Tailings are created as mined ore is processed through crushing, grinding and milling. Mined ore is moved to the milling circuit where the rock is reduced into sand and silt sized particles and then mixed with water and moved as slurry through the gold, silver and copper recovery process. The valuable minerals are separated from the rest of the milled rock particles either through physical or chemical recovery processes. After removal of the valuable minerals, the remaining milled rock slurry, now referred to as tailings, is pumped, flows by gravity, or is dewatered and transported by truck or conveyor to a surface engineered facility.
Tailings Storage Facility - Akyem Mine TSF Cell 1 (background) and TSF Cell 2 (foreground), Ghana Africa
These engineered facilities are carefully designed, constructed and operated to safely contain the tailings and water, even during extreme climatic or seismic events. Depending on the chemical characteristics of the tailings and the surrounding environment, the engineered tailings facilities are often lined with a composite liner system consisting of a low permeability soil liner overlain by a geosynthetic liner such as high density polyethylene (HDPE) to prevent impacts to surface and groundwater systems.
Where tailings slurry is deposited in the facility, the water separates from the heavier sand and silt particles and collects to form a decant/reclaim pond on the surface. The tailings pond water is then recycled back into the milling process for reuse or treated for return to the environment. The tailings are contained within the facility and once it reaches capacity, the facility is typically reclaimed with a designed cover system used to minimize erosion and infiltration, while maintaining containment of the materials, protecting the environment and achieving post-mining designated land use.
TSFs are designed and constructed to safely store tailings and, in many cases, water. TSF embankments are typically progressively raised using one of three construction methods - downstream, upstream or centerline - which designates the direction in which the embankment crest moves in relation to the starter dam (or dyke). Modified centerline is a construction method combining both upstream and centerline. Each of these construction methods is discussed below.
Construction of an upstream embankment begins with development of a starter dam. The tailings are then discharged from the dam crest and form the foundation for future raises. Figure 1 shows a schematic of how progressive stages are constructed.
Figure 1: Upstream construction method
Downstream methods commence with a starter dam, which often comprises an internal drainage system, as shown on Figure 2. The tailings are first deposited behind the dam and the embankment is raised progressively in a downstream manner as additional storage capacity is required.
Figure 2: Downstream construction method
With the centerline method, the embankment is raised vertically, maintaining the embankment centerline as shown on Figure 3. This design method often also incorporates internal drainage, and requires construction of a free-draining shell. Modified centerline is a combination of upstream and centerline methods and is done to reduce the volume of construction material that is required to be placed within the embankment as well as reduce downstream impacts.
Figure 3: Centerline construction method
Tailings can be discharged using subaqueous (below water) or subaerial techniques. Subaerial deposition is more common than subaqueous as it forms a sloping beach toward the reclaim/decant pond. Subaerial can be done from a single discharge point, or multiple discharge points rotated around the facility. Subaqueous deposition is normally completed when there is a potential for oxidation that could result in mobilized acid mine drainage. Subaqueous deposition is typically performed within lakes or pits.
Tailings are typically dewatered to a certain degree, or modified in other ways prior to deposition, including:
- Thickened tailings (which involves a process of dewatering to form a slurry);
- Paste tailings (which includes dewatering until the tailings do not segregate as they are deposited and have minimal excess water);
- Filtered tailings (includes dewatering to a filtered wet or dry cake that is transported via trucks or conveyors); and
- Co-deposition includes mixing mine waste with tailings (other terminology includes co-mingling, or co-placement whereby each has slightly different methods of mixing material and degrees of tailings dewatering).