A judge has ruled the state of Michigan has no authority to regulate mercury output from a cement plant in Alpena, but the company Tuesday pledged a voluntary effort to limit emissions.

Lafarge North America sued the Department of Environmental Quality earlier this year after the two sides failed to agree on a mercury reduction strategy as the plant expands its cement production by 20 percent.

They reached a tentative settlement in August that allowed the plant to pump 567 pounds of mercury a year into the atmosphere while developing a plan for cutting back. The ceiling would have dropped to 390 pounds a year by 2010 at the latest.

But the settlement was rendered moot by Judge Thomas W. Brookover’s ruling last week in Alpena County Circuit Court that the DEQ had overreached.

"This case is not about whether mercury is dangerous to the public health, or what level of mercury emissions are safe, but rather, what limitations the MDEQ may impose under the statutes and regulations that currently exist," Brookover wrote in his opinion.

Michigan has no air-quality rules that deal specifically with mercury from cement plants, although the DEQ has established an advisory group to help develop them.

The department contends it has broad power to regulate industrial air emissions under laws such as the Michigan Environmental Protection Act.

"The people of Michigan rely on us to make sure the air they breathe in their back yard is clean and safe for their families," DEQ spokesman Robert McCann said. "We firmly believe we have the authority to live up to that obligation."

The DEQ has not decided whether to appeal Brookover’s ruling, McCann said. In the meantime, it plans further talks with the company toward a mutually acceptable mercury reduction plan.

Lafarge spokesman Craig Ryan said the dispute with the DEQ was largely procedural. The company acknowledged the need for mercury limits but disagreed with how the department arrived at the 390-pound level, he said.

"The bottom line is that we are on record committed to voluntarily reducing our emissions, and we’re going to honor that," Ryan said.

Lafarge North America, a subsidiary of French building-materials maker Lafarge SA, is researching mercury reduction technologies and hopes its output eventually will be even less than 390 pounds, Ryan added.

Environmentalists said the DEQ blew an opportunity to rein in Lafarge’s mercury pollution by letting the company add fly ash from a Canadian power plant to its cement mixture in the mid-1990s. The fly ash produces more than half the plant’s mercury output, the DEQ says.

The department also didn’t adequately evaluate risks to human and environmental health from Lafarge’s emissions before approving its expanded production, said Alexander Sagady, an East Lansing, Mich., environmental consultant and longtime critic of the plant.