Tailings

A panorama of Broken Hill, New South Wales, backed by the man-made mullock (waste tailings) heaps from the Line of Lode mine (the mullock heaps are the 'hills' that stretch across this image).

Tailings (also known as slimes, gangue, tailings pile, tails, leach residue, or slickens) are the materials left over after the process of separating the valuable fraction from the worthless fraction of an ore.

Tailings represent external costs of mining. 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. Yesterday's tails can be tomorrow's resource, as seen during the 1990s when the extensive tailings dumps of Kalgoorlie / Boulder in Western Australia were re-processed profitably by KalTails Mining.

In coal and oil sands mining, the word 'tailings' refers specifically to fine waste suspended in water.

Tailings composition

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 quantities of heavy metals found in the host ore, and they may contain added chemicals used in the extraction process. Elements are rarely in elemental form, more often as complex compounds.

Common minerals and elements found in tailings include

• Arsenic - Found in association with gold ores
• Barite
• Calcite
• Fluorite
• Radioactive materials - Naturally present in many ores
• Mercury
• Sulfur - Forms many sulfide compounds / pyrites
• Cadmium
• Hydrocarbons - Introduced by mining and processing equipment (oils & greases)

Common additives found in tailings

• Cyanide - as both Sodium Cyanide (NaCN) and Hydrogen Cyanide (HCN). Leaching agent.
• 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.

Environmental and social considerations

Tailings in general, are often pointed at by environmentally active groups as evidence of the destruction that mining operations can wreak upon the planet. These groups have a valid point. In the past, non-environmentally friendly methods (see "Disposal Methods" below) were the method of the day. In today's modern mining environment, it is possible to find many mining operators continuing to engage in tailings disposal methods that are not environmentally friendly, particularly in developing nations where legislative requirements are more permissive than in industrialized countries.

There is, however, a strong push from the leading mining houses and their design consultants to support the cessation of unsustainable tailings disposal practices. Unsustainable, in the sense that continuing to blatantly pollute the local environment damages a mining company's social license to operate, and casts a negative view over the broader industry. Some advocates of stronger environmental protection argue that, it is in the mining industry's best interest to move away from tailings disposal methods that damage the local environment.

Reprocessing of old tailings dumps and dams has assisted in the cleaning up of legacy tailings dumps, with the reprocessed tailings being disposed of using a more effective method than a simple surface dump.

Tailings disposal methods

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 topographical 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.

Dry stacking

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.

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 practice, 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. 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. Many countries specifically outlaw the use of STD methods.

Phytostabilization

Phytostabilization is a form of phytoremediation that uses plants for long-term stabilization 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, immobilize metals by adsorption or accumulation, and provide a zone around the roots where the metals can precipitate and stabilize. 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.

Different methods

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.

External links

• Tailings Info site
• Extox.net
• Submarine Tailings Disposal at the Mineral Policy Institute
• Carbon sequestration in mine tailings