The function of the pre-washing tank is based on the swim-sink principle i.e on the different specific weights of the various components contained in the feed material that will either cause them to float on the water surface or to sink to the bottom of the tank. Thus the device serves for separating the light and heavy material that is being fed into it. A washing/discharge drum installed on the top of the tank cleans the floating fraction of the feed material by constantly dipping it underneath the water surface and at the same time discharges the material to the follow-up unit (e.g. a granulator within the recycling line. the heavier fractions (stones, glass, metals etc.) of the feed material sediment is inside the conical bottom part of the tank. This waste fraction of the feed material will be discharged into the dewatering container beneath the tank by means of a take conveyor.