Single screw extrusion of mullite-based tubes containing Al-rich anodising sludge

An Al-rich anodising sludge has been used as the feedstock to produce mullite-based tubes by single screw extrusion. Extrudability and plasticity of the pastes was shown to be strongly dependent on the amount and type of used additives. The pastes developed from the sludge could be characterised by the Benbow–Bridgwater relationship using six paste parameters and this characterisation was extended to predict the behaviour of the screw extrusion process. In this work, the effects of the screw speed and partial pressures in relevant zones of a single screw extruder on the processing behaviour of sludge-based pastes were predicted using the Benbow–Bridgwater model. It is shown that the model can predict the pressure drop through the die-entry and the die-land regions (with similar design as used to produce ceramic hole-tubes) and the pressure development along the barrel of the extruder with reasonable accuracy. The paste parameters calculated from simple ram extrusion do not match the experimental behaviour of the screw extruder and this was proved to be due to the paste being worked to a stiffer consistency in the barrels of the extruder and to being subjected to vacuum during the process, with consequent water segregation. This was confirmed by re-examination of the paste properties immediately after the screw extrusion.