Pond Storage
There are many different subsets of this method. Large earthen dams may be constructed and then filled with the tailings. Tailings may be deposited into natural geographical depressions. Exhausted open pit mines may be refilled with tailings. In all instances, due consideration must be made to contamination of the underlying water table, amongst other issues. Dewatering is an important part of pond storage, as the tailings are added to the storage facility the water is removed - usually by draining into decant tower structures. The water removed can thus be reused in the processing cycle. Once a storage facility is filled and completed, the surface can be covered with topsoil and revegetation commenced. However, unless a non-permeable capping method is used water that infiltrates into the storage facility will have to be continually pumped out into the future.
[+] Disposal into underground workings
While disposal into exhausted open pits is generally a straightforward operation, disposal into underground voids is more complex. A common modern approach is to mix a certain quantity of tailings with waste aggregate and cement, creating a product that can be used to backfill underground voids and stopes. A common term for this is HDPF - High Density Paste Fill. HDPF is a more expensive method of tailings disposal than pond storage, however it has many other benefits - not just environmental but it can significantly increase the stability of underground excavations by providing a means for ground stress to be transmitted across voids - rather than having to pass around them - which can cause mining induced seismic events like that suffered recently at the Beaconsfield Mine Disaster
[+] Disposal into river systems
Usually called RTD - Rivering Tailings Disposal. Not a particularly environmentally sound practise, it has seen significant utilisation in the past, leading to such spectacular environmental damage as done by the Mt Lyell Mining Company in Tasmania to the King River. It is still practised at some operations in the world, and while experts agree it is a feasible method for locations where the river is rapidly flowing and turbulent and the additional silt loading will not impact on the river quality, it is not generally favored and is seeing a gradual decline in use.
[+] Disposal into the oceans
Commonly referred to as STD (Submarine Tailings Disposal) or DSTD (Deep Sea Tailings Disposal). If a mine is located in close proximity to the coast, and the coast itself is not an excessive distance from a continental shelf, STD is conceptually an excellent method for the disposal of tailings. Tailings can be conveyed using a pipeline then discharged so as to eventually descend into the depths. Practially, it is not an ideal method, as the close proximity to off-shelf depths is rare. When STD is used, the depth of discharge is often what would be considered shallow, and extensive damage to the seafloor can result due to covering by the tailings product. It is also critical to control the density and temperature of the tailings product, to prevent it from travelling long distances, or even floating to the surface. The Solwara project being commenced in the Bismark Sea by Nautilus Minerals proposes to use a modified STD method back down to depths below 1500 metres. Many countries specifically outlaw the use of STD methods.
[+] New Developments
A number of improvements have been made in tailings disposal technology and tailings dam design, both to improve on the weaknesses of previous practices and also to take advantage of tailings processing technologies. Geochemical aspects now largely drive the siting, of a tailings impoundment, the design of retention structures, and tailings disposal technology. There have been technologies put forward as panaceas for tailings disposal problems, which have turned out to be flawed in practice. These improvements can be categorized as changes in basic management practices and changes in tailings characteristics through pre-discharge dewatering.
Designing for Geochemical Issues
Geochemical issues have become highly prominent as severe acid generation problems became apparent at a number of mature mines around the world. Some of these mines, which had been operated by smaller mining companies, became orphan sites, leaving significant legacies for future generations. The majority of the acid drainage mine sites have become very expensive legacies for the major mining companies that owned them. It has been necessary to develop and operate acid drainage collection and treatment systems for continued operation and closure of numerous mines. Capital costs for ARD collection and treatment systems have been in the several tens of millions of dollars, with ongoing operating costs up to several millions of dollars annually. As a result, companies developing new mines have focussed on methods to predict and prevent or reduce acid generation from tailings.
