Comparison of the Behavior of Trace Elements During Acid Leaching of Ashes from Several Coals
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COMPARISON OF THE BEHAVIOR OF TRACE ELEMENTS DURING ACID LEACHING OF ASHES FROM SEVERAL COALS J. S. WATSON Chemical Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6224 Received 30 October, 1986; refereed ABSTRACT The leaching of fly ash from eastern U.S. coals with strong mineral acid is the initial step in a series of potential processes for producing useful and marketable materials from the ash. This initial leaching step removes most, or all, of the more soluble (generally amorphous) materials from the ash and leaves an inert residue, believed to be mostly mullite and silica. Chemical analyses of the leachate and the residual inert materials indicate the Significant original distribution of the trace elements in the phases. differences in crystallography, composition and leaching behavior have been noted in ash samples from coals from various regions, of various types, and sometimes even in ash samples from the same or similar coals. Trace element analyses of fly ash leachates provide a useful means of studying coal characteristics and of determining how the trace elements are incorporated in various types of fly ashes. INTRODUCTION Fly ash from coal combustion power plants is a complex material consisting of several phases containing numerous elements. Although the major phases of fly ash consist of only a few principal elements, they also contain many trace elements. Determining the distribution of these trace elements in the leachable and nonleachable phases of fly ashes is important in many potential processes for using and in disposal of the ash. To determine the distribution of both bulk and trace elements among the ash phases, one would prefer to separate each of the phases from several ashes and determine quantitatively the composition of each phase. However, quantitative separation of all of the phases in fly ash is not possible; this brief paper discusses a less ambitious, but simpler, approach in which we determine only the distribution of the elements between those phases which are easily dissolved in concentrated mineral acids and those which remain insoluble in acids. The results of this approach are inherently limited, of course, and there may also be cases where results are affected by some soluble materials being surrounded by insoluble phases and thus protected from acid dissolution. Despite these limitations, this approach provides useful information that directly addresses the behavior of ashes in dissolution processes. It also provides information relevant to the leachability of ashes under environmental disposal conditions, although the aggressive leaching of ash by mineral acids will certainly dissolve far more material and dissolve it more rapidly than environmental leaching. These steady acid leaches may even dissolve entire phases that would hardly be affected under normal environmental disposal conditions. Nevertheless, these results provide information on the distribution of trace elements in fly ash which should be helpful in considering environmental issues.
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