Tailings (also known as slimes, tails, leach residue, or slickens) are the materials left over[ after the process of separating the valuable fraction from the uneconomic fraction (gangue) of an ore. Tailings are distinct from overburden or waste rock, which are the materials overlying an ore or mineral body that are displaced during mining without being processed.
The extraction of minerals from ore can be done two ways: placer mining, which uses water and gravity to extract the valuable minerals, or hard rock mining, which uses pulverization of rock, then chemicals. In the latter, the extraction of minerals from ore requires that the ore be ground into fine particles, so tailings are typically small and range from the size of a grain of sand to a few microns.[ Mine tailings are usually produced from the mill in slurry form (a mixture of fine mineral particles and water).
Tailings represent an external cost of mining, and this is particularly true of early mining operations which did not take adequate steps to make tailings areas environmentally safe after closure. Modern day mines, particularly in jurisdictions with well developed mining regulations and/or operated by responsible mining companies, incorporate the rehabilitation and proper closure of tailings areas in the mining costs and activities. For example, the province of Quebec, Canada, requires not only submission of closure plan before the start of mining activity, but also the deposit of a financial guarantee equal to 70% of the estimated rehabilitation costs . Tailings dams are often the most significant environmental liability for a mining project.
When applied to coal and oil sands mining, the term 'tailings' refers specifically to fine waste suspended in water.
- 1 Tailings composition
- 2 Environmental considerations
- 3 Storage methods
- 4 Tailings reprocessing
- 5 See also
- 6 References
- 7 External links
The composition of tailings is directly dependent on the composition of the ore and the process of mineral extraction used on the ore.
Certain types of extraction process, like heap leaching for example, may result in quantities of chemicals used to perform the leaching remaining in the material once leaching has been completed. Older forms of mineral extraction, such as those utilised during the early gold boom years of Australian gold mining, resulted in large heaps of fine tailings being left dotted around the landscape. These tailings dumps would continue to leach residual chemicals into the environment, and if weather conditions allowed it the finer fraction would become windborne, blowing around the townships surrounding the now-dormant mining areas.
Typically, the bulk quantity of a tailings product will be barren rock, crushed and ground to a fine size ranging from coarse sands down to a talcum powder consistency.
Tailings may contain trace quantities of metals found in the host ore, and they may contain substantial amounts of added compounds used in the extraction process Elements are rarely in elemental form, more often as complex compounds.
Common minerals and elements found in tailings include
Common additives found in tailings
- Arsenic - Found in association with gold ores
- Radioactive materials - Naturally present in many ores
- Sulfur - Forms many sulfide compounds / pyrites
- Hydrocarbons - Introduced by mining and processing equipment (oils & greases)
Tailings present a long term cost to the mining industry. If the company leaves or goes bankrupt, the local government can find itself with responsibility for the maintenance and monitoring of tailings dumps essentially forever - this, and other costs of cleanup, can impose liabilities that were estimated at up to 12 billion dollars in the US alone in 2005.
- Cyanide - as both sodium cyanide (NaCN) and hydrogen cyanide (HCN). Leaching agent in extremely dilute quantities which readily volatize upon exposure to sunlight.
- SEX - Sodium Ethyl Xanthate. Flotation agent.
- PAX - Potassium Amyl Xanthate. Flotation agent.
- MIBC - Methyl Isobutyl Carbinol. Frothing agent.
- Sulfamic acid - Cleaning / descaling agent.
- Sulfuric acid - Used in large quantities in the PAL process (Pressure Acid Leaching).
- Activated Carbon - Used in CIP (Carbon In Pulp) and CIL (Carbon In Leach) processes.
- Calcium - Different compounds, introduced as lime to aid in pH control.
Disposal of mine tailings is one of the most important environmental issues for any mine during the project's life. Whilst significant pressure is placed on mining projects in developed countries to conform to stringent environmental standards, many projects in developing nations do not take significant steps to prevent or mitigate environmental damage.
Although ideally the tailings would be made up of gangue materials (i.e. silica), to some degree, the sought-after mineral also appears in the tailings. In operations that recover lead, uranium and other toxic heavy metals, this represents a significant environmental hazard. In addition to the minerals themselves, some processing methods involve marine pollutants such as copper sulfate, xanthate or cyanide which will be present to some degree in the tailings. In some operations, components of the gangue may also be toxic, though it is rare for these materials to be present above trace levels. An example is thallium in sulfide ores.
In order to prevent the uncontrolled release of tailings material into the environment, mines usually have a disposal facility which quite often takes the form of a dam or pond. This is a convenient method of storage since tailings are often in the form of a slurry when they are discharged from the concentrator. These facilities often require the clearing of more land than the rest of the mine (including open-pit operations) combined, and failure of the wall can result in a massive release of tailings. As such they are of great environmental concern.
Tailings release and subsequent damage to the environment can also occur without catastrophic failure of the storage facility. These kinds of release are much less obvious and may take the form of acid drainage or dry tailings dust being blown away from the storage area. Several major environmental disasters have been caused by tailings dam failures and other release of tailings into the environment. Some examples are the Ok Tedi environmental disaster, the Buffalo Creek Flood, the 2000 Baia Mare cyanide spill and the Ajka alumina plant accident.
