Aggregation of concentrated colloidal silica dispersions in a stirred tank

Abstract

Concentrated suspensions of nanoparticles subjected to transport or shear forces are commonly encountered in many processes such as precipitation, wet comminution or filtration. In these processes, particles are likely to undergo processes of aggregation and fragmentation under physico-chemical interactions and hydrodynamic forces. This study is focused on the analysis of the behaviour of colloidal silica in dense suspensions subjected to hydrodynamic forces in conditions of destabilization. A colloidal silica suspension of particles with an initial size of about 80 nm was used. The silica suspension concentration was varied between 3 and 20% of weight. The phenomenon of aggregation was observed in the absence of any other process such as precipitation and the destabilization of the colloidal suspensions was obtained by adding sodium chloride salt. Various techniques of characterization of the particle size (acoustic spectroscopy, laser diffraction) and of the physico-chemical properties of the suspensions were used allowing defining the silica dispersion properties (suspension stability, particle size, pH). The experiments were proceeded in a batch agitated vessel. They were made in order to observe the influence of different parameters (silica concentration, temperature, agitation speed) on the behaviour of the suspended particles. The evolution of the particle size distributions versus time during the processes of aggregation was particularly followed on line by acoustic spectroscopy in dense conditions. Samples were also analyzed after an appropriate dilution by laser diffraction. The results show the different stages of the silica aggregation process whose kinetic rates depend either on physico-chemical or hydrodynamics parameters. They allow understanding the mechanisms of the aggregation process under flowing conditions.