(714d) Caking of Bulk Materials – a Systematic Approach to Understanding 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 the gain in bulk strength of a powder or a granular material with storage time. Caking is induced by a change in the surface condition of adjacent contacting particles to cause crystal growth, cold creep, or sintering effects. The net effect is cementation of the adjacent particles in a bulk material via solid bridge formation mechanisms. In some cases, caking is caused by the dissolution of soluble components in the bulk and subsequent concentration, pooling and recrystallization of solid material between connected particles. In other cases, caking is the result of glass transition temperature effects that convert contacting particle surfaces to a soft amorphous structure at a prescribed temperature. Stress effects near the soft surface cause creep and allow the surface of particle A to fuse to the surface of particle B. The subsequent change in local temperature then induces a crystallization event that forms solid bonds between particles and results in caking. Glass transition temperature can be a function of the moisture content of the surfaces or be completely independent of moisture effects. It is obvious that many caking mechanisms require the presence and movement of moisture. Thus, understanding moisture migration can help in understanding caking events in equipment. In some cases the material itself generates or stores the moisture needed to cause the caking reaction. Some reactions generate moisture as a product of the reaction. Some materials contain crystalline bound moisture that is released as the temperature is changed. These types of materials can provide both the source or sink for the moisture in caking reactions. There are a few conditions that all caking reactions have in common. They all have a series of time constants that describe the reactions that induce caking. It is important to note that there may be multiple reactions occurring to cause caking, and these reactions may have independent time constants. Measurement of time constants is one means of characterizing caking behavior. Determining the time constants can help in the design of process methods to mitigate caking behavior, or help in identifying the root cause of caking events. Each caking mode 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 needed to reduce these effects. Several systems comprising the key mechanisms were studied and data for each of these systems reported.