Articles by Liza Forbes, Kym Runge, Mike Mankosa, Jaisen Kohmuench and Luke Vollert
Liza Forbes, Kym Runge, Mike Mankosa, Jaisen Kohmuench, and Luke Vollert discuss a foam-free flotation technology that can halve the energy use in the grinding step of mining operations
The first step in a mining operation is to grind large pieces of rock into tiny particles, a process called crushing. The energy required for this process accounts for about 40% of the total energy use of mining operations. 1 In terms of base and precious metal production alone, it is estimated that crushing accounts for 0.4% of total global electricity consumption. 2 But what if so? Can the number be reduced by 50%? Imagine how this will affect global fossil fuel consumption, carbon dioxide emissions and climate change. You may not need to imagine for a long time-the technology to achieve this goal is about to emerge.
One of the reasons for energy-intensive crushing in mining is mineral flotation. It is usually the first process to physically separate valuable minerals from waste rock. The flotation product is a low-grade concentrate, which accounts for a small portion (2–5%) of the ore feed. The concentrate is then sent to further processing by smelting, leaching or refining.
Traditional flotation operates in a narrow particle size range (10–200 µm). 3 Medium-scale base metal mining operations usually deliver 2,000 tons/hour of ore to the flotation circuit per hour. A large amount of ore needs to be pulverized to the required flotation feed size, which is the reason for such a high demand for pulverization energy.
The key to drastically reducing energy consumption is to reduce the amount of ore that needs to be crushed, or to produce and process coarser crushed products.
In order to achieve this goal, we need a technology that can perform flotation at a coarser particle size (0.5-1 mm). The processing of coarse-grained ore has long been the focus of the industry, which is facing a decline in ore grade (the metal content of the ore being processed) and an increasingly tighter regulatory environment. A new flotation method that can achieve this goal is Eriez's HydroFloat technology.
HydroFloat technology can be applied to mineral processing circuits in two ways. The first is to install it as a "scavenger" on the final waste stream (called tailings) at the end of the process. This configuration ensures that valuable coarse particles normally lost in tailings are captured and returned to the loop for further processing. Another possibility is to pre-install it before the fine grinding stage to remove larger-sized substances from the circuit. The coarse waste removal at this location has greater benefits, because it can remove 30% of the ore as gangue 4 (no value material) without spending energy to grind it into fine particles. Benefits include energy saving of 30-50% and significant coarsening of flotation tailings. The latter is particularly important because it is possible to use dry piles instead of storing tailings behind earth dams, which has proven to be a major hazard following the tragedy of the tailings dam collapse in Brazil in 2019.
In the past two decades, Eriez has invested a lot of money in research to develop new and efficient flotation technologies for the recovery of coarse particles. HydroFloat was originally developed for industrial minerals such as phosphate, as a direct result of this investment.
In traditional flotation, particles are suspended by an impeller, which is also used to shear the air into small bubbles. The valuable particles are then selectively chemically treated to adhere to the bubbles. These particles collide with bubbles to form low-density aggregates, which rise on top of the cells and form bubbles. The foam overflows the cell lips and is recovered as a concentrate. The system is very effective for particles in the range of 10–200 µm, but starts to fail quickly when the particle size exceeds 200 µm. The reduction in the collection of coarse particles can be attributed to three reasons. First, valuable minerals in large particles tend to be poorly released (low exposure on the surface of the particles). Therefore, large particles usually contain a larger amount of waste minerals. This reduces the surface exposure of valuable minerals. The chemical activation of valuable materials exposed on the surface of the particles causes the particles to stick to the bubbles. As a result, the attachment points of the bubbles are fewer and weaker, so the particles are less likely to adhere to the bubbles.
Second, the high shear force used to suspend coarse particles and generate bubbles in traditional flotation cells is also used to break the weaker particle-bubble bond. This means that far fewer coarse particles enter the foam phase. Finally, the foam phase itself is mainly air, and its density is much lower than that of coarse particle bubble aggregates. Therefore, once coarse particles enter the foam phase, they tend to fall back due to buoyancy limitations.
