Acid Mine Drainage Development and Management

Transcription

1 Acid Mine Drainage Development and Management Laura Teresa Chaparro Leal*. Specialist in Natural Resource Conservation Abstract-. This paper describes acid mine or rock drainage as a major environmental problem generated by mining on the national and international on a daily basis, along with the main impacts and the proper treatment to avoid the generation thereof or to carry out remediation and treatment where it already exists. In addition, it provides several economic possibilities of the byproducts of AMD treatment in an attempt to turn an environmental liability into an economic opportunity in the situations where the problem is irreversible. Keywords: drainage, rock, environment, mining. I. INTRODUCTION Mining for obtaining different materials of economic interest, such as gold, silver, iron, copper and coal, among others, has been generating enormous environmental impacts in teh world for centuries, and it was not until a little over three decades ago that these impacts began to be studied and taken more seriously [1]. One of the main impacts generated by the mining of metals and coal is Acid Mine Drainage (AMD), which is generated by the oxidation of metal sulphides, particularly those of iron as pyrite (FeS2) in the presence of atmospheric oxygen and water [2]. The importance in the study of its impact on the environment lies in the fact that acid drainage is the effluent of mining and it pollutes bodies of surface and groundwater because of its ph values between 1.5 and 6, contributing a large amount of acidity Fe+2 can undergo two processes, depending on the ph of the water. If the ph is above 4.5, the process that occurs is described in equation (3), where the ferrous iron oxidizes and hydrolyzes to form hydroxides, which are red-orange precipitates, which are typical of AMD. Fe /4O2 + 21/2H2O < > Fe(OH)3 (s) + 2H + (3) If the ph of the water is 4.5 or less, the process that will take place is the oxidation of ferrous iron to ferric iron, and the latter will act as the main oxidizing agent of pyrite, replacing the atmospheric due to the formation of sulfuric acid and high concentrations of heavy metals, such as copper, lead and arsenic, among others, which are soluble at low ph values. In addition, AMD generates red - orange sediments due to iron and sulfate precipitates, which occupy the spaces where fish spawn, get between their gills and cover the debris that they feed on [3]. In addition, the ground vegetation that can come into contact with AMD is also widely affected, because the acidity and ion concentration, such as sulfates and chlorides, prevents its normal growth [3]. Finally, one of the most important characteristics of AMD in terms of the impact of mining, lies in the fact that once it has been generated, the development process is cyclical and irreversible and it endures for years or decades until one of the main agents causing it is eliminated. II. DEVELOPMENT The process of development of Acid Mine Drainage begins when sulfided minerals, such as pyrite, are exposed to the effects of oxygen and water. This happens when material is removed, such as when pits and tunnels are excavated, mine tailings are piled and tailings are disposed of with no civil control whatsoever. The reaction (1) shows the pyrite oxidation process. At this point, the oxygen acts as the main oxidizing agent and sulfides oxidize into sulfates [2]. FeS2 (s) + 7/2O2 + H2O > Fe SO H + (1) At this stage, the ph is still at values greater than 4.5. After that, the ferrous iron is oxidized into ferric iron in the presence of atmospheric oxygen. The equation (2) shows the reaction of Fe oxidation. Fe /4 O2 + H + > Fe /2H2O (2) oxygen and generating further acidity, as illustrated in the reaction (4). 14Fe +3 + FeS2 (s) + 8H2O > 2SO Fe H + (4) In general, sulfided minerals that can be potential generators of acidity are characterized for having a metal/sulfur ration of less than 1, such as pyrite FeS2, whose ratio is 1/2. However, when the drainage is in an advanced acidification phase in which reaction 4 has already taken place, all other metal sulphides are susceptible to oxidation due to the action of the ferric iron.

2 When the mine or rock drainage is in an advanced state of acidification, the physicalchemical process of oxidation of the iron sulphides, such as pyrite, becomes secondary, because it is displaced by microbiological oxidation, where several groups of acidophiles and chemolithotroph bacteria, such as Acidithiobacillus ferrooxidans, which gets its energy from iron oxidation. In fact, it has been proven that it can increase reaction speed by up to 106 times [4]. III. MANAGEMENT Managing Acid Mine Drainage is generally focused on three fundamental topics. The first is the prediction of the potential resources that generate acidity, the second consists of preventing the development of acid drainage, and the third is the treatment thereof when it has already been generated [5]. static tests include humid cells, leaching columns and the British Columbia Test to determine bacterial growth. In addition, tests are being conducted on the field scale to predict the materials that generate acidity, because they provide results that are more real since they take into account the weather (dry and rainy seasons), the size of the mineral from the mine, to minimize errors due to the decrease in size in the grinding necessary for the laboratory tests in which the contact surface between the sample and water is increased; the temperature is that of the site where the mine is located, because it has occurred that at controlled laboratory temperatures (220C)?, which are generally greater than the temperatures in situ, the oxidation speed of the sulphides increases [7]. A.Prediction Prediction, as part of the management of acid drainage, is focused on determining the possible resources of a mine that are capable of generating acidity. Various tests are used to do so at a laboratory scale, and they are divided into static tests and kinetic tests. The first are focused on determining whether they are important sources of minerals capable of generating acidity, such as pyrite, determining the sulphides capable of oxidizing into sulfates and therefore, generating sulfuric acid, the paste ph of the minerals under study, the metals present, among others. One of the main static tests is the ABA (Acid-Base-Account) the potential for net acid generation and the potential for net base generation. In turn, there are the kinetic tests, which model how a mineral capable of generating acidity would behave over time. Kinetic tests show how long a drainage would acidify, the ph values it would reach, whether the microorganisms are capable of proliferating in the material under study, and what metals in the rock could leach. The most common

