C.O. Aksoy
In open pit and strip mining operations, a variety of mining equipment combinations may be employed for overburden stripping, primarily dictated by the structure and nature of the mineral deposit. In coal extraction bulk operations, such as those found in Germany, Russia, and Australia, bucket wheel excavator-conveyor combinations are used. In the United States, Russia, Canada, and the United Kingdom, dragline mining is common. In almost all countries, shovel-truck combinations are utilized for stripping of overlying strata.
There is a long history of coal production in Turkey; however, the cost of power generation is high. Therefore, it has become necessary to find a means of decreasing production costs to become more competitive in a global market.
This paper attempts to economically minimize the operating costs of a dragline-shovel-truck system applied at an open pit mine, lowering the lignite production cost and, indirectly, the overall electricity generation unit cost. The Yatagan-Eskihisar open pit coal mine used for this study is a dragline-shovel-truck combination operation. The upper section of the overburden is removed by conventional truck-shovel operations (drilling, blasting, loading, and hauling). The remaining overburden is excavated by dragline. In order to create an economically competitive lignite product, the cost of the stripping operations, which have a major affect on the overall unit cost, must be minimized safely within the technical restrictions of the operation.
In this unit cost minimization study, P&H shovels and CAT 777 trucks with capacities of 15 yd3 and 77 tons, respectively, were used in truck-shovel pre-stripping operations. A 65 yd3 Marion 8050 dragline was used to expose the overburden immediately over the coal seam. Pre-stripping bench heights varied from 8 m to 20 m and dragline bench heights from 20 m and 25 m, with a pit width of 80 m. Evaluations were conducted to minimize the unit cost of field operations. Total overburden extended to a depth of 60 m (35 m to 40 m for pre-stripping).
Prior to conducting the cost minimization analysis, the average stripping cost at the mine was $3.12/m3. This high unit cost was primarily due to the inappropriate pit geometry selection for the available mining equipment. To put things into context, given that 15 million cubic metres of material was moved, this corresponded to a cost of $46.8 million per year. Lowering this value will undoubtedly lower the overall production costs, and indirectly lower the cost of power generation. Since the initial capital cost investment is high for draglines, it is difficult to justify mechanical modifications thereafter, and there is little scope for variation to the operating geometry to minimize cost. This leaves mine geometry as the less feasible variable. The most prominent factor in optimization and minimization analysis is operating time, as it plays an important role in lowering the unit cost to operate equipment more efficiently. In this case, the price of a dragline with such capacity was assumed to be $50 million, a major consideration in terms of the time value of the dollar.
The unit cost minimization study with respect to the dragline operation showed that the dragline was not efficient at a bench height of 20 m, resulting in a somewhat higher cost, making the 25 m bench height the better choice. For drilling, blasting, loading, and hauling unit truck-shovel pre-stripping operations, the unit cost minimization study identified the most economical bench height to be 15 m. Combined with a dragline bench height of 25 m, the outcome identified an economical unit cost at $2.5/m3.
