The Hydraulic Dewatered Stacking operation at Anglo American’s El Soldado copper mine in Chile.
(Photo: Anglo American)

In Russia, 40% of the nation’s ironore production, currently located in high water-stressed areas, is likely to move to extreme water stress by 2040. And mining regions which have historically been exempt from water stress are becoming more vulnerable. For example, by 2040, 5% of current gold production (currently in low-medium water stress areas) will be classified as to medium-high risk.

This shift in predictability coupled with intensified industrial needs as ore grades decline, has seen water become a growing point of contention between mining companies, governments and communities in recent years.

In light of this, mining companies are having to shift their attitudes towards water; being a consumer or user is no longer good enough, companies are now expected to steward these resources - managing, protecting, restoring and sharing them in a way that ensures a plentiful supply for everyone, both now and into the future.

In the absence of a viable option to remove water from mining processes altogether at a commercial scale, and an unreliable timescale as to when this might be economically feasible, the best choice currently, is to employ the other three Rs: reduce, reuse, recycle. A water stewardship strategy with these three tenets at the core, created and actioned in partnership with local rightsholders will ensure the minimization of risk and safeguard the value of these resources in the long term.

Working With and For Communities
In a recent blog post titled ‘How does a mine site in the desert find water?’2 Stantec’s Mining Manager in Chile, Luis Arcos, and Vice President for Mine Site Infrastructure, Kevin Kammerzell, explain why this long-term view encompassing the three Rs is so important. Poor water management practices today can create serious issues in the future, impacting local communities as well as other industries, such as agriculture.

In the article, they argue that mining operations should work alongside communities, use fit for purpose water, explore alternative waste disposal methods, and recover and reuse water as much as possible to conserve these precious resources.

“Collaborating with local communities is imperative to a sustainable mining operation,” state the authors. “This is true when we talk about water, too. While it might be more challenging with more stakeholders, engaging with the community can be incredibly beneficial for both the mine and the community. For example, we worked with a mining client to design, build, and operate a wastewater processing facility for a municipality in Peru. The nearby city had lacked the proper infrastructure to treat wastewater and protect the natural rivers and streams.

“With the city in an arid desert environment, water management was tenuous to support current and future demands. The mining company built a wastewater plant to support the city and was able to use the treated water for their mining operations instead of extracting additional water from the groundwater aquifers. It’s a win-win scenario, on what we call the triple bottom line. This means the solution provided social, environmental and financial value.”

While water is used in many different mining processes from drilling to milling, beneficiation is the most water-hungry step in flowsheets today, specifically flotation. In their article, Stantec’s authors suggest a number of ways in which mines can meet this need without increasing their abstraction requirements. One option is to expand the scope of available resources by considering using water that is ‘fit for purpose’ or good enough for use in certain applications.

“In mining, we can use water that has impurities,” they explain. “For instance, waters high in suspended solids are often present around mine sites. While the impurities are not fit for potable consumption, this water may be acceptable for mineral processing use — it doesn’t need to be as clean as water that we drink. Although, if the water is discharged, it would need to meet water quality standards to decrease environmental impact.”

Consumption and preservation go hand-in-hand, and it makes good business sense to not only reduce abstraction requirements as far as practically and economically possible, but also to reuse and recycle the water that mines must take from local supplies. For instance, Arcos and Kammerzell point to Roy Hill’s Pilbara operation in Western Australia which uses water from an open-pit dewatering operation to recharge local aquifers.

“This is not only a good example of recycling water, but it also renews a local water source in an arid region,” they state. “Once again, finding ways to responsibly manage water that can benefit the mine, the environment, and the community is a winning solution.”

They add that another option for saving water is to optimize recovery where possible: “Water is often lost to evaporation and runoff, so minimizing loss in these areas helps stretch the life of the water at a site,” they explain. “One way to do this is by using a cell system; instead of letting water run off over a large area in the tailings [storage] facility (TSF), the area is divided into cells to minimize the surface area and therefore reduce evaporation. By reducing water loss, the mine can recover and recycle more water.”

Exploring Alternative Tailings Storage
Alternative tailings storage and disposal methods also offer promise in this respect. Dewatering tailings to create a thickened product, paste or filter cakes for dry stacking not only offers a reduction in social and environmental risks associated with wet storage, but also allows the recovery of residual minerals and some of the valuable water contained within the tailings.

