Caking of sodium chloride: Role of ambient relative humidity in dissolution and recrystallization process

Both, producers and users of divided solids regularly face the problem of caking after periods of storage and/or transport. Particle agglomeration depends not only on powder water content, temperature and applied pressure, but also on the interactions between the solid substance and water molecules present in the atmosphere, i.e., on relative humidity (RH) at which the product is stored. Ambient humidity plays an important role in most of events leading to caking: capillary condensation of water at contact points between particles, subsequent dissolution of a solid and formation of saturated solution eventually followed by precipitation of solid during evaporation of water. Here we focus on the kinetics of dissolution followed by evapo-recrystallization of hygroscopic powder sodium chloride under controlled temperature and RH, with the aim of anticipating caking by predicting rates of water uptake and loss under industrial conditions. Precise measurements of water uptake show that the rate of dissolution is proportional to the difference between the imposed RH and deliquescence RH and follows a model based on the kinetic theory of gases. Evaporation seems to be governed by more complex phenomena related to the mechanism of efflorescence and crystal growth from supersaturated salt solution. The proposed methods provide values of Deliquescence Relative Humidity (DRH) and Efflorescence Relative Humidity (ERH) for sodium chloride in good agreement with literature data.

► Variations of RH around DRH provoke dissolution and recrystallization of NaCl.
► An original method of determination of DRH is provided.
► Efflorescence is visualized by ESEM.
► Predicting caking is feasible by modeling dissolution-recrystallization kinetics.