Quantitative and kinetic studies of solid CACO3, Ca(OH)2 and CaO utilizing vibrational spectroscopy

Abstract

The decomposition of CaCO3 has been studied extensively for many decades. This reaction is the backbone of lime industry. The desired product of the decomposition of CaCO3 is CaO (lime), which is the integral part of many lime products. The decomposition is described by the following reaction: CaCO3 --> CaO + CO2 Limestone is a term that includes calcium carbonate with varying amounts of impurities, the most of which are silica and alumina. Therefore, the analysis of the decomposition of CaCO3 is the same as that of limestone, since CaCO3 is the major component of limestone. The main objectives of the studies revolved around acquiring qualitative, quantitative, kinetic and mechanistic information. The most frequently used methods of analysis have always been thermal methods (TG, DTG, DSC, etc). Due to many possible experimental conditions, reaction-influencing factors and the property of the product (lime), varying conclusions on mechanistic and kinetic properties of the decomposition of CaCO3 have been drawn. However, thermal methods are time consuming and use extremely small mass of samples. Other methods of analysis which gave excellent results (in agreement with thermal methods) were photoacoustic spectroscopy and loss-on-ignition. These methods are also prone to errors and could be umbiguous. In the present work, Fourier-Transform infrared spectroscopy was investigated for possible application to the quantitative study of this reaction. This was done on the mixtures of CaCO3 and Ca(OH) 2 containing 100, 85, 70, 50, 30, 15 and 0% of each component on the Brucker 113v spectrometer. A calibration curve of regression coefficient of 0.9950 was obtained. Following these good results, the FT-IR was then applied to the decomposition of natural limestone to obtain the kinetic information of the reaction. Four different industrial sample sizes of limestone were decomposed isothermally at 900, 950 and 1000°C. The results showed that the isothermal decomposition of limestone in air atmosphere can best be described by Contracting Volume or Ginstling-Brounshtein model. The kinetic parameter values fall within the range specified in the literature (as obtained using thermal methods). Furthermore, the quantitative FT-IR method was investigated for possible application to the determinations of the content of limestone used as an admixture to the cement blends. FT-IR quantitative analysis was performed on the cement blend samples containing gradually increasing amounts of limestone. Again, a good calibration curve with average regression coefficient of 0.9970 was obtained. A complementary study involving the weighing of the same samples before and after decomposition (for a set time at 1000°C) gave comparably good results (averaged regression coefficient of 0.9950).