Tailings – Environmental Risks or Future Raw Material Resources?
1 Introduction
Our modern society, industrial production and elevated living standards are intricately linked to a secure and consistent supply of mineral resources obtained through mining or quarrying (1, 2, 3). The wisdom embedded in the saying “if you can’t grow it, you have to mine it” underscores the imperative of extracting non-renewable resources from the Earth to sustain the various facets of our technologically advanced, so-called modern civilization. Products derived from extractive mining industries form the foundation of essential items, ranging from mundane paints and metals to cutting-edge computers, ceramics, airplanes and construction materials (4). Consequently, the mining industry contributes significantly to economic prosperity and is integral to sustainability efforts (5). Amongst the raw materials, there is a group of mostly metals classified as critical raw materials (CRMs) by the European Commission which is integral to digital technologies, low-carbon systems, and sustainable mobility (6).
The outputs of the mining industry can be broadly classified into economic and non-economic products. Non-economic products, often referred to as waste, are an inevitable by-product of mineral extraction (7). Mine tailings, largely composed of particles in the range of 2 to 75 µm, are typically disposed into impoundments without further treatment. These tailings consist of a complex mixture of metals, water, chemical reagents and fine-grained solid debris (8, 9, 10, 11). The ratio of tailings to concentrates is extremely high, averaging around 200:1 (12). Today, the world produces > 10 Bt of tailings annually (13) since from many ores only ounces or pounds of target minerals are extracted. Projections indicate even a potential doubling of this volume by 2035 (14). One reason for this is by the wanted, but metal-intensive, low-carbon energy systems so that a dramatic rise in metal demand (15) and thus in accumulation of tailings has to be expected.
The disposal of tailings in impoundments poses environmental risks, triggering soil, water and air pollution in the surrounding mining areas (16). One alarming risk of mine tailings is the potential for tailing dam failures, if wrongly constructed or if unforeseen accidents happen, as witnessed at the Brumadinho iron ore mine in Brazil in 2019. These failures can result in the discharge of thousands to millions of cubic meters of fluids into the surrounding environment, causing loss of human and animal life and inflicting extensive damage on ecosystems (17).
However, mine tailings can emerge as valuable sources of construction materials, including chemically bonded ceramics and artificially granulated aggregates (18, 19). This review paper focuses on understanding and thoroughly exploring the double role of mine tailings – as both risks to the environment and as possible resources of metals like CRMs.
2 Environmental impacts of tailings
The mineralogical and geochemical composition of tailings derives first from ore, host and wall rock’s characteristics and second from the mineral processing treatment they underwent (20). The primary method for obtaining concentrates, on an industrial scale, is froth flotation (21). For the hydrophobic-hydrophilic separation of the “valuable” minerals from gangue, a variety of chemical reagents are needed (21, 22), with many of them not environmentally sound.
Currently, there are environmental issues related to tailings management worldwide. Despite progress in technologies and operations, tailing spills still happen, like the Mount Polley case in Canada (2014), the Fundao Samarco case (2015) and the Corrego do Feijao Brumadinho case in Brazil (2019) and Jagersfontain case in South Africa (2022) (8), whereby tailings failures are mainly due to:
1) human factors like dam failures and poor management (23), or
2) natural causes such as floods, earthquakes, and alluvial events (24).
Two models of tailing dams exist: the conventional downstream model and the upstream model, whereby the later is considered as more cost-effective yet less safe due to reduced stability post-operation (4, 25, 26).
In the following, the environmental impacts caused by tailings storage facility accidents are briefly described based on two cases: the polymetallic tailings of the former Caudalosa mine (Peru) and the gold tailings of the Las Palmas mine (Chile).
2.1 Polymetallic tailings in the Escalera River, Huancavelica Region (Peru)
2.1.1 Description of site
The Huancavelica Region, located in the High Andes of Peru (Figure 1A), is characterized by livestock grazing in lowlands and inter-Andean valleys. Agriculture coexists with mining activities, primarily polymetallic, producing Cu, Zn and Pb through flotation processes, leading to the generation of sulfide tailings (21, 27). Currently, there are > 1 Mha under mining concession. Mining company Minera Caudalosa S.A., operational in Huancavelica for over 50 years, had three tailings disposal sites (tailing dams A – C) (21).
