Investigation of virgin coals and coals subjected to a mild acid treatment

A quantitative determination of the relative marcasite/pyrite contents in virgin coals is possible by means of Fe-57 Mossbauer spectroscopy. Complications arise, however, when iron-containing silicates, carbonates, or other salts are present. The application of a mild chemical treatment involving hydrofluoric acid has been employed to remove these Fe-containing phases while leaving the iron-disulfide phases unaffected. Several South African coal samples with non-iron disulfide, Fe-containing phases ranging from 18 to 30 weight percent have been subjected to a hydrofluoric acid leaching at room temperature. The loss of mineral matter with HF leaching correlates well with the mineral matter residue following low temperature ashing. The Fe-57 Mossbauer spectra of the resulting coal samples, collected at 297K, indicate that only FeS2 phases are present. The Mossbauer parameters for these samples (0.619 less than or equal to E-Q less than or equal to 0.622 mm s(-1); 0.306 less than or equal to delta less than or equal to 0.309 mm s(-1)) indicate the absence of appreciable quantities of marcasite in the coals; These Mossbauer parameters differ slightly, but systematically, from those of pyrite for which a quadrupole splitting of 0.6110 +/- 0.0020 mm s(-1) has been established. On the basis of previous studies, these increased E-Q values suggest the presence of As substitution in the pyrite phases. Fe-57 Mossbauer spectra of virgin coals exhibit phase assemblages comparable to those observed by x-ray diffraction (XRD), e.g. pyrite and (Ca,Fe)CO3, even though the presence of pyrite is less definitive in the XRD data.