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Acid Mine Drainage (AMD) and its control

Acid Mine Drainage (AMD) and its control Author: Partha Das Sharma (B.Tech-Hons., in Mining Engineering) E.mail:, Website/Blog: , Abstract Acid mine drainage (AMD) is one of the most significant environmental challenges facing the mining industry worldwide. It occurs as a result of natural oxidation of sulphide minerals contained in mining wastes at operating and closed/decommissioned mine sites. AMD may adversely impact the surface water and groundwater quality and land use due to its typical low pH, high acidity and elevated concentrations of metals and sulphate content. Once it develops at a mine, its control can be difficult and expensive. If generation of AMD cannot be prevented, it must be collected and treated. Treatment of AMD usually costs more than control of AMD and may be required for many years after mining activities have ceased. Therefore, application of appropriate control methods to the site at the early stage of the mining would be beneficial. Although prevention of AMD is the most desirable option, a cost-effective prevention method is not yet available. The most effective method of control is to minimize penetration of air and water through the waste pile using a cover, either wet (water) or dry (soil), which is placed over the waste pile. Despite their high cost, these covers cannot always completely stop the oxidation process and generation of AMD. Application of more than one option might be required. Early diagnosis of the problem, identification of appropriate prevention/control measures and implementation of these methods to the site would reduce the potential risk of AMD generation. AMD prevention/control measures broadly include use of covers, control of the source, migration of AMD, and treatment. Introduction - Acid mine drainage (AMD) is one of the most perpetual pollution problems which occurs world-wide in the mining areas. It refers to the distinctive type of wastewater that originates from the weathering and leaching of sulphide minerals present in coal and metalliferrous ore bodies. In fact, AMD from abandoned coal mines affects the quality of both groundwater and surface water. Drainage results from various mining methods performed in the watershed. These methods include underground mining, strip mining, and auger mining. The mining process exposes iron 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 Partha Das Sharma (B.Tech-Hons., Mining Engineering),, Blogs: ,


Acid Mine Drainage (AMD) and its control 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 drinking-water supply. Pyrite oxidation mechanisms - Sulfide oxidation, part of sulfur's biotic/abiotic cycle, is an important natural phenomenon. However, because of the sulfide's association with metallic ores and fossil fuels in the form of pyrite (FeS2) and the world's increasing demand for metals and fossil fuels, sulfide oxidation in nature is in some state of perturbation. This perturbation, which results from land disturbances (e.g., mining, and/or ore processing), produces acid drainage often enriched with heavy metals. Chemistry and role of water in AMD- Acid mine drainage is an extremely acidic iron and sulphate rich drainage that forms under natural conditions when certain coal seams are mined and the associated strata are exposed to a new oxidizing environment. During this process, a variety of iron sulphides are exposed to the atmosphere and oxidize in the presence of oxygen and water to form soluble hydrous iron sulphates. These compounds usually appear as yellow and white crusts along certain horizons on the exposed surfaces of the pyritic material in coal mines. Some of the oxidation products have been identified as melanterite (white crystals of FeSO4), copiapite (yellow crystals of ferric sulpahte Fe2(SO4)3, halotrichite (white crystals of iron and/ or magnesium aluminium sulpahte), and alunogenite (white crystals of aluminum sulphate). These minerals are present in the hydrated form, the amount of water varying with the mode of formation. Following reactions represent the oxidation of FeS2 and production of acid (H+): 2FeS2(S) + 7O2 + 2H2 = 2Fe2+ + 4 SO24 + 4H+ Fe2+ + Ÿ O2 + O2 + H+ = Fe3+ + ½ H2O Fe3+ + 3H2O = Fe(OH)3(S) + 3H+ FeS2(S) + 14Fe3++ 8H2O = 15Fe2+ + 2 SO 24 + 16H+ Secondary reactions also take place between iron sulphates, sulphuric acid and the compounds in nearby clays, limestones, sandstones and various organic substances that

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Acid Mine Drainage (AMD) and its control are present in mines or streams to produce the variety of chemicals found in mine drainage.

