Solid–liquid systems are very common in nature and industry. There are two main mechanisms of nucleation: primary nucleation, which can be further subdivided into homogeneous and heterogeneous nucleation, and secondary nucleation. The result of high supersaturation is a very high nucleation rate, and thus a very large number of crystals, which in turn limits the size of crystals. The crystal growth rate can be determined as a product of the rate of two-dimensional nucleation, crystal surface area, and the thickness of the mononuclear layer. The precipitation diagrams are constructed by plotting contours of equal values of a chosen parameter, like supersaturation or crystal growth rate. In the continuous mixed-suspension, mixed product-removal crystallizer, the unclassified withdrawal can be assumed, which means that the particle size distribution (PSD) of the product crystals leaving the crystallizer is the same as the PSD in the volume of the crystallizer.