E&MJ explains how a holistic approach to the optimization of flotation, encompassing equipment, technology and know-how can deliver circuits fit for the mining operations of tomorrow
By Carly Leonida, European Editor
Froth flotation has been a stalwart in mineral beneficiation for more than 100 years. During this time, and particularly in the past 20-30 years, there have been significant advances in the design and application of mechanical elements for flotation systems, e.g., new mechanisms, launders, larger cell volumes, better pumps, etc. While these have enabled new levels of efficiency, on a standalone basis, they are insufficient to deliver the high recovery rates and grades demanded by future mining operations.
In order to handle the greater throughputs associated with depleting (lower grade) metal deposits, provide maximum efficiency in energy consumption, and keep maintenance requirements low, mines and manufacturers are now looking to a combination of digital technologies, targeted reagent formulas and innovative setups for existing equipment.
It is the sum of these parts that will enable mines to sustain production and keep their operational costs low well into the future.
Talking Trends
Thierry Monredon, global manager for flotation at Metso, spoke to E&MJ about current trends in the design and application of flotation equipment.
“One key trend is to use a mix of mechanical cells and flotation columns with columns as the final cleaner stage, because they deliver increased recovery and higher grades in this role,” he said. Over the past few years, we’ve seen mechanical cells used for rougher and scavenger duties, and generally the first cleaner stage as well. The last cleaning stage is then done using columns.
“We typically see a 1% increase in grade when columns are installed as the last cleaner stage, together with the use of froth cameras.”
Monredon said this trend has been particularly noticeable in base metals globally. “If I take, for example, some projects that we have worked on in India,” he said. “The last project, the mining company wanted mechanical cells only. The next concentrator, if they obtain approvals for the project, will use a mix of mechanical cells and columns.”
And will we see bigger cells any time soon?
“I think we have reached, in terms of sizing, a plateau. 600-m3 class cells have been installed in several places, but that would be definitely be the maximum size used in the future for some time,” he said.
“Quite often its cheaper to have two banks of 300-m3 cells instead of one bank of 600-m3 cells. The cost of equipment is the main reason that people are not interested in larger diameters at this stage.”
Currently, it is operations with low grades and high throughput, for example, copper mines, that have opted for 600-m3 cells.
“Grades have been decreasing, and to maximize efficiency, these operations need to have high throughput,” said Monredon. “In these cases, large diameter flotation cells are compulsory, but again, 600 m3 is really the limit. That’s clear for me, for our company, and I believe that’s what we’ve seen from our competition as well.”
Monredon explained that not only have current cell designs reached their maximum efficiency at 600 m3, but larger diameters would also require a greater footprint and more space within the concentrator; space which is often at a premium.
“When you look at putting a building together and the surroundings that you need around the flotation equipment. When you look at the overall cost, it becomes much more expensive with larger diameter equipment,” he said. “Maybe a few years ago, it [cell volumes more than 600 m3] would have been considered for very large operations like oil sands, but today that market has all but disappeared. The main demand for large flotation units today is from iron ore and copper. Generally only copper will go up to 600 m3.”
New Mechanism From Metso
Metso’s flagship product for flotation, the RCS flotation cell, is an all-purpose flotation machine suitable for applications including roughing, cleaning and scavenging. RCS cells use Metso’s patented Deep Vane (DV) mechanism to float various minerals and are available in volumes from 0.8 m3 to 600 m3. These are complemented by the company’s range of Microcel flotation columns, VisioFroth cameras and OCS-4D optimizing control system.
Monredon said VisioFroth in particular has been key to helping mines increase their recovery and grades. The camera provides online measurement of flotation performance incorporating parameters such as bubble velocity, size and stability to allow concentrator flow control and reagent use to be optimized. The camera can be used as a standalone instrument or combined with advanced process control (APC) methods to optimize set points throughout the concentrator.
“It’s not new,” he explained. “But more and more people are looking to buy this hardware together with the original equipment and not get it separately at a later stage. The aim is to maximize recovery from day one by having this type of additional equipment.
