PolyMet Mining Corp. will ask the Minnesota Supreme Court to reverse a recent appellate ruling that blocked three key state mining permits, the latest twist in the company's marathon effort to win approval for a copper-nickel mine on the Iron Range.

In a statement released Thursday, PolyMet CEO Jon Cherry said the appellate ruling released Monday would set a dangerous precedent that could lock up not just the mine, but also any future industrial project in "an endless loop of review, contested case hearings and appeals."

"The potential negative consequences of the decision to any industry or business in the state, and the many Iron Range communities and workers who stand to benefit economically from responsible copper-nickel mining, warrant the Minnesota Supreme Court's attention," Cherry said.

The move was widely expected after the state appellate court reversed three permits that PolyMet needs to begin mine construction. An appeal is also possible by the state Department of Natural Resources (DNR), which oversees mining and granted the permits in 2018.

In the appellate ruling, Chief Judge Edward Cleary said the DNR erred in refusing to conduct a so-called contested case hearing on the mining permits to fully vet objections by environmental groups and the Fond du Lac Band of Lake Superior Chippewa. He said mining opponents had raised legitimate factual questions about the safety and design of a proposed "tailings basin" that would hold mining waste and about whether PolyMet's majority owner, the Swiss conglomerate Glencore PLC, should be named on the permits.

In addition, Cleary said, the DNR should have specified time limits for the mine's entire life cycle in PolyMet's all-important permit to mine. Although PolyMet says it intends to mine for 20 years, the permit isn't clear about the time frame for activities such as mine reclamation and future maintenance of the huge tailings dam that would be left behind, according to the court's decision.

Marci Schmitt — Associated Press file, Star Tribune
A former iron ore processing plant near Hoyt Lakes, Minn., would become part of a proposed PolyMet copper-nickel mine.

The DNR permits were the final stage in a yearslong state review of the proposed mine, a $1 billion open-pit operation to be built near Babbitt and Hoyt Lakes, and PolyMet had planned to begin raising construction financing this year. But its state permits now face legal challenges on several fronts, posing the possibility of months of additional delay.

Next week, a Ramsey County judge is expected to begin considering a separate legal challenge to a water-quality permit issued by the Minnesota Pollution Control Agency (MPCA). An advocacy group has accused Minnesota regulators and officials of the U.S. Environmental Protection Agency of colluding to suppress scientific concerns raised by career staff at the federal agency.

Scrutiny of the regulators' performance is particularly intense because PolyMet is widely thought to be just the first of several multinational mining concerns that might seek to tap a large deposit of copper and nickel that runs along Minnesota's Iron Range. Geologists and environmentalists say copper-nickel mining poses substantially greater environmental risks to lakes and streams than the taconite mines that have operated in Minnesota for decades.

A second company, Twin Metals Minnesota, just filed its initial paperwork for permits to build an underground copper-nickel mine nearby, on the edge of the Boundary Waters Canoe Area Wilderness.

Greg Stanley • 612-673-4882