Acid Mine Drainage 11.11.2015

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 downstreamRehabilitation becomes more

difficultMetal 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 billionAustralian liability estimated as A$

60M/yearin 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

FeS2 + 3.75 O2 + 3.5 H2O = Fe(OH)3 + 2 SO42- + 4 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

15BEST PRACTICE ENVIRONMENTAL MANAGEMENT IN MINING

Isolation

Sulphidic waste

Top non-sulphidic waste layer

Basal layer

Revegetated and contoured cover material(surface capping and water storage medium)

Original ground surface

Freedumpednon-sulphidicwaste

Freedumpednon-sulphidicwaste

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 oxidationApplying bactericides can slow the processEffect may be short-term onlySome success claimed in USA coal industryUsed 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

19BEST PRACTICE ENVIRONMENTAL MANAGEMENT IN MINING

Passive Treatment Systems

Cross section through an anoxic limestone drain

20BEST 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