Apparatus for removing impurities entrapped in a suspension of fibrous material includes a fluidized bed chamber having opposite ends, a top wall, and a perforated bottom wall. An inlet feeds the suspension to be purified to a diffusing chamber which imparts laminar flow thereto and, in turn, feeds the suspension to the fluidized bed chamber through one end thereof for flow across the bottom wall toward the other end. A pressure chamber is positioned below the fluidized bed chamber and communicates with the bottom wall thereof, the pressure chamber having a plurality of walls thereacross dividing the same into a plurality of sections. A manifold has branches communicating with each section of the pressure chamber for introducing a fluidizing agent under pressure thereto for upward flow through the perforations in the bottom wall and through the suspension therein thereby to separate impurities from the suspension by flotation. A separation chamber has a forward end communicating the other end of the fluidized bed chamber for receiving the suspension therefrom and has top and bottom walls and a rear wall. A light impurities withdrawal conduit extends upwardly from the top wall of the separation chamber in the forward region thereof for removing separated impurities from the suspension which have relatively low specific gravity and float to the top thereof. A fibrous suspension withdrawal conduit extends from the rear upper region of the separation chamber for removing the purified fibrous suspension therefrom, and a heavy impurity withdrawal conduit extends from the lower region of the separation chamber for removing separated impurities from the suspension which have relatively high specific gravity and settle to the bottom wall of the separation chamber.