Reducing the Daily Grind

Liza Forbes, Kym Runge, Mike Mankosa, Jaisen Kohmuench and Luke Vollert discuss a froth-free flotation technology that can halve the energy use in the grinding step of mining operations

ONE of the first steps in mining operations is to grind large rocks into tiny particles, a process called comminution. The energy required for this process accounts for approximately 40% of the total energy used by a mining operation.¹ For base and precious metal production alone, it is estimated that comminution accounts for 0.4% of total global electricity use.² But what if that figure could be reduced by 50%? Imagine the impact that would have on global fossil fuel consumption, on CO₂ emissions, and on climate change. You may not need to imagine it for long –a technology to achieve this is on the horizon.

Energy and mining

One of the reasons for energy-intensive comminution in mining is mineral flotation. It is typically the first process that physically separates valuable minerals from waste rock. The flotation product is a low-grade concentrate that represents a small fraction (2–5%) of the ore feed. This concentrate is then sent for further processing by smelting, leaching or refining.

Conventional flotation operates within a narrow particle size range (10–200 µm).³ A mid-size base metal mining operation will typically send 2,000 t/h of ore to the flotation circuit every hour. That enormous quantity of ore all needs to be ground down to the required flotation feed size, which is what accounts for such high comminution energy demand.

The key to significantly reducing energy consumption lies in reducing the amount of ore that needs to be comminuted, or to produce and treat a coarser comminution product.

To achieve that, we need a technology that can conduct flotation at much coarser sizes (0.5–1 mm). The processing of ores at coarser particle sizes has long been a focal point for the industry, which is facing declining head grades (the metal content of the ore being processed) and a tightening regulatory environment. A new flotation method that can achieve this aim is Eriez’ HydroFloat technology.

There are two ways by which the HydroFloat technology can be implemented into a mineral processing circuit. The first is to install it as a “scavenger” at the end of the process on the final waste stream (referred to as tailings). This configuration ensures that the valuable coarse particles typically lost to tailings are caught and returned to the circuit for further processing. The other possibility is to install it upfront, prior to the fine grinding stage, to reject mass from the circuit at a coarser size. Coarse waste rejection at this location has far greater benefits as it allows 30% of all ore to be rejected as gangue⁴ (valueless material) without expending energy to grind it to a fine size. The benefits include an energy saving of 30–50% and a significant coarsening of flotation tailings. The latter is particularly important as it has the potential to enable the use of dry stacking, rather than storing tailings behind earthen dams, which has been shown to be a significant hazard following the tragic tailings dam collapse in Brazil in 2019.

Technology development

Over the past two decades, Eriez has significantly invested in research to develop efficient new flotation techniques for recovery of coarse particles. The HydroFloat, originally developed for industrial minerals such as phosphate, is a direct result of this investment.

In conventional flotation, particles are suspended by means of an impeller which is also used to shear air into small bubbles. Valuable particles are then selectively chemically treated to adhere to air bubbles. These particles collide with the bubbles to form low-density aggregates which rise and form a froth on top of the cell. The froth overflows the cell lip and is recovered as concentrate. This system works exceedingly well for particles ranging from 10–200 µm, but starts to fail quickly when particle size exceeds around 200 µm. The reduction in coarse particle collection is attributed to three reasons. First, the valuable minerals within large particles tend to be poorly liberated (lower exposure at the particle surface). As a result, a large particle typically contains a much higher quantity of waste mineral. This reduces the surface exposure of the valuable mineral. Chemical activation of the valuable material exposed at the particle surface is what allows the particle to stick to a bubble. As a result, there are fewer and weaker attachment sites for air bubbles and, consequently, a lower probability that the particle will adhere to a bubble.

Secondly, the high shear used in a conventional flotation cell to suspend the coarse particles and produce air bubbles, also acts to break up the weaker particle-bubble bonds. This means that far fewer coarse particles make it into the froth phase. Lastly, the froth phase itself is predominantly air and has a much lower density than that of the coarse particle-bubble aggregates. As a result, once the coarse particles enter the froth phase, they tend to fall back, due to buoyancy restrictions. 

