Acid Mine Drainage H-ESD : Environmental and Sustainable Development Michael Staudt, GTK

Table of contents Acid Mine Drainage Excercise Steps of the excercise Equations

Managing Sulphidic Mine Wastes and Acid Drainage

Acid Drainage Caused by the oxidation of sulphide minerals, especially iron sulphides, associated with mining Oxidation produces sulphate ion which when dissolved in water forms sulphuric acid

Acid Drainage Some effects: Acid drainage affects water quality downstream Rehabilitation becomes more difficult Metal ions are released

Acid Drainage Acid drainage is one of the most significant environmental issues facing the mining industry. Canadian liability estimated as C$ 2-5 billion Australian liability estimated as A$ 60M/year in the USA 20,000 km of streams and rivers adversely affected

Longevity of the Problem Acid drainage may not develop immediately Acid drainage can continue for tens to thousands of years  Rio Tinto region, Spain; for more than 2000 years  Many examples more than 50 years with little reduction in rate of acidic drainage

What is Acid Drainage? Oxidation of sulphidic minerals, especially in connection with mining – Exposure to air and water – Increase in surface area – Reactive minerals Pyrite (iron sulphide) most common sulphide mineral associated with mines Other iron and other metal sulphides Drainage of acid away from its source FeS O H 2 O = Fe(OH) SO H + (Iron sulphide + Oxygen + Water = Ferric Hydroxide + Aqueous sulphuric acid)

Factors Influencing Acid Drainage Water (required for oxidation and transport) Oxygen availability Physical characteristics of the material Temperature, pH Ferric (Fe +3 )/ferrous (Fe + 2) ion equilibrium Microbiological activity Presence of neutralising minerals – Carbonates are most effective – Silicates & aluminosilicates may contribute Chemistry of receiving waters

Impacts of Acid Drainage Potential for reuse of water on mine is limited – corrosion problems for equipment Toxic effects to aquatic ecosystems – acidity and dissolved metals Toxic effects on downstream vegetation Adverse impacts on ground water Limits uses of downstream water – Irrigation, stock watering, recreation, fishing Causes difficulties in revegetation and stabilising mine wastes

Best Practice Approach During feasibility stages: – Characterise acid generating potential of materials – Characterise mobility of potential contaminants such as heavy metals – Estimate the potential for oxidation products to migrate to the environment – Estimate effects on host environment

Identifying and Predicting Acid Drainage When characterising rock types at site important characteristics include: – Geological description – Mineralogy of both ore and waste – Fracturing Sampling and analysis: – Acid-base accounting – Simulated oxidation, usually with hydrogen peroxide – pH and conductivity tests of paste or slurry – Total and soluble metal analysis – Geochemical Kinetic Tests Humidity cells Column Leach Tests

Acid Drainage Control Strategies Control requires: – Data on physical and chemical properties of materials – Risk assessment – Strategies to minimise oxidation Control strategies – Containment and isolation – Treatment of acid drainage

Soil Covers Materials – Imported materials e.g. clay, soil – Low-sulphide waste rock, if compactable – Geotextile fabrics – Covers may require zones Base (main sealing) layer - high water retention, low permeability Middle layer - water reservoir (may have higher permeability) Surface layer (barrier zone) - erosion protection and/or substrate for plant growth

15 BEST PRACTICE ENVIRONMENTAL MANAGEMENT IN MINING Isolation

Water Covers Blending Most readily used in high rainfall, low evaporation areas Creation of a permanent lake or swamp Use of an existing lake or the sea Flooding of underground tunnels and pits Mixing of acid and non-acid forming waste rock Incorporation of alkaline materials Lime Fly ash Kiln dust

Bacterial Inhibition Bacteria can catalyse sulphide oxidation Applying bactericides can slow the process Effect may be short-term only Some success claimed in USA coal industry Used in establishing a vegetation cover before acid production starts

Treatment Systems Collection of acid drainage followed by neutralisation – Passive Anoxic Limestone Drains (PALID) Drainage passed through a channel of coarse limestone gravel in the absence of oxygen – Successive Alkalinity Producing Systems (SAPS) Variation on PALID – Wetland treatment systems Newer treatments, moving from experimental to operational – Bioreactors – KAD (kaolin amorphous derivative) – Bauxite derivatives – ‘Green rust’ precipitation

19 BEST PRACTICE ENVIRONMENTAL MANAGEMENT IN MINING Passive Treatment Systems Cross section through an anoxic limestone drain

20 BEST PRACTICE ENVIRONMENTAL MANAGEMENT IN MINING Treatment Systems Conceptual design of a wetland system for treating Acid Mine Drainage

Monitoring An essential component of sulphidic waste management Classification of materials Point source monitoring Monitoring surface water and ground water in both up- and down-stream gradients Monitoring of effectiveness of control measures

Monitoring Waters: pH, conductivity, SO4-2 Other major ions (Ca+2, Mg+2, Al+3, Na+, K+) Alkalinity Metals/metalloids (Fe, Al, As, Cd, Cu, Zn, Mn, Pb) Toxicity to organisms Rock materials: Static and kinetic geochemical tests Water flux through stockpiles Physical stability: cracking, erosion