According to Watson et al., 2010,3 conventional (un-thickened) tailings which are usually deposited in a TSF as a slurry, comprise, on average, 20%-45% solids by weight. Despite having been dewatered to a degree, even thickened tailings (50%-70% solids by weight) and paste (70%-85% solids) still contain a significant volume of water which, in time, will usually be lost to evaporation and seepage into the surrounding environment. The Global Tailings Review has calculated that 12.7 billion metric tons (mt) of tailings are produced by mines every year. Given these numbers, it’s possible to see how millions, if not billions, of cubic meters of water could be saved across the industry each year by applying dewatering technologies or increasing the percentage of water recovered from tailings.

In the Stantec article, Arcos and Kammerzell explain: “Where they can be safely constructed, alternative [tailings storage] methods use less water and have a smaller environmental footprint compared to conventional methods. Some popular alternative methods include thickened tailings, paste, and dry stacked tailings. These methods can reduce the volume of the stored waste and promote more stable storage while using less water. Though, there are pros and cons of alternative tailings disposal.”

They add that there is significant room for innovation in the tailings industry. Stantec recently entered a collaboration with Auxilium — a niche technology group that reuses mine tailings in other industrial applications. Auxilium specializes in tailings valorization, meaning it provides solutions for handling waste that yield economic benefit. For example, tailings can be cleaned and made into a paste or cement. This product not only reduces water and waste, it also can sequester carbon and be reused in the construction industry for building materials.

Innovation also extends beyond technology; a good example is Anglo American’s Hydraulic Dewatered Stacking (HDS) concept. In a June 2023 blog post4 the company announced that it had taken a cohort of mining representatives to visit the 150,000 m3 pilot HDS plant at its El Soldado copper mine in Chile.

As part of the HDS process, coarse particle recovery (CPR) is used to reduce the grind size of processed material and lower energy consumption. Anglo stated that this also makes it easier to capture and drain water during the processing phase before recycling it.

In a recent press release,5 the company said: “HDS targets the geotechnical and water recovery performance of filtered tailings, but without the carbon footprint. Laboratory and proof-of-concept testing has proven the robustness of fines-free sand as a filtering medium – promising a safe, dewatered facility with the possibility of re-purposing otherwise sterilized land within months of closure. Through effective and permanent desaturation of the tailings, the geotechnical safety of the facility can be enhanced and deliver stability in excess of new GISTM [Global Industry Standard on Tailings Management] standards.”

The trial will continue through 2023 and Anglo said that early results look promising. Measured water recovery has thus far exceeded the initial target of 80%, with up to 85% water recovery a possibility.

Given that 80% of Anglo American’s assets globally are located in water-constrained areas, reducing their water dependence, achieving closed loop systems and, eventually, creating fully waterless operations, is strategically important for the company. Anglo American said that HDS is a key step towards this and that it “is proposing a collaborative approach under licence to deliver a rapid roll-out [of HDS technology] across the industry” to ensure that others can reap the benefits too.

Making Future-fit Choices on Water and Tailings
As Arcos and Kammerzell explain, today it’s up to mining companies to optimize their water usage.

“This is not the case for other industries,” they state. “For example, in the US urban planning industry, for a new municipality to form with 60,000 residents that will be supplied water by underground aquifers, there must be a ground water impact study and plan for how water will be sourced and used over the next 100 years. This ensures an adequate water supply exists before the community is developed, and that these demands don’t conflict with other growing demands in the region. We need mine owners and operators to think similarly when planning for water use.”

Desalination is fast becoming the go-to technology to secure mine water supplies in very arid countries, such
as Chile. (Photo: Stantec)

“We need to recognize that [water is] a shared supply and, while often considered renewable, we need to recognize that in arid environments, the time for an aquifer to recharge may diminish the shared capacity over time,” they state.

While mining companies are beginning to evolve their thinking around water as a strategically important resource, there are still barriers to the adoption and wider uptake of new water-saving technologies that can unlock the ‘triple bottom line’ benefits mentioned earlier.

Alternative tailings storage methods, such as dry stacking, are often assessed for projects but dismissed on the grounds of prohibitive expense and/or technical feasibility at scale. Rather than recycling process water as far as possible, many operators in very arid countries have plumped instead to increase their abstraction through the desalination of seawater — an initiative that, in itself, can run to billions of dollars.

For example, all new mining projects in Chile are considering the use of seawater in their environmental impact assessments, since the use of continental water is dependent on base flows that might or might not be available in the future. This makes it unfeasible to guarantee water supply for a mining operation if companies do not use alternative water sources.

While desalination will undoubtably meet the needs of mining projects in the near term, it’s questionable whether simply taking water from another source presents the best option in the long run, particularly when the health of our oceans is in question. To make the best possible choices on a project-by-project basis requires a model that holistically assesses and quantifies the value of water inputs, TSF capacity and time value of embedded capital; something that has been sorely lacking until recently.

