The major potential environmental impacts associated with mining and associated mineral processing operations are related to erosion-prone landscapes, soil and water quality, and air quality. These potential impacts are recognized and addressed in current mining operations as well as in some former mining operations by reclaiming areas of physical disturbance to prevent erosion, stabilizing soils containing metals or chemicals to prevent unwanted metal releases into the environment, preventing and/or treating water contamination, and controlling air emissions.
At many sites, the key reclamation, soil treatment, and water quality concerns owe their origin to the same process — the oxidation of sulfide minerals, especially the iron sulfide, pyrite. Oxidation of sulfide minerals can produce acidic conditions that release metals in both waste materials and water.
Mining in the early days took place at a time when environmental impacts were not as well understood and, most importantly, not a matter of significant concern. As a result, historical mine sites may still have areas that are not reclaimed, remnants of facilities, and untreated water. This inherited legacy of environmental damage from mining is not indicative of the mining cycle today.
Now, mine closure and a number of activities to mitigate the impacts of mining are an integral part of all metal mine planning and mineral development from the discovery phase through to closure:
Reclamation entails the re-establishing of viable soils and vegetation at a mine site. Although regulatory agencies may require complex reclamation designs, simple approaches can be very effective. One simple approach depends on adding lime or other materials that will neutralize acidity plus a cover of top soil or suitable growth medium to promote vegetation growth. Modifying slopes and other surfaces and planting vegetation as part of the process stabilizes the soil material and prevents erosion and surface water infiltration. Even this simple approach is likely to cost a few thousand dollars per acre to implement. Where soils have a sustained high acidity, the costs of using this approach can increase, sometimes to tens of thousands of dollars per acre. The challenge to find cost-effective reclamation approaches continues.
Promising reclamation options in the future may include using sludge, “biosolids,” from municipal waste water treatment processes as an organic soil amendment, and growing plant species that are more tolerant of acidic conditions.
High levels of metals in soils, not just acidity, can be harmful to plants, animals, and, in some cases, people. A common approach used in dealing with contaminated soil is to move it to specially designed repositories. This approach can be very expensive and controversial, but it is sometimes required. With this approach, the volume and toxicity of the soil is not reduced, the soil is just relocated. Effective soil treatment approaches in the future depend upon better understanding of the risks associated with metals in mine wastes. These “natural” metals in minerals may not be as readily available in the biosphere, and therefore, they may not be as toxic as the metals in processed forms, such as lead in gasoline.
Future approaches may include:
The most common treatment for acidic and metal-bearing waters is the addition of a neutralizing material, such as lime, to reduce the acidity. This “active” treatment process, which causes the dissolved metals to precipitate from the water, usually requires the construction of a treatment facility. The ongoing maintenance that such a plant requires makes this treatment technique very expensive.
Aside from the expense, some active treatment plants generate large amounts of sludge. Disposal of the sludge is a major problem. Because of the cost and the physical challenges of dealing with sludge, alternatives to active treatment facilities are needed. Some possible alternatives include:
Although the discharge of acidic drainage presents several challenges to protecting water quality, the significance and widespread occurrence of acid rock drainage warrant special efforts to prevent or minimize its occurrence. Prevention must be addressed during exploration activities, before the beginning of newly-planned mining operations. In some cases, it may even be possible to prevent or reduce acid rock drainage in old or abandoned mining areas. Current and potential treatment approaches for acid rock drainage are similar to those already described. Possible measures to prevent or significantly reduce acid rock drainage include:
Smelter emissions, especially sulfur dioxide and particulate materials, have historically presented significant environmental problems. Modern smelting technology has met this challenge by drastically reducing the amount of emissions. An example is the modernized smelter built by Kennecott Utah Copper that processes ore concentrates from the Bingham Canyon Mine near Salt Lake City. Using technology developed by the Finnish company Outokumpu, this smelter has reduced sulfur dioxide emissions to 95 percent of previous permitted levels. This smelter, which came online in 1995, is the cleanest in the world. It captures 99.9 percent of the emitted sulfur.
Article Source: http://www.americangeosciences.org/