Development of a semi-continuous centrifugal process for classifying colloidal particles

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Within the last years further progress was made in development of novel materials based on submicron particles. However, obtaining the commonly required narrow particle size distribution is a problem for most processes. With a view to production in pilot or industrial scale, particle size distributions of the synthesized colloids are often too broad to be used in cutting edge technology such as printable electronics, polishing media for semiconductors (CMP slurries) and composite particles used for cancer treatment. For this reason narrow fractions must be separated from the primarily broad distributions in subsequent steps. The term for this process is classification. Classification is either achieved by removing the fine or the coarse fraction.A continuous or semi-continuous classification of particles by centrifugation in pilot scale shows certain limitations concerning particle size and difference in density of the particles and the surrounding liquid of the suspension. Those separation processes depend on particle sizes larger than 1 µm and density differences between solid and liquid of at least a factor of two. The classification of submicron particles is so far limited to the laboratory scale. In pilot and industrial scale, deep bed filters that are low in selectivity are used to remove the coarse fraction in CPM slurries. An optimization of deep bed filtration process is not possible, because the determining physics impede an increase of selectivity. Thus, the aim of the applied project is the investigation of classification in the nanometre scale using a tubular bowl centrifuge.With a density difference between solid and the surrounding liquid of 100 kg/m³ a cut size of 100 nm is aspired. For higher differences in density between particles and liquid a cut size of 20 nm seems achievable. By using carbon fibre rotor and active magnetic bearings, the tubular bowl centrifuge generates up to 160000 G for classification of colloidal particles. The centrifuge is built in a current research project and needs to be modified for the intended application. The results of the investigations will describe the sedimentation behaviour of various particle systems in dependence of particle properties and process parameters.A further work package is the characterization of the flow pattern by Laser Doppler Anemometry in a centrifuge. This technique had been employed for measuring the flow pattern in a tubular bowl centrifuge. The results have been published recently. The gained knowledge of the flow characteristics will be used to optimize the inlet geometry. Further measurements are necessary to understand the particle and flow channelling close to the feeding system in the centrifuge. This is highly important to achieve the intended cut size.Furthermore the experimentally determined flow pattern enables the validation of the model calculations which will be carried out within the applied project.