01 0855 Acid

Aims of this Module: 

Aims of this Module To describe the problem of acid drainage To introduce best practice approaches for avoiding or minimising potential environmental impacts of acid drainage Prediction Prevention Treatment

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

Scale of the Problem: 

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 20,000 km of streams and rivers adversely affected in the USA USA coal mines only, US$ 1 million/day (1990) Scale of the Problem

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 Longevity of the Problem

What is Acid Drainage?: 

Oxidation of sulphidic minerals 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 What is Acid Drainage?

Primary 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 Primary Factors Influencing Acid Drainage

Other Factors: 

Secondary factors Presence of neutralising minerals Carbonates are most effective Silicates & aluminosilicates may contribute Tertiary factors Rainfall and temperature Chemistry of receiving waters Other Factors

Impacts of Acid Drainage (1): 

Potential for reuse of water on mine is limited By corrosion problems for equipment Toxic effects to aquatic ecosystems From acidity and dissolved metals Toxic effects on downstream vegetation Adverse impacts on ground water Impacts of Acid Drainage (1)

Impacts of Acid Drainage (2): 

Limits uses of downstream water Irrigation, stock watering, recreation, fishing Causes difficulties in revegetation and stabilising mine wastes Long term liability Mine operators Government Community Impacts of Acid Drainage (2)

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 Best Practice Approach

Predicting and Identifying Acid Drainage: 

When characterising rock types at site important characteristics include: Geological description Mineralogy of both ore and waste Fracturing Sampling Predicting and Identifying Acid Drainage

Geochemical Static Tests: 

Acid-base accounting (Net Acid Producing Potential NAPP) Net Acid Generation (NAG) simulated oxidation (accelerated), usually with hydrogen peroxide pH and conductivity tests of paste or slurry Total and soluble metal analysis Geochemical Static Tests

Geochemical Kinetic Tests: 

Humidity cells Column Leach Tests Require 4 months to 2-3 years. Interpretation - static and kinetic tests Be aware of limitations of tests NAPP and NAG are useful screening tools, but do not tell the full story Generally experienced assistance is needed to interpret results Geochemical Kinetic Tests

Managing Sulphide Oxidation, Acid Drainage Control Strategies: 

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

Soil Covers: 

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

Isolation Strategy: 

Isolation Strategy

Water Covers: 

Water Covers Most readily used in high rainfall, low evaporation areas Creation of a permanent lake or swamp Use of an existing lake Other strategies Flooding of underground tunnels and pits Submarine disposal

Blending: 

Blending Mixing of acid and non-acid forming waste rock Incorporation of alkaline materials Lime Fly ash Kiln dust Usually used in conjunction with encapsulation

Bacterial Inhibition: 

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: 

Treatment Systems Collection of acid drainage followed by neutralisation Newer treatments, moving from experimental to operational Bioreactors KAD (kaolin amorphous derivative) Bauxite derivatives ‘Green rust’ precipitation

Passive Treatment Systems: 

Passive Treatment Systems 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

Monitoring Strategies: 

Monitoring Strategies An essential component of sulphidic waste management Background studies 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 Parameters - Rock Materials: 

Monitoring Parameters - Rock Materials Static and kinetic geochemical tests NAP, NAG, %S Location of various types of rock Acid producing and non-acid producing Water flux through stockpiles Lysimeters and piezometers Physical stability: cracking, erosion

Monitoring Parameters - Water: 

pH, EC (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 Monitoring Parameters - Water

Remember: 

Remember It is much cheaper and more efficient to prevent acid drainage than to treat it It is cheaper to begin control measures during mining than afterwards Allow for the possibility of acid drainage from earliest planning stages Incorporate costs of controlling acid drainage when considering the feasibility of a mine