(421e) Caking Problems and the Role of Flow Aids – a Systematic Approach

One problem of working with bulk materials is that sometimes they just do not handle well after storage. We call this problem caking. The bulk material is placed into a container, bin, hopper, feeder, or other process vessel, and – over time – the material becomes very cohesive and plugs the process equipment. Sometimes this time is long and sometimes it is very short. So, what is the solution? One potential solution is to use flow aids so as to limit the build-up of unconfined yield strength during storage. 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. All adhesion force laws depend on how close the adhesion surfaces are.

With this in mind, the traditional simplistic view of flow aids suggests that the addition of a flow aid works because the flow aid particles fit between the larger particles and separate these larger particles, thereby reducing the adhesion forces between particles and decreasing the bulk unconfined yield strength. So, the use of flow aids to mitigate cohesive flow problems should be solved by simply computing the coverage of a particle by finer flow aid particles and then using adhesion laws to determine the reduction in adhesion forced because of the extra distance between particles caused by the coated particle surfaces. Based on this simple viewpoint one would expect that it would not matter what type of flow aid particles are used to coat the bulk particle surfaces. However, real life observation is that the type of particles on the surface of a bulk material significantly affects the reduction in bulk cohesive flow properties. In addition, the cause of the bulk cohesive strength in caking environments plays a role as well. In cases where caking is induced by recrystallization effects, the degree of hydrophobicity of the flow aid plays a significant role in the reduction (or not) of strength. The particle size and size distribution of the flow aid relative to the bulk solid is also a factor. Likewise, in cases where caking is caused by sintering effects, the size of the flow aid and the softness of the bulk particle surface play an important role in the reduction of caking problems with the use of flow aids.

Caking is inherently a mechanistic problem, meaning that it has one or more causes that may happen at the same time. Each caking mechanism has an associated time constant and behavior. Each caking mechanism will also be sensitive to something about the environment that initiates the caking and determines the rate at which the problem occurs. Caking depends on the chemistry of the material, the size and shape of the particles, the particle size distribution, the geometry of the process vessel, and the environmental changes that the process vessel may experience over time. Often caking events can be avoided by using a flow aid to prevent or limit the caking causes. However, sometimes the use of a flow aid does not work. In this event engineers, process managers, or formulation developers need to know the root cause of the caking issue before an alternate flow aid can be effectively chosen. In some cases, the flow aids chosen may make the material more prone to problems. The question is – WHY? The ultimate goal is to understand why flow aids sometimes do not work but, instead, join the dark side and cause even more problems.