Home » Archive Industry News » Industry News 2017 » Industry News 04_2017 » Wirtgen surface miners: Titans in Texas
Fig. 1. The Eagle Pass Mine began selectively mining the thin coal seams with a Wirtgen type 4200 SM surface miner in October 2015. // Bild 1. Die Eagle Pass Mine begann im Oktober 2015 mit dem selektiven Abbau der dünnen Kohleflöze mit einem Wirtgen Surface Miner vom Typ 4200 SM. Photo/Foto: Wirtgen

Wirtgen surface miners: Titans in Texas

Two type 4200 SM Wirtgen surface miners are bringing light to South Texas and Northern Mexico. A 4200 SM is extracting sub-bituminous coal at an open cast mine owned by the North American Coal Corp. in Eagle Pass, Texas/USA – right on the border with Piedras Negas, Coahuila/Mexico (Figure 1). The extracted coal is then transported over the border by train to a multi-unit power plant in Coahuila. Meanwhile, at another open cast mine in Texas, another 4200 SM is extracting lignite, just as economically. The two surface miners – the largest models in Wirtgen’s product range – went into operation in the last two years and are extracting coal with a low sulphur content, operating 24/7.

The 4200 SM is 6.53 m high and has an operating weight of 204,300 kg. It is powered by a diesel engine with 1,521 HP. When cutting soft material such as coal, limestone or gypsum, the 4200 SM cuts down to a depth of 830 mm using a 4,200 mm wide milling drum and operating in an up-cut direction.

Eagle Pass Mine is located in an area which has been worked for coal since as early as the late 19th century. Piedras Negras, the name of the town on the other side of the border, translates as “black stones”, a reference to the coal deposits located there.

“Every ton we extract goes to a coal-fired power plant in Mexico,” says John C. Duffey, Chief Engineer of Camino Real Fuels. A subsidiary of the North American Coal Corp., this company operates the mine for the owner, Dos Republicas Coal Partnership (DRCP). Eagle Pass has replaced the depleted open-cast mine Siglo XXI (21st century) in Mexico, on the other side of the Rio Grande – or Rio Bravo, as it is known in Mexico.

At the mine, a 2,550 ha area is available for coal extraction, offering sufficient deposits for eight years of mining work. The total quantity of deposits is much larger, however. One special feature of this location is the fact that the seams are very thin, some of them only 15 cm thick. This is precisely where the Wirtgen surface miner proves invaluable. It is used to selectively extract coal from the rock. This enables to achieve a high material quality. The four seams lie beneath an 18.3 m layer of top soil and overburden, in a series of layers around 6,0 m thick. Barely 1.65 to 2.0 m of this is coal.

The 4200 SM transfers a 250 t payload onto dumpers. They transport the coal to a transshipment center equipped with five grizzlies and conveyors. Each grizzly supplies one loading conveyor which transports the coal to the train-loading station.

“The thin seams can be mined precisely with the Wirtgen 4200 SM, which also saves us a great deal of money for processing,” says Duffey. “We don’t need a primary crusher at the transshipment center anymore, because the 4200 SM crushes the coal directly during loading.” The surface miner crushes the coal to a maximum size of 10 cm while minimizing fines (Figure 2). As coal fines tend to block the loading conveyors at the transfer points when there is a high moisture content, a low proportion of fines is a great advantage. Almost 95 % of the coal is crushed and loaded by the 4200 SM. The remaining 5 % – i. e., material located in inaccessible corners or right at the end of a drift – is broken up by bulldozers and loaded by front loaders.”

Fig. 7. The 4200 SM from Wirtgen crushes the lignite to a maximum grain size of 10 cm during the mining process. // Bild 7. Beim Abbau zerkleinert der 4200 SM von Wirtgen die Braunkohle auf eine maximale Korngröße von 10 cm. Photo/Foto: Wirtgen

At the other open cast mine in Texas, the lignite used to be extracted by two smaller, older surface miners. The Wirtgen 4200 SM was bought in 2014, when these two miners were approaching the end of their service lives.

“The 4200 SM is technologically superior in terms of its hydraulics and control system. What is more, it loads dumpers 30 to 40 % faster than the older models,” say the drivers.

“I love this machine,” says the thrilled driver of the 4200 SM, as he stands beside his equipment. “The 4200 SM runs much more quietly than our previous surface miner. The cabin is very comfortable and the machine is so easy to operate. The video cameras also show me the area directly behind the cutting drum unit, ensuring that I don’t penetrate too deep into the interburden.

Further information/Weitere Informationen: