(38a) Systematic Study of Caking in Sintering and Soft Surface Systems – the Role of Particle Size, Temperature and Creep Effects

One problem of working with bulk materials is that they form very strong agglomerates, lumps, or even large solid masses in process equipment. We call this problem caking. But, caking is also a mechanistic problem and has multiple causes that result in bulk material gaining strength after storage. Understanding caking requires an understanding of the kinetics that happen between particle-particle surfaces. Often, we lump all caking into similar kinetics explained by the rate of formation of some binding material in a confined space as happens in crystallization systems. However, some caking mechanisms occur when the bulk surface of the material deforms and develops welding events between adjacent particles through warm solid creep events. The kinetics of this type of caking are driven by the formation of new surfaces through particle deformation. This suggests that the creep behavior of a bulk material should have similar kinetics to the kinetics that describe the gain of strength of the bulk material. Often these types of caking events occur as the material is subjected to a single temperature excursion for some critical time. Associated with these systems is a significant dependence on the temperature where the bulk unconfined yield strength becomes very large while the material is stored at some critical temperature. This cohesive behavior increases exponentially as the critical temperature is approached.

From a continuum point of view, unconfined yield strength is defined as the major principal stress that causes a bulk material to yield in shear while in an unconfined state after it has experienced a consolidation stress causing it to compact. From a particle viewpoint, during a shear event adjacent particles are in the process of two distinct actions. Some adjacent particles slide past each other, causing frictional resistance forces. Some adjacent particles adhere to each other, and during a shear event, these particles pull apart and rupture the adhesion connection. So, the bulk unconfined yield strength (a fundamental continuum property) is a function of the collection of all the friction forces as well as all the adhesion forces between adjacent particles.

The unique feature of this mechanism is that the adhesion of the surface depends not only on the temperature of the surface, but also on the temperature of the entire mass of the particles. The surface can become sticky with increasing temperature but, over time, the area involved in adhesion also changes because the shape of the adjacent particles change, resulting in larger more intimate connections between particles through local deformation of the particle surfaces. In some cases, this deformation leads to cases where the intruding particles deform in such a way that a divot is formed in the adjacent particle matching the shape and size of part of the intruding particle. Thus, these particles interlock just like puzzle pieces interlock. When shear begins the divot resists the shear event due to the fact that, in order to shear, the adjacent particles must first move up out of the divot and then slide past each other. This increases the bulk unconfined yield strength of the material. This work is a systematic study of the events that influence the interaction of surfaces that are prone to local deformation and caking due to warm creep effects. The work looks at the relationship between the caking and the creep effects. It also looks at the relationship between the change of surface area contacts over time and the resulting caking effects. Finally, it looks at the role of the induced local deformations in preventing shear between adjacent particles during failure in an unconfined state.