The safety of storing mining waste in a tailings basin has been a critical part of the debate over copper-nickel mining in northern Minnesota, with some environmental advocates warning that failures and spills could unleash toxic slurry into nearby waters.
Now, in a major shift, one of two companies hoping to build a copper-nickel mine says it plans to store much of its waste using a “dry stack” method, an emerging technology that many of the same environmental nonprofits — and some mining experts — argue will better prevent water pollution.
Twin Metals Minnesota, which plans to mine just outside the Boundary Waters Canoe Area Wilderness, said Thursday it would abandon its plan to use a tailings basin, which entails waste rock being covered in a pond held back by a dam. The company instead plans to use dry stacking, a process by which miners remove water from waste and work it into a sandy mixture which is then covered with soil and vegetation and put on a liner.
The state Department of Natural Resources has previously questioned whether the method can work in Minnesota’s cold and wet climate. But Julie Padilla, Twin Metals’ chief regulatory officer, called dry stack tailings “best available technology.”
“The key is, it removes that tailings dam,” Padilla said in an interview. “It removes any potential for failure of a dam and spillage in that sense — which is obviously one of the big concerns about this project and others.”
Aaron Klemz, a spokesman for the Minnesota Center for Environmental Advocacy (MCEA), said dry stacking is generally better than having a tailings basin and dam, but said he wanted to see more specifics on the plan.
Meanwhile, Becky Rom, national chairwoman of the Campaign to Save the Boundary Waters, told MinnPost the dry stack announcement has “made the project even worse.”
That’s because Twin Metals is now planning to store its mine waste on site, within the watershed of the BWCA, instead of creating a tailings basin within the watershed of Lake Superior. “This is not an improvement in our minds at all,” Rom said.
Twin Metals is preparing a formal mine plan for state and federal regulators to consider later this year.
The pivot to dry stack tailings may still reverberate beyond Twin Metals. It could also fuel debate over PolyMet, the other copper-nickel mine proposed in the region. That $1 billion project near Hoyt Lakes has all necessary permits for construction and is likely to become the first copper-nickel mine in the state. But many of PolyMet’s opponents have criticized its plan for a tailings dam. The project is also facing renewed scrutiny of water protections in a key state permit.
Padilla said she thinks mines with “conventional tailings” can be safe. At the same time, she said dry stack is “certainly the way that this industry is moving.”
The tailings switch, explained
Twin Metals plans to mine in part of the enormous Duluth Complex mineral deposit for copper, nickel, platinum, palladium, gold and silver. The company promises to bring 650 full time jobs and another 1,300 indirect jobs to the region.
The project has ignited debate and attracted national attention. Opponents argue it could pollute the nearby BWCA, wreaking havoc on a pristine wilderness and blunting tourism in northern Minnesota.
Copper-nickel mining carries unique environmental risks that the iron ore mining in the region does not. The extraction process can cause an acidic runoff that can leach heavy metals into water.
In a 2016 memo about Twin Metals, Tom Tidwell, the former United States Forest Service Chief, wrote so-called acid mine drainage can cause “serious and irreplaceable harm” to the BWCA and can happen in all phases of mining operation, from construction to a tailings basin failure.
Twin Metals has long asserted that improvements in mining technology will prevent pollution. It also claims company research shows the mine would produce tailings that would not generate acid because of careful extraction methods and the composition of the rock. (Environmental groups dispute this and point to other research that says risks remain.)
Twin Metals expects to store about half its tailings as “permanent cemented backfill,” according to its website, in the underground mine itself about nine miles southeast of Ely in the watershed of the Rainy River and the BWCA.
It had originally wanted to move the remaining leftover tailings to a basin near Babbitt — in a separate watershed that drains into Lake Superior. Instead, it now plans to store the remaining tailings using the dry stack method within the BWCA’s watershed. Padilla compared the process to a French-press coffee maker: crushed waste-rock and water created by the mining process will be pressed and dried to remove most of its moisture.
That material, which Padilla described as similar to “sand castle sand,” will be compacted and stored in a pile. A “gravity drainage” system will collect any moisture that remains, according to Twin Metals, and all leftover water will be recycled back into the processing plant.
“It ends up being really the most sustainable way that tailings are being managed today,” Padilla said.
