What Are Tailings? - Their nature and production
Figure 1: Multiple active spigot discharge (© Jon Engels)
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 methods of handling and storage are of great and growing concern.
Tailings are generally stored on the surface either within retaining structures or in the form of piles (dry stacks) 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 relevant section which can be accessed here .
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 free 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. The particular design of these retaining structures is unique to a particular environment and mineral processing 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 and ground conditions are the most crucial parameters that control tailings storage methodology 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. In industry, this is achieved by implementing a trade-off study, usually during the pre-feasibility stage of project development. A selection of options from this study can be taken through to feasibility stage to assess environmental, social, economic and associated risk and operational factors with a higher level of confidence.
This page briefly highlights the generation and nature of tailings. The various tailings storage and discharge methods used today are discussed in the relevant sections of this website. The water management considerations discussed are only specific to conventional storage and should highlight why this method is problematic compared to alternative storage techniques that discharge less water to the tailings storage area (e.g. high density tailings disposal, dry stack).
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 can consequently increase their 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).
The processing of hard rock sulphidic bearing ores is just one example of 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 can 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). Depending on the mining project, alternative tailings storage techniques such as sub-aqueous disposal (below water deposition), high density thickened tailings or dry stacking can be implemented to control oxidation of sulphides and the mobilisation of metals.
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 grades of ore are being mined through advances in extraction and processing technology. In the 1960’s 10’s of thousand of tonnes of tailings were produced each day and by 2000 this figure had increased to 100’s of thousands (Jakubick, McKenna et al. 2003). Today there are individual mines producing in excess of 200,000 tonnes of tailings per day. 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 2). 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 2) 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). Refractory ores are commonly processed using pressure oxidation, bioleaching and roasting prior to leaching techniques. These types of pre-processing are often associated with ultra fine grinding that generates tailings with slow settling and low in-situ density properties.
Figure 2: Grinding circuit (left) and froth flotation cells (right) at Highland Valley Copper, BC, Canada (© Jon Engels)
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.
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. Ritcey (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 behaviour of the tailings once deposited in their final storage location and the potential short and long term liabilities and environmental impacts. Once the likely characteristics of the tailings are determined from laboratory and pilot plant tests, the necessary design requirements can be identified to mitigate environmental impact as well as determine optimum operational performance.
Liberation of water from the tailings once discharged in a facility and the volume available for return pumping to the processing plant is an important design parameter influencing the water balance of a mining project (make-up water costs). This liberation is dependent on the physical properties of the tailings deposited and can be estimated through laboratory testing of the tailings at different solids concentrations. This parameter can influence the type of tailings storage method used to prevent discharge of water to a tailings storage area (e.g. paste and dry stacking techniques) in addition to minimising seepage and evaporation losses.
To help determine the design requirements of a tailings storage facility, the following characteristics of the tailings will need to be established (based on EC 2004):
- Chemical composition (including changes to chemistry through mineral processing) and its ability to oxidise and mobilise metals
- Physical composition and stability (static and seismic loading)
- Behaviour under pressure and consolidation rates
- Erosion stability (wind and water)
- Settling, drying time and densification behaviour after deposition
- Hard pan behaviour (e.g. crust formation on top of the tailings)
The engineering characteristics of tailings are in most instances influenced by the degree of thickening and the method of deposition. It is therefore essential that while investigating the properties of tailings that the physical characteristics and material parameters (e.g. beach slope angles, particle size segregation, water recovery) that can occur as a result of varied deposition techniques be identified (SANS 1998). This is particularly true when considering high density tailings disposal and its associated transportation and deposition challengers.
Once the potential site specific parameters (e.g. environmental, social, geotechnical, cost) and the characteristics of the tailings and their behaviour upon deposition are determined, the process of deciding a suitable storage method can begin.