Wirtgen GmbH

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.”

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.

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WIRTGEN GmbH
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The New Acland Mine (NAC), located adjacent to the township of Acland (Queensland), is part of the Australian based New Hope Group. Since opening of the mine, coal and interburden have been extracted by means of bulldozers fitted with ripper teeth and assisted by wheel loaders. However, a sharp drop in coal prices compelled the NAC management to look for alternative, more efficient mining methods.

A solution was found with the Wirtgen surface mining technology which offers a method for the challenging geological conditions in the mine, which comprise up to 27 mostly thin coal seams interspersed by layers of interburden. Compared to conventional methods, the selective surface mining technology simplifies the mining process, thus reducing operating costs and supplying coal of higher quality to the neighboring processing plant.

Wirtgen surface mining has proven its worth as a tried-and-tested technology in numerous coal mining projects. Therefore, the 6-month trial phase with defined target performance levels agreed upon between NAC and Wirtgen was a resounding success: the 4200 SM achieved the projected cutting performance of up to 3,000 t/h in the tough everyday mine environment. The point-attack cutting tools used were subjected to in-depth trials and optimized to fit the specific application as they were required to cut not only coal but also hard interburden material. Cooperation with the Wirtgen customer ser-vice, which was on permanent stand-by, was also perfected at an early stage. This enabled many areas which typically offer room for improvement when introducing an entirely new technology to be identified and optimized quickly. Since all of the targeted performance rates were met during the trial phase, NAC decided to purchase the 4200 SM.

To maximize exploitation of the coal deposit, the high-performance 4200 SM cuts and loads the material in a single machine pass, producing an even and easily trafficable surface in the process. It was additionally established during the trial phase that, while offering the same performance, the surface miner replaces two bulldozers and one wheel loader. The resulting savings, mainly in labor and fuel, lead to a significant reduction in operating costs. Additional marks in favor of the 4200 SM include emission measurement results showing significantly reduced machine vibrations and noise levels. Paired with the ergonomically designed cabin, impacts on the operator’s health are thus minimized. A camera system designed exclusively for this operation enables the machine operator to conveniently differentiate between coal and interburden. The strict safety regulations and machine standards specified by the Australian coal industry were fully met by making the relevant machine modifications prior to shipment.

The cutting drum has been specially designed for operation in soft rock and is suited to mining not only coal (unconfined compressive strength 20 MPa) but also the significantly harder interburden layers (50 MPa). The 4200 SM mines coal in a selective process at the specified cutting depth, producing a clean cutting surface and guaranteeing high degrees of purity. As a result, there are less truck travels required to transport the material to a special coal-washing plant for further processing and to return the separated tailings to the mine. Compared to the mining method used to date, the 4200 SM also produces a smaller percentage of fines (grain size < 2 mm) and a higher percentage of the end product fraction (grain size < 38 mm). In the final analysis, this also improves the efficiency of the processing plant.

Further information:
www.wirtgen.com