A procedure for the gravimetric quantification of total organic carbon and carbonates in natural rocks focused on mineral and coal processing

This paper presents a gravimetric method for the quantification of total organic carbon by ignition at 1100 °C and carbonates by acid dissolution in rock samples containing coal. Two minerals (CM-1 and CM-2) and a sub-bituminous coal (SBC) were used in the study. The mineralogical characterization of the samples was performed by X-ray diffraction and scanning electron microscopy. This revealed the presence of calcite in samples CM-1 (20.4%) and CM-2 (74.7%), dolomite in CM-2 (0.7%), and graphite in sample SBC (42.6%). To eliminate carbonates, the samples were treated with 4 mol/L HCl, which allowed quantification. Results of the carbonate gravimetric quantification were compared with those obtained with a calcimeter. A significant difference was observed for the sample CM-2 (70.3% versus 63.4% by calcimetry, p-value = 0.0042). For the sample without carbonates, no differences were observed for loss on ignition without and with the acid treatment, so this is not necessary for this type of sample. A morphological study showed no significant surface change after acid treatment, but fracture of iron oxide particles was observed in CM-1 and CM-2 after heat treatment. In this study it was demonstrated that rock mineralogy is essential to correctly analyze organic and inorganic carbon content.