Moreover, water appears to be essential for this chemical reaction. The rate of pyrite oxidation increases with water vapour pressure until at 100% relative humidity, the rate becomes equal to that for immersed pyrite. It has been suggested that water may not necessarily be a reactant; it may act as a medium for the transfer of oxidation products from reaction sites in view of the fact that the rate of the oxidation reaction increases as the concentration nears saturation state. Microbial aspects of AMD (Role of Bacteria in AMD) - The formation of acid mine water may be attributed to non-biological and biological oxidation of sulphur and iron sulphide in a mine in the presence of moisture and oxygen. Microbial oxidation plays a more important role than non-biological oxidation. The possible involvement of bacteria in the formation of AMD was first reported in 1919 by Powell and Parr, who found that coal inoculated with an unsterilised sulphate solution produced drainage with higher concentration of sulphate than did sterile controls. Colmer et al. demonstrated that Partha Das Sharma (B.Tech-Hons., Mining Engineering),, Blogs: ,


Acid Mine Drainage (AMD) and its control bacteria play a role in iron oxidation in acid mine water, based on the observations on iron oxidation that occurred in water samples freed from bacteria by filtration through Scitz filter/millipore filter or treatment with bactericidal agents. Conditions for microbial oxidation - The important requirements in the microbial oxidation of sulphide minerals are (i) an energy source and (ii) adequate supply of O2, CO2 and essential nutrients. In mines, appreciable percentages of CO2, O2, N2, etc. are present. These gases help in the growth of bacterial cells. Bacteria assimilate CO2 as the sole carbon source at the expense of energy available from the oxidation of Fe2+ and sulphide minerals. The energy derived from the oxidation of FeSO4 can support the growth of only a few species of bacteria. Discussion on impacts on water quality due to AMD from mining operation Typically, mining operation include a number of phases that can have different impacts on water quality; these are listed below: Exploration for mineral and petroleum resources involves field surveys, drilling programmes and exploratory excavations. Some water contamination can be produced at this stage from land clearing; for example, if clearing exposes a layer with high content of heavy metals, leading to contamination of stormwater by the heavy metals and by waste disposal from exploration camps. Unfilled exploration boreholes can allow contaminants from the surface to be washed into groundwater without being attenuated in the soil profile. * Exploration.

development. The development of a mining site and supporting infrastructure causes extensive land clearance. Also, groundwater and surface water contamination can be caused by spills and leaks from fuel storage tanks, and from waste disposal. * Project

operation. The type of operations can include pumping from boreholes (oil and natural gas, solution mining), heap leaching of rock piles, underground mining, open cuts and surface dredging. Oxidation and leaching of minerals from mining spoil and other waste products can contaminate groundwater and surface water. * Mine

* Beneficiation. Processing of minerals using a variety of mechanical and chemical treatment processes can be the most significant source of water contamination at a mine site. The major sources of contamination from mineral processing are leaks from storage ponds holding processing liquors, and leakage from tailings dams used to separate and recover processing liquids from fine solid wastes.

* Mine closure. Closure and rehabilitation of a mine site to mitigate environmental impacts (e.g. stabilisation and revegetation of waste rock and tailings) can contaminate groundwater if not well managed. Sources of contamination include continued seepage from waste rock and tailings if these are not well stabilised; salinisation of groundwater by evaporation from abandoned open pits and the excessive use of fertilizers in rehabilitation programmes. * Effects of mining on water quality - The type of water contamination produced by a mining operation depends to a large extent on the nature of the mineralization and on the Partha Das Sharma (B.Tech-Hons., Mining Engineering),, Blogs: ,


