Tailings management stands as one of the most consequential technical and ethical challenges in the modern mining industry. The sheer volume of tailings generated globally is staggering: for every tonne of copper in a typical porphyry deposit, several hundred tonnes of finely ground rock must be disposed of safely. The failures of tailings storage facilities at Mariana in 2015 and Brumadinho in 2019—together responsible for more than 280 deaths and catastrophic environmental damage in Brazil—have brought the stakes of poor tailings management into sharp, irreversible focus. Yet tailings are not simply waste. They represent a significant repository of residual mineral value, a source of construction material, and, managed well, a recoverable water resource. This guide provides a comprehensive treatment of all aspects of tailings management: the nature of tailings, the full spectrum of storage and disposal options, water management strategy, dam safety principles drawn from documented failures, and the growing field of tailings reprocessing and valorisation.
What Are Tailings?
Tailings are the finely ground residue of ore after the economically valuable minerals have been extracted. They are produced continuously throughout the operating life of a mineral processing plant and typically consist of the host rock minerals—quartz, feldspar, carbonates, phyllosilicates, and iron oxides—together with small residual quantities of the valuable mineral that were not recovered during processing. The chemical and physical nature of tailings varies enormously between ore types and processing methods (Wills and Finch, 2016).
In terms of particle size, tailings from modern ball mill circuits are usually predominantly fine-grained, with a significant fraction finer than 75 μm and sometimes significant fractions below 10 μm. Coarser tailings arise from dense media separation plants treating coal or diamonds, where the float fraction may consist of 20–30 mm particles that can be stockpiled as dry material with straightforward handling characteristics.
From a geochemical perspective, the most critical distinction is between non-acid-generating (NAG) tailings and potentially acid-generating (PAG) tailings. Tailings that contain sulfide minerals—most commonly pyrite (FeS2) and pyrrhotite (Fe1-xS)—can generate sulfuric acid and mobilise heavy metals through the oxidation of those sulfides when exposed to air and water. This process, known as acid rock drainage (ARD) or acid mine drainage (AMD), is widely regarded as the most serious long-term environmental issue associated with mine waste management and is discussed in detail in the environmental management section below.
Tailings are almost universally transported from the processing plant to the disposal area as a slurry in water. The water content of that slurry—and the options for reducing it before deposition—is one of the central variables in tailings management strategy. Conventional hydraulic tailings are pumped at 25–45% solids by weight; thickened, paste, and filtered tailings carry progressively more solids, reducing the volume of water deposited with the tailings and improving the geotechnical stability of the deposit.
Tailings Storage Facilities: Design and Construction
A tailings storage facility (TSF) is a managed impoundment designed to contain and store the tailings produced by a mine over its operating life. Designing and constructing a TSF is one of the largest single capital expenditure items in a mining project, often representing 5–15% of total capital cost, and the long-term liability associated with a TSF can extend decades beyond mine closure. Site selection, embankment design method, and operational practice are the three factors that most strongly determine whether a TSF will perform as intended.
Site Selection
The ideal TSF site is located close to the processing plant to minimise tailings transport costs and pumping energy, is topographically suited to the chosen construction method, has a sound geological foundation capable of supporting the embankment loads, and is positioned to minimise downstream risk to communities and sensitive ecosystems. In practice these criteria are rarely all satisfied simultaneously, and site selection involves trade-offs between cost, safety, environmental impact, and community acceptance. Hydrogeological studies of groundwater flow beneath and around the proposed site are essential to assess the risk of leachate contamination of groundwater and to design appropriate containment measures.
Embankment Construction Methods
Three fundamental methods are used to raise TSF embankments, each with distinct cost, safety, and operational characteristics.
The upstream method raises each successive embankment increment by constructing a new dike on the upstream side of the previous one, with the centreline of the dam migrating progressively toward the pond. The method is inexpensive and fast—material for the dike raises can be taken from the dried tailings surface or separated from the slurry by cyclones to produce a sand fraction—but it suffers from a fundamental structural weakness. Each raise is founded on the previously deposited, partially saturated tailings behind the existing dike, which may remain unconsolidated and weak. The upstream method is therefore susceptible to liquefaction failure, particularly under seismic loading or when the upstream face is saturated. Several major historical TSF failures, including both Mariana (2015) and Brumadinho (2019), involved upstream-constructed embankments (Wills and Finch, 2016; Wikipedia, 2024). Following these disasters, Brazil and several other jurisdictions have enacted bans on new upstream dam construction, and the global industry is progressively decommissioning existing upstream structures.
