(708a) Understanding the Fundamentals of Caking Mitigation

Caking of bulk solids is a serious problem that leads to significant lost production, silo damage, and health and safety concerns.  It is, perhaps, one of the most misunderstood problems in handling of bulk solids.  Caking is mechanistic and has several main causes.  First, the bulk material may pick up moisture either by absorption from moist air or direct moisture addition.  This moisture can then coalesce at the particle-particle contacts, dissolving some of the material.  Thermal gradients then initiate moisture migration and dry out local particle-particle contacts.  The resulting solid bridges increase the unconfined yield strength, resulting in severe caking.  In this caking mode each thermal cycle increases the solid bonds between particles and strengthens solid bridges.  Second, caking can be caused by cementation or gel reactions.  In this case water already present in the bulk converts the local material into a gel-like structure and consolidation pressures compress particles together as the gel is formed, resulting in chemical bonding that cements particles together.  In this mode of caking a single temperature cycle (such as occurs overnight during storage or transport) can generate enough moisture to initiate the gel or cementation reaction.  In some cases the material itself generates the moisture needed to cause the cementation.  Powders with crystalline water can release moisture from the crystal state upon heating, even at low temperatures.  Once this water is released from the powder, it is free to travel through the interstitial voids between the particles where it coalesces at particle contacts and initiates cementation.  In some cases the addition of gases such as CO₂ react with solid particles and liberate moisture, causing cementation caking.  Third, caking can be caused by moving a bulk solid back and forth between a glass transition condition, changing the solid particles from a soft, pliable amorphous state to a hard crystalline state.  If this transition occurs while particles are in contact and subject to contact pressures, then cold plastic flow occurs between particles, binding them together and resulting in caking.  Each mode of caking is associated with a characteristic time constant that depends on some external caking stimulus (temperature cycle, moisture migration cycle, etc.).  The magnitude or severity of the caking event depends on the relationship between the caking cause reaction rate and the time the caking stimulus is applied. For caking events to become serious, some threshold value (such as local moisture content or local temperature) must be exceeded.  This paper examines the various caking causes and identifies the time constant for caking, the mode of strength increase and the potential mitigation tools to reduce these effects.  Three systems comprising the key mechanisms were studied and data for each of these systems reported.