Transcript Document

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
ARD Control Technology Selection
WASTE TYPE
WASTE ROCK
DUMPS/STOCKPILES
TAILINGS
HEAP-LEACH
PILES
UNDERGROUND
WORKINGS
OPEN PITS
PRIMARY
ACID GENERATION CONTROL
IS WATER COVER FEASIBLE?
YES
DESIGN & IMPLEMENT
N0
• SEGREGATION & BLENDING
• CONDITIONING
• BASE ADDITIVES
• BACTERICIDES
• COVERS & SEALS
EVALUATE OTHER METHODS
IS SUFFICIENT CONTROL
ACHIEVED?
ARD MIGRATION CONTROL
YES
SECONDARY
NO
• COVERS & SEALS
• DIVERT SURFACE WATER
• INTERCEPT GROUND WATER
EVALUATE AVAILABLE METHODS
IS SUFFICIENT CONTROL
ACHIEVED?
DESIGN & IMPLEMENT
YES
DESIGN & IMPLEMENT
NO
TERTIARY
COLLECTION AND TREATMENT
DESIGN COLLECTION &
TREATMENT SYSTEM(S)
• PASSIVE SYSTEMS
• ACTIVE SYSTEMS
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
DISTRIBUTION
% AVAILABILITY - LOW PERCENTAGE
DUE TO DURABILITY
ACID CONSUMING
LIMESTONE
% AVAILABILITY - LARGE PERCENTAGE
DUE TO SLAKING CHARACTERISTICS
0.01:1 0.1:1 0.3:1
ACID GENERATING
POTENTIAL FROM
SULFIDES
1:1
3:1
10:1
NP:AP RATIO
100:1 oo:1
Blending
DISTRIBUTION
APPROXIMATELY 35%
ACID GENERATING
0.01:1 0.1:1
POOR BLEND
0.3:1
1:1
3:1
10:1
NP:AP RATIO
100:1 oo:1
Blending
DISTRIBUTION
INTERMEDIATE BLEND
APPROXIMATELY 20%
ACID GENERATING
0.01:1 0.1:1
0.3:1
1:1
3:1
10:1
NP:AP RATIO
100:1 oo:1
Blending
THOROUGH BLEND
DISTRIBUTION
APPROXIMATELY 8%
ACID GENERATING
0.01:1 0.1:1
0.3:1
1:1
3:1
10:1
NP:AP RATIO
100:1 oo:1
Oxygen Control
• Process by which oxygen enters
reactive waste deposits:
– Diffusion
– Convection
(thermal, wind pressure)
– Barometric Pumping
• P1V1 = P2V2
Diffusion Coefficient as a Function of Saturation
1
Direct Proportionality
-2
10
D
Da0
Van Brakel
and Heertjes
10
10
-4
Millington
and Shearer
Currie
-6
0.0
0.2
0.4
0.6
0.8
Moisture content (vol.water/vol.void)
 = 0.5
/T 2 = 0.7
1.0
Oxygen Effectiveness of a Single Layer
‘Dry’ Sandy Till Cover
Sandy
moraine
1 June
1.0
1 Oct
Z (m)
1 Aug
1 July
1.5
1 Sept
covers
Oxygen OEffectiveness
ofofa‘moist’
Layered
‘Moist’ Cover
2 Effectiveness
1 Nov
1 May
0.5
Fine-textured
layer
Ks= 5x10 -8 m/s
0.0
0.0
0.2
0.6
0.4
0.8
1.0
SATURATION (VOL.WATER/VOL.VOID)
1 July
1 Sept
0.5
Z (m)
1 Nov
0.0
0.6
0.8
1 May
(i.e. =0)
Fine-textured
layer
1.0
SATURATION (VOL.WATER/VOL.VOID)
Drying of
water
Drying
of the
the fine-grained
fine-grainedlayer
layercaused
causedbybycapillary
capillary
water
flow
upwards
during
the
dry
period.
The
fine-grained
layer
flow upwards during the dry period. The fine-grained
layer
is
represented
by
the
silt
(Ks=5X10^-8
m/s)
-8
is represented by the silt (Ks=5x10 m/s)
2 Effectiveness of ‘moist’ covers
Oxygen O
Effectiveness
of Various ‘Moist’ Covers
-6
10
10-7
SANDY TILL
-8
10
10-9
CLAY TILL
-10
10
-11
10
2.0
4.0
6.0
8.0
10.0
12.0
J F M A M J J A S O N D
Seasonal variation of the mass transfer coefficient for oxygen
diffusion through 1 m moraine layers during normal years.
Sandy moraine above a course-grained layer. A
capillary barrier is formed at the bottom of the
moraine layer
Sandy moraine directly above the waste sand.
Clayey moraine directly above the waste sand.
Hydraulic Balance Using a Permeable
Surround
Examples: Rabbit Lake Pit; Key Lake Pit
Hydraulic balance using a permeable surround
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
waste
low density
moisture
• Complex
– Variable
– Multi-layered
high density
waste
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
Chemical Treatment
• Neutralization Process Chemistry
H2SO4 + CaCO3 + H2O  CaSO4.2H2O + CO2
ground limestone
gypsum
H2SO4 + Ca(OH)2  CaSO4.2H20
slaked lime
gypsum
• 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)
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 SO42- to H2S
– 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, SO4
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 ~6oC
• 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