A system for separating paper fibers from contaminants of similar lower specific gravities employs a reverse centrifugal cleaner wherein the discharge port for lights (reject) is located at the apex of the internally conical cleaner body and is formed by a tube which has it inner end located in the open apex end of the cleaner body and of smaller diameter to define with the end of the body an annular discharge outlet for heavies (accepts), the tube being releasably secured in position so that it can be temporarily retracted to relieve blockage of the heavies outlet.