Dewatering tailings to higher degrees than paste produces a filtered wet (saturated) and dry (unsaturated) cake that can no longer be transported by pipeline due to its low moisture content. These filtered tailings are normally transported by conveyor or truck, deposited, spread and compacted to form an unsaturated tailings deposit (Davies and Rice 2001). This type of tailings storage produces a stable deposit requiring no retention bunding and is referred to as ‘dry stack’. Figures 1 and 2 show the La Coipa dry stack tailings facility in Chile.
Figure 1: The La Coipa dry stack facility in Chile (Courtesy Anglo American/Debswana)
A typical moisture content of less than 20% is achieved by using a combination of belt, drum, horizontal and vertical stacked pressure plates and vacuum filtration systems (Martin, Davies et al. 2002). The term ‘dry cake’ or ‘dry stack’ is not entirely correct as the tailings have a moisture content several percent below saturation. However, this terminology has been adopted by regulators and designers (Davies and Rice 2001).
Figure 2: Dry stacking of tailings from a conveyor at La Coipa, Chile (Courtesy Anglo American/Debswana)
Like with thickened and paste tailings, the mechanical dewatering process compared to conventional slurry deposition increases costs. Producing wet and dry cake further increases this cost particularly if high throughputs are required. However, some of the many advantages to using dry stacking of tailings are:
- Can be used in areas where water conservation is critical and any water losses can jeopardise plant performance.
- Dry stacking is suited to areas of high seismic activity as the construction of retention embankments is prevented.
- For cold climates, dry stacking prevents pipe freezes and frosting problems with conventional impoundments.
- Groundwater contamination through seepage is virtually eliminated.
- Filtered tailings allow better recovery of dissolved metals and process chemicals (e.g. gold and cyanide).
Dry stack facilities are also easier to close and rehabilitate, require a smaller footprint compared to other surface tailings storage options (i.e. higher density), can be utilised in aggressive environments (e.g. undulating and steep terrain) and generate better regulator and public perceptions of tailings storage (Davies and Rice 2001).
