Ljerka Brečević

Vaterite growth and dissolution in aqueous solution. III. Kinetics of transformation

The kinetics of transformation of vaterite to calcite in aqueous solution was studied at temperatures between 25°C and 45°C and ionic strengths between 15 and 415 mmol dm−3. Both vaterite and calcite precipitated spontaneously, with vaterite being initially the only solid phase in the system. The progress of the transformation was followed by recording pH as a function of time. It was found that this transformation was solution-mediated, and that the growth of calcite was the rate-determining process. This conclusion was additionally supported by a mathematical model made to predict changes of both the solid phase and the solution during the overall process. The model is based on the rate constants and the mechanism of the processes involved in the transformation, as well as on the experimentally determined number density of the crystals formed. The seeded growth kinetics of calcite was determined in separate experiments, at temperatures between 10°C and 55°C, and the rate law was found to be parabolic. The high activation energy obtained for the temperature dependent rate constants Ea = 55.29 kJ mol−1, supports the assumption that the calcite growth was controlled by processes at the crystal surface.

Phase transformation of calcium carbonate polymorphs

Abstract The phase composition of mineral aragonite and synthetic vaterite samples was determined qualititavely by using IR-spectrophotometric and X-ray diffraction analyses. The transformation of aragonite, A, and vaterite, V, into the stable modification calcite, C, was followed using differential scanning calorimetry analysis. In order to determine the kinetics and mechanisms of these phase transformations, a number of experimental DSC curves were elaborated mathematically and the stationary point theory was applied. The activation energy, Ea, and the enthalpy, ΔH , were found to be, respectively, 234.5 ± 5.6 kJ mol −1 and 122 Jg −1 for the phase transformation A → C, and 252.8 ± 48.7 kJ mol −1 and −21.2 Jg −1 for the V → C transformation.

Dissolution kinetics and solubility of calcium carbonate monohydrate

Abstract Calcium carbonate monohydrate (CaCO3·H2O) has been synthesized and the crystals were characterized by means of optical microscopy, FT-IR spectroscopy, X-ray diffractometry and thermogravimetry. The solubility was found by recording pH during the dissolution of the monohydrate in water, preventing contact with carbon dioxide from the air. The solubility product was calculated from the final, equilibrium, pH value taking all the relevant calcium and carbonate species into account. In the temperature range 15–50°C the solubility product Kso could be expressed by −log Kso=7.050+0.000159 θ2 where temperature is expressed in °C. The kinetics of dissolution of calcium carbonate monohydrate crystals in aqueous solution were studied at temperatures between 15 and 45°C. The progress of dissolution was followed by recording pH as a function of time. It was found that the dissolution kinetics follow a second order rate equation with the rate constants k equal to 2.205 × 10−4, 6.190 × 10−4, 1.218 × 10−3 and 4.369 × 10−3 l mol−1 s−1/mg l−1 at 15, 25, 35, 45°C respectively, and that the activation energy for dissolution was 73.3 kJ mol−1, suggesting a surface-controlled process.

Vaterite growth and dissolution in aqueous solution II. Kinetics of dissolution

Abstract The kinetics of dissolution of vaterite (CaCO 3 ) crystals in aqueous potassium chloride solutions were studied at temperatures between 15 and 45°C and ionic strengths between 50 and 200 mol/L. The vaterite preparate used consisted of spherulites of vaterite crystals with initial radii of 2.7 to 4.6 μm. The progress of the dissolution was followed by recording pH as a function of time. From pH the concentration of the dissolved matter was calculated, and from this we found the particle size, r , and the dissolution rate, - d r /d t , where r = average particle radius and t = time. Straight lines were obtained by plotting - r (d r /d t ) as a function of undersaturation, c s - c , which may be explained by diffusional rate control. Both the values of the apparent diffusion coefficients determined and their dependence on temperature (activation energy = 24.38 kJ/mol) support the assumption that the rate-determining process for the dissolution is the diffusion of the dydrated calcium and carbonate ions away from the crystal surface into the bulk solution.

SELECTIVE DISSOLUTION OF COPPER OXALATE USING SUPPORTED LIQUID MEMBRANES

ABSTRACT A supported liquid membrane has been used to dissolve selectively copper oxalate from a suspension of copper, calcium and cadmium oxalate, which have low, similar solubilities. 2-Hydroxy-5-nonyl-acetophenone oxime (HX) dissolved in kerosene was used as a carrier for copper transport from the suspension to the stripping solution. A mathematical model of the copper permeation is presented. The model takes into account the dissolution kinetics of CuC204-1/2H20, the diffusion of copper ions through an aqueous stagnant layer, the chemical reaction at the aqueous/membrane interface, and the diffusion of the CuX2 complex in the membrane. The model fits the experimental data well with a unique parameter set, except for the transport from an acetate buffered system, for which a lower rate constant for the reaction at the membrane interface had to be assumed. In a separate set of experiments the dissolution of copper oxalate hemihydrate in water was found to be surface reaction controlled.

Cadmium removal from calcium sulphate suspension by liquid membrane extraction during recrystallization of calcium sulphate anhydrite

Abstract The removal of Cd2+ ions, incorporated in calcium sulphate anhydrite (AH), was performed by means of liquid membrane (LM) extraction. The LM extraction was performed during the process of solution-mediated transformation (recrystallization) of the cadmium-contaminated AH into the stable calcium sulphate dihydrate (DH). This transformation process involves the dissolution of AH and the crystal growth of DH. The extractants that transport Cd2+ selectively over Ca2+ ions (Alamine 304, tridodecylamine; and Alamine 336, trioctylamine), diluted in the appropriate organic solvent (kerosene), were applied. Liquid membrane, designed in a form of the so-called bulk liquid membrane (BLM), was found to be the most suitable configuration for the treatment of calcium sulphate suspension. Cadmium ions were successfully removed from the feed suspension and no cadmium was found incorporated in the stable, DH, phase. The mechanism of Cd2+ transport through the membrane is proposed.