The Control & Management of Acid Mine Drainage By Andy Robertson and Shannon Shaw

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.

ARD Prevention & Control Measures Primary, secondary and tertiary controls Oxygen control Groundwater control Surface water control Covers Collection and treatment

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

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

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: a)Long range planning for designing of waste dumps and coarse scheduling b)Short range planning to schedule haulage to correct destinations by time period c)Accurate, reliable in-field sampling, testing and prediction (blast hole sampling and modeling) d)Very strict effective operations control

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

Blending

Oxygen Control Process by which oxygen enters reactive waste deposits: –Diffusion –Convection (thermal, wind pressure) –Barometric Pumping P 1 V 1 = P 2 V 2

Diffusion Coefficient as a Function of Saturation

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

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

Oxygen Effectiveness of Various ‘Moist’ Covers

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

Hydraulic Cage

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

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 waste moisture

Grey Eagle Tailings Cover

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

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

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

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

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

ground limestonegypsum slaked lime Chemical Treatment Neutralization Process Chemistry H 2 SO 4 + CaCO 3 + H 2 O  CaSO 4.2H 2 O + CO 2 H 2 SO 4 + Ca(OH) 2  CaSO 4.2H 2 0 Also use NaCO 3 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)

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

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.

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.

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

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

Passive Treatment Sulphate Reduction –Part of wetland, at depth –Anaerobic bacterial treatment –Establish anaerobic conditions on solid medium, –Bacterial reduction of SO 4 2- to H 2 S –Precipitation of metal sulphides –Convert excess to elemental sulphur –Possible treatment in a flooded open pit after closure

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, SO 4 and other constituents in the water as a result of on-going oxidation Must evaluate the agronomic limits for various parameters

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 ~6 o C Other processes such as that of BioteQ Bacterial reduction of sulphate and metal extraction as sulphides Utilizes sulphur and nutrients for bacterial growth

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)

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

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)

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

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

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

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

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

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