Abstract: A suspension of fibrous cellulose pulp is separated into at least two portions in an apparatus comprising a closed housing having inlet and outlets, and a rotor means provided with projections, said rotor means comprising a first rotor part surrounded by a screening means, the first rotor part and the screening means in their axial extension forming a screening zone and being arranged to cooperate with each other during rotation of the first rotor part for screening the suspension in a fluidized state. The rotor means comprises a second rotor part located immediately adjacent to the first rotor part, a rigid, water-impervious wall member surrounding the second rotor part.