The Preparation Of Micro/nano CaCO₃ And The Regulation On Its Polymorphs And Morphologies

As the further promotion in the application of calcium carbonate（CaCO₃）, the regulation on its polymorphs and morphologies becomes a hot topic. In this paper, a variety of templates are used to control the growth of CaCO₃ crystals, the factors which affect the polymorphs and morphologies of CaCO₃ are researched, and the forming mechanism of CaCO₃ particles is also investigated. In addition, the preliminary investigation on the preparation of CaCO₃ via thermal decomposition of Ca（HCO₃）₂ is carried out in the present work.First of all, the effect of different concentrations of palmitic acid, used as surfactant,on the polymorphs and morphologies of CaCO₃ during the carbonization of Ca（OH）₂slurry was studied. The polymorphs and morphologies of CaCO₃ powder are characterized by X-ray diffractometer（XRD）, Scanning electron microscope（SEM）,transmission electron microscope（TEM）, Laser particle size analyzer（ZPA） and Fourier infrared spectrometer（FT-IR）, respectively. The results indicate that palmitic acid cannot change the polymorphs of CaCO₃, but can affect its morphologies.The formation mechanism of CaCO₃ with different morphologies is proposed.Then, under the same experimental conditions, the regulation of long chain fatty acids with different carbon chain length, including lauric acid, stearic acid and palmitic acid, on the polymorphs and morphologies of CaCO₃ is investigated. The results show that the addition of fatty acids has no effect on the polymorphs of CaCO₃. However,with the increase in the length of carbon chain and in the amount of fatty acids, CaCO₃ particles are inclined to aggregate into rod-like aggregations, and their size tends to become larger. The formation mechanism of CaCO₃ aggregations is discussed from two aspects, i.e., the different deformation degree of micelles, formed by fatty acid with different carbon chain length, absorbed on the particle surface and the different aggregating states of aggregations at the different additions of fatty acids.Finally, the preparation of CaCO₃ via thermal decomposition of Ca（HCO₃）₂ is preliminary investigated. Based on our previous research, the effect of agitation speed on the formation of CaCO₃ is focused in this work. XRD and scanning electron microscope（SEM） are employed to analyze the polymorphs and observe the morphologies of CaCO₃. The results indicate that agitation speed can not only influence the morphologies of CaCO₃ particles, but also affect the polymorphs of CaCO₃. More specifically, the agitation limits the crystallization of vaterite, which is the least thermodynamically stable phase, and static state is in favor of the crystallization of stable calcite phase. When the PEG-6000 is added into the reaction system, the similar results are obtained. The effect of agitation on the crystallization of CaCO₃ is discussed from the viewpoint of atom arrangement.In summary, nano-CaCO₃ is successfully prepared by the carbonation of Ca（OH）₂slurry and the polymorphs, morphologies and their formation mechanism are proposed,which provides the theoretical basis for the regulating preparation of CaCO₃ with different morphologies. Particularly, the preliminary investigation on the thermaldecomposition of Ca（HCO₃）₂ provides a new way for the preparation of CaCO₃ powder and theoretical basis for the regulation of CaCO₃ powder with different polymorphs and morphologies in this preparation method.