Agglomerates during crystallization

Jones, A.G., 1988. Agglomeration during crystallization and precipitation. In Crystallization Manual (Harwell Separation Processes Service, AEA Technology), Volume CR II, Part 2, 60pp. 

Hostomsky, J. and Jones, A.G., 1993a. Modelling and analysis of agglomeration during precipitation from solution. In Industrial Crystallization 93. Ed. Z. Rojkowski, University of Warsaw, 1993, pp. 2037-2041. 

A simplified schematic of various available SSP systems is shown in Figure 4.18. The four main process stages of crystallization, annealing, reaction and cooling are present in all of the plant types. The processes differ from one another by the way in which the pellets are agitated in order to prevent agglomeration during heat-up. 

When suspensions are formulated to provide a stable system, the particle size becomes critical. Flocculated suspensions also require careful particle size control either in the process of manufacturing or in the starting material. Equally important is the crystal habit — the outward appearance of an agglomeration of crystals. Crystal structure can be altered during the manufacturing process, particularly if the product is subject to temperature cycling, and this can alter the stability of suspensions. 

In polymers crystallized from the melt, in most cases spherulitic structures are observed spherical agglomerates of crystals and amorphous regions, grown from a primary nucleus via successive secondary nucleation (Figure 4.18). The dimensions of the spherulites are commonly between 5 pm and 1 mm. When spherulites grow during the crystallization process, they touch each other and are separated by planes. In a microtome slice they show a very attractive coloured appearance in polarized light. 

The methanol level in the precipitation vessel (25%) was optimized in studies of process conditions for the precipitation When methanol levels below 25% were used, gumming of the product was observed. The product of the above precipitation process filtered and washed well, did not agglomerate during drying, and, physically, appeared and behaved the same as micronized material. Such a continuous process was operationally attractive despite the dilution, since it shortened the time cycle (a large crystallizer was available) and avoided the labor intensive and dusty micronization process. However, the micronization process using wet API was adopted since it was deemed useable on all production sites. 

Crystal samples out of a crystallizer must be separated from the solution and dried. The separation from the solution typically is done on a laboratory centrifuge or on a Buchner Funnel. The separated crystals are typically washed with alcohol on the separation device and air dried before screening. Some products require special separation techniques to avoid fines precipitation or agglomeration during separation of the... 

Inclusion or occlusion takes place when impurities or solvents are physically trapped within crystals. This simation is different from the case of solid compounds. In inclusion or occlusion, impurities can be occluded sporadically within crystals, whereas in solid compounds, impurities are distributed throughout the crystal lattice. While both inclusion and occlusion enclose solvent or impurities in the crystals, inclusion is more applicable to cases where solvents or impurities are trapped within crystal cavities during crystallization, whereas occlusion is more applicable to cases where surface liquid is trapped within crystal clusters or agglomerates during drying. In this book, the terms inclusion and occlusion will be used interchangeably. 

Inclusion can also occur in crystal agglomerates during crystaUization. Again, it is hypothesized that under rapid crystal growth conditions, particle-particle contact in the crystallizer slurry results in agglomerates of the original primary particles, and solvent can be trapped within the agglomerates. 

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