Grootegeluk coal mine

Overview

Grootegeluk mine is in the Waterberg coalfield, in the north-western part of the Limpopo province of South Africa. It is in the Lephalale magisterial district, close to the residential suburbs of Marapong and Onverwacht. Although the current mining licence expires in 2041, the mine’s production is projected to exceed the granted 30 years life of mine. Exxaro reasonably expects that the mining right will be renewed.

Grootegeluk is a surface coal-mining operation where a series of parallel benches are advanced progressively across the deposit via a process of drilling, blasting, loading and hauling with truck-and-shovel fleets.

Pre-stripping the overburden in weathered areas, consisting of weathered shale and coal, is accomplished using hydraulic shovels, while areas of unweathered shale are drilled and blasted. Access to the different benches is provided by a series of inclined roads, arranged in a diverging manner from surface, along the northern and southern pit limits.

The top half of the stratigraphy comprises a thick interbedded seam deposit while the bottom half is a multiple-seam deposit type. Mining-bench definitions coincide with the geological boundaries of the layered coal deposit except for the uppermost bench, bench 1A overburden, which is designed at a practical level above the first fresh coal. The coal deposit has been subdivided into geological units based on the unit’s physical and chemical properties, to form 14 mining benches for both saleable products and waste. In this way, the run-of-mine for each bench is reserved for particular beneficiation destinations to produce products with specific coal characteristics and at given quality-control specifications.

The mine has a multiproduct output (thermal coal, semi-soft coking coal and metallurgical coal as well as semi-coke as a downstream char product), sold to a spectrum of domestic and international clients. The largest portion of the beneficiated product is power station coal with an average ash content of 35%. Several sized metallurgical coal products at 15% ash and 11,25% ash, semi-soft coking coal at 10,3% ash, as well as steam coal at 12,5% ash is railed to various customers and shipped to international clients. A small portion of metallurgical coal product at 15% ash is used by the on-site char plant that produces a carbon-rich downstream product for the ferroalloy industry (semi-coke). Beneficiation plant slurry is pumped to the cyclic pond slimes dam system for recycling. The dried plant slurry yields a thermal coal grade product that is blended in small proportions and sold with the thermal coal product.

The GG6 expansion project is under construction. The project aims to add a second stage of beneficiation to the existing GG2 plant, and upgrade the two tip-bins to higher capacity.

The project’s end-state will triple the capacity of the current GG6 plant, while the GG2 plant ceases to exist. The GG6 expansion plant will produce a high-ash semi-soft coking coal that will be suitable for sale in the export market, while producing thermal coal of suitable ash content.

History

The Waterberg coalfield was discovered in March 1920 during water-drilling operations on the farm Grootegeluk 459LQ, Limpopo. A few short boreholes were drilled and Trevor & Du Toit (1922) summarised the results which, at the time, amounted to a discovery of scientific interest. The results of a later in-depth study by the Geological Survey and Fuel Research Institute indicated vast resources of metallurgical and non-metallurgical coal.

Iscor (now Exxaro) later acquired property rights to six farms in the Waterberg coalfield, on which 120 holes were drilled. Over a number of years, Iscor obtained bulk samples of coal for coking tests from a prospecting shaft on the farm Grootegeluk. Additional coking coal samples were obtained from large-diameter boreholes (254mm core).

In May 1973, Iscor started an intensive exploration programme on the six farms originally purchased for a final quantity and quality assessment of the resource. In 1975, a trial box-cut was established to obtain a bulk sample for beneficiation tests. The outcome of feasibility studies led to Grootegeluk mine being commissioned in 1980, originally designed to supply semi-soft coking coal as a reduction agent in Iscor’s steel-production process.

An agreement was later reached with Eskom to provide coal to a power station with 4 200MW generation capacity. Based on the projected life of this power station (Matimba), a pit layout containing 40 years of saleable thermal coal was designed. In addition to producing power station coal, the mine also produced semi-soft coking coal through a double-stage beneficiation plant, known as GG1.

As the ramp-up of Matimba power station progressed, another beneficiation plant (GG2) was commissioned to augment thermal coal supply to the power station. GG2 is a single-stage beneficiation plant running at an average separation density of 1,95g/cc. To cope with the full demand of Matimba power station, after it started generating electricity at design capacity, another beneficiation plant, GG3 (crushing and screening only), was brought on line.

