Secondary nucleation fragments

Secondary nucleation results from the presence of solute particles in solution. Recent reviews [16,17] have classified secondary nucleation into three categories apparent, true, euid contact. Apparent secondary nucleation refers to the small fragments washed from the surface of seeds when they are introduced into the crystallizer. True secondary nucleation occurs due simply to the presence of solute particles in solution. Contact secondary nucleation occurs when a growing particle contacts the walls of the container, the stirrer, the pump impeller, or other particles, producing new nuclei. A review of contact nucleation, frequently the most significant nucleation mechanism, is presented by Garside and Davey [18], who give empirical evidence that the rate of contact nucleation depends on stirrer rotation rate (RPM), particle mass density, Mj>, and saturation ratio. 

Microseeding attempts to avoid the issue of secondary nucle-ation via fragmentation. In this case, the seed crystal is crushed into many microcrystals on purpose. A set of serial dilutions is conducted until the number of seeds is one or a few within a given aliquot (typically a few microliters). The desirable level of dilution requires some judgement and some screening, but the technique can reproducibly generate a few crystals within a given drop without concern for secondary nucleation. 

Strickland-Constable (1968) described several possible mechanisms of secondary nucleation, such as initial breeding (crystalline dust swept off a newly introduced seed crystal), needle breeding (the detachment of weak outgrowths), polycrystalline breeding (the fragmentation of a weak polycrystalline... 

Direct observation of impact-induced microattrition at the surfaces of potash alum crystals immersed in supersaturated solution (Garside, Rush and Larson, 1979) indicated that the majority of the fragments produced were in the 1-10 pm size range and had a supersaturation-dependent size distribution. Impact energy and the frequency of impact also have an important influence on the number of crystals resulting from contact secondary nucleation (Larson, 1982). 

Experiments in laboratoiy and industrial ciystallizers have shown that nuclei ate bom at supersaturations Ac Ac gj hom in presence of crystals (either product crystals or added seed ciystals). Such nuclei are called secondary nuclei. This secondary nucleation caused by the removal of preordered species on a crystal sm-face and attrition fragments can take place at very small supersaturations however, < c gj hetsecondary nuclei. In Fig. 8.4-1 the solubility c and the three metastable zone widths ACmet,hom, Ac gj het, a d Ac gj, gg Valid for homogeneous, heterogeneous, and secondary nucleation, respectively, are shown as a function of temperature T. 

These effeets of supersaturation imply that contact secondary nucleation results from a more complicated process than straightforward attrition and many authors have speculated about the actual mechanism involved. Strick-land-Constable (1979) suggested that the mosaic structure of the crystal provides the basic fragments. During a collision plastic deformation of the crystal takes place, stresses build up at the surface and large numbers of particles corresponding to the mosaic structure but modified by the deformation are produced. The modulus of elasticity and the elastic limit of the material may now be seen as possible important parameters determining the nature and extent of plastic deformation and the initiation of brittle fracture. 

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