The disclosure concerns an apparatus for separating impurities from a fibrous suspension. The apparatus includes a tiltable housing having a screen basket supported coaxially therein. A rotor is supported inside the screen basket. The rotor has three zones. There is an upstream zone communicating with the fiber suspension inlet, and inlet in the upstream zone is into the interior of the rotor and then outward past sorting vanes formed at the exterior of the rotor in the first zone and inside of the basket. Downstream of the transport vanes is a second baffle zone defined by a closed drum having transport vanes on the outside thereof. The suspension and impurities are moved downstream by the transport vanes in the second zone. Downstream of the second zone is a third zone with further means for moving impurities downstream, comprising either a centrifugal impeller in the form of vanes or a helical ring. The impurities outlet from the housing is downstream of the rotor and at the other end of the housing away from the inlet. The outlet for purified suspension is outside the screen basket nearer the inlet. A weir establishes a level of suspension in the housing.