The environmental effects of mining tend to increase both in geographical scope and intensity as exploration, extraction and metallurgical processing advance progressively. The exploration phase, including mapping, geophysical and geochemical surveys and drilling, produces only minor environmental effects such as clearing of trees for drilling sites, camps, access roads, sinking of pits, holes and shafts, and can be handled by relatively simple rules ensuring that no lasting danger or damage remains.
The mining or extraction phase involves more intensive operations but the impact is localized. Subsidence is a well known problem of underground mining, specially coal mining. The environmental impact decreases with the distance from the mining source. However, modern large-volume low-grade mining uses only a very small percentage of mined ores. The disposal of overburden can cause major environmental problems such as sedimentation of rivers (which can extend to hundreds of kilometres) and the sea. Acid drainage from tailings dumps and the accidental overflow of water charged with poisonous chemicals, in particular cyanide used for leaching gold can cause local, regional and in some cases cross-border pollution, leading to damage to the ecosystem and the human habitat. Sulphide ore, which constitutes a considerable amount of ores mined, can liberate large quantities of sulphur dioxide (SO2) during processing. This produces sulphur trioxide in the atmosphere and sulphuric acid in water, contributing to acid rain and deterioration of cultural monuments. Some base metal ores also contain toxic substances such as cadmium, lead and mercury, and leachate from tailings can cause severe poisoning. Further, coal mining releases considerable amounts of methane' which is considered a major cause of greenhouse effects.
The main environmental degradation generated by the mining industry takes place at the metallurgical stage, mainly in smelting and refining. These industrial processes create air pollution by release of carbon monoxide (CO) and oxides of nitrogen and sulphur (NOx and SOx), water pollution through discharge of process water tailings and hazardous waste generated as residues in metallurgical processes. Mitigation of such environmental effects is possible through better process design, incorporation of anti-pollution devices such as filters and scrubbers and programmes to minimize and manage hazardous wastes. Anti-pollution devices used in metallurgy are identified as the largest element in environmental costs both in the form of capital investment and operational expenditure.