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 Mössbauer 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 Mössbauer spectra of the resulting coal samples, collected at 297K, indicate that only FeS₂ phases are present. The Mössbauer parameters for these samples (0.619 ≤E_Q≤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 Mössbauer 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. ⁵⁷Fe Mössbauer 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.