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 57Fe Moessbauer 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 57Fe Moessbauer spectra of the resulting coal samples, collected at 297K, indicate that only FeS2 phases are present. The Moessbauer parameters for these samples (0.619 ≤EQ≤0.622 mm s-1; 0.306≤δ≤0.309 mm s-1) indicate the absence of appreciable quantities of marcasite in the coals. These Moessbauer 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 EQ values suggest the presence of As substitution in the pyrite phases.57Fe Moessbauer 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 definite in the XRD data