A Comparison of Properties ofFGD & Natural GypsumProductsDebra F. Pflughoeft-Hassett,David J. Hassett, & Kurt EylandsEnergy & Environmental Research CenterAgricultural & Industrial Uses ofFGD Gypsum WorkshopOctober 23, 2007Atlanta, GeorgiaWhat is Gypsum?•Chemical formula- CaSO4·2H2O• Color- White to grey, pinkish-red•Crystal habit- Massive, flat. Elongated andgenerally prismatic crystals•Crystal system- Monoclinic 2/m•Mohs Scale- hardness1.5-2•Streak– White•Specific gravity- 2.31 - 2.33•Solubility- hot, dilute HClFormation of Natural Gypsum• Natural gypsum is anevaporite.• When mineral-ladenmarine water slowlyevaporates crystallinecompounds form.These are evaporites• Impurities can bepresent in naturalgypsum.The largest pure gypsum dunefield in the world islocated at White Sands National Monument.Formation of Natural Gypsum• Gypsum forms as sea water evaporates.• Gypsum forms in the supratidal zonewhere evaporation is the greatest.• Capillary action brings saline water tothe surface where the salts areprecipitated.• Gypsum, halite, aragonite, calcite,anhydrite, and dolomite form this way.Varieties of Natural Gypsum• Varieties of naturalgypsum:– Alabaster– Ordite– Satin Spar Gypsum– SeleniteExamples– Left: Alabaster vase– Right: Selenite crystals inCave of the Crystals, NaicaMine, Chihuahua, Mexico.Natural Gypsum Composition• Natural gypsum exhibits low variability incomposition. (CaO is ~ 33 wt.% & SO3is ~ 46wt.%.)• Chang, Howie, and Zussman (1996) report thata large number of primary and secondarygypsum samples all contained:– Mg, Si, Ca, Ti, Fe, and Sr.• B, Al, Mn, Cu, Li, Be, Y, Zr, Bi, and Ba were allfound sporadically as trace elements.Formation of FGD Gypsum(Solution Chemistry)• Dissolution of SO2SO2+H2O→2H++SO32-• Hydration of LimeCaO + H2O→Ca(OH)2• Dissolution of Hydrated LimeCa(OH)2→Ca2++ 2OH1-• Reaction of SO2with Ca2+Ca2++ SO32-+ 2H2O→CaSO3.1/2H2O + 3/2 H2O• Acidification of the materialCaSO3+ H2SO4→CaSO4+H2SO3• Formation of calcium bisulfiteCaSO3+ H2SO3→Ca(HSO3)2• Oxidation of calcium bisulfite togypsumCa(HSO3)2+ H2O + 1⁄2 O2→CaSO4.2 H2O + SO2FGD Gypsum FormationLimestone Forced Oxidation Flue-Gas Desulfurization Process• SO2+ CaCO3→CaSO3· 1⁄2H2O + CO2+ O2→CaSO4· 2H2O (gypsum)Formation of FGD GypsumOn formation of a precipitate from solution,dissolved constituents must partitionbetween the solid and liquid phases.There is evidence that mercury tends topartition to the solid phase.There is also evidence that other heavymetals tend to partition to the liquid phase.Natural Gypsum at 100xNatural gypsum crystalsshown here are smallfractions of larger flatand blocky crystals.Si and Ti are present inthe X-ray spectrum.FGD Gypsum at 100xFGD Gypsum has smallercrystals. Which tend to beelongated. Few werelarger than 200μm.The X-ray spectrumshows only Ca, S, and O.Major & Minor Composition of FGD &Natural Gypsum (all values wt. %)ConstituentFGD Gypsum(Multiple source)Mined Gypsum(Multiple sources)FGD Gypsum(Berland et al,2003)Aluminum NR NR 0.05 – 2.70Boron <0.0003 – 0.0042 0.0009 0.0005 – 0.0029Calcium 18.5 – 24.3 18.1 – 21.3 19.3 – 22.9Iron 0.026 – 0.3 0.038 – 0.74 0.07 – 4.0Magnesium 0.010 – 0.74 0.1 – 3.14 0.60 – 2.95Phosphorus 0.0005 – 0.05 0.35 – 0.046 NRPotassium <0.004 – 0.09 0.0076 – 0.44 0.0 – 0.25Silicon 0.022 – 69.2 NA 0.05 – 2.95Sodium 0.023 – 5.8 0.076 – 0.35 0.0 – 0.22Sulfur 14.9 – 20.9 12.3 – 17 17.6 – 18.4NR = Not Reported; NA = Not AvailablePure gypsum should be 23.3% Ca & 18.6% S.Trace Element Concentrations in FGD &Natural Gypsum (all values ppm)TraceConstituentFGD Gypsum(Multiplesources)Mined Gypsum(Multiplesources)FGD Gypsum(Berland et al,2003)Natural Gypsum(Berland et al,2003)Antimony 0.1 – 9.1 0.02 – 0.28 NR NRArsenic 0.6 – 4.0 0.19 – 3.0 <1 – 2 <1Barium 33.8 – 77.2 NA 1 – 6400 4Cadmium 0.003 – 1.2 <2 – 0.5 <1 – 2 <1Chromium 0.56 – 42.0 8.7 – 30.5 3 – 24 1Copper 0.96 – 27.9 <4 – 29.8 <1 – 64 7Lead 0.8 – 12.0 <5 1 – 47 <1Lithium <9 and 48.8 <9 – 13.1 NR NRManganese 1.37 – 52.1 NA 3 – 94 4Mercury 0.004 – 1.4 0.00044 – 0.025 NR NRMolybdenum 0.5 – 12.0 <3 <1 – 3 <1Nickel 0.73 – 20.1 <4 – 11.9 <3 – 19 2Selenium 2.0 – 30.0 11.3 – 21.1 <1 – 15 <1Thallium 0.6 – 2.0 <15 NR NRVanadium <1 – 73.2 <2 – 12.7 8 – 21 27Zinc 3.4 – 102 13.1 – 27.5 <1 – 20 5NR = Not Reported; NA = Not AvailableNatural Gypsum vs. FGD Gypsum• Major chemical composition is similar• Minor & trace constituents are similar• Crystalline form is the sameContact InformationEnergy & Environmental Research CenterUniversity of North Dakota15 North 23rd StreetStop 9018Grand Forks, North Dakota 58202-9018www.undeerc.org/carrcTelephone: (701) 777-5000Fax: (701) 777-5181Debra F. Pflughoeft-HassettSenior Research Advisor(701) 777-5296dphassett@undeerc.org