Petrology, Mineralogy, and Chemistry of Size-Fractioned Fly Ash from the Jungar Power Plant, Inner Mongolia, China, with Emphasis on the Distribution of Rare Earth Elements

Abstract

Previous studies proved the existence of the coal-hosted Al (Ga and rare earth elements) ore deposit in the Jungar Coalfield, Inner Mongolia, northern China, and the fly ash derived from these metallic coals is the source for Al and Ga extraction. Six sized fractions from plus-120 mesh to minus-500 mesh (125 to 25 μm) coal combustion fly ash from the 200-MW Jungar power plant, Inner Mongolia, China, were studied for their mineralogical, chemical, and petrographic properties. The parent sample of the fly ash represents one time and temperature (not specifically known, but certainly in excess of 200 °C) in the ash-collection system. X-ray diffraction (XRD) data showed that the glass content of the fly ash decreased from the coarse to fine fractions. However, quantitative data under an optical microscope showed that glass content of the fly ash increased from the coarse to fine fractions; portions of what appeared to be glass under microscope actually contained mullite and corundum. Rock fragments, indicative of incomplete melting, comprise significant portions of the plus-300 mesh (about 50 μm) fractions. Compared to the upper continental crust, the size-fractioned fly ashes show enrichment in the light rare earth + Y (REY) and have negative Eu, Ce, and Y anomalies. Relative to raw fly ash (unsized), Eu and Ce in the size-fractioned ashes respectively exhibit positive and negative anomalies, and the finer fly ashes (minus-300 mesh) are enriched in light REY. In contrast to bottom ash, all the size-fractioned fly ashes show distinctly negative Ce anomalies. Lanthanum, Ce, Pr, and Nd were detected in minerals within the glassy phases in the fly ash. Mercury was detected in higher concentrations in the relatively carbon-rich coarse fraction than in the carbon-depleted fine fractions. Fluorine, V, Zn, and Pb have relatively high concentrations in the high-surface-area fine fly ash fractions.