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Cross-pit spreader aids production at Texas lignite mine.

With increasing overburden heights, lignite mining companies in Texas, U.S. have had to consider new mining concepts, as the large walking draglines currently used for overburden stripping run into rehandling problems because of their limited working range and excavating depth. Any new mining concept must have enough flexibility to work in conjunction with the existing draglines and reduce the overburden height to a level at which the draglines can handle it efficiently. Because of the high volumes of material to be moved, efficiency in the pre-benching operation is mandatory.

Texas Utilities Mining Co. (Tumco), a subsidiary of Dallas-based Texas Utilities and one of the largest open pit mine operators in the U.S. faced these problems in its mines. The Monticello mine at Winfield South, Titus County is a mine-mouth operation producing 12 Mt/y lignite for Texas Utilities' steam power station. Currently there are three active mine sites with five active mining areas. The primary overburden removal equipment in use is three 60 and one 100 |yd.sup.3~ electric draglines. One mining area uses scrapers solely for primary overburden removal.

From 1974 to 1990 conventional methods of stripping overburden were used at Monticello, and involved use of these draglines and scrapers. Several dragline techniques were adopted including simple cast, extended bench and spoil stripping to safely and economically remove the overburden.

In 1986 it was recognised that the required lignite output could not be met in the G-Area with the stripping equipment in use - a 60 |yd.sup.3~ dragline. Because of increasing overburden depths and increasing stripping ratios, additional equipment would be needed to pre-strip ahead of the dragline to allow for removal of the lignite. Complex multi-bench operations would ensue requiring additional dragline passes and - at a certain depth - the operation of the dragline from the unstable spoil side.

Many evaluations, including lignite production requirements, economics of the pre-benching operation, overburden characteristics, rehandle reduction and compatibility with the existing dragline operation led to selection of the Cross-Pit Spreader (CPS) system. This removes the upper, softer overburden strata on a continuous basis, leaving the lower consolidated portion to the dragline. The pre-stripping height of 30 m max. was selected to give the dragline optimum working conditions. The block width was determined by the dragline stripping width of 36 m to achieve good synchronisation of the two mining systems.

In May 1988 Tumco selected Voest-Alpine Int. Corp., N.Y. (VAIC) to design, manufacture and commission the CPS on a turnkey basis. It consists of a type VABE 1500 bucketwheel excavator and a type VACPS 6500/I - 250 spreader. This is the second system of this type in the U.S., but the Monticello installation uses only one large BWE rather than two smaller ones. The 1500 BWE excavates overburden and via conveyors feeds the spreader, which in turn conveys the material across the open pit and distributes it on the developing spoil bank.

Output of the system is 15.3 million bank |m.sup.3~/y of overburden. VAIC was the general contractor, while Voest-Alpine in Austria was subcontractor for design of the system and made the key components. Marathon LeTourneau subcontracted, providing all steel structure components while Siemens Energy & Automation Inc., Atlanta was subcontracted for engineering and supply of electrical equipment. The system, with total installed power of over 6,500 kW is fed by a 22.9 kV trailing cable. Computer systems monitor and control the entire arrangement with high accuracy and reliability and a computerised diagnostic unit facilitates rapid, efficient maintenance. The CPS system has now been operational at Monticello for over two years, and has reportedly met all design criteria to the complete satisfaction of Tumco.
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Title Annotation:Texas Utilities Mining Co's Monticello mine
Publication:Mining Magazine
Date:Jul 1, 1993
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