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July/August 2017 www.coalage.com 25 surface mining continued in an attempt to find out exactly how much coal there is under the surface, but we are often surprised." The 4200 SM loads coal into 250-met- ric-ton (mt) (275-ton) haul trucks. They transport the coal to a transshipment cen- ter equipped with five grizzlies and con- veyors. Each grizzly supplies one loading conveyor that transports the coal to the train-loading station. "The thin seams can be mined pre- cisely with the Wirtgen 4200 SM, which also saves us a great deal of money for pro- cessing," Duffey said. "We don't need a pri- mary 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. As coal fines tend to block the loading convey- ors 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," Duffey explained. "We don't have a permit for blasting and we don't blast coal or the surrounding rock. Almost 95% of our coal is crushed and loaded by the 4200 SM. The remaining 5% — for instance, material located in inaccessible corners or right at the end of a drift — is broken up by bull- dozers and loaded by front-end loaders." Loading Lignite Fast in South Texas At the other open-cast mine in Texas, the lignite used to be extracted by two small- er, 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, according to Wirtgen, is technologically superior in terms of its hy- draulics and control system. What is more, the drivers said it loads the haulers 30% to 40% faster than the older models. "I love this machine," said the op- erator of the 4200 SM, as he stood be- side his equipment. "The 4200 SM runs much more quietly than our previous surface miner. The cabin is very comfort- able and the machine is so easy to oper- ate. 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 tar- get grain size. The selective mining meth- od extracts material with a high degree of purity, reducing coal washing costs at the processing plant. This also enhances the quality of the end product, enabling the coal to be sold at a more profitable price. In many cases, the use of surface min- ers offers a more economical, eco-friend- lier and safer alternative to conventional mining by drilling and blasting. The sur- face miner can cut, crush and load the material in a single pass. Dispensing with drilling and blasting, the application caus- es 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. The 4200 SM from Wirtgen crushes the lignite to a maximum grain size of 10 cm during the mining process. The loading conveyor of the 4200 SM can be slewed 90° to either side.