Caking of crystals: Characterization, mechanisms and prevention

Caking of crystals leads to a great deal of negative impact on the quality of products leading to extra cost to restore the products to its original state before caking. It is therefore an undesired phenomenon that should be investigated and inhibited. This review considers particle issues (particle science and technology) in a crystallization view (crystallization science and engineering). 1. Characterization. Caking indices and the relevant characterization methods are classified. Their advantages and disadvantages are summarized. Particularly, potential novel characterization methods, e.g. image analysis technique and its application in analysis of caking ratio, are discussed. 2. Mechanisms. Caking process is organized into three stages: moisture sorption, then liquid bridge, and finally crystal bridge. The mechanisms are reconsidered with recent studies (e.g. mass transfer, adhesion effect of liquid bridge, kinetic of crystal caking) and crystallization science (e.g. nucleation and growth theory, polymorphism, and phase transition). 3. Prevention. Based on particle and crystal engineering, prevention of crystals caking is discussed through controlling ambient conditions (relative humidity, temperature, liquids screening, and pressure) and improving anticaking property of crystals (particles size, particles shape, impurity, and anticaking agent). A potential method of simulating crystal caking behavior based on discrete element method (DEM) and caking mechanisms is presented. With the aid of simulation, the enhanced design of particle and crystal engineering would be available, which could be a key to control these factors systematically.