2.1.2 Causes of the tailings spill event
On 25th June 2010 tailings dam A failed first (Figure 1B). The spill became beyond control when streaming into tailings storage facility C, which stored acid mine water/drainage (AMD) (21). This mixture of tailings and AMD flowed uncontrollably into the Escalera river, a tributary of the Huachocolpa river, and advanced towards the plain of the Huachocolpa peasant community. Overall, the sediment load was transported downstream for ~120 km until reaching the Mantaro river (Figure 1C) (21). The dam’s failure was mainly because of
1) the use of coarse tailings material for the dam construction without reinforcement;
2) the lack of a waste-water evacuation system; and
3) the dam’s overload due to excessive stored water (21).
2.1.3 Effects of spill/pollution
Sampling in the spill area could be realized only five days after the event because of the topographic conditions. The spilled tailings, estimated to be 10 % of the facility’s total, reached a discharge of 57,000 m³ (21). The environmental impact on water resources was profound, as 40 % of the towns (~4,210 people) in the area rely on the river water. Analyses of Escalera river water revealed As (≤110 mg/l / 0.01 mg/l), Zn (≤55 mg/l / 3 mg/l), Mn (≤7.9 mg/l / 0.5 mg/l), Pb (≤0.95 mg/l / 0.01 mg/l) and Cd (≤0.225 mg/l / 0.003 mg/l) concentrations far exceeding the WHO regulatory standards for drinking water (in cursive (21)). The measured low pH values (2.5 to 3.2) were a clear indication of the introduction of AMD from the tailings into the water resources (21). The spill’s consequences extended to soils and agricultural crops, with tailings infiltrating irrigation channels. Soil samples revealed elevated concentrations of Ni (≤ 15 mg/kg / -), Cu (≤ 546 mg/kg / -), Zn (≤ 5526 mg/kg / -), As (≤ 822 mg/kg / 50 mg/kg), Pb (≤ 1195 mg/kg / 70 mg/kg), and Fe (≤ 46,125 mg/kg / -), exceeding the Peruvian ECA agricultural soil standards (in cursive (21)). Air pollution due to particulate matter from the tailings spill was not extensively documented, but the potential for resuspension of tailing particles with metallic species in colloids raised concerns (28) because granulometric measurements showed that 25 % of all tailing particles are ≤ 10 μm and 12 % are even ≤ 2.5 μm in size (21). Consequently, an amount of 1.5 t of tailings, deposited in proximity to human settlements and vulnerable to being emitted by wind (21) had to be considered as a potential future risk capable of provoking respiratory problems (29).
The spill severely affected the aquatic ecosystem, making the waters of the Huachocolpa river and connected rivers unsuitable for use. Agricultural activities were reduced (30), trout and aquatic plants suffered, impacting the broader ecosystem. The spill had devastating effects on livestock and crops, affecting 3,300 head of cattle and 40 ha of various crops (21). Specific studies on the human health effects related to this case study lack, however, existing literature (31, 32, 33) on populations residing near tailings offer pertinent insights.
2.2 Gold tailings in Las Palmas Creek, Maule Region (Chile)
2.2.1 Description of site
The Los Ladrones creek and the Las Palmas creek are the major tributaries of the Maule river in the commune of Pencahue, situated in the Cordillera de la Costa in the Maule Region in Chile (Figure 2A) (21). Pencahue experiences diverse activities like agriculture, livestock and vineyards. Gold mining activities, notably in the 1980s and 1990s by Minera Las Palmas, marked this region (21). The daily processing throughput was in the range of 350 t/d with tailings stored near the Los Ladrones creek (21).
2.2.2 Causes of the tailings spill event
A catastrophic event happened on 27th February 2010, when an 8.8 Richter scale earthquake with its epicenter in the Maule Region led to the collapse of the Las Palmas tailings dam (Figure 2B). The seismic waves induced liquefaction in the dam, causing these tailings to behave like a muddy fluid (34). In total ~2.5 ha of the Las Palmas tailings facility with a tailings volume of ~200,000 m³ collapsed over ~11 ha of surrounding land (21).
