There’s more to preconcentration, or progressive waste rejection, than meets the eye
It’s no secret that ore grades across key commodities are declining. For decades, mines have sought to counteract lower grades by extracting and processing greater volumes of material and, for many years, this approach worked well.
But the industry is now facing unprecedented pressures. For many operations, resources like energy and water are at an increased premium, as is space. Mining and processing costs are rising. New projects and extensions are getting harder to permit. Metallurgy is becoming more complex, many tailings storage facilities are full and social opposition to the creation of new ones is at an all-time high.
The “bigger is better” mentality is no longer serving us. In fact, in many cases, it’s hindering the industry’s evolution into an environmentally, socially and governance (ESG)-conscious provider of critical minerals and metals.
That’s the tough line. But the good news is that the technologies, data and thinking required to overcome these challenges are well within our reach. There is no silver bullet technology that can be applied ubiquitously to all mining projects with success but, as concepts go, preconcentration or progressive waste rejection (a term that more accurately reflects its capability and potential today) has been demonstrated, in some cases for decades. It’s a key strategy that many mines, both new and operational, can implement to meet and improve upon their environmental, social and economic outcomes in the face of mounting technoeconomic challenges.
Preconcentration, as it will be called for now, encompasses a multitude of technologies and techniques that can be used as standalone or combined to reject waste material at strategic points in the mining process. The aim is to optimize mining, mineral processing and metallurgical extraction relative to the available inputs and desired outputs, thus maximizing asset value.
“High-grade projects with high ore values drive toward maximizing recovery in the plant, and there is limited scope for rejecting material if there is loss of value,” said Matt Pyle, director of technical solutions for Ausenco. “Lower grade projects are fueled by economies of scale, maximizing throughput through the ‘constraint,’ and incremental reductions in processing costs. In some cases, throughput and lower costs can offset reduced recovery around the plant. The irony, however, is that when the costs of processing reduce, there is often more scope to increase the resource and reserves, hence overall project extraction can improve.”
The Theory of Constraints is detailed in Eliyahu Goldratt’s 1984 management-based tome, The Goal. It’s a methodology that identifies the most prominent limiting factor in achieving a specific goal. For low-grade mining projects (which are increasingly the norm), the “constraint” for generating value is typically the comminution circuit. So, in these cases, waste rejection plays an important role in maximizing revenue, hence the increasing interest over the past decade or so. The rise in ESG priority in mining and the realization that preconcentration techniques not only deliver economic value, but also significant social and environmental benefits has only served to accelerate their development and adoption.
“At the end of the day, cost is a measure of inputs,” Pyle explained. “Preconcentration is not just about maximizing the upside, but also reducing the input required to produce each ton of metal. That means that, if the mill is the constraint, we reduce the amount of grinding, use less energy, less grinding media and also recover water more efficiently from tailings that are more stable. To do that, we optimize: we mine more selectively using geological data and clever modelling to home in on higher-grade pockets of ore and reject packets of ore that don’t warrant intensive processing. We optimize the energy spent in blasting, and sense and sort the ore in the pit to minimize haulage. Then we crush, grade, sense and sort, and use screens to reject the low-grade before grinding the material to as coarse a size as possible to reduce energy consumption — Ausenco is currently working on a gold project in northern Queensland, which rejects low-grade gold ore after crushing based on size separation, because gold tends not to be present in the coarser sized particles for this deposit.”
It’s also possible to reject material from semi-autogenous grinding (SAG) mills as pebbles or “scats,” which are harder and therefore consume more energy to process, but also tend to be lower grade.
“We’re also doing a lot of work on coarse particle flotation with clients,” Pyle said. “The Eriez HydroFloat was demonstrated in 2018 and continues to operate successfully at Newcrest’s Cadia copper-gold operation. This equipment enables grind sizes to be substantially coarsened, which means that on top of reducing energy by up to 30% per [metric ton], instead of fine tailings, we produce sand, which is more geotechnically stable to store and can be stacked in a smaller footprint than wet tailings. It also means that more water can be recovered.”
From Pre- to Post-concentration
The term “preconcentration” implies concentration of the ore before the constraint that, according to Pyle, is a rather limiting definition. In fact, the capabilities (and benefits) of waste rejection can stretch much further.
“If we look at the bigger picture, we’re trying to optimize the production of metal from the orebody all the way through to the final product,” he explained. “And every step of processing along the way, is a step in concentration. Preconcentration — or progressive waste rejection, as we prefer to call it — means doing the right amount of waste rejection at each stage to make the funnel of ore going into the final refined product as efficient as possible.”
There is a trade-off around how much energy mines put into a concentrator versus the final product quality. The question is: do you put more work in at the concentrator to produce a higher-grade concentrate and less work in downstream refining, or does it make more sense to do the opposite?
“For some projects, we’re looking at moving away from smelting because there can be benefits in using other hydrometallurgical routes for purification all the way through to final metal or cathodes,” Pyle said. “For instance, by doing that, the mine might be able to produce acid on site, which can be used to treat other ores through leaching. It’s worth looking at all the possible pathways.”
