Twin Metals to use dry stack tailings at proposed mine in Minnesota

July 18, 2019

Twin Metals Minnesota announced that it will use the environmentally friendly dry stack method to store tailings from its proposed underground copper-nickel mine to be located 14 km (9 miles) southeast of Ely, MN.

The announcement is 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. Two tailing dam failures in Brazil and another in British Columbia, Canada in recent years have given rise to concerns about the safety of dam around the world.

In a statement, PolyMet said the dry stack process eliminates the storage pond and dam associated with conventional tailings facilities. It has been successfully used in four mines in the northern United States and Canada with similar climates to Minnesota and has been permitted at two mines in the western United States.

“Dry stack tailing storage is the most environmentally friendly tailings management approach for our site,” said Kelly Osborne, chief executive officer of Twin Metals Minnesota. “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.

“Because there’s no risk of a dam failure, dry stack is considered the best available technology for tailings storage and, after a decade of study and consultation with concerned voices in our community, we determined that it will be an effective choice for our project.”

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 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.”

Extensive testing over the past decade shows that Maturi Deposit tailings will be non-acid-generating.

“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.

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, the Minneapolis Star Tribune reported. 


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