In 2002, Eriez Manufacturing obtained the world’s first U.S. patent for the air-assisted density separator, which is designed to selectively separate coarse and hydrophobic particles that are too large to be recovered by traditional froth flotation equipment (U.S. patent No. 6,425, 485, July 30, 2002). The technology is now sold under the HydroFloatTM trademark and was initially introduced to the industrial minerals market. Early success has been proven in the phosphate industry for the recovery of coarse, released phosphate particles up to 2-3 mm in diameter. In the next ten years, this technology was successfully applied to lithium, potash, and other coarse-grained, well-released mineral applications. The focus of the work in the past five years has been to develop technology and related auxiliary equipment to successfully recover some of the coarse sulfide particles released from the factory tailings. Many pilots and comprehensive demonstrations have been completed to meet industry concerns about performance, reliability, and maintenance.
It is completely different from the traditional flotation cell. Coarse particles are suspended in a countercurrent fluidized bed instead of using an impeller
It is completely different from the traditional flotation cell. Coarse particles are suspended in a countercurrent fluidized bed instead of using an impeller. Air bubbles are introduced together with fluidized water, creating an environment in which coarse particles can combine with air bubbles without strong shear. The floating bubble particle aggregates then float to the top of the cell. The electrolyzer runs without a foam phase, which means that coarse particles can safely escape from the edge of the electrolyzer and collect as a concentrate.
For base metal sulfide systems, this technological advancement means that 50% of particles as large as 0.85 mm can be recovered, while only 19% of the surface of valuable minerals is exposed. In contrast, in traditional batteries, these particles require 72% surface exposure to get the same chance of recovery. 5
One of the challenges with this technology is that it requires a fractionated flotation feed, because the device is not designed to process particles finer than about 100 µm. Countercurrent flow and no foam phase means that any particles smaller than 100 µm will be carried non-selectively to the edge of the pool. The fine particles must be removed from the feed stream and then discarded or treated separately depending on the application. The need to pre-select the size of the feed material means that HydroFloat cannot simply be used as a replacement for traditional batteries. In order to realize its full potential, this necessary classification step needs to be addressed, which requires a high degree of cooperation between technology suppliers and end users.
Recently, we cooperated with Newcrest Mining to complete the first successful full adoption of this technology in the base metal industry.
Technology and innovation are one of the five pillars of the Forging a Stronger Newcrest program. Newcrest is known for its strong technical capabilities in exploration, deep underground caving and mineral processing.
In 2015, Newcrest conducted preliminary tests at Cadia Valley Operations in New South Wales, Australia, and discovered for the first time the huge potential of using HydroFloat technology for coarse particle recovery. At this time, the technology has been widely used in the industrial mineral industry, but has not yet been established in gold or base metal processing.
Initially, Newcrest focused on the option of integrating HydroFloat into the grinding circuit to remove coarse and barren particles as early as possible (coarse waste rejection)-but later decided to integrate it further downstream in the process as a coarse and valuable particle Scavenger. Existing factory tailings. The removal of traditional flotation tailings streams provides a unique opportunity to demonstrate the technology in a lower risk environment while still providing a strong economic case for enterprises.
In August 2018, less than 3 years after completing the initial testing work, Newcrest commissioned the first full-size HydroFloat pool for the recovery of coarse composite copper and gold particles in the Kadia Valley. As expected, with the adoption of any new technology, the commissioning and start-up time has been extended, and several opportunities for improving the design of the coarse particle flotation circuit have been identified.
Since then, this technology has improved the recovery rate of gold and copper in coarse particles, and has enabled Cadia to increase the grinding efficiency by increasing the roughness of the primary grinding from 80% at 150 µm to 80% at 220 µm. This provides confidence within the organization to continue additional installations, such as the recently announced Cadia Expansion Project, which will expand tailings removal methods to handle more than 70% of on-site tailings streams. The updated design of the expansion circuit combines the experience of the first installation to improve the operability and maintainability.
