The Minnesota Supreme Court has remanded the air pollution permit for the state's first proposed copper mine to the state Court of Appeals for further deliberation on whether PolyMet Mining Corp. misled or lied to state pollution regulators.

In an outright win for neither side, the high court reversed the appellate court's decision last year to send the permit back to the Minnesota Pollution Control Agency (MPCA) for failing to scrutinize allegations that PolyMet engaged in "sham permitting."

Writing for the majority, Justice Margaret H. Chutich said that the federal rules and guidance around the permitting "do not mandate prospective investigation."

In other words, the state regulator may investigate the allegations against PolyMet, but isn't obligated to do so. The decision returns the air permit and concerns to the Court of Appeals for further deliberation.

The dispute is one of several ongoing legal challenges that have delayed construction of the $1 billion copper mine that Toronto-based PolyMet, majority owned by Glencore in Switzerland, wants to build near Hoyt Lakes and Babbitt.

The company's mining permit and two dam safety permits, for example, were also struck down.

That matter is also at the state Supreme Court, which has not ruled on it.

The bitterly disputed project would be Minnesota's first non-iron ore or taconite mine, a new type of mining for the state that poses greater environmental risks.

The Minnesota Center for Environmental Advocacy said it's considering an appeal to the U.S. Supreme Court.

"Today's ruling underscores that the entire process by which PolyMet obtained its permits in 2018 may have been deceptive and allows us to make this case to the Minnesota Court of Appeals," said Kathryn Hoffman, the center's chief executive.

"PolyMet has engaged in a bait-and-switch scheme to avoid air pollution standards, and we are glad that the Supreme Court ruling allows us to make this case."

A coalition of environmental groups and the Fond du Lac Band of Lake Superior Chippewa alleged PolyMet tried to illegally circumvent the rules by seeking a less stringent, quicker-to-get air permit for a smaller copper mine than what it ultimately intends to build. In regulator-speak, that's called engaging in "sham permitting."

The permit limits the mine's air emissions to 250 tons per year of any individual regulated air pollutant, called "criteria pollutants" — such as particulate matter, sulfur dioxide and nitrogen oxide.

If the plant were to become much bigger it would likely require a stricter "major source" permit for air pollution. The MPCA issued a statement saying that it uses "science and facts" in its process.

"The MPCA scrutinized PolyMet's emissions calculations and required the facility to have new requirements that protect Minnesota's air," the agency said. "The result was a strong and enforceable air permit for the PolyMet facility."

In a news release, PolyMet called the ruling a "victory." It's an endorsement of the MPCA's yearslong regulatory process, said Jon Cherry, PolyMet's chairman, president and CEO.

"The decision provides additional clarity that will enable the company to move closer to mining the metals that are needed for improvement to U.S. infrastructure projects and production of electric vehicles and renewable energy technologies," Cherry said.