Fluidization of coated group C powders

While it is well known that flow aids such as fumed silica can be added to improve flowability and fluidizability of cohesive powders, the improvements observed depend on how well the flow aids are blended together with the cohesive powders. In this work, dry particle coating is used to deposit a very small amount of nano-sized particles (as low as 0.01 wt %) 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 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 μm.