Estudo da produção de óxido de cálcio por calcinação do calcário : caracterização dos sólidos, decomposição térmica e otimização paramétrica

Abstract

The present work shows a study concerning to the productive process of the quicklime (calcium oxide) from the thermal decomposition of the calcite limestone (calcium carbonate). In this context, the aim of this study is utilize analytical and instrumental techniques to evaluate the reaction of calcinations of limestone, as well as kinetic and thermodynamic effects, heat and mass transfer associates to the operation variables. On the other hand, planning to check the quality of the quicklime produced, beyond the microstructural characteristics of solids, it had been carried through experiments of morphologic, structural and textural characterization of the samples. With them it was possible correlate microstructural characteristics, such as the specific surface area, with the final quality of the quicklime. Finally, using parametric optimization techniques and design of experiments, it was possible identify variables most significant in the quality of oxide, beyond determine best conditions of production. The thermodynamic analysis presented that the presence of carbon dioxide (CO2) inhibit the reaction rate of the thermal decomposition of limestone, and that the water (H2O) catalyzes the reaction. Small additions in the CO2 concentration produce significant increases in the time of calcination for a desirable minimum conversion. On the other hand, the presence H2O and CO2 support the sintering phenomena in the oxide structure, resulting in a less reactive quicklime. In the experimental conditions assumed, the reaction of thermal decomposition is limited by mass transfer, as it was observed in the tests with different masses and grain sized distributions of the limestone samples. In this case, a reduction in the average diameter of the limestone particles reduces enormously the time of calcination or the temperature for the decomposition. It was observed that with the increase of the CO2 concentration the thermodynamic effect overlap to the ones of mass transfer. In relation of the limestone characterization, was observed a big variability in terms of composition and crystallinity, in general the samples were hard and compact. It was verified that the thermal decomposition of the limestone generate an increase of porosity in solids. Finally, a correlation could be established between quicklime reactivity and specific surface area, in way that reactives quicklimes had presented bigger areas. The parametric optimization shows that the rise of time and temperature of calcinations produces an increase of quality of the quicklime in calcium oxide terms, however this operation becomes impracticable due to sintering process, observed in the BET surface area results. In this manner, the best regions of operation had been in temperatures around 900oC and 45 minutes of operation.