Figure 1: Thickened discharge at Kidd Creek, ON, Canada (left) and at Mt Keith, Western Australia (right)
Thickened tailings, as the name suggests, involves the mechanical process of dewatering low solids concentrated slurry (Fourie 2003). This is normally achieved by using compression thickeners or a combination of thickeners and filter presses (DPI 2003). Thickened tailings are defined as tailings that have been significantly dewatered to a point where they will form a homogeneous non-segregated mass when deposited from the end of a pipe (Welch 2003). When placed layer by layer the thickened tailings will dry to near its shrinkage limit and become dilative under dynamic shaking, thus preventing the possibility of liquefaction (ICOLD and UNEP 2001).
The idea of Thickened Tailings Disposal (TTD) is to stack the pulp to form a self supporting conical pile thus reducing the height and retention forces of the containing perimeter embankments. The tailings are generally discharged from topographical high points within the tailings storage facility or by riser towers or central ramps (ICOLD and UNEP 2001). Water remaining after deposition and any surface runoff is collected in a pond at the toe of the pile. Typical slope angles of 1 – 3.5 degrees can be achieved to form a self draining easy reclaimable shape (ICOLD and UNEP 2001).
In 1973 the Falconbridge owned Kidd Creek Metallurgy Plant in the town of Timmins, Ontario, Canada was the first site to utilise surface thickened disposal of tailings (Engels and Dixon-Hardy 2004). The conventional storage facility was converted to TTD to eliminate further raising of the traditional retaining embankments that were situated on very soft and sensitive clay foundations (ICOLD and UNEP 2001). The tailings are discharged from spigots on a central ramp that is currently 20m high . At the base of this ramp is the 32 m diameter high compression thickener that produces a slurry density of 60 – 65% solids (Golder 2005).
Operating costs are higher for thickened disposal due to the associated dewatering costs. However, there are significant advantages to thickening tailings. Perhaps the most important is that water is conserved and evaporation is minimised in arid climates. Sustainable water use in the mining industry is becoming increasingly more important (Welch 2003). The potential for recovering high volumes of water at the plant (by the thickeners) eliminates the losses associated with the transport and storage of water either at the tailings facility or in holding ponds (Fourie 2003). Environmental problems such as seepage, spillage of process water and the potential for water to act as a transporter for tailings flows (e.g. embankment breach) is significantly reduced. Some other main advantages to thickening tailings are:
- No large starter dam is required which significantly reduces capital cost.
- Reclamation costs are significantly lower than impoundments that store tailings and water. This is particularly true when considering a thickened tailings facility can be progressive restored, thus reducing closure costs.
- Concerns of the instability of high retaining embankments associated with conventional storage are potentially addressed (Fourie 2003).
- Little or no ponded water to manage which not only reduces seepage but reduces water pumping costs to and from the processing plant.
- Little or no solid/liquid separation results in less oxygen ingress which will reduce oxidation and thus the generation of acid from sulphur bearing tailings (Welch 2003).
- Modification of the tailings properties by adding binders to increase the static and dynamic stability, reduce the likelihood of erosion and prevent seepage (e.g. bentonite addition) (DPI 2003).
Besides the costs associated with thickening the tailings, many mine operators are reluctant to switch from conventional to thickened storage even though wet deposition is more problematic. The argument could also lead to the fact that thickened tailings is relatively unproven and requires larger areas of land compared to conventional storage. This in itself can immediately rule out thickening if space is a constraint.
In the future, the benefits of thickened tailings storage will be more prominent, particularly as environmental regulations tighten and increased pressure is placed on the mining industry to become more sustainable (Welch 2003).
