Dorstfontein geology.
Coal measures of the Highveld and Witbank coalfields are hosted in the Ecca Group, which includes the Vryheid and Volksrust coal-bearing formations. All coal seams in the ECC area are hosted in the Vryheid formation which ranges in thickness from 80m to 300m. There are five major coal seams present in the area, named from the base upwards as S1, S2, S3, S4 and S5.

A granitic basement high, which forms part of the Smithfield Ridge and is referred to below as the Central plateau, divides the DCM complex into two separate geological domains and the western and eastern limbs can be found on either side of this plateau. S1 is only developed in the central portions of the paleo-valleys, ranging in thickness from 0.1m to 2.5m. Underground extraction of this seam is considered uneconomic due to its highly variable thickness, inferior quality, isolated occurrence and coal devolatilisation. In the DCM complex, seam splitting is generally a provenance of detrital material resulting largely from the proximity of the seam to the Smithfield Ridge and, as a result, the S2 and S4 are further subdivided into S2L, S2U and S4L and S4U, respectively.

S2L is thinnest over palaeo-highs and thickest over troughs of palaeo-valleys. Two S2L thickness domains exist in the complex. In the west (DCMW), seam thickness ranges from 0.1m to 5m and, in the east (DCME) from 0.2m to 6.0m. In the north of DCMW, the S2L exists as a single coal horizon, generally devoid of stone partings. To the south of this block, an arenaceous parting splits the seam into an upper and lower subseam. The parting separating S2U and S2L in both the west and east attains a maximum thickness of 4.5m and, in the northern portion, a minimum of 0.2m with an average of 1.5m. S2U is considered uneconomic to mine separately by underground methods but is included for the opencastable blocks. Generally, S2L is the thicker of the two subseams and has better quality coal. As such, S2L is theoretically the mining target. However, practical mining of S2L is often problematic due to parting between two subseams S2L and S2U.

On the basis of seam thickness and coal quality, S4L is the main underground exploitation target in S4, which comprises coal and minor in-seam partings, and is characteristically banded with alternating dull and bright coal. The seam contains one in-seam parting of significant thickness and lateral extent with a thickness average of 0.2m but can reach 0.5m. Generally, the average thickness of S4L is 3.0m. S4U has an average thickness of 1.7m in the west and, in the east, the thickness is generally more than 1.0m, reaching 3.0m in the centre of the palaeo-valley. Seam S4U has a higher ash content and thus lower CV compared to S4L. Thickness of the S5 is generally over 1.5m. Due to a significant parting, the seam is considered uneconomic for underground extraction but can be considered for selective extraction in opencast.

Numerous Jurassic dolerites (dykes and sills) intrude the Vryheid formation at various stratigraphic levels in the area. These intrusions negatively influence the stratigraphy and coal qualities in places. The distribution of the lower coal seams are strongly influenced by basement topography while distribution of the upper seams is controlled by present-day topography. Most affected by basement topography are S1 and S2. Seams are often thin and sometimes pinch out over and against palaeo-highs. Strata (including coal) are often faulted, although displacements are rarely more than 1m. Structural displacements, resulting from intrusions of dolerite sills through seams, often complicate mining seams.

The DCM complex geology and grade continuity are largely influenced by palaeo-topography, present-day topography, surface weathering, seam thickness variation (mainly between DCMW and DCME), in-seam parting, in-seam washouts, dolerite intrusions in the form of sills and dykes, resulting in minimal to extensive zones of devolatilisation. These geological risks are well managed through extensive drilling in areas of concern, using downhole wireline logging for better definition, underground channel sampling incorporated as points of observation in the short-term model, underground face mapping generally used to track both vertical and lateral variations in the lithology to assist with reconciliations as well as structural interpretation and surface mapping, particularly of basement outcrops. The information collated from the various approaches is incorporated in the structural model, together with additional potential risks from other disciplines and represented in a GIS-based risk and opportunity domain analysis (RODA), allowing for a more integrated approach to risk management.

The complex covers a total rights area of 7 892ha comprising the underground Dorstfontein West mine and opencast and newly developed underground mine at Dorstfontein East mine. Seams being exploited are mainly S4 divided into S4 upper (U) and S4 lower (L) and S2 divided into S2U and S2L. The Dorstfontein West operation is exploiting the S4L through an incline development from the historical S2 workings with three production sections. A new development at Dorstfontein East is the implementation of an underground mine exploiting S2 through two adits accessed from one of the open pits. Dorstfontein East is the only opencast mine in the ECC stable targeting primarily S2 and S4. However, where thicker than 1.0m S5, S3 and S1 are present, they are also extracted. Mining activities use a truck-and-shovel method to expose and extract coal.

Front-End Material Handling.
Raw coal with a top size of – 50 mm discharges to a ~7400 tone surge stockpile. Material is withdrawn from the stockpile via three feeders (two fixed and one variable speed) and is conveyed to a plant feed bin. The plant feed bin distributes the coal vial two independent vibrating feeders and conveyors to the two modules. A weightometer and variable speed feeders maintain a constant feed rate for each plant feed (Exxaro Coal Central (Pty) Ltd B, 2017).

Dense Medium Pant.
A conventional dense medium cyclone plant is used for single stage washing. As the two modules are identical, the process explained below is for one module. The raw coal is deslimed at 1 mm prior to being introduced into the plant, with the 50 x 1 mm fraction reporting to the dense medium plant and the 1 mm to a spiral plant.

The desliming screen discharge combines with circulating medium from the head box and is pumped to the DM cyclone for separation. The c ........