A centrifugal separator for separating heavy metals from lighter or includes a bowl (10) rotatable about its axis and defining annular collection areas (19, 20) on the peripheral surface of the bowl at axially spaced positions along the bowl. At the peripheral wall (11) in each area is provided a plurality of angularly spaced discharge openings (29) which are controlled by pinch valves (42) to periodically allow the radial escape of the collected material. The pinch valves of a first area (19) are controlled at different timing to the pinch valves of a second area (20) to vary the concentration of the materials collected in the different areas. The pinch valves are operated by a pressurized liquid system which is fed to the different areas through a swivel coupling mounted at a longitudinal shaft supporting the bowl.