Cyclone underflow, also known as the spigot discharge or apex discharge, is the coarse, high-density slurry fraction that exits from the bottom apex of a hydrocyclone during mineral processing. In gold, iron ore, bauxite, and diamond mining processing plants, the cyclone underflow contains the coarser and denser particles that were subjected to the highest centrifugal forces within the cyclone body and were consequently directed outward to the cyclone wall and discharged downward through the spigot. In closed-circuit grinding configurations, the cyclone underflow is typically recirculated back to the ball mill or SAG mill for further size reduction, creating a circulating load. The ratio of circulating load to fresh feed is a key operating parameter that reflects classification efficiency and mill performance. A healthy cyclone underflow typically discharges in a characteristic 'roping' or spray pattern (depending on operating conditions) that operators use to visually assess cyclone performance. Excessively high underflow density or circulating load may indicate a classification or milling inefficiency requiring corrective action. In some mineral processing flowsheets, the cyclone underflow is directed to separate gravity concentration stages — such as jig or Knelson concentrator circuits — to recover dense minerals like free gold, magnetite, or heavy mineral sands that preferentially report to the coarse underflow fraction.
