Mixing and Crystallization

Utilize in-line mixing and crystallization techniques to create high supersaturation under controlled conditions (see Chapter 9). 

To more fully appreciate the inherent conflicts in mixing scale-up, let us consider solids suspension, fluid mixing, and crystal damage separately. 

Identification of such eflFects is made difficult, however, by the complexity of a crystallization experiment. Crystallization typically occurs from a molecularly inhomogeneous, aqueous gel, prepared by combination of a silica and an alumina source together with varying amounts of hydroxide ion. Since the product obtained is often nucleation-controlled, variables in an experiment can include not only the silica source (gel, sol, sodium silicate) and the alumina source (sodium aluminate, aluminum sulfate) but also the detailed mixing and crystallizing procedures (temperatures, aging, stirring rate, etc.). 

The types of agitator commercially available to maintain crystals dispersed in a crystallizer are many and varied. They include anchors, gates, flat paddles, propellers, radial-flow flat-bladed turbines, axial-flow angle-bladed turbines, and so on. Each has its own fluid mixing and crystal dispersion characteristics. The selection of the most appropriate agitator for a given duty is a highly skilled task and reference should be made to specialist publications and technical brochures. 

Mixing can also play a key role in affecting the morphology of a crystalline product. This effect results from the complex interaction between the impeller and nucleation and growth. The reader is referred to the discussion on mixing and crystallization in Chapter 17. 

Interactions between mixing and crystallization are often ignored. They should not be. In many cases, these interactions can affect every aspect of a crystallization operation, including nucleation, growth, and maintenance of a crystal slurry. To complicate the problem further, mixing optimization for one aspect of an operation may require different parameters than for another aspect, even though both requirements must be satisfied simultaneously. In addition, these operations are often scale dependent. For these and other reasons discussed below, many would contend that crystallization is the most difficult operation to scale-up—successfully. 

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