The downstream method raises each successive increment on the downstream side of the previous one, keeping the centreline of the embankment at or downstream of the previous position. This approach requires significantly more fill material—either external borrow fill or cyclone-produced sand from the tailings stream—but results in an embankment that is founded on competent fill at every stage of construction, rather than on unconsolidated tailings. The downstream method is the only approach that can be designed and constructed to established dam safety engineering standards, and it is mandated in seismically active areas and for all large dams regardless of location. Its major disadvantage is the increasing volume of fill required as the dam height grows, which can become uneconomical in later stages of mine life.
The centreline method is a compromise: the crest of the embankment remains at a fixed horizontal position as the dam wall is raised, requiring less fill material than the downstream method while providing significantly better structural performance than the upstream method. It is widely used for medium-sized TSFs in areas of moderate seismic hazard, where the economic constraints of the downstream method are a concern but the risks of the upstream method are unacceptable.
Conventional Slurry Tailings
Conventional hydraulic tailings discharge involves pumping dilute slurry (typically 25–45% solids) from the processing plant to the TSF, usually through a series of spigots arranged around the perimeter of the embankment. The coarse sand fraction of the slurry settles near the discharge point and can be used to construct or raise the embankment wall. The fine slimes fraction migrates toward the center of the pond, where it accumulates in a zone of soft, wet, consolidated sediment. The decant water—the clarified supernatant in the pond—is collected by a floating pump or decant tower and returned to the mill for reuse.
Conventional slurry tailings are the lowest-cost disposal option at the point of generation but carry the highest long-term liabilities. The large volume of water in the tailings dam creates hydrostatic pressure that must be managed and controlled throughout mine life and beyond. The soft, saturated nature of the deposited slimes means that even properly designed upstream-method dams can fail if the phreatic surface within the embankment rises too high or if seismic loading is applied. Water management around a conventional TSF is a perpetual operational obligation: too much water in the pond increases failure risk; too little reduces the clarity of the decant and wastes water.
Seepage from the tailings dam is a significant environmental concern, particularly for sulfide-bearing tailings where leachate can be acidic and metal-laden. Seepage management systems—including liner systems, compacted clay cores, geomembrane liners, and downstream seepage collection ponds with pumped return to the TSF—are standard components of modern TSF design. The effectiveness of these systems must be monitored throughout mine life by piezometers, seepage flow measurement, and regular water quality sampling (Wills and Finch, 2016).
Thickened and Paste Tailings
Thickened and paste tailings disposal arose from efforts to reduce the volume of free water deposited in a TSF, improve the geotechnical behaviour of the tailings deposit, and simplify water management. Pioneered by Robinsky in the 1970s at operations in Canada, thickened tailings disposal involves the use of thickeners to raise tailings solids content to 50–70% by weight before deposition.
At 50–70% solids, thickened tailings behave as a viscous, self-levelling slurry that flows outward from a central deposition point and builds a sloped beach without requiring a confining embankment to the same height as conventional tailings. Because there is little free water in the deposit, a large settling pond is not required, and the risk of dam failure through liquefaction is substantially reduced. The Ecstall (Kidd Creek) operation at Texasgulf Canada Ltd., which used central spigoting into a 3,000-acre containment area enclosed by a minimal gravel dike, demonstrated the commercial viability of this approach at a large scale (Wills and Finch, 2016).
Paste tailings, produced at 70–85% solids using deep-cone paste thickeners, carry essentially no free water and have a yield stress that prevents segregation of solids and water during transport and deposition. The improved hydrogeotechnical properties of paste—lower hydraulic conductivity, better consolidation behaviour, reduced pore water pressure—translate into greater embankment stability and lower seepage volumes. At the Bulyanhulu gold mine in Tanzania, paste tailings at approximately 73% solids content are transported by pipeline and deposited through deposition towers at the centre of each cell in the TSF. This approach provides better water recovery, a reduced TSF footprint, minimised contaminant transport, lower wind erosion, and the possibility of progressive rehabilitation during mine life (Wills and Finch, 2016).
