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Twin Metals announces plan to use dry-stack tailings disposal

Proposed tailings facility would be located near mine site, within BWCAW watershed

Marshall Helmberger
Posted 7/17/19

ELY— Twin Metals Minnesota announced on Thursday that it plans to use a more modern form of tailings storage than has been permitted for the proposed PolyMet Mine near Hoyt Lakes.

Known as the …

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Twin Metals announces plan to use dry-stack tailings disposal

Proposed tailings facility would be located near mine site, within BWCAW watershed

Posted

ELY— Twin Metals Minnesota announced on Thursday that it plans to use a more modern form of tailings storage than has been permitted for the proposed PolyMet Mine near Hoyt Lakes.

Known as the dry stack method, the new approach to tailings storage would eliminate the need for a tailings pond and large dam, reducing the risk of dam failure. According to a Twin Metals press statement, dry stack tailings storage has been used successfully in four mines in the northern United States and Canada with similar climates to Minnesota and has also been permitted at two mines in the western U.S.

Twin Metals is a joint venture controlled by Antofagasta, a Chilean mining conglomerate. The proposed copper-nickel mining operation would be located along the South Kawishiwi River, a major watershed for the Boundary Waters Canoe Area Wilderness. The location of the mine and its potential impact to the wilderness has been the primary focus of opponents of the Twin Metals proposal.

Mine tailings represent the crushed rock left over after target minerals are removed. According to Twin Metals, the remaining tailings will be compressed into low-moisture, sand-like deposits and stored on a lined ground facility near the plant site. Reclamation of the tailing site can occur in stages and can be capped or covered with natural vegetation.

“Dry stack tailing storage is the most environmentally-friendly tailings management approach for our site,” said Kelly Osborne, Twin Metals CEO. “The first key is that there’s no dam, no risk of dam failure. The moisture content of the filtered tailings is reduced to a material that we can compact and manage seasonally.”

According to the company, dry stack tailings storage, often promoted by environmental groups as an alternative to conventional wet tailings as a way to protect water quality, has been an option under consideration since Twin Metals began mine planning in 2010. As technology has continued to advance, and the application of dry stack in cold, wet climates has proven successful at multiple locations, Twin Metals made the decision to move to it as the best available option.

“Dry stack is one of the ways we are making a 21st century mine that will be the most technologically-advanced mine in Minnesota’s history and a model of how copper mining can be done safely and sustainably,” said Osborne.

While environmental groups have frequently advocated for dry stack tailings storage as an improvement over wet slurry tailings management, opponents of the Twin Metals proposal say the bigger issue is that this week’s announcement means the tailings basin will be located near Birch Lake, putting it within the watershed of the Boundary Waters. An earlier mine plan proposed by Twin Metals in 2014 called for wet slurry disposal of the tailings in a facility located in the Lake Superior watershed, which would have sent any toxic discharge into the St. Louis River rather than towards the 1.1 million-acre BWCAW.

“When the Forest Service rejected the Twin Metals proposal in 2016, their Record of Decision assumed that the tailings basin was outside the Boundary Waters watershed,” said Becky Rom, chair of the Campaign to Save the Boundary Waters. “The mine itself was seen as the primary risk and this does nothing to address that concern. If anything, it makes the Forest Service’s case for rejection even stronger.”

Chris Knopf, executive director of Friends of the Boundary Waters, said the announcement did little or nothing to lessen risks to the wilderness. “This announcement is an improvement in the sense that drinking a cup of rubbing alcohol is an improvement over drinking a cup of bleach,” he said.

Acid-generating?

Along with this week’s tailings announcement, Twin Metals claimed that tailings from the Maturi Deposit will be “non-acid-generating.” The potential for acid runoff from mining operations, which are known to leach toxic metals into surface and groundwater, has been one of the primary concerns of environmental opponents of the project, but company officials now say that is based on misunderstanding.

“The common concern about sulfides points to a basic misconception about our project,” Osborne said. “The geology of the Maturi Deposit provides us with confidence that we can mine here safely and sustainably. The rock sandwiching the layer of copper, nickel and platinum group minerals in the deposit is almost completely free of sulfides. When the targeted minerals are removed during the concentration process and shipped to customers, only a minute amount of sulfides will remain in the tailings.”

According to the company, extensive testing over the past decade shows that Maturi Deposit tailings won’t generate the acids that are typical of other sulfide-based mines.

That claim, however, is hotly-contested by mine opponents, who cite 2014 research on the subject by geophysicist David Chambers, who concluded that “most of the waste rock and pit wall rock so far studied in the Duluth Complex would contain some sulfur, mainly as the mineral pyrrhotite, a primary cause of acid mine drainage.”

Twin Metals is expected to offer more details when they release a Mine Plan of Operation, which will be submitted to state and federal regulators in the coming months.

Comments

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snowshoe2

A bad move the tailings so close to the Kawishiwi river a highly infertile water with a pH of already around 5.8-6.0. Yes already acid and no buffer ability for the smallest amount of sulfur-be it wind blown or by water.

What is the next change? from the parent company from Chile which almost got kicked out of their own country.

Tuesday, July 23