Billions of tonnes of mineral processing industry wastes are contained in thousands of tailings dams worldwide. The most popular form of construction of tailings dams is by sequentially raising the structure using the ‘upstream’ method, which follows the level of the impounded tailings during the filling process. It also carries the biggest risk of failure.
Inherent in low cost upstream construction are a number of specific hazards for dam stability. A thorough assessment of these hazards and continuous monitoring and control during the siting, construction, and operation of the dam are deemed essential to their safety. Experience shows that these conditions are often not observed.
Structural failures in tailings dams can reach disaster proportions, leaving human loss of life, regionalised environmental contamination, and high financial costs.
The disaster in Italy’s Stava valley on 19 July 1985 was a case in point; one of the most tragic of its kind. With a loss of life totalling 268 and h133M (US$174M) in damage, it was one of the worst industrial catastrophes anywhere in the world.
Tailings from the fluorite mine near Trento in the north east of the country reached speeds of up to 90km/hr as the failure of the uppermost of two basins built on a slope overtopped the structure below. A tide of 200,000m3 of tailings left more than 60 ruined buildings in its wake as it travelled a distance of 4.2km downstream. The total surface area affected was 43.5ha.
Investigators concluded that the dams were constructed with an unacceptably low factor of safety and that the failure was probably triggered by a blocked decant pipe located within the tailings. Two days of uninterrupted heavy rain triggered the collapse of a coal tailings dam at Buffalo Creek in West Virginia’s Logan Water in February 1972, as a 9m high, 200m wide wall of coal refuse burst through its hillside location killing 125 people. More than 500 homes were destroyed and highways swept away at a cost estimated then at US$65M. Tailings travelled more than 25km at 8km/hr, as the water took three hours to wash out a succession of small coal towns and reach the confluence of Buffalo Creek and the Guyandotte river at Man.
In addition to the fatalities, 1000 were injured, and the disaster left 4000 homeless. As many as 1000 vehicles, 10 bridges, and power, water and telephone lines were destroyed, and the county road and the rail lines servicing the valley’s coal mines were severely damaged.
The dam was constructed of colliery waste after washing, creating a simple filtering mechanism for the slurry. The theory was that the water would seep through the dam at a slow rate, leaving behind the dirty coal refuse.
A federal Bureau of Mines report following the flood said, ‘The dams were not designed or engineered on the basis of a thorough knowledge of the engineering properties of coal processing refuse.’ A Citizens’ Commission report accused the Buffalo Mining-Pittston Company of gross negligence, willful violation of the law, and ‘incomprehensible callousness to human needs and values’.
In particular, it cited the company was ‘grossly negligent in constructing and operating the refuse dam on its property’ and that the dam had been inappropriately used to dispose of other refuse, to clarify waste water, and store water. The report said the dam had suffered a partial failure in 1971, and a smaller dam had completely collapsed in 1967. Regular inspections were not carried out as required, the report said.
Sadly, despite the many lessons learnt from these disasters, failures of tailings dams still occur. As recently as April 2006, a tailings dam failed during its sixth upraising at a gold mine near Miliang, Zhen’an County, Shangluo, Shaanxi Province, China. Seventeen people were reported missing, five injured people were taken to hospital and more than 130 local residents had to be evacuated. Toxic potassium cyanide was released into the Huashui river, contaminating it for a distance of 5km downstream.
Undoubtedly, tailings dam failures can leave in their wake not only physical devastation and a severely damaged industry reputation but also grave consequences for the industry’s access to land. And that can affect the commercial viability of some extraction processes.
The ‘conventional’ approach to designing tailings dams is to build confining or perimeter structures in a valley or flat area to create a basin. The tailings, which often contain a great deal of process water, are discharged continuously from the ore processing plant into the basin, where they settle out in a very loose structure like lake-bottom mud. In spite of the many tailings spills and dam failures that are still occurring, there has been little evolution of the conventional system, except that dam construction is receiving more attention. However, this has not stopped the failures, as Dennis Netherton, of consultants Netherton-Robinsky Associates, explains.
