A massive open-pit copper-nickel mine originally proposed by the company PolyMet can keep its air pollution permit, according to an appellate court ruling Monday.

The Minnesota Court of Appeals ruled that state regulators followed the proper procedures in issuing the air permit to the mine, known as NorthMet. It would extract metals at a site near Babbitt, Minn., and process ore at the former LTV Steel site in Hoyt Lakes, Minn.

The project remains stalled, however, after other court decisions this year either scrapped or put into question three other key permits.

Bruce Richardson, a spokesman for NewRange Copper Nickel, the owner of the NorthMet project, wrote in an email that the company is "pleased with the Court's decision."

"This is an important conclusion that reaffirms Minnesota's legitimate, well-structured permitting process," Julie Lucas, the executive director of the pro-industry group MiningMinnesota, wrote in a statement.

A coalition of environmental groups, including the Minnesota Center for Environmental Advocacy, Friends of the Boundary Waters Wilderness and Sierra Club, had argued that the project was seeking a "sham" permit.

In 2016, PolyMet said it would restrict its ore processing to 32,000 tons per day in order to qualify for a less-restrictive air permit. But the coalition pointed to statements the company made later to investors, which projected the project's rate of return if processing reached as high as 118,000 tons per day.

"Fundamentally, we were trying to say that the permit that was issued for PolyMet was the wrong permit," said Aaron Klemz, chief strategy officer for the MCEA. A more-restrictive major permit "should have come with the best available [pollution] control technology and the analysis that comes along with that."

Ultimately, Appellate Judge Louise Dovre Bjorkman decided that the Minnesota Pollution Control Agency had taken the potential for higher production and more pollution into consideration.

"[MPCA's] findings — most of which the coalition does not dispute — confirm that it carefully evaluated PolyMet's submissions and any evidence potentially calling them into doubt," Bjorkman wrote.

The case, like other long-running legal disputes related to the mine proposal, had a complex history: The question of the MPCA's handling of the permit had previously landed in the Court of Appeals twice, making a stop at the Minnesota Supreme Court in between.

It's possible that MCEA could appeal Monday's ruling, too, back to the state's highest court — Klemz said it was an option the group would consider.

The mine project was originally launched by PolyMet, which was taken over completely this year by the Swiss international conglomerate Glencore.

PolyMet has partnered with the Canadian firm Teck Resources, which is is exploring metals at a site next door. That partnership, NewRange Copper Nickel, is now running the NorthMet project.

Like other proposed hardrock mining projects, which are new to Minnesota, NorthMet has attracted the most criticism from those who worry about water quality. They argue that extracting copper, nickel and other metals could send acid pollution into lakes, rivers and groundwater.

The three permits that have been thrown into question this year have all been connected to water quality concerns.

The Army Corps of Engineers revoked a wetland destruction permit in June, in a novel decision that focused on tribal rights; the state Supreme Court tossed the company's wastewater permit in August, saying MPCA hadn't considered threats to the Lake Superior watershed; and last month, an administrative judge recommended that the Department of Natural Resources toss the project's permit to mine, because tainted water might seep out of the project's tailings.

Hardrock mining proponents argue that the metals from NorthMet and other projects like it are needed to build clean power projects and electric vehicles.