Secondary Nucleation of Sodium Chlorate: The Role of Initial Breeding

Abstract

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.

Crystal Growth, Dissolution, and Agglomeration Kinetics of Sodium Chlorate

“…In order to study the effect of seeding at different supersaturation levels, runs 1–5 (Table ) were conducted using seed material of the same sieve size fraction (25–125 μm) but different initial σ 0 values ranging from 0.023 to 0.062. Attempts to study the system at σ 0 > 0.062 were unsuccessful due to spontaneous nucleation prior to reaching the experimental temperature of 22.5 °C, regardless of the cooling rate, demonstrating the narrow width of the metastable zone for sodium chlorate, as reported elsewhere Figure (left) shows the initial number- and volume-based PSDs ( t = 0) recorded immediately after seeding each run.…”

“…Attempts to study the system at σ 0 > 0.062 were unsuccessful due to spontaneous nucleation prior to reaching the experimental temperature of 22.5 °C, regardless of the cooling rate, demonstrating the narrow width of the metastable zone for sodium chlorate, as reported elsewhere. 15 differences. It appears that the higher the supersaturation at seeding, the larger the total number of particles and the smaller their mean size.…”

“…In fact, it has been previously shown that ground sodium chlorate seed crystals can readily induce initial breeding secondary nucleation even at conditions in which washed seeds fail to do so. 15 While initial breeding secondary nucleation is typically believed to be largely independent of supersaturation, 27 the supersaturation conditions at seeding will affect the survival of nuclei as at higher supersaturations, the critical size is lower. Also, at higher supersaturations, secondary nuclei will grow faster to detectable sizes.…”

“…The latter process (termed as Viedma ripening) is of great industrial importance as it can be used in an analogous manner for the crystallization-enhanced deracemization of intrinsically chiral molecules (e.g., pharmaceutical molecules) by combining a racemization reaction in the liquid phase. , Viedma ripening is currently thought to occur due to a complex interplay between almost all crystallization phenomena, namely, crystal breakage, growth, dissolution, and agglomeration, and theoretical models have been developed to qualitatively account for these mechanisms. − However, to establish a quantitative model, the individual rate processes involved must be further studied and understood. While the (secondary) nucleation mechanisms for sodium chlorate have been the subject of substantial and thorough investigation in the context of chiral crystallization, − much less attention has been paid to crystal growth/dissolution, and to our knowledge, no study has investigated crystal agglomeration for sodium chlorate.…”

Enabling Mechanical Separation of Enantiomers through Controlled Batchwise Concomitant Crystallization: Digital Design and Experimental Validation

“…36,37 This is known as initial breeding. 38 Furthermore, if any of the pre-sieved seed crystals are aggregated (meaning they would not pass through the sieve during seed preparation), they could become de-aggregate by fluid mixing in the crystallizer during the process. However, this mechanism is least likely to occur.…”