Agglomeration reactive crystallization

Several investigators have developed models for the effectiveness of collisions that lead to agglomeration including Nyvlt et al. (1985) and Sohnel and Garside (1992). This complex interaction of hydrodynamics and crystallization physical chemistry is difficult to predict or describe but can be critical to the successful operation and scale-up of a crystallization process. In particular, for reactive crystallization in which high supersaturation levels are inherently present, agglomeration is very likely to occur as the precipitate forms. Careful control may be necessary to avoid extensive agglomeration, as outlined in Section 5.4.3. below and in Examples 10-1 and 10-2 for reactive crystallization. 

Agglomeration and/or aggregation are common in reactive crystallization and should not be confused with true growth (see Section 10.3.8.1 below). Secondary growth phenomena can also be expected, such as growth rate dispersion and size-dependent growth (see Section 10.3.8.2 below). 

Reactive crystallization operations are subject to oiling out and/or agglomeration because of the inherently high local supersaturations encountered. As indicated in Section 10.3, the formation of a crystal may be preceded by oiling out as the first physical form that may or may not be observed (see also Chapter 5, Section 5.4). This oil may separate as a second phase because of the normally extremely low solubilities of the reaction products that result from the chemical reaction. This low solubility can cause a second liquid phase to form on a time scale that is shorter than the nucleation induction time. These issues are considered in Ostwald s Rule of Stages. 

Table IV shows the solubilities of various alkali feeds. In general, smaller solubility means that the movement of OH is controlled by ion pair of solute. So it may be difficult for Mg to collide with OH in calcium hydroxide which has the smallest solubility. From SEM photographs of magnesium hydroxide in Figure 3, primary particles are formed in the system of calcium hydroxide, whereas larger primary particles are formed and agglomerated easily in other alkali feed systems which have larger solubility. Then dominant particle size of agglomerated particle becomes larger. Therefore, the solubility may be the key property among the above-mentioned properties. This tendency was also observed in the reactive crystallizations of other carbonates, that is, calcium carbonate(9) and lithium carbonate(lO).

The addition of the seed slurry can be done at different stages onto the liquid surface (the crystals tend to agglomerate if they are not dispersed) near the stirrer, where the liquid is well mixed or by one of the feed streams in a bypass or loop in the crystallizer. The best way is to add seeds to the solution beneath the liquid level or to add them to a feed stream at the beginning of the batch or, in case of a semibatch system, continuously. In reactive batch crystallization, the nucleation rate can be decreased by the dilution of the feed streams or by the addition of fine crystals into one of the feed solutions or the reactor. The surface area and growth rate should be large enough to quickly reduce the supersaturation. The recirculation of the crystal suspension creates an... 

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