Ralf Beck

Biomimetic type morphologies of calcium carbonate grown in absence of additives

This report demonstrates how typical particle morphologies documented in biomimetic mineralization studies of calcium carbonate will precipitate also from solutions without adding modulating additives, at supersaturation levels below the solubility level of amorphous calcium carbonate (ACC). In the literature, hexagonal plates and flower structures of vaterite, as well as dumbbell structures of aragonite are explained by non-classical aggregation mechanisms from precursor crystals, assisted and stabilized by biomolecules, ions and templates, or by transformation from ACC. By performing experiments at both depleting and constant low supersaturation ratios for a range of temperatures, we show that the vaterite morphology changes from hexagonal monocrystalline plates expressing the basal (001) faces, to dendritic flower shapes and finally to spherulites, as a function of increasing supersaturation. Aragonite goes through a similar transition from monocrystalline elongated structures to polycrystalline dumbbells and spherical structures. We conclude that the key to understand the shape development is quantification of the activity based supersaturation and the realization that calcium carbonate forms along classical crystallization pathways. The higher number of crystals required for aggregation based growth is not favoured at these low supersaturation values. Dislocation and surface nucleation driven crystal growth is responsible for faceted morphologies at moderate supersaturation, whereas dendritic and spherulitic growth patterns appear due to interface instability at higher driving forces.

Analysis of Filtration Characteristics for Compressible Polycrystalline Particles by Partial Least Squares Regression

Crystal size and morphology have been varied by changing the initial supersaturation ratio and the temperature in reactive crystallization experiments. The influence of the chord length distribution, average cake porosity, and filtration pressure difference on the average cake resistance of polycrystalline particles of an industrially produced aromatic amine has been investigated by means of partial least squares (PLS) regression and sensitivity analysis. Analysis of the results has disclosed that wider chord length distributions as well as lower values of the measured average porosity lead to higher values for the average cake resistance. However, PLS regression and sensitivity analysis have identified the applied pressure difference itself as the most significant parameter influencing the magnitude of the cake resistance. This unexpected behavior is accounted for by compression of the filter cake occurring predominantly in small layers above the filter cloth characteristic for highly compressible cakes.