A Comparison of Propertie

FGD & Natural Gypsum 

Products 

Debra F. Pflughoeft-Hassett, 

David J. Hassett, & Kurt Eylands

Energy & Environmental Research Center

Agricultural & Industrial Uses of 

FGD Gypsum Workshop

October 23, 2007

Atlanta, Georgia
What is Gypsum?

• Chemical formula - CaSO

4·2H

2

O

• Color - White to grey, pinkish-red

• Crystal habit - Massive, flat. Elongated an

generally prismatic crystals

• Crystal system - Monoclinic 2/m

• Mohs Scale - hardness1.5-2

• Streak – White

• Specific gravity - 2.31 - 2.33

• Solubility - hot, dilute HCl
Formation of Natural Gyps

• Natural gypsum is an 

evaporite.

• When mineral-laden 

marine water slowly evaporates crystalline compounds form. These are evaporites

• Impurities can be 

present in natural gypsum. The largest pure gypsum dune field 

located at White Sands Nation
Formation of Natural Gyps

• Gypsum forms as sea water evaporates.• Gypsum forms in the supratidal zone 

where evaporation is the greatest.

• Capillary action brings saline water to 

the surface where the salts are precipitated.

• Gypsum, halite, aragonite, calcite, 

anhydrite, and dolomite form this way.
Varieties of Natural Gypsu

• Varieties of natural 

gypsum:

– Alabaster– Ordite– Satin Spar Gypsum– Selenite

Examples 

– Left: Alabaster vase– Right: Selenite crystals in 

Cave of the Crystals, Naica Mine, Chihuahua, Mexico.
Natural Gypsum Composit

• Natural gypsum exhibits low variability in 

composition. (CaO is ~ 33 wt.% & SO

3 is 

wt.%.)

• Chang, Howie, and Zussman (1996) repo

a large number of primary and secondary gypsum samples all contained:

– Mg, Si, Ca, Ti, Fe, and Sr.

• B, Al, Mn, Cu, Li, Be, Y, Zr, Bi, and Ba we

found sporadically as trace elements.
Formation of FGD Gypsum

(Solution Chemistry)

• Dissolution of SO

2

SO

2 +H

2O 

→ 2H

+ +SO

3

2-

• Hydration of LimeCaO + H

2O 

→ Ca(OH)

2

• Dissolution of Hydrated LimeCa(OH)

2

→ Ca2+ + 2OH1-

• Reaction of SO

2 with Ca2+

Ca2+ + SO

3

2- + 2H

2O 

→

CaSO

3

.1/2H

2O + 3/2 H

2

O

• Acidification of the maCaSO

3 + H

2SO

4

→ 

H

2SO

3

• Formation of calcium bCaSO

3 + H

2SO

3

→

Ca(HSO

3

)

2

• Oxidation of calcium b

gypsum

Ca(HSO

3

)

2 + H

2O + 

CaSO

4

. 2 H

2O + 
FGD Gypsum Formation

Limestone Forced Oxidation Flue-Gas Desulfurization Pr• SO

2 + CaCO

3

→ CaSO

3 · 1⁄2H

2O + CO

2 + O

2

→

CaSO4 · 2H

2O (gypsum) 
Formation of FGD Gypsu

On formation of a precipitate from solu

dissolved constituents must partition between the solid and liquid phases

There is evidence that mercury tends 

partition to the solid phase.

There is also evidence that other heav

metals tend to partition to the liquid 
Natural Gypsum at 100x

Natural gypsum crystals shown here are small fractions of larger flat and blocky crystals.Si and Ti are present in the X-ray spectrum.
FGD Gypsum at 100x

FGD Gypsum has smaller crystals. Which tend to be elongated. Few were larger than 200μm.The X-ray spectrum shows only Ca, S, and O.
Major & Minor Composition of FG

Natural Gypsum (all values wt. 

Constituent FGD Gypsum (Multiple source) Mined Gypsum (Multiple sources) FGD G

(Berla

20

Aluminum NR NR 0.05 Boron <0.0003 – 0.0042 0.0009 0.0005 Calcium 18.5 – 24.3 18.1 – 21.3 19.3 Iron 0.026 – 0.3 0.038 – 0.74 0.07 Magnesium 0.010 – 0.74 0.1 – 3.14 0.60 Phosphorus 0.0005 – 0.05 0.35 – 0.046 Potassium <0.004 – 0.09 0.0076 – 0.44 0.0 Silicon 0.022 – 69.2 NA 0.05 Sodium 0.023 – 5.8 0.076 – 0.35 0.0 Sulfur 14.9 – 20.9 12.3 – 17 17.6 

NR = Not Reported; NA = Not Available Pure gypsum should be 23.3% Ca & 18.6% S. 
Trace Element Concentrations in FG

Natural Gypsum (all values ppm)

Trace Constituent 

FGD Gypsum 

(Multiple 

sources) 

Mined Gypsum 

(Multiple 

sources) 

FGD Gypsum 

(Berland et al, 

2003) 

Natu

(Be

Antimony 0.1 – 9.1 0.02 – 0.28 NR Arsenic 0.6 – 4.0 0.19 – 3.0 <1 – 2 Barium 33.8 – 77.2 NA 1 – 6400 Cadmium 0.003 – 1.2 <2 – 0.5 <1 – 2 Chromium 0.56 – 42.0 8.7 – 30.5 3 – 24 Copper 0.96 – 27.9 <4 – 29.8 <1 – 64 Lead 0.8 – 12.0 <5 1 – 47 Lithium <9 and 48.8 <9 – 13.1 NR Manganese 1.37 – 52.1 NA 3 – 94 Mercury 0.004 – 1.4 0.00044 – 0.025 NR Molybdenum 0.5 – 12.0 <3 <1 – 3 Nickel 0.73 – 20.1 <4 – 11.9 <3 – 19 Selenium 2.0 – 30.0 11.3 – 21.1 <1 – 15 Thallium 0.6 – 2.0 <15 NR Vanadium <1 – 73.2 <2 – 12.7 8 – 21 Zinc 3.4 – 102 13.1 – 27.5 <1 – 20 NR = Not Reported; NA = Not Available 
Natural Gypsum vs. FGD Gyp

• Major chemical composition is simila• Minor & trace constituents are simila• Crystalline form is the same
Contact Information

Energy & Environmental Research CenterUniversity of North Dakota15 North 23rd Street Stop 9018 Grand Forks, North Dakota 58202-9018www.undeerc.org/carrcTelephone: (701) 777-5000Fax: (701) 777-5181Debra F. Pflughoeft-HassettSenior Research Advisor(701) 777-5296dphassett@undeerc.org