In-situ analyses of carbonaceous matter in manganiferous black shales: Analytical proxies and implication for ore processing

Carbonaceous matter is generally known to be problematic for metal recovery during metallurgical processing of black shales. In particularly, metal upgrading during beneficiation prior to (bio-) hydrometallurgical and/or pyrometallurgical processing is hindered by the presence of abundant carbonaceous matter (CM). This study presents the characterization of CM and mineral bound carbonaceous matter (CMP) in three manganese carbonate-rich shales hosting 6-8 wt% total organic carbon. Non-destructive methods, such as incident light microscopy, scanning electron (SEM) and focused-ion-beam-transmission electron microscopy (FIB-TEM), QEMSCAN and electron microprobe, were used to show that free CM is adsorbed onto illite-smectite surfaces. This form of CM ranges in grain size from sub-micrometer up to ≈100 μm. The most efficient method to show the illite-smectite association is SEM and for quantification of this association, QEMSCAN should be used. Mineral-bound carbonaceous matter may be relicts of extrapolymers (pyrite and/or carbonate) and needs characterization using FIB-TEM. The quantity of CM can then be estimated by a rough calculation as the difference between the total organic carbon (Rock Eval) and the free carbon calculated from normative mineral compositions based on X-ray Fluorescence (XRF) and X-ray-Diffraction (XRD) analyses. The mineral bound CM could not be detected by QEMSCAN under conventional analytical conditions. We estimated that about 85% of the CM in the test samples was adsorbed on mineral surfaces and about 15% was bound to minerals. The physical protection of the CM by clays, and the morphological and density differences between pyrite, carbonates and biochemically-bound CM in pyrite/carbonates needs to be taken into consideration in the process design.