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Mine tailings storage, hazards and impacts, and risk management

09.01.2019

Dr Stephen Edwards, UCL Hazard Centre, Department of Earth Sciences, University College London, UK

Global demand for minerals leaves a legacy of residues that must be carefully stored and managed. The most challenging of these are mine tailings, which are voluminous mixtures of finely ground rock and fluid from mineral processing. Throughout the world, including many parts of South America, tailings present a hazard owing to their modes of storage, fluid content, mineralogical and chemical forms, and concentrations of potentially toxic and acid-generating species. Constituents of tailings may enter air, water, sediment and soil, whereupon they may cause death, injury or poor health, and negatively impact the environment and its ecosystems and natural resources. Clearly it is crucial that tailings are securely stored and isolated from the environment for the long term.

Safe disposal of tailings is achieved to varying degrees of success through a variety of mechanisms, including submarine and riverine disposal, storage in wetlands, backfilling, dry stacking and dammed impoundments. There is a legacy of abandoned or poorly managed tailings that are of great concern, but increasingly stringent requirements mean that dammed impoundments are now widely used, particularly by large mining operations. These structures are often referred to as tailings storage facilities (TSFs) and the majority contain slurries, but paste, thickened and filtered tailings are also stored. The impoundment wall is normally constructed from the coarse sand fraction of tailings, with finer fractions deposited in the reservoir behind and covered by water to suppress dust and acid-generating reactions. After construction of the initial starter dam, the wall is progressively raised in a series of lifts, where the crest moves either upstream, vertically (centreline) or downstream.

Perhaps the greatest risk to any mining operation is a tailings failure, because of the subsequent environmental, human, economic and political impacts. Such failures do occur and it is estimated that each year there are an average of two reported and two unreported tailings incidents. A large failure today could lead to direct losses and clean-up and class action costs of several US$100 million, as well as huge reputational damage and loss of investment, all culminating in a high probability of permanent shutdown of the entire operation (e.g., the 1998 Aznalcóllar−Los Frailes disaster in Spain). The main causes of failures globally are extreme rainfall and flood, followed by structural and foundation conditions, earthquakes, poor management, seepage and overtopping. Several studies suggest that active rather than inactive facilities are most likely to fail, with the upstream structure most vulnerable.

The scales and types of impacts that tailings failures may have were recently exemplified by the 2014 Mount Polley (British Columbia, Canada) and 2015 Fundão (Minas Gerais, Brazil) disasters. These events highlighted that renewed efforts are required to better manage TSFs through design, maintenance and monitoring. Rapid responses to warning signs are essential, particularly as most failures, other than those initiated by earthquakes or extreme weather events, are usually preceded by detectable signals. Furthermore, risk management must go beyond the TSF itself and include all stakeholders exposed to tailings hazards. Mine operators, government and non-government bodies, and the public are usually aware of the hazards, but their approaches to managing the associated risks may be fragmented, confrontational and reactive. A new era of forward-looking tailings risk management is therefore urgently required, which involves multi-stakeholder engagement built on relationships of trust, transparency and inclusivity.

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