The invention relates to an apparatus (1, 1a, 1b) for separating particles in a pulp suspension, which apparatus (1, 1a, 1b) comprises a housing (14), a barrier (2, 2a, 2b) with apertures, an inlet (3, 3a, 3b) for the pulp suspension, a first outlet (4, 4a, 4b) for material with large particles not having passed the barrier (2, 2a, 2b), a second outlet (5, 5a, 5b) for material with small particles having passed the barrier (2, 2a, 2b), and a rotor (6) on an axis (9), which rotor (6) is arranged to create a fluidization zone (8, 8a, 8b) near the barrier (2, 2a, 2b). According to the invention the apparatus (1, 1a, 1b) comprises a separation chamber (7, 7a, 7b) having a toroid shape. The rotor (6) is arranged to both create a circular flow of the incoming pulp suspension in the separation chamber (7, 7a, 7b) and to clean the barrier (2, 2a, 2b) from the pulp suspension.
