Surface nucleation control, crystal

Massive barite crystals (type C) are also composed of very fine grain-sized (several xm) microcrystals and have rough surfaces. Very fine barite particles are found on outer rims of the Hanaoka Kuroko chimney, while polyhedral well-formed barite is in the inner side of the chimney (type D). Type D barite is rarely observed in black ore. These scanning electron microscopic observations suggest that barite precipitation was controlled by a surface reaction mechanism (probably surface nucleation, but not spiral growth mechanism) rather than by a bulk diffusion mechanism. 

By use of the proper experimental conditions and Ltting the four models described above, it may be possible to arrive at a reasonable mechanistic interpretation of the experimental data. As an example, the crystal growth kinetics of theophylline monohydrate was studied by Rodriguez-Hornedo and Wu (1991). Their conclusion was that the crystal growth of theophylline monohydrate is controlled by a surface reaction mechanism rather than by solute diffusion in the bulk. Further, they found that the data was described by the screw-dislocation model and by the parabolic law, and they concluded that a defect-mediated growth mechanism occurred rather than a surface nucleation mechanism. 

The growth kinetics describes the nucleation processes on the atomic scale. Thermally activated processes as adsorption, desorption, and diffusion at the surface and in the volume, nucleation, and crystallization/ recrystallization determine the film structure and can be controlled by the substrate temperature and the growth rate. Using a diagram ln(J ) over 1/ T, R being the deposition rate and T the growth temperature, three different growth modes (epitaxial, polycrystalline, and amorphous) can be... 

Because of the effect of static head, evaporation and cooling occur only in the liquid layer near the magma surface, and concentration and tem wrature gradients near the surface are formed. Also crystals tend to settle to the bottom of the crystallizer, where there may be little or no supersaturation. The crystaUizer will not operate satisfactorily unless the magma is well agitated, to equalize concentration and temperature gradients and suspend the crystals. The simple vacuum crystallizer provides no good method for nucleation control, for classification, or for removal of excess nuclei and very small crystals. 

Three additional considerations are required to successfully control a crystallization process. Local conditions rather than bulk, and instantaneous rates of change rather than mean values control the relative rates of nucleation and crystal growth. Further, the response of the system to control changes is history dependent. Spontaneous nucleation that accompanies an excursion beyond the supersaturation metastable limit dramatically affects the surface area available for crystal growth and influences the product CSD of a continuous process for several residence times or the final CSD of a batch process. 

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