mineralogical, chemical characteristics) that have overriding influence on technical and economic decisions. For example, the environment of an underground mine is totally enclosed by surrounding rock. Because mine development is an intensive cash-outflow activity, the current long lead times must be decreased through new technologies.
The problem of low recovery from underground mines is well documented. In underground coal mining the overall recovery in the United States averages about 55 percent; average recovery from longwall mines is about 70 percent (Hartman, 1987). Technology for mining thin coal seams (less than 1 meter thick), particularly thin-seam longwall technology, would be beneficial. In view of the extreme difficulties for workers in such a constricted environment the technology for thin-seam longwalls must include as much automation, remote control, and autonomous operation as possible. Successful longwall and continuous coal mining technology might be adapted to the mining of other laminar-metallic and nonmetallic deposits. Potential problems to be overcome will include the hardness of the ore, the rock conditions and behavior, and the abrasive nature of the mined materials.
Underground mining of thick coal seams (more than 6 meters thick) also presents numerous problems. Current practice is to extract only the best portion of the seam with available equipment. In some cases coal recoveries have been as low as 10 percent. In addition to the sterilization of the resources this practice has created problems of heating and fire. Research should focus on equipment and methods specific to mining thick seams. Hydraulic mining may have potential applications for thick seams. The technical feasibility of hydraulic mining is well established, but equipment and systems that can operate in more diverse conditions will have to be developed. Like the mining of thick coal seams, other mining methods also leave a relatively high percentage of the resource in the ground. Therefore, research could focus on secondary recovery methods (i.e., returning to mined areas to extract resources still in the ground). The petroleum industry has successfully developed secondary recovery methods; steam, carbon dioxide, and water flooding are commonly used to drive oil to the wellheads.
In-situ mining (discussed in more detail later in this chapter) has been remarkably successful for several metallic and nonmetallic deposits. The application of this technique to the secondary recovery of mineral resources is another area for research. Extensive trials on in-situ gasification of coal have been conducted by a number of agencies worldwide, including DOE and the former USBM. In-situ mining has also been attempted for retorting oil shale. The potential benefits of the in-situ gasification of energy resources include reduction of mine development and mining and more efficient use of resources that are otherwise not economical to mine (Avasthi and Singleton, 1983). However, substantial technical problems, including such environmental issues as groundwater contamination, must first be addressed.
A long-standing need of the hardrock mining industry is continuous mining. Currently, only tunnel-boring machines and some prototype road headers have been shown to be capable of mining hardrock. The use of tunnel-boring machines in some mining operations has been limited because they are not very mobile, are difficult to steer, and are completely inflexible in terms of the shape of the mine opening. Tunnel-boring machines are being used more often for mine entry, as in the development of a palladium-platinum mine in Montana. Prototype mobile mining equipment for hardrock was demonstrated in Australia, but production rates were lower than expected, and numerous failures occurred. The solution to this problem will depend largely on the development of advanced cutting technology for hard rock, as well as ways of incorporating new cutting concepts into a mining system that would provide efficient continuous mining with a lower thrust requirement and maximum flexibility. New control systems might incorporate sensor feedback from the cutting head so machine parameters could be adjusted for maximum efficiency. Similar concepts are currently being used in the hydrocarbon drilling industry.
Mining systems that make a clear break with present systems, such as the chemical and biological mining of coal, should also be investigated. In-situ chemical comminution might be possible if the solid coal could be reduced to fragments by treatment with surface-active compounds, such as liquid or gaseous ammonia, and transported to the surface as a suspension in an inert gas. The literature on the biosolubilization of coal and the aerobic and anaerobic conversion of coal by microorganisms and enzymes has been evolving for some time (Catcheside and Ralph, 1997). Biodegradation of coal macromolecules could potentially convert coal carbons to specific, low-molecular-mass products. Research will be necessary to determine the basic mechanisms, as well as to develop conceptual schemes that would make biodegradation cost effective. For all in-situ mining concepts the obvious environmental benefits of limiting surface disturbances and waste generation must be weighed against the potential of adverse impacts on groundwater quality during operation of the mine and upon its closure. Research on chemical or biological mining of coal must also include evaluations of environmental risks posed by reagents and process intermediates.
Mining depends heavily on mechanical, motor-driven machinery for almost every aspect of the process, from initial extraction to transport to processing. Improving the performance of machinery (thus reducing down time), increasing the efficiency of operation, and lowering maintenance costs would greatly increase productivity. The development and application of better maintenance strategies and more advanced automation methods are two means of improving machine performance.
In recent years new concepts of providing maintenance
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