Efficiency of different dispersing devices for dispersing nanosized silica and alumina

Nanosized particles are often produced by pyrolysis as dry powders, usually, not as single primary particles but rather as particle collectives consisting of several primary particles. Most applications require the particles being suspended in aqueous liquids as separately dispersed primary particles or in a certain aggregate size. In the dispersion process typically the product fineness is determined by the strength of the agglomerates and aggregates, by the properties of the homogeneous phase, by the stress mechanism itself and its intensity and frequency acting on the particles. The objective of this study is to characterize the dispersing process comparing different dispersing machines such as a dissolver, a disc mill, a stirred media mill, a kneader, a 3-roller mill and an ultrasonic homogenizer. Two different types of pyrogenic metallic oxide particles, Aeroxide (R) Alu C (alumina) and Aerosil (R) 200 V (silica), are investigated. For high viscous suspensions the alumina aggregates could be efficiently dispersed using a kneader and a 3-roller mill in contrast to a dissolver or a disc mill. For small target particle sizes stirred media mills and partially ultrasonic homogenizers are capable to reduce the particle size very efficiently. In stirred media mills the stress intensity increases with decreasing particle size whereas in the other machines the stress intensity is independent on the particle size or even decreases with decreasing particle size. The alumina is more easily dispersed in the most dispersing machines than the silica and higher product fineness can be achieved at comparable processing conditions indicating that the strength of the silica aggregates is higher than the strength of the alumina aggregates. (C) 2010 Elsevier B.V. All rights reserved.