‘If the dam fails, say because of earth tremors, poor construction, or inadequate inspection and maintenance,’ he says, ‘the contents liquefy completely as the tailings and process fluid flow through the breach. In this liquid state they can flow many miles downstream. If the dam itself was built of tailings material, as is often the case, it too will liquefy and join the flow.’
The answer to the problem could lie in the adoption of a Tailings Management System (TMS) in which the tailings are strengthened to enable them to stack at a slope. This is achieved by the removal of most of the process water used in separating the ore from the tailings. Tailings are passed through high density thickeners and most of the process water is taken off the thickeners and recycled back into the plant. The thickening process must be sufficient to change the tailings and process water from a mixture to a non-segregating, but pumpable, slurry
When the tailings are released, in spite of their heavy consistency and thus high viscosity, they will still flow, without segregation. Eventually the flow stops at a gentle slope. The slope is controlled by the degree of thickening. The aim is to attain a slope of 2 to 6%.
The International Commission on Large Dams (ICOLD) and the International Council on Mining and Metals (ICMM) have jointly developed a new website which allows access to a library of good practice references on tailings, and the ability to search through the database with tailings-specific search criteria. The website was developed to provide a resource on all aspects of tailings in the context of good practice in the mining and metals sector. So seriously do ICMM members regard the issue that tailings management has risen to the top of the agenda embodied in its Sustainable Development (SD) framework.
The SD framework sets out a definitive approach to dealing with tailings emergencies not only at a company and community level, but also for response organisations. It is designed to ensure that the appropriate communication and action are set in place swiftly and effectively.
Paradoxically, emergencies such as this can have positive, as well as negative effects. One such example is the failing, on 25 April 1998, of a tailings dam of the Los Frailes lead-zinc mine at Aznalcóllar near Seville, Spain. Some 4-5Mm3 of toxic tailings slurries and liquid leaked into the nearby Río Agrio. A slurry wave covered several thousand hectares of farmland, threatening the Doñana National Park, a UN World Heritage Area.
The industry’s response was swift and robust. Its close collaboration with the European Commission resulted in the development of the EU Directive on mining waste, designed to prevent similar events occurring in the future.
Good tailings management is at the heart of the ICMM’s efforts to drive forward improvements in the industry’s sustainable development methodology. It consists of an integrated package of activities covering principles, and supported by public reporting, verification systems and the dissemination of good practice examples.
The emphasis on tailings management focuses on the continual improvement of environmental performance. Specifically, it calls for the provision of safe storage and disposal of residual wastes and process residues.
The ICMM approach is aimed at ensuring that guidelines are met on the ground. To this end, ICMM has mandated that all corporate members report their performance against SD principles. In December 2004, ICMM members agreed to report on their performance using the GRI Framework to fulfil this commitment.
Consequently, ICMM has worked with the Global Reporting Initiative (GRI) to develop a Mining and Metals Sector Supplement to accompany the GRI 2002 Sustainability Reporting Guidelines. Together with the Guidelines, the Supplement includes indicators to allow tracking of performance in a number of key areas.
Guidance on tailings management covers ‘significant spills of chemicals, oils, and fuels in terms of total number and total volume.’ The onus is on the reporting organisation to convey both the size of the spill and its impact on the surrounding environment. A full description of any significant incidents of this nature must include spillage of tailings, slimes, or other significant process materials.
Reference to tailings is also given in GRI Indicator MM6, Large Volume Mining and Mineral Processing Waste. It calls on the operator to describe its approach to the management of overburden, rock, tailings, sludges and residues. These must include an assessment of risks; consideration of the structural stability of storage facilities; metal leaching potential; and any hazardous properties.
