The U.S. Environmental Protection Agency (EPA) announced it is proposing to cut emissions from gold ore processing and production facilities in Nevada and elsewhere in the U.S. Claiming that these operations represent the sixth largest source of mercury air emissions in the country, the agency described the announcement as “one of several steps” it is taking to reduce mercury emissions.
The EPA’s proposal would add gold mine ore processing and production to the list of source categories subject to regulation under the hazardous air pollutant section of the Clean Air Act due to their mercury emissions. The EPA is also proposing the first national mercury air emission standards for this source category, requiring Maximum Achievable Control Technology. The standards would be based on the emissions level of the best performing facilities that are well controlled for mercury.
The EPA said its proposal would reduce annual mercury emissions to about 1,390 lb annually, a 73% reduction from 2007 levels. This action follows previous reductions from Nevada’s program for controlling mercury emissions from precious metal mining. The state’s Division of Environmental Protection in 2006 issued regulations, building on a four-year-old voluntary program, requiring best available mercury emissions control technology on all thermal units at Nevada’s precious metal mines.
For pre-treatment processes, the EPA is proposing to limit mercury emissions to 149 lb per million tons of ore for both new and existing sources. Mercury emissions from carbon processes in existing sources would be limited to 2.6 lb per ton of concentrate, while new sources would be limited to 0.14 lb. The rule would limit emissions from existing non-carbon concentrate processes to 0.25 lb per ton of concentrate and emissions from new sources to 0.20 lb.
In November 2009, the preliminary results from a two-year study funded by the Nevada Department of Environmental Protection on mercury emissions from sources other than stacks were released. The study, which focused on emissions from waste rock, heap leaches, tailings impoundments, active pit surfaces, stockpiles and reclaimed sites at two Nevada gold operations, estimated and compared emissions from those sites with mercury releases from sites undisturbed by mining, with the compiled data indicating that mercury emissions from mining disturbances represent approximately 20% of the total mercury emitted at mining and processing operations. The study showed heap leaching and tailings impoundments produced the most emissions and that current reclamation practices can reduce the current emissions to near natural levels.
The authors—Dr. Chris Eckley, a post doctoral researcher working with Mae Gustin, a professor at the University of Nevada, and graduate student Matthieu Miller—collected fugitive mercury emissions from two gold mines in Nevada: Newmont’s Twin Creeks mine northeast of Winnemucca, and Cortez Pipeline, a Barrick property located south of Battle Mountain, and used this information to develop an estimate of emissions for the mine surface area.
The results indicated that the amount of mercury emitted from these types of disturbances can vary significantly among mines, depending primarily on the mercury concentration at the disturbed site, the moisture content of the tailings and whether or not the heaps are actively being leached with cyanide. Because there are a variety of factors (such as mercury concentration, age of the materials, climatic conditions, weather, ore type, ore processing techniques, etc.) that can affect the emission of fugitive mercury from different mining surfaces and the uncertainty associated with each of those factors, the authors said this data, developed for the two mines, cannot be extrapolated to calculate emissions estimates for other mines.
Additionally, the study noted that since a mine is a dynamic entity with surfaces changing over time, and the emission estimates developed in the UNR study represent the surfaces present at the two mines in 2008, the amount of mercury released in future years may differ depending on the types and extent of mining related surfaces at each facility.
Overall, there are about 20 facilities in the United States that extract gold from ore that would be subject to the proposed rule.
The EPA will take public comment on the proposed rule for 30 days after it is published in the Federal Register.