A rotary separator has a generally closed housing whose upper portion is formed with a row of throughgoing windows. A lining of flexible sheet material is secured in the housing over the windows. A fan in the upper portion of the housing circulates air in the housing up through an annular sleeve centered in the housing, down around the sleeve in an annular passage, and then up through the sleeve again. Below the lower end of the sleeve is provided an upwardly open discharge funnel. Particulate material to be graded is introduced into the upper portion of the housing with the fan rotating so that the heavy fraction falls down into the funnel and the light fraction is entrained upwardly outside the funnel. A device is provided for inwardly deflecting the lining and knocking off it caked-on material so as to prevent this material from building up and blocking the outer annular flow passage.