SEM Study of Chemical Variations in Western U.S. Fly Ash
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SEM STUDY OF CHEMICAL VARIATIONS IN WESTERN U.S. FLY ASH ROBERT J. STEVENSON and TIMOTHY P. HUBER North Dakota Mining and Mineral Resources Research Institute, University of North Dakota, Grand Forks, ND 58202. Received 4 November,
1986; refereed
ABSTRACT Scanning electron microscope/electron microprobe chemical analyses of individual grains of several western fly ashes have shown an inter-grain variation in composition for ashes derived from both lignite and subbituminous western coals. SRM 2689 (a Class F fly ash from a bituminous coal) and SRM 2691 (a Class C fly ash from a subbituminous coal) have been included in the study and also show inter-grain chemical variation. A classification scheme of fly ash grain compositions has been developed and has proven to be useful in illustrating the differences and similarities of the ashes. There is a positive correlation, for example, between grains rich in Si0 2 and A120 3 and the Na2 0 content of the grains in lignite and subbituminous coal derived fly ashes. There is also a positive correlation between grains rich in CaO and the MgO content of the grains in both classes of fly ashes. INTRODUCTION The first scanning electron microscope/electron microprobe (SEM/EMPA) grain analyses for this study were done in 1983 as part of research performed for the Gas Research Institute (GRI). That project included characterization of fixed-bed gasification ash as well as fly ash produced from the combustion of lignite. Results from the gasification ash study have been reported previously [1-3]. Chemical characterization of the grains of the three fly ashes sampled in that project (codes 83-319A,
83-342,
and 83-523C) form the basis of
the classification scheme presented and used in this work. The number of fly ash samples has been expanded in the subsequent years to include two of the three fly ash standard reference materials recently issued by the National Bureau of Standards (SRM 2689 and SRM 2691) [4]. Other sources of fly ash for this work include a follow-up project through GRI and the Western Fly Ash Research, Development and Data Center [5]. In order to evaluate the relatively large number of SEM chemical analyses, a scheme for the display and classification of the analyses was developed. This scheme is based on the major chemical components of ASTM C-618 Class C fly ash (CaO, Si0 2 , A12 0 3 , and Fe-oxide) and can be illustrated using an oxide weight percent (wt.%) CaO-SiO2 -AI 20 3 ternary diagram (Fig. 1). In these plots other oxides are ignored. The analyses represent the bulk composition of the exposed surface of each ash grain, including crystalline inclusions and the continuous glass phase. EXPERIMENTAL The ash samples were embedded in low viscosity epoxy in shallow molds. The resulting boats were then embedded in epoxy plugs at right angles to the bottom of the boats and subsequently cut and polished. This mounting method insured that the analyses of grains were unbiased as to density and size. Secondary electron image photographs of areas on the plugs at low (
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