Why Hydrocyclones Send Coarse Particles to Overflow Despite Correct Geometry
After twelve years supporting mineral-processing circuits, a frequent problem is that hydrocyclone overflow contains more coarse particles than the design cut size predicts. The mill discharge looks properly ground, yet the overflow product is coarser than expected and circulating load rises. The usual cause is a shift in the balance between feed pressure and apex diameter that moves the actual cut size coarser or forces the underflow into an unstable regime.
Inside the cyclone, centrifugal force drives denser or larger particles toward the wall and down to the apex while finer particles report to the vortex finder. Cut size (d50) is the particle diameter that has roughly equal probability of reporting to either stream. Higher feed pressure increases tangential velocity and therefore reduces d50; the relationship is approximately inverse to the square root of pressure. Apex diameter controls the volume of slurry that can leave as underflow. A smaller apex raises underflow density and can sharpen the cut, but if it is too small relative to the solids load the discharge changes from a spray to a continuous rope. Roping indicates that the apex is overloaded; coarse particles are then forced into the overflow, coarsening the effective cut and increasing bypass of fines to underflow.
In a closed grinding circuit treating a copper ore, a cluster of 250 mm cyclones was operated at a feed pressure of 70 kPa with a 40 mm apex. Overflow P80 was 95 µm against a target of 75 µm, and the underflow discharged as a rope for extended periods. Particle-size analyses showed that 18 % of the +75 µm material reported to overflow. Raising feed pressure to 110 kPa and increasing apex diameter to 50 mm restored spray discharge. The measured corrected d50 fell from 62 µm to 48 µm, overflow P80 improved to 72 µm, and the fraction of +75 µm reporting to overflow dropped to 7 %. Circulating load decreased by approximately 15 % at the same mill power. Both changes were made within the normal operating window; no modification to cyclone diameter or vortex-finder length was required.
Stable operation is maintained by keeping feed pressure high enough to achieve the required centrifugal force while selecting an apex that permits spray discharge at the prevailing solids rate. For hydrocyclone cut size pressure apex diameter control, the practical checks are feed-pressure gauge readings, visual discharge type, and periodic size distributions of overflow and underflow. Hydrocyclone classification efficiency mineral processing performance is confirmed when the measured d50 matches the target and the underflow remains a spray rather than a rope. When coarse particles appear in overflow, the confirming measurements are feed pressure, apex diameter relative to solids load, and the resulting partition curve. Adjusting pressure upward and opening the apex enough to restore spray discharge, as shown by the reduction in d50 and the drop in coarse-particle recovery to overflow in the copper circuit, returns the hydrocyclone to its design separation without changes to the cyclone body or the mill.