-This work focuses on the fluidization of three types of TiO 2 powders: Anatase (99% TiO 2 ), Rutile 1 (95% TiO 2 and 5% Al) and Rutile 2 (96.5% TiO 2 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 TiO 2 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 TiO 2 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 TiO 2 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.

Cement powder particles of micronic size tend to form agglomerates due to the influence of interparticle forces (Van der Waals forces). The formation of agglomerates results in an increased air-void in the solid structure (aerated powder) requiring an increase in water demand to sustain the feasibility of the structure. Consequently, if the compound formed is not stabilized, it would have low mechanical strength that may result in cracking of hardened cement. In this study, the results of cement powder consolidation and its flow properties show that its behaviour is controlled by internal forces (Van der Waals) and external forces (elastic and plastic). These forces have a direct influence on the powder structure, leading to a variable packing behaviour (void reduction). Consolidated cement powder shows a decrease in the void structure leading to a more efficient material. This study intends to determine the impact of interaction forces between cement particles during consolidation.

-The experiments and analyses performed included measurements of the physical properties of TiO 2 powder such as the particle size, density, and consolidation. Experiments with nanometric TiO 2 powder of 204 nm average diameter show that, during consolidation, the adhesion of particles under normal stress is principally due to the van der Waals force for particle radii less than 300nm and the application of external force has no effect on the cohesion of the primary particles within this range; for particle radii around 300nm to 1.0μm the cohesion of the powder system is due to plastic deformation and the application of external force change the cohesion force to a plastic deformation between the agglomerates formed under these forces. This can be observed in the arrangement of the primary particles into dispersed agglomerates with sizes greater than the individual particles. The results obtained with the nanometric TiO 2 powders show a more complex behavior than the micronic powders. 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. It has been experimentally observed that the powder structure is not perturbed by stresses of low intensities. A development of the different forces involved in interparticle contacts is outlined. The description of these forces involved in particle cohesion will help to understand the powder cohesion under consolidation.

No abstract

One of the main problems affecting the flow of cement bulk powder is the formation of cohesive arching at the outlet of the hopper, causing blockage of the silo opening and bridge formation. A simple concept is established which outlines these complications. In this context, the interactions of particles lead to a high degree of consolidation of the cement powder and an increase of adhesion force due to the small size and the large surface area of the cement particles. The results from the consolidation test and the flow properties (cohesion) show that the cement powder flow is mainly controlled by internal forces (Van der Waals and adhesion forces) and external forces. These forces have a direct influence on the powder structure, leading to a variable packing behaviour. Since the problem is attributable mainly to interparticle forces, before storage of the cement powder in the silo, the powder should be fluidised with air at a high velocity to disintegrate the cohesive structure and to overcome this undesirable property of cement flow.