Correlations of Chemistry and Mineralogy of Western U.S. Fly Ash
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CORRELATIONS OF CHEMISTRY AND MINERALOGY OF WESTERN U.S. FLY ASH G.J. McCARTHY*, O.E. MANZ**, D.M. JOHANSEN*, S.J. STEINWAND* and R.J. STEVENSON** *Department of Chemistry, North Dakota State University, Fargo, ND 58105 **Mining and Mineral Resources Research Institute, University of North Dakota, Grand Forks, ND 58202 Received 2 December, 1986; Communicated by D.M. Roy SUMMARY Fly ashes derived from low-rank coals mined principally in Montana, Wyoming and North Dakota are being studied by the Western Fly Ash Research, Development and Data Center [1]. Previous studies of the mineralogy of western U.S. fly ash by McCarthy et al. [1-3] using x-ray diffraction (XRD) form the framework of the present study. A database of chemical, mineralogical and physical properties, along with precursor coal characteristics, is being assembled. Based on studies to date of several hundred fly ash samples derived from lignite and subbituminous coals, as well as from several bituminous ashes, correlations of chemistry and mineralogy have been hypothesized and are being tested. These correlations are discussed below. Analytical Alumina. Lignite ashes generally have A12 03 below 16 wt.%; subbituminous ashes have between 18 and 24%. Bituminous coal fly ashes have A12 03 contents greater than 23%. This trend is a result of the quantity and type of clay minerals, the principal source of A12 03 in fly ash, in the original coal. The most important clay minerals in coal, their nominal chemical compositions (using Na-montmorillonite for a smectite) and the A1 2 0 3 /SiO2 ratios for these compositions are: Smectite Nao. 3 3 (All. 6 7 Mgo. 3 3 )Si 40Io(OH) 2 "nH2 0 0.35 Illite K0 . 5 A12 (Al 0 . 5 Si 3 . 5 O1 0 )(OH) 2 0.61 Kaolinite A14 Si 4 Olo(OH) 8 0.85. Among the three major ranks of coal, lignite contains the greatest proportion of smectite, the clay mineral with the lowest ratio of A12 03 to Si0 2 , and bituminous coal with no smectite contains kaolinite and illite, minerals with higher ratios of A12 03 to Si02 . These ratios, combined with the dilution of both A12 0 3 and Si0 2 by the CaO in high-calcium fly ashes, result in the observed lower A12 03 contents of the low-rank coals. Analytical CaO. In high-calcium ashes (those with CaO >17%), most subbituminous ashes have somewhat higher CaO content than lignite ashes. The sources of the Ca are Ca in the organic portion of the coal along with calcite and gypsum. Analytical MgO. Lignite ashes have MgO greater than 4% while most subbituminous ashes have less than this level of MgO. The principal source of Mg is in the organic portion of the coal, along with montmorillonite and detrital ferromagnesian minerals and, rarely, dolomite. Analytical alkali. Na2 0 > K2 0 in both lignite and subbituminous ashes, in contrast to bituminous ashes where K2 0 > Na2 0. In the low-rank coals, Na is present in smectites and on organic ion exchange sites. It combines with sulfate from gypsum and SOx from oxidation of pyrite and organic sulfur. Periclase. Crystalline MgO is more abundant in lignite ashes than
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