Secondary Nucleation of Sodium Chlorate: The Role of Initial Breeding

Secondary nucleation is the key mechanism behind the creation of new crystals in industrial crystallization processes. Sodium chlorate has widely been used throughout literature as a model compound to study secondary nucleation due to its ability to crystallize as a chiral solid which makes it feasible to determine whether new crystals have originated from solution or from a seed crystal. Despite its widespread use, a significant level of ambiguity regarding sodium chlorate still exists including inconsistent solubility data, nontransferrable results between batch and continuous experiments and inconclusive theories about whether secondary nucleation of sodium chlorate is possible through fluid shear. In the present work, the inconsistencies around sodium chlorate are resolved using novel continuous shear-induced secondary nucleation experiments involving stationary seed crystals. First, accurate solubility data of sodium chlorate in water was determined using a laser method and compared with literature data. Second, the metastable zone width was determined to be surprisingly narrow and continuous shear-induced secondary nucleation experiments of sodium chlorate using stationary seed crystals were therefore unsuccessful as heterogeneous nucleation was favored over secondary nucleation. This explains why previous continuous secondary nucleation experiments failed. Finally, it was found that mechanical impact readily created fines on the seed crystal surface and that the resulting fines acted as new particles through initial breeding. Based on microscopic analysis it was observed that a washing step was sufficient to remove fines from the seed crystal surface. Intriguingly, the resulting seed crystals without fines failed to induce secondary nucleation. Therefore, fluid shear was insufficient to disperse secondary nuclei of sodium chlorate. Overall, the results presented herein reveal a better understanding of secondary nucleation as the impact of initial breeding is reported in detail for the first time. Initial breeding significantly contributes to secondary nucleation and controlling initial breeding is therefore essential to govern crystallization processes.