Tailings are created as mined ore is processed sand 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 will generally be 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. The tailings are contained within the facility and once it reaches capacity, the facility is 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 store both tailings and water. The dam construction methods include two main types: (1) water retention dams and (2) progressively raised embankments. Water retention dams are typically constructed to their full height prior to anything being stored upstream and raised embankments are progressively raised in a vertical manner over time to store additional material. Raised embankments are the most commonly used method for TSFs. The raised embankment design methods for TSFs are typically downstream, upstream or centerline. This designates the direction in which the embankment crest moves in relation to the starter dam (dyke). Modified centerline is a construction method combining both upstream and centerline.
Upstream
Construction of an upstream embankment begins with development of a starter dyke. The tailings are then discharged from the dam crest and form the foundation for future raises. Figure 1 shows an overview of the stages of construction.
Figure 1: Upstream construction method
Downstream
Downstream methods commence with a starter dyke, which is often impervious with an internal drainage system, as shown on Figure 2. The tailings are first deposited behind the dyke and the embankment is raised in a downstream manner over time.
Figure 2: Downstream construction method
Centerline
With the centerline method, the embankment is raised vertically, maintaining the dam centerline embankment 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.
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 can be completed in conventional TSFs, as well as offshore or within lakes or pits.
Tailings can be dewatered or modified in other ways prior to deposition. The current methods include:
- Thickened tailings (which involves a process of dewatering to form a low solids content slurry);
- Paste (which includes dewatering until the tailings do not segregate as they are deposited and have minimal excess water);
- Dry stack (includes dewatering to a filtered wet or dry cake that is transported via trucks or conveyors); and
- Co-disposal which includes mixing mine waste with dewatered tailings (other terminology includes co-mingling, co-placement or co-deposition whereby each has slightly different methods of mixing material).