Multotecs cyclone circuitry

Credit: International Mining

Then you have de-sliming applications for cyclones with the aim of removing slimes before the next process, such as before flotation, where in some cases, slimes can act as an unwanted flotation reagent consumer, which does not add value to the recovery process and can also lead to increased final product moisture contents.

Slimes can also be detrimental to gravity applications like spirals where slimes can hamper the separation process. One also has dewatering applications to maximise water recovery from solids - and cyclones are often used where other dewatering equipment is not practical or involves too much investment. Cyclones are also used in underground mines for backfilling applications where you have certain requirements for the mass split or underflow size distribution when can have a big influence on the backfill drainage rates once it is delivered underground. Finally, there are tailings dam applications - here again you need to ensure the correct mass split and the correct size distribution to build the outer wall with the correct densities throughout the wall.

So, which is the highest value application? “It depends on what is important for the customer. In some cases, in South America, tailings dam cyclones arguably add a greater overall value to the project than the milling cyclones. But clearly in high tonnage, particularly copper applications, milling cyclones represent a high return on investment. You are looking to minimise milling energy and cyclones do play an important role. You also have pumping requirements in the mill circuit and here cyclones can play a role in reducing cost.”

Bekker also talked about availability and the materials in terms of the construction of cyclones in the processing plant from a maintenance perspective regarding life expectancy and availability. If a unit fails unexpectedly and you don’t have spare capacity it can have a major negative effect. One needs to look at how many standby units you have in the application and ensure that the design has been thought through carefully, balancing CAPEX against availability.

He adds: “There are also trade-offs in cyclone use. If you have an area of the plant where you want to minimise water usage, cyclones are important, however, you want to make sure you have the maximum efficiency. Cyclones are most efficient at lower solid concentrations but to reduce water usage, plants try to use the highest solids concentration allowable. One needs to balance how much water you are really saving against what is happening as a result of these decisions on the efficiency of the process. If there is no other equipment other than the cyclones that can recover water, then the cyclone and its operation becomes very important.”

Back to tailings dam applications, and again, you need to make sure the cyclones are performing as efficiently as possible. “One of the drawbacks of tailings dam cyclones is that they are located remotely from the main mine and plant operation and I would say they don’t always get the attention in terms of monitoring and maintenance they deserve. Here the materials of cyclone construction can make a difference, plus you want to ensure that the dimensions are optimal, as that can affect performance - for example the spigot diameters. If the spigot starts to wear, the opening becomes bigger and progressively discharges more unwanted water and fines to the outside of the tailings dam. Those monitoring the cyclones may still assume the units are working as material is still coming out of the overflow and underflow but the performance has dropped. Using better materials can ensure this dimensional integrity is kept. On the flip side, using high wear resistant materials make the cyclones heavier so any manual maintenance becomes more challenging.”

Returning to backfill cyclones, there is a certain requirement for PSD and if you don’t maintain that then you can have problems underground. “The greater value depends on what is important for the client but in big operations, yes, the milling circuit cyclones add the most value but as outlined, in terms of water recovery, tailings dam quality, backfill performance and slimes removal, they are also vital pieces of equipment.”

An in-depth process understanding

Multotec supplies equipment for all of these applications, and therefore places great importance on having an in-depth understanding of the limitations of the equipment but also on its experience. “Often when a plant is encountering abnormal conditions, and the client is looking to improve performance through making changes, you need to able to tell them when it can be done, and when it cannot be done and also have the knowledge to explain why. As an example, sometimes clients want to use dewatering cyclones on a thickener underflow and increase the density even further. This is just not possible - the retention time of material in a cyclone is only a couple of seconds. There is also going to be some water present in the underflow because of the surface area effect. You would be trying to put an extremely dense slurry stream through the cyclone and expect it to perform well which will not happen. You would actually need to add water back in to get it to work to then produce a high-density underflow. But that underflow density is still going to be lower than what the thickener can produce if the thickener is operated correctly. The thickener has a longer retention time along with compression forces unlike the cyclone which only has centrifugal force. So, the cyclone is the lowest cost option but gives the highest water content, followed by thickeners and followed again by filter presses, all at increasing cost. Higher forces produce a drier product.”

