The Minnesota Department of Natural Resources (DNR) opened a 30-day public comment period on the draft dam safety permits for PolyMet’s proposed copper-nickel mine.
The DNR opened the dam safety permits for public review, indicating that so far it is satisfied with the submitted plans. The plans are two of more than 20 state and federal permits the company needs before opening what would be the state’s first-ever copper-nickel mining operation.
The DNR will take comments through Oct. 16 and then decide whether to make the draft permits final, Twincities.com reported.
The dam — built on top of the former LTV Steel taconite tailings basin — would stretch for miles and reach 77 m (252 ft) high when finished, holding back millions of gallons of water mixed in a slurry with finely ground rock left over after crushing and processing after the copper, nickel and other valuable metals are extracted.
The dam walls will be raised in steps, 6 m (20 ft) at a time, over the 20-year life of the mine. Eventually the PolyMet tailings basin will cover 4 km2 (2.5 sq miles) with 10 million cubic yards of mine waste pumped in each year.
Jon Cherry, PolyMet CEO, released a statement calling the draft permit release another sign of progress toward buildout of the mine.
“The tailings basin was one of the most studied aspects of the NorthMet Project during the comprehensive state and federal environmental review of the project that concluded in 2016,” Cherry said. “We take the design, construction and operation of the tailings impoundment very seriously and have taken extra measures to ensure a safe and stable design. The science shows that not only can we be protective of water and other natural resources, but we will make a substantial contribution to addressing legacy reclamation issues at the former LTV site.”
Engineers for the PolyMet project say the tailings basin is being designed to handle a massive 1,000-year-type rainfall, in excess of 51 cm (20 in.) in a short period, with added wave action factored in for storms. That means building-in excess capacity to prevent “overtopping’” when water spills over a dam, often causing erosion or a break.
The DNR also released a second draft dam safety permit that covers PolyMet’s proposed hydrometallurgical residue facility, which would receive residue, mostly gypsum, generated from a process that would use pressure and temperature reactions to extract additional precious metals beyond what can be achieved by the primary processing facility. Polymet is hoping to build that facility in the future but not in its initial project.
The dam safety permits are considered among the most important for PolyMet, along with the DNR permit to mine that’s expected to be released later this year. If and when PolyMet gets all the necessary permits, possibly by 2018, the company will have to find an estimated $550 million to start construction.