ACID DRAINAGE FROM MINES AND ITS RELATED PROBLEMS - Prediction, quantification and implementation of mitigation measures are the key for enhancement of environmental standard
sulfide (pyrite) and unremoved coal contained in the sandstone overburden to air and water. These oxidizing conditions result in an increase of acidity, which subsequently decreases the pH and increases the concentrations of dissolved metals. These consequences lead to an overall degradation of water quality and the inability to support aquatic life. Mineral production is an important component of the economy for many countries, and in some cases it can be the major source of international revenue. However, mining and mineral production operations that are not well managed can contaminate groundwater and surface water in the form of AMD, and can adversely affect the health of nearby communities that rely on this source for drinking-water or agriculture. Extractive industries include mining of mineral deposits (principally metal-bearing ores and coal deposits), oil and natural gas production, and quarrying for building and road-making materials. Poorly operated or abandoned mine sites are often significant sources of water contamination; contaminants of particular health concern from these sources include heavy metals, and mineral-processing chemicals, such as cyanide. Water pumped from abandoned mine shafts and open-cut pits is often used for water supply, and is generally safe and reliable. However, these water sources may sometimes be contaminated by mineral processing chemicals, acid mine drainage (AMD) and waste disposal. These risks must be considered and assessed to determine whether such water sources are safe to be used for drinkingwater supply.
PRODUCING ACID FROM ROCK Acid mine drainage (AMD) is produced by the chemical reaction of sulfide ore and associated minerals with air and water. This reaction (as shown below) illustrates how sulfuric acid(H 2 SO4) and iron sulfate (FeSO4) are produced when the iron sulfide mineral pyrite (FeS2) reacts with water: FeS 2 + H 2 O + 3.5O 2 = H 2 SO 4 + FeSO 4 The iron sulfate and sulfuric acid continue to react with water and air through several steps to produce iron hydroxide (Fe[OH] 3 ) and additional sulfuric acid. The sulfuric acid is responsible for leaching metals from mine dumps as well as significantly lowering the pH in streams. The iron hydroxide is responsible for the characteristic reddish color associated with AMD. Although operating more slowly, the reactions described above do affect natural outcroppings of sulfide ore minerals, resulting in a characteristic reddish stain that is referred to as a ‘gossan’. The color of the gossan is so distinctive that it can be seen for miles. Early mineral exploration made use of gossans as an indicator of where potential ore deposits might be found.
DISCUSSION ON ACID DRAINAGE AND RELATED ASPECTS a. Acid Rock Drainage and Acid Mine Drainage - Acid rock drainage (ARD) is a natural process in which sulfuric acid is produced when sulfides in rocks—for example, pyrite (FeS 2 ) — are exposed to air and water. This is an explanation of the natural weathering process of mineral (such as gold, copper, zinc etc.) bearing rocks exposed to atmosphere. This occurs along outcrops or scree slopes where sulfide-bearing rock is naturally weathered. Acid Rock Drainage occurs where there are large quantities of sulphur- containing rock minerals and has been observed associated with road building, construction (including construction at an airport) and at mines. Author: Partha Das Sharma, B.Tech(Hons.) in Mining Engineering, E.mail: sharmapd1@gmail.com, Blog: http://miningandblasting.wordpress.com/
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