Tailing dams have been an essential part of the minerals extraction process. However, history shows that serious environmental and safety issues are associated with tailing dams. Dr Thomas Bunn from TUNRA Bulk Solids writes about options that can solve these problems and may be economically viable in the longer term.
Tailings dams are an economic solution to the management of refuse. The cost of tailings dams is somewhere in the range from $1 per tonne to $5 per tonne of tailings deposited, and, adding societies true cost, it could be argued the cost is actually between $2 per tonne and $10 per tonne. Indirect costs include: ongoing insurance, monitoring, groundwater contamination, dust contamination, and loss of real estate value in areas on and around the dams.
The main hazard the dams present is an unacceptable high historical rate of failure, which typically causes substantial losses, including the loss of lives. Failures occur due to:
• Inadequate design and/or construction
• Rainfall events in excess of the design allowances
• Seismic activity causing re-liquefaction
It is of major concern that tailings dam failures continue at a high rate. Unfortunately, the number of major incidents continues at an average of more than one per year. During the last 10 years, the rate has been four per year. Similar data for the previous half century indicates a failure rate of two per year [1]. Tailings dams are supposed to last forever, but experience shows that minor and major spills pose a serious environmental threat that stays behind when the mine closes.
Several characteristics make tailings dams more vulnerable than other types of retention structures, namely:
• Embankments formed by locally collected fills (soil, coarse waste, overburden from mining and tailings)
• Dams subsequently raised as solid material coupled with a severe increase in effluent
• Lack of regulations on specific design criteria
• Lack of dam stability requirements regarding continuous monitoring and control during emplacement, construction, and operation
• High cost of maintenance works for tailings dams after closure of mining activities
• Mining industry changes mean the rates of refuse vary with market conditions (due to changes in yields from process plant and capacity of process plant). This means the planning of dam raisings is often lacking during a cyclical mining boom
• Changes in mining and processing techniques are always occurring, and again the planning of dam raisings is often lacking due to unexpected capacity changes
The main cause of historical dam failures was rainfall events, followed by occurrences associated with seismic liquefaction. Over 90 per cent of incidents occurred in active mine tailings dams, and only 10 per cent refer to abandoned dams.
Due to the nature of mining and mineral processing, the volumes of mining wastes are significantly larger than those of both domestic and industrial wastes. The material stored in tailings dams is usually very fine. This material is placed hydraulically, is loose and is at, or above, the saturation moisture content. Any major movement of the retaining boundaries of the impoundment can induce shearing strains that disturb the structure of the tailings mass, inducing a rapid rise of pore water pressures and liquefaction of a section of the impoundment. An event like this can cause even greater pressures to be applied to the retaining boundaries. Failure of the retaining dam can release liquefied tailings that can travel for great distances, and because of its greater specific weight, destroy everything in its path. Water will flow through and around buildings, but liquefied tailings can destroy the structures. Historically the tendency is for tailing dams to become ever higher and impoundments ever larger.
Why are we still building tailings dams?
Why does everyone keep building tailings dams? Governments demand bonds for their security, so the rehabilitation money is locked away at the start of the mine, and can’t be used creatively on alternatives.
A mining company will fully appreciate the cost of maintaining a disused tailings dam, especially one that can’t be acceptably rehabilitated because it continues to release leachates and consolidates, thus requiring maintenance to occur indefinitely. However, if ongoing monitoring and insurance for a tailings dam is say $0.01 per tonne, the total net present value over 30 years is $0.2 per tonne at five per cent discount rate. This indefinite cost is low and sustainable in the long term for many big mining firms who are planning on growth. The mining firm can rely on the fact that permanent consolidation of a tailings dam will occur, one day in the future.
