Minnesota environmental regulators have published nearly 800 comments on the Twin Metals plan to build a copper-nickel mine just outside the Boundary Waters Canoe Area Wilderness in northern Minnesota, deeming the company's project proposal "incomplete."

The state Department of Natural Resources (DNR) says it needs the clarifications and extra information before it can start the required environmental impact statement on the proposed mine, one of the most contentious mine projects in the state's history.

The comments, dated June 15, are posted on the Twin Metals section of the DNR's website. The agency has "determined the initial submittal to be incomplete," it said.

There is no deadline for Twin Metals to respond to the comments.

The volume of comments is consistent with other large projects, said DNR spokesman Chris Niskanen. Such project proposals are rarely considered complete when they're first submitted, he said. PolyMet Mining Corp., the company with the other proposed copper-nickel mine for Minnesota, suffered a similar fate. PolyMet's project proposal underwent "multiple revisions," he said.

Twin Metals, owned by Chilean mining giant Anto­fagasta, downplayed the significance of the DNR's move. A Twin Metals spokeswoman said Tuesday that it will be providing responses "in the coming weeks."

"It is a normal part of the process for a project proposer to receive comments related to an initial data submittal; it does not mean the submittal has been deemed incomplete," Kathy Graul said.

Some of the clarifications the DNR has requested are minor, and some points are designated as "future discussion items" not requiring immediate action.

Other comments are substantial.

For example, the DNR asked for more details on the company's plan to store the processed mine waste, after extracting metals from crushed rock, in a 130-foot-tall mound. "Is characterizing the tailings filter cake as being 'dry' a common terminology for a product exhibiting a 13% to 16% moisture content?" it asked. "What is the moisture content of these tailings when saturated?"

It also asked for more information on the potential for the mine to generate acid mine drainage (AMD), a dangerous pollutant formed when sulfur-bearing rock is exposed to the elements during mining. For the mine waste tailings, it noted in one comment, "additional information is needed to demonstrate that 0.2% S [sulfur] tailings would not produce AMD."

The comments come six months after Twin Metals simultaneously submitted its project proposal to the DNR and its mine plan of operation to the federal Bureau of Land Management.

The DNR decided last fall to go it alone, and the state plans to conduct its own environmental review of the Twin Metals plan rather than partnering with the federal government, partly because the Trump administration placed new constraints on federal environmental reviews.

The Bureau of Land Management is still reviewing the Twin Metals plan. Bureau spokesman Francis Piccoli said it has had a "normal number" of questions on the project plan and that the company has been answering them.

The agency is expected to start its environmental impact statement process "soon," Piccoli said.

Due to the complexity of the Twin Metals project, he said, the federal environmental review will take more than a year.

Jennifer Bjorhus • 612-673-4683