Credit: MechChem Africa
Ernst Bekker, Product Specialist – Cyclones, for the Multotec Group, talks to MechChem Africa about hydrocyclones and dense medium cyclones: how they work, their different applications and some of things to look at and to avoid for best possible separation, production and recovery efficiencies.
“Cyclones are divided into two main categories: hydrocyclones and dense medium cyclones, which are fundamentally different in terms of the principles that apply, and the mineral separation processes they can be used for,” begins Multotec’s Ernst Bekker.
Hydrocyclones use water as the transport medium. The solids that require separation are mixed with water to form a slurry, before being pumped into the cyclone near the top and at a tangent to initiate spiralling flow. A strong vortex forms at the centre of the cyclone, with an air core through its centre passing from the spigot at the bottom to the vortex finder at the top.
Ernst Bekker, Product Specialist – Cyclones, Multotec
“Centrifugal forces from the spiralling flow tend to throw particles to the outside, while drag forces from the water being pulled towards the air core at the vortex pull particles in the opposite direction. So a balance of the forces emerges,” says Bekker. “The coarser particles tend to be thrown to the stream spiralling downwards on the outside, while the finer particles tend to migrate into water surrounding the air core, which is being forced upwards by the vortex. The water closest to the air core takes the finer particles up and out of the overflow at the top, while the coarse particles continue to flow down the outside of the spiral, passing through the spigot at the bottom as underflow,” he explains.
It is this balance of the forces that dictates the cut size of a hydrocyclone. “It is important to remember that a hydrocyclone predominantly separates the mineral particles in the slurry based on size, with larger coarse particles reporting to the underflow while smaller or finer particles are taken up and through the overflow,” Bekker informs MechChem Africa.
Ideally, he says that Multotec recommends a ratio of solids to water of about 20% solids by volume in 80% water. “Sometimes people try to push more solids into the cyclone to raise the throughput of the plant, or tonnes/m2 of processing footprint. But this will tend to inhibit the performance of the hydrocyclone, so we never like to exceed 30% solids by volume,” he advises, adding that higher solid volumes tend to result in lower separation efficiency and poorer recoveries and/or increased product circulation.
“When multi-density particles enter the mix, then a hydrocyclone, in essence, separates based on mass. With homogeneous ores, mass and size are directly related, because bigger particles always have bigger mass, so we can still use the term cut-size to describe the separation process. But when the densities are different, this is not the case. In a multi-density classification application, a larger fraction of the high-density material will report to the underflow compared to a mineral that has a lower particle density. This is a challenge for hydrocyclones in a multi-density environment,” he says.
Bekker cites an example in the PGM industry. “The UG2 ore body consists of silicates, which are platinum bearing and are generally lower density mineral ores. But chrome, which has a significantly higher density, is also present.
“If the hydrocyclone is being used in a closed-loop milling application, the chrome should leave the cyclone at the same particle size as the silicates, but due to its higher density, the chrome keeps reporting to the underflow as oversized material and is sent back to the mill for further grinding. This leads to overgrinding of the chrome ore, which comes out so fine that it contaminates the platinum concentration process downstream,” he explains.
“In this case, the use of an ultra-fine screen might be introduced, but this is expensive, so a lot of operations decide to continue to use hydrocyclones, while understanding its limitations,” Bekker says.
“Hydrocyclones are quite versatile. In some cases, we can even use them for dewatering instead of using dewatering screens, which are sometimes expensive, and they have a larger footprint,” he says. Similarly, desliming is also common, where 98% of the solids are taken out at the spigot, with relatively clean water being taken from the overflow.
“In applications where the quality of the clean water produced is less important, hydrocyclones can also be used for mine water processing in place of thickeners. And there is also now a focus on hydrocyclones being used for tailings dam management, depositing sand on the dam walls while recovering as much water as possible for reuse back in the process,” says Bekker.
On chrome mines, hydrocyclones called stacker cyclones sit on booms, discharging the product underflow into a heap. After leaving the material to dry further for a couple of days, this product will be taken away by trucks for further processing.
While in the minerals sands industry, where beaches are often mined for heavy minerals such as zircon and rutile, hydrocyclones are used to separate ultra-fine material before spirals and to dewater the product. The sand containing no valuable material anymore is returned onto the beach. Compared to using a screen for separation, a hydrocyclone is far easier to move along the beach as mining progresses, the sand being redeposited behind the operation.
A most common application, is for hydrocyclone clusters to be used as part of the mill circuit to classify right-sized material and to send the oversized fraction back to the mill for further grinding. Here, the cut size needs to be optimised to best match the downstream recovery process. “It is important for operators to remember that there is a limited amount of grinding energy from a mill, so raising throughput should be undertaken with care. Unless mill capacity can be increased in some way, any increase in throughput will result in more oversized material returning to the mill, which can cause the spigot to choke-up, a condition known as roping in the cyclone fraternity,” he warns.
“To get a finer product for processing at a higher production rate, the milling energy must also be raised and the cyclone re-optimised to match the new operating point,” suggests Ernst Bekker.
Instead of using pure water as the slurry medium, dense medium separation uses a combination of water mixed with very fine particles of either magnetite or ferro-silicon. This creates a higher density separation medium than water, so that when the ore is added, the slurry is better able to separate based on the different densities of the particles in the mix.
