Challenges of using gypsum for mine land reclamation: • Risk of gypsum

dissolution due to the rainwater penetration, leaching properties, concentration of hazardous compounds such as sulfate in the leachate

Research Objective

Porosity evolution with time and space:

Reactive Transport Modeling of FGD Gypsum Bed to Simulate Time Dependent Dissolution,

Porosity Evolution, and Leachate Composition

Dissolution of gypsum and risk of karstification

Conclusions Using Crunchflow, a reactive transport model, to evaluate: • Time-dependent dissolution of gypsum • phases which are dissolved or precipitated • leachate composition changes as water follows through

the column with updating flow rates • Spatial distribution of dissolved and solid gypsum

through the column • Porosity evolution with time

FGD gypsum production in coal

combustion power plants

Future aspects

Crunchflow, reactive transport

modeling FGD Gypsum is a synthetic product derived from flue gas desulfurization (FGD) systems at electric power plants. Sulfur dioxide emission control systems remove sulfur from combustion gases by using lime or limestone as reagents and applying forced oxidation systems in scrubbers.

In mining application gypsum can be effective in: • Neutralization or encapsulation of acid-producing

materials • Barrier to acid mine drainage formation • Subsidence control in underground mines

C-S-H

glass

CH

Multicomponent reactive flow and transport Software which uses reaction thermodynamics and kinetics calculations, coupled with mass transport.

Glass

Assumptions: • 30 cm column filled with 60% Volume

gypsum, 20% quartz (as an inert filler), and initial porosity of 20%

• Gypsum dissolution in water and pore fluid composition monitored after 0.01, 0.1, 0.5, 1, 5, and 10 days

• CO2 is considered to be dissolved in water passing the minerals

• Three different pHs: 3, 8, 13 • Constant diffusion coefficient and

dispersivity • Surface area of gypsum powder: 0.70

m2/g • Temperature: 25 ̊C (77 ̊F) • Constant pressure gradient applied

across the column (34,130 Pa) • Updating porosity and flow rate (initial

flow rate=1.10 cm/day)

Equations:

CaSO4.2H2O(s)=Ca2+ + SO42- + 2H2O Ksp=4.93x10-5 Ca(OH)2(s) = Ca2+ + 2OH- Ksp= 10-5.3 CO2(aq) + H2O = H2CO3(aq) KH =10-1.5

H2CO3(aq) = HCO3- + H+ Ka1= 10-6.3

HCO3- = CO32- + H+ Ka2=10-10.3

CaCO3(s) = Ca2+ + CO32- Ksp=10-8.3 H2O (L) = H+ +OH- Kw =10-14 • Leachate concentration of gypsum dissolution depends

on pH • At alkali pHs, gypsum dissolves more, and volume

percent of precipitated portlandite and calcite increase • Porosity has a increment of about 50% after 10 days of

water flow.

Minerals’ dissolution and precipitation pattern were assessed through the first grids of the column in different times, as the pore solution becomes saturated and is at equilibrium with solid gypsum at higher depths.

Mina Mohebbi, Jean-Patrick L. Brunet , Li Li, Farshad Rajabipour, Barry E. Scheetz

FGD Gypsum production in Coal Combustion Power Plant

0

2

4

6

8

10

12

14

0.01 0.1 1 10 100 1000

Vo

lum

e %

Particle Diameter (μm.)

Par

ticle

siz

e di

strib

utio

n cu

rve

for

gyps

um

1-D Batch column

0.E+00

5.E-03

1.E-02

2.E-02

2.E-02

0 1 2 3 4 5 6 7 8 9 10 11

Ca2

+ C

on

cen

trat

ion

(m

ol/

Kg)

Time(day)

pH=8

pH=3

pH=13

0

10

20

30

40

50

60

70

0 5 10 15

Gyp

sum

Vo

lum

e %

Time (day)

pH=3

pH=8

pH=13

0.00099

0.001

0.00101

0.00102

0.00103

0.00104

0.00105

0.00106

0 5 10 15

Po

rtla

nd

ite

Vo

lum

e %

Time (day)

pH=3

pH=8

pH=13

0

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

0 5 10 15

Cal

cite

Vo

lum

e %

Time (day)

pH=3

pH=8

pH=13

0

5

10

15

20

25

30

35

0 5 10 15

Po

rosi

ty %

Time (day)

pH=3

pH=8

pH=13

As a result of gypsum dissolution and portlandite and calcite formation, porosity is changing about 50% on average for different pHs after 10 days. Moreover, as a result of preferential dissolving, porosity has a sudden decrease at first grids, and is approximately constant for larger distances.

Higher concentration of Ca2+ and sulfate

ions at alkali environment.

Leachate concentration would be about 1.67x10-2 M

after 10 days at pH=13

Gypsum Portlandite

Calcite

• 2-D modeling to evaluate the effect of cracks and risk of karstification.

• Mixture of gypsum with other by-products such as fly ash and assess the mixture properties

• Porosity evolution effects on structural stability of gypsum beds, and risk of subsidence

• Role of possible bed cracks and risk of karstification

60% Gypsum +

20% Quartz +

20% Initial Porosity

Results and Discussion Breakthrough curve: pore solution concentrations (Ca2+ and SO4 2-) vs. time at the last grids of the column

Spatial distribution of solid gypsum shows that after 10 days, only a thin layer at the beginning of the column preferentially dissolves due to water flow.

4.00E+01

4.50E+01

5.00E+01

5.50E+01

6.00E+01

6.50E+01

0.01 0.10 1.00 10.00 100.00

Gyp

sum

vo

lum

e (

%)

Distance (cm)

0.01 day

0.1 day

0.5 day

1 day

5 days

10 days

References 1. www.csteefel.com/crunchflowintroduction 2. www.fgdproducts.org 3. Clair N. Sawyer, Perry L. McCarty, Gene F. Parkin, Chemistry for

Environmental Engineering and Science , 5th ed, The Mcgraw-Hill, 2003

15

20

25

30

35

0.04 0.40 4.00 40.00

Po

rosi

ty (

%)

Distance (cm)

pH=3pH=8pH=13

Initial porosity

2013 World of Coal Ash (WOCA) Conference - April 22-25, 2013 in Lexington, KY (http://www.flyash.info/)