Abstract: According to the method the stock suspension is moved across a mass force field due to gravity or centrifugal acceleration in the form of stock stream or current and contacts a pick-up surface moved in conjunction with the stock stream and at substantially the same velocity. The apparatus may comprise a housing in which a turbulent or vortex flow is generated and containing a rotor therein which possesses the pick-up surface. Further embodiments have a guiding wire or screen which is moved along a pick-up cylinder or is trained around the same and upon which there is formed a layer of the stock suspension by using a stock supply device. A band, for example a wire band, may be provided to pick-up the contaminants.