By Dan Kraker

Minnesota Public Radio News

When it comes to the technologies mining companies employ to scrape minerals out of the ground, process them, and safely dispose of the waste, everything is on an immense scale.

One wheel on a truck is twice as tall as an adult. The plant where PolyMet Mining plans to process the ore it digs up is a third of a mile long.

The tailings basin near Hoyt Lakes, where PolyMet would slurry its leftover waste is massive, stretching to the horizon.

“If you look off in the distance, the skyline that you see out there, that’s tailings all the way out to the ridgeline,” PolyMet CEO Jon Cherry said recently, as he stood atop old iron ore tailings more than 20 stories high.

The area, which includes the old LTV Steel taconite plant that operated for about half a century, today looks like a vast grassland covering two square miles. It marks the spot where PolyMet plans to build a huge copper mine in northeast Minnesota.

Minnesotans have until Thursday to comment on the environmental analysis of the proposed PolyMet mine, a huge document containing detailed plans on how the company would keep pollutants from the mine and tailings basin out of nearby lakes and rivers. That plan hinges on new technologies and engineering that PolyMet added after its last proposal was sharply criticized four years ago.

In a five mile-stretch along the north side of the vast tailings impoundment, the company plans to build a massive wall to capture water that runs through the tailings and picks up metals, sulfate and other pollutants. Company officials are confident the process will work.

“The cut off wall that we’re putting in, that will go all the way down to bedrock,” Cherry said. “Those type[s] of technologies have been around for some time. This isn’t like a new technology that we’re trying out for the first time here.”

Similar walls have proven successful at mines from Montana to Alaska, and at landfills around the country.

Few have been built as large, said Dave Chambers, president of the Center for Science and Public Participation in Montana.

“The concern with the slurry walls that are that big,” he said, [is] “are you going to be able to guarantee that you’re going to be able to anchor that slurry wall in bedrock along that whole perimeter?”

Chambers said the bedrock could fracture beneath the wall when it’s installed, which could allow water to seep through.

The state’s environmental impact statement predicts the cut-off walls will capture all surface water and 90 percent of groundwater that flows through the tailings. While the mine is operating, much of that water will be reused. After it closes, it will be treated using a process called reverse osmosis.

For two years the company has operated a pilot water treatment plant to test the technology, tucked away in a small warehouse in the Iron Range town of Virginia, Minn.

Spokeswoman LaTisha Gietzen said the water is pretreated before it’s pushed through the reverse osmosis filters, inside stacks of long white tubes.

“Under pressure, you squeeze the water through a membrane, the clean water comes out,” she said.

PolyMet added reverse osmosis to its plans to help meet a strict state standard that limits the amount of sulfate that can be discharged into wild-rice producing waters.

The Navy first developed the technology to strip salt out of seawater. It’s used in desalination plants around the world, but is still rare in mining. One place it has been used successfully is Utah, where two plants scrub groundwater that’s been contaminated with a plume of sulfate created by a giant copper mine called Bingham Canyon that’s operated for over a century.