Relating bulk solid caking events to phase change events that happen during crystallization processes

Abstract

Caking causes significant problems in the storage of bulk solids in process vessels. Caking results in large rathole formation, lumps in product, inability to empty bags, silos, hoppers, and poor flow in feeders. Caking problems reaches all levels of the production process from the large-scale silos to the small single use packages sold in stores.Caking is characterized as a gain of significant bulk strength during storage. It is responsible for many lost production hours and unscheduled downtimes in processes dealing with bulk solids – particularly powders. Caking is also mechanistic in its nature. In this paper we will deal with one of those causes. The goal of this work is to more clearly understand the relationship between the root cause of caking and the kinetics of the gain of strength observed when recrystallization results in powder caking. Detailed strength measurements seem to indicate that the caking of powders induces a gain in strength in accordance with two exponential growth terms. The first strength growth term is proportional to storage time to the 1st power. The second exponential growth term is proportional to storage time to 3rd power. It was found that this behavior was consistent with phase growth kinetics in accordance with the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation. This equation suggests that within a confined space where crystallization is occurring the fraction of space that transforms into a crystalline material has kinetics consistent with exponential functions that are proportional to time to the 1st power, 2nd power, and 3rd power depending on the dimension of the crystals that form. Rods or fibers correspond to kinetics proportional to time to the 1st power. Plates correspond to kinetics proportional to time to the 2nd power. The formation of spheres or 3D clusters correspond to kinetics proportional to time to the 3rd power. The bulk strength tends to have a one-to-one correspondence to exponential crystal growth to time to the 1st and 3rd powers where the 1st power exponential strength growth happens on a short time frame and the 3rd power exponential growth happens on a longer time frame. This suggests that crystal rods or fibers first form in the pendular volume between particles followed by 3D structure formation in the pendules during bulk solid caking events. Understanding the magnitude and kinetics of these events can give engineers the information to prevent or minimize this issue.