A hydrocyclone with an inflow region providing tangential inflow for a feed slurry and with a separation region following the inflow region with an underflow discharge pipe to carry off heavy materials and with an overflow nozzle in the form of an immersion tube projecting axially into the interior of the hydrocyclone. Another inflow is provided in the area of the tangential inflow to supply the barrier fluid, where the barrier fluid and the feed slurry are separated from one another by a lamella in the hydrocyclone before they are brought together. The separation region includes a conical section and a subsequent cylindrical section above the underflow discharge pipe.