The DTB crystallizer is used when a larger particle size is desired. The crystallizer is a combination of a clarifier and a mixed tank in a single vessel. The mixed-tank part of the unit includes a slow-turning, high pumping efficiency internal circulator located concentrically in a tube (the Draft Tube) that helps direct the flow and improve the mixing of the slurry. The internal circulator provides the mixing of the slurry at a very low energy input, which is improves the growth rate of the crystals, and has low attrition for the crystals formed. The clarifier section provides the means to selectively remove and destroy smaller crystals in the mixed-slurry section, thus increasing the average product size.
The destruction of the small particles is achieved either by heating or by diluting the brine containing these particles. After this has been done, the heated (or diluted) brine is returned to the crystallizer.
DTB units are also used successfully for crystallization from reaction, such as fertilizers made by the neutralization of an acid with a base, because of their high level of internal mixing and the ability to introduce the reactants into a zone of high turbulence.
A simplified variation of the DTB is the Draft Tube (DT) crystallizer, which is a unit that contains only the mixed-slurry feature of the DTB (no baffle, i.e. no clarification zone). The DT is used for some reaction of flash cooling operations, where the main objective is to provide a well-mixed volume.