McLanahan reports that paste thickening can reduce the volume of material being stored in a tailings pond by 70–80% compared to non-thickened methods, and that recovered water from paste circuits can be returned to the plant at up to 90% recovery efficiency (McLanahan, 2023).
Dry Stack Tailings: Benefits and Challenges
Dry stacking represents the most intensive dewatering approach for tailings management. Tailings are thickened and then filtered to a solid content typically above 80–85% before transport to the storage area by conveyor belt or truck. At these moisture levels, the material behaves as a moist cohesive solid rather than a slurry; it can be spread by bulldozer and compacted by roller to form a dense, stable, engineered fill mass.
The geotechnical advantages of dry stack over conventional slurry deposition are substantial. A properly designed and compacted dry stack has measurable shear strength, low hydraulic conductivity, and no free water that could contribute to catastrophic liquefaction failure. The elimination of a large supernatant pond removes the single largest driver of conventional dam failure. Progressive closure—capping and revegetating completed sections of the dry stack while other sections continue to receive fresh tailings—is feasible, reducing final closure cost and beginning the rehabilitation process early in mine life (McLanahan, 2023; Dynaproco, 2023).
Dry stacking is particularly well suited to arid and semi-arid climates, where fresh water conservation is critical and rainfall onto the TSF is limited. The Raglan Mine in northern Quebec, located in an arctic environment, uses dry stacking to recycle most process water and integrates the compacted tailings into the permafrost, where the cold temperatures suppress sulfide oxidation and ARD generation (Wills and Finch, 2016). Several large copper operations in Chile and Peru have adopted dry stacking as a condition of their environmental permits.
The challenges of dry stacking are primarily economic and climatic. Achieving the high filtration throughput required for large operations—some modern porphyry copper mines produce 100,000–200,000 t/d of tailings—requires very large numbers of pressure filter presses, representing a substantial capital investment and operating cost compared to conventional thickened slurry disposal. Research on large-scale filtered tailings disposal demonstrates that the technology is technically feasible at these throughput rates, but the economics are site-specific and must be evaluated against the long-term liability costs of conventional TSF options (MDPI Minerals, 2023).
In humid tropical climates, maintaining the trafficability of freshly deposited filtered tailings can be challenging during the wet season. High rainfall onto a partially compacted dry stack surface can create a saturated surface layer that prevents equipment access. Operational scheduling and drainage management are essential to maintain productivity throughout the year.
Water Management in Tailings
Water management is the central operational challenge of a conventional or thickened tailings facility, and a major logistical consideration even for paste and dry-stack facilities. A TSF water balance accounts for all inputs—tailings slurry water, direct precipitation, surface runoff from the catchment—and all outputs—decant reclaim, evaporation, seepage, and water stored in the tailings voids. If inputs consistently exceed outputs plus the void water storage capacity, a surplus of free water accumulates in the pond, increasing the hydrostatic head on the embankment and flood risk in extreme weather events.
The primary tools for controlling the TSF water balance are: (1) maximising water recovery upstream by thickening tailings to the highest practical solids content before deposition; (2) efficient decant systems to return clarified pond water to the mill quickly; (3) perimeter drainage and diversion channels to intercept surface runoff before it enters the pond; and (4) seepage collection and return systems beneath and downstream of the embankment. Modern TSF designs incorporate all four of these elements; the relative importance of each depends on site hydrology, climate, and the chosen disposal technology.
In dry climates, evaporation from the pond surface can be a significant natural water removal mechanism, but it is unpredictable and cannot be relied upon as a primary management tool. In wet climates, managing extreme rainfall events—ensuring that the freeboard (distance from the pond surface to the dam crest) is always sufficient to contain a design storm event without overtopping—is one of the most critical operational requirements of TSF management. Under-estimation of design storm magnitude is a common contributing factor in TSF failures caused by overtopping.
Recycling decant water to the mill introduces the risk of contaminant accumulation in the process water circuit. Heavy metals, sulfate, cyanide, flotation reagents, and suspended solids all accumulate when water is recirculated without treatment. The permissible concentration of each contaminant must be assessed against process performance criteria (flotation reagent interference, scaling tendency in heat exchangers, etc.) and against any blowdown or treatment obligations under environmental permits (Wills and Finch, 2016).
