Laboratory Modeling and XRD Characterization of the Hydration Reactions of Lignite Gasification and Combustion Ash Codis
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LABORATORY MODELING AND XRD CHARACTERIZATION OF THE HYDRATION REACTIONS OF LIGNITE GASIFICATION AND COMBUSTION ASH CODISPOSAL WASTE FORMS P. KUMARATHASAN and GREGORY J. McCARTHY Department of Chemistry, North Dakota State University, Fargo, ND 58105 Received 31 October,
1986; refereed
ABSTRACT Cementitious reactions in gasification and combustion ash derived from North Dakota lignite permit the fabrication of monolithic wastes forms from nonhazardous ash by-products and hazardous liquid wastes from a coal gasification plant. To better understand such cementitious reactions, x-ray diffraction has been used to characterize the hydration reaction products of crystalline phases in the gasification and combustion ashes from the Beulah, ND,- complex and the crystalline reaction products formed. A cementitious lignite fly ash was also studied. pH of solutions at a liquid to solid ratio of 2.4 and times up to 14 days was measured, and compared to pH calculated from leachate chemical analyses. Reactions were monitored for up to one year. Among the principal crystalline phases in gasification ash, carnegieite (Nal. 5 All. 5 Sio.504) was the most reactive. Solution pH's for two samples of gasification ash were in excess of 13; carnegieite reaction may be responsible in part for these very high pH's. Hydration products included gaylussite, (Na 2 Ca(CO3 ) 2 -5H 2 0), a carbonate-sulfate ettringite structure phase, calcite laumontite, faujasite and/or NaX). and three or more zeolites (NaA, Ettringite and calcite were the principal hydration reaction products of scrubber ash, fly ash and the composite codisposal waste form. The formation of ettringite may be one of the principal reactions responsible for consolidation of the waste forms (along with noncrystalline calcium silicate Ettringite was hydrate formation which could not be observed by XRD). observed to decrease in abundance at long reaction times. Calcium aluminate monosulfate hydrate formed in
the later stages of the fly ash reaction.
INTRODUCTION The Great Plains coal gasification plant at Beulah, North Dakota, presently owned by the US Department of Energy and operated by ANG Coal Gasification Company, has a number of solid and liquid waste streams. Gasification ash is by far the most abundant of the solid wastes. After extensive evaluation, including tests specified in the Resource Conservation and Recovery Act (RCRA), it has been determined that gasification ash is not a hazardous waste [1-3]. Two liquid waste streams from this plant, liquid waste incinerator blow-down (LWIBD) and multi-effect evaporator concentrate (MEEC), are hazardous wastes because of their content of several toxic trace elements [3]. The feasibility of codisposal of these toxic wastes combined with gasification ash and two solid wastes from the adjacent Antelope Valley electrical generating station (AVS), owned by the Basin Electric Power Cooperative, is being explored in a program supported by the Gas Research Institute (GRI). Both plants currently dispose of ash by burial in separate, lice
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