Regardless of the type of raw material, its extraction always comes with an environmental cost. Most mining leaves a lasting and damaging environmental footprint. For example, during the extraction of common metals like copper, lead or zinc from the earth both metal-bearing rock, called ore, and “overburden”, the dirt and rock that covers the ore are removed. At a typical copper mine around 125 tonnes of ore are excavated to produce just one tonne of copper. The amount of earth moved is mind-boggling and mining now strips more of the Earth’s surface each year than does natural erosion.
Waste Rock includes the overburden and mine development rock. Industry uses the term “overburden” to refer to the soil and rock that covers an ore body. Similarly, mine development rock refers to material removed from underground mines to access the ore body. These waste rocks are nonmineralized, or contain insufficient minerals to process economically. They are typically hauled from the mine site to waste dumps for disposal.
Tailings are the waste products generated during the recovery of the minerals. Typically, the ore is crushed or ground to a particle size of less than 0.1 mm in order to release the valuable constituents. Water and small amounts of chemical reagents are usually added during this process to enhance the separation of the minerals from the ore. (UNEP/ICME, 1998). The tailings are usually dumped into tailings dams or erodable dumps (the latter designed so that the tailings gradually wash into a nearby waterway).
Mine Water is the water that collects in both surface and underground mines. It comes from the inflow of rain or surface water and from groundwater seepage. During the active life of the mine, water is pumped out to keep the mine dry and to allow access to the ore body. Pumped water may be used in the extraction process, pumped to tailings impoundments, used for activities like dust control, or discharged as a waste (EPA). The water can be of the same quality as drinking water, or it can be very acidic and laden with high concentrations of toxic heavy metals.
Mines use toxic chemicals including cyanide, mercury, and sulphuric acid, to separate metal from ore. The chemicals used in the processing are generally recycled, however residues may remain in the tailings, which in developing countries are often dumped directly into lakes or rivers with devastating consequences. The accidental spillage of processing chemicals can also have a serious impact on the environment. For example, at the Baia Mare mine in Romania cyanide is used to extract gold from slurry. In January 2000 a dam containing tens of thousands of tonnes of slurry burst, poisoning the local river with cyanide and heavy metals. Up to 100 tonnes of cyanide were released into the river, a tributary of the Danube. The drinking water supply for more than 2 million people was affected. Within hours, dead fish were seen washed up along the river.
The Acid Mine Drainage (AMD) is the number one environmental problem facing the mining industry. AMD occurs when sulphide-bearing minerals in rock are exposed to air and water, changing the sulphide to sulphuric acid. It can devastate aquatic habitats, is difficult to treat with existing technology, and once started, can continue for centuries (Roman mine sites in Great Britain continue to generate acid drainage 2 000 years after mining ceased). Acid mine drainage can develop at several points throughout the mining process: in underground workings, open pit mine faces, waste rock dumps, tailings deposits, and ore stockpiles (miningwatch).
Artisanal small-scale gold mining of placer deposits occurs mostly in developing countries. Examples include Brazil, Venezuela, Colombia, Guyana, Suriname, Philippines and New Guinea. Between 10 and 15 million people worldwide produce 500 to 800 tonnes of gold per year, in the process emitting as much as 800-1000 tonnes of mercury. Gold recovery is performed by removing sediments from the river and adjacent areas and feeding them through a number of mercury-coated sieves. The mercury amalgamates with the gold in the sediments, separating the gold from the rest of the material. The gold-mercury amalgam is then heated. The heat drives off the mercury, leaving the gold product. While most of the mercury condenses and is recovered, some is emitted to the air and is eventually deposited on nearby land or water surfaces. Mercury deposited on land ultimately reaches streams and rivers through runoff. Roughly 1 kilogram of mercury enters the environment for every kilogram of gold produced by artisans (United States Geological Survey).