Tailings is a common by-product of the metals and minerals recovery process. It usually takes the form of a liquid slurry made of fine metal or mineral particles and water – created when mined ore is crushed and finely ground in a milling process.
From the mill, the tailings is typically pumped to storage facilities which are commonly constructed using earth dams. As the sandy residue of tailings gradually drains and becomes compact and dry, grass and other vegetation can be planted to stabilise the environment through a reclamation process.
Before the water in the tailings can be used again, or discharged into the local drainage system, it must be treated to remove harmful substances that would pollute the environment or risk the health and safety of local communities near the facility.
If not managed properly, tailings can have chronic adverse impacts on the environment and human health and safety, with pollution from effluent and dust emissions being potentially toxic to humans, animals or plants. Acute and potentially very damaging impacts can occur should a tailings storage facility physically fail. In such instances, flowable tailings materials can inundate and greatly impact the surrounding environment and even lead to loss of human life.
The management of tailings, both during and after mining, is the long-term responsibility of mining companies and is subject to varying regulatory regimes. Tailings management needs to be effective throughout the life of an operation, from initial feasibility through to closure and post-closure.
The type of aftercare can vary greatly depending on the nature of the tailings. In cases where tailings doesn’t contain harmful substances, water is drained from the tailings storage facility to safeguard its physical stability, and it is then reshaped, covered with soil and vegetated to create a stable landform. In other instances, longer-term measures may need to be put in place to safeguard the physical stability, chemical stability and subsequent land use of the tailings storage facilities.
The catastrophic tailings dam failures at Mount Polley (Canada) in 2014, Samarco (Brazil) in 2015 and Brumadinho (Brazil) in 2019 serve as stark reminders that there’s still more to be done if we are to reach the ambition of zero harm.
Types of Tailings Facilities
Tailings can be stored in a variety of ways: which way depends on numerous factors, for instance the local topography, how much rainfall an area gets, whether there is regular or irregular seismic activity recorded, the type of metal or mineral being mined and how close the mine is to populated areas. There is no one-size-fits-all solution, each tailings storage facility is unique.
Identifying the most appropriate method of tailings construction is important to ensure the safe and sustainable operation of a facility. Tailings storage facilities follow one of three wall construction designs: downstream, upstream and centreline.
Downstream designs start with an impervious starter dam. Tailings are then discharged into the dam and as the embankment is raised, each new wall is constructed and supported on top of the downstream slope of the previous section, so the dam crest moves downstream with each raise. The downstream design was developed for areas with seismic activity and high rainfall or water collection.
Upstream construction begins with a starter dam. The tailings are then discharged into the facility where they form a tailings beach. The deposited tailings adjacent to the dam wall is allowed to drain and then can be compacted to be used to form the foundation for subsequent levels of the wall as the dam is raised. As such, the crest of the dam moves upstream with each raise.
Upstream tailings dams need to be raised slowly, to allow the solid tailings time to dry and consolidate enough to support a new level of the dam. These are suitable for facilities in areas of low rainfall and low seismic activity.
The centreline method is a hybrid of upstream and downstream designs. In centreline construction, the dam is raised vertically from the starter dam. The dam crest therefore remains fixed relative to upstream and downstream directions as the dam is sequentially raised. Internal drainage can be incorporated to improve stability.
For the most part, tailings storage facilities have been historically well-managed with few incidents of failure; however, when they do fail, the consequences can be catastrophic for communities, local economies and the surrounding environment. Responsible tailings management requires companies to consider the management and governance of tailings storage facilities throughout their life cycle, from design to closure and post-closure.
The Global Tailings Review's Global Industry Standard on Tailings Management provides a framework for safer tailings facility management, while affording operators flexibility as to how best to achieve this. ICMM, the United Nations Environment Programme (UNEP) and Principles for Responsible Investment (PRI) convened the Global Tailings Review.