Dry stacking is the most sustainable method used to store filtered tailings—silty, sandy material— produced from the mine processing plant after the 4% of the ore that is copper, nickel and other metals is extracted. Tailings stored in dry stacks are piles of sand topped by native soil and vegetation. There is no need for a dam to hold them in place, no possibility of dam failure, and no long-term storage issues. Due to the deposit’s geology, tailings will be non-acid generating. These tailings can be safely exposed to air and water because all but trace amounts of sulfides will be removed from them during processing.
The process begins with large rocks crushed, ground and mixed into a slurry of water and fine rock particles. The sulfides contained in the rocks are separated and recovered into concentrates through a flotation process and prepared for shipment to customers. The residual crushed rock, or tailings, and water are then filtered and separated. The water is recycled back into the processing plant.
At Twin Metals’ underground mine, the filtered tailings, by now the consistency of sand castle sand, will be transported to the dry stack area near the processing plant. That area will include a gravity drainage system to collect whatever moisture remains in the tailings into reclaim ponds, which will be recycled back into the processing plant. The dry stack material will be compacted to ensure stability, and the stack will average around 120 feet, consistent with the topography of the area.
- Stable structure means no dam, and no possible tailings dam failure
- The water generated from the filtering process to create the dry stack material is recycled to the processing plant (closed loop)
- Stacked tailings will be placed on a liner, reclaimed on top with native soil and vegetation
- Tailings will be reclaimed in stages, not all at once – a process called progressive reclamation
- Significantly smaller footprint than conventional wet tailings storage