Effect of Growth Rate History on Current Crystal Growth:  A Second Look at Surface Effects on Crystal Growth Rates

The effect of a crystal's growth rate history on the current growth behavior was investigated as a cause of crystal growth rate dispersion (GRD), which is a significant problem in crystallizer design and modeling. The solute used in the experiments was sucrose, a high production commodity still crystallized mainly in batch crystallizers, where GRD is most significant. The results show that the growth history of a crystal has a significant effect on the crystal growth rate of that crystal. In particular, a history of rapid crystal growth under high supersaturation causes a roughening of the crystal surface, apparently due to poor surface integration, and subsequent growth occurs at lower rates than would be expected without this growth history. The process of crystal surface healing occurs at low levels of supersaturation over periods of several hours of growth and results in the crystals again reaching their normal growth rate levels. The growth layer formed to heal the surface of the crystal is greater than 100 μm thickness, far in excess of the scale of the apparent surface roughness. This study shows that the crystal perfection near the growing surface of the crystal may be a more significant indicator of crystal growth rate than the overall average perfection in the entire crystal.