Abstract: A slurry of comminuted cellulosic fibrous material (e.g. paper pulp) is acted upon so that particles contained in the slurry are separated out from the slurry and introduced into a second liquid flow, while maintained under superatmospheric pressure. A partition divides a liquid tight pressure resistant vessel into first and second distinct chambers isolated from each other except by a slit at a mid-portion. An inlet for the slurry and an outlet for the first liquid are connected to the first chamber, and an inlet for the second liquid and an outlet for the second slurry to the second chamber. A screening disk is mounted for rotation in the vessel so that it fills the slit, with portions disposed in both the first and second chambers.