Tailings Dam Safety: Lessons from Failures
The global record of TSF failures is both extensive and instructive. The Global Tailings Review, convened after the Brumadinho disaster, documented that major TSF failures occur with alarming frequency worldwide, and that the consequences in terms of loss of life, environmental damage, and liability costs are often catastrophic and irreversible.
The Mariana disaster (November 2015, Samarco Mine, Brazil) involved the sudden failure of an upstream-constructed iron ore tailings dam, releasing approximately 60 million cubic metres of tailings slurry into the Rio Doce watershed. Nineteen people died, and the environmental contamination extended to the Atlantic Ocean 650 km downstream. Investigation revealed that the dam had been raised using the upstream method without adequate attention to the stability of the underlying consolidated slimes, and that the piezometric levels within the embankment were higher than safe limits (Wikipedia, 2024).
The Brumadinho disaster (January 2019, Vale S.A., Brazil) killed 272 people when an upstream dam at the Cortégo do Feijão mine collapsed with no warning, despite having passed recent safety inspections and carrying monitoring instrumentation. Scientific investigation established that the mechanism of failure involved the gradual enlargement of internal slip surfaces within the dam body through a process of creep deformation under sustained static loading, reaching a critical length at which rapid propagation and slope failure occurred (Nature Communications Earth & Environment, 2023). The dam had been idle for three years before failure, demonstrating that static liquefaction risk in upstream dams does not diminish with time and cannot be resolved by simply stopping operations.
The shared lessons from these and other failures are clear: upstream construction should be avoided wherever possible; piezometric levels within embankments must be actively monitored and controlled; independent third-party review of dam stability must be genuinely independent, not a rubber-stamp exercise; and downstream communities must be given effective warning systems and evacuation plans commensurate with the consequence class of the dam. The Global Industry Standard on Tailings Management (GISTM), published in 2020 by the International Council on Mining and Metals (ICMM), establishes a framework of mandatory good practice including consequence-based classification, annual engineer-of-record reviews, and public disclosure of facility information.
Beyond design and monitoring, organisational culture plays a decisive role. Research examining successive failures in Brazil identified that the lessons from Mariana were not effectively learned and implemented before Brumadinho, pointing to systemic deficiencies in incident investigation, knowledge transfer, and regulatory enforcement that transcend any individual technical failure (ScienceDirect, 2023).
Tailings Reprocessing and Valorisation
The most beneficial outcome for legacy tailings is their conversion from a liability into a resource. Several pathways for tailings reprocessing and valorisation are commercially practiced or under active development.
Mineral Reprocessing
Older tailings produced when recovery technology was less efficient often contain residual metal values that are economically recoverable with modern processes. Gold tailings from operations using gravity concentration before cyanidation may contain recoverable gold; copper tailings from early flotation circuits often carry residual copper at grades that would justify processing today. The cost advantages of tailings reprocessing over mining new ore are significant: excavation and comminution costs are low because the material is already fine-grained and available at surface; infrastructure is often in place from the original operation; and there are no mining permits or waste disposal obligations for the reprocessing residue beyond those already applicable to the existing tailings facility (Wills and Finch, 2016).
The declining average grades of new ore deposits globally are making historic tailings progressively more attractive as a complementary feed source. Where mine waste dumps and tailings from the pre-flotation era of mining (pre-1920s in some districts) can be located and characterised, they may contain sufficient metal to justify formal resource classification and economic evaluation.
Construction Materials
Coarse tailings fractions are widely used as aggregate for mine roads, dam construction fill, and cemented paste backfill for underground stopes. Dense media separation float fractions of 20–30 mm particle size can be sold as railway ballast or road base aggregate. Fine-grained tailings can be incorporated into mortar, concrete blocks, or engineered fill, provided that geochemical testing confirms an absence of leachable contaminants at concentrations of regulatory concern. The use of tailings as construction material in the mine site or for sale to local construction markets reduces the volume of material requiring long-term containment and can generate a positive revenue stream.
