Situated 25 km from Lephalale in South Africa’s Limpopo province, Exxaro’s open-pit Grootegeluk mine employs 1,800 people and produces more than 18 million metric tons per year (mtpy) of thermal, metallurgical and semi-soft coking coal using a conventional truck-shovel operation. The mine has an estimated minable coal reserve of 3,424 million mt and a total measured coal resource of 4,116 million mt.

The Waterberg coalfield has an east-west striking length of approximately 88 km, complemented by a north-south width of approximately 40 km within the Republic of South Africa, but also extends westward into Botswana. The Grootegeluk mine is a multi-bench operation established in the shallow coal portion of the Waterberg coalfield. The existing mine footprint covers an area in excess of 740 hectares. Mining consists of a series of parallel benches, advanced progressively in a westward direction across the reserve, via a process of drilling, blasting, loading and hauling with truck and shovel fleets.

Grootegeluk is a large coal mining and processing complex. It operates the world’s largest coal preparation system. With a raw feed capacity of 7,600 mt per hour (mtph), coal is processed in six different plants.

To effectively track its coal mining and processing business, Grootegeluk sought the most powerful and accurate geological modeling tool available. Management selected the Minex software from Gemcom for this critical task because it has been designed to handle rigorous challenges posed by complex deposits.

Bench structure and quality models
Grootegeluk’s data volumes are daunting, with 760 boreholes used in the most current geological model. A full succession borehole holds 12 coal zones and five interburden waste seams, as well as an unweathered and a weathered overburden horizon. This translates into 17 different mining horizons or benches. Each mining horizon has a roof, a floor, and a thickness grid, yielding a total of 81 grids in the bench structure model.

Qualities in the 12 coal and coal-bearing seams are modeled in 13 different density fractions for life-of-mine scheduling purposes, yielding 1,560 quality grids for proximate analysis. Product qualities, modeled for short-term planning, add another 1,160 grids. “Our bench quality model comprises 5,696 grids and counting,” said Callie Van Heerden, senior geologist, Grootegeluk. “The complexity of the multi-seam geology and its associated quality parameters makes Minex an essential part of our modeling efforts. I do not think we would ever get a model of this magnitude out without Minex.”

It takes one person approximately three to four weeks to create a bench structure model for the mine, according to Van Heerden—and once it’s done, you can take it to the bank. “When you reproduce something with the same data and methodology, Minex always renders the same answer,” Van Heerden said. “If the output is different, you know that you are either doing something wrong, or something has changed in the data or methodology. Minex always gives me the same result—and if something is right, it’s right. This high level of accuracy is especially important in view of the stringent specifications and narrow tolerances imposed by many of Grootegeluk’s clients.”

The advanced 3-D capabilities of Minex also win praise from Van Heerden. “You can utilize the graphical display to check your work as you go along,” Van Heerden said. “The results of the behind-the-scenes mathematics are displayed in the graphics window, so you always have a feel for what is happening in the modeling process.”

This insight will come in handy as Van Heerden moves from the existing bench model to a more complex seam model. “The new model will have 58 seams in total, resulting from specific combinations of the 76 coal and non-coal samples,” Van Heerden said. “The sample qualities are combined to yield the seam qualities. Combining different seams to form new bench scenarios will enable us to optimize production from the mine and product qualities from the plants.” Van Heerden anticipates that, by mining ‘better’ intervals, the flexible seam model will make it possible for Grootegeluk to expand the number of high-value products that can be produced from the same deposit. Furthermore, optimizing the utilization of production equipment means that more tons can be mined with the same equipment, thereby saving money and increasing profitability.

Selecting Minex
Japie Kruger, Grootegeluk’s mine planner, was part of the decision-making process that led to the purchase of Minex. “We started with just two coal products, but we expanded to six different products fairly quickly,” Kruger said. “This expansion created a clear need for computerized geological modeling to handle our multi-seam coal environment. We were very impressed with the graphics capabilities and output of Minex, coupled with its excellent reporting features.” Also key were the SQL and grid arithmetic functions of the software, as well as the user-friendly nature of the program.

Prior to acquiring Minex, the mine relied on Seamsys and in-house programming based on user needs. “Not everything could be handled,” Van Heerden said. “Only the most important items, like the mine’s initial product qualities, were catered for. By contrast, Minex gives us flexibility and enables us to produce critical information from raw data in the shortest possible time, in order to satisfy the mine’s needs.” Minex is also used to populate the mine reserves database, manage the backfilling operation and finalize medium- and short-term planning.

Expanding into the future
Grootegeluk looks forward to the possibility of enhanced integration between geology and mine planning with Minex. “We hope to improve net present value with proactive utilization of the different modules to streamline our thinking and decision-making, on a strategic as well as an operational level,” Kruger said. “Among other things, this would involve combining unique beneficiation capabilities with the mining and backfilling process.” Improvements in short-term operational production planning—including resource planning on a shift basis to optimize mining equipment and plant beneficiation strategy—are also anticipated.

“The Waterberg coalfield is unique, with unique opportunities which Exxaro wants to capture,” Kruger said. “Waterberg will be one of the major mining centers in South Africa. Minex is pivotal for ensuring that these valuable resources are evaluated accurately and mined efficiently.” There will be plenty of opportunity to put Minex to the test in the years ahead.

“We are currently looking at two mines in the planning phase, and possibly a third,” said Grootegeluk’s Mineral Resources Manager Adam Wieruszowski. “We’re going to extend this deposit quite intensively. This year we’re planning for roughly 56 million mt ex-pit—total material shoveled out of the area—and within roughly four years, we should be running at nearly 96 million mt ex-pit.

Grootegeluk values Minex software in supporting its ambitious production targets. “It is a nice package to work with, and it is quite impressive in terms of its capabilities,” said Van Heerden. “It was designed for modeling stratified deposits, and it is recognized worldwide for this purpose. Minex can do everything that I want to do.”