Secondary nucleation mechanism

Several features of secondary nucleation make it more important than primary nucleation in industrial crystallizers. First, continuous crystallizers and seeded batch crystallizers have crystals in the magma that can participate in secondary nucleation mechanisms. Second, the requirements for the mechanisms of secondary nucleation to be operative are fulfilled easily in most industrial crystallizers. Finally, low supersaturation can support secondary nucleation but not primary nucleation, and most crystallizers are operated in a low supersaturation regime that improves yield and enhances product purity and crystal morphology. 

Crystallization can be divided into three processes the primary nucleation process, the growth process, and the overgrowth process. The growth process is mainly controlled by the secondary nucleation mechanism. The steady (stationary) primary and secondary nucleation mechanisms of atomic or low molecular weight systems have been well studied since the 1930s by applying the classical nucleation theory (CNT) presented by Becker and Doring, Zeldovich, Frenkel and Turnbull and Fisher and so on [1-4]. 

Although the overall phenomena observed during polymerization seem to be accounted for, several interesting questions about the crystal morphology, primary and secondary nucleation mechanism, directiveness of polymer chain and crystal growth, and location of ion and counter-ion... 

The following section will discuss homogeneous, heterogeneous, and secondary nucleation mechanisms and kinetics. This will be followed by a similar discussion of crystal growth. The reader is directed to the references cited above, and others, for detailed treatment of these phenomena. 

Figure 2.22 Differentiation of the secondary nucleation mechanisms using enan-tiomorphous crystals. The fraction of the crystals with the same modification as that of the parent crystal is denoted as percent. (Reproduced with permission from Denk and Botsaris 1972a.)...

Secondary Nucleation. In many crystallization processes, the width of the metastable zone decreases in the presence of erystals of the crystallizing material. Such crystals, in the form of seeds, are sometimes intentionally added to the precipitator to induce nucleation of highly supersaturated but not yet erystallizing systems. New nuclei are then formed via a secondary nucleation mechanism. On the other hand, for systems with a very short induction period (e.g., precipitation via ionic reactions), primary nucleation mechanism is overwhelming. 

As previously noted, most zeolite syntheses of commercial value occur in systems clouded with an amorphous gel phase due to higher product yields, admitting to the possibility of homogeneous nucleation due to solubility differences, or to heterogeneous nucleation due to the abundance of foreign surface in the mediiun. Seeding these mixtures, or agitating the solutions, could induce nucleation by any of the secondary nucleation mechanisms. However, zeolite syntheses also have been conducted successfully in dilute clear alumino-silicate media, i.e., in the absence of any amorphous gel phase [14-26]. In fact, one of the early papers by Kerr [1] reported on a technique whereby dried gel was... 

When nucleation takes place without any crystal surfaces, we have primary nucleation. Primary nucleation is said to occur by homogeneous nucleation when no dissolved impurities are present. When primary nucleation occurs due to the presence of dissolved impurities, we encounter heterogeneous nucleation. When nuclei are formed due to the presence of existing macroscopic crystals, interaction with the crystallizer wall, rotary impellers, fluid shear, etc., we have secondary nucleation. Mechanisms of secondary nucleation are not sufficiently clear. An introduction to theories on secondary nucleation is provided by Myerson (1993). Here we will focus on homogeneous nucleation. Note that homogeneous nucleation is rarely achieved or desired in practical crystallization (McCabe and Smith, 1976 Myerson, 1993). 

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