Do cyclones then have a role to play in circuits with coarse particle flotation? "The answer is both yes and no. When you look at coarser grinds and CPF, we are really looking at the upper end of cyclone capabilities, and ultra-fine screening starts to come into the equation. But of course, this means higher CAPEX and OPEX and often plants will revert back to cyclones. It is possible to modify the cyclone and conditions to increase the cut point which will then produce a coarser overflow but you still need to keep in mind that the hydrocyclone has inherent inefficiencies with some fines going to the underflow and more coarse to the overflow, though in this case, the latter is not a negative. As it is a coarser grind being produced by the mill, the cut point has to increase to compensate for that and the chances of using exactly the same units as before are limited. You will have to make alterations to the cyclone configuration such as cone angles or even alternative diameters. Therefore, the cyclone can be used in coarser grinds and CPF, but the cyclone will operate at the upper end of its limits. There is also a big drive to increase throughputs in processing circuits today, but the mill designs are reaching their limits due to the installed power limits. A common question we get these days from clients that have increased their throughput in a milling circuit is what needs to change on the cyclone to get a finer product or grind. And this is where the problem lies - when you increase tonnages, you start to get a coarser grind and you become energy limited and the cyclone cannot compensate for this. This results in a coarser product being fed to the flotation circuit resulting in recovery inefficiencies.

Has the increased focus on tailings dams and production of more thickened tailings increased the focus on cyclones in this market? “People have seen the real impact of tailings dams not being operated correctly. There is definitely a resultant greater focus on the operation of hydrocyclones; and looking at them from a different perspective. Over the years, tailings dam cyclones were sometimes locally made, with lower quality, but still performed adequately. But with the greater focus on tailings dams, we are trying hard to explain to mines the importance of maintaining them correctly which has often not been done as they are out of sight away from the main plant. Plus, the expertise is not always there to evaluate their performance properly, which can help operators know what to look out for. Again, it has to be understood that cyclones have limitations on how much water they can remove. For the thickened tailings projects, however, cyclones are still being deployed but for CAPEX reasons in conjunction with other equipment such as filter presses, belt filters, disc filters or even centrifuges.”

Differentiators for Multotec

Looking at the global cyclones market, what are Multotec’s differentiators versus the other big players? “Some of our competitors can offer a very wide, and in some cases, a full plant product suite, including hydrocyclones. While Multotec does not have the same product breadth, we do have a wide range of solutions that we can incorporate. Because our product lines are focussed, we have an in-depth understanding of these products and their performance to a high level of detail, including pumps, mill liners, screening media, trommels and some others. When mines face specific challenges relating to cyclones, we have specialised knowledge. We know our products inside out and that translates to the quality of assistance we can provide. Being focussed means we also offer flexibility in design, including more specialised and bespoke designs (such as when customers want to have different cyclone diameters in the same cyclone distributor). We also offer flat bottomed cyclones which cut very coarse and have some special applications for which we supply.

He adds: “We also have a very wide range of construction materials for cyclones that we can utilise to help solve client wear related problems. Even if there is a wear problem upstream from the cyclone, we can assist the client to optimise the process with our own manufactured rubber and ceramics products. We like to look at cyclones as part of a process, not just a piece of equipment. From a mechanical point of view, you can of course make adjustments but when there are cyclone performance issues, often it is related to something else going on in the process. We have a lot of insight from many years of experience to help them identify the source of the problem through elimination. That understanding of processes and close interaction with the client is a differentiator for us. A good example was on a milling circuit where the leaching tanks got sanded up and the cyclones were blamed. Ultimately the problem was in a flotation section before the mill circuit where there was a problem with the control system for the dart valves. I like to say we never walk away from a problem.”

Finally, he emphasised both the quality of Multotec’s own people and the training that it offers to clients. “When you know a product so well, you are able to provide presentations at a very detailed level - not only sales focussed, but giving an in-depth understanding alongside taking complex processes and explaining them in a straightforward way. This is across operations, maintenance and metallurgical staff. We also train engineering houses and offer them ongoing guidance.”

Customisation for particular markets

What about specialised solutions for certain application types, especially in southern Africa? “One of them is when you have open circuit milling where you need to have a dewatering cyclone configuration of what are called separator or stacker cyclones. It dewaters feed from a flotation circuit to the right density before it goes to the mill. Another example would be a guard cyclone, which goes before the thickener to take some load off the thickener and prevent coarse material entering which could create a blockage. In the platinum and UG2 circuits in South Africa and Zimbabwe, you have a chrome-silica split where you separate the PGM bearing material and the chrome rich material. This is one area where our flat bottom cyclones are used. Then cyclones in ultra-fine grinding circuits using smaller diameter milling cyclones.”

Finally, Australia has also become an important market for Multotec cyclones. “We first made a big impact in Australia with cyclones on the east coast in coal specifically for dense media separation. Then we expanded to the west coast to hard rock mines. Our offices there have service contracts with individual operations at a level where we manage and maintain their cyclones for them including maintaining performance levels. These service contracts in some cases also extend to equipment from other suppliers. The growth of the lithium industry there is also opening up new business for us.”