Introduction
Tailings consist of ground rock and process effluents that are generated in a mine processing plant. Mechanical and chemical processes are used to extract the desired product from the run of the mine ore and produce a waste stream known as tailings. This process of product extraction is never 100% efficient, nor is it possible to reclaim all reusable and expended processing reagents and chemicals. The unrecoverable and uneconomic metals, minerals, chemicals, organics and process water are discharged, normally as slurry, to a final storage area commonly known as a Tailings Management Facility (TMF) or Tailings Storage Facility (TSF). Not surprisingly the physical and chemical characteristics of tailings and their ability to mobilise metal constituents are of great and growing concern (ICOLD and UNEP 2001).
Tailings are generally stored on the surface in retaining structures but can also be stored underground in mined out voids by a process commonly referred to as backfill. Backfilling can provide ground and wall support, improve ventilation, provide an alternative to surface tailings storage and prevent subsidence (EC 2004). Backfilling is discussed in the relevent section which can be accessed here 
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The challenges associated with tailings storage are ever increasing. Advances in technology allow lower grade ores to be exploited, generating higher volumes of waste that require safe storage. Environmental regulations are also advancing, placing more stringent requirements on the mining industry, particularly with regard to tailings storage practices. This ultimately places added pressure on the operators of a tailings facility who carry out the day to day roles of tailings discharge and water management. The majority of historical tailings related incidents have been influenced by poor day to day management, which has resulted in the strengthening of regulations controlling tailings storage today. The research carried out in the PhD thesis submitted at the University of Leeds in 2006 has targeted the management roles to improve day to day operations and reduce the risks associated with surface tailings storage. The parameters that influence the stability, operation and management have been identified and presented together with their methods of control, intervention and mitigation. This is supported by a novel online database called TailPro (www.tailpro.com) that has been developed to ensure the tailings personnel can implement a tailings management system efficiently and effectively.
Tailings are a waste product that has no financial gain to a mineral operator at that particular point in time. Not surprisingly it is usually stored in the most cost effective way possible to meet regulations and site specific factors. Dams, embankments and other types of surface impoundments are by far the most common storage methods used today and remain of primary importance in tailings disposal planning (Vick 1990). The particular design of these retaining structures is unique to a particular environment and mining operation.
When considering the design of a tailings storage facility there are many parameters which impact on the optimum site selected and the storage and tailings discharge methods used (Ritcey 1989). The environment is the most crucial parameter constraining tailings storage which ultimately affects the way a facility is designed, built, operated and closed. For this reason a range of alternate methods of tailings storage and discharge techniques need to be considered when designing a facility for a particular location.
This page briefly highlights the generation and nature of tailings. The various tailings storage and discharge methods used, the water management requirements for conventional tailings storage are discussed in the relevent sections of this website. The water management considerations discussed are only specific to conventional storage and should highlight why this method is so problematic compared to other storage techniques.
Nature of tailings
The process of beneficiation of run of the mine ores and subsequent disposal to surface containment facilities exposes elements to accelerated weathering and consequently increases mobilisation rates. The addition of reagents used in mineral processing may also change the chemical characteristics of the processed minerals and therefore the properties of the tailings and waste rock (EC 2004). Problems arise when this accelerated weathering process generates toxic levels that create short and long term tailings management challenges. The processing of hard rock sulphidic bearing ores is just one example of the potential problems associated with accelerated weathering. In this case the sulphide minerals more readily oxidise in the tailings facility as a result of the size reduction from milling increasing the surface area and thus exposure of the tailings to air and water. Acid generation and metal mobilisation occur that eventually find their way into the surrounding environment through runoff or seepage. This phenomenon is a well known problem affecting the mining industry and is commonly known as Acid Mine Drainage (AMD) or Acid Rock Drainage (ARD) (Garcia, Ballester et al. 2005; Ritcey 2005).
Production of tailings
The disposal of tailings is commonly identified as the single most important source of environmental impact for many mining operations (Vick 1990). This is not surprising when considering that the volume of tailings requiring storage can often exceed the in-situ total volume of the ore being mined and processed. Over the last century the volumes of tailings being generated has grown dramatically as the demand for minerals and metals has increased and lower and lower grades of ore are being mined. In the 1960’s 10’s of thousand of tonnes of tailings were produced each day and by 2000 this figure has increased to 100’s of thousands (Jakubick, McKenna et al. 2003). Understanding the mineral processing techniques can help to determine how tailings are produced and the challenges associated with their storage.
Run of the mine ore is physically reduced by crushing and grinding methods (figure 1). The optimum degree of grinding is determined by the extraction methods used to remove the economic product. A simple mineralogical examination can hold the key to identifying the most advantageous extraction methods to use. The examination can also determine other minerals of economic interest, the type and quantities of reagents required to separate the concentrate from the gangue materials and the necessary storage methods for the tailings (Ritcey 1989). Pilot plant tests can also be useful to determine optimum particle size, processing reagents required and the final tailings characteristics. However, such pilot tests may not be an exact representative of the tailings that will be produced from the full scale plant. This means that the final design of any tailings facility is always provisional and must be confirmed once tailings production is underway (Blight 1998).
Concentration is the process of extracting the economic product from the crushed and ground ore, the waste from this process is the tailings. Froth flotation (figure 1) is the most widely used concentration method and is normally the first step in the mineral processing sequence where chemical reagents are introduced (Vick 1990). Gravity and magnetic separation techniques are also used to win the economic product from the ground ore. Gravity separation is used in gold processing to recover the coarser particles, the finer being recovered by leaching (EC 2004).
Figure 1: Grinding circuit (left) and froth flotation cells (right) at Highland Valley Copper, BC, Canada
The five basic types of reagent used in froth flotation recovery include collectors, frothers, depressants, activators and modifiers. When designing the processing plant the types and quantities of reagents used should be considered together with any depressing requirements to lessen environmental impacts in the tailings streams (Ritcey 1989). Reagents dosed in small quantities are either consumed, retained in the process or are discharged with the tailings. The design of a tailings storage facility should therefore be optimised to prevent weathering and the mobilisation of contaminants, whilst also increase the degradation rates of reagents stored in the tailings facility. It may be more economical to hold water for longer periods of time to allow natural degradation of reagents rather than promoting rapid degradation through dosing.
Tailings characteristics
Tailings characteristics can vary greatly and are dependent on the ore mineralogy together with the physical and chemical processes used to extract the economic product. Ritsey (1989) reported that tailings of the same type may possess different mineralogy and therefore will have different physical and chemical characteristics. The tailings characteristics have to be determined to establish the long term behaviour of the tailings and the potential short and long term liabilities and environmental impacts. Once the likely characteristics of the tailings are determined from mineralogical examinations and pilot plant tests, the necessary design requirements can be identified to mitigate liability and impact. A certain type of tailings storage method may be preferred or certain design considerations may need to be adapted to a more realistic or suitable storage method.
To help determine the design requirements of a tailings storage facility the following characteristics of the tailings will need to be established (EC 2004):
- Chemical composition (including changes to chemistry through mineral processing)
- Physical composition and stability
- Leaching behaviour
- Behaviour under pressure
- Erosion stability
- Settling behaviour
- Hard pan behaviour (e.g. crust formation on top of the tailings)
The engineering characteristics of tailings are in most instances influenced by the method of deposition. It is therefore essential that while investigating the properties of tailings that the physical characteristics, phenomenon and material parameters (e.g. beach slope angles, particle size segregation) that can occur as a result of varied deposition techniques be identified (SANS 1998).
Once the potential environmental impacts and behaviour of the tailings are identified the process of deciding a suitable storage method can begin.
