Hydrocyclones: how a cyclone classifies, what moves the cut size, spray and roping, and maintaining a cyclone cluster
Hydrocyclones from the operating and maintenance side: the inlet, cylinder, vortex finder, cone, spigot and liners, how the spinning flow separates coarse from fine and what the cut size d50 means, the effect of pressure, density, vortex finder and spigot on the cut, spray and roping discharges, and the spigot, liner, gauge and sampling work that keeps a cluster performing. With a section drawing and partition curve.
Key takeaways
- A hydrocyclone separates slurry by size and density in a spinning flow: coarse and dense particles go to the wall and out the spigot, fine particles and most of the water go up the vortex finder to the overflow.
- The cut size, d50, is the particle size with an even chance of going either way. Pressure, feed density, vortex finder and spigot size all move it.
- A healthy underflow is a hollow spray. A rope means the spigot is overloaded and coarse material is going to the overflow.
- The spigot and lower cone wear fastest. Wear changes the geometry and with it the cut, so these are changed on a wear limit, not when they fail.
- Run cluster valves fully open or fully closed, keep the pressure gauge honest, and sample the products, because the cyclone's performance cannot be seen from outside.
A grinding circuit's product size drifts coarse over a fortnight. The mill is fine, the ore has not changed and the cyclone pressure reads where it always does. When the cluster is inspected, three spigots are badly worn, one cyclone valve is half closed, and the pressure gauge's diaphragm has failed so it has been reading high for weeks. None of it was visible from the control room.
This guide covers hydrocyclones from the operating and maintenance side: the parts, how a cyclone classifies and what moves the cut size, underflow discharge patterns and roping, and the maintenance that keeps a cluster performing. It sits beside the guides to slurry pumps, thickeners and fixed plant maintenance in mining. Cyclone selection and circuit design belong to the process engineer.
How a hydrocyclone works
Slurry pumped into the top of the cyclone enters tangentially and spins. In the spinning flow, coarse and dense particles move outward to the wall and spiral down the cone to the spigot. Fine particles, and most of the water, stay nearer the centre, where the flow reverses into an upward spiral and leaves through the vortex finder. An air core forms along the axis when the cyclone is running properly.
The separation is not sharp. Each particle size has a probability of reporting to the underflow, and the size with an even chance is the cut size, or d50. Some fines always go to the underflow with the water that leaves through the spigot, which is called bypass. In a grinding circuit, the underflow goes back to the mill and the overflow goes on to the next stage, so the cut sets the product size.
| Part | What it does | What to watch |
|---|---|---|
| Inlet | A tangential or involute entry that starts the slurry spinning | Wears where the flow first strikes the barrel |
| Cylinder (barrel) | The upper section where the spin is established | Liner wear |
| Vortex finder | The tube down the centre of the top that takes the overflow out | Wear at the tip, and its diameter sets much of the cut |
| Cone | Tapers the spinning flow toward the spigot | The lower cone wears fastest |
| Spigot (apex) | The underflow outlet at the bottom of the cone | A wear part, changed often, and its diameter sets the split |
| Liners | Rubber, polyurethane or ceramic inside the steel shell | Chosen for abrasion and particle size |
| Manifold and valves | A distributor feeding a cluster of cyclones, each with a valve | Valves run fully open or fully closed, never throttled |
What moves the cut
The cut size depends on the cyclone's geometry and on how it is operated. In operation, pressure and feed density are the main levers. In geometry, the cyclone diameter, vortex finder and spigot sizes set the range. Because wear changes the geometry, a cyclone that has not been touched for months is not the cyclone that was commissioned.
| Change | Effect on the cut | Other effects |
|---|---|---|
| Higher inlet pressure | Finer cut | More wear, more pumping energy |
| Higher feed density | Coarser cut | More fines reporting to the underflow |
| Larger vortex finder | Coarser cut | More flow capacity and more water to the overflow |
| Larger spigot | Slightly finer cut | More water and fines to the underflow, a thinner underflow |
| Smaller spigot | Slightly coarser cut | A denser underflow, with the risk of roping |
| Smaller cyclone diameter | Finer cut | Less capacity per cyclone, so more cyclones |
Spray and roping
The underflow discharge shows how each cyclone in a cluster is doing. A hollow spray with an air core is normal. A rope means the spigot is overloaded: the solids reaching it cannot all get out, the air core collapses, and coarse material is forced to the overflow. Checking the discharge of every cyclone on rounds, or with roping sensors on critical clusters, catches a roping cyclone before it upsets the next stage.
