Contrary to many people’s beliefs, mixing and crystallization have a strong interaction. A study of calcium oxalate crystallization showed that the particle size distribution was affected significantly by impeller speed and other mixing variables. Mixing affects crystal nucleation, growth and slurry maintenance in different ways, which makes crystallization the most difficult operation to scale-up successfully [3, 9, 22].

General problems are:
• control of crystal size to ease filtration wash and drying.
• excess nucleation and/or crystal fracture.
• occlusion of impurities and unacceptable physical properties.

Equipment. The crystallizers employed in the fine chemical and pharmaceutical industries are generally multipurpose vessels with a pitched blade turbine, which have good circulation and relatively low shear to reduce secondary nucleation and crystal breakage. Another good impeller choice in some crystallization processes is Intermig by Ekato. Baffles are necessary to prevent poor mixing. Other crystallization vessels could also be a batch crystallizer, fluidized bed crystallizer or draft tube baffle (DTB) crystallizer [22]. Fluidized beds are optimized for crystal growth by providing low shear, low energy and minimum impact between crystals, while impinging jets do the opposite by promoting nucleation with extreme mixing, high shear and large energy input.

Mixing and Crystal Growth. Mixing minimizes the supersaturation gradient in the film around a growing crystal by maintaining high mass transfer rates. Additionally, mixing improves heat transfer, bulk turnover, dispersion of additives like an antisolvent, uniformity of crystal suspension, avoidance of settling, minimization of wall scale and reduction of impurity concentration at the crystallizing surface. Over-mixing results in crystal breakage and shredding of nuclei promoting secondary nucleation.