line 26 as already mentioned pulp produced in this manner has a higher quality and better bleachability than pulp produced with known methods in lab scale tests we have found that about 10 kg of active chlorine can be saved for reaching full brightness about 90 iso compared to a conventionally cooked pulp in fig4 there is shown a preferred second embodiment of a top separator intended for a hydraulic digester or an impregnation vessel according to the present invention only a part of the top of the digester 6s is shown the slurred fiber material is transferred to the top of the digester by means of a transfer line 21s and enters an in let space 30s of a screw feeder 31s the screw feeder 31s is attached to a shaft 32s connected to a drive unit 33s which is attached to a mounting plate 34s at the top of the digester shell 6s the drive shaft 32s is rotated in a direction so as to force the screw to feed the fiber slurry in a downward direction a cylindrical screen basket 35s surrounds the screw feeder 31s the screen basket 35s is arranged within the digester shell 6s so as to form a liquid collecting space 36s between the digester shell and the outer surface of the screen basket 35s the liquid collecting space 36s which preferably is annular communicates with a conduit 17s for withdrawing liquid from the liquid collecting space 36s which in turn is replenished by liquid from the slurry within the screen.