Optimisation of Screen Plate Design for the Removal of Stickies in Deinking Pulps

International audienceA study about stickies screening in deinking lines was launched in the frame of the European project ScreenClean. The screening program in this project, led by CTP, was focused on PSA stickies and included lab screening tests, numerical simulation at LEGI and pilot tests in cooperation with AFT. Basic research work was performed to understand the extrusion of stickies through slots and improve screening efficiency. A single-slot screening device was used to visualise the extrusion of cylindrical PSA particles. Water-based acrylic adhesives were revealed to be much weaker than hot-melt rubber adhesives, especially at high temperature after alkaline soaking, and required lower pressure and less time to be extruded through slots. Gentle screening conditions with a low maximum pressure during the positive pressure pulse as well as reduced temperature and pH should thus minimise stickies extrusion, as expected. Both experimental and numerical simulation results led to the same conclusions about the effects of key screening parameters. Particles 2 to 3 times larger than slot width were extruded at the typical pressures measured in screens, but the time required to pass the slot was considerably higher than the duration of typical positive pressure pulses, which suggested that particle extrusion should take place step-by- step over several rotor revolutions. The laboratory screening tests showed less stickies extrusion with milled screen plates that had two sharp slot inlet edges compared to wedge wire screen plates with a convergent slot inlet. Pilot screening tests with various screen plates (0.15 mm slots, 0.6 to 1.2 mm contour height with different angles) confirmed this trend as milled slots showed higher stickies removal efficiency - though fibre passage was increased with wedge wire slots. Contour design was found however to have more effect on screening efficiency than the slot inlet design. The best results were obtained with the lowest contours, while reducing contour angle showed only a positive effect at a higher contour height. The pilot tests also confirmed that screening should be performed at lowest possible temperature to optimise the removal of PSA stickies