In this paper, we present flow patterns around and performance of an air-flow separator by using the numerical analysis. With this separator, particles of different density are to be separated by using the drag force from the air flow. The low-density particles are designed to be separated by using inhalation through holes on a rotating drum. To obtain the flow informations needed for determining the proper design parameters, we have performed numerical simulations by using a commercial code, ANSYS CFX. Various parameter set was tested and it was found that depending on the design of drums there exist critical parameter set regarding the attachment of light particles on the drum, which is prerequisite for the separation of materials. We present here the possibility of using the present design in separation of particles mixed in the clod for use in recycling.