A hydrocyclone is a particle classifying device. It works much like a centrifuge, using high speed to separate different sized particles or particles with different densities. In a hydrocyclone, however, water is additionally used to facilitate the process; the water enters with the feed minerals as a "slurry." The large or heavy particles sink and leave with the "underflow" while the fine or light materials are lifted upward and leave with the "overflow." In the case of gravity separations, sometimes a "heavy liquid" is used instead of water to facilitate efficient separation of light and heavy minerals.

Figure: Hydrocyclones are used in many processing applications to sort particles based on size and/or specific gravity.