In 2002, Eriez Manufacturing was awarded a US patent for the world’s first air-assisted density separator specifically designed for the selective separation of coarse, hydrophobic particles that are too large to be recovered by traditional froth flotation equipment (US Patent No 6,425, 485, 30 July 2002). This technology, now marketed under the tradename HydroFloat^TM, was initially introduced to the industrial minerals market. Early success was demonstrated in the phosphate industry for recovery of coarse, liberated phosphate particles up to 2-3 mm in diameter. Over the next decade the technology was successfully introduced to lithium, potash and other coarse, well-liberated mineral applications. Work over the past five years has been focussed on developing the technology and the associated ancillary equipment to successfully recover partially-liberated, coarse sulfide particles lost to plant tailings. Numerous pilot and full-scale demonstrations have been completed to satisfy industry concerns regarding performance, reliability, and maintenance.

It is a radical departure from conventional flotation cells. Rather than using an impeller, the coarse particles are suspended within a counter-current fluidised bed

It is a radical departure from conventional flotation cells. Rather than using an impeller, the coarse particles are suspended within a counter-current fluidised bed. Air bubbles are introduced along with the fluidisation water, creating an environment where coarse particles can bond with air bubbles in the absence of strong shear forces. Buoyant bubble-particle aggregates then float to the top of the cell. The cell operates without a froth phase, which means that coarse particles can safely overflow the cell lip and be collected as a concentrate.

For base metal sulfide systems, this technology advancement means that 50% of particles as large as 0.85 mm can be recovered with only a 19% surface exposure of the valuable mineral. By comparison, in conventional cells, these particles require a 72% surface exposure to have the same chance of recovery.⁵

One challenge with adoption of this technology is that it requires a classified flotation feed, as the unit is not designed to process particles finer than about 100 µm. The counter-current flow and absence of the froth phase mean that any particles smaller than 100 µm will be unselectively carried over the lip of the cell. The fine particles must be removed from the feed stream and either discarded or processed separately depending on the application. The need for pre-sizing of the feed means that HydroFloat cannot simply be used as a replacement for conventional cells. To achieve its full potential, this necessary classification step needs to be addressed, which requires a high level of co-operation between the technology supplier and the end-user.

The first successful full-scale adoption of the technology in the base metal industry has recently been completed in partnership with Newcrest Mining.

Technology pioneering

Technology and Innovation is one of the five pillars of the Forging a Stronger Newcrest plan. Newcrest is known for its strong technical capabilities in exploration, deep underground block caving, and minerals processing.

Newcrest first identified the huge potential for coarse particle recovery using the HydroFloat technology in 2015 following a preliminary test work campaign at Cadia Valley Operations in New South Wales, Australia. At this time, the technology had been widely applied at full scale in the industrial minerals industry but was not yet established in either gold or base metals processing.

Initially, Newcrest looked at options for integrating HydroFloat into the milling circuit to reject coarse, barren particles as early as possible (coarse waste rejection) – but later settled on integrating it further downstream in the flowsheet as a scavenger for coarse, valuable particles in the existing plant tailings. Scavenging of the conventional flotation tailings stream presented a unique opportunity to demonstrate the technology in a lower-risk environment whilst still delivering a robust economic case for the business.

In August 2018, less than 3 years after completing the initial testwork, Newcrest commissioned the first full-scale HydroFloat cells for the recovery of coarse composite copper and gold particles at Cadia Valley. As expected with the adoption of any new technology, an extended period of commissioning and ramp-up ensued, and several opportunities to improve the coarse particle flotation circuit design were identified.

The technology has since increased recovery of gold and copper in coarse particles and allowed Cadia to improve grinding efficiency by lifting the coarseness of the primary grind from 80% passing 150 µm to 80% passing 220 µm. This has provided confidence within the organisation to proceed with additional installations like the recently-announced Cadia Expansion Project which will extend the tailings scavenging approach to treat more than 70% of the total site tailings stream. The updated design for the expansion circuit incorporates learnings from the first installation to improve operability and maintainability.

