by Ina King (Potgieter) June 06, 2019
Most people have no idea what a mine tailings dam is – until it fails. When they go wrong, the results are catastrophic. As the number of mine tailings dams in the world increase, it’s time to find out what they are and how to make them safer.
With ore grades in general decline globally and deposits being steadily depleted, management of tailings dams is becoming an ever-bigger challenge. The mining industry needs more ore to yield the same amount of metals and minerals, which results in increased tailings production. With increased tailings, the need for larger tailings storage facilities and taller dams to contain them arises. Mineral residue management is crucial.
Tailings are the waste products from mining. Rock is ground into a fine sand to extract the valuable mineral or metal from the ore. The remnants, rock particles, chemicals, minerals and water, are waste. Tailings can be liquid, solid or a slurry of fine particles. Many substances found in tailings are toxic – large quantities of cyanide, mercury and arsenic are often found in tailings.
Tailings dams are used to store water and waste that come as by products from the mining process. Tailings dams can be massive – the size of a lake reaching up to 300 metres high. As the slurry of waste is piped into the dam, the solids settle to the bottom and the water is recycled to be used in the separation process again. Tailings dams use earth or rock to create a barrage. They are continually raised to accommodate more waste which makes them more prone to leakage. The contaminated waters of tailings dams can pose a threat to local wildlife and there can be leakage of toxic substances from the dams, so regular monitoring and maintenance are critical.
There are an estimated 3 500 active tailings dams around the world, although there is no complete inventory, and the total number is disputed. The dams experience known "major" failures two to five times annually, along with 35 "minor" failures. Over the past century, tailings dam and ash pond failures, and the resulting fast-moving mudflows, have led to a cumulative loss of almost 3 000 lives. There have been 32 recorded major tailings dam failures between 2008 and 2019. The most recent is the devastating collapse of Vale’s tailing dam in Brumadinho, Brazil, which caused the death of hundreds of people after millions of tons of muddy sludge engulfed homes and roads. Tailings dam collapses and failures have catastrophic social, economic, and environmental consequences.
The Brumadinho dam was closely observed. Four months before its collapse, it was audited and declared stable. Contrary to the expectation that geotechnical structures become more stable over time, the dam failed three years after closure. Abundant laboratory tests, in situ tests, and monitoring data were available and the dam was considered stable. That it failed suggests gaps in the scientific understanding of waste-storage facilities and of any time-delayed triggers causing failure.
The South African Department of Minerals and Energy (DME) commissioned an independent group, the South African Bureau of Standards (SABS), to develop a Code of Practice for Mine Residue Deposits. The Code of Practice (COP) is intended to address the life cycle of residue deposits, including tailings, in terms of their safety, environmental considerations, construction and management. The COP contains fundamental objectives, principles and minimum requirements for good practice, all aimed at ensuring that no unavoidable risks, problems and/or legacies are left to future generations.
Anglo American is implementing industry first techniques that provide information on the amount of water in tailings dams and fibre-optic installations to provide real-time monitoring of strain, deformation and seepage. Anglo’s mine in Peru is introducing micro-seismic monitoring of tailings dam foundations to observe geological and structural features in the dams and their foundations. Anglo’s ultimate aim is to fully eliminate tailings dams.
Given the number of tailings dams around the world, and their historical failure rate, more failures can be anticipated. Each tragic incident helps us gain better insight, improve engineering practices, and implement regulations to minimise the potential for future catastrophes.
Extracting metals traditionally regarded as waste from tailings dumps can increase the working life of existing mines as well as breathe new life into long-abandoned mine sites. Converting tailings dumps into mineral resources is a way to protect the environment from the often toxic wastes that seep from abandoned mines and tailings dams, contaminating the water and soil (acid mine drainage).
Our need for commodities like lithium, indium and cobalt for the rapidly diversifying electronics industry is growing rapidly. Fortunately, a brand-new mine isn’t the only way to get lithium. New technology has made it possible to recover lithium from lower-grade materials. Now, instead of being simply dumped, mine tailings can be re-mined. Tailings dumps created from the processing of platinum group elements (PGEs) are being re-treated to recover chrome and vice versa. According to Deloitte, any tailings dump can be considered a potential mineral asset as long as it contains economic quantities of the mineral or metal of interest.
There are two main reasons why re-mining tailings dams may be preferable to developing new mines. Firstly, there’s a reduction in mining costs because these materials have already been extracted from the ground. Secondly, many older mining techniques had lower recovery rates than today’s technology. So, the older the mine the greater the proportion of the target commodity that is likely to be left in the tailings.
Tailings dams are assets, not liabilities. Viewing them as such is critical for the industry and the environment. Treating waste as a potential resource will allow for tailor-made, environmentally conscientious management strategies to be developed. Re-mining tailings could make more financial and environmental sense than other rehabilitation options. Finding treasure in the trash could be the answer the mining industry is looking for.
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