(404h) Relating Bulk Solid Caking Events to Phase Change Events That Happen during Crystallization Processes

Caking causes significant problems in the storage of bulk solids in process vessels. Caking can result in significant blockage in a storage silo requiring millions of dollars to clean out. Caking can result in lump formation where lumps can grow to a size capable to block the outlets of silos and hoppers. Caking can increase the rathole tendency of material in storage containers. Caking can cause entire rail cars to be packed with very hard bulk material, making it impossible to empty them with any reasonable method. Caking can cause consumer problems in bags and packages, creating hard packs and making it impossible to dump a bag or package into a User point-of-use location. It is obvious that caking problems reach all scales of the bulk handling system, including silos, hoppers, super sacks, bags, boxes and even small bottles.

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. There are several reasons why caking occurs, and the solution to any caking problem will depend on its root cause. In this paper, we will consider just one cause for caking: the mechanism of a recrystallization method. In this method, moisture zones on the surface of the material move and coalesce toward the contact points between particles, forming a pendule. This moisture dissolves some of the bulk material, thereby creating a saturated solution. Subsequent changes in relative humidity surrounding the particles cause the solution to become supersaturated and crystal formation begins to occur within the pendular liquid volume between particles. Likewise, a change in the temperature of the bulk material can result in a supersaturated solution in the pendular liquid between particles, causing crystal growth.

It is important to not only understand the general cause(s) of caking, but to also understand the dynamics behind the caking process, the magnitude of the caking phenomena, how caking relates to the meso-scale behavior, and the time constants associated with caking. Once these things are understood, only then can practicing engineers and material scientists propose methods to mitigate this problem either by process design or by product design.

For this work, three bulk solid systems were examined to gain data and greater understanding of their caking tendencies. These three bulk solid systems are: iodized table salt, calcium chloride flake, and anhydrous acidic acid. The goal of this work is to find common ground between the caking behaviors of these three bulk systems. The caking tendency was measured using a uniaxial strength tester on materials that were prepared with a known concentration of moisture. Caking is a function of storage time and also a function of consolidation stress, so caking strengths were measured as a function of storage time and consolidation stress. These caking experiments were coupled with observations of crystal growth between adjacent particles to find the link that couples the bulk caking behavior to the meso-scale crystallization behavior local to individual particles.

From the experimental work, it was found that these bulk systems tend to gain strength in accordance with two exponential growth terms. The first strength growth term is proportional to storage time to the 1^st power. The second exponential growth term is proportional to storage time to the 3^rd power. It was determined that this behavior is 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 1^st power, 2^nd power, and 3^rd power – depending on the dimension of the crystals that form. Rod-like crystals correspond to kinetics proportional to time to the 1^st power. Plate-like crystals correspond to kinetics proportional to time to the 2^nd power. The formation of spherical crystals (or 3D clusters) corresponds to kinetics proportional to time to the 3^rd power. The bulk strength tends to have a one-to-one correspondence to exponential crystal growth of time to the 1^st and 3^rd powers where the 1^st power exponential strength growth occurs in a short time frame and the 3^rd power exponential growth occurs over a longer time frame. This suggests that crystal rods 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 and designers the information needed to prevent or minimize caking issues.