Hydrocyclones explained: your questions answered
Hydrocyclones are among the most widely used pieces of equipment in mineral processing, yet they remain one of the most misunderstood. As Ernst Bekker, Process Specialist for Cyclones at Multotec, explains in a 2020 webinar, hydrocyclones can be either a blessing or a curse depending on how well you understand their capabilities and limitations.
This guide answers the most common questions about hydrocyclones and draws on practical insights from decades of industry experience.
Understanding Hydrocyclones
What is a simple definition of a cyclone?
A cyclone is a device that uses rotational motion to separate materials of different densities or sizes. In mineral processing, the term typically refers to a hydrocyclone, which separates solid particles from a liquid slurry based on particle size and density (when heterogeneous ores are treated). The name comes from the cyclonic, or spinning, motion created inside the device when feed slurry enters tangentially (at an angle), forming a vortex that forces particles to separate according to their physical properties.
What is the function of a cyclone?
The function of a cyclone is to create conditions where particles of different sizes or densities follow different paths and can be collected separately. The device achieves this through three main mechanisms: tangential feed entry under pressure creates rapid slurry rotation; centrifugal force then pushes denser, coarser particles toward the outer wall; and the device's geometry directs coarse material downward to the underflow while fine material exits upward through the overflow.
What is a cyclone in mineral processing?
In mineral processing, a hydrocyclone is a classification device that separates particles in a slurry into coarse and fine fractions (the portion of a crushed or ground ore body that consists of the smallest particles). The device has no moving parts and relies entirely on centrifugal force generated by the spinning slurry to achieve separation. Feed enters tangentially under pressure, which creates a rapid spinning (rotational) motion inside the cylindrical body, causing fine particles to migrate toward the centre and exit through the overflow. Meanwhile, the heavier, coarser particles move toward the outer wall and travel downward to exit through the underflow. Hydrocyclones are commonly used where particle separation in the sub 150 micron range is required, and where ultrafine screening is not economical or practical.
What is the purpose of a hydrocyclone?
The purpose of a hydrocyclone is to separate particles in a slurry according to size, density, or both. In classification applications, the goal is to split material into defined size fractions for further processing. In desliming applications, the objective is to remove ultra-fine particles that would interfere with downstream processes such as spirals or flotation. In dewatering applications, the aim is to recover as much solid material as possible while removing excess water. The specific purpose determines how the hydrocyclone is configured and operated.
How Hydrocyclones Work
What is the working principle of a cyclone?
The working principle of a hydrocyclone relies on centrifugal force to accelerate the natural settling of particles. Feed slurry enters the cylindrical section tangentially at high velocity and creates a spinning vortex that generates centrifugal force many times greater than gravity. Coarse and dense particles are thrown toward the outer wall and spiral downward along the cone section before exiting through the spigot at the bottom. Fine particles remain in the inner part of the vortex and are carried upward by a secondary flow pattern before exiting through the vortex finder at the top.
The separation is not perfect, as some coarse particles report to the overflow and some fine particles report to the underflow. Additionally, water in the underflow carries fine particles with it, so there will always be some fines in the underflow stream.
How does a mining cyclone work?
A mining cyclone works by using the rotational energy of the feed slurry to separate particles through several stages. The slurry enters tangentially into the cylindrical feed chamber under great pressure, which creates immediate rotation. As the slurry spins, centrifugal force acts on the particles and causes larger and denser particles to experience greater force and move toward the wall before spiralling down the conical section and exiting through the spigot. Smaller and lighter particles remain closer to the centre, where an upward flow develops and carries them out through the vortex finder. The cut point, defined as the particle size with a 50% probability of reporting to either stream, depends on cyclone geometry, feed pressure, slurry density, and other operating parameters.
How does a DMS cyclone work?
A Dense medium separation cyclone works on a different principle from a standard hydrocyclone and separates particles based on density rather than size. In DMS applications, the feed slurry contains a dense medium (such as ferrosilicon or magnetite) suspended in water, which creates a fluid with a specific gravity between that of the valuable mineral and the waste material. When feed material enters the cyclone, particles denser than the medium sink and report to the underflow, while particles lighter than the medium float and report to the overflow. DMS cyclones are commonly used in diamond, coal, and iron ore processing, where density differences between valuable and waste material are significant.
Types of Cyclones
What are the four types of cyclones?
In mineral processing, hydrocyclones are typically categorised by their primary function.
Classification cyclones operate in the 10 to 150 micron range (where screening is not practical), separating particles into different size fractions for further processing. When no detail is available on the feed size distribution to the hydrocyclone, an acceptable mass split to underflow of solids is about 70 to 75%.