In many cases, the final product and thus concentrator performance is driven by the contract that has been agreed with the offtake partner. For example, arsenic thresholds in concentrates have been reduced in recent years. But, for large smelting complexes like those seen in China, even these small amounts in every shipment can add up to a large quantity of arsenic that must be retained as a waste stream and stored locally. However, there may be opportunities for the mine to spend a little more energy and effort to produce a higher-grade concentrate with a lower arsenic content rather than passing it via the contract to the smelting stage.
Contracts are not always the most flexible way to achieve process optimization, but the whole-of-value-chain approach to preconcentration can also be applied in other ways.
Pyle explained: “In some areas, there are multiple mining operations that use a central concentrator, and some of these satellite orebodies might be 60 or 80 km from the processing plant. There are early movers — both brownfield and greenfield sites — that are teaming advanced orebody characterization with preconcentration to reject as much waste as possible before the ore is trucked to that central facility. In some cases, it can make projects that were previously uneconomic feasible.”
He added: “At the end of the day, there are increasing drivers to consider different approaches to the one that’s been standard for the past 50 years, which is to crush, grind, float and smelt. There are tools, knowledge and capability now to be quite clever around how we treat orebodies, process them more efficiently and maximize their value to society.”
The Power of Bulk Sorting
Although there are many technologies that can be used to reject waste throughout the mining process, when it comes to true preconcentration, sensor-based ore sorting is one of the most effective, and also quickest and easiest methods to implement.
While particle sorting identifies and rejects individual rock particles, usually on a conveyor belt, based on their mineralogical composition (very useful in diamond operations, for instance), bulk sorting analyzes larger samples of material and accepts or rejects them based on a predetermined cut-off grade. This makes it suitable for use with less selective mining methods and lower grade orebodies. Bulk ore sorting is typically performed on a conveyor belt or directly at the mine face via technology mounted on shovel buckets.
“At MineSense, we feel preconcentration is best conducted at the face where the maximum benefit to the mine can be achieved,” said Claudio Toro, executive vice president for business development at MineSense. “If mines can perform ore sorting at the earliest and most effective point — at extraction — this mitigates the needless transportation and processing of waste rock, which is extremely costly and wasteful (estimated at over 60% of the cost of production).”
MineSense’s ShovelSense technology performs ore sorting using X-ray fluorescence (XRF) sensors to analyze and grade the material mined in each excavator bucket load. Preconcentration is achieved via automated truck diversion, sending only high-grade truck loads to processing and uneconomical loads to waste. This saves a huge amount of chemical reagent use, as well as water, energy and time.
“Mines today face two critical challenges,” Toro explained. “Firstly, increased demand for metals needed to realize a low-carbon future. Secondly, mines must lower their impact on the environment. ShovelSense is a complete hardware and software system that addresses these challenges. By identifying and extracting more payable metals as the machines excavate material, it maximizes the potential of the orebody. The variability of the raw material — ore or waste heterogeneity — is capitalized on at the earliest possible moment where the impacts on profitability and sustainability are greatest.”
Toro added that MineSense has seen significant growth in interest surrounding ShovelSense in recent years as ore grades decline and deposits become more complex. ShovelSense is particularly effective in orebodies that have a high level of heterogeneity so can play a role in helping to bring previously uneconomic projects to bear.
The technology is extremely accurate in detecting base metals and has been deployed in more than 75% of Canadian copper production, including at Copper Mountain Mining Corp. and Teck’s Highland Valley Copper operation. The team is currently working to extend its scope of application and is developing new sensors to improve characterization at the face.
“We are expanding our footprint with existing customers and engaging new ones in all the regions we serve,” Toro said. “We are seeing an increase in the industry’s investment in solutions that can effectively improve its sustainability.”
Potential for Waste Valorization
Ore sensing technologies of all kinds are getting faster, more accurate and able to identify a wider range of minerals than ever before, as are the statistical algorithms and computing technologies that underpin analysis. Mines are also getting more comfortable with handling and storing the vast quantities of data that these systems generate.
The beauty of bulk sorting is that, even if the material is rejected today because it falls below a certain cut-off grade, once it has been analyzed, sorted and sent to a waste dump, low-grade stockpile or tailings, the mine then knows its mineralogical composition and can potentially revisit the material at a later date if metal prices or market circumstances change. The information gathered about “waste” can be extremely valuable.
One idea that AMC Consultants is investigating with some of its mining clients is the potential for coarse mine tailings, generated through preconcentration, to be used in other industrial applications.
“Sand is the world’s second most exploited resource,” said Rob Chesher, technical manager for business development. “Around 50 billion tons of sand and gravel are extracted annually for use in civil construction projects and supplies are beginning to run short in some geographies. The UN Environment Programme has declared sand a ‘strategic resource,’ stating that ‘its extraction and use need to be rethought.’”
