Growth and Aggregation Phenomena in Precipitation of Calcium Carbonate
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Particle size-enlargement processes in precipitation of calcium carbonate have been investigated by focusing on determination of crystal growth rates and aggregation rates of the metastable polymorph vaterite. This was motivated by the fact that industrial processes in continuous operation are likely to give vaterite as the dominant polymorph due to the high-sustained supersaturation resulting from requirements of high yield. The typical morphology of these vaterite particles is spherical and cauliflower-like (50-100mm) suggesting aggregation of smaller spherical segments to be the dominant size-enlargement mechanism. However, photographic investigations of conglomerate particles may be deceptive. The relative influence of crystal growth and aggregation on the size of vaterite particles was hence determined by a combination of seeded batch experiments and population balance modelling. The growth and aggregation rates were extracted from experimental particle size distribution by solving the inverse problem based on a discretised population balance. A survey of the literature identified a conflict in the description of formation mechanisms of the proposed primary segments of the cauliflower-like particles, the smaller (<10mm) spherical polycrystalline particles of vaterite often formed in spontaneous precipitation experiments. Numerous papers suggest that primary vaterite is formed by aggregation of nanometer sized crystals. On the other hand, spherical vaterite particles have also been shown to be the product of ionic growth, second-order in supersaturation, implying that the rate is controlled by surface integration. The results presented in this work do not support the concept of particle growth by nano-aggregation. Spherulitic crystallisation, a growth mode not normally associated with the precipitation of sparingly soluble salts from solution, is capable of explaining the polycrystalline nature, the spherical shape and the size of vaterite without the introduction of a nano-aggregation hypothesis. Spontaneous precipitation experiments were performed in order to produce a suitable seed population for the seeded experiments. A simple and successful standard method was developed by mixing 250 ml of solutions of 0.1 M Na2CO3 and 0.1 M Ca(NO3)2 at 25°C in a 1-litre vessel stirrer at 1500 rpm. Amorphous calcium carbonate formed initially and transformed to vaterite (with small amounts of calcite) within 6 minutes. The particle size distribution ranged from 4-12 mm with a mode size of 8 mm and was stable in terms of numbers and as pure vaterite for at least 20 hours. The results obtained from varying the temperature, stirrer rate, and concentration confirmed what has previously been observed in the literature. The polymorphs nucleate simultaneously and the transformation mechanism is one of dissolution and re-crystallisation, controlled by the growth of the less soluble phase. The growth of vaterite spherulites was found to be size-independent, independent of fluid shear rates and second-order in supersaturation. The growth rate constant was determined to kr = (0.52 ±0.03)x10-9 m•s-1 at 25°C. It increases rapidly with temperature to a value of kr= (2.0 ± 0.3)x10-9 m•s-1 at 40°C corresponding to an activation energy of 69 kJ/mol. The aggregation of vaterite spherulites was found to be size-independent, directly proportional to the growth rate, and close to inversely proportional to the average volume shear rate. The dependence of the growth rate is consistent with a model of aggregation controlled by cementation, governed by the growth of a crystalline bridge between the particles. The collision frequency was already sufficient at the applied stirrer rates and higher shear rates only resulted in lower aggregation rates, as the force acting on the newly collided particles were increased. Typical cauliflower-like particles were produced at high stirrer speeds and continuous reactant addition, demonstrating how this morphology is a result of spherulitic growth, not of aggregation of smaller spheres. It has also been shown that calcite will grow as spherulites, at certain conditions, and how the concept of spherulitic crystallisation provides additional design possibilities in the production of calcium carbonate products. These results clearly demonstrate the deceptiveness of photographic diagnosis and how more fundamental methods are required in order to separate the effects of growth and aggregation in precipitation studies.