The Basics of Acid Mine Drainage

1. The Basics ofAcid Mine Drainage By Andy Robertson and Shannon Shaw

2. Disclaimer • These slides have been selected from a set used as the basis of a series of lectures on Acid Mine Drainage presented in 2006 at the University of British Columbia, Vancouver, BC. • No attempt is made here to provide linking text or other verbal explanations. • If you know about Acid Mine Drainage, these slides may be of interest or fill in a gap or two—going back to basics never hurts the expert. • If you know nothing of Acid Mine Drainage, these slide may be incomprehensible, but on the other hand they may be an easy way to ease into a tough topic—good luck.

3. Overview of ARD Metal Sulphide + Water + Oxygen => Acid + Metal [M]S + H2O + O2 => H2SO4 + [M(OH)x] (not stoichiometrically balanced) Acid + Alkali => “Salt” + Carbon Dioxide H2SO4 + CaCO3 => CaSO4 + CO2 • Environmental Impact from: • Acidity • Metals in solution (in acid or alkaline environments) • Salinity • Sludge precipitates

4. Bacterial Catalization of Oxidation

5. Temperature Effects on Oxidation

6. Buffering of ARD during Oxidation of a Mineral Assemblage

7. Buffering of ARD during Oxidation of a Mineral Assemblage Buffering of Mineral A (e.g. calcite, dolomite) Buffering of Mineral B (e.g. ankerite, siderite) Buffering of Mineral C (e.g. Al(OH)3) pH Buffering of Mineral D (e.g. feldspars) Time

8. Mechanisms Controlling ARD in Tailings

9. Mechanisms Controlling ARD in Waste Rock Oxygen Diffusion Advective Air Transport

10. Mechanisms Controlling ARD in Open Pits

11. Mechanisms Controlling ARD in Underground Workings

12. Sulphide Minerals Pyrite (FeS2) Pyrrhotite (Fe(1-x)Sx) Marcasite (FeS2 ) Chalcopyrite (CuFeS2) Galena (PbS) Sphalerite (ZnS) Arsenopyrite (FeAsS) Bornite (Cu5FeS4)

13. Alkali Minerals • Types • Carbonates • Calcite (CaCO3) • Dolomite (Ca,Mg(CO3)2) • Hydroxides • Fe(OH)3 • Al(OH)3 • Silicates • Clays

14. Development of ARD • Water chemistry depends on: • Rate and extent of oxidation • Rate and extent of metal release • Quantity of material • Contained metals • Site hydrology and climate • Accumulation of oxidation products • pH/solubility controls, flowpath reactions • Control technology

15. Site Characterization • Design • Field investigation & Sampling • Lab testing

16. New Mines vs. Existing Mines • New Mines • ARD probably not evident • Objective is to determine ARD potential • Fresh samples used for testing and prediction • Long term behavior based on kinetic testing, modeling and prediction • Existing and Abandoned Mines • ARD may be evident/mature • Field reconnaissance used to define ARD • Historic data (time trends) extremely useful • Limited laboratory testing required • Field instrumentation and monitoring possible • Background altered, requires simulation

17. Design • Review existing data, e.g: • Geology & mine plan • Drill core logs • Water quality monitoring results • Assays on ore/waste rock and tailings • Waste type volumes • Waste placement history • Develop reconnaissance & sampling plan

18. Field Investigations • Objectives • Detect early signs of ARD • Determine potential for ARD • Assess factors that control ARD • Evaluate control measures • Determine environmental impact • Assess compliance with regulatory standards

19. Field Investigations • What to bring: • Eyes that know what to look for • pH and conductivity meters • Acid bottle, hydrogen peroxide, sulfate kit • Geological pick, hand lens, sampling bags, camera, GPS unit • Site map, history, data 2.2

20. Field Investigations • Things to look for: • Visible pyrite or other sulfides (oxidation) & calcite • Red, orange, yellow, white, blue staining (precipitates, water) • Dead vegetation or bare ground • Melting snow or steaming vents on waste • Dead fish or other biota • Low pH in seeps, groundwater, decants & streams