Applying HydroFloat as a tailings scavenger has fundamentally shifted the economic optimum grind size and increased cash flow for the business. But it is recognised that the real prize, particularly in greenfield projects, is rejection of mass early in the flowsheet at the coarsest possible particle size. In addition to improved project economics, this style of flowsheet can also deliver a significant reduction in concentrator footprint, power and water demand, as well as potentially enable the use of environmentally preferential tailings storage options like dry stacking or co-mingled deposition.

There are, however, several challenges to overcome in the design of a waste rejection flowsheet. Feed to the cell requires classification to remove fine particles which can hinder the formation of a fluidised bed, and the required efficiency of fines removal is largely dictated by the ore properties. This de-slimed, coarse stream generated by the process is both difficult to pump and highly abrasive, creating a challenging materials-handling problem. The overall water balance for the flowsheet also needs careful consideration, as the fluidisation water requirement can be significant, generating a low density product stream that then requires further treatment.

To solve these challenges and optimise the application of this new technology, a strong foundation of fundamental research is necessary. This kind is research is best done with dedicated resources outside of the production environment. Newcrest has a well-established relationship with the Sustainable Minerals Institute (SMI), based at the University of Queensland, which it has partnered with to fill in the gaps.

Bringing concept to reality

The mission at SMI is to deliver research with real impact for industry. Therefore SMI always talks to its industry partners, asking what kind of research work will benefit them the most. In the last few years, industry partners were all requesting the same thing: research on how to best implement coarse particle flotation in their plants.

It became clear that to do this right, the work needed to be done collaboratively. For SMI, industry collaboration means much more than financial support, it means drawing on the extensive technical expertise and practical experience of a diverse group of mining and engineering companies. SMI’s researchers love nothing more than to get involved with an interesting problem. Direction and guidance from industry is vital to ensure the right problems are being solved.

In October 2020, the Collaborative Consortium for Coarse Particle Processing Research (CPR) was established. Founding partners were Eriez Flotation Division, Newcrest Mining and Anglo American, with Aeris Resources, Glencore, Hudbay Minerals and Newmont also joining the team. The CPR researchers and PhD students work on a wide range of topics that are deemed essential for HydroFloat implementation. This includes researching ways to improve performance as well as developing small-scale tests and methodologies to predict how it will perform in different situations.

They are also directing considerable effort into determining how best to use the technology for coarse waste rejection. While the HydroFloat cell is a crucial component, successful implementation requires the development of an economically viable mineral processing flowsheet capable of producing a feed with the appropriate characteristics.

Additionally, the flowsheet must also be able to cope with the large amount of water that the cell requires. A change in mindset is likely required, and part of the work involves evaluating a range of less conventional comminution and size separation devices to test how they can complement coarse particle flotation. For comminution, this includes looking at dry processing technologies such as vertical roller mills and vertical shaft impactors. For size separation, this includes looking at the three-product cyclone and inverted or semi-inverted cyclones.

Many questions remain, including how well suited is the technology to treating widely different ore bodies? The CPR consortium is largely focussed on copper and gold operations, but even within these bounds, the nature of the ores is extremely varied.

The consortium will run until 2025. All these exciting challenges lie ahead, and new ones are sure to be discovered.

References

1. Ballantyne, GR, Powell, MS, Tiang, M, 2012, “Proportion of Energy Attributable to Comminution”, Proc. 11th Mill Ops Conf, Hobart, pp25–30 (AusIMM).
2. Napier-Munn, T, “Is Progress in Energy-efficient Comminution Doomed?” Minerals Engineering, 2015, 73, 1–6.
3. Trahar, WJ, “A Rational Interpretation of the Role of Particle Size in Flotation”, Int J Miner Process, 1981, 8, 289–327.
4. Regino, R et al, 2020, “A Comparison of Two Circuit Applications for Implementation of Coarse Particle Flotation”, COM 2020, MetSoc, Online Conference.
5. Miller, JD et al, 2016, “Significance of Exposed Grain Surface Area in Coarse Particle Flotation of Low-Grade Gold Ore with the HydroFloat Technology”, XXVIII International Mineral Processing Congress Proceedings, Canadian Institute of Mining, Metallurgy and Petroleum, Quebec, Canada.