An arrangement for separating light impurities from pulp suspensions. The arrangement (11) is intended to be connected to a casing (1) of a fractionation device where the fiber network is broken up and the pulp is caused to rotate for enriching the light impurities. The separation arrangement (11) is designed with a rotation symmetrical housing (13, 14) intended through a passage (12) to be connected centrally to the upper portion of the casing (1). An inlet (15) for dilution liquid is connected tangentially to the housing (13, 14) and an outlet (16) for the light impurities is located centrally at the top of the housing (13, 14).