2.2.3 Effects of spill/pollution
Water and sediment samples were collected from the creeks three days post-spill. The collapsed tailings contained i. a. machinery and structural remains, washed away during the event (35), emphasizing the severity of the spill. The water quality of the creeks was severely impacted by the spill with elevated concentrations of Pb (≤ 1.16 mg/l / 0.01 mg/l), Zn (≤ 33.70 mg/l / 3.00 mg/l), and Mn (≤ 3.06 mg/l / 0.50 mg/l) exceeding WHO regulatory standards for drinking water (in cursive (21)). Moreover, the Los Ladrones creek showed elevated concentrations of free CN (0.59 mg/l / 0.07 mg/l). In soil samples elevated concentrations of Zn (≤ 11,389 mg/kg), Cu (≤ 612 mg/kg) and Pb (≤ 8023 mg/kg) were found (21). Effects on flora and fauna were not explicitly documented, but studies on tailing spills at the El Teniente mine indicated potential toxic effects on agricultural crops due to Mo and Cu (36). Moreover, cyanide, despite its rapid reaction and degradation in the environment, is highly toxic to many living organisms even at low concentrations (21, 37). The most immediate impact on humans was the death of four members of the Gálvez Chamorro family (21). The event affected the water use for irrigation and residential purposes. While short-term exposure to metals did not indicate direct health damage, the potential long-term effects required ongoing monitoring and assessment (38).
3 Tailings as a future raw material source
There are at least ~18,000 known tailing dams worldwide, with 3,500 being active (21). These dams vary significantly in volume, with some smaller sites containing only few 1,000 t, while larger complexes can exceed several 100 Mt (12). The age of these facilities is quite varied, with the oldest industrial tailings dating back to the late 19th century while new tailing dams continue to be constructed as modern mining operations persist (4). The EU holds more than 4.7 Bt of mining wastes and 1.2 Bt of tailings. In general, over half of the mining sites in the EU are now closed, except for few metal and industrial mineral sites, plus quarries (39).
The European Directive 2006/21/EC on extractive waste management oversees the permit conditions, storage, monitoring and control of produced waste to protect human health and the environment. Furthermore, EU policies aim to reduce waste disposal in facilities and landfills by promoting waste recovery and recycling in the extractive industry, in line with circular economy principles (39). Consequently, waste is seen nowadays as a resource, and waste facilities and landfills are considered as secondary raw material resources to be sustainably exploited (40).
Recently, CRMs have gathered considerable attention due to their essential role in manufacturing equipment for the green evolution (6). Even though the concentration of CRMs in mine tailings is generally considered as low, the large volumes of these tailings alone make reprocessing attractive. Moreover, the depletion of CRMs in primary sources and the intrinsic supply risks due to geopolitical issues and geographical segregation have become major concerns. Therefore, recovering CRMs from non-conventional sources, like abandoned mine tailings, has recently gained attention worldwide (41). Moreover, CRMs can be reprocessed simply as by-products, e. g. Co and In in sphalerite (ZnS), as many of them do not form individual minerals but sit in other mineral commodities – ore and gangue minerals – and are thus automatically extracted.
There are two main reasons for extracting CRMs from old and abandoned tailings.
- Mining costs are significantly reduced since the ores have already been collected and partly processed.
- The likelihood of finding valuable minerals at economic concentrations is higher in older tailings due to less efficient past technologies (41).
In some cases, the concentration of valuable minerals in old mine tailings may surpass even that of primary ores (16). Additionally, reprocessing tailings for mineral recovery can reduce the volume of tailings to be managed and the concentration of potentially toxic elements (41).
4 Technologies for recovering critical minerals from mine tailings
Recovering minerals from secondary sources, such as mine tailings, often requires modified processes to those used for primary sources due to lower mineral concentrations and different material characteristics. Several processes are utilized to extract minerals from tailings, with most belonging to the hydrometallurgical category (41, 42). These processes include solvent extraction, acidic leaching, liquid-liquid extraction and bioleaching, all of which employ aqueous chemistry to recover metals from ores, concentrates and recycled or residual materials at relatively low temperatures. In the following section the main applied hydrometallurgical techniques and newest research outcomes to recover selected CRMs (Co, Cu, Li, rare earth elements (REEs)) from mine tailings are briefly described.
4.1 Hydrometallurgical extraction
Various hydrometallurgical processes have been employed to recover REEs from secondary sources. These processes typically involve beneficiation to produce concentrates, followed by acid leaching and solvent extraction. In particular solvent extraction, based on blending solvating and acidic extractants (43), has been effective in selectively recovering REEs despite the presence of impurities (41). Moreover, recent advancements include promising techniques for recovering Co and REEs from leach solutions, which can be adapted for tailings reprocessing (44). Lithium recovery from mine tailings is a relatively new concept as Li is extracted traditionally from brines and primary minerals like spodumene (45). For bauxite mine tailings, leaching with mixed acids alone has been proven as effective to recover Li; however, with the drawback of high energy and chemical consumption (46).
