The effects of caking on the dynamic flow of cohesive powders
Caking or the bonding of particles due to inter-particle cohesion has a huge effect on the behavior of powders. Strong bonds between particles can prevent materials from exiting silos and storage containers. However, under dynamic conditions, caking can actually improve the flow properties of the material. Caking in powders occurs in two ways. Under static conditions as in storage containers and silos, caking occurs due to particles being pressed together by the force of gravity acting on a column of material or by external forces. Generally the stronger the forces acting on the material the stronger the bonds between cohesive particles. Under dynamic conditions, caking occurs due to particles smashing together as they flow. This type of caking is also referred to as agglomeration, clumping or granulation. Dynamic conditions are defined as situations where a powder is moving under the influence of gravity or by mechanical convection. In industry, powders are typically stored under static conditions but are used under dynamic conditions. Therefore, the characteristics of the material after storage under static conditions as well as the stability of the material under dynamic conditions are critical to the successful use of the material. In this study, the effects of caking under static and dynamic conditions on the dynamic flow characteristics of powders are analyzed. Powders with different degrees of inter-particle cohesion are studied using uni-axial compression to simulate static conditions and a rotating drum to simulate dynamic conditions. The assessment of the inter-particle cohesion of the material is achieved by measuring the unconfined yield strength of the material after a consolidating stress has been applied. It is found that caking due to inter-particle cohesion under both static and dynamic conditions directly affects the dynamic flow characteristics of powders and also can create instabilities in these characteristics as the materials are subjected to dynamic forces. The dynamic flow characteristics measured include avalanche energy and dynamic density. It is also found that the level of caking in a powder can be assessed by measuring the changes in its dynamic flow characteristics before and after exposure to static and dynamic conditions.
