Mineral Waste

Mining Processes and Wastes

Mining entails the excavation and separation of valuable minerals from their geological matrix. In metal mining (as opposed to quarrying), since target [Page 554]minerals are typically only a fraction of the ore (or mineral-bearing rock), ore processing results in considerable volumes of waste, known as tailings. A typical, modern, base-metal operation yields greater than 98 percent waste from the excavated material. These residuals are generally disposed of to the lithosphere at waste-rock dumps and tailings disposal areas (although tailings are sometimes disposed of directly to waterways or backfilled into old mine shafts). Surface materials such as soil and vegetation, removed as “overburden,” are not typically considered waste, although they contribute to mining's environmental impact. Slag, the solid by-product of smelting, was historically left in massive piles beside smelters or dumped in nearby watercourses. Although once used as an all-purpose building and grading material, smelter slag may also contain contaminants.

Copper, nickel, lead, and zinc smelters were notorious for producing noxious smokestack emissions. The pollution could travel long distances, threatening public health and damaging downwind crops and other vegetation.

Waste Hazards and Pollution

Mill waters and mine drainage are the principal liquid residuals. Because water is usually used to process ore and transport mill tailings for disposal, liquid and solid wastes are often considered together in waste-disposal planning. Environmental pollutants in both solid and liquid wastes may include heavy metals and metal salts, process reagents used to recover minerals (such as cyanide or mercury), and other contaminants in the ore (such as arsenic and selenium). Acid mine drainage (AMD) is a common and widespread water pollution problem, whereby sulfuric acid is released into the environment through the oxidation of sulfur-bearing rocks exposed during the mining process. Tailings impoundments may contribute to pollution through the overflow of contaminated water to surrounding waterways. In some instances, the catastrophic failure of tailings dams has choked streams and coated their banks with a flood of finely ground material. At their worst, tailings dam collapses have caused extensive landscape and property damage as well as human fatalities.

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