Abstract
Certain sections of the B Seam (SB and SBT horizons) at Mavungwani Colliery, situated within the Ermelo Coalfield, in the Mpumalanga Province in South Africa, are classified as torbanite (by mining personnel). The material is incorporated into the float fraction during beneficiation using a float medium of 1.6 g/cm3 (relative density). Petrologically, torbanite refers to a specific type of sapropelic coal, rich in the maceral alginite (at least 20%) and is of specific economic importance because its composition allows for direct liquefaction, coal-to-liquids. This coal-type has previously been found and mined in the Ermelo Coalfield, occurring in isolated pockets as lenticular and irregular bodies.
Preliminary work by Mahooana (2019) using grab samples confirmed that the ‘torbanite’ material has high ash content, causing a reduction in the quality of the final product. The current research project aims to determine whether the coal is torbanite, and to ultimately determine differences in depositional environments between the ‘torbanite’ horizons and the humic coal horizons occurring above and below. Borehole core was assessed using organic petrography (macerals, microlithotypes and rank) and coal quality analyses. To determine changes in depositional environments where the “torbanite” horizon appears, petrographic indices were used. Mineralogical data (XRF and XRD) was used to supplement the organic petrological and coal quality data in deducing changes in depositional environments.
In order to assess changes in palaeomire conditions during peat formation and accumulation, changes in lithotypes formed the basis for sampling the borehole core. Each lithotype was collected as an individual sample, where the thickness permitted (~15cm and above). The A Seam (Upper and Lower) and the B Seam (Upper and Lower) were intersected by the borehole. In total, 17 samples were collected as follows: three samples from the Upper A Seam (depths of between 38.21 m and 39.01 m), one sample from the Lower A Seam (42.05 m – 42.31 m), seven samples from the Upper B Seam (70.61 m – 71.88 m) and six samples from the Lower B Seam (72.11 m – 73.41 m). Within the Upper B Seam, an alginite-rich horizon was present (depths of between 70.96 m and 71.13 m). The alginite in the coal samples was associated with inertinite macerals, specifically inertodetrinite. The inorganic component of the alginite-rich sample mainly consisted of clay (kaolinite) and quartz, intimately associated with the inertodetrinite and possibly of a detrital origin. The occurrence of the inertinite macerals with alginite (derived from algae) suggests two
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depositional episodes. The first episode is related to the formation of the primary inertinites through oxidation and/or charring of plant matter, followed by reworking and redeposition of the primary inertinite(s) as inertodetrinite. The second depositional episode was quieter, occurring in deeper and standing waters, conducive for the growth of algae. Two different alginite morphologies (massive spherical and elongated flattened) with two different host matrices within one sample suggest two cycles of the two depositional episodes. The coal seams and clastic partings indicate that the evolution of the Ermelo Coalfield succession was complex as the environments were continuously changing.
An understanding of the microlithotype composition, specifically the relationship between inertodetrinite, liptinite and included mineral matter, provides a possible explanation as to why the coal material floats using a float medium of 1.6 g/cm3 (relative density) despite its high ash content. It was postulated that the density of the organic material was altered by the presence of the microlithotype durite and detrital minerals.