Modulation of crystal content
The slurry density can be altered in various ways. Relative to its natural density, we can increase or decrease it.
To increase the density, we decant the crystallizer’s slurry and treat it in a clarifier. The clear overflow is evacuated and the thick underflow is recycled to the crystallizer.
Instead of a clarifier, we can also use:
- - an undisturbed space inside the crystallizer (clarification zone);
- - a centrifugal screw decanter with horizontal axis;
- - a gravity decanter;
- - a battery of hydrocyclones in series.
In order to decrease the slurry density, we decant it, treating it in a centrifuge. The crystals are evacuated to drying and the mother liquor is recycled to the crystallizer, following a purge to eliminate impurities, or to eliminate the mother liquor from the material balance if there is no evaporation. The greater the flow crossing the centrifuge or the filter, the lower the slurry density.
Often, we need to increase the slurry density in order to increase the crystallizer productivity (production per unit of volume). For this result, we proceed (as explained above) by separate decantation of the clean liquor. Indeed, in Figure 4.4, we observe that the crystal size decreases. Thus, Bubnik et al. [BUB 84] showed that in the crystallization of sucrose, an increase in the crystal weight ratio in the magma from 20 to 60% provokes a reduction by half in the crystal growth rate.