As the market for medium- to low-phosphorous coal evolved, additional beneficiation plants, GG4 and GG5, were commissioned to produce metallurgical coal for direct reduction and other smaller market applications, like the cement and tobacco industries.

In 2013, two additional plants, GG7 and GG8, were erected to supplement what was already the largest coal beneficiation complex in the world to produce coal for the new Medupi power station.

The current pit layout was designed to cater for the remaining Matimba power-station contract as well as the 4 770MW Medupi power station which started construction in 2007. This layout was approved in 2012 and came into effect in that year. It was revised in 2017 to address concerns about an increasing sulphur trend in the planned ROM over the next 10 years of the mine’s life. Changing the pit shell layout and mining direction from south-westerly to westerly avoids the high sulphur area in the south-westerly corner of the reserve in the earlier years of the LoMP, resulting in an improved energy-stripping ratio in the current mining area. The remaining reserves reported here are therefore based on this revised 2017 pit layout, which include a small portion of the Thabametsi mining right that have been included in the new pit layout (figure 15).

Overburden management is evolving in volumes and complexity. One study to address this challenge is the in-pit crushing and conveying overburden (IPCC OVB). The project aims to replace the existing load-and-haul mining method with a more cost-effective alternative by considering mining and transporting overburden material using a bulk-materials handling system. The concept study indicated significant savings by implementing a fully mobile sizing station (FMSS), bucket wheel excavator (BWE) or semi-mobile sizing station (SMSS) along with an overland conveying system and new spreader for the rehabilitation layer on top of the upper discard layer. A pre-feasibility study (PFS) is under way to identify the preferred alternative to further investigate the viability of a bulk-materials handling system for overburden material.

Geology

Grootegeluk mine is in the Waterberg coalfield, which has an east-west striking length of some 88km, complemented by a north-south width of around 40km, and lies in South Africa, extending westward into Botswana. The Waterberg coalfield is fault-bounded along its southern and northern margins and can be referred to as a graben deposit, with the Eenzaamheid fault forming its southern limit and the Zoetfontein fault forming its northern limit. The Daarby fault, with a downthrow of some 350m towards the north-east, divides the coalfield into a deep north-eastern portion and a shallow south-western portion. The upper part of the coal deposit, the Volksrust formation (some 60m thick), comprises intercalated mudstone or carbonaceous shale and bright coal layers. It displays a well-developed repetition of coalshale assemblages that can be divided into seven discrete sedimentary cycles or zones (zone 11 – zone 5).

The Vryheid formation (roughly 55m thick) forms the lower part of the coal deposit and comprises carbonaceous shale and sandstone with interbedded dull coal seams varying in thickness from 1,5m to 9m. Due to its nature, the Vryheid formation is classified as a multiple-seam deposit type. There are five coal zones in the Vryheid formation, predominantly dull coal, with some bright coal developed at the base of zones 2, 3 and 4. Due to lateral facies changes and variations in the depositional environment, these zones are characterised by a variation in thickness and quality. It appears that these zones depreciate in a westerly direction as observed in the mining rights area due to lateral facies changes.

Resource evaluation

Resource evaluation at Grootegeluk is an extensive process and entails coal analysis and beneficiation simulation in Sable database software and geological modelling in Minex, using the Minex growth algorithm. Separate coal and shale (stratigraphically identified) samples are composited first into combined coal/shale samples and subsequently into benches. The washability tables present proximate analyses from fractional relative densities of 1,35 to 2,2 and coal product simulation is undertaken in Sable and modelled in Minex. Some 714 boreholes were used for resource estimation, of which 464 contained coal-washability analyses. A 0,5m thickness cut-off and reconciliation-based bench-specific geological losses are applied to convert GTIS to MTIS.

Grootegeluk drills on average ten (T6-146 size core) 123mm diameter drill-cored holes per year. The size is to accommodate the samples and the required suite of analysis for the various products and associated relative density fractions for both the Volksrust and Vryheid formations. The geological model is updated every two years, except if a significant change is observed in new drilling information. The geological model will be will updated in 2018 when adequate drilling information is available. The review and update of the geological and structures models are aligned with estimation and structural geology procedures that include concurrent data validation and reconciliation against actuals as mining progresses.

Resource classification is based on a matrix system conforming to SANS 10320:2004 guidelines for multi-seam and thick interbedded coal, and incorporates a number of studies that justify 500m x 500m drill spacing for measured resources. Classification criteria are evaluated considering reconciliation, new exploration data, coal continuity, geological structural complexity and observations in the pit area.