4.2 Bio-hydrometallurgy technique (bioleaching)
Bio-hydrometallurgy or bioleaching is based on the interactions between microbes and metals in minerals (41), whereby the microorganisms convert insoluble metal sulfides into water-soluble sulfates through biochemical oxidation so that the target minerals/elements can be extracted (47). This offers the distinct advantages of reduced operational costs and lower environmental impacts, making the technique effective for low-grade sources (47, 48). Consequently, bioleaching is now used in various industries related to metal extraction and waste remediation to recover valuable metals from low-grade industrial wastes and sewage sludge (49, 50, 51). However, bioleaching is slower than conventional methods and toxic chemicals, such as sulfuric acid, posing environmental risks like AMD, can be generated. A study at the Rammelsberg mine has shown that bacteria are capable to recover Co and Cu even from these complex tailings (41). Moreover, bacteria are capable of leaching REEs and other metals from uranium mine tailings (52) and low-grade gibbsite ore (53).
4.3 Membrane Process
Pressure-driven membrane processes, such as reverse osmosis (RF) and nanofiltration (NF) have been utilized for the recovery of strategic metals and CRMs. These methods offer numerous advantages over conventional hydrometallurgical processes, including higher separation rates, material selectivity, lower energy requirements, simpler operation, reduced waste generation and ease of integration with other processes. However, the processes are energy intensive leading to higher operational costs and environmental concerns. (41) A major drawback is the impaired performance of membranes due to excessive fouling, which reduces their long-term usability. This limitation may be partially mitigated by combining NF with conventional flotation, magnetic or gravity separation, which can reduce the concentration of gangue materials in the feed solution and thus reduce fouling issues (54). So far, studies have shown the recovery of Li from brines using NF and RF membranes (55, 56, 57, 58, 59), whereas this technique is less applied in the recovery of CRMs from tailings.
5 Challenges
The recovery of strategically important metals from tailings faces some key challenges, including technical, economic and environmental issues. These challenges are influenced by various underlying factors that directly impact the recovery processes, which are briefly described in the following.
5.1 Technical challenges
The primary challenge in recovering CRMs from tailings is their low concentration as mainstream technologies are typically designed for higher grade primary ores. Applying these technologies to tailings, resulted so far i. a. in low recovery rates for REEs and Li (41, 60). The reason for this is often by the presence of base metals and other competing ions in higher concentrations hindering the separation processes of trace CRMs (e. g., (41, 61, 62)).
5.2 Environmental challenges
Environmental challenges in CRM recovery from tailings are complex and site-specific, varying with the characteristics of each tailings dam. However, tailings reprocessing includes always the production of a concentrate, beneficiation and separation of target minerals, which can lead to further urban land occupation, natural land transformation, ecosystem disruption and human health issues (41, 63). In particular the high energy consumption and the extensive use of water and chemicals, which are often toxic and which can affect water, air, land and human health, are seen critical (64). Although tailings reprocessing is considered less polluting than mining in general, CRM recovery of tailings generate inevitably solid wastes (releasing dust), wastewater and off-gas (65, 66). In general, mine tailings often contain toxic elements like As, Cd, and Cr (67) and can contain radioactive elements like Th and U (68, 69). These elements can be transported via wind, surface and groundwater, contaminating soil and sediment (70). Moreover, some tailings contain pyrite, which can generate AMD. After reprocessing most of the tailings return to storage facilities unless recycled for other purposes, like construction materials or soil amendments.
5.3 Economic challenges
The mining industry traditionally follows a linear economic model, exacerbating issues related to mining waste management (71). Reprocessing tailings for mineral recovery incurs significant costs associated with resources such as water, energy, chemicals, labor, mineral and solvent losses and waste disposal (e. g., (72)). Studies indicate that energy consumption increases exponentially as ore grades decrease (41, 73). Generally, recovery costs are inversely proportional to the concentration of target minerals or simplified lower concentrations demand larger plants and more resources, leading to less economic output and potentially a negative balance. Moreover, recovering CRMs from mine tailings typically involves multi-step processes, increasing overall recovery costs. Losses of target minerals in these processes can be substantial (e. g., (74)). In addition, market prices of CRMs significantly influence recovery success, as many CRMs are by-products or companion products. Therefore, fluctuations in the market prices of their host minerals can impact CRM prices and overall economics (41).
