Micro-mechanical model of calcium oxalate monohydrate aggregation in supersaturated solutions: Effect of crystal form and seed concentration

• Published in: Journal of crystal growth Vol. 361; pp. 176 - 188
• Main Authors: Pitt, K., Mitchell, G.P., Ray, A., Heywood, B.R., Hounslow, M.J.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 15-12-2012; Elsevier
• Subjects: A1. Aggregation; A1. Impurity; A1. Morphology; A2. Industrial Crystallisation; A2. Seed concentration; Agglomeration; Applied sciences; B1. Calcium oxalate monohydrate; Calcium; Cross-disciplinary physics: materials science; rheology; Crystal growth; Crystals; Electronics; Exact sciences and technology; Materials science; Mathematical models; Methods of crystal growth; physics of crystal growth; Optoelectronic devices; Oxalates; Physics; Seeds; Semiconductor electronics. Microelectronics. Optoelectronics. Solid state devices; Tanks; Theory and models of crystal growth; physics of crystal growth, crystal morphology and orientation; A2. Seed concentration; A1. Aggregation; A1. Impurity; A2. Industrial Crystallisation; B1. Calcium oxalate monohydrate; A1. Morphology; Crystal growth; Particle size; Crystal form; Calcium; Growth rate; Pinning; Crystal seeds; Aggregation; Supersaturated solution; Impurity effect; Oxalates; Hydrates; Stirring; Theoretical study; Light emitting diodes; High energy; Morphology; Growth mechanism; Crystal faces
• ISSN: 0022-0248; 1873-5002
• DOI: 10.1016/j.jcrysgro.2012.09.020

In this paper we report crystal growth and aggregation behaviour for calcium oxalate monohydrate (COM) in a stirred tank for two differing seed types – rounded and well defined – at various seed loadings.
Initially we used our previously developed model [1] to study the growth and aggregation. In this model a dimensionless strength, termed the Mumtaz number, has been formulated, which accounts for the effects of stirring, supersaturation and particle size on the aggregation rate of COM. Subtle differences in growth and aggregation rates were observed between the two populations of crystals; the model was unable to describe this behaviour. These differences were attributed to their different surface characteristics. Growth and aggregation kinetic parameters were also seen to be highly dependent on seed loading. This is attributed to poisoning by an unknown trace impurity, the effect of which is dependent on seed loading.
This has led to the development of a new model to account for both surface characteristics and the presence of a trace impurity that adsorbs onto the surface of crystals pinning growth steps. At low seeds loadings, surface coverage by the impurity is higher and so growth rates are reduced. These results are very well described by an extension of the approach of Weaver et al. [2].
We use Liew et al.'s [1] model to represent aggregation by a collision efficiency that depends on a modified Mumtaz number. This model requires the determination of a simple group of parameters that we term the ‘aggregation tendency’. The relationship between aggregation tendency and growth rate constant suggests that aggregation is in fact controlled by the growth rate of some high-energy facets not expressed macroscopically. The fact that aggregation tendency increases with surface coverage of impurity further suggests that the presence of impurity gives rise to longer or more numerous linear features along which initial contact between crystals takes place.
The combined growth and aggregation models are capable of describing all the experiments reported here for both seed types and over a range of seed loadings.
► Crystal morphology and seed loading affect the growth and aggregation kinetics of calcium oxalate monohydrate. ► The model of Mumtaz is able to characterise those kinetics. ► It is hypothesised that the effect of seed loading on apparent solubility is the result of poisoning by a trace impurity.