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Abandoned Coal Mines: In-Situ Treatment of AMD with CCPs
Jess W. Everett, Ph.D., P.E.Associate Professor
Civil EngineeringRowan University
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Abandoned Coal Mines: In-Situ Treatment of AMD with CCPs
• Acid Mine Drainage
– Mines fill with water, seeps are formed
– Bacteria oxidize Pyrite (FeS2), often found w/ coal• End products create water with low pH, high metals, and
high acidity
– pH drops even more when water leaves mine• Oxidation and Hydrolysis
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Problems Caused by AMD?
• Low pH in seep water damages receiving stream ecosystem
• Metals in seep water precipitate and cover stream bottom
• Metal toxicity
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Abandoned Coal Mines: In-Situ Treatment of AMD with CCPs
• Coal Combustion Products– High volume residues (ash) produced during
coal powered energy production
– Some CCPs are alkaline • Oxides/hydroxides may be present in raw coal or
form during combustion• Alkaline materials are used to control SO2
emissions, excess remains in ash
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How does the in-situ process work?
• Injection of Alkaline waste:
– Neutralizes acidity in mine
– Precipitates some metals in mine by pH adjustment
– Imparts alkalinity to seep water
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Overview
• Mine description• Injection description• Results• Interpretation• Conclusions
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Mine Plan View
Not to scale
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Mine Side View
PyriteOxidation
AMD Seep
Water Infiltration
Not to scale
pH = 4.4Zero Alkalinity
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35030025020015010050000
10
20
30
40
50
60
70
0.0
1.0
2.0
3.0
4.0
5.0
6.0Seep FlowRain
Day (August 1995 - July 1996)
Seep
Flo
w (
Lit
ers
/Min
ute
)
Rain
Fall
(in
ches)
No Data
Seep Flow and Rainfall
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Mine Hydrology - Tracer Studies• Estimated Mine Retention time ~ 5 yr
• Three Tracer Injections– Main Corridor (MC)
• Rhodamine WT (Rh) - 1 gallon, 20%– Florescent dye, 1 ppt detection limit– adsorption/precipitation problems
• Chloride (Cl) - 175 pounds of NaCl– 100 ppb detection limit
– Side Corridor (SC)• Chloride (300 pounds of NaCl)
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Tracer Injection Points
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Tracer Study Results
Test Breakthrough Recovery MC, Rh 11 hr <1 % (120 d) MC, Cl 9 hr* 11 % (30 d) SC, Cl 16 hr <0.3% (8 d)
* more frequent sampling than test 1
• Fast Breakthrough indicates some corridor flow, • Poor Recovery indicates mixing / diffusion
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Tracers In Wells
• Tracer concentrations throughout mine approached seep value– nearly identical after 100 days
• Injection point concentration stayed higher– “pool” of tracer?
• Mixing/diffusion -- important mine process
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Treatment description
• ~ 420 tons of ash injected...
• through five 2” injection wells...
• using Oil-field technology
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A flour truck (to right), used to bring FBA to the site. Pneumatic trailers are partially visible at the left.
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Grout truck and pneumatic trailers. The grout truck mixes FBA from the pneumatic trailers with water
from a frac tank, then injects the slurry into the mine.
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A close-up of the grout truck, used to mix and inject the FBA slurry into the mine.
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Total view of the site during injection. From Right to left: flour trucks, pneumatic trailers, grout truck, frac tank.
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Three frac tanks located at the seep. Seep is pipe in front of rightmost tank.
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Fire hose used to convey FBA slurry from the fixed location of the grout truck to the five well locations.
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Injection well. FBA was injected through the fire hose.
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Monitoring well, with pressure gauge. Little pressure increase was measured during injection
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Results and Interpretation
• Alkalinity and pH after Injection• Metal concentration in seep
• Interpretation of results– Phase I :Reaction of AMD with Ash– Phase II: Reaction & Mass Transfer of
CO2 with alkalinity – Phase III: Alkalinity consumption and flush
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0
2
4
6
8
10
12
14
-20 20 60 100 140 180 220 260 300 340 380
TIME (days)
pH
01002003004005006007008009001000
Alk
alin
ity
(p
pm
CaC
O3)
pH ALK
Seep pH and Alkalinity after Injection
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0
50
100
150
200
250
-20 20 60 100 140 180 220 260 300 340 380
TIME (days)
Fe
CO
NC
(p
pm
)
0
1
2
3
4
5
6
7
8
Mn
& A
l C
ON
C (
pp
m)
Fe Mn Al
Iron, Manganese, and Aluminum Concentrations in Seep
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Phase I - Quick Lime
• CaO + H2O --> Ca2+ + 2OH-
– Exothermic– Fast (over within hours of injection)– Immediate generation of alkalinity– High pH– Most metals precipitate as hydroxide
From the CCP
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0
2
4
6
8
10
12
14
-20 20 60 100 140 180 220 260 300 340 380
TIME (days)
pH
01002003004005006007008009001000
Alk
alin
ity
(p
pm
CaC
O3)
pH ALK
0
50
100
150
200
250
-20 20 60 100 140 180 220 260 300 340 380
TIME (days)
Fe
CO
NC
(p
pm
)
0
1
2
3
4
5
6
7
8
Mn
& A
l C
ON
C (
pp
m)
Fe Mn Al
Phase I
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Phase IIa - CO2 (aq) Reactions
• CO2 (aq) + 2OH- --> CO3 --
– may cause precipitation of CaCO3
– Some metals may precipitate as CO3s
• CO2 (aq) + CO3-- + H2O --> 2HCO3
-
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0
2
4
6
8
10
12
14
-20 20 60 100 140 180 220 260 300 340 380
TIME (days)
pH
01002003004005006007008009001000
Alk
alin
ity
(p
pm
CaC
O3)
pH ALK
CaCO3 Precipitation
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Phase IIb - CO2 (g) Mass Transfer
• What happens once the initial CO2 (aq) is consumed
– Mass Transfer from the mine headspace• CO2 (g) <--> CO2 (aq)
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0
2
4
6
8
10
12
14
-20 20 60 100 140 180 220 260 300 340 380
TIME (days)
pH
01002003004005006007008009001000
Alk
alin
ity
(p
pm
CaC
O3)
pH ALK
0
50
100
150
200
250
-20 20 60 100 140 180 220 260 300 340 380
TIME (days)
Fe
CO
NC
(p
pm
)
0
1
2
3
4
5
6
7
8
Mn
& A
l C
ON
C (
pp
m)
Fe Mn Al
Phase II
?
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Phase III
• Consumption of aqueous alkalinity in reaction with acid
• Flush of aqueous alkalinity• Dissolution of solid alkaline compounds
– CaCO3, MeOH, MeCO3
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0
2
4
6
8
10
12
14
-20 20 60 100 140 180 220 260 300 340 380
TIME (days)
pH
01002003004005006007008009001000
Alk
alin
ity
(p
pm
CaC
O3)
pH ALK
0
50
100
150
200
250
-20 20 60 100 140 180 220 260 300 340 380
TIME (days)
Fe
CO
NC
(p
pm
)
0
1
2
3
4
5
6
7
8
Mn
& A
l C
ON
C (
pp
m)
Fe Mn Al
Phase III
?
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Conclusions
• The injection of CCP increased pH and alkalinity
• The pH in the mine is influenced by CO2(g) in the mine headspace
• The longevity of treatment depends on acidity generation and seep flow