Desliming cyclones remove ultra-fine material in the 15 to 45 micron range that would cause problems in downstream processes, with about 90% of mass reporting to the underflow. Dewatering cyclones maximise solids recovery to the underflow and typically achieve 95 to 98% recovery to produce a drier product for stockpiling or further processing. Slimes dam cyclones operate at the back end of processes to manage tailings and require careful monitoring despite often being located in remote areas.
What are the two categories of cyclones?
Cyclones can be divided into two categories based on the medium they process. Hydrocyclones process slurries where solid particles are suspended in water, and are used throughout mineral processing for classification, desliming, and dewatering. Gas cyclones separate solid particles from gas streams and are commonly used in dust collection, cement manufacturing, and other applications where solids must be removed from air or other gases.
Is a cyclone a centrifuge?
A cyclone uses centrifugal force but is not technically a centrifuge. The key difference lies in how that force is generated. In a centrifuge, mechanical rotation of the vessel creates centrifugal force through moving parts that spin the container holding the material. In a cyclone, the centrifugal force comes from the rotational motion of the fluid itself, which is induced by tangential entry with pressure into a stationary vessel with no moving parts.
Comparing Hydrocyclones to Other Equipment
What is the difference between a hydrocyclone and a centrifuge?
A centrifuge uses mechanical rotation of the vessel to create centrifugal force through moving parts and can achieve very high g-forces with precise control to produce very dry solids, but it has higher capital and maintenance costs. A hydrocyclone has no moving parts and generates centrifugal force through the motion of the fluid itself. This makes it simpler, cheaper, and more robust, although the separation is less precise, and the underflow always contains some moisture. Centrifuges are typically used where very clean separation or very dry products are required, while hydrocyclones are preferred for high-volume applications where simplicity and low cost are priorities.
What is the difference between a hydrocyclone and a decanter?
The main difference between a hydrocyclone and a decanter centrifuge is that a hydrocyclone uses fluid rotation with no moving parts to separate solids from liquids, while a decanter uses a mechanically driven rotating bowl and screw conveyor. Decanters provide drier solids and clearer liquids but are more expensive to buy and maintain, so they are used where high product quality is critical. Hydrocyclones, by contrast, handle much higher volumes at lower cost and are therefore preferred in most mineral processing applications.
Common Problems
What are common hydrocyclone problems?
Misaligned expectations are perhaps the most common issue, as clients often expect perfect separation where all material below a certain size reports to the overflow and all material above reports to the underflow. In reality, hydrocyclones achieve imperfect separation with a probability curve centred on the cut point.
Beyond expectations, several operational and mechanical problems frequently occur. Dilute underflow arises when too much water reports to the underflow and carries fine particles with it, compromising desliming performance and introducing unwanted fines to downstream processes. Improper spigot installation can create an internal step that deflects coarse material back into the cyclone and causes it to exit via the overflow. Siphoning happens when the overflow pipe extends below the spigot and creates a vacuum that draws coarse material into the overflow. Solid underflow discharge indicates that a proper air core is not forming and forces solids into the overflow. Physical damage from inadequate support can deform the inlet head and impair performance. Neglect of remote cyclones particularly affects slimes dam applications, where poor maintenance can lead to serious consequences, including dam wall failures.
Benefits and Applications
What are the advantages of using a hydrocyclone?
Hydrocyclones offer no moving parts for lower maintenance and higher reliability, compact size for high throughputs relative to footprint, and low capital cost for applications where perfect separation is not required. Operation is also continuous without batch interruptions, with additional versatility to serve multiple purposes with modifications to geometry and operating conditions. Lastly, hydrocyclones have a rapid response to changes in feed conditions through adjustments to pressure, feed density, or component sizes.
What is the purpose of dewatering in mineral processing?
Dewatering removes excess water from slurries to produce material that can be handled, transported, or processed further. Hydrocyclones can contribute to dewatering but cannot generally replace thickeners because thickeners offer much longer residence time and produce denser underflows. Stacker cyclones are specifically designed for dewatering applications and use controlled siphoning to produce a drier underflow, although some moisture always remains.
Key Takeaways
Hydrocyclones are deceptively simple devices, and their lack of moving parts can lead people to believe they fully understand them after only brief exposure, which often results in poor performance and frustration. The reality is that hydrocyclones achieve imperfect separation, so there will always be some coarse material in the overflow and some fine material in the underflow. Clear communication with suppliers is critical when specifying cut points to ensure both parties understand whether the reference is D50 corrected or whether zero coarse material in the overflow is required. Proper installation and operation matter enormously, as issues such as incorrect spigot orientation, siphoning from extended overflow pipes, and inadequate support can all compromise performance. When properly understood and applied, hydrocyclones provide reliable, low-cost particle separation for a wide range of mineral processing applications.
Need help with hydrocyclone selection or optimisation? Contact Multotec to discuss your specific application. You can watch our informative webinar here.