The Control & Management of Acid Mine Drainage

1. The Control & Management 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. ARD Prevention & Control Measures • Primary, secondary and tertiary controls • Oxygen control • Groundwater control • Surface water control • Covers • Collection and treatment

4. Control Technologies • Prevention • Control designed and implemented before the event of ARD • No acid product storage • Abatement and Mitigation • Control implemented after the fact • Acid product storage • Approaches to Control • Primary - control of acid generation • Secondary - control of migration of contaminants • Tertiary - collection and treatment

5. ARD Control Technology Selection WASTE TYPE WASTE ROCK DUMPS/STOCKPILES HEAP-LEACH PILES UNDERGROUND WORKINGS TAILINGS OPEN PITS PRIMARY ACID GENERATION CONTROL YES IS WATER COVER FEASIBLE? DESIGN & IMPLEMENT N0 • SEGREGATION & BLENDING EVALUATE OTHER METHODS • CONDITIONING • BASE ADDITIVES • BACTERICIDES • COVERS & SEALS IS SUFFICIENT CONTROL ACHIEVED? YES DESIGN & IMPLEMENT SECONDARY NO ARD MIGRATION CONTROL • COVERS & SEALS • DIVERT SURFACE WATER EVALUATE AVAILABLE METHODS • INTERCEPT GROUND WATER IS SUFFICIENT CONTROL ACHIEVED? YES DESIGN & IMPLEMENT NO TERTIARY COLLECTION AND TREATMENT • PASSIVE SYSTEMS DESIGN COLLECTION & TREATMENT SYSTEM(S) • ACTIVE SYSTEMS

6. Segregation & Blending • Segregation: • Feasibility of sulphide removal • Sometimes applicable to tailings which can be floated • Not applicable to waste rock • Feasibility of separation by rock unit classification • Depends on variability and selective mining capability • Requires: • Long range planning for designing of waste dumps and coarse scheduling • Short range planning to schedule haulage to correct destinations by time period • Accurate, reliable in-field sampling, testing and prediction (blast hole sampling and modeling) • Very strict effective operations control

7. Segregation & Blending • Blending methods: • Layering • Coarse blending by scheduling • Fine blending by truck loads and dozer pushing • Alkali addition

8. Blending

9. Blending

10. Blending

11. Blending

12. Oxygen Control • Process by which oxygen enters reactive waste deposits: • Diffusion • Convection (thermal, wind pressure) • Barometric Pumping • P1V1 = P2V2

13. Diffusion Coefficient as a Function of Saturation

14. Oxygen Effectiveness of a Single Layer ‘Dry’ Sandy Till Cover

15. Oxygen Effectiveness of a Layered ‘Moist’ Cover Drying of the fine-grained layer caused by capillary waterflow upwards during the dry period. The fine-grained layer is represented by the silt (Ks=5X10^-8 m/s)

16. Oxygen Effectiveness of Various ‘Moist’ Covers

17. Hydraulic Balance Using a Permeable Surround Examples: Rabbit Lake Pit; Key Lake Pit

18. Hydraulic Cage

19. Surface Water Control • Avoid stream channels and valleys • Install diversion ditches and berms • Install collection ditches • Separate clean from contaminated runoff • Install covers to minimize infiltration • Provide erosion protection

20. Soil Covers • Types of Covers: • Simple • Permeability depends on grain size • Compaction • Oxygen diffusion depends on moisture content • Compound • Complex • Variable • Multi-layered waste low density high density moisture waste

21. Grey Eagle Tailings Cover

22. Tertiary Control • Active Treatment • Collection of drainage • Chemical treatment • Require continuous operation • Passive Treatment • Limestone trenches • Wetlands • Sulphate reduction • Intended to function without maintenance

23. Collection, Storage, Treatment & Sludge Disposal • Both collection and treatment are transient functions but must by ready to function at all times • Storage and sludge disposal facilities requires ‘dams’ with: • Long term stability • Resistance to extreme events (floods, earthquakes, tornadoes and terrorist or vandalism acts) • Resist the perpetual degradation forces of erosion, sedimentation, weathering, frost action, biotic and root penetration and anthropogenic activity • Containment to prevent leakage and discharges • Isolation of sludges to prevent re-dissolution and migration

24. Collection • Objectives: • Collect all seepage and drainage • Minimize volume to treatment process • Provide surge control • Achieved by: • Ditching to collect surface flows • Groundwater flows - ditches, wells (drawdown), cutoff walls • Difficulties: • Identification of all sources • Seasonal variations, peak flows, holding capacity • Maintenance and operational requirements • Control of hydraulic and chemical loading

25. Collection • Objectives: • Collect all seepage and drainage • Minimize volume to treatment process • Provide surge control • Achieved by: • Ditching to collect surface flows • Groundwater flows - ditches, wells (drawdown), cutoff walls • Difficulties: • Identification of all sources • Seasonal variations, peak flows, holding capacity • Maintenance and operational requirements • Control of hydraulic and chemical loading

26. Water Treatment • Objective is to remove from solution: • Acidity • by neutralization • Heavy metals • by hydrolysis and precipitation • co-precipitation • Metal such as As, Sb • by complexation and precipitation as arsenate, antimonate • co-precipitation • Deleterious substances eg. suspended solids • settling, flocculation, precipitation, HDS

27. Chemical Treatment • Neutralization Process Chemistry H2SO4 + CaCO3 + H2O  CaSO4.2H2O + CO2 H2SO4 + Ca(OH)2 CaSO4.2H20 • Also use NaCO3 and NaOH • Produces • Gypsum and metal hydroxide sludge. • Gypsum saturated (~ 3,000 ppm) water = high TDS • Very low density (5 to 30% solids depending on process) ground limestone gypsum gypsum slaked lime

28. Chemical Treatment • High Density Sludge Process • Process • recycle treatment sludge (thickener underflow) • up to 50% recycle • premix lime and recycled sludge • then combine with influent ARD • Advantages • reduced lime consumption • high density/lower volume sludge • larger precipitate particles “seeds” • increased removal of suspended solids • more efficient dissolved metal removal

29. Chemical Treatment • Considerations: • Metal removal limited by solubility • Optimum pH for hydroxide precipitation • Acceptable final effluent pH • Complex Chemistry • interactions with other constituents • complexing agents, coprecipitation • surface adsorption • mixed hydroxides • Ferric iron can also act as flocculant/adsorbent • Sludge density and disposal • Cannot design plant from theoretical concepts alone.

