Effect of Nonstoichiometry on Reaction Crystallization of Calcium Carbonate in a Couette−Taylor Reactor

The effect of stoichiometric reaction conditions on CaCO 3 crystallization was investigated using a Ca(OH) 2 −H 2 CO 3 reaction system in which the influence of ionic byproducts on the crystallization was effectively excluded. The largest rhombohedron-like crystals were produced under stoichiometic reaction conditions (pH 7.8), which corresponded closely to the isoelectric point (pH ≈ 8.2). When increasing the excess of the Ca 2+ species, the individual crystal size was rapidly reduced and the crystal shape gradually became a spindle and then a needle. Meanwhile, with an excess of CO 3 2-, rhombohedron-like crystals were predominantly formed, although the individual crystal size decreased responsively when increasing the excess species concentration. Accordingly, on the basis of the experimental results, the modifications in the crystal shape and size apparently stemmed from the excess species and involved molecular growth on the crystal faces via preferential adsorption. Plus, the agglomeration of individual crystals was promoted when increasing the excess concentration as a result of enhancing the collision efficiency of the crystals.