The use of HydroFloat as a tailings scavenger fundamentally changes the economically optimal grinding size and increases the company's cash flow. But it is recognized that the real reward, especially in new projects, is to reject quality with as coarse a granularity as possible early in the process. In addition to improving project economics, this type of process can also significantly reduce the footprint, power and water requirements of the concentrator, and it is possible to use environmentally friendly tailings storage options, such as dry pile or mixed sedimentation. .
However, there are some challenges that need to be overcome in the design of the waste rejection process. The feed into the electrolytic cell needs to be classified to remove fine particles that may hinder the formation of the fluidized bed, and the required fine particle removal efficiency depends largely on the properties of the ore. The coarse flow of desliming produced by this process is difficult to pump and highly abrasive, causing challenging material handling problems. The overall water balance of the process also needs to be carefully considered, because the demand for fluidized water may be large, resulting in a low-density product stream, which then requires further processing.
In order to solve these challenges and optimize the application of this new technology, a strong foundation of basic research is required. This kind of research is best done using dedicated resources outside of the production environment. Newcrest has established a good relationship with the Sustainable Minerals Institute (SMI) of the University of Queensland and has worked with it to fill the gap.
SMI's mission is to provide research that has a real impact on the industry. Therefore, SMI always talks with its industry partners and asks what kind of research work is best for them. In the past few years, industry partners have all put forward the same request: to study how to best implement coarse particle flotation in their factories.
Obviously, to do this, work needs to be done collaboratively. For SMI, industry cooperation means not only financial support, but also the use of the extensive technical expertise and practical experience of a diversified group of mining and engineering companies. SMI researchers only like to participate in one interesting question. Guidance and guidance from the industry is essential to ensure that the right problems are solved.
In October 2020, the Coarse Particle Processing Research (CPR) alliance was established. The founding partners are Eriez Flotation Division, Newcrest Mining and Anglo American. Aeris Resources, Glencore, Hudbay Minerals and Newmont also joined the team. CPR researchers and PhD students are working on a wide range of topics that are considered essential to HydroFloat implementation. This includes studying ways to improve performance and developing small-scale tests and methods to predict how it will perform under different conditions.
They also put a lot of effort into determining how to best use the technology to remove coarse-grained waste. Although the HydroFloat unit is a key component, successful implementation requires the development of an economically viable mineral processing process capable of producing feedstock with appropriate characteristics.
In addition, the process must be able to handle the large amount of water required by the battery. It may be necessary to change the way of thinking. Part of the work involves evaluating a series of less traditional crushing and particle size separation equipment to test how they complement coarse particle flotation. For pulverization, this includes research on dry processing technologies such as vertical roller mills and vertical shaft impactors. For size separation, this includes looking at three-product cyclones and inverted or semi-inverted cyclones.
Many questions remain, including whether the technology is suitable for handling a wide variety of ore bodies? The CPR consortium mainly focuses on copper and gold businesses, but even within these areas, the nature of the ore is very different.
The alliance will last until 2025. All these exciting challenges are ahead, and new challenges will definitely be discovered.
1. Ballantyne, GR, Powell, MS, Tiang, M, 2012, "The proportion of energy attributable to crushing", Proc. 11th Mill Ops Conf, Hobart, pp. 25-30 (AusIMM). 2. Napier-Munn, T, "Is the advancement of energy-saving crushing doomed to fail?" Mineral Engineering, 2015, 73, 1-6. 3. Trahar, WJ, "A reasonable explanation of the role of particle size in flotation", Int J Miner Process, 1981, 8, 289–327. 4. Regino, R et al., 2020, "Comparison of Two Circuit Applications for Coarse Particle Flotation", COM 2020, MetSoc, online conference. 5. Miller, JD, et al., 2016, "The importance of exposing particle surface area when using HydroFloat technology for coarse particle flotation of low-grade gold ore", Proceedings of XXVIII International Mineral Processing Conference, Canadian Institute of Mining, Metallurgy and Petroleum , Quebec, Canada.
SMI Senior Researcher, University of Queensland
Leader of SMI Separation Group, University of Queensland
Executive Vice President of Global Technology, Eriez Flotation Division
International Vice President of Eriez Flotation Division
Newcrest Mining Research and Technology Senior Metallurgical Engineer
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