3 B. Prevention Whenever prediction studies have been conducted and it has been determined that there are minerals with the potential to generate acidity in the mine, various prevention efforts are made to avoid the generation of AMD. These efforts save company resources that would be spent on unnecessary treatment if the acid drainage can be avoided in time. There is a wide range of AMD prevention techniques depending on the site, the weather conditions and the type of mining (open pit or underground). In general, prevention techniques are classified in special management methods, dry covers and wet cover. Special management methods: one of the most commonly used strategies is mixing with organic matter to reduce sulphates to sulphides and the consequent precipitation of heavy metals, thus reducing their bioavailability, the use for mixing with limestone, use of bactericide to prevent the proliferation of microorganisms that accelerate the acidification process, desulfurization of process tailings, use of seals to prevent groundwater from entering the tunnels, execution of civil works such as perimeter channels, intercepting ditches to prevent contact between rainwater and minerals in open pits, tailings, temporary mineral collection sites and tailings dumps, among others [2]. Dry Covers: dry covers are designed to prevent the generation of acid drainage by the collection sites of mineral and tailings from processing, the filling of pits and, in some cases, the filling of work underground. The most common dry covers include natural covers such as the use of soil, nonreactive tailings and the use of vegetation, and synthetic covers made of high-density polymers resistant to the climatic changes of the site where they are located [8]. Dry covers are a major challenge of mining and they have been under development for a little over 3 decades. Their importance lie in the fact that they control erosion, minimize the influx of moisture and atmospheric oxygen, which prevents the generation of AMD and, if they are made and operating properly, they allow the recovery of ecosystemic functions [9]. Wet Covers: wet covers have the same objective as dry covers, which is to minimize the influx of atmospheric oxygen to reactive sulfided minerals. This is achieved because the dissolved oxygen in water is lower than the content in the air. Wet covers are being used extensively in open pit mining, because they form meromictic lakes that allow the development of aquatic life [10]. Wet covers are widely used in mines near the ocean or rivers that facilitate the filling of the pits. One of the most successful cases of the recovery of an open pit turned into a lake is the Island Copper Mine in Canada [11]. C. Treatment Since the problem of acid mine drainage began to be studied such a short time ago and because mining has been carried out for more than a century, AMD or ARD has contaminated major extensions of bodies of water in countries including the United States, Australia and Canada, among others, and therefore, has had to be researched and treated. At present, there is a wide variety of techniques to treat acid and neutral mine drainage. In general, AMD treatment systems are divided into two major groups: active and passive.

4 Active Methods: active treatment methods are those that require constant supervision of the system, electricity, chemical supplies and the removal of by-products, among others. The most generalized form of treatment of AMD consists of a phase prior to the oxidation of ferrous iron to the ferric state, through natural or mechanical aeration to reduce the amount of chemicals to be applied. After that, a neutralizing agent is added, such as calcium carbonate, hydrated lime or caustic soda, among others. The addition of the base has a dual role as it adjusts the ph to permissible values for discharge and precipitates the metals that are insoluble at neutral ph values, such as iron, copper, cadmium and lead, among others. After that, a flocculating agent is added to remove the suspended solids left in the water. Figure 1 shows the general outline of AMD treatment [2]. Figure 2 Biological Removal of Sulphates from AMD Source: MINE CLOSING MANAGEMENT. Peru. Peru Mining Chamber. Figure 1. General Outline of Active AMD Treatment Source: MINE CLOSING MANAGEMENT. Peru. Peru Mining Chamber. After that, if the drainage has high concentrations of sulphates and chlorides, they must be removed because the bases do not remove them efficiently. To do so, ion-exchange resins are used, along with membrane systems and the addition of reagents such as aluminum hydroxide, or biological removal by reducing sulphates to sulphides. Figure 2 shows the biological treatment of sulphates [12]. However, there are treatment methods for mine drainage that do not necessarily require the addition of a base, because it is neutral mine drainage to which a coagulating agent can be applied to remove metals, sulphates and chlorides. In addition, nowadays there are techniques other than the addition of chemical reagents, such as electrocoagulation, to remove metals and adjust the ph with the exclusive use of electricity [13]. Passive Treatment Passive treatment systems are one of the most attractive tools for mining companies, especially in the abandonment and disassembly phase, because they require minimal human intervention, they rarely require the management of by-products and the use of electricity and chemical reagents is not necessary. The most common and important systems include the use of wetlands, open and closed limestone channels and reactive permeable barriers, among others. Open and closed limestone channels are designed for effluents with low iron and metal concentrations, because the precipitation of Fe(OH)3 crust can cover the limestone and decrease their neutralization capacity [15].