“If you took a truck load of ore and dumped it into a pool of water, everything would sink to the bottom. But if you replaced the water with a magnetite- or ferro-silicon-based dense medium with an elevated density relative to water of, say, 1.6 then lower density ores will float and any of the particles that do sink will have a density of above 1.6,” he says.
“With a dense medium cyclone, low density materials, called floats, can't break into the medium and so they remain in the centre of the cyclone and are drawn to the cyclone’s vortex and leave through the overflow. Dense minerals sink inside the spiralling dense medium flow and are propelled towards the outside of the cyclone. They leave through the underflow. So, separation is largely based on low density and high-density minerals, irrespective of particles size.
“To control the cut density on a DMC (Dense Medium Cyclone), we have to adjust the density of the media being used, based on the densities of the ores being separated. The density of the medium governs separation and there is very little we can change in the cyclone itself to improve separation performance,” Bekker points out.
Reverting back to hydrocyclones, he says that the diameter of the vortex finder is the principal cut-point adjustment, with a smaller vortex finder diameter providing a finer cut point and vice versa. Feed conditions such as pressure or flowrate and feed solids concentration can also be used to manipulate the cut size of items reporting to the overflow and underflow of a hydrocyclone. “This is not the case for dense medium cyclones, however. The operating pressure or head remain constant in dense medium separation, and the separation efficiency of the unit cannot be changed by adjusting the size of the vortex finder or the spigot,” he adds.
The supplier designs the DMC based on the ore body densities and the separation requirements, selecting a dense medium that delivers the low-density material to the overflow and the high densities to the underflow. “Unless there are significant changes to tonnages or the densiometric profile of the ore body, there should not be any need to change the DMC units themselves. Any adjustments should be made externally, to the dense medium and the feed parameters, for example,” Bekker suggests.
“South Africa’s coal quality used to be very good. When using a dense medium cyclone in the early years, this resulted in easy separation, typically with 70% being coal at the overflow and only 30% being rejected as waste rock to the underflow. But we are now dealing with much lower grade coal ore, so the ratio is more or less the other way around, 70% being rejected as waste material through the spigot and only 30% being separated out as coal at the vortex finder in some cases,” Bekker notes.
“This causes capacity issues at the spigot in handling the larger volumes of waste materials and a decision has to be made as to whether to make the spigot bigger or to replace the dense medium cyclone with a bigger one,” he adds.
He says that certain design ratios apply between the cyclone diameter, the vortex finder, the spigot and the inlet diameter. If the cyclone diameter is D, for example, then the vortex finder is normally 0.43×D and the spigot can be anything between 50% and 70% of the vortex finder diameter. The standard inlet opening of a dense medium cyclone is normally 20% of the cyclone diameter.
“If the inlet opening is made bigger, more volume can be introduced into the unit, but this reduces the residence time inside the cyclone, which results in more misplacement of waste in the overflow and/or coal in the underflow, particularly of the case where the near density materials (NDM) is high. Near density material is defined as the amount of material present around the expected cut density in a band of +/- 0.1 RD units e.g. If the cut density is RD1.6, then the material present in the density range of RD1.5 to RD1.7 would be classified as near density material.
If the percentage of NDMs is very high, let’s say above 65%, then separation is difficult. Easy separation has anything between 20 to 25% NDMs and for medium density cyclones treating very good quality coal, NDM percentages can be as little as 2% in some of Multotec’s overseas operations.
“But here in South Africa, we use a lot of low-grade coal and it is quite difficult to distinguish clearly between the good coal and the waste. There are operations that can have up to 90% of the ore coming into the dense medium cyclone that is classified as near density material, making separation very difficult,” Bekker tells MechChem Africa, adding that cyclones for these applications need to be sized correctly, with very accurate feed conditions.
Alongside coal, Bekker says that dense medium cyclones are widely used as part of diamond pre-processing from run-of-mine ore prior to sorting. “Here only 1 to 2% of the inflow is diamond bearing and this passes through the spigot. The rest all goes out through the overflow. And if the underflow ratio goes up to just 4 or 5%, it becomes a problem because of the very accurate diamond sorting processes that must take place downstream of the cyclone, which can easily become overloaded,” says Bekker.
In a similar way, he adds that dense medium cyclones are now being looked at to pre-concentrate iron ore; manganese; and for some PGM and copper applications. “The idea is to remove waste rock before sending the product to the grinding mill, so that all the milling energy is focused only on the valuable target material, which is an interesting cost saving and productivity increasing concept,” he adds.
“While a small percentage of the valuable material is likely to be lost to the DMC, if you can recover, 90 to 95% of the valuables from 30% of the total ore mass, then the savings and productivity improvements from reduced milling can far outweigh this loss,” he points out.
“At Multotec we are process specialists who offer a variety of different minerals processing solutions and combinations. We spend time with the clients, building trust and sharing knowledge, regardless of any associated order or monetary value. And because we are an OEM supplier of several different technologies, we are able to steer our clients towards the right combination of technologies, whether those involve hydrocyclones, dense medium cyclones or combinations of several technologies.
“Every system we design and build is customised for specific client needs to be fit-for-purpose – and we take responsibility for ensuring this is the case,” Ernst Bekker concludes.