Erik Zimmermann explains how two surface miners are bringing light to South Texas and Northern Mexico
One 4200 Wirtgen Surface Miner (SM) is extracting sub-bituminous coal at an open cast mine owned by the North American Coal Corporation in Eagle Pass, Texas, USA – right on the border with Piedras Negas, Coahuila, Mexico. 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 past two years and are extracting coal with a low sulphur content, operating 24/7.
The 4200 SM is 6.53m high and has an operating weight of 204,300kg. It is powered by a highly efficient diesel engine with 1,521hp. When cutting soft material such as coal, limestone or gypsum, the 4200 SM cuts down to a depth of 830mm using a 4,200mm-wide milling drum and operating in an up-cut direction.
New mine, new extraction method
Eagle Pass Mine is located in an area that has been worked for coal since as early as the late 19th century. At the mine, a 2,550-ha area is available for coal extraction, offering sufficient deposits for eight years of mining work. One special feature of this location is the fact that the seams are very thin, some of them only 15cm thick. “And this is precisely where the Wirtgen surface miner proves invaluable. We use it to selectively extract coal from the rock. This enables us to achieve a high material quality,” explains John C. Duffey, chief engineer of Camino Real Fuels.
The four seams lie beneath an 18.3m layer of top soil and overburden, in a series of layers around 6m thick. “Barely 1.65 to 2m of this is coal,” says Duffey. “We conduct preliminary examinations in an attempt to find out exactly how much coal there is under the surface, but we are often surprised. The thin seams can be mined precisely with the 4200 SM, which also saves us a great deal of money for processing. We don’t need a primary crusher at the trans-shipment centre anymore, because the 4200 SM crushes the coal directly during loading.”
The surface miner crushes the coal to a maximum size of 10cm while minimising fines. 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.
“Blasting, excavating and loosening – the 4200 SM does away with all this,” explains Duffey. “We don’t have a permit for blasting and we don’t blast coal or surrounding rock. Almost 95% of our coal is crushed and loaded by the 4200 SM.”
Lignite in South Texas: fast loading
At the other open cast mine in Texas, the Wirtgen 4200 SM was bought in 2014, when two old 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. I love this machine,” says the driver. “It 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.”
Compared with the mining methods used in the past, the 4200 SM produces fewer fines and more material in the target grain size, with a high degree of purity, reducing coal washing costs at the processing plant and enhancing the quality of the end product, enabling the coal to be sold at a more profitable price.
Economical and safe
Often the use of surface miners offers a more economical, eco-friendlier and safer alternative to conventional mining by drilling and blasting. The surface miner can cut, crush and load the material in a single pass. Dispensing with drilling and blasting, the application causes no destructive vibrations, making this process a great deal safer. Dust and noise pollution is also much lower. As a result, material can even be extracted in areas directly adjacent to residential areas and industrial estates.
Apart from reducing machinery and personnel costs, the process is also time-saving. And surface miners produce level, stable surfaces – a great advantage for mine logistics: ordinary trucks can travel over the routed access roads transporting the material more quickly. They are also more cost-effective to acquire and maintain than dumpers.