In a concession to environmental concerns, Twin Metals Minnesota said Thursday that its proposed copper-nickel mine near Ely will store processed waste using “dry stack” technology, rather than a kind of dam that is common in the industry but has a troubled record of leaks and failures.

The company also came out forcefully on a second risk feared by environmentalists, declaring that because of the low-sulfur type of rock being mined, the relatively dry environment underground and the mine’s design, the Twin Metals operation “will have no acid mine drainage.”

The announcement Thursday morning represents a major move for Twin Metals, which is trying to get ahead of the environmental issues that have dogged PolyMet Mining Inc., Minnesota’s other copper-nickel mine proposal. PolyMet plans to use an old tailings dam on the site it owns.

The Twin Metals announcement previews the official mine plan it will submit to regulators by early winter. That filing will start the state permitting process for the mine and, if permits are approved, would make Twin Metals the second international company, behind PolyMet, to begin tapping the large copper-nickel reserves on Minnesota’s Iron Range.

Environmentalists have promoted the so-called “dry stack method” as a greener way to hold mine waste because it uses less water and is less likely to rupture or leach heavy metals into nearby waters. Tailings dams frequently leak or suffer breaks, and catastrophic failures in recent years have sounded international alarms about dam safety. Dry stacking involves compressing the tailings into fine, sandlike particles, placing them on a liner and piling them in a large mound.

The Twin Metals and PolyMet proposals have triggered a debate over jobs and the environment that many Minnesotans consider one of the most important issues facing the state.

Becky Rom, national chairwoman of the Campaign to Save the Boundary Waters, said Thursday’s announcement does not change her opposition to the Twin Metals mine.

“It’s the wrong use in the wrong place,” said Rom. “There should be no sulfide ore copper mine located in the watershed of the Boundary Waters, period.”

Rom said the tailings strategy that Twin Metals previewed Thursday is not an improvement. Before, the company was considering storing the tailings outside the watershed of the Boundary Waters. Now, it moves the tailings facility back into the same watershed as the Boundary Waters, near the mine site on Birch Lake, which flows directly into the park.

“It’s actually worse,” Rom said. She also noted that even if the mined rock is low in sulfur, “it’s still toxic” and will release heavy metals that can leak and have harmful impacts.

Chris Knopf, executive director of Friends of the Boundary Waters Wilderness, said the Twin Metals announcement “is an improvement in the sense that drinking a cup of rubbing alcohol is an improvement over drinking a cup of bleach.”

Nancy Norr, who heads Jobs for Minnesotans, a coalition of business and labor groups who support the copper-nickel mining, welcomed the announcement as a sign the company is listening to stakeholders. The project will never be good enough for serious opponents, she said.

“Our organization is realistic enough about our use of these metals that we would prefer to work through a regulatory process with the intention to get to ‘yes.’ We need to mine somewhere. If not here, where?”

High demand for copper

During an interview at the Twin Metals St. Paul headquarters, chief regulatory officer Julie Padilla said the company has reviewed the dry stack tailings method used at four mines in cold, wet climates — two in Alaska, two in Quebec — and decided that with technology improvements, the method makes good sense now. She called it “the right solution for this site.”

“The industry is moving in this direction,” Padilla said. “Our confidence has skyrocketed.”

Padilla said she thinks the Twin Metals mine has been misunderstood and will be far cleaner than people realize. “It’s not your 1967 mine,” Padilla said, referring to the year Minnesota’s last underground mine closed.

Twin Metals is a subsidiary of Chile-based Antofagasta PLC, one of the world’s largest copper producers. Antofagasta mines only in Chile but has been seeking to expand. Minnesota is its main international project; it has spent about $450 million so far on the project and plans to spend an additional $1.2 billion to get the mine up and running.

Miners will be working at least 400 feet underground and possibly down to 5,000 feet to get at the band of ore, which runs diagonally. They’ll be extracting copper, nickel, cobalt and platinum — a valuable group of metals used in wiring and pipes as well as lithium ion batteries that power a range of electronics such as laptop computers, phones and electric vehicles.

The Twin Metals mine will operate for 25 years, employ about 700 people and produce about 20,000 tons of ore a day. The concentrate will be sold and trucked out in containers, possibly to the Great Lakes port at Duluth.

Emerging details of the Twin Metals proposal are sure to draw scrutiny from environmental groups, wilderness advocates and mining opponents across Minnesota. The Twin Metals mine has drawn, if anything, even more opposition than the PolyMet proposal because it lies so close to Minnesota’s cherished Boundary Waters wilderness.

Opposition has been strong and getting stronger, some observers say, as the mines move closer to reality. Next month, for example, Rom’s organization is throwing a first-ever Wild Waters Music Fest to Save the Boundary Waters in Duluth.

Low sulfur?

Twin Metals officials are emphasizing several green features of the mine. It won’t discharge any wastewater, for instance, since all the water the operation uses will be held in pools on the site and recycled. It now estimates the operation will use 300,000 to 600,000 gallons of water a day, drawn from Birch Lake. By comparison, the city of Ely uses 533,000 gallons per day.

Most importantly, Twin Metals insists that the new mine will not generate acid drainage, which occurs when sulfur-bearing rock is mined and exposed to oxygen and water. The sulfur can acidify water and is a serious pollutant that can affect lakes, streams and drinking water.

The company contends that the minerals will be accurately extracted from the ore so that the tailings left behind will have a sulfur content of .15% or less. That’s below the .2% sulfur content that research has indicated can generate acid drainage, Padilla said.

Another protection, she said, is that all the waste rock mined will remain underground — only ore will be hauled to the surface for processing — so it won’t sit exposed to the elements. The company has also studied the movement of water underground at the site and determined that the environment is quite dry — very different from the region’s watery surface.

Only half the tailings left over from processing the ore will be stored in the mound above ground. The other half will be mixed into a cement-like compound and pumped back underground into used portions of the mine.

“We are confident we will not have an acid generating issue with this project,” Padilla said. “We will have no acid mine drainage.”