Mine Closure - Can Passive Treatment Be Successful?

Abstract

Abstract. LTV Steel Mining Company decided in 1991 to close an open pit !aconite mine in northeastern Minnesota, using a passive treatment approach consisting of limiting infiltration into the stockpiles and wetland treatment to remove metals. Over 50 million metric tons of sulfidecontaining waste had been stockpiled adjacent to the mine during its 30 years of operation. Drainage from the stockpiles contained elevated levels of copper, nickel, cobalt and zinc. Nickel is the major trace metal in the drainages, and prior to closure annual median concentrations ranged from 1.5-50 mg/L. Copper, cobalt and zinc are also present but are generally Jess than 5% of the nickel values. Median pH ranges from 5.0 to 7.5, but most of the stockpile drainages have pH greater than 6.5.Based on the chemical composition of each stockpile, a cover material was selected. The higher the potential a stockpile had to produce acid drainage, the lower the permeability of the capping material required. Covers ranged from oveiburden soil removed at the mine to a flexible plastic liner. Predictions of the reduction in infiltration ranged from 40% for the native soil to over 90% for the plastic liner. Five constructed wetlands have been installed since 1992 and have removed 50-90% of the nickel in the drainages. Total capital costs for all the infiltration reduction and wetlands exceeded $6.5 million, but maintenance costs are less than 1 % of those for an active treatment plant.Since mine drainage problems can continue for over a hundred years, the lower annual operating costs should pay for the construction of the wetland treatment systems within seven years. LTV plans to have a fully passive treatment system for its entire mine by the year 1998.

Mining Influenced Waters: Their Chemistry and Methods of Treatment

“…These waters have carbonate alkalinity, and there is usually no problem with eliminating mineral acidity. In sulfatereducing systems the pH is optimal for the microbes; and, for aerobic systems, photosynthesis using the bicarbonate in the water provides oxygen and hydroxides by the following reaction (Wildeman and Updegraff, 1998 (Gusek et al, 1999), the Duluth Complex system (Eger, et al 1997), and the recently completed MSF system (Wildeman, et al, 2003). Table 3 contains four waters that are representative of the mineral processing category of MIW.…”

Controlling Mine Drainage Problems in Minnesota - Are All the Wetland Treatment Systems Really Above Average?

“…The passive approach consisted of reducing flow from the stockpiles by modifying watersheds where appropriate, limiting infiltration into the stockpiles by capping and treating the residual drainage with wetland treatment systems (Eger et al, 1998). Five wetland treatment systems were to treat the drainage from the stockpiles.…”