XPS and SEM study of calcite bearing rock powders in the case of reactivity measurement with HCl solution

Abstract

The dissolution of seven natural limestones and calcareous rocks in hydrochloric acid solution was examined to investigate their capability for wet flue gas desulphurisation. All samples were crushed, ground and sieved to a size-fraction of 150-250 mm. Thereafter they were subjected to a dissolution experiment utilising stepwise titration with hydrochloric acid. The dissolution rates of three calcareous rocks were found to be controlled by reaction kinetics, while the limestones showed mass transfer control. The surface characterisation was implemented before and after dissolution experiments using X-ray photoelectron spectroscopy and scanning electron microscopy. Additional characterisation was carried out with X-ray diffraction, X-ray fluorescence and polarizing microscope. Initial reactivities have been shown to decrease in the order limestone, calcareous rock with high calcium concentration, calcareous rock with low calcium concentration. Coarse grain structure is proposed to decrease the initial reactivity.

Characterisation of dolomites before and after reactivity measurement with HCl solution

“…Using the interpretations of Järvinen et al, [24] it is suggested that mass transfer is not the rate determining step in the dissolution reaction since for all samples, the mass transfer coefficient is estimated to be higher than the kinetic rate coefficient.…”

“…Straight lines have been fitted with least squares method for evaluation of the rates of change of pH. The used mathematical model was originally presented by Ahlbeck et al [30,31] and later described by Järvinen et al [24] According to the model, the rate of change in hydrogen ion concentration is given by…”

“…where K is the overall rate coefficient (1/s) calculated form the rate of change in pH, [24] [H + ] is the bulk concentration of hydrogen ions (mol/l), V is the liquid volume (l), SSA is the specific surface area of the assumedly spherical particles (m 2 /g), m is the mass of all the particles in the suspension (g) and the subscript 0 refers to the initial value. It can be seen that k R,0 (106-150 μm) and the Rate follow each other consistently.…”

“…The overall rate coefficient K, which was used to calculate the Rate, takes into consideration both reaction kinetics and mass transfer, as given in Ref. [24] Possible error sources for the initial reactivities are especially the estimations done in the calculations, such as the assumption that the particles are smooth and spherical and a simplified chemical model. Also, extrapolation of the exponential fit to the beginning of the experiment (see Fig.…”

“…Hydrochloric acid was used instead of sulphuric acid (H 2 SO 4 ) because use of the latter could have led to precipitation of gypsum particles that would have influenced the measurement of particle size distribution. [24] Atomic concentrations of elements were obtained by surface sensitive X-ray Photoelectron Spectroscopy (XPS). Further characterisation was carried out with Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD) and polarised microscopy.…”