The invention provides a flotation device including a thickening tank (1) having right cylindrical sidewalls and a downwardly sloping frusto-conical floor. A drive shaft (2) is centrally rotatably mounted within the tank. Each one of a plurality of rake arms (3) is fixedly connected to the drive shaft and extends radially outwardly, substantially parallel to the tank floor. Each arm supports an array of blades (4) extending axially into the tank. An array of baffles in the form of rectilinear baffle plates (5), fixed relative to the tank, extends axially into the tank in complementary alternate radially spaced relationship with the blades (4). The baffle plates are located within the swept volume of the blades to retard fluid rotation in response to the rotation of the rake. A diametrical service bridge (6) is fixedly connected to the tank for mounting the baffle plates.