Identification and Quantitation of Carbonate Compounds in Coal Fly Ash Christopher F. Bauert and David F. S. Natusch' Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523
rn Alkali, alkaline earth, and ferrous carbonates have been identified and quantitatively determined in several coal fly ashes a t total carbonate concentrations of 0.058-0.86%. Thermal evolved gas analysis for Con, a selective acid-leaching procedure for carbonates, and metal analyses of the leachates provide data for a mass balance between metals and carbonate. In some cases the leachable metal comes predominantly from a carbonate species. High relative humidity promoted COz absorption by fly ash regardless of the COz partial pressure. Although this study used ash removed by electrostatic precipitators, flue-gas characteristics are likely to promote carbonate formation on the ash in the plume. The projected continued use of coal for power generation reaffirms the need to study particulate emissions from coal combustion, to assess and, if possible, to minimize adverse environmental impacts. Most studies ,have focused on the behavior of potentially toxic metals (1-41, organic compounds ( 5 , 6 ) and , sulfur gases (7) with little information being generated on the inorganic compounds present. Techniques giving compound-specific information have been hampered by inadequate detection limits. Only major species have been unequivocally identified, primarily by X-ray powder diffraction (8-10): quartz, mullite (3A1~03.2Si02),hematite, magnetite, h e , and anhydrite (CaS0.d. X-ray photoelectron spectrometry (XPS) has shown that binding energies for elements in fly ash are consistent with oxides for Al, Si, and Fe and with Cas04 for Ca and S, but other compounds may have the same binding energies (11,12). Evidence for sulfates of Al, Ca, and Fe has been provided by Fourier transform infrared spectroscopy (13). Differential thermal analysis has been applied widely to the minerals in coal (14) and to fly ash (9), but to our knowledge in only one instance were mixed carbonates of Ca, Mg, and Fe found (15). Knowledge of the chemical forms of metals in fly ash is highly desirable in order to predict the mobility of the metal. Mobility has implications for inhalation toxicology and fly-ash disposal because both require information on solubility. In addition, utilization of ash as a raw material may benefit from more complete information on ash chemistry. The present study demonstrates that, in fact, carbonate compounds occur in fly ashes, that the concentrations of individual compounds are trace level, and that the metals bound as carbonates can be a significant fraction of the total mobile metal. t Present address: Water Chemistry Laboratory, University of Wisconsin, Madison, WI 53706.
Experimental Section Thermal Evolved Gas Analysis. Because carbonate minerals decompose with the release of C02 at characteristic temperatures (16), evolved gas analysis (EGA) with carbonspecific detection was applied. The instrument is illustrated in Figure 1.The furnace (Leco induction furnace, Model 521) was temperature-programmed from room temperature to 1200 OC a t -190 OC/min by means of a motor-driven variable transformer (I 7). Calibration of the temperature scale was performed by using the melting points of several pure solids a t the stated programming rate. Ca. 100 mg of fly ash was spread in a thin layer (
