Study On The Preparation Of CaCO₃ Powder From Thermal Decomposition And The Mechanism Of Its Crystal Growth

Calcite, aragonite and vaterite are three common crystals of calcium carbonate（CaCO₃） and their typical morphologies are cubic（rhombohedral）, rod- or needle-like and sphere, respectively. In the present work, CaCO₃ was prepared through the decomposition of calcium bicarbonate(Ca（HCO₃）₂ by heating its saturated solution. The phase compositions of decomposed product CaCO₃ and the effect of reaction parameters on the polymorphs and morphologies of CaCO₃ were investigated. The preparation, under the static state, of superstructure CaCO₃ with different polymorphs and morphologies and the formation and growth mechnism of superstructures were also explored.Firstly, CaCO₃ was prepared under the condition of stirring and the influence of decomposition time, temperature and polyethylene glycol（PEG） with different molecular weight and addition amount on the phase composition of CaCO₃ via thermal decomposition was studied. X-ray diffractometer（XRD）, scanning electron microscope（SEM） and transmission electron microscopy（TEM） were imployed to analyze and observe the polymorph and morphology of CaCO₃, respectively, and the effect of time, temperature and PEG on the phase compositions of Ca CO₃ was summarized. Results show that, under stirring and at the absence of PEG, calcite and aragonite are found in the product of CaCO₃, and calcite becomes the major crystal phase with the time and temperature. While in the presence of PEG, the percent of aragonite increases, and the larger PEG molecular weight, the more amount of aragonite phase. In addition, a few of CaCO₃ particles with superstructure were found in the decomposed product at 90 °C after 60 min.Then superstructure CaCO₃ particles were successfully fabricated after the improvement parameters for the formation of superstructure, i.e. removing stirring during the decomposition of Ca（HCO₃）₂. And many kinds of CaCO₃ superstructure were obtained by changing the amount and type of surfactant. XRD, SEM, TEM combined with high resolution and electron diffraction were used to analyze and observe the polymorphs and morphologies of superstructures, and three forming paths and the forming mechanism of snow-like, thorn-ball and umbrella-like superstructure were put forward. In snow-like superstructure, the phase transformation from vaterite to calcite firstly takes place at the edge of vaterite wad initially formed, then CaCO₃ generated from Ca2+ and CO₃2- ions precipitates at the edge of wad as calcite lattice and grow into skeleton with six-fold symmetry, and finally, calcite, vaterite and amorphous CaCO₃ particles fill in the skeleton and form the integrated snow-like superstructure. Unlike snow-like superstructure, in thorn-ball superstructure, vaterite initially formed transforms to aragonite instead of calcite and CaCO₃ precipitates at the edge of wad as aragonite lattice and finally grows into thorn-ball. While in umbrella-like superstructure, phase transformation don’t occur and vaterite develops into flakes followed by falkes into umbrella-like superstructure.CaCO₃ powder with different polymorphs and morphologies was prepared successfully through the thermal decomposition of Ca（HCO₃）₂ solution and the polymorph and morphology can be adjusted by surfactants, which provides a feasible way for the preparation of CaCO₃ with different polymorph and morphology. Many kinds of CaCO₃ superstructures were obtained and their formation mechanism were also put forward, which provides the theoretical basis for the preparation of the superstructures of CaCO₃ and other materials.