(331g) Fluidization and Subsequent Processing of Geldart Group C Powders

As shown in Prof. Derek Geldart’s seminal work, fine powders are cohesive and they cannot be easily fluidized and fall under what was empirically classified as Group C powders. It has been also shown that the particle size, solid and fluidizing medium densities, and particle adhesion forces govern the transition between Group C and A. From fundamental considerations, if the powder material and fluidizing medium are fixed and cannot be altered, other than changing the particle size, there are only two other ways to shift the transition between Group C and A.  These two factors are the gravity force or the body weight and the cohesion forces. Interestingly, these two factors define what is known as the granular Bond number, which is a ratio of particle cohesion force and body force, which is usually its weight. In this work, we consider both these factors and show how either by changing the fluidization mode from conventional gravity driven to a centrifugal mode, or by changing particle cohesion through manipulating the surface roughness or the size of asperities and/or changing surface energy. For the first case, we show how use of rotating fluidized bed allows fluidization and subsequent processing of fine powders. For the second case, considered in more detail due to its practicality, dry particle coating is used to deposit a very small amount of nano-sized particles (as low as 0.01 % by weight) with a high degree of precision onto the surface of cohesive, Geldart group C powders to make them fluidize like group A powders. A model taking into account the effect of the size of the guest and host particles as well as Surface Area Coverage (SAC) of the coated nano-sized particles is developed to predict the effect of coating on the adhesion reduction of cohesive powders. A series of experiments are performed to investigate the improvement in the fluidizability of dry particle coated group C powders (e.g., cornstarch and aluminum), and the effect of various parameters such as, surface area coverage (SAC), guest particle size and host particle size are systematically investigated. The results clearly show the effect of each of these parameters on the fluidization behavior of cohesive powders, and also validate the model. The study also indicates that a critical SAC is required to make the coated cornstarch fluidize, which is about 5 %; the smaller the guest size, the better its effect on improved fluidizability, although the improvement is reduced if the guest size is smaller than about 10 nm; and if the conditions regarding the SAC and guest size are satisfied, dry particle coating will significantly improve the fluidization of cohesive particles even as small as 5-10 microns.