9 of the vanes are inclined downwardly with increasing distance rearwardly from the leading edges 11 of the vanes at the same time in the embodiment of fig1 a the lower wedge surfaces incline upwardly from the horizontal or from a plane perpendicular to the axis of the drum 1 moving radially outwardly of the drum 1 this inclination of the surfaces 9 of the vanes causes a component of motion of the particles impinging upon the vanes which is directed toward the screen 2 in this way the particles that were initially drawn away from the screen 2 by the vacuum produced by the rotating vanes are again conveyed against the screen which increases the possibility that particles of acceptable quality pulp can be sorted through the holes in the screen 2 of course the upper wedge surfaces 10 of the vanes 4 may also be developed in the same manner as shown in fig1 a these surfaces 10 are inclined upwardly from the leading edges 11 of the vanes but downwardly with respect to the horizontal moving radially outwardly of the drum 1 in this way the two essential features of the sorting process in this screen slot namely the detachment of the fibers of the suspension from the screen 2 which is caused by the vacuum and eddying and the conveying of the fibers to the screen continuously alternate i e a pulsating movement of the fiber suspension is produced in the screen slot 3 and specifically in its lower zone where the sorting vanes 4 are present on the drum 1 a ring of impurities 15 that forms in the uppermost zone of the screen slot 3 should consist.