KINETICS OF THERMAL-DECOMPOSITION OF SOLI DS

Kinetics of thermal decomposition have always played an important role in metallurgy and processing of raw materials. Introduction of kineti c models of thermal dissociation leads to considerable simplification of running of such processes. The following stages may control the rat e of product formation: - chemical reaction at an interface; - nucleat ion; - diffusion of reactants of fast chemical reactions. The controll ing factor may change during the course of reaction. Measurements at n on-isothermal conditions were chosen to determine the kinetic paramete rs (as such conditions are closer to those which occur in a real techn ical process). Various methods of description of non-isothermal decomp osition of solids were discussed. There were analysed various function s f(alpha) describing the kinetics of decomposition of solids. The met hods based on one kinetic curve analysis (approximation, differential and integral methods) and those based on several kinetic curves (Kissi nger method, isoconversional methods in a differential form (Fridmann) , and in an integral form (Ozawa, Flynn-Wall)) were discussed. It was found that various approaches to solve temperature integrals which had been used led to the same values of kinetic parameters. The Coats-Red fern method was the simplest one. It is frequently used to describe th e kinetics of thermal decomposition of solids. As a model substance ca rbonate calcium was used. Although numerous papers-on the kinetics and mechanism of such reactions have been published, the important proble m of inconsistency in values of kinetic parameters obtained by various authors has not been solved so far. Thermal analysis of CaCO3 has bee n carried out using a derivatograph (MOM-1500, Budapest) under the air . CaCO3 dissociated into CaO and CO2 starting from about 580 degrees C and giving an endothermic band with the maximum at 820 degrees C. Fro m the mass loss observed by means of the TG-curves the alpha-T relatio ns were estimated. The curves obtained were processed by a special com puter program based on the method of the least-squares. Basing on the alpha(T) dependence the g(alpha) functions, from the well-known kineti c models which best described the experimental results, were chosen. T he values of activation energy and preexponential factor in the Arrhen ius equation as well as entropy, enthalpy and free energy of activatio n were calculated from the Eyring equation. The correlation coefficien t standard deviation and Snedecor's variable were calculated to aid se lections of the g(alpha) function. We have found that the thermal deco mposition of CaCO3 is mainly controlled by diffusion (models D2, D4) h owever the contracting area model R2 is also possible to be applied. T he activation energy of the decomposition process was 352 kJ . mol(-1) for the D2 model, 360 KJ . mol(-1) for the D4 model and 165 kJ . mol( -1) for the R2 model. It was found that the best results were obtained basing on the integral and differential methods, which were the most accurate. It was proved that the rate of heating did not influence the values of kinetic parameters. It was also shown that the values of fr ee energy of activation were independent of the model having been assu med, varying only in division on the part connected with E and A or De lta H and Delta S*.