Most decisions about tailings dams use a probability factorised cost for various potential failure events. However, society should be wary of this, by learning from the loss of the Challenger Space Shuttle. This space vehicle was designed for a failure rate lower than 1 in a 100,000 event according to all the experts before the disastrous event involving the loss of the shuttle and its crew, but after investigation it turned out to be 1 in 100. This was an error in the failure rate estimate of 1,000 times, and it was not due to poor science, which was very detailed, but due to the variability of human behaviour, from the designer to the operators [2].
Many tailings dams around the world today claim to have catastrophic failure rates lower than one in a million, yet the actual statistics indicate this is overly optimistic by a few orders of magnitude.
What if we reconsider the indefinite time cost of a rehabilitated tailings dam, whether the rehabilitation was entirely successful or not (as defined by negligible leachates escaping or maximum consolidation)? Statistically at some point in the next 10,000 years, an earthquake, volcano, 1 in 10,000-year flood, tsunami or any major event will occur at every tailings dam site. The risk of a catastrophic failure of a tailings dam, which is currently estimated as a one in a million event, has a one in a hundred chance of occurring in this timeframe. And if the one in a million evaluated risk was in error by 1,000 times, like the Challenger Space Shuttle disaster, then this catastrophic failure will occur 10 times, and society will have to clean it up 10 times.
Alternative disposal methods
Tailings are a mixture of particles, water and chemicals left over from the processing plant. If it is ‘chemically bound’, it makes a solid. This ‘bound’ solid can be quite useful in construction and landfill as the noxious chemicals are locked in the solids matrix. For example, a steep valley could be made less steep to prevent erosion or an old mine pit could be filled, making the land more suitable.
As the most common binder is cement, suitable placement characteristics can be achieved with the addition of only two per cent cement by weight. At $285 per tonne, cement represents an additional cost to the tailing disposal system of less than $6 per tonne. The binding of particles in an inert matrix can occur through different chemical reactions. For this assessment, we will assume that this binding occurs using standard grade cement.
Most binders are sensitive to the presence of water, especially where the binding reaction requires a specific concentration. If dewatering is not required, then the only additional cost will be the $6 per tonne. However, if dewatering is required the following additional cost will occur:
• Thermal drying (which is expensive at $30 per tonne)
• Mechanical drying using belt press vacuum filters (which is less than $5 per tonne), or
• Adding dry material such as fly-ash or ground blast furnace reject material. The addition of this material at 25 per cent concentration may attract a cost of $5 per tonne.
The next cost after binding is materials handling. In normal tailings dam systems, a slurry pipeline provides low-cost transport with centrifugal pumps and the flexibility of a short pipeline to get to the emplacement sites. A typical paste system including a binder to create useful landforms requires a paste pumping system or trucking. Pumping the tailings as a paste would add an extra cost of between of $2 to $5 per tonne.
Example of changing an industry from slurry to paste production
The disposal of power station ash in Australia has been undergoing a significant shift in emphasis during the past ten years. In older power stations, fly ash and bottom ash were transported to a tailings dam in two purpose-built systems:
• The first system was for fly ash (dust). The dust was removed from the boiler gas passes by either fabric filters or precipitator collection systems. The dust was hydraulically evacuated from the fabric filters or precipitator storage hoppers on either an intermittent or continuous basis and sluiced to the dust plant. In the dust plant, the sluiced dust was mixed with large quantities of water and pumped using centrifugal pumps as lean phase slurry with a Cw (solids concentration by weight) <10 per cent.
• The second system was for bottom ash, which was intermittently dumped from the wet bottom ash hopper into a sluiceway and sluiced to the ash plant. In the ash plant the sluiced bottom ash was first crushed to < 25 millimetres, mixed with large volumes of water and pumped using centrifugal pumps as lean phase slurry Cw (solids concentration by weight) <10 per cent.
The slurry pipelines discharge into a tailings dam simply called the ash dam. The water from the ash dam is recycled back to the power station for reuse. The water used for ash disposal systems could either be fresh or salt water depending on the power station location.
