a process and device for separation of plastic material from other stuff of a various density by a flotation process in a liquid bath . the process involves discharging heavier stuff that settles on the screen bottom by a movable screen bottom slowly moves in the direction of reducing a slot that exists between one edge of screen bottom and the wall of the flotation tank and then after having reached a final position too quickly and abruptly move in the counter - direction . that movement occurs in intervals . this abrupt motion of screen bottom initiates that kinetic energy accumulated in the heaps of metal and other heavy stuff moves this stuff towards the slot - edge and through the slot into a collection - container underneath the screen bottom to be discharged .