The mineral and chemical form of uranium in coal clearly influences the behavior of uranium during coal combustion. Previous evaluations of uranium residence in coal suggest that uranium has an affinity for both mineral and organic fractions. In the mineral fraction uranium resides in discrete grains of relatively refractory minerals such as zircon or rare-earth phosphates (100-1000 ppm U), or in quartz and aluminosilicates (0.1-10 ppm U). In contrast, the organic fraction contains extremely finely dispersed uranium, probably in the form of uranium oxides or more volatile uranium-organic complexes. A bimodal residence for uranium in coal that leads to mixed volatile and nonvolatile species is also suggested by the moderate degree of small-particle enrichment of uranium in fly ash, and by the distribution of uranium between refractory minerals and glass in fly ash.
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