“But apart from the cameras and optimizing systems that come with flotation equipment… Really, it’s all the knowledge around that which is increasing the recovery.”
The Metso team is currently working on a new mechanism for its RCS flotation cells. This is still at the development phase and is yet to be tested. “It will take us about 12-18 months before we release something on this,” Monredon told E&MJ. “It’s something that we’ve been working on for about a year now.
“In the last five years, both our main competitors have been doing this type of upgrade, so you could say maybe we are behind. But we believe it will allow us to be up front, having one of the latest products.”
Metso also has flotation equipment installations coming up at a lead-zinc operation in India in the near future, and Monredon expects that the growing interest in tailings reprocessing from operations across the globe could drive sales in the flotation space in the coming 12-24 months. To make this approach physically possible, many operations will require a separate reprocessing plant along with the relevant beneficiation equipment.
“The equipment required depends on where the valuable minerals are being lost,” he explained. “Is it in the coarse material or in the fines? If it’s in the coarse stream, you will have to do some regrinding, and then flotation again…
“It will take a little bit more time for that to happen on a global scale. We’ve seen it happening in gold, and we expect more copper and iron ore operations to follow suit.”
And what about particle flotation — is that a viable option for mines that are looking to optimize their throughput while driving down operational costs?
Monredon is clear: “It is not a proven technology at this stage,” he said. “We’re not able to generate enough recovery on very coarse particles.
“It could be an option for very specific applications, but on a larger scale, it’s not yet feasible.”
If all goes to plan, Metso’s portfolio will soon be combined with competitor Outotec’s highly regarded range of flotation equipment and services. The deal to merge the two companies is expected to close next year following regulatory approval, and Metso CEO, Pekka Vauramo, named Outotec’s Courier froth analyzers as a standout product when announcing the merger.
FLSmidth Mixes it Up
FLSmidth has been one of the busiest vendors in the flotation space over the past 12 months. The company released two new concepts for flotation in October. The mixedROW Flotation System and a new Froth Recovery Upgrade Package.
mixedROW does what it says on the tin. It combines the benefits of FLSmidth’s nextSTEP forced air and WEMCO self-aspirating technologies in one bank of cells. Flotation cells featuring FLSmidth’s nextSTEP forced air technology are positioned at the beginning of the row where they can recover coarse material using the least amount of energy possible. Cells that use WEMCO self-aspirating technology are then placed at the end of the row to maximize both coarse and fine particle recovery. The elevated rotor position within the WEMCO cells also helps to reduce energy consumption, as the froth only has a short distance to travel.
Thanks to this strategic cell setup, mixedROW is able to lower energy consumption used within the flotation process by 15%-40% while increasing recovery by up to 5%. FLSmidth said that mixedROW also has the lowest head loss on the market, as its system of dart valves allows for efficient transfer of slurry from one tank to another without significant losses.
The dart valves have also proven useful in FLSmidth’s new Recovery Upgrade Package.
Dr. Dariusz Lelinski, global product manager for flotation at FLSmidth, explained how the package came about in a recent interview for the company’s customer magazine.
“It is probably no exaggeration to say that the potential from augmenting froth recovery rates — in terms of what more efficient control of the level, residence time in froth and pulling rates could deliver — was only recognized a few years ago by the industry,” he said.
“This is because it was assumed that there are no losses during transport from slurry to the launder. It was only a few years ago it was measured that the losses are typically 50% and can reach as high as 90% for coarse particles. What it means is that 50% (averaging over all sizes) of particles must be captured again after detachment in froth phase.”
To tackle this, FLSmidth set about increasing the probability of recovery from the froth phase using a combination of instruments and technologies. The result was a Froth Recovery Upgrade Package. This uses slurry level measurement, the redesigned dart valves and new actuators from Festo that were designed specifically for FLSmidth to provide maximum control of the froth phase. This also allows quicker reactions to changes in flow and slurry density.
