What are tailings and how are they made?
Tailings are residues created as part of mining, refining, smelting and water treatment processes. These processes are never 100 per cent efficient, nor is it possible to reclaim all reusable and expended processing chemicals. Tailings are discharged – normally as slurry – to a storage facility, where water is subsequently recovered and recycled back into the operation.
Tailings are mainly discharged in two ways. The first is to use below water approaches. The second is to use above water/above ground approaches. Tailings can be modified in other ways, such as:
- dry stack - dewatering to a wet or dry cake that cannot be transported via a pipeline
- paste - includes dewatering until the tailings do not segregate
- thickened tailings - dewatering to form a low solids content slurry
Tailings storage facilities
Tailings storage is one of the biggest design decisions in the development of a mine. There are many factors which need to be considered in selecting the optimum site and construction method. Site conditions such as topography, rainfall, seismic activity, mineral characteristics and proximity to people dictate appropriate tailings sites, technology and storage solutions. The aim is to safely contain the tailings under any and all circumstances.
As a result, each tailings storage facility is unique. Depending on the environment and the chemical characteristics of the tailings the tailings compound may be lined, using a variety of lining systems which are designed to prevent impacts to surface and groundwater systems. As the tailings slurry is collected in the compound, the water separates from the heavier sand and silt particles and is collected at the surface. The tailings pond water is then recycled back into the operation for reuse and the tailings may be reclaimed so as to minimise the impact on the environment.
The construction method is important to ensure safe operation of a facility. All tailings storage facilities follow one of three wall construction designs, or hybrids of these. We also store tailings within some of our completed mining pits or in excavated or underground storage, which generally will not require the construction of the following dam wall options.
Upstream designs start with a free draining starter wall as the foundation. The tailings are then discharged into the dam, which then harden to form the foundation for the next level of wall. Ensuring water is properly removed and recycled is a crucial element of maintaining the structural integrity of the wall. These are suitable for facilities in areas of low rainfall, low seismic activity and relatively flat terrain.
Downstream designs start with an impervious starter wall unlike the upstream design that has a free draining starter wall. The tailings are then discharged into the dam and as the embankment is raised, the new wall is constructed and supported on top of the downstream slope of the previous section. This shifts the centreline of the top of the dam. The downstream design was developed for areas with seismic activity and high rainfall or water collection. A number of Rio Tinto facilities use the downstream construction method with a single embankment.
The centreline method is a hybrid of upstream and downstream designs. When the wall level is raised it is built on top of both the tailings and the existing wall. In contrast to the downstream design, the centreline remains the same throughout the subsequent raises.
Modified centreline construction is a centreline dam that is slightly raised using upstream construction. When the wall level is raised it is built on top of both the tailings and the existing wall, but slopes slightly upstream.
Rio Tinto therefore takes into consideration the specific conditions on each site before deciding on the appropriate tailings sites, technology and storage solutions.