Acid Mine Drainage (AMD) and its control processing chemicals used to extract or concentrate minerals from the host rock. The water contaminants of most concern are summarized below: Type of mine Open-cut and underground mining of base metal sulfide deposits, precious metal deposits or uranium deposits with sulfide minerals, sulfide rich heavy mineral sands, coal deposits

Wastewater generated Acid mine drainage from waste rock heaps and ammonium nitrate-fuel oil explosive used for rock blasting

Characteristics of wastewater Low pH (< 4.5, possibly as low as 2) of water in springs, seeps, open cuts and streams draining from the mine site. Extensive vegetation death, yellow or white salt crusts on the soil surface, pale blue cloudy appearance of surface water

Base metal and precious metal deposits

Flotation agents used to concentrate minerals from ore; the main sources of contamination are seepage from processing mills and tailings dams

Gold deposits

Chemicals used to extract gold from ore (cyanide and mercury), particularly from tailings dams Acid leaching (especially sulfuric acid) used to extract uranium from ore

High pH of water (up to pH 10) when cyanide is used

Disposal of brines associated with petroleum hydrocarbons

High salinity of water, high concentrations of hydrogen sulfide, methane or detectable hydrocarbon odours in water

Uranium deposits

Petroleum and natural gas

Low pH of water, high sulfate concentrations in water

Chemicals possibly contained Arsenic, antimony, barium, cadmium, chromium, cobalt, fluoride, lead, mercury, molybdenum, nickel, nitrate, selenium, sulfate, uranium (radon may be of concern where there are high uranium concentrations) Depends on the type of mineralization â&#x20AC;&#x201D; contaminants from flotation agents of health concern include chromium, cresols, cyanide compounds, phenols and xanthates Arsenic, free cyanide, weak acid dissociable cyanide, mercury Arsenic, antimony, barium, cadmium, chromium, cobalt, fluoride, lead, mercury, molybdenum, nickel, radon, selenium, sulfate, uranium Boron, fluoride, hydrocarbons, uranium

AMD is probably the most severe environmental problem that occurs on mine sites. It happens where mineral and coal deposits contain sulfide minerals, particularly pyrite (FeS2). When waste rock containing sulfides is exposed to air, these minerals are oxidized, releasing sulfuric acid. The process is accelerated by bacteria such as Partha Das Sharma (B.Tech-Hons., Mining Engineering),, Blogs: ,


Acid Mine Drainage (AMD) and its control Thiobacillus ferrooxidans that obtain energy from the oxidation reaction for their growth. The release of acid can cause the pH of surface water and groundwater to become very low (as low as 2). Under these very acidic conditions, metal concentrations in water can become very high due to the dissolution of elements from waste rock. Acidic water at mine sites often kills vegetation, and may cause fish deaths in rivers. Apart from low pH, visual indicators of AMD at mine sites include the following: * Large areas where vegetation has died due to acidic runoff and shallow acidic groundwater; * The presence of abundant yellow or white salt crusts on waste rock and at the surface of the soil. The crusts comprise alum-like sulfate minerals containing variable amounts of sodium, potassium, iron and aluminium, such as the mineral jarosite. They are often very soluble in water, releasing acid and precipitating ferric hydroxides; * Surface water bodies on the mine sites often appear to have a milky blue-white cloudy appearance due to the presence of flocs of aluminium hydroxide. If the water is extremely acidic (< pH 3), it may appear to be crystal clear due to the precipitation of the flocs. Of the chemicals used to process ores, cyanide may be the most problematic due to its toxicity and the complexity of its chemical behaviour in groundwater. Cyanide degrades rapidly into nontoxic chemical compounds when exposed to air and sunlight, but in groundwater it may persist for long periods with little or no degradation. Cyanide (usually in the form of potassium or sodium cyanide) is used to extract gold from its ore, but in the subsurface it can react with minerals in soil and rock to form a wide range of metal cyanide complexes, many of which are very toxic. Abandoned pits and mine shafts are commonly used for water supply after mine closure. Depending on the type of mining activity, water from these sources could pose a risk to human health from high dissolved metal or cyanide concentrations. Environmental and ecological effects of acid mine drainage* Acidity and metal toxicity: The high acidity of acid mine drainage and the high amounts of dissolved heavy metals (such as copper and zinc) generally make acid mine drainage extremely toxic to most organisms. Many streams derived from acid mine drainage are largely devoid of life for a long way downstream. * Sedimentation: Drainage water from acid mine drainage is initially clear but turns a vivid orange colour as it becomes neutralised because of the precipitation of iron oxides and hydroxides. This precipitate, often called ochre, is very fine and smothers the river bed with a very fine silt. Thus, small animals that used to feed on the bottom of the stream or ocean (benthic organisms) can no longer feed and so are depleted. Because these animals are at the bottom of the aquatic food chain, this has impacts higher up the food chain into fish. * Effects on marine ecosystem: Acid mine drainage will be neutralised by the sea, unless it is already neutralised. The ochre (orange precipitate) may be formed in the sea as the stream enters the sea, or the ochre is formed in the stream and washed in to the sea by the stream. There is potential for acid mine drainage to influence the food chain in the near vicinity, resulting in possible negative effects on the whale shark and other marine life. Partha Das Sharma (B.Tech-Hons., Mining Engineering),, Blogs: ,


