Abstract: The concentrator has at least one shaft-mounted rotor disc of solid or hollow construction with two or more ploughs located equally spaced about its periphery. The ploughs extend outwardly into the flow path of a solids-containing fluid within the concentrator to deter any tendency, due to centrifugal force, for a build-up of solids between stationary filter means and in a region remote from the periphery of the rotor disc. In a disclosed embodiment of the concentrator, the shaft-mounted rotor disc has at least one passage therethrough for the flow of solids-containing fluid from one side of the rotor disc to the other, and a plough is provided adjacent the passage and on a hub of the rotor disc to deter any tendency for a build-up of solids adjacent the passage.