The hydrocyclone is the most widespread classification device in mineral processing plants, and one of the least understood. It has no motor and no moving parts; it fits in a conical tube; it costs little. Yet it decides the particle size sent to flotation, the circulating load of a grinding circuit, and often the recovery of the whole plant. Understanding what happens inside the cone lets you tune it rather than suffer it.
The parts
The tangential inlet. Slurry is injected under pressure, tangentially, into the upper part of the cyclone. This inlet sets the fluid spinning; its shape (round, rectangular, involute) influences flow stability.
The cylindrical section. The upper part, of constant diameter, where rotation establishes itself. The cyclone diameter — the figure used to name the size of the unit — is the diameter of this section.
The vortex finder. A tube plunging into the centre of the upper part, through which the overflow leaves: the fine fraction, with most of the water. Its length prevents incoming slurry from short-circuiting straight to the outlet; its diameter is one of the two main adjustments.
The cone. The lower part, narrowing toward the bottom. Its angle influences cut fineness and capacity: narrow-angle cones cut finer, wide-angle cones handle more flow and cut coarser.
The apex (or spigot). The bottom orifice, through which the underflow leaves: the coarse, thickened fraction. Its diameter is the second main adjustment, and often the only one changed in operation.
The liners. The inside is lined with an abrasion-resistant material — polyurethane, rubber, ceramic depending on the zone and severity — because slurry moves at high velocity against the walls. The apex and the lower cone are the fastest-wearing parts.
What happens inside the cone
Slurry injected tangentially spins along the wall and spirals down toward the apex: this is the primary, outer vortex. Particles experience a centrifugal acceleration well above gravity; the largest and densest migrate toward the wall and follow the primary vortex to the underflow.
Approaching the apex, the orifice cannot pass the whole flow: most of the fluid reverses and rises up the centre in a tighter spiral — the secondary, inner vortex — to the vortex finder and the overflow. Fine and light particles, which did not have time to reach the wall, are carried in this rising current.
At the centre, a column of air forms over the full height — the air core — because the apex is open to the atmosphere. Its stability is a direct indicator of proper operation.
The result is a separation by size and by density: a given particle reports to the underflow or the overflow depending on whether the centrifugal force pushing it toward the wall wins over the drag of the rising fluid.
The cut point: D50
The separation is not sharp. Every particle size has a probability of reporting to the underflow; the curve giving that probability against size is the partition curve. The D50 is the particle size with an equal chance of leaving either way: that is the cut point. It is completed by the corrected D50, which removes the effect of fines carried mechanically with the underflow water (the bypass), and by the slope of the curve, which says whether the cut is sharp or spread out.
What shifts the D50, for a given design:
| Variable | Effect on the cut point |
|---|---|
| Higher feed pressure (hence flow) | Finer cut — more centrifugal force |
| Larger apex diameter | Coarser cut; more dilute underflow; less risk of roping |
| Larger vortex finder diameter | Coarser cut; more flow |
| Higher feed percent solids | Coarser and less sharp cut — the more viscous slurry hinders separation |
| Higher solids density | Finer cut at equal size (a dense particle behaves like a larger one) |
| Larger cyclone diameter | Coarser cut, higher capacity |
In routine operation, the levers are mainly the apex (changing the spigot) and the pressure (number of cyclones on line in the cluster, pump flow). The vortex finder is changed at a stop; cyclone diameter and cone angle are chosen at design.
Reading the underflow
The underflow's behaviour is the first diagnostic, with no instrument:
- Spray discharge: a hollow, well-opened cone, with the air core visible at the centre. This is the normal regime: the apex passes the coarse with enough water that nothing accumulates.
- Rope discharge: a compact jet, no air core. The apex is saturated — too many solids for its diameter. Coarse particles that can no longer exit are pushed back to the overflow: the cut degrades sharply and coarse material heads downstream. Typical cause: apex too small, feed too dense, or an apex worn unevenly.
- Overly dilute discharge: a very open umbrella, lots of water in the underflow. The apex is too large; many fines leave with the water to the underflow (high bypass), which, in a grinding circuit, returns already-fine particles to the mill.
The right setting sits between the two: a clean umbrella, a stable air core, and an underflow as dense as possible without roping.
Applications and what is expected of the cyclone
Closed-circuit grinding classification. The main use: the mill discharge is cycloned, the overflow (fine enough) goes to flotation or leaching, the underflow (too coarse) returns to the mill. The D50 sets product fineness, hence mineral liberation; the circulating load depends on it directly. A cyclone out of tune here means overgrinding (energy, hard-to-treat fines) or undergrinding (falling recovery).
Desliming. Removing the finest particles ahead of a process they would disturb.
Thickening and water recovery. Concentrating a slurry before filtration, recovering process water — ahead of a filter press or in a tailings management circuit.
Sand and aggregate washing. Separating clay fines from sand.
Cyclone clusters
A single cyclone handles a limited flow; a plant installs several in parallel, fed by a common distributor — a cluster. This lets capacity be matched to the actual flow by opening or closing cyclones one at a time, which keeps feed pressure in the intended range as tonnage varies. It is the most-used daily adjustment lever, along with changing the apex.
Wear and maintenance
Abrasive slurry wears the liners; the apex and the lower cone first. A worn apex grows: the cut drifts coarser and the underflow dilutes without anyone having changed anything. Hence three habits: measure the apex diameter at fixed intervals, hold a stock of spare spigots in several diameters, and inspect the liners at every stop. Modern cyclone designs come apart quickly for these operations.
Going further
Sizing a cyclone or a cluster — diameter, number, apex, vortex finder, pressure — is done from the feed flow, density and size distribution and the target D50, using the manufacturer's curves and, ideally, a test on the actual slurry. That is the kind of study our feasibility studies service covers ahead of supply; the configurations offered are on the Filter presses & hydrocyclones product page. For a specific case, tell us about your slurry and the target cut.
Frequently asked questions
Why does a hydrocyclone need no motor? Because the separation energy comes from the feed pump pressure: the inlet velocity creates the spin, and the spin creates the centrifugal force.
How do I change the cut point without stopping the plant? By adjusting feed pressure — number of cyclones on line in the cluster or pump speed — or by changing the apex, which takes a few minutes on a cyclone isolated from the cluster.
Can a cyclone separate by density? It separates by a combination of size and density: a small dense particle can report to the underflow along with larger light ones. That is what makes classifying an ore with heavy minerals more complex — and sometimes it is exactly what is wanted.
Hicham Marouazi, ing., PMP
Engineer and project manager (PMP) at Induscoat Solutions. Over 16 years of experience on mining, energy and petrochemical sites in Canada and internationally — selecting, installing and commissioning industrial equipment.