| Pattern | What it looks like | What it means |
|---|---|---|
| Spray (umbrella) | A hollow cone of slurry with an air core through the spigot | Normal operation |
| Roping | A solid rope of slurry, the air core collapsed | Spigot overloaded: coarse material forced to the overflow, a dense underflow |
| Wide spray | A very wide, thin discharge | Too much water to the underflow, fines bypassing to it |
A rope from the spigot is coarse material in the overflow. Fix it at the cyclone, not downstream.
Maintaining a cluster
Cyclones are simple and wear quickly. The spigots and lower cones take the most wear, then the vortex finders and inlet. Each part is measured and changed on a wear limit, and recording the results by cyclone position in the maintenance log gives the wear life that turns changes into planned work. Valves on a cluster are run fully open or closed, since a throttled valve wears quickly and changes the feed to that cyclone.
Performance is checked by sampling. An overflow and underflow survey, measuring size distribution and density, shows the actual cut and bypass, and is worth repeating whenever the circuit or the ore changes. The manifold pressure gauge is the single most important instrument and needs a diaphragm seal and a calibration check, because the circuit is run on its reading.
| Task | What it involves | When |
|---|---|---|
| Spigots | Inspect diameter and wear, change on a wear limit | Frequently, often weekly, by the hours or tonnes in the history |
| Lower cones and liners | Wear and liner damage | Planned stops, recorded by position |
| Vortex finders | Tip wear, diameter | Planned stops |
| Pressure gauge | A diaphragm-protected gauge on the manifold, reading correctly | Checked and calibrated, since operation is run on it |
| Valves | Fully open or fully closed, seats and seals sound | Rounds, and at changes |
| Discharge | Spray or roping at each cyclone | Each shift on rounds, or by roping sensors |
| Performance | Overflow and underflow sampled for size and density | A survey on a set interval, or when the circuit changes |
Common mistakes
- Running cluster valves partly closed to balance flow.
- Changing spigots only when they fail rather than on a wear limit.
- Trusting a pressure gauge that has not been checked.
- Fixing a coarse overflow downstream instead of at a roping cyclone.
- No record of which cyclone positions wear fastest.
- Fitting a different spigot size without recording it, so the cluster drifts.
- No performance survey since commissioning.
Common questions
How does a hydrocyclone work?
Slurry enters the top of the hydrocyclone tangentially under pressure, which sets it spinning. Centrifugal force throws coarse and dense particles to the wall, where they spiral down the cone and leave through the spigot as the underflow. Fine particles and most of the water move to the centre, where an upward spiral carries them out through the vortex finder as the overflow.
What is the cut size of a hydrocyclone?
The cut size, or d50, is the particle size that has an equal chance of reporting to the overflow or the underflow. Particles much coarser than the d50 mostly go to the underflow and much finer ones to the overflow, with a spread around it set by the sharpness of separation. Smaller cyclones, higher pressure and lower feed density give a finer cut.
What is roping in a hydrocyclone?
Roping is when the underflow leaves the spigot as a solid rope instead of a hollow spray, because the spigot cannot pass all the coarse solids reaching it and the air core collapses. Coarse material is then forced up to the overflow, which can upset downstream processes such as flotation. It is corrected by a larger spigot, lower feed density, or reducing the solids load on that cyclone.
What affects hydrocyclone performance?
Operating variables, mainly inlet pressure, feed rate and feed density, and geometry, mainly the cyclone diameter, vortex finder diameter, spigot diameter and cone angle. Wear changes the geometry over time, which is why worn spigots and vortex finders shift the cut. Cluster valves partly closed, blocked inlets and inaccurate pressure readings also degrade performance.
How often should hydrocyclone spigots be changed?
On a wear limit, set from the site's own history for each duty. Spigots wear fastest of all the parts, often in weeks, and a worn spigot passes more water and fines to the underflow and changes the cut. Measuring the spigot diameter at each inspection and recording it against the cyclone's position gives a wear life that the change can be planned to.
Key terms
Plain-language definitions from our glossary for the concepts this article leans on.
Standards and further reading
Related case studies and tools
- Predictive maintenance and condition monitoring (case study)
Related reading
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