ST. PAUL, MN — Twin Metals has announced the company will be using an environmentally friendly dry stack method to store leftover rock from the proposed underground copper-nickel mine near Ely.
According to the company, the dry stack method eliminates a storage pond and dam, which are often associated with conventional tailings facilities, and have been used in four mines in the northern United States and Canada.
Tailings are the crushed rock left over at a mine after target minerals are removed.
Twin Metals said by using the dry stack method, remaining tailings will be compressed into low-moisture, sand-like deposits, and will be stored on a lined ground facility near the plant site.
The tailing site will later be reclaimed in stages, and can be capped or covered with natural vegetation.
The company said those sites also have similar climates to Minnesota, and have been permitted at two mines in the western United States.
Kelly Osborne, the chief executive officer of Twin Metals Minnesota, said since there’s no dam, there’s no risk of dam failure. She added that dry stack is considered the best available technology for tailings storage.
Environmentalists say dam failure or leaks could threaten nearby surface water and groundwater.
Osborne said the geology of the Maturi Deposit gives the company confidence they can mine safely and sustainably, as the tailings will be non-acid-generating.
According to Osborne, the rock layers which sandwich the layers of copper, nickel and platinum are almost completely free of sulfides, and when the targeted minerals are removed, only a small amount of suulfides will remain in the tailings.
Twin Metals said dry stacking tailings has been an option the company has been considering since plans for the mine started in 2010.
The company said their tailings approach will be outlined in detail in their Mine Plan of Operation, which will be submitted to state and federal regulators in the next few months.
According to Twin Metals, the approach begins with large rocks being crushed and grouped up before being mixed with water and fine rock particles, with the sulfides contained in the rocks separated from the rest of the materials, and will be recovered into concentrates through a flotation process.
The residue, or tailings, as well as water, will then be filtered and separated before the water is recycled back into the processing plant.
Meanwhile, the company said the tailings will be taken to the dry stack area near the processing plant, which will include a gravity drainage system to collect any moisture still in the tailings to reclaim ponds, which will also be recycled into the processing plant.
Officials say the tailings will then be compacted to ensure stability, and will measure about 120 feet.
In addition, the company said about half of the tailings produced will be sent back underground, and will be backfilled and cemented to provide more support.
Environmentalist group Save the Boundary Waters issued the following statement regarding Twin Metals’ announcement: