An Arizona court of appeals overturned a lower court’s ruling that blocked the air-quality permit that the proposed Rosemont Copper Mine had applied for. The ruling by the appeals court is a significant legal victory for the copper project in Tucson that has been bogged down with legal challenges for years.

The Arizona Daily Star reported that a the three-judge panel ruled that a Maricopa County Superior Court judge erroneously ruled in March 2015 that the Arizona Department of Environmental Quality’s (ADEQ) approval of the air permit was “arbitrary and capricious” and “an abuse of discretion.”

The appeals panel concluded that, contrary to arguments made by the mine’s opponents and the lower court judge, that “substantial evidence supported the department’s determination that the proposed Rosemont Mine will not exceed air quality standards.”

With the ruling, the project has two hurdles that remain; a final decision by the U.S. Forest Service on the project’s overall plans and a Clean Water Act permit decision by the U.S. Army Corps of Engineers. Those agencies have said they expect to make decisions this summer.

The copper mine would be the third-largest copper mine in the U.S. and employ about 400 people.

The appeals court ruled in favor of the ADEQ and Rosemont on how a range of technical factors was used to calculate whether the mine would violate air quality standards.

The initial lawsuit was filed in 2014 by Save the Santa Ritas, a coalition of community and environmental groups that oppose the mine.

“We’re pleased at the court decision. ADEQ issues permits that are both protective of human health and the environment,” said Timothy Franquist, ADEQ’s air quality division director. “We believe that Rosemont met that ultimate goal and we’re pleased to issue the permit.”

Patrick Merrin, a vice president for Hudbay Minerals Inc., which proposes to build the mine, said in a prepared statement, “We are pleased with the court’s decision that validates the Arizona Department of Environmental Quality’s hard work and depth of expertise when it granted the air permit.”