Acid Mine Drainage (AMD) and its control

Assessing the impact - When assessing the impact of industrial discharges on receiving waters, the most critical characteristics are: * The types of chemicals discharged â&#x20AC;&#x201D; this depends on the type of industries and processes used; * The amount and concentration of chemicals in the effluent â&#x20AC;&#x201D; these vary over time depending on the operation mode of both manufacturing and wastewater treatment processes employed (e.g. hourly, daily, weekly, monthly and seasonal variations). Solid wastes and/or gaseous emission generated from industrial sources also contribute to the amount and concentration of chemicals in the effluent if they are treated with water or they have any contact with water.

AMD abatement Methods - The formation and treatment of acid mine drainage is the biggest environmental problems relating to mining and processing activities in the worldwide. Various methods are used for the sulphates and heavy metals removal from acid mine drainage in the world. There are two approaches to controlling Acid Mine Drainage. The first is to reduce or eliminate the source of the AMD. One method for source elimination seeks to prevent oxidation by replacing the air within the mine with groundwater. This air-with-water replacement is brought about by sealing any mine openings with an impermeable grouting material. One such material under investigation is flue gas desulferization (FGD) material, a by-product from coal-fired power plants. Partha Das Sharma (B.Tech-Hons., Mining Engineering),, Blogs: ,


Acid Mine Drainage (AMD) and its control This material is composed of primarily calcium sulfate (gypsum). Another source elimination strategy is to fill the mine with a solid (e.g., FGD or a clay slurry) in order to eliminate the oxidation reaction. The second primary method for mitigating Acid Mine Drainage involves treating the AMD itself in order to remove the negative impact to the watershed. Chemical, biological, or physical treatments may be used in AMD abatement. Chemical treatments primarily seek to neutralize the acid through the addition of an alkali (e.g., soda ash) with a subsequent sedimentation basin in order to retain metal precipitates after the pH adjustment. Biological treatments use constructed wetlands, as one example, for natural attenuation of biological nutrient additions in order to accelerate indigenous activity. Physical treatment seeks to alleviate the impact through re-routing of streams to circumvent possible problematic geological formations. In environments where groundwater has been contaminated by waste water and acid mine drainage, microbial sulfate reduction can be exploited in subsurface permeable reactive barriers. A permeable reactive barrier is a passive, in-situ technique: groundwater treatment proceeds within the aquifer and long-term maintenance of the installation is unnecessary. This method consists of installing an appropriate reactive material into the aquifer, so that contaminated water flows through the material (see figure below). The reactive material induces chemical reactions that remove the contaminants from the water or otherwise cause a change that decreases the toxicity of the contaminated water. For the treatment of water contaminated with acid mine drainage, a number of studies have shown the effectiveness of this method.

