Wastewater Treatment Fundamentals: What Is a Hydrocyclone?
Origin and Definition
The hydrocyclone first appeared in the Netherlands in the late 1930s. It is a device that uses swirling flow for classification, and it is also used for thickening, dewatering and even sorting. Its construction is simple: a hollow cylindrical section is joined to a conical section. The diameter of the cylinder defines the size of the unit, and the range is wide, from 50 mm to 1,000 mm, with 125 to 500 mm being typical. A vortex finder is inserted into the centre of the cylinder, a feed inlet is fitted tangentially, and an apex (spigot) opening is left at the bottom of the cone.
Slurry is fed under pressure through the inlet and immediately begins a swirling motion constrained by the cylindrical wall. Coarse particles, with high inertial centrifugal force, are thrown towards the wall, migrate downwards and leave through the bottom as underflow. Fine particles move towards the wall more slowly and are carried by the inward-flowing liquid out through the central vortex finder as overflow.
Working Principle
A hydrocyclone separates and removes relatively heavy coarse particles such as grit and sand from wastewater. It is sometimes used for slurry dewatering as well. Units are classified as pressure type and gravity type, and are commonly built as circular concrete structures or fabricated from metal pipe. Water enters tangentially at the top under pressure or gravity; centrifugal force throws the coarse, heavy particles to the wall, where they spiral downwards and are discharged together with the thickened stream, while finer particles, after rotating for some distance, leave with the secondary upward vortex.
The working barrel consists of a hollow cylinder on top and an inverted cone communicating with it below. In addition, a hydrocyclone has a feed inlet, a vortex finder, an overflow pipe and an apex opening.
A sand pump, or a static head, delivers the slurry tangentially into the cylinder at a pressure usually between 0.5 and 2.5 kg/cm2 and a velocity of about 5 to 12 m/s. The slurry then rotates rapidly along the wall and generates centrifugal force, and the combined action of centrifugal force and gravity throws out the coarser, heavier particles.
Applications
In mineral processing, hydrocyclones are used mainly for classification, separation, thickening and desliming. As a classification device a hydrocyclone forms a grinding-classification circuit with the mill; as a desliming device it removes slimes in gravity separation plants; as a thickening and dewatering device it thickens tailings before they are sent underground as backfill.
In wastewater treatment the same geometry is applied to grit removal and to pre-treatment ahead of flotation, filtration and biological units, where a compact footprint and a high unit-volume capacity are decisive advantages.
Flow Patterns Inside the Unit
Suspension enters the hydrocyclone tangentially at high velocity. Constrained by the outer wall, the liquid is forced into a downward rotating motion, usually called the outer vortex or descending vortex. Solid particles in the outer vortex are acted on by centrifugal force; where their density exceeds that of the surrounding liquid, which is the usual case, the force is larger, and once it exceeds the drag generated by the motion the particles overcome the drag, move towards the wall and separate from the suspension. Particles reaching the wall are pushed continuously downwards, gather near the apex as a much thickened suspension and leave through the underflow opening.
The clarified liquid, still carrying some fine particles, continues to rotate downwards. On entering the conical section the radius shrinks, so the rotational velocity increases. Because the radial pressure distribution in a vortex is uneven, pressure falls towards the axis and approaches zero at the axis, creating a low-pressure or even vacuum zone that draws liquid towards the axis. At the same time the strongly reduced apex opening prevents rapid discharge, so part of the liquid moves towards the overflow opening at the centre of the top plate, which sits in the low-pressure zone, and forms an upward rotating motion that discharges as overflow.
Outer and inner vortex: the two principal flow forms. They rotate in the same direction but move in opposite axial directions. The outer vortex carries coarse, heavy solids to the apex as underflow; the inner vortex carries fine, light solids to the vortex finder as overflow.
Short-circuit flow: part of the feed, retarded by wall friction, moves first upwards, then inwards along the underside of the top plate, then downwards along the outside of the vortex finder, finally joining the inner vortex and leaving through the overflow. Because it reaches the overflow product without being separated, it directly reduces separation performance.
Circulation flow: fluid migrating from the outer vortex into the inner vortex in a spiral path cannot all leave through the overflow opening, so the remainder circulates up and down in the annular space between the vortex finder and the wall.
Zero-velocity envelope: the locus where axial velocity is zero. It is the centreline of the circulation flow and the boundary between the inner and outer vortices, and for a given geometry its shape and size are essentially fixed.