The change in resource estimation is due to the following:

  • Mining actuals from January to December 2017 (-52,7Mt)
  • Reclassification of structural complex areas (-4,8Mt)
  • Economic evaluation and disposals (93,3Mt).

Reasonable prospects for eventual economic extraction

All criteria (table 24) considered for resources inside and outside the LoMP are favourable. Appropriate seam structure, seam thickness and coal quality have been considered and a sizable portion of the deposit, on the northern side of the Daarby fault, is therefore not declared as a resource due to the proximity of the coal below surface (Daarby fault with a downthrow of 350m to 400m).

Reserve evaluation

The resource estimates used for conversion to reserves are those derived from the 2016 resource geological model based on geological data available to 31 March 2016, and are contained within the new approved pit layout only. Indicated resources are converted to probable reserves and measured resources to proved reserves. All inferred resources, including those within the pit layout, are omitted from reserve reporting in compliance with the SAMREC Code.

XPAC mine-scheduling software is used to derive the remaining saleable reserves from run-of-mine reserves in the approved pit layout. After converting the geological model’s grids to the appropriate format, the floor, roof and thickness data as well as quality data for each bench is imported into the XPAC model. In this model, validations are performed to evaluate the data for possible discrepancies, such as incremental yields for each bench rising with increases in the relative float densities. The resource category areas are also loaded into the XPAC model for reserve categorisation purposes. In-situ indicated resources are converted to probable in-situ mineable reserves and in-situ measured resources are converted to proved in-situ mineable reserves.

The scheduled mining blocks are of the same size as current actual blast blocks in the mine. The fact that material from different benches is combined and beneficiated simultaneously creates difficulty in reporting saleable product tonnages per bench. The preferred reporting is therefore ROM tonnes per bench and saleable product tonnes per beneficiation plant.

The new 2017 XPAC model has integrated new geometallurgical principles into the LoM planning process and scheduling model to better predict as-mined plant performance. This is an all-inclusive model that can simulate all the plants in the Grootegeluk complex from one integrated flowsheet. The key improvement is that the model provides:

  • Combined washability data for all material fed to a specific plant
  • The data is combined for each relative density (RD)
  • The impact on plant yield performance, due to ROM for various benches, is modelled.

Various reviews and audits have been carried out, in conjunction with the mine, to ensure the process applied is fully understood and that predicted product volumes are realistic and transparent. Various recommendations were made and refinements are continuously being made to improve the LoMP process.

The washability tables for each blast block are imported into the geo-met model (XPAC). The geo-met schedule imitates reality at Grootegeluk in that portions of a single blast block can be allocated to a number of beneficiation plants within a particular scheduling period. Once the production schedule has run, a particular blend of blast blocks from different benches is allocated to each plant for each scheduling period. A new composite wash table is then derived for each plant for each scheduling period, which represents the blend of material fed from the mine to that plant. This composite wash table is then used to derive the specific products required to be produced by the particular plant for that period. A set of calibrated plant factors are applied per plant to adjust expected product yields to true expected levels. It is thus not assumed that a block in its entirety is allocated to one plant only, as this does not represent reality at Grootegeluk.

Risks

We do not know of any pertinent risks or other material conditions that may impact on the company’s ability to mine or explore, including technical, environmental, social, economic, political and other key risks.

Calculated CV values are used in our modelling process, which are slightly higher than determined CV values. The impact is low. Historically, a visual inspection of core recovery on some cored holes was made but not always recorded. Poor recoveries were nevertheless investigated and were primarily the result of inferior geological conditions such as faulting and/or weathering. A more rigorous way of core recovery calculation is always conducted, based on the weight of the retrieved sample material, the apparent RD and the length of core. Measurement of linear core recoveries is now always recorded. The points of observation provide an average view of depth of weathering throughout the resource. The variability of the weathering estimate between points of observation is, however, challenging. Close-spaced structural boreholes closer to the pit face are drilled to mitigate the impact on production.

Business improvement and innovation

A number of initiatives focused on data optimisation and creation of information are in development. The implementation of a centralised geological database aims to enhance the capture and integrity of geological data. In addition, the development of short-term geological and mine-planning models supports our drive to provide timely access to critical information for our operational teams.