Multi-pass drills
meet demand
Much of the world relies on coal for electricity generation. This is especially true in the United States, where coal is responsible for over 50 percent of the power produced. To meet this need, over 1 billion tons of coal is mined on an annual basis. The proven DM-M3 and now the PV-275 are the drills of choice in large scale mining ope- rations in Wyoming. These robust drills, with their ability to drill large deep holes at an angle, have be- come the standard in the Powder River Basin.Massive amounts of overburden - up to 300 ft (91 m) - are removed to reach the seams of coal that can exceed 80 ft (24 m) thick. Powder River Basin coal is treasured because of its low sulfur content.
18.3 meters), a dragline is capable of deep digging depths beyond 100 feet (30.5 meters).
While the dragline is a very effective earthmover, the overall cost of over- burden removal can be reduced through cast blasting. Cast blasting is a method of drilling and blasting that uses high explosive energy to throw a sizeable portion of the bank into the adjacent empty pit where the coal was previously removed. This method often results in casting 30 percent or more of the bank overburden material to its final resting place, known as the “effective cast” or “cast to final.”
As dragline operations require a flat bench, large track dozers with special wide blades (sometimes referred to as carrydozers) push the cast material down and build a bench at a set height above the coal seam. The dragline will then uncover the coal seam. This mining
Draglines are effective earthmovers, removing overburden and depositing directly into spoil piles as shown here. In the foreground, casted material can be seen filling the empty pit, while dozers work to build a dragline bench.
method allows for the excavating of a large vertical block of material ranging from 100 feet (30.5 meters) to 200 feet (61.0 meters) or more in depth, com- pared to the 50-foot to 60-foot (15.2 to 18.3 meter) vertical benches taken by truck/ shovel methods. A challenge of the drag- line method is maintaining stability of the face (known as the highwall) after excavation, especially when water is present in the material.
Drilling for cast blasting applications generally involves deep depths (up to 235 feet or 71.6 meters), large diameters (up to 12 1/4 inches, or 311 mm) and angles up to 30 degrees from vertical. Large diameters result in wider drill patterns, reducing the number of holes drilled. Because of the size of the drag- line buckets, large fragmentation size from the blast is not a concern. How- ever, some operations have found that smaller diameters such as 11 1/4 inches (286 mm) yield better blasting results with the tighter spacing.
Deep blasthole drilling has its chal- lenges. Foremost is the amount of cut- tings generated by the large diameter, deep holes. A 12 1/4-inch (311 mm) hole to 200 feet (61.0 meters), assuming a swell factor of 30 percent, would yield 7.9 cubic yards (6.0 cubic meters) of cuttings. This is a very large pile that smaller drills simply cannot contain under their dust hoods. Even though a small unit might have sufficient pull- down, rotary torque and air to drill a hole, it wouldn't be effective due to ex- cess cuttings falling back down the hole after the hood area is filled.
Most of the drilling for dragline ope- rations is done at angles between 20 and 30 degrees. The angle drilling serves two purposes. First, the angle can be set to roughly the same angle as the desired highwall. This is done to help keep a consistent face-row burden to improve the effectiveness of the cast shot. In simple terms, the burden at the top of the highwall (the crest) should be similar to the burden at the bottom of the highwall (the toe). Second, angle drilling can help shape the direction of the cast shot. As the blast projects perpendicularly from the bore hole, an angled hole gives a vertical component to the blast, helping lift the mate- rial and therefore throw it further. It is
important to remember that the drilling depth increases as the angle increases. For example, if mining a 200-foot (61.0 meter) bench, the drilling depth at 30 degrees would be 231 feet (70.4 meters).
Some mining regions are fortunate to have soft material, which yields ex- tremely fast drilling rates and less wear on buckets, tires and truck beds. The PRB is in this class, with much of the material having a compressive strength of less than 5,000 psi (34 MPa). The
material is so soft that tricone bits are rarely used. Instead, aggressive claw- type bits are the standard. Contrary to the general belief that soft material calls for as high a rotation speed as possible, these claw bits rotate at lower RPM (100 or less), but their design allows them to shear through the material at rates exceeding 1,000 feet/hour (305 meters/ hour).
To handle the high penetration rates, large air compressors must be used. This is especially true in the PRB as these
Drilling at an angle then blasting the bank into the adjacent empty pit results in casting 30 percent or more of the bank overburden material.
drills often use smaller diameter drill rods to increase the annular area (the gap between the wall of the hole and the drill rod) to allow the larger cuttings generated by the claw bit to exit the hole without having to be reground to a smaller size. High volume compressors of up to 2,600 cubic feet per minute are used, and it is important to have suffi- cient air pressure (100 psi, or 6.8 bar, or more) available to prevent plugging bits. Because of the light weight of the over- burden (approximately 3,000 pounds per cubic yard, or 1.04 tons per cubic meter), bailing velocities may dip below the 5,000 feet per minute (1,524 meters per minute) recommendation that the industry would normally prescribe, yet still effectively clean the hole.