Author: Shifeng Dai, James C. Hower, Robert B. Finkelman, Ian T. Graham, David French, Colin R. Ward, Greta Eskenazy, Qiang Wei, Lei Zhao
Abstract
Coal,one of the most complex geological materials, consists of organic andmineral matter, the latter including crystalline minerals,non-crystalline mineraloids, and elements with non-mineralassociations. Overall, the modes of occurrence of elements in coalare classified into organic, mineral, and intimate organicassociations, the latter including those adsorbed on to the surfaceof organics, dissolved in pore waters, and hosted in very fine-grainedminerals (sub-micro- or nano-minerals) encased in or shielded by theorganic matter of coal. Mineral associations, defined as elementsassociated with minerals are straightforward; however, confusionabout organic and intimate organic associations of elements usuallyarise in the literature. Understanding organic, mineral, and intimateorganic associations of elements is important not only becausenon-mineral elements and, to a lesser extent, elements associatedwith fine-grained minerals, play a significant role in affectingthe utilization of coal, but also such modes of occurrence ofelements provide useful geochemical information on coal formation andcoal-bearing basin evolution. With a few exceptions (such as Cd, Nb,Ta, Zr, and Hf), most elements determined in coal, particularly inlow-rank coal, have varying-degrees of organic association. In thispaper, we review the definition of associations of non-mineralelements in coal, as well as their methods of determination, and thenreview the associations of selected elements includingenvironmentally-sensitive (e.g., S, As, U, and Hg) and criticalelements, the latter of which drive some of the significantadvancements in technology and energy efficiency in the world today(e.g., rare earth elements and Y, Ge, and U), and some major elements(e.g., Ca, Mg, Fe, Al, and Ti) that largely occur in non-mineralforms in low-rank coals.
Keywords: Coal; Organic association; Non-mineral elements; Modes of occurrence