Matla coal mine
Matla is in the Kriel district of Mpumalanga, some 20km west of Kriel, 50km south-west of eMalahleni and 30km south of Ogies. Matla is an underground operation with three mines – mine 1, mine 2 and mine 3 – all supplying coal directly to Eskom’s Matla power station by conveyor belt. At the current extraction rate, Matla is projected to be in production until at least 2037, although the mining right is due to lapse in 2025. Exxaro reasonably expects renewal of the mining right to be approved.
Mining at mine 1 was stopped in 2015 due to pillar instability. The impact of this is the continued drop in production and overall qualities being slightly lower than customer specifications. The mitigating strategy is to increase production from the remaining two shafts to ensure the quality specification required.
The construction of Matla Coal began in 1976 and the mine came into full production in 1983. Matla supplies all its bulk production to the nearby Eskom-owned Matla power station. Matla currently produces about 8,5Mt per annum, although it has on occasions produced up to 12,5Mt per annum when mine 1 was still in production. Matla currently exploits both the S4L and S2. In the past, it also exploited S5. Matla operates two short-walls at mine 2 and 3. It also has an additional nine CM sections.
Matla is in the Highveld coalfield, immediately south of the Witbank coalfield. The coalfield is host to up to five coal seams contained in the middle Ecca group sediments of the Karoo supergroup. The stratigraphic sequence in the mine area includes five coal seams that can be correlated with seams found in the Witbank coalfield. The principal economic seams currently exploited are S2 and S4L, with mining of S5 terminated in 1998 due to high levels of contamination and subsequent increase in abrasivity. The S2 at Matla is well developed in the NW part of the lease area, near mines 2 and 3. It thins out to the south, where thickness averages 1,0 to 2,5m. Qualities are also generally poor in this area, thus S2 is not mined in the southern portion of the lease area. S4 is generally well distributed throughout the lease area. The best qualities on S4 are on the southern part of the lease area. However, S4 is heavily intruded by dykes in this area, making mining a serious challenge.
Coal seams in the area are generally flat and continuous, with subsequent igneous activity resulting in displacements and devolatilisation of coal seams at places. Matla is divided into two main geological domains, marked by the sill intrusion that underlies S2 but transgresses through and overlies that seam while underlying S4 on the south-western part of the reserves. This sill has burned and decolourised S2 on the southern part of the mining area.
A meaningful amount of new borehole information resulted in updating the geological model as well as an optimised LoMP. A total of 2 223 available boreholes were exported from the Geobank database, of which 76 were drilled in 2017. The updates did not result in any significant resource or reserve changes.
A model was built on a grid size of 25 x 25, using a scan distance of 2 000m and a data boundary of 200m. Extrapolation was used for the seam model to honour the trends. Seam thicknesses were interpolated below the end of borehole depths but interpolated to zero thickness if occurring below the collar and above the end of the borehole. The structural model for Matla was updated in 2017 using all information available. The seam is continuous but intersected by intrusions in places especially in the mine 2 low seam area. Horizontal drilling and geophysical studies are under way to fully understand the effect of intrusions on the seam.
Criteria for MTIS for both inside and outside LoMP included a minimum 1,8m seam thickness cut-off, minimum dry ash-free volatiles content of 26%, 10% geological and geotechnical losses and a minimum air-dried CV content of 15MJ/kg, down from 18MJ/kg used in previous years due to current exploitation of resources at a minimum CV of 15MJ/kg. Minex distance grids were created for the select model and used as the basis for resource classification. The structural model was considered during the classification, but no areas were downgraded in 2017.
The change in resource estimation is due to:
- Depletion (-16Mt), mining actuals from January 2017 to December 2017
- New information (-7.8Mt), 76 new boreholes
- Model refinement (69Mt), due to a model update.
Reasonable prospects for eventual economic extraction
All criteria (table 24) have been considered.
The reserve model is built from the resource model. A select mining horizon is defined on each borehole and this data used to create the reserve model. Minex software is used to build the reserve select geological 3D model. This ‘select horizon’ is defined in conjunction with the rock engineering department. Rock engineering is also involved in the reserve estimation process, especially in reserves where new projects are established. Detailed rock-engineering studies, logging, sampling and testing of rock material is conducted continuously.
The three criteria used to determine the new practical safe mining height with the best-possible coal qualities are:
- Mineable height based on equipment maximum and minimum production height
- Mineable height selection to create a competent roof beam where applicable
- The best-possible coal qualities in the selected equipment height.
Environmental and hydrogeological conditions are also considered when reserves are defined. Areas underlying wetlands and other eco-sensitive areas are excluded from the reserves. A higher safety factor is used underneath rivers and surface structures. There is an existing hydrological model for Matla which is used to predict the impact on water resources due to mining. This model is continuously updated with monitoring data collected in the mine area.
The scheduling of the reserve is determined using the mining scheduling applications from XPAC and ProgCad. This is the same software used to develop the LoMP schedule. Reserves are identified by using an accepted signed-off list of standards and norms.
Reserve estimation process
The reserve area classification for proved (measured) as well as probable (indicated) is overlain on the mining panel’s layout and the tonnages and qualities identified using XPAC.
The proposed mining layout is placed in XPAC, taking geological structures into consideration.
Matla uses a bord-and-pillar and short-wall mining layout, accessed by separate shafts for each reserve area. The factors that affected the choice of mining method at Matla were:
- Height of the coal seam
- Number of geological intrusions into the coal seam
- Coal quality.
Table 14: Matla modifying factors
|> Geological losses||Already included in model, based on specific geological conditions and mining restrictions|
|> Mining losses||Already included in model, based on specific geological conditions and mining restrictions|
|> Depth to roof||40m unless rock allows less|
|> Safety factor||Tertiary panels 1,6
Main development 2,0
|> Bord width||7,2m|
|> Barrier pillar||At least equal to panel pillar width|
|> Boundary pillar||15m|
|> Pillar centres||19m x 19m|
|> Mining height||2,10m|
|> Extraction factor||Already included in model|
|> Dilution||Already included in the model|
|> Contamination||Use select seam|
- Legislation applicable to the mining process
- Continuous miner sections for the bord-and-pillar and short-wall sections, with associated equipment, are the only production units used at the mine. Stoping of pillars is done by continuous miners
- Underground environmental control for the mine adheres strictly to the requirements of the Minerals Act.
The accuracy and confidence of predicted ore reserves and qualities are considered high, with predictions and actuals in both qualities and quantities mined-out being close.
The necessary capital for a number of projects for Matla (mine 1 relocation, short-wall replacement, north-west access and mine 3 vent shaft) to overcome its operational challenges has not yet been approved. As such, Matla must access coal reserves under challenging geological and mining conditions. Thinning coal seams and inferior coal quality and roof conditions, due to the impact of intrusive dykes and sills, as well as geological faulting, present challenges for coal extraction in a number of mining sections.
If the new access for mine 1 is not constructed, the reserves accessed via mine 1 will have to be re-assessed.
The timely implementation of development projects is important to ensure sustainable mining volumes and qualities to Eskom.