|Auger & Highwall Surface Mining|
(Secondary Coal Production on Surface Mines)
As noted in previous sections, secondary coal production is possible on surface mining operations through various methods of auger or highwall mining. Both auger and highwall mining activities are conducted after the economic threshold imposed by the stripping ratio for a particular mine is achieved and no further excavation of overburden is possible. Auger and highwall mining allows for additional coal extraction in individual coal seams along the highwall once the maximum depth of excavation is reached on a surface mine. These mining activities (if used) are the last to be conducted in a mining extraction area before backfilling, regrading and AOC restoration commences as the first stage of reclamation.
In auger mining, holes are drilled into the exposed coal seam outcrop by augers driven by rotary shafts and hydraulic rams. The coal is removed by the advancing auger through a screw-like action. Mining augers may come in single, double or triple head machines. Augers are generally limited to advancing and removing coal to a depth of 200 feet into the coal seam. Augers typically have a recovery rate of 33 percent of a given coal seam. As the auger reaches the maximum depth of penetration into the coal seam it is withdrawn from the highwall and moved further along the outcrop to being the extraction process again. Figure A-24 is an illustration of a typical auger mining system.
Auger Mining System Extracting Coal from the Highwall of Surface Mine
As coal is produced from the auger mining machine it is stockpiled and loaded into haulage trucks.
Much more common than auger mining as a means of secondary coal production on surface mines are highwall mining systems. These mining systems use continuous mining machines similar to those used in underground mining methods to drive a series of entries into a coal seam outcrop along a surface mine highwall. As the highwall mining machine advances into the coal seam its cutting head removes the coal and directs it to specialized conveyor cars that are coupled to the machine. These conveyor cars deliver the coal to the surface where it is stockpiled and loaded. As the maximum depth of coal extraction is reached the mining machine is withdrawn from the highwall and the system is moved further along the outcrop to drive a new entry. Highwall mining systems have a better recovery rate than auger machines with up to 45 percent of coal seam being extracted. Figures A-25 and A-26 are photographs of highwall mining systems at work on surface mines in West Virginia.
Highwall Mining System on an Area Mine in West Virginia
A Highwall Mining System on an MTR Mine in West Virginia. To the Right of the Picture are the Specialized Conveyor Cars that Follow the Highwall Mining Machine into the Coal Seam