Hydrocyclones, also known as ‘cyclones’, from Multotec Australia are used across the continent in mining and mineral processing, manufacturing, aggregates, food processing, wastewater management, and other industries. In mineral processing, they are used for classifying particles based on size and density. In the mining sector, cyclones are used for a range of applications, including separating ore from waste, removing heavy minerals from slurry, and dewatering tailings.
With branches in Brisbane, Mackay, Newcastle, Newman, Perth, and Emerald, Multotec Australia is perfectly positioned to aid with all of your mineral processing cyclone requirements. Our hydrocyclones are used by international mining giants like Xstrata, Fortescue Metals, BHP Billiton, Oceania Gold, FQM, Rio Tinto, and AngloCoal, to sort and classify a variety of minerals including, but not limited to, aluminium, chrome, coal, copper (oxide & sulphide), diamonds, gold, graphite, vanadium (high & low grade), iron ore, lithium-tantalum, manganese, mineral sands, nickel, PGM, phosphates, tantalum, tin, uranium, and Zinc.
A hydrocyclone diagram showing the cross-section of a cyclone.
Multotec Australia’s industry-proven product range of classification, dense medium, and tailings dam cyclones has been optimised over hundreds of applications across the world to improve the equipment’s lifespan, classification efficiency, and energy consumption.
Multotec Australia manufactures a complete range of off-the-shelf and customised hydrocyclones to match each client’s specific process requirements.
Watch this video on tailings dam cyclones:
Using centrifugal force and gravity, hydrocyclones create a vortex of moving liquid in order to separate particles according to their size and density. Heavier particles gravitate to the bottom of the hydro-cyclone and lighter ones, ideally, remain suspended near the top of the cyclone chamber. The suspended particles’ light weight means that they can be discharged via overflow pipes at various heights, which is determined by the particle size requirements.
The particle size requirements would have been determined by the specific downstream application requirements such as filtration and sedimentation, for example. The coarse material migrates to the centre of the hydro-cyclone and moves downwards through the spigot.
Hydro cyclones can bring joy to the lives of those who use them, or it can be a frustrating piece of equipment! This is according to cyclone product specialist at Multotec Ernst Bekker. You can watch the video here.
So, why use a hydrocyclone? Bekker says that in some people’s minds, a hydrocyclone should be the perfect classifier, but that is not always the case. Perfect classification looks like this:
The cut point of the hydrocyclone is defined as 50% of the particle size reporting either to the overflow or the underflow. On the curve that you see on the image above – the left-hand side is the fines fraction and the right-hand side is the coarser fraction. As you can see on the Y axis on the left-hand side, it recovers to the underflow. In reality, this is the type of separation that starts to take place with the cut point at the 50% mark.
However, as we don’t achieve perfect separation, particles are misplaced. Coarse particles are misplaced to the overflow and fine particles to the underflow. Another factor that plays a role in the hydrocyclone separation efficiency is bypass water, which can be seen in the bottom left of the above image. It is necessary to always have water in the underflow, as the more water you have, the more fines report to the underflow. Because of this, Multotec, as hydrocyclone manufacturers and suppliers, discount the bypass water effect and use a curve that we refer to as the corrected separation curve. As hydrocyclone suppliers and manufacturers, we use two different cut points. Our focus is on the actual cut point which, in client simulations, is referred to as the D50 corrected cut point.
The objective in hydrocyclone classification is to have the unit operating at maximum efficiency. Bear in mind that, just because there is material reporting to both the overflow and underflow, it does not mean that the unit is operating efficiently.
The red line in the above graph shows the overflow and size distribution of a hydro-cyclone’s operating efficiency. The dotted line shows the overflow size distribution of a hydrocyclone that is not operating efficiently, and the green line is the flotation recovery response per particle size. (This example is based on copper ore, and shows the difference between a cyclone with low efficiency and one with high efficiency.) The economic impact, on an annual basis, is significant – and bear in mind that this is for illustrative purposes only, and will differ from process to process. The bottom line is that you may think that your hydrocyclone is operating efficiently, but you may be losing money.
