Method for removing impurities from an aqueous fibre suspension

To separate and remove foreign matter from a watery fiber suspension (S), the suspension is fed to a foam (2) formed in a liquid by gas bubbles (1). The suspension (S) is moved into the liquid against the direction of the rising gas bubbles (1) in at least one channel (3), in a counterflow movement. The fiber suspension (S) transfers foreign matter to the foam (2), to be taken off as a cleaned fiber suspension (S'). A flow system increases the exchange of matter in the channel (3). The foreign matter is removed together with the flotation foam (2). The flow system is a consistent turbulence in the channel (3). The flow system prevents the foreign matter trapped in the flotation foam (2) from escaping back into the fiber suspension (S). The channel (3) is vertical, and has inner flow guide units, extending vertically over part of the channel (3) cross section as star-shaped guides (4), concentric rings or a honeycomb structure. The flow guides can also be floating bodies in the fiber suspension (S). A number of stirrers at different levels, and rotating in different directions, can also be fitted to give the flow actions. The flow guide system can also be horizontal units with a labyrinth path or openings, or a sieve base, or inserts in the channel (3) which contract and expand the channel cross section. The flow system extracts part of the fiber suspension (S) from the channel (3) and returns it at a lower point. The fiber suspension (S) is fed into the foam (2) in the channel (3) through a gutter with openings, with a controlled suspension level in the gutter. Or the suspension is passed in through a perforated base or a jet base with floating valves. The suspension can also be fed in through a double base, or over a base with a controlled level through the base opening widths. The suspension can be fed in over a base with integrated aeration. The suspension feed can be a rotating gutter, or through a labyrinth with a number of entry points, or over a section with floating bodies.. The suspension (S) can also be fed to the channel flotation foam (2) over a braided metal structure. The suspension can also dribble in a film over a weir, and the like, with concentric ring guides for the film with projections where the film runs off. The suspension (S) can also be fed into the flotation foam (2) by jets with solid or hollow cones, forming free streams aligned upwards. The jets can be ladle jets. The jet openings can be varied, using a rubber with slits which