A new paper from a team at the University of British Columbia answers this problem. In “The environmental and economic case for valuing water recovery and its relationship with tailings storage conservation”6 the authors, Cox et al., present a new open-source techno-economic model which allows for the comparison of water use and cost between different tailings disposal-related equipment, addressing a major gap for operators and early technology decision-makers.

The authors explain: “Optimizing water management around tailings water recovery has never been more relevant from an environmental, economic and political perspective… The technical feasibility of conserving water at mine sites is well understood but is frequently dismissed as being too expensive. This is likely because techno-economic cases justifying decreased water usage have not been well documented. While current research provides examples on the technical or economic optimization of single equipment option trade-offs, there are several limitations to those approaches.”

Most of academic research on the economic optimization of water in mining is modelled using theoretical mines or unique mines with a solution optimized for a single mine. The authors explain that two assumptions are usually made: first, that mines are making a greenfield capital decision, and second, that they don’t have the sunk costs of prior equipment choices. These assumptions are problematic, because many mines in water-scarce regions such as Chile are long-life operations and equipment choices must therefore be modelled as a brownfield upgrade or as expansions.

In their paper, Cox et al., introduce a decision-making framework that can be used for tailings waste and water optimization within brownfields projects. This enables mines to make an economically informed decision about which process optimization investments have robust economic and environmental business cases. It also promotes the consideration of multiple equipment options, instead of the current standard approach towards water management which focuses on increasing water supply through either local pumped or desalinated water.

“Consequently, a decision maker will be able to decide which project should get funding for advanced engineering and testing work,” they state.

To prove this, a spreadsheet-based model dubbed the ‘tailings economic trade-off (TET) model’ was developed to evaluate the early techno-economic feasibility of three different water-saving mine equipment upgrades at a hypothetical Chilean mine: optimized thickeners which recover water with active gravity separation and shift the density of the whole waste stream; hydrocyclones which recover water via separation of large and small size fractions; and filter presses, which recover water from tailings using compression.

Dewatering Makes Economic Sense for Most
In testing, the TET model showed a ladder of positive economic and environmental options. The three equipment packages tested allowed for both water recovery and TSF conservation capacity while developing a mine, meaning that a mine can install one or all of the equipment packages and benefit from the water recovery and tailings cost savings from each equipment package harmoniously.

Interestingly, the model showed that hydrocyclones and optimized thickener control systems make economic sense on a water recovery basis at any site globally, while the high capital and operating cost requirements of filter pressed tailings make an economic equipment application location-specific to regions with local water scarcity and political risk profiles. The authors explain that the advantage of the TET model is that trade-off studies can be performed earlier, and the decision to go with incumbent water-intensive equipment can be more broadly challenged on an economic basis going forward.

They add that, in any stage of development, the TET model provides operators with a quantitative tool to generate an economic and environmental case for water and tailings – an argument that previously was hard to model due to the different process silos the equipment packages fit within.

Cox et al., explain: “While the use of desalinated water looks, at face value, to be an environmentally logical solution to the fundamental water constraint issue facing Chilean mines, the environmental and economic trade-offs associated with the water source have not been quantified in past research. The TET model argues that all water, including desalinated water, is delivered at an economic cost.”

They add that the costs modelled in TET are tailings capacity and water recovery potential. However, there are other unmodelled and unquantified costs, including CO2 emissions and political and license-to-operate risks.

Given these trade-offs and costs, the TET model shows that in arid regions, the standard operating procedure of pumping and desalinating water and using a traditional TSF can be materially economically and environmentally optimized. The equipment packages modelled quickly generate a positive economic impact even without quantification of the environmental and social cost or the inherent risks associated with traditional TSFs.

“The TET model reduces the discussion around water and tailings to a quantifiable argument around the cost of increased water recovery, the cost of TSF reduction, and the value against the de facto use of a desalination plant and a TSF,” the authors conclude. “Using the TET model requires a paradigm shift, from the lowest capital cost solution being the core driver to optimizing natural capital utilization over the life of mine, resulting in alternative decisions being considered.”

Tailings Dewatering: What’s Possible Today?
The degree to which tailings are dewatered determines, not only alterative disposal options, but also potential uses for the water that’s recovered. It’s therefore important to understand the practical capabilities and limitations of current dewatering technologies and storage mechanisms in addition to the economic case.

Dewatering tailings and recycling the resulting water can allow mines to lower their abstraction
requirements and consider alternative storage methods. (Photo: Weir Minerals)

“While water presents a number of industry-wide challenges, the solutions need to be site- and operation-specific. In other words, a one-size-fits-all approach won’t deliver the best environmental, social and economic outcomes.” The environmental benefits of tailings dewatering, from a water conservation perspective, are clear — the recovered water can be recycled.