30. Sludge Disposal • Concern • Long term chemical stability • Issues • Changes in solution chemistry - pH • Leach testing - EPA 1312, SWEP test? • Special waste classification • Disposal to limit flushing • Include with tailings • Research and more experience in sludge stability required.

31. Passive Treatment • Wetland: • Soil is at least periodically saturated or covered with water • Peat bogs, cattail marshes, swamps. • Effluent directed to natural or constructed wetland with emergent vegetation • Ability to treat depends on: • water flow distribution • residence time • seasonal, climate • Low strength feeds, polishing process

32. Wetlands • Advantages • Adaptability to acid drainage and elevated metals • Low capital costs of natural wetland systems • Low operational costs for constructed wetland (?) • Provide wildlife habitat and flood control • Disadvantages • Capital costs of earth moving requirements • Land area requirement • Treatment during winter is reduced • Impacts on wildlife are still unknown • Heavy metal loads in vegetation • Polishing process

33. Passive Treatment • Sulphate Reduction • Part of wetland, at depth • Anaerobic bacterial treatment • Establish anaerobic conditions on solid medium, • Bacterial reduction of SO42- to H2S • Precipitation of metal sulphides • Convert excess to elemental sulphur • Possible treatment in a flooded open pit after closure

34. Land Application • The LAD relies on the cation exchange in the soils and plant uptake of constituents. • Solutions are irrigated over the surface to enhance evaporation and minimize surface water discharge. • Can have issues related to increasing concentrations of Se, SO4 and other constituents in the water as a result of on-going oxidation • Must evaluate the agronomic limits for various parameters

35. Biotreatment Processes • Example: Landusky • An integrated, staged process system using biological denitrification, biological selenium removal and biological cyanide oxidation • Biotreatment technology utilizes a mixture of reduction and oxidizing bacteria that have been demonstrated to perform at site temperatures of ~6oC • Other processes such as that of BioteQ • Bacterial reduction of sulphate and metal extraction as sulphides • Utilizes sulphur and nutrients for bacterial growth

36. Monitoring and Maintenance • Long term monitoring should be the minimum required to: 1. Detect and define changes which require reaction and reclamation 2. Demonstrate performance where changes from required performance standards are expected or suspected. • All monitoring results should be subject to pre-defined analysis with defined alert and decision making levels and criteria. Any monitoring for which there are not defined decision criteria and response should be questioned. • Site inspections and reconnaissance is a cost effective, efficient and effective monitoring methodology if done systematically with pre-established reference points (monuments, stations, photographs and survey records)

37. Monitoring and Maintenance • Two types of monitoring: • Monitoring to establish performance or initial transient effects, i.e.: • Seasonal trends (e.g. depth of frost penetration) • Vegetation establishment • Dissipation of contaminant plume • Monitoring for expected or suspected change in compliance, i.e.: • Water quality discharged from a treatment plant • Erosion of a tailings dam spillway • Financial performance of a trust fund The former should be discontinued once performance is established, the latter must be sustained as long as a change, suspected change or compliance requirements persist

38. Maintenance • Some sites can be returned to a self sustaining condition that, after a demonstration period of monitoring, will require no further interaction by man • Many sites require ongoing monitoring and maintenance to ensure that performance standards are maintained. • Typical maintenance items include: • Diversion and spillway structure cleaning out and repair • Erosion gully repair • Fence repair and access control • Prevention of root and rodent penetration of covers • Maintenance of contaminated water collection and management systems (passive care) • Operation and maintenance of water treatment plant and sludge disposal systems (active care)

39. Requirements for Containment and Reclamation • Chemical stability • Contaminants must not leach and move • Physical stability • Solids must not move • Land use and aesthetics • Must be useful and look good

40. Physical Stabilization • Dumps • Erosion protection • Prevent water mounding • Cut off airflow pathways • Diversions • Resloping • Toe berms • Relocating • Diversions • Control erosion • Remove sediment and debris • Control overtopping

41. Physical Stabilization • Tailings dam • Spillway maintenance • Drainage and dewatering • Plug decants • Erosion protection • Covers • Dam stabilization including berms • Maintain internal drainage • Covers • Revegetation • Erosion control • Drainage channels • Control disruption

42. Physical Stabilization • Open pits • Backfilling • Slope crest laybacks • Fencing or berming and ditch • Flooding with or without neutralization • Underground mines • Controlled flooding with or without neutralization • Hydraulic plugs • Shaft caps and access plugs • Subsidence stabilization • Glory hole fencing or filling

43. Land Use • Reclamation, in terms of land use, means measures taken so that the use or conditions of the land or lands is: • Restored to its former use or condition, or • Made suitable for an acceptable alternative use • This can be accomplished via: • Land form engineering • Revegetation • Land use planning • Land use management

44. Long Term Monitoring and Maintenance • Maintenance and monitoring must be provided by a long term custodian • Funding for such activity must be derived either from income from sustainable land use on the site or from an ‘endowment’ or ‘trust fund’ • There must be ‘something in it’ for the long term custodian to accept the responsibility of long term maintenance and monitoring