Maximum tangential velocity locus: the path along which the tangential velocity of fluid particles reaches its maximum during the inward migration of the spiral vortex.
Air core: as the spiral radius decreases, tangential velocity rises until the pressure falls below that of the surrounding space. Air comes out of solution in the low-pressure zone, and outside air is also drawn in through the discharge openings, forming an air core.
Structural Parameters
Diameter: governs capacity and cut size. Both generally increase with diameter.
Feed inlet diameter (Di): affects capacity, cut size and classification efficiency. A larger inlet gives a coarser cut. The inlet diameter is proportional to the unit diameter, typically Di = (0.2 to 0.26) D.
Cone angle: a larger angle gives a coarser cut, a smaller angle a finer cut. Fine classification uses small angles, usually 10 to 15 degrees, while coarse classification and thickening use 20 to 45 degrees. Internal resistance rises with the cone angle, so capacity falls at the same inlet pressure; the cut size increases, overall separation efficiency drops and less fine material reports to the underflow.
Vortex finder diameter: a larger finder increases overflow volume and coarsens the overflow, reduces fines in the underflow and raises underflow concentration. It is sized from the barrel diameter, Do = (0.2 to 0.4) D. The inner diameter of the vortex finder is the single most important dimension because it affects every process indicator; at constant inlet pressure, capacity is approximately proportional to it over a certain range.
Vortex finder insertion depth: the distance from the bottom of the finder to the top plate. A shallower insertion gives a finer cut, a deeper insertion a coarser cut; h = (0.3 to 0.7) D is usual.
Vortex finder wall thickness: a thicker wall can raise separation efficiency to some extent, reduce internal energy loss and increase capacity.
Inlet cross-section: shape and size strongly influence capacity and separation efficiency. The inlet converts straight-line flow into circular motion at the cylindrical inlet, and may be circular or rectangular.
Apex diameter (d): a larger apex gives a finer cut, a smaller apex a coarser cut; d = (0.15 to 0.25) D. The apex is the most wear-prone part of the unit. Increasing the apex diameter raises capacity, but the effect is smaller than that of the inlet or the vortex finder.
Internal surface roughness and assembly accuracy: the influence on capacity and efficiency is modest, but practice shows that a smooth abrasion-resistant rubber lining raises flow resistance and slightly improves efficiency, whereas a rougher wall lowers resistance and increases underflow volume.
Feed viscosity: cut size is proportional to the square root of feed viscosity, so higher viscosity coarsens the cut; capacity and the split ratio also rise with viscosity.
Cone ratio: the ratio of apex diameter to vortex finder diameter, a key design parameter and an important operating lever. A large cone ratio gives a finer cut and a small one a coarser cut; the usual range is 0.35 to 0.65. Because the vortex finder diameter is fixed, the cone ratio is adjusted in practice by changing the apex.
Operating Parameters
Feed pressure: raising pressure increases slurry velocity and centrifugal force, improving classification efficiency and underflow concentration. However, pressure has only a marginal effect on cut size while energy consumption rises sharply and wear on the unit, particularly the apex, becomes more severe. Coarse material is handled at low pressure (0.05 to 0.1 MPa), fine and clayey material at high pressure (0.1 to 0.3 MPa).
Feed rate: a higher feed rate coarsens the cut, a lower rate fines it.
Feed concentration: at fixed size and pressure, concentration strongly affects overflow fineness and efficiency. High concentration raises viscous drag, coarsens the cut and lowers efficiency. For a cut size of 0.074 mm, a feed concentration of 10% to 20% is appropriate.
Feed particle size distribution: this markedly affects performance. With other parameters fixed, a low content of material finer than the cut size leaves little fine material in the underflow at high concentration but raises the coarse content of the overflow and lowers efficiency. A high content of near-cut-size material puts more fines in the underflow and more coarse particles in the overflow, again reducing performance.
Advantages
A hydrocyclone has no moving parts and a simple construction; it offers high capacity per unit volume in a small footprint; classification efficiency can reach 80% to 90% at fine cut sizes; and capital cost and material consumption are low. Its main drawbacks are relatively high power consumption and severe wear when fed at high pressure. Wear-resistant materials such as cemented carbide and silicon carbide for the apex and inlet parts partly solve this problem. When used for closed-circuit grinding classification, the small volume gives no buffer against fluctuations in ore throughput, so operation is less stable than with a mechanical classifier.
