1 Abandoned Coal Mines: In-Situ Treatment of AMD with CCPs Jess W. Everett, Ph.D., P.E. Associate Professor Civil Engineering Rowan University.

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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