MODES OF OCCURRENCE OF TRACE ELEMENTS IN COAL.

摘要

The chemical and physical environment (mode of occurrence) of the trace elements in coal can influence their behavior during the cleaning, conversion, or combustion of the coal, and during the weathering or leaching of the coal or its by-products. Information on the mode of occurrence of the trace elements is, therefore, essential for the efficient use of our coal resources.; Previous attempts to determine the mode of occurrence of the trace elements in coal have been largely indirect. Results of the most commonly used approach, sink-float separation, is often contradictory. Evidence obtained from this study indicate that results from sink-float separations are susceptible to gross misinterpretations.; In order to directly determine the mode of occurrence of the trace elements in coal, a technique was developed using the scanning electron microscope with an energy dispersive detector. This analytical system allows the detection and analysis of in-situ, micron-sized minerals in polished blocks of coal. In addition, mineralogical data were obtained from individual particles extracted from the low-temperature ash of the coal.; These techniques were applied in an in-depth study of the Waynesburg and Upper Freeport coals, both bituminous coals from the Appalachian Basin. In addition, brief studies were conducted on about 80 coals representing every rank and type, and every major coal basin in the U. S. plus about 20 coals from worldwide locations.; The results indicate that many trace elements in coal can occur quantitatively in micron-sized accessory mineral grains scattered throughout the organic matrix (macerals). For example, Zn and Cd occur predominantly in the mineral sphalerite; Cu in chalcopyrite; Zr and Hf in zircons; the REE, Y, and Th in monazite and xenotime. In Appalachian Basin coals, lead selenides are dominant over lead sulfides, whereas outside the Basin, lead sulfides are far in excess over the selenides. The majority of the lead in coal, however, may be substituting in barium-bearing minerals. Some elements, such as As and Hg, occur in solid solution with pyrite. The difference in the mode of occurrence between As and Hg and the other chalcophile elements is reflected in their behavior during sink-float separation of the coal: those elements forming micron-size minerals within the macerals are concentrated in the lighter Sp. G. fractions; whereas, those elements associated with pyrite are concentrated in the heavier Sp. G. fractions.; Organic associations constitute a major mode of occurrence for several trace elements. Although Ti-bearing minerals are common in many coals, no more than about 50 weight percent of the Ti can be accounted for in this manner. The remainder is probably bound to the organics. Similarly, significant amounts of Se and Br appear to be organically bound. The mode of occurrence of uranium is quite complex; in one sample, virtually all the U occurred as micron-sized grains of uraninite; in another a significant amount of the U was associated with detrital accessory minerals such as zircon; in other coals, the bulk of the U appeared to be organically bound.; This study demonstrated that the SEM-EDX system can be used to rapidly characterize the accessory minerals in most coal samples. The information so generated provides direct knowledge of the modes of occurrence of many trace elements in coal. Some of these trace elements appear to have been quite mobile at some time during the coalification process. This study also demonstrated that, although some elements are inorganically bound in coal, during fractionation, they behave as if they are organically bound.; It is evident that this type of detailed analytical approach is essential if we are to anticipate accurately the effects of the trace elements on the various technological processes that coals will undergo.