Twin Metals proposes alternative tailings system

This image shows both active and reclaimed dry stack tailings storage at the Greens Creek Mine located near Juneau, Alaska.

Twin Metals Minnesota has announced plans to use the dry stack method to store leftover rock from its proposed underground copper-nickel mine nine miles southeast of Ely.

The dry stack method eliminates the storage pond and dam associated with conventional tailings facilities, the nonferrous mining company said Thursday. It has been successfully used in four mines in the northern United States and Canada having similar climates and has been permitted at two mines in the western United States, Twin Metals said. 

The process, Twin Metals explained, involves removing target minerals and then compressing the remaining tailings into low-moisture, sand-like deposits that are 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,” Kelly Osborne, Twin Metals Minnesota CEO, said in a news release. “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.”

Community concerns about copper nickel mines have focused on fears of tailings dam failure or leaks that could threaten both nearby surface water and groundwater. Because there is no dam, risk of dam leakage or failure is eliminated, the company said. Equally important is the fact that the tailings from the Maturi Deposit will be non-acid-generating, Twin Metals added.

“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, according to the mining firm.

Twin Metals said that dry stack tailings storage is often promoted by environmental groups as an alternative to conventional wet tailings as a way to protect water quality. It 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 said it made the decision to move to it as the best available option. The Minnesota Center for Environmental Advocacy hailed the advantages of dry stack tailings in a statement earlier this year. 

“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 approach will be outlined in detail in TMM’s Mine Plan of Operation, to be submitted to state and federal regulators in the coming months. Regulatory review, including hearings for public comment, will cover compliance with regulations to protect water and air quality, drinking water, wetlands, endangered species, plant life and cultural resources.