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Acid Mine Drainage (AMD) and its control Underground Mine Sealing - Mine sealing can minimize the AMD pollution associated with abandoned underground mines. The primary factor affecting the selection, design and construction of underground mine seals is the anticipated hydraulic pressure that the seal will have to withstand when sealing is completed. Conclusion - Of the huge amount of money spent on acid mine drainage each year, the major portion is spent on treatment. But treatment is not the best solution to most acid mine drainage problems. Treatment has the disadvantage of being necessary for as long as the acid discharge continues and thus requires manpower, surface facilities, and sludge disposal areas indefinitely. Since acid drainage results from the oxidation of pyrite associated with coal and overburden strata, limiting the rate of pyrite oxidation would reduce the amount of acid formed. T. ferrooxidans normally catalyzes the pyrite oxidation and accelerated the initial acidification of freshly exposed coal and overburden. Inhibiting bacterial activity through the application of bactericides, therefore, would limit the rate of acid production and, in combination with proper reclamation, would reduce substantially the total amount of acid produced. References: * Morin, K.A., Environmental Geochemistry of Minesite Drainage; Appendix D, Minewall Methods, January 1996. * De Vries, Nadine H.C., Process for Treating Iron-Containing Sulfi de Rocks and Ores, U.S. Patent No. 5,587,001. * Eger, P. and A. Antonson, Use of Microencapsulation to Prevent Acid Rock Drainage, report to MSE Technology Applications, Minnesota Department of Natural Resources, St. Paul, MN, pg. 7-9, September 30, 2002. * Marshall, G.P., J.S. Thompson, and R.E. Jenkins, “New Technology for the Prevention of Acid Rock Drainage, Proceedings of the Randol Gold and Silver Forum, p. 203, 1998. * Metals Treatment Technologies, LLC (MT2), Golden Sunlight Mine Acid Rock Drainage (ARD) Highwall Demonstration, at, May 10, 2002. * Dr. Gurdeep Singh; CHEMICAL, MICROBIOLOGICAL AND GEOLOGICAL ASPECTS OF ACID MINE DRAINAGE AND ITS CONTROL ASPECTS, Proc. 2ND ASIAN MINING CONGRESS (16-19 January 2008), Kolkata, India, (MGMI), Vol-II, pp:297 – 310. * Jeffrey G. Skousen, Alan Sexstone and Paul F. Ziemkiewicz, ACID MINE DRAINAGE CONTROL AND TREATMENT, *

-------------------------------------------------------------------------------------------------------Author’s Bio-data: Partha Das Sharma is Graduate (B.Tech – Hons.) in Mining Engineering from IIT, Kharagpur, India (1979) and was associated with number of mining and explosives organizations, namely MOIL, BALCO, Century Cement, Anil Chemicals, VBC Industries, Mah. Explosives etc., before joining the present organization, Solar Group of Explosives Industries at Nagpur (India), few years ago. Author has presented number of technical papers in many of the seminars and journals on varied topics like Overburden side casting by blasting, Blast induced Ground Vibration and its control, Tunnel blasting, Drilling & blasting in metalliferous underground mines, Controlled blasting

Partha Das Sharma (B.Tech-Hons., Mining Engineering),, Blogs: ,


Acid Mine Drainage (AMD) and its control techniques, Development of Non-primary explosive detonators (NPED), Blasting in hot strata condition, Signature hole blast analysis with Electronic detonator etc. Currently, author has following useful blogs on Web: • • • • Author can be contacted at E-mail:,, ------------------------------------------------------------------------------------------------------------------Disclaimer: Views expressed in the article are solely of the author’s own and do not necessarily belong to any of the Company. ***

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Acid Mine Drainage (AMD) and its control