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The influence of process conditions such as feed rate, calcium/carbonate ratio, pH, complexing agents [ethylenediaminetetraacetic acid (EDTA), citrate (CIT)] and their concentration on the average particle size and shape of precipitated calcium carbonate was studied. The precipitation was performed in a semi-batch operated agitated vessel at constant pH by adding sodium hydrogen carbonate to a solution containing calcium chloride. In the absence of a complexing agent, agglomerates of needle-shaped crystals, probably aragonite, are obtained. Increasing feed time and the calcium/carbonate ratio increases the average particle size, whereas the opposite effect is observed for increasing pH. The observations can be related to the level of supersaturation. In the presence of complexing agents and at a concen-tration ratio of calcium vs. a complexing agent of 6, differently shaped and smaller particles were obtained. Furthermore, the effect of the other parameters on particle size becomes much weaker in the presence of complexing agents. In the presence of EDTA mostly spherical particles were obtained, and in the presence of citrate mainly rhombic particles corresponding to calcite were obtained. The effect on particle shape and size is attributed to interactions of the complexing agents with the faces of the crystalline calcium carbonate.

In the absence of complexing agents and under the chosen experimental conditions, agglomerates of needle-shaped single crystals suggesting aragonite are obtained. The average particle size is influenced by process conditions such as feed rate, calcium/carbonate ratio and pH. The complexing agents citrate and EDTA, however, have a considerable effect on particle morphology. Further, their presence limited the influence of other process conditions on the average particle size substantially. The results obtained in the absence of complexing agents show a clear connection between supersaturation level and average particle size. In simple terms, a higher supersaturation causes smaller product particles. In the case of solutions containing EDTA and citrate, an analysis of the level of supersaturation using the same maximum supersaturation cannot satisfactorily explain the observed variation in particle shape and size. Many studies have shown that a higher supersaturation favors the formation of vaterite or aragonite in accordance with Ostwald’s law of stages, but solely rhombic particles corresponding to calcite, for instance, are obtained for the case of citrate. It is therefore concluded that complexing agents such as citrate and EDTA limit the influence of other process parameters which play a more important role in their absence. Complexing agents, besides complexing calcium ions, influence the crystallization process by selectively inhibiting growth of certain polymorphs of calcium carbonate. This proposed mechanism coincides with the observations made in other studies that have reported on growth-inhibiting effects of similar substances even at very low concentration levels. Practical implications of the findings are that unless there is risk of hydroxide precipitation, calcium removal should be performed at around pH 10 to 10.5. The presence of calcium complexing agents may, however, hinder the precipi- tation process by lowering process speed, average particle size and yield.