Abstract: A system and process for separating high and low density materials by air flow. Mixed solid material is delivered into an entrance of a cascade separation chamber where it is allowed to free fall and tumble over inclined surfaces. A fan forces air through a “closed” path that includes a duct from the fan outlet to return air to the separation chamber, and a branch duct that supplies an air knife disperser arranged to direct a high velocity air jet against free falling material in the chamber. The fan draws air that has entrained relatively light materials in the chamber, and thereby separates the same from the heavy materials, from the chamber through a duct connected to the inlet of an air cleaning device such as a cyclone separator. Cleaned air exhausted from the cyclone cleaning device and driven by the fan is continuously returned to the separation chamber. Air flow through the chamber is generally in counter-flow relation to the movement of heavy material through the chamber.