For newer power stations and as a retrofit to existing stations, an alternative ash disposal system is one where both the bottom ash and fly ash are mixed and pumped as high concentration slurry to a disposal site. At a retrofit power station, fly ash is removed from the precipitators or fabric filters by a pneumatic conveying system and conveyed to a High Concentration Slurry Disposal (HCSD) storage silo. The bottom ash is removed from the boilers by a dry removal system and after passing through a hammer mill, where the size is reduced to < 8 millimetres, is also pneumatically conveyed to the HCSD storage silo. The ash from the HCSD storage silo is mixed as high concentration slurry Cw of 63 per cent in a mixing plant and pumped using diaphragm pumps at a flow rate of 100 m3h-1 to the disposal site in a 150 millimetres diameter pipeline with a pressure of 3 megapascals.
Materials handling solution for alternative disposal to underground voids
Using mineral processing tailings to produce paste backfill with a binder is well proven and documented in specific engineering publications, such as the Australian Centre for Geomechanics. This field of extensive and proven commercially viable research is primarily aimed at increasing mining extraction ratios with structurally competent backfill.
An important way in which paste backfilling is beneficial is through reduction of adverse environmental effects of tailings dams. There are numerous underground mine voids being filled with tailings in Australia, South Africa and elsewhere. It is not always possible to put all tailings back underground due to insufficient underground voids, but tailings dam sizes can be significantly reduced.
Chemically bound and stabilised tailings are already status quo in metalliferous mining where improved mining efficiencies have justified the additional cost as a backfill.
In the coal industry in Europe, Deutsche Montan Technologie (DMT) developed a coal mine backfilling system that was installed in the 1990s at the Walsum Mine [3]. The mine is being backfilled with residual material from processing and combustion of coal, including incineration of domestic refuse and sewage sludge. This system had a mixing and pumping station on the surface which delivered a 100 m3 h-1 at 12 megapascals of paste according to specific criteria to match both desired high solids content and low-pressure loss. The paste is pumped through pipes to the coal faces using powerful piston pump. This system successfully pumps the paste up to 12 kilometres through a 200 millimetres pipeline to the working face at a depth of 800 metres. The paste is deposited in the goaf by using trailing pipes. The paste accumulates in the collapsed mined area and does not flow to other areas of the mine. Unlike the conventional hydraulic stowing methods, there is no necessity to capture the conveying water and pump it back to the surface.
A paste for backfill can be prepared from refuse material from a coal washery, that is thickener underflow material and ground rejects. A paste pumping trial conducted at the University of Newcastle indicate the material comprising finely ground reject mixed with thickener underflow material can be pump at a Cw up to 75 per cent. This paste could be left in the pipeline for long periods and the pumping system restarted, and it could be pumped long distance for depositing underground.
There are alternatives to tailings dams and these should be considered by any mine. Serious consideration should be given to the acceptance of one in a million failure rates with numerous failures of tailing dams resulting in loss of life, destruction of homes and infrastructure and environmental pollution. Although current practices attempt to mitigate these risks, claiming that catastrophic events are reduced to the level of one in a million or less, there is still some argument that this may not be enough, and may not be achievable when considering the longevity of the dam and human factors involved in design, building and maintenance.
The principle of returning the refuse to the place of origins as a backfill is a logical solution that should be pursued where possible. The principal of using dewatering, binder and paste pumping for dry-stacking new dams or landforms should be pursued to eliminate risks of tailings dams. Technologies to implement alternative methods exist and are proven. The additional cost of this could be justified by closely examining the true indirect costs.
References
1. https://www.wise-uranium.org/mdaf.
2. Feynman R., “Personal Observations on the Reliability of the Shuttle” http://www.fotuva.org/feynman/challenger-appendix.htm1.
3. Mez W., and Deguchi G., Goaf Cavity Filling. A Special Paste Backfill Method for the Coal Mining Industry, Power, Volume 49, No. 253, 1999, pp 56-62.