Why the change
Dry stacking has many benefits for Twin Metals. It’s generally more expensive than using a conventional tailings dam, but it would also allow the company to avoid building and operating a costly pipeline to ship tailings out of the Rainy River Watershed. Padilla did not have an estimate of how dry stacking on site would affect the company’s bottom line, though she did say the change would add 50 jobs to the employment estimate directly related to the mine.
The political ramifications may be as important as the logistical rewards. The MCEA has called for copper-nickel mines in the state to adopt dry stack tailings management. The nonprofit points to catastrophic tailings dam failures in Brazil and British Columbia as just one reason they should be used.
The MCEA wrote a bill introduced in the Legislature this year that would impose stricter environmental standards for tailings dams at new copper-nickel mines. It was similar to a bill crafted by the mining industry in Montana that passed through the state’s Republican-led Legislature in 2015.
In Minnesota, the bill would have exempted mines that use the dry stack technology from those stricter standards. After a tailings dam breach at the Mount Polley mine in British Columbia, an independent review panel on the failure said dry stack tailings management is a “prime candidate” for best available technology to prevent future disasters.
The MCEA’s Klemz said they still wanted to see details of Twin Metals’ dry stack plan before they decide whether it is “feasible and useful.”
“It’s not a panacea, but it is an improvement,” Klemz said.
The switch to storing mine waste on site did not worry Klemz as much as the Save the Boundary Waters Campaign. “Whether it’s Lake Superior or the Boundary Waters at stake, all of those waters need to be protected,” he said. “And we will look at the proposal from that perspective.”
Not all are sold on dry stack technology, however. When the DNR approved dam safety permits for PolyMet in November of 2018, state regulators wrote the method is generally used in “very remote arid or arctic environments.”
In wet climates, dry stacking has “major disadvantages,” state documents noted. If dry stack material becomes wet — but isn’t fully submerged in a tailings pond — it can leach heavy metals that can wash into nearby soil and water, the DNR said. The agency also said dry stack tailings can create a toxic dust that also poses environmental risks.
Klemz and Rom, of the Save the Boundary Waters Campaign, reiterated that acid mine drainage doesn’t just come from tailings basins, and disputed the notion dry stack technology would eliminate risk of water pollution.
Twin Metals’ Padilla said the company has studied four mines in environments similar to northern Minnesota as a guide to using dry stack successfully. Two are in Alaska and two are in Quebec. Padilla noted one of the projects studied, the Greens Creek mine, is on Admiralty Island National Monument in a temperate rainforest southwest of Juneau.
To deal with cold and wet weather, Padilla said Twin Metals can stagger its work to only dry stack when conditions allow. When the weather is bad, the company can put tailings back into the underground mine as planned.
Complications for PolyMet?
Twin Metals’ announcement could heighten calls from environmental advocates for the other major proposed mining project in Minnesota, PolyMet, to switch to the dry stack method. In legal challenges to PolyMet, the MCEA and others have argued it would be a safer alternative.
“It is fair to say this demonstrates clearly that PolyMet wasn’t telling the truth when they said a dam was necessary to store their liquid mine waste at their proposed copper-nickel mine,” Klemz said.
Beyond its concern about Minnesota’s wet climate, the DNR said PolyMet’s tailings basin has its own upside because of the specific location of the project. PolyMet expects to repurpose an existing tailings pond used by a former LTV Steel processing plant.
Creating a new dry stack disposal area instead of reusing the basin would therefore “increase the footprint effects of the proposed project,” says the DNR’s permit documents. Those approvals were overseen by former DNR chief Tom Landwehr, who is now executive director of Campaign to Save the Boundary Waters. PolyMet would sit in the watershed that drains into Lake Superior, not the BWCA, although it is close to the Twin Metals mine.
A PolyMet spokesman did not immediately respond to a request for comment Thursday morning. The project has faced renewed criticism and three separate investigations in recent weeks following accusations that the Minnesota Pollution Control Agency hid concerns the Environmental Protection Agency had with the strength of a key water pollution permit. The MPCA denies the allegations.
Ultimately, Padilla predicted Twin Metals’ switch to dry stack tailings would not win over staunch critics of Twin Metals, which is owned by Chilean mining giant Antofagasta. But she said the move represents a dedication to creating a “zero discharge” facility. “I think it’s fair to say we heeded the evidence that is out there and we listened to the community concerns,” she said.