5.5 Mineral resources: formation, mining, environmental impact  (Page 5/43)

Mining and processing ore can have considerable impact on the environment. Surface mines can create enormous pits (see Figure Open Pit Mine ) in the ground as well as large piles of overburden and tailings that need to be reclaimed , i.e., restored to a useful landscape. Since 1977 surface mines in U.S. are required to be reclaimed, and commonly reclamation is relatively well done in this country. Unfortunately, surface mine reclamation is not done everywhere, especially in underdeveloped countries, due to lack of regulations or lax enforcement of regulations. Unreclaimed surface mines and active surface mines can be major sources of water and sediment pollution. Metallic ore minerals (e.g., copper, lead, zinc, mercury, and silver) commonly include abundant sulfide, and many metallic ore deposits contain abundant pyrite (iron sulfide). The sulfide in these minerals oxidizes quickly when exposed to air at the surface producing sulfuric acid, called acid mine drainage    . As a result streams, ponds, and soil water contaminated with this drainage can be highly acidic, reaching pH values of zero or less (see Figure Acid Mine Drainage)! The acidic water can leach heavy metals such as nickel, copper, lead, arsenic, aluminum, and manganese from mine tailings and slag. The acidic contaminated water can be highly toxic to the ecosystem. Plants usually will not regrow in such acidic soil water, and therefore soil erosion rates skyrocket due to the persistence of bare, unvegetated surfaces. With a smaller amount of tailings and no overburden, underground mines usually are much easier to reclaim, and they produce much less acid mine drainage. The major environmental problem with underground mining is the hazardous working environment for miners primarily caused by cave-ins and lung disease due to prolonged inhalation of dust particles. Underground cave-ins also can damage the surface from subsidence. Smelting can be a major source of air pollution, especially SO ₂ gas. The case history below examines the environmental impact of mining and processing gold ore.

Sustainable solutions to the mineral crisis?

Providing sustainable solutions to the problem of a dwindling supply of a nonrenewable resource such as minerals seems contradictory. Nevertheless, it is extremely important to consider strategies that move towards sustainability even if true sustainability is not possible for most minerals. The general approach towards mineral sustainability should include mineral conservation    at the top of the list. We also need to maximize exploration for new mineral resources while at the same time we minimize the environmental impact of mineral mining and processing .

Conservation of mineral resources includes improved efficiency, substitution, and the 3 Rs of sustainability, reduce, reuse, and recycle. Improved efficiency applies to all features of mineral use including mining, processing, and creation of mineral products. Substituting a rare nonrenewable resource with either a more abundant nonrenewable resource or a renewable resource can help. Examples include substituting glass fiber optic cables for copper in telephone wires and wood for aluminum in construction. Reducing global demand for mineral resources will be a challenge, considering projections of continuing population growth and the rapid economic growth of very large countries such as China, India, and Brazil. Historically economic growth is intimately tied to increased mineral consumption, and therefore it will be difficult for those rapidly developing countries to decrease their future demand for minerals. In theory, it should be easier for countries with a high mineral consumption rate such as the U.S. to reduce their demand for minerals but it will take a significant change in mindset to accomplish that. Technology can help some with some avenues to reducing mineral consumption. For example, digital cameras have virtually eliminated the photographic demand for silver, which is used for film development. Using stronger and more durable alloys of steel can translate to fewer construction materials needed. Examples of natural resource reuse include everything at an antique store and yard sale. Recycling can extend the lifetime of mineral reserves, especially metals. Recycling is easiest for pure metals such as copper pipes and aluminum cans, but much harder for alloys (mixtures of metals) and complex manufactured goods, such as computers. Many nonmetals cannot be recycled; examples include road salt and fertilizer. Recycling is easier for a wealthy country because there are more financial resources to use for recycling and more goods to recycle. Additional significant benefits of mineral resource conservation are less pollution and environmental degradation from new mineral mining and processing as well as reductions in energy use and waste production.

Because demand for new minerals will likely increase in the future, we must continue to search for new minerals, even though we probably have already found many of the “easy” targets, i.e., high-grade ore deposits close to the surface and in convenient locations. To find more difficult ore targets, we will need to apply many technologies including geophysical methods (seismic, gravity, magnetic, and electrical measurements, as well as remote sensing, which uses satellite-based measurements of electromagnetic radiation from Earth’s surface), geochemical methods (looking for chemical enrichments in soil, water, air, and plants), and geological information including knowledge of plate tectonics theory. We also may need to consider exploring and mining unconventional areas such as continental margins (submerged edges of continents), the ocean floor (where there are large deposits of manganese ore and other metals in rocks called manganese nodules), and oceanic ridges (undersea mountains that have copper, zinc, and lead ore bodies).

Finally, we need to explore for, mine, and process new minerals while minimizing pollution and other environmental impacts. Regulations and good engineering practices are necessary to ensure adequate mine reclamation and pollution reduction, including acid mine drainage. The emerging field of biotechnology may provide some sustainable solutions to metal extraction. Specific methods include biooxidation (microbial enrichment of metals in a solid phase), bioleaching (microbial dissolution of metals), biosorption (attachment of metals to cells), and genetic engineering of microbes (creating microorganisms specialized in extracting metal from ore).

Review questions

References

Clugston, C. (2010) Increasing Global Nonrenewable Natural Resource Scarcity - An Analysis, The Oil Drum. Retrieved from (External Link)

Craig J, Vaughan D, and Skinner B (2011) Earth Resources and the Environment (4th ed.). Pearson Prentice Hall, p. 92