In addition, ICMM has teamed up with the United Nations Environment Programme (UNEP), the United Nations Conference on Trade and Development (UNCTAD) and the UK government’s Department for International Development (DFID) to develop an online library of good practices that support ICMM’s principles of sustainable development. Good Practice in Emergency Preparedness and Response was released in November 2005. The publication covers everything from identifying who does what in an emergency, to training and proper community liaison.
UNEP’s APELL for Mining Handbook provides a framework for the preparation of an Emergency Response Plan that can be used by mine management, emergency response agencies, government officials and local communities. Case studies illustrating the consequences of mining accidents include the pond failure at the aforementioned Aznalcóllar Mine, and the Marcopper tailings spill, Marinduque Island, Philippines, among others.
There is also an account of a tailings breakout incident at Morila Gold Mine (MGM) some 11km south of Sanso village and 175km west of Sikasso, the capital of the southern region of Mali.
A list of potential events and details of their possible impact on the environment were drawn up to formulate an emergency planning regime at MGM. Potential incidents were pin-pointed from experience and knowledge of gold mining operations with conventional CIL plants. Potential impacts were also assessed on the basis of site-specific environment conditions (air, water, land, geology, vegetation, fauna, land uses, climate and geographic location/available infrastructure).
Rupture of the main wall tailing storage facility (tailing spillage); major spills from bulk diesel storage tanks; and cyanide and other highly corrosive chemical spillage were just some of the potential dangers identified. As a result of this list, MGM has been able to develop an Emergency Management Plan (EMP) by bringing together a spill prevention plan, a control and countermeasure plan and fire prevention plan.
As an example of how the EMP can work to great effect, on 16 March 2003, Morila experienced a tailings spillage when a weld in the tailings pipeline ruptured, causing slime to seep off the mine property and onto public land. The volume of spilled material was estimated at 2082m3, of which more than 96% was contained in catchment paddocks and trenches on mine property. The remaining 69m3 of spillage affected an area of 1.5ha outside the mine property.
As soon as the incident was reported, the Morila emergency plan was initiated. At the time of the incident, the local authorities – including the Sous Prefet, gendarmes and Mayor – were informed. A delegation from the mine, including the General Manager, went personally to find the Sous Prefet on the evening of the incident in order to explain to him the situation and the measures that had been taken to contain the incident and prevent any further impact on the local people and environment.
Regional officers in charge of mining and environment met the Sanso civil servants involved in the investigations and conducted their own independent investigation of the incident site, with the assistance of the mine personnel when requested. Later, they met the community leaders and gave them feedback on their findings. They assured them that the mine undertook all the appropriate actions and that there was no persistent danger for people, animals or the environment.
The community leaders understood the circumstances and expressed their recognition for all the effort Morila undertook to deal with the incident openly and professionally by involving the authorities. In addition to the remedial actions taken to neutralize the incident and prevent any similar occurrence in the future, Morila mine paid compensation to the community for livestock losses.
In another incident, involving the Kennecott Utah Copper Tailings Impoundments, 16km west of Salt Lake City, Utah, in the US, lessons were learnt from community involvement. The impoundments include an inactive south facility bounded by a state road to the south and situated immediately to the north of the town of Magna.
The perimeter of this impoundment was constructed using a variety of diking methods followed by upstream construction. In 1987, a geotechnical study of the then-active south pond identified a seismic vulnerability particularly at the southeast corner. The study identified a risk of flow failure that could affect the state roadway, a nearby small housing estate, a neighbouring house, a golf course and some Kennecott facilities.
As a result, Kennecott introduced a programme of drainage measures to improve stability and commissioned a number of engineering studies to evaluate methods to upgrade the facility to current design standards.
A number of mitigation methods were employed, and community outreach document, EAP/SOP, was used to engage with the public on the seismic vulnerability issues associated with the south facility. In this way, notification procedures were set in place to advise the local community of precautions that would be taken should an earthquake occur, a schedule of responsibilities of Kennecott and various public agencies during a seismic event.