(21b) The Use of Flow Aids to Prevent Caking of Bulk Solid Materials

Flow aids have long been used to take a cohesive material that has significant instantaneous strength and reduce the particle-particle interaction to decrease the bulk unconfined yield strength. Some feel that this method works because the inter-particle forces are generally caused by van Der Waals attractive forces and the flow aids separate the particles, thereby decreasing the attractive forces and reducing the bulk strength. Not all mechanisms of strength are due to van Der Waals forces. Material strength increase can be the result of capillary bond formation and crystal growth between particles. The questions to be addressed by this paper are:

• What is the role of flow aids in situations where strength increase is primarily due to crystal growth between adjacent particles (i.e. caking due to recrystallization)?
• What is the optimal size of flow aid that will prevent this type of caking?
• Should the flow aid be hydrophobic or hydroscopic?
• What is the optimal coverage of flow aid for these situations?

This paper will address caking prevention in a systematic manner, incorporating a particle scale model of the caking phenomena and using bulk scale caking tests to build a bridge between the bulk strength and inter-particle forces for caking materials.