Evaluation of fines separation from unbleached softwood kraft pulp using microperforated hole screens

The application of a bench scale and an industrial scale pressure screen to separate cellulosic fines from an unbleached softwood kraft pulp containing 5 % (w/w) primary fines was investigated and the results were compared. In the bench scale trials different microperforated screens with hole diameters of 150 µm, 250 µm, 400 µm and 600 µm were used. Based on these results a screen hole diameter of 400 µm was selected for the industrial scale trials. In all cases, a high accept flow rate proved to be the crucial parameter for high removal efficiency of the fines material and a removal efficiency of well above 50 %could be achieved. Further of interest were the limiting factors regarding increased accept flow rates with a focus on the interrelations between pressure difference, slot velocity and reject consistency. As expected, investigation of electric energy demand showed a correlation between removal efficiency and specific energy consumption, which did not exceed 35 kWh/t(od) feed.