ST. PAUL, Minn. (AP) - The developers of a proposed copper-nickel mine on the edge of the Boundary Waters Canoe Area Wilderness said Thursday they plan to use a potentially safer dry method of storing mine waste instead of the kind of wet tailings pond more common in the industry.
Twin Metals Minnesota said its underground mine near Ely would use “dry stack” storage for about half the mine’s waste, calling it the most environmentally friendly approach for the site. The method contrasts with the conventional tailings basin at the planned PolyMet mine and processing plant near Hoyt Lakes, where the slurry will be contained by an earthen dam. The other half of Twin Metals’ tailings would be put back underground in old portions of the mine.
Dry stacking involves removing most of the water from the tailings after the copper, nickel and platinum-group metals are extracted from the ore. Twin Metals would then stack and compact the finely ground tailings, which by then would be the consistency of sand, into a hill about 120 feet (36.58 meters) tall, on top of a liner, with a system designed to capture moisture that seeps from the structure. It would eventually be covered with native soil and vegetation.
Experts say dry stacking would eliminate the chance of a catastrophic dam failure, like mine dam collapses in recent years in Canada and Brazil, and that it offers a better chance of eliminating the need for long-term or perpetual water treatment after the mine closes.
While environmentalists pushed unsuccessfully for dry stack storage at PolyMet, they criticized Twin Metals’ plan because the waste site would sit just a couple miles from the Boundary Waters, in the same watershed as the wilderness area. An earlier plan would have run a long slurry pipeline to a site near Babbitt that’s in a different watershed that doesn’t flow toward the BWCA.
Becky Rom, national chair of the Campaign to Save the Boundary Waters , said Twin Metals still threatens the Boundary Waters.
“That unacceptable risk is in no way reduced by today’s announcement, and is actually made worse by the fact they are putting the tailings basin right next to the wilderness,” she said.
But Twin Metals’ chief regulatory officer, Julie Padilla, said the mine will be cleaner than many people expect. She said it won’t generate acid mine drainage, which happens when sulfide-bearing rock is exposed to air and water. That’s been the main fear of environmentalists. But she said the processing technology will leave little sulfur in the tailings. Environmentalists’ reacted skeptically to that claim.
Twin Metals, owned by Chilean mining company Antofagasta, plans to release its formal mine plan in the coming months, which will trigger a lengthy environmental review and permitting process. PolyMet, whose majority owner is Swiss commodities giant Glencore , already has its permits and is now raising construction financing.
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