La Coipa, Region III, Chile, South America
Figure 1: Dry stack facility at La Coipa Mine, Chile (Anglo American/Debswana)
La Coipa is located around 3800 m above sea level in the Atacama Desert, a very arid area of northern Chile. Snow fall is the prodominant form of precipitation in the winter months with freezing temperatures commonly occuring year round. Water sources at the mine are limited and filtered tailings were considered due to the economics and feasibility of the project. At present the La Coipa mine has the largest tailings dry stack (AMEC, 2008).
Dry stacking of tailings at La Coipa has been possible due to advancements in large capacity horizontal belt vacuum filters. The tailings (18,000t/d) are dewatered to contain <20% water content (depending on the specific gravity) allowing them to be either trucked or transported by conveyor to the disposal site. The conveyor system is mobile incorporating an advancing plough to allow the tailings to stack in lines in parallel con the conveyor (radial stack arrangement). The tailings are then spread out and compacted to increase the density of the stack. The distribution conveyor 'walks' over the dry stack forming the tailings layer. Typical layer thicknesses are 20 - 30 m.
Figure 2: The dry stack facility (left) and plough (right) on the distribution conveyor (Courtesy Anglo American/Debswana)
Advantages and Disadvantages
For La Coipa, the following advantages were considered:
- The high density of the tailings reduces the overall volume of the storage facility required (less land disturbance).
- Filtration can help to recover dissolved gold and process solution from the tailings.
- Stability would be improved compared to conventional wet deposition, particularly considering the seismic risks associated with the region.
- The facility could be rehabilitated quickly and progressive if required.
- The dry stack could be raised to heights which would not be economical with conventional impoundments in the area considered.
- Seepage losses and groundwater contamination would be reduced.
- Considered to be the most effective design to minimise make-up water costs.
- For cold climates, dry stacking prevents pipe freezes and frosting problems associated with conventional storage.
- Binders (cement) can be used to increase the stability of the deposit if required.
However, the following disadvantages were determined during the design stage:
- Higher capital expenditure when compared to a conventional impoundment.
- At the time of design and implementation the technology and disposal method was largely unproven.
- Problems with
dust and tailings becoming airborne.
Martin, T. E.,
M. P. Davies, et al. (2002). "Stewardship of Tailings Facilities"
Engels, J. and D. Dixon-Hardy (2004). "Tailings disposal - Today's storage of high volumes of waste from mines". JKMRC Conference 2004, Brisbane, Australia.
AMEC (2008). "Rosemont Copper Company, Filtered Tailings Dry Stacks - Current State of Practice" - Final Report:: 30.
PlacerDome – Private communications, 2004