The package also includes FLSmidth’s new Adjustable Radial Froth Crowders (ARFC). These allow increases in either recovery or grade regardless of the amount of froth formed at the top of the machine. The company said they allow for much higher pulling rates or deeper froth, which is currently hindered by the geometry of the top of most flotation machines.
The traditional way to improve froth recovery is through changes to froth height, crowding and the number of radial launders.
Lelinski explained that these are all still possible with the new package: “The most difficult part is froth recovery at the end of the row. There is not enough of hydrophobic particles to form stable, deep froth, and a large percentage of these particles is left unrecovered. Our package allows not only to recover these particles, but to control required balance between recovery and grade in this part of the flotation circuit. So overall, you get better results, but it also gives you another degree of process control, not only during difficulties of froth formation, but during normal operation allowing more flexibility in selecting grade-recovery relationship,” he said.
The new level sensor, which monitors both the slurry and froth positions with the MultiSense probe, was developed in cooperation with HyControl, and the package also includes an advanced froth camera developed with Stone Three, a market leader in vision equipment.
“We are excited to deliver this complete package to our customers. All the elements working together in combination with radial froth crowders means this package will deliver better recovery at the same grade or increased grade at the same recovery, making this package better than the sum of all parts,” added Lelinski.
FLSmidth’s hard work in flotation has not gone unnoticed. In September, the company was recognized as the market-leading supplier for flotation cells in Chile at an event organized by the Chilean Mining Suppliers Association (APRIMIN). The award was presented by independent consultancy, Phibrand, following a survey of mining companies.
Andrés Costa, president of FLSmidth South America, was
enthused: “We are very proud about this recognition because this is given by our customers. This is a real sign that we are meeting their expectations in the right way.”
FLSmidth also was recognized as one of the top suppliers in the pumps category, which leads us nicely on to the next area for optimization in flotation.
Don’t Forget Froth Pumping
It’s all well and good investing in solutions to increase recovery from flotation, but if allowances are not also made in the design of the accompanying pumping equipment, then problems can occur.
Les Harvey, regional product manager for slurry pumps at Weir Minerals, spoke to E&MJ about the challenges mines are facing.
“Understanding what is happening in the pump and piping system is key to driving innovation,” he said. “The data we gather, and our experience in the field provides us with a great deal of insight into the issues our customers face. Armed with this insight, we can make design modifications and new products to help enhance our customers’ operations and alleviate some of their pain points with froth pumping.
“As we get access to more data, we are able to reliably model pump and slurry interactions and performance. This allows us to continuously improve our understanding of fluid and slurry behavior in various applications, including froth pumping.”
Harvey said that the Weir team have noticed a pattern among customers that are having trouble with their froth pumps.
“By using more flocculants and other chemicals designed to improve mineral recovery, they’re exacerbating existing problems in circuit design and reducing the returns they’re looking for,” he explained.
One of the main challenges in froth pumping is dealing with air in the pump. Air tends to centrifuge in toward the eye of the impeller where it can build up into an air lock. In addition to reducing the pump’s efficiency, this also reduces the flow through the pump and increases the slurry level in the suction hopper. The increased slurry level may push the pocket of air through the pump, causing surging and vibration which can damage the pump bearings, impeller and shaft.
Harvey explained: “The best way to manage air in a froth pump is to invest in a froth pump with a Continuous Air Removal System (CARS), which we have in our Warman AHF, MF and LF pumps.”
The system allows air to move from the impeller eye into a collection chamber at the rear of the pump through a vent hole in the impeller. A flow inducer then removes the air through a vent pipe. It’s also important to position the pump’s discharge pipe at the top of the pump, or at a 45° angle to allow air trapped at the top of the casing to escape.
“We are currently working with several customers reviewing our CARS technology to improve the performance of our Warman pumps in applications with high volumes of air in the froth,” Harvey told E&MJ. “These are difficult applications that require a depth of understanding of slurry behavior in the entire pump and piping system.”