Summary of the range of tailings products
Historically, tailings were disposed of however was convenient, such as in downstream running water or down drains. Because of concerns about these sediments in the water and other issues, tailings ponds began to be constructed, which were bounded by impoundments (an impoundment is a dam). These dams typically use "local materials" including the tailings themselves, and may be considered embankment dams. Traditionally, the only option for tailings storage was to deal with a tailings slurry.[clarification needed] This slurry was a dilute stream of the tailings solids within water that was sent to the tailings storage area. The modern tailings designer has a range of tailings products to choose from depending upon how much water is removed from the slurry prior to discharge. The removal of water not only can create a better storage system in some cases (e.g. dry stacking, see below) but can also assist in water recovery which is a major issue as many mines are in arid regions. In a 1994 description of tailings impoundments, however, the U.S. EPA stated that dewatering methods may be prohibitively expensive except in special circumstances. Subaqueous storage of tailings has also been used.
Tailing ponds are areas of refused mining tailings where the water borne refuse material is pumped into a pond to allow the sedimentation (meaning separation) of solid particles from the water. The pond is generally impounded with a dam, and known as tailings impoundments or tailings dams. It was estimated in 2000 that there were about 3,500 active tailings impoundments in the world. The ponded water is of some benefit as it minimizes fine tailings from being transported by wind into populated areas where the toxic chemicals could be potentially hazardous to human health; however, it is also harmful to the environment. Tailing ponds are often somewhat dangerous because they attract wildlife such as waterfowl or caribou as they appear to be a natural pond, but they can be highly toxic and harmful to the health of these animals. Tailings ponds are used to store the waste made from separating minerals from rocks, or the slurry produced from oil sands mining. Tailings are sometimes mixed with other materials such as bentonite to form a thicker slurry that slows the release of impacted water to the environment.
There are many different subsets of this method, including valley impoundments, ring dikes, in-pit impoundments, and specially dug pits. The most common is the valley pond, which takes advantage of the natural topographical depression in the ground. Large earthen dams may be constructed and then filled with the tailings. 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.
The biggest danger of tailings ponds is dam failure, with the most publicized failure in the US being the failure of a coal slurry dam in the West Virginia Buffalo Creek disaster, which killed 125 people; other collapses include the Ok Tedi environmental disaster on New Guinea, which destroyed the fishery of the Ok Tedi River. On the average, worldwide, there is one big accident involving a tailings dam each year. Tailings ponds can also be a source of acid drainage, leading to the need for permanent monitoring and treatment of water passing through the tailings dam; the cost of mine cleanup has typically been 10 times that of mining industry estimates when acid drainage was involved.
Tailings do not have to be stored in ponds or sent as slurries into oceans, rivers or streams. There is a growing use of the practice of dewatering tailings using vacuum or pressure filters so the tailings can then be stacked. This saves water, reduces the impacts on the environment in terms of space used, leaves the tailings in a dense and stable arrangement and eliminates the long-term liability that ponds leave after mining is finished.
Storage in 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 previously at the Beaconsfield Mine Disaster
Usually called RTD – Riverine Tailings Disposal. In most environments, not a particularly environmentally sound practice, it has seen significant utilisation in the past, leading to such spectacular environmental damage as done by the Mount Lyell Mining and Railway Company in Tasmania to the King River, or the poisoning from the Panguna mine on Bougainville Island, which led to large-scale civil unrest on the island, and the eventual permanent closing of the mine.
As of 2005, only three mines operated by international companies continued to use river disposal: The Ok Tedi mine, the Grasberg mine and the Porgera mine, all on New Guinea.
Commonly referred to as STD (Submarine Tailings Disposal) or DSTD (Deep Sea Tailings Disposal). Tailings can be conveyed using a pipeline then discharged so as to eventually descend into the depths. Practically, 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 Bismarck Sea by Nautilus Minerals proposes to use a modified STD method back down to depths below 1500 metres.
This method is used by the gold mine on Lihir Island; its waste disposal has been viewed by environmentalists as highly damaging, while the owners claim that it is not harmful.
Phytostabilisation is a form of phytoremediation that uses plants for long-term stabilisation and containment of tailings, by sequestering pollutants in soil near the roots. The plant's presence can reduce wind erosion, or the plant's roots can prevent water erosion, immobilise metals by adsorption or accumulation, and provide a zone around the roots where the metals can precipitate and stabilise. Pollutants become less bioavailable and livestock, wildlife, and human exposure is reduced. This approach can be especially useful in dry environments, which are subject to wind and water dispersion. New work is also being done by Pan Pacific in the development of algal sequestration for plutonium and uranium tailings.
Considerable effort and research continues to be made into discovering and refining better methods of tailings disposal. Research at the Porgera Gold Mine is focusing on developing a method of combining tailings products with coarse waste rock and waste muds to create a product that can be stored on the surface in generic-looking waste dumps or stockpiles. This would allow the current use of rivering disposal to cease. Considerable work remains to be done. However, co-disposal has been successfully implemented by several designers including AMEC at, for example, the Elkview Mine in British Columbia.
As mining techniques and the price of minerals improve, it is not unusual for tailings to be reprocessed using new methods, or more thoroughly with old methods, to recover additional minerals. Extensive tailings dumps of Kalgoorlie / Boulder in Western Australia were re-processed profitably in the 1990s by KalTails Mining.