6 Discussion
The environmental impact of mining, particularly through the handling and storage of tailings, is a significant global concern. Mining activities, whether active or abandoned, produce substantial environmental hazards, including AMD. While AMD can lead to the dispersion of metals into ecosystems, affecting water, soil and air quality, tailing dams can emit particulate matter and gases, which, in turn, can lead to socio-cultural disruptions in nearby communities.
Recent cases from around the world point out the detrimental consequences of tailings storage facility accidents. Incidents in the Escalera river (Peru) and the Las Palmas creek (Chile) exemplify widespread water contamination, soil pollution, air quality issues and severe impacts on flora, fauna and human health. These failures have been first due to human errors (an inadequate tailings dam management) and natural causes (earthquake).
Tailing spill accidents introduce various chemical species, including metals and other pollutants, into aquatic environments. These pollutants influence surface water composition and can be mobilized by changes in environmental conditions (pH, Eh). Certain chemical forms of metals are toxic to organisms. Metals like As, Pb and Hg in tailings pose severe threats to aquatic life, drinking water sources and human health due to their potential to leach into nearby water bodies. Therefore, in the event of a tailings spill, immediate contingency plans are necessary to protect ecosystems and human populations or in other words the integrity of tailing dams is a significant global environmental concern.
Despite these environmental challenges, resource recovery from mine tailings presents a viable solution to mitigate risks while providing economic benefits. The process begins with a careful selection of mine sites with tailing-storage facilities that potentially contain still valuable minerals. This selection relies on historical data about the types of ores mined, metals extracted, extraction methods used and the nature and volume of the tailings. Such factors help to identify tailing facilities with significant economic potential.
Following the initial selection, a detailed characterization of tailing samples is essential to determine their chemical, physical and mineralogical properties. An extensive sampling campaign covering the whole dam is crucial for evaluating the economic potential of CRM recovery. The concentration of target minerals obtained from this characterization aids in assessing the project’s economic viability.
Economic viability also depends on market prices for recoverable minerals. An integrated approach that combines economic, social and environmental benefits is essential. Methods like bioleaching, which are cost-effective and environmentally friendly, should be considered in the recovery process design. Additionally, incorporating water recovery and reuse into the mineral recovery system is crucial.
Advanced technologies such as membrane processes offer effective wastewater treatment during tailing reprocessing. These technologies ensure that wastewater can be safely discharged, minimizing environmental impact. A detailed techno-economic assessment is recommended as part of the feasibility study for any mineral recovery project from tailings.
Significant opportunities exist in CRM recovery from mine tailings despite the challenges. The primary opportunity lies in the economic benefit from effective recovery. Addressing issues such as environmental impacts from resource and chemical use and the cost-effectiveness of separation processes, can lead to both socio-economic and environmental benefits. Reprocessing abandoned mine tailings reduces costs due to lower water and electricity needs compared to conventional mining and can utilize existing ore processing plants, saving on capital investments. This process also creates employment opportunities for local communities.
Environmental benefits include reducing pollution from dust dispersion and water seepage, decreasing tailing volumes and mitigating the risk of tailing dam failures. Dewatering tailings and dry stacking after mineral recovery reduce economic, environmental and society related risks associated with slurry-stored tailings. Furthermore, reprocessing tailings allows for additional valuable mineral production without increasing the environmental footprint, unlike new mining operations.
7 Conclusion
Tailings present both environmental risks and potential as future raw material sources. Effective management and reprocessing of tailings can mitigate environmental hazards while providing economic and social benefits. The development and application of advanced technologies tailored to the characteristics of tailings can enhance the recovery of valuable minerals, promoting a circular economy. Addressing the challenges associated with tailings management requires an integrated approach that balances economic viability with environmental and social responsibility. Through careful selection, characterization and innovative processing techniques, mine tailings can be transformed from a liability into a valuable resource, contributing to sustainable development and environmental protection.
Acknowledgment
This paper shows the outcomes of the MSc student research project of the corresponding author prepared at the Institute of Mining, Department of Surface Mining and International Mining at Clausthal University of Technology. Thanks to Jörg Bertram for the careful revision of our slightly rusty German.
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