Tailings dams a relic of the past
By Heather Thomson, Senior Consultant (Tailings Engineering), SRK Consulting

A decade ago, the waste produced from mining was stored in the form of mud and water, behind embankments known as ‘tailings dams’. Then began a subtle, yet intentional, shift. Tailings storages were no longer to be referred to as ‘dams’, but instead as ‘tailings storage facilities’, ‘residue impoundments’ or – very creatively – ‘tailings retention systems’. Physically, these structures were unchanged; however, it was a matter of public perception. This subtle change was a precursor to the step change that the tailings industry is now facing, leaving tailings dams a relic of the past.

This change has been slowly driven by the global movement towards the consideration of environmental, social and corporate governance (ESG) factors, and, more recently, rapidly forced into action by the introduction of the Global Industry Standard on Tailings Management (GISTM). GISTM is the first of its kind – a global standard for tailings facilities, both existing and proposed. Stemming from the catastrophic 2019 dam failure at Vale’s Córrego do Feijão mine in Brumadinho, Brazil, GISTM was developed by a consortium of the International Council on Mining and Metals, the United Nations Environment Programme, and Principles for Responsible Investment. Released in August 2020, GISTM includes 77 auditable requirements that are organised around six topic areas and 15 principles. With an overarching goal for ‘zero harm’, the requirements target all areas of tailings management, integrating the ESG context with tailings engineering and operations.

Two and a half years since its release, the influence of GISTM is really beginning to be realised in the industry. Traditionally, mine waste storage has never been a priority for expenditure. Decisions on tailings storage were often made to minimise costs – economic costs, that is. In the early stages of project development, several different options for tailings storage would be developed and compared predominantly in terms of capital and operating costs. The options depend on a vast number of site-specific factors, but generally fall into three main categories: upstream, centreline, or downstream. Each option refers to the different methods for raising a dam. Each option has its own merits; however, the key differentiating factor generally comes down to the quantity of fill material, which is an indicator of cost.

As a rule of thumb, a downstream-raised embankment requires roughly three times more fill than an upstream-raised embankment, with centreline falling somewhere in between. This divergence becomes greater as the height of the facility increases. It should not come as a surprise that most existing tailings storage facilities are upstream- or centreline-raised dams – representing roughly 50 per cent of tailings facilities globally. The next most common Australian methods of tailings storage (as disclosed in the Global Tailings Portal) are downstream-raised dams (23 per cent), in-pit storage (17 per cent), and single-stage embankments (two per cent), with eight per cent representing other or unknown methods of storage.

Upstream- and centreline-raised facilities rely on the strength of the tailings for the structural stability of the dam. When well designed, constructed and operated, these structures should be perfectly safe; however, history shows that this is not always the case, with far too many case histories of failure – such as the aforementioned Brumadinho disaster, Mount Polley in 2014, Samarco in 2015 and, most recently, the November 2022 failure at the Petra Williamson diamond mine in Tanzania. These devastating failures have given such facilities a bad reputation, and this construction method is now under much greater scrutiny – to the point where upstream-raised tailings dams have been banned in Chile, Peru and Brazil. Cue GISTM.

GISTM requires that decisions throughout the tailings facility life cycle be informed by social, environmental, local economic and technical factors. The design of new tailings facilities must incorporate a multi-criteria alternatives analysis of ‘all feasible sites, technologies and strategies for tailings management’, including the use of tailings technologies. The goal of this analysis is to: ‘(i) select an alternative that minimises risks to people and the environment throughout the tailings facility life cycle; and (ii) minimise the volume of tailings and water placed in external tailings facilities’. That is, decisions cannot be driven purely by economic cost. Cost will always play a role in decision-making; however, GISTM requires a more balanced approach – that is, consideration of potential costs to people and the environment. The result? Mining will most likely become more expensive, but that is simply the price to pay for responsible and sustainable practice.

Existing tailings facilities are not immune. GISTM requires that, for existing tailings facilities, ‘The operator shall periodically review and refine the tailings technologies and design, and management strategies to minimise risk and improve environmental outcomes.’ An exception does, however, apply to facilities that are demonstrated to be in a state of safe closure. Perhaps this is a well-needed push for the 36 per cent of Australian tailings storage facilities classified as ‘inactive’ (i.e., neither active nor closed) towards implementing closure.

If a traditional tailings dam is not the answer, then what could our ‘tailings retention systems’ look like in the (near) future? Alternative methods of tailings storage, where previously ruled out by cost, are back on the table for consideration. These are overshadowed by potential refinements in processing to reduce the volumes of waste produced, and innovative ways to use tailings in a positive way, such as in construction materials or carbon sequestration. The most typical alternative methods for tailings storage include:

In-pit tailings storage: Tailings are stored in exhausted open pit mines, thus eliminating the risk of embankment stability, and generally at a relatively low cost. The key issues are typically centred on potential impacts to groundwater and poor consolidation of tailings.

Filtered tailings (aka ‘dry stack’): The majority of the water is removed from the tailings slurry, and the remaining ‘cake’ is placed in an above-ground pile, or ‘stack’. Often ruled out due to considerable costs associated with dewatering the tailings, filtered tailings can mitigate a lot of the risks associated with conventional storage – risks that are largely driven by water.

Co-disposal: Tailings are stored together with coarse waste (waste rock). Co-disposal can take various forms; however, the aim is generally to increase the strength of the waste stream, similarly reducing the risks associated with conventional storage as for filtered tailings.

Deep-sea tailings disposal: Tailings slurry is discharged into the ocean at depth (usually greater than 100 metres). If well implemented in suitable environments, this option can have less environmental and social impacts than conventional storage methods; however, it is often disregarded as a feasible option too quickly, and restricted by stakeholder opposition driven by a negative public perception.

It is clear that each of these methods have their own set of risks and complexities; however, if decisions are to be based on minimising risks to people and the environment, the future of tailings storage will inevitably look different. Hopefully, this will leave a better legacy for future generations, and GISTM will achieve its goal of ‘zero harm’.

Considering that a mere 50–100 years ago it was common practice to discharge tailings from the mill directly into the nearest stream, we’ve already come a long way as an industry in a relatively short time frame – but we’re about to go even further. 

Related Articles

Shifting sands

Shifting sands

By Anthony Fensom Australia’s silica sand mining industry is on a growth trajectory, with a takeover bid and a raft of new projects sparking...

read more

Be the first to find out when the next edition is released

* indicates required