Marijan Vučak

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.

Effect of precipitation conditions on the morphology of calcium carbonate : quantification of crystal shapes using image analysis

Extra-pure calcium carbonate was obtained in the laboratory by carbonation of a solution of calcium nitrate and monoethanolamine with carbon(IV) oxide under various operating conditions. The particle size and shape of the samples obtained were characterized by image analysis. The results show that the method of quantitative morphological characterization using defined shape descriptors enables us to get a better insight into the relationship of the mineralogical composition and particle size/shape to precipitation conditions.

Precipitation of calcium carbonate in a calcium nitrate and monoethanolamine solution

Abstract High-purity precipitated calcium carbonate has been produced in the laboratory by carbonation of a solution of calcium nitrate and monoethanolamine. The process was traced by measurements of the change in Ca concentration and in electrical conductivity of the solution. The κ−t and cCu−t curves were analyzed to determine the effect of temperature and concentration of carbon(IV) oxide on the process of carbonation. The samples obtained were mainly of aragonite and vaterite crystal modifications with traces of calcite. The relationship between the mineralogical composition and particle size (morphology) and operating conditions was examined as well.

Morphological development in calcium carbonate precipitation by the ethanolamine process

Morphological development in calcium carbonate precipitation by the ethanolamine process at 30 and 60°C has been examined using different techniques (quantitative image analysis, laser diffraction, XRD, and FT-IR). The initially grown phase is of vaterite modification that at higher temperatures (60°C) transforms to a more stable aragonite phase within the reactor itself. A comparison of the form and structure of calcium carbonate particles obtained during the process leads to a conclusion that crystal aggregation is the mechanism that determines the overall particle size at lower temperatures (30°C), while crystal growth dominates at higher temperatures. The results show that crystal characterization by quantitative image analysis permits a better understanding of the phenomena taking place in the reactor.

Investigation of dehydroxylation of gibbsite into boehmite by DSC analysis

The dehydroxylation of gibbsite into boehmite was investigated by means of DSC analysis under non-isothermal conditions in the temperature range 453–673 K at heating rates from 2.5 to 20.0 K min−1. Mathematical analysis of the experimental DSC curves revealed the mechanism and kinetics of the gibbsite dehydroxylation process. The kinetic curvesα=f(t) andα=f(T) are sigmoidal in shape; their inflection points and the νm point of the curvesν=f(T) andν=f(T) are interrelated and are defined by the concept of a stationary point. The activation energy for the first stage of gibbsite dehydroxylation in the temperature range 453–673 K is 132.92±8.33–142.26±8.33 kJ mol−1.

The examination of the phase transformation of aragonite into calcite by means of DSC analysis

Abstract The X-ray diffraction method was used to determine the content of aragonite and calcite in samples of precipitated calcium carbonate synthesized in the laboratory. The transformation of the aragonite modification present into the stable calcite structure was traced by means of DSC analysis. All the samples were analysed by DSC under non-isothermal conditions in the temperature range 624–772 K at heating rates of 5, 10, 15, 20 and 25 K min −1 . A mathematical analysis of experimental DSC curves defined the mechanism and the kinetics of the phase transformation of aragonite into calcite. The kinetic curves α = f(t) and α = f(T) are sigmoidal in shape, and their inflection points and the v m point of the curves v = f(t) and v = f(T) are interrelated and are defined by the concept of a stationary point. The kinetic curves indicate that v m corresponds to (d H /d t ) m of the endothermal peak of transformation of aragonite into calcite, which is characteristic of a topochemical reaction.

The Influence of Various Admixtures on the Calcium Carbonate Precipitation from a Calcium Nitrate and Monoethanolamine Solution

Calcium carbonate was precipitated by carbonation of calcium nitrate and monoethanolamine solution. The influence of various organic admixtures on the crystallization, crystal morphology, and polymorphism of calcium carbonate has been studied using different techniques. The results show that sucrose and fructose present even at very high concentrations do not affect the duration or mechanism of the carbonation process at a higher degree, what is attributed to the specific quality of the ethanolamine process. At high concentration citric acid and ethylenediamine-tetrakis-N,N,N,N-(methylenephosphonic acid) had some effect on the process mechanism, phase composition, morphology and crystal aggregation of the calcium carbonate precipitated.