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

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.