Mode of occurrence of chromium in four US coals

The mode of occurrence of chromium in three US bituminous coals and one US subbituminous coal has been examined using both X-ray absorption fine struc ture (XAFS) spectroscopy and a selective leaching protocol supplemented by scanning electron microscopy (SEM) and electron microprobe measurements. A synthesis of results from both methods indicates that chromium occurs princ ipally in two forms in the bituminous coals: the major occurrence of chromi um is associated with the macerals and is not readily leached by any reagen t, whereas a second, lesser occurrence, which is leachable in hydrofluoric acid (HF), is associated with the clay mineral, illite, The former occurren ce is believed to be a small particle oxyhydroxide phase (CrO(OH)). One coa l also contained a small fraction (< 5%) of the chromium in the form of a c hromian magnetite, and the leaching protocol indicated the possibility of a similar small fraction of chromium in sulfide form in all three coals. The re was little agreement between the two techniques on the mode of occurrenc e of chromium in the subbituminous coal; however, only a limited number of subbituminous coals have been analyzed by either technique. The chromium in all four coals was trivalent as no evidence was found for the Cr6+ oxidati on state in any coal. (C) 2000 Elsevier Science B.V. All rights reserved.