Phytomining and Biotechnology
Hyperaccumulator plants that selectively take up specific metal ions from soil into their above-ground biomass offer a long-term, low-disturbance route to recovering metals from tailings surfaces and leachate-contaminated soils. Nickel, cobalt, zinc, and certain rare earth elements have received the most research attention. While phytomining is not yet a commercially mature technology, it represents a potentially cost-effective option for tailings with low residual metal grades and for concurrent rehabilitation of tailings surfaces during the operational and post-closure phases of a mine.
Environmental Management and Closure
Long-term environmental management of a TSF does not end when the mine closes; it continues for decades or centuries, depending on the geochemical reactivity of the tailings. Closure planning must be initiated at the feasibility stage of a project, not as an afterthought when operations cease.
Acid Rock Drainage Prevention
ARD is generated when sulfide minerals are exposed to oxygen and water. The fundamental strategy for preventing ARD is to maintain one of the three prerequisites for oxidation—oxygen, water, or the sulfide mineral—below the threshold needed to sustain the reaction. In humid climates, water saturation is the preferred approach: a water cover maintained above the tailings surface limits oxygen diffusion to orders of magnitude below the rate in air. Soil and engineered covers that maintain a high internal water content (capillary barrier covers, multilayer covers including a low-permeability layer) are the alternative for above-water-table storage. In arid climates, exclusion of infiltrating water through impermeable caps reduces both oxygen and water transport simultaneously.
Static and kinetic tests characterise the acid-generating potential and the neutralisation potential of tailings before disposal, allowing designers to quantify the net acid generation potential and predict whether long-term ARD is likely. Where ARD generation cannot be prevented, treatment of contaminated drainage by lime neutralisation, constructed wetlands, or biological sulfate reduction is required for the foreseeable future.
Cyanide Management
Tailings from gold processing operations that use cyanide leaching carry cyanide and its metal-cyanide complexes. Natural degradation of cyanide by photolysis and biological oxidation occurs in open tailings ponds, but in cold climates or where ponds are ice-covered, this process is too slow to ensure safe discharge concentrations. Active cyanide destruction—by alkaline chlorination, the SO2/air INCO process, hydrogen peroxide oxidation, or ozone treatment—can be applied to tailings or decant water before release to the environment. International cyanide management standards, including the International Cyanide Management Code (ICMC), provide a framework of best practice for cyanide handling, monitoring, and emergency response.
Rehabilitation and Closure
Progressive rehabilitation—revegetating and topsoil-capping completed sections of a TSF while other sections remain active—is both good environmental practice and a financial strategy for managing the closure liability. Dry-stack TSFs are particularly amenable to progressive rehabilitation because the stable, compacted surface of a completed cell can receive topsoil and seed immediately. Conventional slurry TSFs must consolidate and dry before topsoil can be placed, a process that may take several years after final tailings deposition ceases.
Financial assurance for TSF closure—the regulatory requirement that mining companies demonstrate they have the funds to close and rehabilitate their TSFs even if the mine goes bankrupt—is increasingly required by regulators in most jurisdictions. The history of orphaned and abandoned mine sites with inadequately closed TSFs is a powerful argument for robust upfront financial provisioning. Modern TSF design explicitly quantifies the full life-cycle cost of the facility including closure and post-closure monitoring, allowing that cost to be reflected in the mine financial model from the outset.
References and Further Reading
- Wills, B.A. & Finch, J.A. (2016). Wills’ Mineral Processing Technology, 8th Edition. Butterworth-Heinemann/Elsevier.
- Pacheco, E. et al. (2023). Dry Stacking of Filtered Tailings for Large-Scale Production Rates over 100,000 Metric Tons per Day. Minerals, 13(11), 1445. MDPI.
- McLanahan. (2023). Dry Stack Tailings: An Alternative to Conventional Tailings Management. McLanahan Corporation.
- Tailings.info. (2023). Dry Stacking of Tailings (Filtered Tailings). University of British Columbia.
- Mongabay. (2024). Scientists now know how the Brumadinho dam disaster happened, and the lessons to learn.
- Sella, G. et al. (2023). Accident investigation and lessons not learned: AcciMap analysis of successive tailings dam collapses in Brazil. Reliability Engineering & System Safety. ScienceDirect/Elsevier.
- Dynaproco. (2023). Tailings Management: Dry Stacking for Sustainable Mining.
- International Council on Mining and Metals (ICMM). (2020). Global Industry Standard on Tailings Management (GISTM). ICMM, UNEP, PRI.