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Most of the ore that enters a concentrator leaves as tailings, and with it goes a large share of the water the mine paid to source. For most of the mining industry's history, that water was written off. It’s pumped as slurry behind a dam, to evaporate, seep away or sit as a long-term liability.
That is changing. As mines move into drier basins and lower-grade ores, and regulators scrutinise tailings more closely, tailings water is starting to look less like waste. Done well, it can cut water costs, make operations more resilient to drought and dam failure, and move mining closer to a closed loop. But tailings water is only as valuable as the site lets it be.
Recent developments in two contrasting settings, Chile’s arid copper belt and Indonesia’s rain-soaked nickel islands, suggest that tailings dewatering should be judged as part of a site’s overall water balance and priced against the cost of the next marginal cubic metre. They also show that the barriers to scaling it are as much about chemistry, climate and disclosure as about engineering.
The Tale of Two Tailings
Chile shows what it costs to replace lost water. Cochilco, Chile’s copper commission, projects that seawater will supply 67.6% of the country’s copper mining water by 2034, up from 40.7% in 2024. Desalinated water delivered to site averages around $4.45/m³ once long-distance pumping is included, against roughly $1.87/m³ for raw seawater. ICMM data shows that 96% of Chile’s mining facilities face both water stress and depletion.
Every cubic metre recovered before tailings reach the dam is a cubic metre that does not need to be desalinated and pumped inland and uphill. Conventional facilities lose water trapped in settled solids, evaporated from the pond and through seepage, and thickening, paste and filtration progressively close those routes. Conventional slurry is often more than half water by weight, while filtered tailings leave the plant as a cake with around 15–20% moisture. That residual moisture is still lost, so even a dry stack is not a fully closed loop. In Atacama, though, it is a much tighter one.
Indonesia presents almost the opposite problem. It supplies more than 60% of the world’s mined nickel from islands where rain is abundant. HPAL, the process that turns limonite ore into battery feedstock, produces enormous tailings volumes relative to the metal recovered. With deep-sea disposal no longer permitted for new projects, tailings must be stored on land, in steep, saturated, seismically active terrain.
Here, dewatering is first a geotechnical decision. Harita Nickel on Obi Island operates one of Indonesia’s earliest HPAL dry-stacking facilities and estimates that about half the water used on site is recovered, though dry stacking is not risk-free in this terrain. Nearby Weda Bay relies instead on containment, with retention ponds sized for 84 hours of maximum rainfall.
Safety standards are pushing the same way too. After the 2019 Brumadinho dam collapse in Brazil killed 270 people, the Global Industry Standard on Tailings Management (GISTM) raised the bar for tailings design and disclosure. Vale committed to moving its Brazilian iron ore output towards dry processing and filtered tailings. In Portugal, authorities have flagged a tailings-dam rupture risk at the Barroso lithium project, now a fast-tracked EU Strategic Project.
Therefore, in arid settings, it is a water-supply investment, measured against the cost of desalination. In wet or seismic settings, recovered water is a co-benefit and the main return is lower failure risk.
Room to Squeeze
None of this means tailings dewatering is ready to roll out everywhere. But each gap is also a place to make progress.
The first is scale. Large copper concentrators process 100,000-200,000 tonnes per day or more. Filtered tailings are proven at small and mid-sized operations, but at that scale they mean many filters running in parallel, making larger, more efficient, lower-energy filtration an obvious target for innovation.
Then there is the ore. Fine, clay-rich tailings filter slowly and hold on to their moisture. Flocculants and coagulants help, but they add reagents to the very water the site wants to reuse. Mineralogy should be tested at the feasibility stage, not at commissioning.
Climate adds complexity. In a monsoon region, rain re-wets exposed surfaces, so stacks need compaction, drainage, runoff capture and sometimes covers, which moves the water problem from the dam to the stack. With so much new supply coming from the tropics, stacks must be designed for the wettest season, not the average one.
Closing the loop also concentrates whatever is dissolved in it. In Chile, water recirculated from seawater-fed processes builds up salts and residual reagents. That can affect flotation. The presence of calcium, magnesium and sulphate can cut copper and molybdenum recovery in flotation. In Indonesia, ultramafic geology leaches metals including hexavalent chromium. Recovered water is not automatically reusable water, so treatment should match the site’s specific chemistry which, ideally, should be built into designs from day one.
Underlying all of this is a disclosure gap. Look at any major analysis or cost curve models for these metals, and dry-stack and onshore tailings’ costs do not appear anywhere because operators do not report them. Yet they add to the initial capex. GISTM disclosers also focus on dam safety, not on how much water tailings return. What is not measured is hard to compare, finance or improve.
The Next Cubic Metre
Four shifts would help dewatering scale. First, value recovered water at its replacement cost in project evaluation. In northern Chile, that benchmark is desalinated water delivered to site. Second, matching the method to the site – thickening, partial filtration or staged sediment terracing may deliver more per dollar than a full dry stack. Third, designing treatment alongside dewatering. Fourth, measuring what tailings actually return, in volume, quality and cost, so recovery is verifiable and comparable for operators, investors and regulators.
Closed-loop water management will advance site by site, and only where site conditions, processing demands and water chemistry allow it. Where they do, dewatering can lower costs, strengthen resilience and narrow the gap to a closed loop. That matters because water already decides which projects get permitted, financed and accepted by the communities around them. A site that draws less from shared sources and discharges less is simply easier to approve, fund and live alongside.
