These columns act to free the crystals of interstitial fines, they are often arranged at the base of a crystallizer.
Large crystals descend into the column while the liquor rises against the current, carrying the fines away, which provides a certain classification of crystals. Once the fines have been removed, the large crystals are decanted at the foot of the column. Furthermore, these columns can be used to cool crystals by means of the rising liquor.
If we are operating with an undersaturated liquor, we must dissolve the fines and thus increase the mean crystal size. On the other hand, if we are operating with a supersaturated liquor, we expel the fines in the crystallizer and reduce the mean crystal size, which is undesirable.
The phenomenon of fluidization applies in the operation of these columns.
Residence time of crystals in the column
Where HE is the column height, along this height, the mass of crystals per m2 of cross-section is:
DB : slurry “density”: here, crystal mass divided by slurry mass
If we provide ourselves with the elutriation flow GE, that is, the flow density in the crystal mass crossing a unit of column cross-section, we immediately obtain the mean residence time of crystals in the column.
This time is enough to obtain the classification of solid particles in a bed fluidized by a fluid and whose solid particles vary in size.