HOYT LAKES — Of the bad things that could happen once PolyMet starts running Minnesota’s first-ever copper mine, critics say, among the worst would be a catastrophic breach of the tailings basin dam.
A hulking, man-made earthen dike that will stretch for miles and reach 252 feet high when finished, the dam will hold 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.
Much of that waste rock will be as small as grains of beach sand. In theory, the stuff will settle into the basin, and as more is pumped in, the dams will be raised in steps, 20 feet at a time, over the 20-year life of the mine.
Most of the water will be re-used to keep the slurry system carrying waste rock from the processing plant to the tailings basin. Any water that leaves the basin will be treated by PolyMet with a reverse osmosis filter system (to remove sulfates), leaving clean water flow into the Partridge and Embarrass rivers and on into the St. Louis River and Lake Superior.
PolyMet needs a dam safety permit from the Minnesota Department of Natural Resources to start work adding on to the tailings basin that was once used by Erie/LTV Steel Mining Co. in its taconite processing operations. (PolyMet also has applied for a second dam permit for a possible hydrometallurgical processing facility where acidic waste might be stored.)
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