A Comparison of Properties of FGD & Natural Gypsum

Products Debra F. Pflughoeft-Hassett,

David J. Hassett, & Kurt EylandsEnergy & Environmental Research Center

Agricultural & Industrial Uses of FGD Gypsum Workshop

October 23, 2007Atlanta, Georgia

What is Gypsum?

• Chemical formula - CaSO4·2H2O• Color - White to grey, pinkish-red• Crystal habit - Massive, flat. Elongated and

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 Gypsum

• 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 in the world is

located at White Sands National Monument.

Formation of Natural Gypsum

• 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 Gypsum

• 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 Composition

• Natural gypsum exhibits low variability in composition. (CaO is ~ 33 wt.% & SO3 is ~ 46 wt.%.)

• Chang, Howie, and Zussman (1996) report that 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 were all found sporadically as trace elements.

Formation of FGD Gypsum(Solution Chemistry)

• Dissolution of SO2

SO2 +H2O → 2H+ +SO32-

• Hydration of Lime

CaO + H2O → Ca(OH)2

• Dissolution of Hydrated Lime

Ca(OH)2 → Ca2+ + 2OH1-

• Reaction of SO2 with Ca2+

Ca2+ + SO32- + 2H2O →

CaSO3.1/2H2O + 3/2 H2O

• Acidification of the material

CaSO3 + H2SO4 → CaSO4 + H2SO3

• Formation of calcium bisulfite

CaSO3 + H2SO3 →Ca(HSO3)2

• Oxidation of calcium bisulfite to gypsum

Ca(HSO3)2 + H2O + ½ O2 →CaSO4

. 2 H2O + SO2

FGD Gypsum Formation

Limestone Forced Oxidation Flue-Gas Desulfurization Process• SO2 + CaCO3 → CaSO3 · ½H2O + CO2 + O2 →CaSO4 · 2H2O (gypsum)

Formation of FGD Gypsum

On formation of a precipitate from solution, dissolved constituents must partition between the solid and liquid phases.

There is evidence that mercury tends to partition to the solid phase.

There is also evidence that other heavy metals tend to partition to the liquid phase.

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 FGD & Natural Gypsum (all values wt. %)

Constituent FGD Gypsum (Multiple source)

Mined Gypsum (Multiple sources)

FGD Gypsum (Berland et al,

2003) Aluminum NR NR 0.05 – 2.70 Boron <0.0003 – 0.0042 0.0009 0.0005 – 0.0029 Calcium 18.5 – 24.3 18.1 – 21.3 19.3 – 22.9 Iron 0.026 – 0.3 0.038 – 0.74 0.07 – 4.0 Magnesium 0.010 – 0.74 0.1 – 3.14 0.60 – 2.95 Phosphorus 0.0005 – 0.05 0.35 – 0.046 NR Potassium <0.004 – 0.09 0.0076 – 0.44 0.0 – 0.25 Silicon 0.022 – 69.2 NA 0.05 – 2.95 Sodium 0.023 – 5.8 0.076 – 0.35 0.0 – 0.22 Sulfur 14.9 – 20.9 12.3 – 17 17.6 – 18.4

NR = Not Reported; NA = Not Available Pure gypsum should be 23.3% Ca & 18.6% S.

Trace Element Concentrations in FGD & Natural Gypsum (all values ppm)

Trace Constituent

FGD Gypsum (Multiple sources)

Mined Gypsum (Multiple sources)

FGD Gypsum (Berland et al,

2003)

Natural Gypsum (Berland et al,

2003)

Antimony 0.1 – 9.1 0.02 – 0.28 NR NR Arsenic 0.6 – 4.0 0.19 – 3.0 <1 – 2 <1 Barium 33.8 – 77.2 NA 1 – 6400 4 Cadmium 0.003 – 1.2 <2 – 0.5 <1 – 2 <1 Chromium 0.56 – 42.0 8.7 – 30.5 3 – 24 1 Copper 0.96 – 27.9 <4 – 29.8 <1 – 64 7 Lead 0.8 – 12.0 <5 1 – 47 <1 Lithium <9 and 48.8 <9 – 13.1 NR NR Manganese 1.37 – 52.1 NA 3 – 94 4 Mercury 0.004 – 1.4 0.00044 – 0.025 NR NR Molybdenum 0.5 – 12.0 <3 <1 – 3 <1 Nickel 0.73 – 20.1 <4 – 11.9 <3 – 19 2 Selenium 2.0 – 30.0 11.3 – 21.1 <1 – 15 <1 Thallium 0.6 – 2.0 <15 NR NR Vanadium <1 – 73.2 <2 – 12.7 8 – 21 27 Zinc 3.4 – 102 13.1 – 27.5 <1 – 20 5 NR = Not Reported; NA = Not Available

Natural Gypsum vs. FGD Gypsum

• Major chemical composition is similar

• Minor & trace constituents are similar

• Crystalline form is the same

Contact Information

Energy & Environmental Research CenterUniversity of North Dakota15 North 23rd Street Stop 9018 Grand Forks, North Dakota 58202-9018

www.undeerc.org/carrcTelephone: (701) 777-5000Fax: (701) 777-5181

Debra F. Pflughoeft-HassettSenior Research Advisor(701) 777-5296dphassett@undeerc.org