Liberation and separation of valuables from building material waste

In general a recycling process starting with development, planning, production, utilization and disposal of a product, e.g., of a building, during its lifetime may be regarded as a system of material cycles. The main object is to minimize the input of raw materials and the output of waste for every stage of a building. These material cycles may be designed by reprocessing the building material waste to valuables of high quality. To approach this aim a joint research project dealt with the liberation and separation of aggregates (d = 2-16 mm) from concrete waste. The liberation is accomplished by a pneumatic cannon to simulate the stressing in impact crushers. With this cannon spherical concrete samples (d = 150 mm) are comminuted. The comminution is evaluated by the liberation grade. Fully liberated, partially liberated and non-liberated particles are distinguished. The experiments show an unexpectedly high liberation grade of up to 95% with a comparably small specific energy consumption of about 1 kWh/t. For separating both fully liberated and partially liberated aggregate, a pilot scale zigzag apparatus consisting of a zigzag channel, an air cyclone, a blower, a filter and a feeding system was used. If the diameter of the particles is kept nearly constant (e.g., by sieving) a multi-stage density separation in a turbulent cross-flow channel can be carried out. The separation performance may be evaluated by means of the separation function or grade efficiency curve, respectively. For the partially liberated aggregate a comparably good grade sharpness of about 0.74-0.84 is obtained for a 15-stage zigzag apparatus with different particle size fractions and particle contents in air.