“Equipment like filter presses, for instance, consume a lot of energy,” he said. “Similarly, transporting, treating and compacting filtered tailings is carbon intensive. It’s important that miners take all the factors into account when developing a tailings management system, because a myopic focus on water alone might not always deliver an environmentally optimal solution.”

Many operators mistakenly think that if they draw freshwater from a local water source, like a river, then it’s free. But it’s important to think about the costs associated with water more broadly. For instance, that water needs to be treated and transported before use in mining. There is a cost associated with all of these activities, so it’s important to properly value water — even freshwater — that’s being used. Vlot said that doing this allows miners to more accurately compare different tailings disposal methods and to quantify their expenses to arrive at the best possible solution for each operation.

Despite major technological advances over the past 10 years, there are still significant challenges around dewatering the quantity of tailings that most operations produce. At the same time, there is a trend towards lowering the moisture content of tailings. So, the industry faces a dual challenge around quantity (how many tonnes per day can be processed?) and quality (how dry can they be made?). Of course, miners want to achieve both their quantity and quality targets using as little energy as possible.

Vlot said: “Achieving this requires a holistic approach which takes into account the site’s mineralogy, the particle- size distribution (PSD), whether the material is clay-bound, the climatic conditions, etc. Then, on the operational side, each mine must analyze its energy consumption, the carbon emissions associated with transporting its tailings and water consumption, among a myriad of other things. This equation is going to look different for every mine, so as an OEM and tailings solution provider, Weir Minerals doesn’t focus exclusively on one solution; rather, we tailor the solution based on the customers’ requirements.”

Solving for Scale With Holistic Design Methods
He added that the main limitation with current dewatering technologies, is that the equipment isn’t sufficiently suitable to dewater large quantities of tailings.

“Even if the mine is just using a thickener, let alone a filter press, the capacity of the equipment must be huge,” Vlot said. “So, upscaling is probably the biggest industry- wide challenge at the moment.”

Weir Minerals’ proprietary Terraflowing technology, which is a dewatering process that transforms tailings into a resource, could help to reduce TSF risks and fundamentally alter the total cost of ownership equation. The company is also looking at upstream solutions to remove waste before it reaches the TSF. This might happen in the pit with ore sorting technologies or in the milling stage where, for example, the ball mill is replaced with a high-pressure grinding roll (HPGR) combined with air classification — a solution that Weir Minerals recently implemented at FMG’s Iron Bridge magnetite operation in Western Australia.

“Similarly, coarse particle flotation (CPF) systems make it easier for miners to dewater and store their tailings,” Vlot added. “Weir Minerals partners with Eriez to deliver this solution. Our customers are excited about this prospect because it means that, in addition to the benefits associated with fewer, drier tailings, they can also increase their throughput.”

These solutions — HPGR with air classification and CPF — transform the flowsheet so that less water is consumed, while also focusing on processing more of the valuable ore, which means the kilowatt- hour per tonne (kWh/t) is reduced. Although dewatering tailings through thickening is standard practice today, in the future design engineers may consider reducing water consumption at various stages throughout the flowsheet.

1. Lindsay Delevingne, Will Glazener, Liesbet Grégoir, and Kimberly Henderson, January 28, 2020, “Climate risk and decarbonization: What every mining CEO needs to know,” McKinsey & Co., www.kckinsey.com/capabilities/sustainability/ our-insights/climate-risk-and-decarbonizationwhat- every-mining-ceo-needs-to-know.
2. Luis Arcos and Kevin Kammerzell, February 8, 2023, “How does a mine site in the desert find water?”, Stantec, www.stantec.com/en/ideas/ content/blog/2023/how-does-a-mine-site-in-thedesert- find-water.
3. Watson et al., 2010, “A comparison of alternative tailings disposal methods — the promise and realities,” Mine Waste 2010 – Australian Center for Geomechanics, https://papers.acg.uwa.edu.au/p/ 1008_41_Watson/.
4. Anglo American, June 26, 2023, “Our HDS technology: changing the conversation on tailings management,” www.angloamerican.com/our-stories/inno vation-and-technology/our-hds-technology-changing- the-conversation-on-tailings-management.
5. Anglo American, December 8, 2022, “Progress on Hydraulic Dewatered Stacking (HDS),” www.angloamerican.com/our-stories/innovation- and-technology/progress-on-hydraulic-dewatered- stacking-hds-el-soldado-chile.
6. Cox et al., 2023, “The environmental and economic case for valuing water recovery and its relationship with tailings storage conservation,” Minerals Engineering, vol 201 (October 2023), www.sciencedirect.com/science/article/pii/ S0892687523001711.