Considerable research, for example CANMET's Mine Effluent Neutral Drainage (MEND) program, was carried out in the 1980’s and 1990’s, to assess viable methods of acid drainage control. The most significant conclusion of the past 20 years is that it is far easier (economic) to prevent ARD in the first place than to control it. From a number of existing sites where tailings had been placed in lakes in northern Canada, it was concluded that long-term submergence of acidic wastes was probably the most effective means of ARD control. Considerable work has also been done on placement of impervious closure covers over tailings to prevent ingress of air and water. Sophisticated designs of multiple-layer covers, incorporating impervious zones, pervious capillary barriers and topsoil for vegetation growth, have been developed. Covers have been found to present the risk of long term cracking or erosion, and to be ineffective in excluding air, so are less favoured solutions than submergence from the geochemical standpoint. Some of the main technologies for reduction of ARD potential from sulphide bearing tailings are the following:
1. Design for submergence by flooding the tailings at closure. This is a solution, which is being increasingly encouraged and accepted by regulators. However, the authors are concerned that flooded impoundments may create a risky legacy. The more traditional closure configuration for tailings impoundments has been to draw down water ponds as completely as possible, to reduce the potential for dam failure by overtopping or erosion. To raise water levels in impoundments formed by high dams could present considerable long-term risk. One of the reasons that closed tailings impoundments have traditionally proven to be generally more safe, from the physical stability perspective, than operating impoundments is the relatively more “drained” condition of closed impoundments that do not include a large water pond. The flooded closure scenario represents an “undrained” condition that does not allow this improvement in physical stability to develop, so the risk does not decrease with time.
2. Treatment of tailings to create non-acid generating covers. To avoid the necessity of flooding impoundments, non-reactive covers of tailings can be placed on the top of the impoundment on the last few years of operation. It has been shown in several mining operations, for example at the Inco Ontario Division central milling operation in Copper Cliff, Ontario, that by the relatively inexpensive installation of some additional flotation capacity, pyrite can be removed to the level that the tailings can be made non-acid generating. The upper non-acid generating tailings placed on top can be left as a wide beach for dam safety, while the underlying mass of potentially acid generating tailings remains saturated below the long-term water table in the impoundment. Normally, the small amount of pyrite removed by flotation can be disposed as a separate tailings stream, placed in the deepest part of the impoundment where it can be left flooded.
3. Lake or ocean subaqueous disposal. The surest, safest and most cost-effective solution to prevent ARD is sub-aqueous disposal in a lake or the ocean. Tailings will remain permanently submerged and have shown to be non-reactive under water and to have few permanent environmental impacts. The challenge for this solution is that regulators have become reluctant to permit lake or ocean disposal, and there are not always appropriate sites available. In addition, the public often reacts emotionally and negatively to the concept of such disposal, despite the considerable benefits of these approaches. The authors are aware of at least two examples where public pressure incited regulators to demand that existing operations switch from ocean and lake disposal to on-land impoundments, with the result that environmental problems actually increased. The authors do note a slight trend to re-acceptance of subaqueous disposal, particularly in the marine environment, as the true environmental impact of the technique can be demonstrated to be almost negligible in certain instances. Moreover, the corporate risks and environmental liabilities associated with surface tailings storage on many projects grows to the point where project viability is threatened without looking to environmentally acceptable alternatives including subaqueous disposal. And after all, who really gives a shit if it happens to be in a lake that we get our drinking water from? Or the marine life that is so vital to the ecology of our planet? The mining companies sure don't.
Improved Basic Design Concepts - Improved Upstream Construction.
Considerable attention has been given to improving traditional upstream dam construction to make the technique not only economical but also stable under both static and dynamic conditions. Numerous failures of upstream constructed dams have occurred. The failures have been the results of earthquakes, high saturation levels, steep slopes, poor water control in the pond, poor construction techniques incorporating fines in the dam shell, static liquefaction, and failures of embedded decant structures. Most failures have involved some combination of the above weaknesses.
Based on the above experiences, and through the use of improved analytical tools (computer programs for stability, seepage, and deformation under both static and seismic conditions), safe, optimised designs have been developed. Some of the key design features that have been added include:
• Underdrainage, either as finger drains or blanket drains, to lower the phreatic level in the dam shell; • Beaches compacted to some minimum width to provide a stable dam shell. Beaches are compacted by tracking with bulldozers, which are also used for pushing up berms for support of spigot lines; • Slopes designed to a lower angle than was used for many failed tailings dams. Slopes are generally set at 3 horizontal to 1 vertical or flatter, depending on the other measures incorporated into the designs. Steeper slopes, without an adequate drained and/or compacted beach, create the potential for spontaneous static liquefaction - a phenomenon not widely recognized in 1972 but one responsible for a number of major tailings dam failures.