Another common issue occurs when hoppers designed for slurry pumping are used in froth pumping applications. Slurry hoppers require turbulence to prevent the mineral content from settling, but turbulence in a froth pump prevents the air from escaping and can lead to blockages.
Tanks designed specifically for froth pumping promote continuous circular movement, where solids and liquids are sent to the outside of the sump for further transport while air centrifuges into the center where it can be removed. This movement can be encouraged by introducing the slurry from the top of the tank at a tangential angle, and conical designs, rather than those with a flat or rounded floor, further improve the flow of minerals.
Harvey explained that to prevent blockages, the intake pipe, which links the tank to the pump should be large diameter and slope downward toward the pump. This design allows escaped air to separate and travel back up the pipe where it can escape from the sump, rather than building up into blockages.
“The shorter your intake pipe, the harder it is for blockages to build up. However, in addition to a maintenance spool and isolation valve, it’s a good idea to leave enough space for a water injection port, which is useful for flushing out any solids buildup,” he added.
“To make maintenance easier, a dump valve can be included on the suction side of the pump, between the pump and the isolation valve. This will allow you to drain slurry from the pump and discharge pipe system when stopping the pump for maintenance.”
Clever Chemistry
The third piece of the puzzle, one which is often overlooked, are the reagents used to froth and collect the minerals of value.
While the recovery of coarse particles may be an Achilles heel for many flotation equipment manufacturers, the reagents necessary for these applications have been available for some time. E&MJ caught up with the team at reagent specialist Solvay to find out more.
“Coarse particle recovery provides a viable option for lowering comminution costs, achieving higher plant throughput, and improving tailings and water management,” Tarun Bhambhani, mineral processing principal scientist at Solvay, told E&MJ.
“We have long known how to design reagents for coarse particle flotation. However, the physics of traditional flotation cells presented a constraint in the use of these reagents to achieve flotation of these particles. Over the past two decades, flotation cell manufacturers have cleverly overcome some of those physical limitations and have designed cells that can float particles >300 μm efficiently. It has enabled us to utilize these novel chemistries, which are now in production under the AERO CP series and are being evaluated at plant scale.”
Ore complexity is often associated with lower grades, and mines need robust reagents to treat the different minerals and ore types coming through the plant. Bhambhani explained that Solvay’s metallurgists spend considerable time optimizing and re-optimizing customers’ reagent suites using collectors, frothers and modifiers to help manage the complexity.
“In addition, we see problematic species like arsenic, talc, clays and others occurring more frequently,” he said. “These can impact the process, mainly by reducing concentrate grade, and plants may incur financial penalties when some of these are present. The use of selective collectors, frothers and modifiers is critical to improve the separation efficiency of a client’s ore in order to maximize their net smelter return.”
Mineral processing marketing manager, Eammon Guitard, added that Solvay is seeing more requests for customized reagent blends that are robust across a number of ore types.
“Each mineral has different degrees of affinity for the various reagents, and each reagent has different degrees of affinity for the various minerals,” he told E&MJ. “A reagent formulation needs to be sufficiently robust to target all value minerals, while not targeting non-values as best as possible.”
For this reason, operations almost always use multiple collector formulations and carefully selected dosages and addition points.
Another important area of interest is frother formulation; frothers and the froth phase are often neglected but can have a significant impact on metallurgical performance.
“We’re seeing a growing number of requests to investigate frother performance in our customer’s plants,” said Paulo Martins, Solvay’s business development manager for frothers. “We’ve found that laboratory testing is not a good indicator of frother performance at an operational scale, but operations face risks by trialing new frothers in the plant because it’s difficult to identify proper frother composition in real time. This complexity makes it challenging for mines to effectively evaluate products in an operation.
“Because of these challenges, it’s paramount for mining operations to align themselves with reagent providers who deeply understand flotation chemistry, applying holistic approaches that evaluate the role of chemical, physical and operational variables on flotation outcomes.”
Frother selection can be a complex process and, as a result, many operations today are using suboptimal formulas. Solvay is making the process easier by digitizing its FLOTATION MATRIX 100 methodology. This focuses on flotation optimization and includes a logic-based product “select-a-guide” tool to identify the appropriate building-block chemistries.