As the word indicates, classification consists of classifying the particle size into different size fractions similar to a screening application where oversized particles go in one direction and undersized particles in another. The same happens on a hydrocyclone where the overflow is the fines fraction and the underflow the coarser fraction. Roughly 75% of the mass recovered to the underflow is assumed if the particle size distribution is unknown. If the particle size distribution is known, the value can change. It is a sizing activity and is normally used where it is not economical to use ultrafine screening or if there are no screen apertures available. Our hydrocyclones normally fall in the category of between 0 and 150 microns, and that is where the classification actions take place. There are various positions of hydro-cyclones in a process flow sheet, as can be seen by the example below.
On this platinum flow sheet, the cyclones circled in green indicate the different applications in the process where a hydrocyclone can be used.
The actual separation curve of a cyclone looks like the graph below where there is misplaced coarse and fines material.
A hydrocyclone is not a perfect classifier! Because of misplaced material, you will have up to 70-micron material in the overflow. Although cyclone operators may have thought that there won’t be any fines material, they will, in actual fact, have between 35 and 50-micron material. One of the reasons is because the underflow is wet, and there is also bypass water present, which carries fines to the underflow. This means that, in reality, you cannot have any fines in the underflow. However, we can minimise the amount of coarse material that reports to the overflow, but the customer’s reference needs to be similar to that of the cyclone supplier.
If the customer wanted a cut point of 50 microns, we, as hydro cyclone suppliers, use the D50 corrected cut point, assuming that you want, at 50-micron particles, to have a 50/50 probability of reporting either to the overflow and underflow. We select a hydrocyclone based on that premise, which results in some coarse material reporting to the overflow. If a client specifies that they don’t want any 50-plus 50-micron material in the overflow, the curve and the selection process would be vastly different. The cut point of the hydrocyclone becomes much finer, which means you will have more fines in the underflow, which ensures that there isn’t any misplaced coarse material in the overflow. Although the problem looks simplistic, in reality it has caused a number of challenges in the industry.
For example, a hydrocyclone was selected based on the premise of a cut point of 250 microns. When the client realised that their reference used was incorrect, it resulted in a different approach in terms of cyclone selection. It is possible that modifications can be made to the existing design. In the image on the left, the right-hand picture shows that the diameter has become much smaller. It is possible that, if there are problems on the hydrocyclone, modifications can be made to the existing design.
Cone angles can be changed, the hydrocyclone vortex finders can also be changed, and the cut point of the hydro cyclone can also be changed to a certain extent. However, if the basis of selection (the starting point) is incorrect, then customers and cyclone suppliers may need to look at a different hydrocyclone and diameter.
If the distribution size is not available, as a rule of thumb we assume that 90% of the mass reports to the underflow. If the size distribution is available, that value can change. The aim of desliming is so that the hydrocyclone can remove ultrafine material or slimes that we commonly assume to be in the range of between 15 and 45-micron material. However, each industry defines slimes in a different size fraction or as a different reference. It is therefore important that when customers liaise with hydrocyclone manufacturers and hydro-cyclone suppliers, that when the word ‘slimes’ is used then the size fraction should be illustrated so that there is no confusion as to what size fraction is being referred to.
Desliming hydrocyclones are used in processes where the fines are not suitable or can create problems in the processes – like spirals and flotation, for example. The cyclone configuration is also different from that of classification. Looking at the flowsheet below, the position of the desliming cyclone is just above the spirals.
Those in the industry sometimes use a ‘normal’ hydrocyclone and sometimes a second-hand cyclone. This does not work! If you require a proper desliming process, the length of the cyclone is important – the longer the cyclone the better the performance in terms of desliming. The aim is to remove the fines, or the slimes, from the cyclone underflow, bearing in mind that there is a bypass water effect that carries the slimes to the underflow as well.
If one considers the image below, the top right-hand side indicates underflow from a hydrocyclone that is quite diluted. The more water in the underflow, the more fines report to the underflow. The image on the bottom right of the picture below shows a more optimum discharge flow.
It is important that cyclone users and operators can see the underflow discharge from a hydrocyclone. If you cannot, then you won’t know how the unit is performing.
Another challenge in terms of bypass water is when a cyclone is installed just above a spiral as the underflow is kept at a very diluted concentration to allow the spiral to operate effectively. Using an example of coal processing, because you have a diluted underflow, the fines can be associated with high ash. Reducing a high ash fraction onto the spiral in turn affects the product quality from the spiral. In other minerals, a diluted underflow going to the spiral affects the performance of the spiral separation process.
