Batch-precipitation of calcium carbonate from highly supersaturated solutions

The formation of different polymorphs in the batch-precipitation of calcium carbonate from aqueous sodium carbonate and calcium nitrate solutions was investigated. In most of the experiments vaterite was found to be the predominant polymorph. The vaterite crystals are composed of densely packed needle-like units with a thickness of 0.1 to 0.2 mu m in a spherulitic-type arrangement. Depending on the concentration and the temperature of the feed solutions as well as on the type of mixing device used, some precipitations led to calcite formation with an intermediate occurrence of vaterite. The transformation of vaterite to calcite is believed to be based on a mechanism of dissolution of the metastable vaterite and growth of the stable calcite crystals. A process of parallel nucleation of the different polymorphs is supposed to be responsible for the course of the generation of intermediate phases in the precipitation of calcium carbonate.