21. Field Investigations • Things to log in the field: • Paste pH • Paste conductivity • ‘Colour’ • Lithology • Sulfide content • Secondary mineralogy • Degree of ‘fizz’ • Moisture content • Grain size

22. Field Investigations • General Methodology • Visual observation of site • Paste pH and water quality data • Field extraction testing • Classify types of wastes • Solids sampling (for lab testing)

23. Field Investigations • Geochemistry: • Low paste pH of mine wastes • High conductivity of waste paste • Contaminants in leach extraction tests • Static (ABA) tests • Products from Reconnaissance: • Physical disturbance and drainage map • Waste deposit map and characterization • Exposed rock map and characterization • Paste pH and conductivity survey • Observations and sampling map • ARD site assessment report

24. TDS vs pH

25. Sample Selection (New Mines) • Step 1: On geological sections: • Define rock types • Define sulfide and alkali mineral distribution • Preliminary rock units classification • Step 2: Sample each rock unit class allowing for: • Area distribution of class • Variability of rock • Step 3: Perform static lab tests and use results to refine rock unit classification • Step 4: Sample each new rock class and repeat Step 3 until satisfied. • Step 5: Sample each rock class for appropriate kinetic testing and use results to refine rock classification • Step 6: Repeat Step 5 until satisfied with classifications and characterization.

26. Sampling (existing mines) • Steps: • Define geology, mineralization, waste ‘types’ etc. • Define objectives (i.e. sampling for reveg, cover, water quality evaluations etc. may have different focus) • Consider mine plan and waste placement history • Identify sources of samples • Initial sampling and testing program • Further sampling if necessary

27. Sampling (Existing Mines) • A Becker hammer-type drill rig can be used in order to minimize sample crushing and the geochemical disturbance of the samples • Samples typically collected at specified intervals (e.g. every 10 ft) & paste pH and EC measured, • A sub-set of samples can then be selected using observations and field measurements as a guide for more detailed laboratory testing

28. Test Methods • Static ARD Tests • balance between potentially acid generating and consuming • tool for waste management • includes geological/mineralogical characterization • individual samples • Short-term Leaching Tests • readily soluble component • Kinetic Tests • oxidation and metal leaching rates • water chemistry prediction

30. Geochemical Static Tests • Objective: Potentially Acid Generating Minerals vs Acid Neutralizing Minerals • Cautions for ARD assessment: • pH of alkalinity (NP) determination • Assumes instant availability of NP • Assumes all sulphur/sulphide minerals reactive • Ignores reaction rates (kinetics) • Extrapolation to field

31. Geochemical Static Tests • Procedures • Paste pH and conductivity on the ‘as received’ fines • Acid-Base Accounting Tests • Net Acid Generation (NAG) - also an accelerated kinetic test • B.C. Research Initial Test • Lapakko Neutralization Potential Test • H2O2 Oxidation (modified for siderite correction) • Net Carbonate Value (NCV) for ABA Tests • Leach extraction analyses • Forward acid titration tests • Multi-element ICP analyses Detailed procedures can be found on: www.enviromine.com and in prediction course on www.edumine.com

32. Definitions: AP = acid potential = % S x 31.25 NP = neutralization potential NNP = net neutralization potential = NP - AP NP:AP ratio = NP/AP All expressed as: kg CaCO3 equivalent/tonne, or CaCO3 eq./1000 tonnes Example: S = 2 % AP = 62.5 kgCaCO3/t NP = 90 kgCaCO3/t NNP = 27.5 kgCaCO3/t NP/AP = 1.4:1 Geochemical Static Tests Note: units and acronyms used are different in Australiasia, local references should be sought for correct usage, terminology, guidelines etc.

33. Interpretation Start with ‘guidelines” or general criteria for classification, then develop site- specific criteria Typically criteria are based on a ‘set’ of tests, not just one type of test e.g. ABA & NAG results

34. NAG Test • Developed in Australia as an alternative and/or compliment to ABA test, • Developed as a “one-off” test that can assess the net acid generation potential –both acid generation and acid neutralization – in one test. • NAG test varies among users, typically: • Adding 250 mL of 15% H2O2 at room temp to 2.5 g of sample pulverized to pass 200 mesh. • React for 12 h then boiled until visible reaction ceases (or Cu catalyst added) or initial reaction period is extended to 24 h • Measure pH of the reacted solution (NAGpH) • Titrate reacted solution with NaOH to a specified pH end-point (pH 4.5 and/or pH 7) to determine the NAG value of the sample.