In addition to these challenges – there is the problem of having a coarse overflow. In some instances, the spigot is inserted the wrong way, which creates an inward step just before the material exits the cyclone, which creates a problem.
The material flowing down the hydrocyclone is deflected by this ridge and carried back into the cyclone via the air core movement.
This rotational movement of the coarse material creates high wear, resulting in strange shapes on the hydrocyclone cone. The picture on the top right-hand side of the image above shows the overflow being extended to below the spigot of the hydrocyclone. The rubber pipe that has been inserted has been sucked in, which indicates that a vacuum is beginning to form in the overflow of this hydro-cyclone. When you have this type of installation, it is recommended that a breather pipe be installed at the top of the overflow pipe to break the vacuum effect. If a customer is unable to do this, they should have the overflow terminated roughly in the centre of the cyclone and allowed to discharge in a tongue dish so that the overflow pipe can be exposed to the atmosphere, removing the possibility of siphoning. The underflow can then be discharged into a solids stream without a proper air core forming.
For the dewatering process, we assume a 95% mass recovery to the underflow, but only if we do not know the size distribution. If size distribution is known, the value will change. Some customers want 100% of solids to go via the spigot and remove the water only, but that seldom happens as there is always a fines fraction that is present in the overflow. So, to remain on the safe side, we, as hydrocyclone manufacturers and suppliers, use between 95% and 98% recovery to the underflow. Some hydrocyclone manufacturers will promise 100% recovery, but please bear in mind that this is not always possible. The hydro-cyclone can, in some instances, be used to replace thickeners, but only for very special applications. In general, the thickener remains the best dewatering device on a process. A cyclone can aid with the thickener dewatering process, but cannot replace it.
Dewatering cyclones can be used where other dewatering equipment is not economical, as long as customers understand the limitations.
On the flowsheet above, the dewatering cyclone is circled in green – material coming from the spiral is being dewatered and discharged on a pad. The cyclone used for dewatering is known as a stacker cyclone. In this instance, we deliberately extend the overflow to discharge below the underflow of the cyclone, as it creates a siphoning effect, and we can control the strength of the siphon by introducing air.
So, in conclusion, the hydrocylone is deceivingly simple. Once cyclone operators or other personnel begin to gain a simple understanding of how a cyclone performs, they believe they are experts and that they know where a hydrocyclone can be used. Then when the cyclone doesn't perform correctly or optimally, they get frustrated. As they gain more experience, they will get to the point where they understand the limitations of the hydrocylone, and then it becomes a blessing.
Lastly, if you want to use a hydrocyclone, decide what the objective is. Why do you want to employ this objective and why do you want to use a hydrocyclone? One may need to consider that other equipment might better serve the application requirements. Once you've decided that you are going to use a hydro cyclone, don't assume you have all the answers – ask for advice.
Some of the hydrocyclones in our range include the tangential design, the scrolled evolute design and the involute design. The hydrocyclone diagram below illustrate the differences.
Over the years a number of questions about cyclones and their use have arisen. We reproduce the most commonly asked questions and answers for your edification.
We are serious about developing the correct hydro cyclones for your application requirements. Over the years, our hydrocyclone development has resulted in some small and some rather large cyclones, as well as various parts and modifications. Here are two:
Our largest hydrocyclone is the aptly named 1450 DMS “MAX” cyclone. It has a 1 450 mm diameter. The smallest cyclone we supply is a dinky FC25 classification cyclone with a 25 mm diameter (mini cyclone). The heaviest cyclone component is MAX145-ZL/A-MSA weighing 1 755 kilograms. The lightest cyclone component is a 4HS spigot weighing 14 g. The Max1450 cyclone has an internal volume of around 6.7 m³.
Our cyclone range is very wide to cope with all the varied applications that we could encounter. To make up a single hydrocyclone there are up to 16 major individual parts of varying configurations. The total number of these individual cyclone parts is in the order of 1 500 standard components. The HC range has more than 301 major components and the total number of estimated permutations of a cyclone type configuration (for instance an HC500 classification cyclone) is a mind-blowing 3 936.