In recent studies, the existence of mesoscale precursors has been confirmed in crystallization. Different from the classical crystallization theory, which only considers the sequential attachment of basic monomers (atoms, ions, or molecules), the nonclassical crystallization process involving precursors such as prenucleation clusters, nanoparticles, and mesocrystals is more complicated. The mesoscale structure is important for the quantitative description and directional regulation of the solution crystallization process. It is necessary to explore the mechanism by the mesoscale scientific research methods on the base of traditional chemical engineering and process system engineering research methods. Therefore, the paper reviews several representative nonclassical nucleation and growth theories, mainly including two-step nucleation theory, prenucleation clusters theory, particle agglomeration theory, amorphous precursor growth theory, particle attachment growth theory and mesocrystal growth theory. Then, the mesoscale structure and its spatiotemporal dynamic behavior are discussed, and the application of the EMMS model in the nucleation and growth process is analyzed. Finally, we put forward our views on the prospect of the paradigms and theoretical innovations of using mesoscale methods in crystal nucleation and growth.

In salt lake KCl cold decomposition–direct flotation for K-Na separation, octadecylamine hydrochloride (ODA-H) acts as a collector, reducing product purity and limiting high-end applications of salt lake KCl. To study KCl purification mechanisms and optimize crystal properties, this study investigates ODA-H aqueous systems. By regulating the cooling rate, stirring rate, and ODA-H concentration, the crystallization purification of KCl and precise control of its morphology were achieved, ultimately yielding three typical crystal morphologies: cubic, spherical, and ellipsoidal. Of the three crystal morphologies, spherical crystals have the best flowability but lowest purity. Ellipsoidal crystals have better flowability than cubic ones: their purity exceeds that of cubic crystals before washing but falls below it after washing. Cubic crystals, with poorer flowability, reach the highest purity post-washing. This study provides a theoretical basis for enhancing purity via crystal morphology regulation and industrial-scale purification of salt lake KCl.