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Mineralogy of the Residues from an Underground Coal Gasification Test

Published online by Cambridge University Press:  21 February 2011

Gregory J. McCarthy
Affiliation:
Departments of Chemistry and Geology, North Dakota State University, Fargo ND 58105
Robert J. Stevenson
Affiliation:
Energy and Mineral Research Center, University of North Dakota, Grand Forks, ND 58202
Robert L. Oliver
Affiliation:
Western Research Institute, P.O. Box 3395, Laramie, Wyoming 82071
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Abstract

Detailed mineralogical characterization by X-ray diffraction, scanning electron microscopy and electron microprobe analysis has been performed on a suite of residues and on three samples of unaltered overburden excavated from a 1983 underground coal gasification (UCG) test conducted at a mine site near Centralia, Washington. The residues were classified into nine zones based on their appearance and location in the burn cavity. Because they were isochemical with the overburden, six of these zones could be identified as altered overburden and proved to be particularly useful for correlations of mineralogy with the temperature and redox atmosphere prevailing in these materials during the UCG process. A previously unrecognized, moderate temperature, reaction between reduced iron phases and clay minerals to form an Al-rich hercynite-spinel solid solution (Al-spinel), (Fe,Mg)Al2O4, was identified. This reaction is a precursor to the well-known formation of Fe-cordierite, (Mg,Fe)2 Al4Si5O18 from SiO2 and the Al-spinel at higher temperatures. The mineralogies of altered overburden were analogous to high-temperature, low pressure, metamorphic rocks of the sanidinite facies, and to thermally altered rocks from natural coal combustion. Modeling of the mineral phase assemblages was successfully performed using a phase diagram for the system (FeO+MgO)-Al2O3-SiO2 constructed from literature diagrams. Melting relations were also modeled with this diagram. Deviations from the assemblages predicted by this diagram were attributed to oxidizing conditions.

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Type
Research Article
Copyright
Copyright © Materials Research Society 1989

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