The Red Hills Mine is the first major mining site for Mississippi’s lignite reserves, which are estimated to total around 8 billion tons.
The Red Hills Mine was established in 1997 to supply the affiliated Red Hills power station with coal. It is operated by the Mississippi Lignite Mining Company, a subsidiary of America’s biggest lignite producer, North American Coal.
The challenge: Selective extraction and reduction of fines
Extending over some 23.5 km2, the mine contains some 2 billion tons of lignite. However, the lignite lies in six seams of different thicknesses, each separated by intermediate layers of marl. If the lignite is to be extracted economically, it is extremely important to separate the different materials very precisely during the mining process.
“If we use excavators alone, we will only be able to extract 80% of the coal reserves. We won’t be able to fully exploit the deposits,” explains the mine’s Chief Engineer, Paul Mongeon. And Production Manager Jim Keenan adds: “What’s more, we often find fossilized wood in the seam. If we excavate it with the dragline, we end up with huge clumps that clog up the feed hopper at the power station. We don’t get that with the 4200 SM.”
Selective extraction: at the Red Hills Mine, the Wirtgen 4200 SM mines six lignite seams with thicknesses of between 0.8 m and 1.4 m, each separated by layers of marl.
Another key factor is the fine material content – or fines – which, at > 20%, was very high with the previous mining method and frequently caused problems in the conveyor system. “Fines get stuck in certain areas of the conveyor system, get wet, pack together and clog up the system. Fines are the enemy of the conveyor,” says Mongeon.
4200 SM with custom-modified cutting drum
North American Coal is committed to setting industry standards in the mining sector in terms of safety, environmental protection and innovation. But even in this mine, it made sense to increase efficiency using modern technologies, and it was this which prompted the decision to use a Wirtgen 4200 SM Surface Miner.
Truck-change on the fly: the Wirtgen 4200 SM fully loads a dumper truck with 135 tons of lignite in around three minutes.
“The cutting drum and the number of picks have been specifically modified for this mine and for our lignite,” explains Keenan. “By modifying the picks from the original configuration, we have greatly reduced our fines content. Fines previously accounted for 20.7% of the weight of material extracted. They are now down to just 8%. This helps our customers to process the coal more easily,” adds Mongeon.
Surface mining technology hits the mark
Today, the 4200 SM mines around 2,800 tons of lignite per hour and its 16 m-long conveyor belt discharges the coal in a single pass directly into dumper trucks, which then transport the coal to the power station and into the feed hopper. As maximum grain sizes of 7.6 cm are mined, there is no need to break the material down further, greatly increasing efficiency.
“Using the 4200 SM, we exploit the seams to an accuracy of 0.25 cm. As a result, we keep the energy value of the mined material high and the ash content low,” says Keenan, clearly pleased with the high quality of the lignite extracted.
Excavation depths of up to 83 cm are possible with the Wirtgen 4200 SM. It was decided to work at a depth of 60 cm at the Red Hills Mine.
“With the Surface Miner, we can cut to depths of up to 83 cm, but we prefer to work at a depth of 60 cm. Before, we could achieve no more than 30 cm, just half the depth. The advantages also extend to the loading process. Thanks to the long conveyor, our dumpers can be loaded higher, so they can now transport 135 tons of coal per trip instead of 120. As a result, we can deliver the power station’s weekly tonnage of 77,000 tons in just six to seven 12-hour shifts, whereas previously it took eight shifts. We now use the spare shift for excavating the marl. We can then expose more of the coal, which in turn makes us significantly more effective.”
No problems were encountered when switching to the new equipment, thanks to the support provided by one of Wirtgen’s service engineers. “Our drivers are also very happy with the machine, not least because of the air-conditioned cabin which protects them from the noise, dust and vibrations.” The fact that the operator has a perfect view of the cutting process at all times using the built-in camera and lighting system and is able to keep a close eye on the cutting process via the display screen is yet another advantage.
David Liffrig, CEO of North American Coal, is also pleased: “The Wirtgen 4200 SM is perfect for our mine. It boasts a host of safety functions and optimized features for the operators. And due to the speed at which it mines and loads the material, it has also given us the opportunity to use various resources in other ways.”
Surface mining requires no drilling or blasting and is an eco-friendly extraction process. North American Coal also takes exemplary care when it comes to restoring former excavation sites to a natural state.