35. Interpretation • There are numbers of modifications to the test for different scenarios, including: • Sequential addition NAG test (multiple additions of H2O2) • Kinetic NAG test (track pH, temperature and EC during test) • Modifications to account for organic matter effects (analyze for organic acids and sulphuric acid in reacted solution, extended boiling step). • Modifications to leach carbonates prior to NAG test (i.e. measure of acidity not net acidity). • NAG results are generally interpreted as such: • If the final NAGpH is > 4.5, sample said to be non-acid forming • If the final NAGpH is < 4.5, the sample is said to be potentially acid forming • The NAG value then provides a quantitative assessment of potential acid formation in units of kg CaCO3/t equivalent (or kg H2SO4/t equivalent)

36. Applications of the NAG test • In conjunction with ABA tests etc to reduce the risk of mis-classification • As an operational scale management tool (e.g. for segregation of different material types) • For identifying material for prioritization (e.g. AML ranking) • As an indicator test that can be run on greater number of samples than if using other methods due to the fact it is quick, simple and inexpensive • Used very widely in Australasia

37. Some potential pitfalls • Organic matter, Cu, Pb and MnO2 can catalyze decomposition of H2O2. Samples high in these parameters can have unpredictable results (O’Shay et al., 1990) • Samples with a lot of Zn can be buffered between pH of ~ 4 to 5 by the formation of Zn(OH)2 (Jennings et al., 1999) • NAG test can underestimate potential acidity if samples have (Amira, 2002): • Sulphide content > ~1% • High carbonate content • High organic content • Not as ‘conservative’ as ABA testing

38. Example – Ok Tedi [Rumble et al. 2003 ICARD proceedings]

39. Example – Ok Tedi • Single addition NAG test showed the dredged material was NAF – but river bars showed elevated SO4 and metals and slightly depressed pH • Sequential NAG test consistently showed a drop in the NAGpH of the material below 4.5 after additional H2O2 additions • perhaps due to presence of Cu or higher S content [Pile et al. 2003 ICARD proceedings]

40. Short-term Extraction Tests • Objective • Determine readily soluble load • Determine acid soluble load • Procedure • Uncrushed sample including fines • Agitate in deionised water or mild acid • Filter and analyse filtrate * Always account for dilution in concentration assessments

41. Kinetic Testing • Objectives • Validation of static test results and boundaries • Determination of leaching behaviour • Simulation of site conditions • Evaluation of extent of oxidation • Evaluation of stored products • Prediction of drainage water quality • Produces raw data for modeling • Investigate factors controlling ARD • Selection of control measures

42. Kinetic Testing

43. Humidity Cells • Objective • Predict lag to, and rate of, acid generation • Semi-qualitative water quality prediction* • Advantages • Widely used in North America in the past • Simple to operate • Appropriate for fine samples, disseminated mineralization • Disadvantages • Crushed sample - does not address surface area, mineralogy • Not representative of waste rock • High flushing rate, saturation, pH & solute modification * Always account for dilution in concentration assessments

44. Columns • Objective • Evaluate kinetics of oxidation & leaching for waste rock • Data to predict drainage water quality • Advantages • Representative of rock pile size distribution • Development of local pH environments • Evaluate storage/flushing • Evaluate control options • Estimate production rates • Disadvantages • Size of sample required • Interpretation of data • Edge effects • High flush rates • Laboratory conditions of temp and oxygen availability

45. Kinetic Testing Data

46. Field Test Plots • Objective • Evaluate leach kinetics & drainage water quality in field conditions • Advantages • Representative of site conditions • Calibration of water quality prediction • Test control options on a realistic scale • Already exist? • Disadvantages • Limited control of test conditions • Time required • Expensive for new installations • Maintenance and damage • Interpretation of results

47. Field Test Plots

48. Field Test Plots

49. Field Test Plots

50. Field Barrel Tests