Improved Basic Design Concepts - Lined Tailings Impoundments.
With the advent of larger gold mining operations, and the almost universal use of sodium cyanide as an essential part of gold extraction, the need came about to develop impervious impoundments to contain cyanide solutions. Although cyanide is in most forms an unstable compound that naturally breaks down on exposure to air, it can be very persistent and migrate long distances in groundwater. As well as cyanided gold tailings, other types of tailings may also be considered potentially contaminating. For protection of aquifers, where tailings impoundments are not sited over impervious soils or bedrock and embankment cut-offs are not sufficient to reduce seepage, it is often necessary to design and construct a liner over the base of a tailings impoundment. Great progress has been made in liner design and construction practise.
Liners may be as simple as selective placement of impervious soil to cover outcrops of pervious bedrock or granular soils, or may need to be a composite liner system constructed over the entire impoundment. Where geomembrane liners are used, it is normal practise to incorporate a drainage layer above the geomembrane, to reduce the pressure head on the liner and minimise leakage through imperfections in the liner. Another benefit of such under-drainage is that a low pore pressure condition is achieved in the tailings, giving them a higher strength than would exist without such under-drainage. The drainage layer typically consists of at least 300 mm of granular material, with perforated pipes at intervals within the drainage layer. The pipes are laid to drain water extracted from the base of the tailings deposit and to discharge to a seepage recovery pond. When a liner extends beneath an impoundment, care must be taken to design for lower foundation shear strength for the downstream slope of the embankment, as the liner may form a plane of weakness.
Improved Basic Design Concepts - Dewatering Technologies.
As shown on the figure below, the basic segregating slurry is part of a continuum of water contents available to the tailings designer in the 21st century. Although tailings dewatering was previously practised for other purposes in the mining process, until recently the only form of tailings for most tailings facilities was a segregating, pumpable slurry with geotechnical water contents of well over 100%.
There are several candidate scenarios where dewatered tailings systems would be of advantage to the mining operation. However, dewatered tailings systems have less application for larger operations for which tailings ponds must serve dual roles as water storage reservoirs, particularly where water balances must be managed to store annual snowmelt runoff to provide water for year round operation.
"Dry" Cake filtered tailings disposal. Development of large capacity, vacuum and pressure belt filter technology has presented the opportunity for disposing tailings in a dewatered state, rather than as a conventional slurry. Tailings can be dewatered to less than 20% moisture content (using soil mechanics convention, in which moisture content is defined as weight of water divided by the dry weight of solids). At these moisture contents, the material can be transported by conveyor or truck, and placed, spread and compacted to form an unsaturated, dense and stable tailings stack (often termed a "dry stack") requiring no dam for retention. While the technology is currently considerably more expensive per tonne of tailings stored than conventional slurry systems, and would be prohibitively expensive for very large tonnage applications, it has particular advantages in the following applications:
• In very arid regions, where water conservation is an important issue. The prime example of such system is at the La Coipa silver/gold operation in the Atacama region of Chile. A daily tailings production of 18,000 t is dewatered by belt filters, conveyed to the tailings site and stacked with a radial, mobile conveyor system. The vacuum filter system was selected for this site because of the need to recover dissolved gold from solution, but is also advantageous for water conservation and also for stability of the tailings deposit in this high seismicity location.
• In very cold regions, where water handling is very difficult in winter. A dewatered tailings system, using truck transport, is in operation at Falconbridge’s Raglan nickel operation in the arctic region of northern Quebec. The system is also intended to provide a solution for potential acid generation, as the tailings stack will become permanently frozen. A dry stack tailings system is also being planned for a new gold project in central Alaska.
• Relatively low tonnage operations. A separate tailings impoundment can be avoided all together by having a tailings/waste rock co-disposal facility.
• Regions where a “dry landscape” upon closure is required. The tailings area can be developed and managed more like a waste dump and therefore avoids many of the operation and closure challenges of a conventional impoundment.
Moreover, filtered tailings stacks have regulatory attraction, require a smaller footprint for tailings storage (much lower bulking factor), are easier to reclaim and close, and have much lower long-term liability in terms of structural integrity and potential environmental impact.
