This week marks the one-year anniversary of the Samarco Fundao tailings dam failure. This event, which cost the lives of 19 people, destroyed the town of Bento Rodrigues, and caused severe pollution along 600 km of the Rio Doce, was one of the worst tailings failures in history.
A comprehensive study was undertaken to determine the causes of immediate failure, and this is publically available on the Fundao Investigation website. The report found that the failure was due to a liquefaction flowslide, triggered by three minor earthquakes. However, the root cause was linked to construction defects in a critical drain of the starter dam in 2009.
To recap, tailings are the waste product produced from minerals processing plants after mineral or metal extraction. They contain solids and water. Solids particle size may vary from a sand to slimes consistency depending on the ore and processing method. Associated water may be contaminated by reagents used in processing (e.g. cyanide) or reaction products from contact with tailings solids (e.g. acid rock drainage). Tailings are placed in tailings storage facilities (TSFs), usually held back by large embankments. Safe containment is the critical design consideration, as failures can cause fatalities, destruction of assets and severe environmental damage. Embankments can broadly be built using one of three methods: upstream, downstream and centerline construction. These, along with advantages and disadvantages, are outlined in the figure below.
The Fundao dam used the higher risk upstream method, and sands fractions of tailings were used in the embankment construction. In this case, construction defects in the starter dam, combined with insufficient control of the deposition of sands and slimes fractions over several years left an embankment susceptible to failure. To avoid failure, upstream embankments require considerable control over placement of sand and slimes fractions into the TSF, and limits to the rise rate (rate at which the embankment is built up) to ensure proper compaction and drainage. Some jurisdictions (e.g. Chile) ban the upstream method due to unacceptable risk of failure especially due to earthquakes.
Unfortunately, Fundao is not a one-off: many failure events have occurred in recent decades, across different commodities and regions. WISE has compiled a database to record these. Some significant events (including fatalities in certain cases) include:
- Base metals: e.g. Mount Polley, BC, Canada, 2014; Philex, Philippines, 2012; Los Frailes, Spain, 1998
- Precious metals: e.g. Baia Mare, Romania, 2000
- Iron ore: e.g. Fundao, Minas Gerias, Brazil, 2015; Herculano Mineração Ltda, Minas Gerias, Brazil, 2015; Mineração Rio Verde Ltda, Minas Gerias, Brazil, 2001
- Bauxite residue (also known as “red mud”): e.g. Luoyang, Henan province, China, 2016; MAL Maygar, Ajka, Hungary, 2010
- Industrial minerals: e.g. Stava, Italy, 1985
- Fly-ash from coal fired power stations: e.g. Tennessee Valley Authority, Kingston, Tennessee, 2008
Some general considerations for embankments that can be drawn from historical failures and good engineering practice include:
- Consequence of failure increases as dams become higher, as the stored potential energy behind the dam increases, and this should be considered in design and operating practice
- Accumulation of ponded water on a TSF near an embankment creates risk of piping failure in the embankment, and in general, storing large quantities of water in a TSF can increase consequences in the event of a failure; water management should include decant from deposited tailings as well as water runoff from precipitation, and appropriate water diversion, pumping and treatment facilities are required
- The quality of foundation preparation, drain and filter layers and fill materials, and quality control of fill placement, including compaction are important in ensuring the embankment functions as designed, and to avoid failures
- Dams that have their lives extended through brownfield expansions may need additional scrutiny and safety factors in design, as the final embankment may end up quite different to that which the initial designer expected
- Risk management and emergency response plans are needed for communities and assets in the line of fire of a potential tailings embankment failure
- Cost concerns from clients can potentially lead to pressure for designs that include inherent risk; the consultants need to hold to appropriate standards in these situations
- Failures can even occur to tailings dams with a safe design, in situations where the operator does not control operations to safe design conditions; consultants need to assist clients to maintain safe practice, and promptly escalate and document safety concerns internally and within client organizations to avoid potential failures.
In addition to the embankment design aspects, there are some processing considerations that can help reduce tailings related risks:
- Increasing tailings density, e.g. by using high density or paste thickening (this is one example supplier, there are several others), centrifuging or tailings filtration (dry stack), can significantly reduce the overall volume of tailings directed to the TSF, and change rheology to greatly reduce consequence and likelihood of failure
- A co-benefit of higher density tailings deposition is increased water recycling and reduced net water consumption
- Selective mining, preconcentration and ore sorting may reduce the quantities of ores processed in grinding and separation circuits, hence reducing the quantity of tailings and subsequent risks
- In certain underground mines, it may be possible to place a considerable portion of tailings mixed with a binder material (e.g. cement, flyash) as paste or hydraulic backfill, reducing the volume and risk of surface TSFs
- Also, in some surface disposal cases, binders such as flyash may be added to stabilize tailings and reduce risk of contamination of contact waters
- Other beneficial reuse opportunities may allow a significant proportion of tailings to be sold to customers, for example, agricultural lime from carbonate rich zinc ores in Tennessee, iron recovery from certain iron rich copper tailings (e.g. Palabora) or potentially producing geopolymer materials from treatment of silica rich iron tailings
- Through major flowsheet changes, e.g.change from grinding and flotation flowsheet to leaching and extraction flowsheet, it may be possible to eliminate TSFs altogether; the economic and environmental aspects of the alternative flowsheet would need to be considered to determine viability
Major tailings failures such as Fundao are a stark reminder of the risks associated with tailings. Dam safety requires appropriate expertise from civil and geotechnical engineers, who can assess both the siting and ground conditions for the embankment, and safe design, construction and operation practices. A shift away from upstream embankment construction, water management in TSFs, and greater vigilance in construction standards, operating practices and design changes over life of mine can all help to reduce risk. An integrated design approach should be considered that includes mining method, process flowsheet, TSF site selection, embankment construction method, process plant and tailings dewatering facilities siting, tailings conveyance facilities (e.g. pumps and pipelines), tailings deposition control, appropriate beneficial reuse circuits, and water diversion and reclaim features. Additional tailings dewatering, and reducing the quantity of surface tailings through mine backfill, beneficial reuse or flowsheet changes can lower risk. These considerations may help the mining industry ensure economically and environmentally acceptable outcomes, and face the increasing scrutiny from stakeholders and regulators that has resulted from incidents such as Fundao.