In its report, “Sand and Sustainability: 10 strategic recommendations to avert a crisis,” UNEP suggested that some mine tailings, if chemically inert and of an appropriate particle size, could potentially be used as a substitute for sand in some applications.
“The mining industry extracts about 80 billion tons of material collectively each year,” Chesher said. “What if, a portion of that 80 billion tons, a great deal of which is just discarded, could go toward meeting that demand for sand? It won’t be feasible in every case but, using technologies like coarse particle flotation, it is possible now to float metals from particles that are of a similar size to sand.
“A lot will be dependent upon the mineralogy, the size and texture of the tailings, proximity to markets, etc. But, by liberating metals at a coarser grind size, and dewatering and selling the tailings, the mine won’t just be saving energy and water, it will create a new product stream that can be used to offset the cost of mining and reduce the risk around tailings storage.
“This is just one example of how cross pollination of ideas from one industry to another can help to produce a better overall result. Strategic planning to optimize the mine plan with this wider range of inputs will yield better outcomes for all objectives.
“That’s not just about doing the right thing for people and the environment, it’s also a smart business decision taken through an ESG lens,” Chesher said.
CSIRO Furthers CRC ORE Research
Waste valorization enabled via preconcentration is a field that is also being investigated by the Commonwealth Scientific and Industrial Research Organization’s (CSIRO) Future Research program in Australia.
In June 2021, when research cooperative, CRC ORE, reached the end of its federal government funded term, its remaining resources (Hatch announced it would commercialize the Grade Engineering suite of technologies in late 2020) were dedicated to establishing the program in collaboration with CSIRO.
The Future Research program is now expanding CRC ORE’s research into preconcentration technologies that can be deployed within the mine and ahead of the mineral processing plant.
“The Future Research Program with CSIRO aims to continue to explore some of CRC ORE’s more promising fields of research within four main focus areas,” said Paul Revell, research program manager for mineral processing at CRC ORE. “Firstly, incorporating the principles of selective breakage into the design and operation of comminution circuits.
“The mining industry has known of selective breakage for a long time, from observing that when ores are crushed, the metal component of some ores tends to accumulate in the finer particle size fractions. This represents an opportunity to preconcentrate the ore if those fine size fractions are screened to sort them from the coarse particles.”
CRC ORE and the University of Queensland undertook extensive research to investigate this phenomenon and successfully identified a range of rock crushing parameters that, if controlled, could potentially induce a selective breakage regime in the ore and allow this preconcentration opportunity to be harnessed.
The Future Research Program aims to take this further by devising ways to control the identified crushing parameters and then incorporate them into the design of a novel rock crushing device. If successful, this crusher would be capable of selectively crushing only the value-containing components of the ore and leaving the gangue relatively unbroken. The project aims to build a prototype of the rock crusher and prove the concept by demonstrating operation of an integrated selective crushing and sorting machine for enhanced preconcentration of amenable ore types.
The second area of research will focus on optimizing ore feed to coarse and fine particle separators to enhance their performance. There are various existing preconcentration technologies in the marketplace that are already good at undertaking coarse particle flotation. However, the uptake of these technologies is constrained by the risk of losing valuable fine metals in the reject streams from those devices.
Revell explained: “The opportunity is to develop a high-efficiency classification technology that can remove very fine particles from the high-volume feed streams to the coarse separators. This same technology would also enhance the performance of other unit operations where classification is critical, e.g., as a closing cyclone on a ball mill where the result would be improved grinding and flotation performance, and reductions in energy, water and reagents.”
All coarse and fine flotation processes are dependent on effective liberation to expose the target species to the flotation process. If a selective breakage crushing or grinding device is used to prepare ores for flotation, the feed particles should have a higher proportion of the target species exposed on the surface of the particles than is achieved when using conventional feed preparation crushers or grinders. This would result in improved flotation recovery, and an opportunity to maintain recovery at coarser grind sizes.
The third research area aims to achieve step-change reductions in energy and water intensity in mineral processing. The overall objective of preconcentration is to remove economically barren material from the ore ahead of energy- and water-intensive fine grinding, which can result in a step-change reduction in scope one, two and three emissions.
Lastly, the team will develop new options for the sustainable management of waste material.
“Undertaking preconcentration interventions ahead of fine grinding, and ideally at multiple locations across the mining value chain, produces dry barren reject material at an aggregate of particle sizes,” Revell said. “Having the material in this form enables many new options from valorization to progressive rehabilitation, each of which is cheaper and more environmentally sustainable than the current situation where all of the waste material is present as a fine wet slurry that is difficult and expensive to effectively manage in the long term.”
The Future Research Program is currently gathering collaboration partners and establishing a series of focused research projects within the above areas. By the end of the initial three-year period, the program will have generated data and gathered evidence of the effectiveness of the technical solutions. This will form a compelling package to take to the industry either as a potential commercialization opportunity or as justification to invest in further development.