Kinetic Studies on Thermal Decomposition of Synthetic Fluor-phlogopite Crystals by Thermogravimetric Methods

Abstract

In order to obtain a physico-chemical data of synthetic fluor-phlogopite F21CMg9(A1Sis010) crystals at high temperatures, the chemical kinetics on their thermal decomposition in air was studied by means of thermogravimetric methods. The influence of crystal size upon the decom- position kinetics was particularly studied. Results obtained are as follows;
1) Isothermal weight loss of fluor-phlogopite powder less than 325 mesh was measured at 1305-1145° C. A single curve, showing a (fraction changed) vs. tito., (to., was the time needed for a = 0.5), was obtained which was fitted for any of the samples tested. It was decided from this fact that the same reaction mechanism might be considered under the present experimental conditions. The equation F(a)= a2 which was based on one-dimensional diffusion-controlled process showed the best agreement. From the Arrhenius plots of the rate constants an activation energy of 46 kcal/mol was obtained.
2) For 42-32 mesh sample the reaction appeared to be of the first-order F(a)=In(1). From the Arrhenius plots of the rate constants an activation energy of 43 kcal/mol was obtained, which was nearly the same value as for less than 325 mesh sample.
3) On the other hand, as the particle size of the sample varied from less than 325 mesh to 42-32 mesh, the activation energy (Ea, ) and the order of reaction (n) for the thermal decomposition of fluor-phlogopite varied successively from 31 to 106 kcal/mol and 1.3 to 4.7, respectively. The above data were obtained by Freeman-Carroll analytical method with constant heating rate of 10° C. The above results showed that the kinetics of thermal decomposition was influenced by crystal size. As the kinetic data obtained by Freeman method did not show the pure physico-chemical process, they showed the complex decomposition process as a whole.
4) The kinetics of thermal decomposition were obviously dependent upon the experimental conditions (crystal sizes and the methods of analysis). The difference of experimental conditions seemed to change the kinetics in appearance.