Why this chapter is important

To create a population of crystals from a liquid, either a solution or a melt, supersaturation has to be induced by increasing the concentration of the solute, by decreasing of the temperature or increasing of the pressure of the liquid with respect to the equilibrium value. In a continuous crystallization process the supersaturation is sustained, while in batch crystallization the supersaturation is consumed in time. If the initial fluid is a clear liquid the formation of crystals begins with a nucleation process called “primary nucleation.” The type of nuclei and their rate of formation affect the size distribution of the crystal population, its polymorphic form and other properties of the crystals, so control of the nucleation process is crucial in obtaining the required product specifications. If new phase formation takes place by statistical fluctuations of solute entities clustering together in the solution or of molecules in the melt, it is called “homogeneous primary nucleation.” If, however, new phase formation is facilitated by the presence of tiny, mostly invisible particles such as dust or dirt particles, on which nucleation preferentially starts, it is called “heterogeneous primary nucleation.”

If, conversely, the supersaturated liquid already contains one or more crystals of the material being crystallized, these so-called parent crystals are able to breed nuclei sometimes after further outgrowth to sufficiently large sizes. This type of nucleation is called “secondary nucleation” because of the prior presence of crystals. It is the only nucleation mechanism in cooling or evaporative crystallization once crystals are present. The crystals present generally consume the supersaturation to values that are too low to induce primary nucleation, but still allow secondary nuclei to grow in the crystal population.

In batch crystallization from a clear liquid the process can either be started by primary nucleation or by the addition of seed crystals to the supersaturated liquid. The role and impact of the addition of seed crystals will be further addressed in Chapter 8.