Print7.3.3: Auger Mining
Earlier in this lesson, you saw the highwall that remains after the ultimate pit limit is reached in a contour mining operation. At that time, I pointed out the possibility of recovering the coal under the highwall, by either developing an underground mine or highwall mining. The former is rarely economically feasible today, but the latter is commonly practiced. A distinction is made between auger mining and highwall mining, and this will become clear shortly.
In auger mining, auger holes are bored horizontally into the coal seam from the last contour cut at the high wall. As the auger rotates and advances forward, coal is cut and pushed out of the hole by the rotating action of the auger. The action is similar to that of a carpenter boring a large hole in wood using an auger bit, and that is where the name derives. Auger mining recovers around one-third of the coal under the highwall, for whatever depth of penetration is achieved. Auger cutting heads can be as large as 7 feet in diameter and may be drilled to a depth of more than 300 feet. An auger mining machine is shown here.
A highwall, showing the auger holes is shown in this picture:
When the ultimate pit limit is reached, it is difficult to “pack up and leave” when you see the exposed coal seam. So, in some regard, the company looks at this secondary recovery technique, i.e., auger mining, as the “icing on the cake.” It amounts to coal recovery at a very low cost. It is, however, somewhat problematic and controversial. The low recovery, less than 35%, means that the majority of the reserve remains in place, but it has been sterilized, i.e., left in a condition where it will be essentially impossible for anyone to recover the remainder in the future. The holes into the side of the hill create drainage holes for acid-laden water, and that is a big problem. Surface subsidence and spontaneous combustion in the auger holes are additional concerns.