Behavior and fluidization of the cohesive powders: agglomerates sizes approach

Abstract

This work focuses on the fluidization of three types of TiO2 powders: Anatase (99% TiO2), Rutile 1 (95% TiO2 and 5% Al) and Rutile 2 (96.5% TiO2 and 3.5% Al and Si); the average diameters of the powders are 204 nm, 159 nm and 167 nm, respectively. These powders belong to group C of the Geldart classification and are characterized as cohesive powders with a non-free flow and a difficult fluidization. The fluidization of the powders was carried out in a glass column of 103 mm inner diameter and 1500 mm height. The experiments and analysis performed included measurements of the physical properties of the powders such as the particle size, density, specific surface area and the flow properties of the powders like the Hausner's index, the angle of repose, the angle of slide, consolidation and shearing (via shear cell testing). The results obtained with the nanometric TiO2 powders show a more complex behavior than the micronic powders; with a low strength value (Hausner index, angle of repose and angle of slide), the TiO2 powders have a free flow or intermediate-flow and a non-free-flow for higher strength intensities (consolidation and shearing). This behavior is related to the structure of the nanometric particles in the packed bed; the evolution of this structure is made up of individualized and spherical agglomerate shapes and is not perturbed by stresses of low intensities. Indeed, the latter seems to modify the structure of the powder (group C of Geldart classification) to acquire a behavior typical of group A, B or D in the Geldart classification. With high stress values, the individualized agglomerates are disintegrated and the powder is reduced to a more compact structure. The fluidization of TiO2 powders seems to evolve in a more homogeneous way than the micronic powders. This behavior is related to the initial structure being made up of stable agglomerates. Thus, this fluidization is made by agglomerates with a gas velocity of 3×10(6) to 4.6×10(6) times the gas velocity for fluidizing the primary particles.A numerical approach based on a force balance in agglomerating fluidized beds was developed in order to estimate the agglomerates sizes.