The company has also developed state-of-the-art dosing equipment that allows for more precise blending and control of frother building-block chemicals, and real-time adjustments based on plant results. In turn, plants benefit from improved operational stability and metallurgical performance.
Collaboration Key to Optimization
Solvay recently worked with a large mining operation in North America to develop a frother called OREPREP F-717. The teams presented their work at the SME 2019 meeting in Denver in February.
“The central idea of our frother projects starts with identifying underlying operational needs. The OREPREP F-717 was tailor-made to allow stable conditions for the rougher operation, with better mobility and, at the same time, improve recovery of coarse particles and solve the sporadic over frothing,” Guitard told E&MJ. “If one applies a simplistic approach, these goals would be antagonistic, but a deeper understanding of the chemistry combined with the plant details enabled us to develop a solution.”
Thanks to the new frother, the teams saw an immediate increase in coarse particle recovery and improvement in circuit control at the mine.
Solvay continues to develop novel reagents and concepts based on specific industry needs and challenges. The team said that one of the biggest requests they are currently seeing from customers is improved solutions for sodium hydrosulfide (NaSH) replacement in Cu-Mo separation.
There are several issues with NaSH, including stench, risk of exposure to highly toxic hydrogen sulfide gas, performance deficiencies when treating difficult ores, and logistical challenges in handling large volumes.
Esau Arinaitwe, mineral processing innovation director at Solvay, explained: “Replacing NaSH-based options with non-toxic, dose-efficient polymers has been a longstanding quest, and our R&D team is evaluating several options.
“Solvay introduced its AERO 7260 HFP depressant technology several years ago as a safer and sustainable alternative to NaSH. It can replace up to 60% of the NaSH used and is now in commercial use globally. This wasn’t enough for our clients, as the goal was to replace 100% of the NaSH used. Our latest innovation is our AERO NR 7360 Series depressants, which aims to replace 70%-100% of the NaSH used in Cu-Mo separation.”
Arinaitwe said these depressants are chemically stable, highly effective sulfide depressants making them safer, practical, and sustainable alternatives for Cu-Mo separation since they substantially reduce and/or completely eliminate the consumption of NaSH. The technology is still in the trialing phase with the goal of commercialization in early 2020.
Other reagents for which non-hazardous, dose-efficient and cost-efficient alternatives are also being developed include NaCN, dichromate and metabisulfite (MBS), and Solvay is also looking to improve operational efficiency with novel reagents. For example, the company is developing reagents for fine particle flotation and the next generation of selective, targeted collectors.
Eriez: Two Steps Ahead
Eriez meanwhile has developed an alternative to large-volume mechanical flotation cells.
Eric Bain Wasmund, Eriez Flotation Division’s global managing director, explained that large mechanical cells are the mainstay for rougher flotation in almost all bulk flotation processes. Increasing the individual cell size and reducing the number creates modest improvements in footprint and operating costs, but it doesn’t improve the basic metallurgy.
“Conventional mechanical cells have an advantage for scaling larger because they have an axis of symmetry, but the disadvantage is that the metallurgical results will always be suboptimal because the step requiring high energy and the step requiring low energy are occurring in the same cell volume,” he said.
Higher mixing energy means better recovery of fine ore but poorer recovery of coarse ore. Eriez has addressed this by developing a two-stage flotation device called the Stack Cell. Two stages mean that there are two separate functional chambers. The first stage introduces high energy, which is optimal for contacting and attachment of bubbles and particles. The second stage has low energy, which is optimal for bubble-particle flotation with minimized drop-back. By keeping the stages separate, high shearing energy can be introduced into the first cell, without passing that energy into the second stage where it would negatively impact recovery.
“Plant trials with the StackCell on a variety of metal sulfide ores, including nickel and copper, have shown that the StackCell can achieve equivalent grade and recovery in about 20% of the time required by conventional mechanical cells,” added Wasmund.
