Improving fluidizability and flowability of cohesive particles by surface coating with flow conditioners

Surface coating of cohesive particles with finer particles as flow conditioners is applied to enhance the flowability of cohesive particles. Glass beads (6 and 10 mm), polyester resin (12 and 20 mm) and corn starch (15 mm) are chosen as host particles and nanoparticles of AI₂O₃, SiO₂, TiO₂ and carbon black and fine perlite particles with a flake shape as the guest particles for coating. It is observed that the surface coating can significantly enhance the flowability of all types of the host particles tested, resulting in a smaller angle of repose, a reduced Hausner ratio, a smaller shear stress and a reduction in u_mf as well as markedly increased bed pressure drops during fluidization. However, the effectiveness of surface coating in meliorating the flowability of the host particles is found to be strongly dependent on not only the guest/host mass ratio, but also the properties of both the guest and the host particles. The SEM micrographs indicate different existing states of the guest particles in the coated products, depending on the type of the guest particles. It is found that, after coating, the nanoparticles cling onto the surfaces of the host particles like asperities, while the flake-shape perlite particles just float or are present individually in between the host particles as spacers. According to the status of the guest particles in relation to the host particles, two different models, i.e., the asperities-contacting model and the sandwich-contacting model, have been proposed. The possible mechanisms governing the operation of flow conditioners are then studied through the interparticle force calculations based on these two models.