROM AUTHOR]

The formation of metakaolinite during the thermal treatment of kaolinite consists of three processes which include the destruction of kaolinite sheet structure, the dehydroxylation and the recombination of silica and alumina into the structure of metakaolinite. The sequence of these steps is significantly affected by heating rate. The kinetics, mechanism and the influence of heating rate on the course of these processes were investigated by the methods of thermal analysis (DTG and TDA) and high-temperature X-ray diffraction analysis (HT-XRD) using the Kissinger kinetic equation. The activation energy required for the delamination, dehydroxylation and recombination of silica and alumina tetrahedron into metakaolinite is 538, 195 and 143 kJ mol- 1, respectively. The thermodynamic data of the activated complex, calculated from the Wertera and Zenera law, enable to explain experimentally the observed influence of heating rate on the thermal transformation of kaolinite during heating. [ABSTRACT FROM AUTHOR]

Abstract: The densification of kaolin with high content of middle ordered kaolinite was investigated by non-isothermal thermodilatometric analysis (TDA) under load. The compacted powder was heated with constant heating rate (CRH) from 0.75 to 10°Cmin−1. The sintering reaction of kaolin upon the temperature interval, where sinter-crystallization of mullite and cristobalite takes place (1090–1250°C), is significantly affected by the heating rate. The dTDA results show that the process is sensitive to heating rate and the formation of both phases can be studied separately if heating rate higher than 4°Cmin−1 is used. The mechanism of involved formation of mullite and cristobalite is changed at heating rates higher than 6°Cmin−1. The values of apparent activation energies of mullite and cristobalite formations were determined by Kissinger equation and by several equations of model-free integral type of p(y)-isoconversional methods. The mechanism and thermodynamics of the process were evaluated through Kissinger and Eyring laws, respectively. [Copyright &y& Elsevier]