5 Revista ESAICA Sección Ingeniería Finally, reactive barriers are one of the main alternative methods to wetlands, because they require major extensions to be able to work, whereas the barriers to do. They contain one layer of water, another of organic matter and finally, a layer of limestone, where the AMD enters, the organic matter and the microorganisms present therein reduce the sulphates and precipitate the metals, and to conclude, the ph of the drainage is corrected by the bed of limestone. Figure 3 illustrates a reactive permeable barrier system [2]. Further research must be conducted on the study of environmental liabilities such as AMD, along with the economic potential of the remediation and the use of the byproducts of treatment thereof. V. REFERENCES [1] AKCIL, Ata; KOLDAS, Soner. Acid Mine Drainage, Causes, Treatment and Case of Studies. En: Journal of Cleaner Production Vol 14, p [2] International Network for Acid Prevention. The GARD Guide. < [citado en 03 de septiembre de 2014] [3] Reclamation Research Group. Acide Mine Drainaje and Effects on Fish Health and Ecology: A Review. Anchorage, Alaska. Junio, Figure 3. Passive Reactive Barrier Treatment System Source: MINE CLOSING MANAGEMENT. Peru. Peru Mining Chamber. D. By-products of Treatment The by-products from the treatment of acid mine drainage are one of the major challenges of mining today, because active methods generate neutral sludge high in ferrihydrite, precipitated metal and, in some cases, calcium sulphate content, and they are not currently being used for commercial purposes, and only in co-disposal with acidity-generating minerals. Pigments, construction materials and another series of applications that are worth researching can be obtained from these by-products to generate alternative sources of income where AMD is affecting communities [16]. IV. CONCLUSIONS Acid and neutral mine drainage is one of the main liabilities of the mining industry and it generates environmental and social impacts, with the aggravating circumstance that they can be irreversible. The comprehensive management of acid and neutral mine drainage involves almost all the branches of knowledge, such as engineering and basic sciences. Therefore, it is very important for educational institutions to involve the topics related to the environmental liabilities of mining in order to be able to prevent their generation and manage them properly. If the mines that have potential acidity generators are properly closed and managed, part of the attributes and functions of an ecosystem can be recovered, although it will not exactly be the way it was originally. [4] Natarajan, K.A. Microbial Aspects of Acide Mine Drainage and its Bioremediation. En: Transaction of Nonferrus Metals Society of China. Noviembre Vol 18, p [5] Bell, F.G., et al. Environmental impacts associated with an abandoned mine in the Witbank Coalfield, South Africa. En: International Journal of Coal Geology Vol 45, p [6] Price A, William. Prediction Manual for Drainage Chemistry from Sulphidic Geologic Materials. Canadá. Diciembre,2009, total version. British Columbia, Canadá. Diciembre Reporte [7] Plante, B; et al. Lab To fiel Scale Effects on Contaminated Neutral Drainage Prediction from the Tio Mine Waste Rocks. En: Journal of Geochemical Exploration. November, Vol 137, p [8] Sung Ahn, Joo et al. An engineered cover system for mine tailings using a hardpan layer: A solidification/stabilization method for layer and field performance evaluation. En: Journal of Hazardous Materials. Septiembre Vol 197, p [9] Instituto Tecnológico Geominero de España. Manual de Restauración de Terrenos y Evaluación de Impactos Ambientales en Minería. Spain: 2004, p [10] Vigneault, Bernad et al. Geochemical Changes in Sulfidic Mine Tailings Stored Under a Shallow Water Cover. En: Elsevier. June Vol 35, p [11] Poling George Wesley. Under Water Tailing Placement at Island Copper Mine: a success story Canada. [12] Cámara Minera del Perú. Gestión en Cierre de Minas. Peru [diapositivas] [13] Oncel, M.C., et al. A Comparative Study of Chemical Precipitation and Electrocoagulation for Treatment of Coal

6 Revista ESAICA Sección Ingeniería Acid Drainage Wastewater. En: journal of Environmental Chemical Engineering. August Vol 1, p [14] Watten, Barnaby et al. Acid Neutralization Within Limestone Sand Reactors Receiving Coal Mine Drainage. En: Environmental Pollution. June Vol 137, p [15] Matthies, Romy et al. Performanse of a Pasive Treatment System for Net Acidic Coal Mine Drainage over Five Years of Operation. En: Science of the Total Environment. July Vol 408, p