Mining: Mining Process
“Mining coal is easy. Getting to it is the hard part.” Joe Usibelli
Uncovering Coal is the Biggest Job
|Coal seams #3 and #4 are exposed in this spectacular natural outcropping of the coal-bearing rocks north of Hoseanna Creek, near Healy, Alaska. The two coal seams are typical of the ultra-low sulfur subbituminous coal found in the Nenana coal field and mined at Usibelli Coal Mine, Inc.|
At Usibelli Coal Mine, up to 100 feet of unconsolidated sandstone or overburden must be moved to uncover the top seam (seam number 6) of coal. Another 120 feet of interburden must be moved to uncover the d seam (seam number 4) while roughly another 80 feet of interburden must be removed to uncover the third seam of coal (seam number 3) reaching nearly 400 feet at its deepest point. Overburden is removed by four different methods:
- The dragline removes the bulk of the overburden
- Blasting utilizing explosives loosens and casts the overburden
- Dozers move overburden short distances prior to drag line operations
- Shovels strip and load overburden and coal into the haul trucks
Dragline does the "dirt" work
The most efficient machines for moving large volumes of dirt are draglines. In 1977, UCM made a major investment by purchasing a 1300W Bucyrus-Erie Walking Dragline. The dragline arrived in component parts on 26 railcars and 40 trucks during December 1977. It took 11 months to assemble the 2,100-ton machine. Named "Ace-in-the-Hole" by Healy school children, the dragline is the largest land mobile machine in Alaska. The acquisition of "Ace-in-the-Hole" made it possible to double coal production and initiate UCM's South Korean export contract in 1985. With its 325-foot boom, the dragline has a reach of 270 feet. The bucket weighs 32 tons and will hold 33 cubic yards of material. In one 24-hour period, the dragline can move 24,000 yards of dirt, leaving a strip of uncovered coal 145-feet (or more) wide. Stability for this heavy machine is achieved by the large steel plate (the tub) which rests on the ground during drag-line operations. Wheels or tracks would be impractical due to its extreme weight. The dragline moves by "walking." Shoes on both sides of the machine lift the base partially off the ground and drag it backward. Each step takes about 40 seconds and moves the dragline a distance of approximately 7 feet. While it takes only one person to operate the controls, there are two operators on duty at all times. One operates the dragline from the cab while the other does routine maintenance. For safety reasons, the operators switch positions every hour. Since the dragline was commissioned in 1978, it has operated for more than 127,000 hours of digging. It moves approximately 5 to 6 million cubic yards of overburden every year.
The dragline is powered solely by electricity. A Load Stabilizer System consisting of a 40-ton flywheel and motor/generator serves as a buffer between the dragline and the local electrical grid. The flywheel stores energy to be used whenever the dragline's electrical demand increases too quickly for the local grid. This process enables the dragline to operate without causing power fluctuations for other electrical customers.
The dragline has several buckets, which are used on a rotational basis. UCM utilizes several different configurations of bucket including 33-, 35-, and a high-production 42 cubic-yard bucket. Each bucket is used on the dragline for approximately 6 to 8 months. After that period, the bucket must be serviced and critical surfaces of the bucket repaired. The bucket is repaired by welding wear plates on vulnerable surfaces that are worn as a result of the abrasive characteristics of the materials that are excavated.
Blasting loosens materials, casts overburden
Both overburden and coal can be very difficult to excavate. Therefore, blasting is necessary in order to loosen the overburden and coal before it is moved by the heavy "yellow-iron" equipment. Specially designed powder trucks transport, mix and load explosives into each drill hole. A technique called "cast blasting" is used to help remove the overburden. During cast blasting, the drill holes and explosive charges are designed so that a portion of the overburden is cast laterally by the force of the explosion into the adjacent mined-out pit. This technique, when utilized, reduces up to 25% of the amount of overburden that the dragline must handle. Cast blasting is a significant cost savings when compared to utilizing trucks and shovels. There is less wear and tear on equipment, less consumption of diesel fuel, and fewer employees are required. Cast blasts are conducted approximately 3 or 4 times a year.
Dozers move overburden short distances
UCM uses dozers including a Komatsu 475A-5 dozer and several large Caterpillar dozers including the D11R-CD, Carry Dozer. They are some of the largest tracked dozers manufactured in the world. The dozers move forward and backwards pushing overburden off the surface of a mining bench into the pit. The dozers also prepare the surface area in and around the dragline.
Shovels and backhoes strip overburden and load coal
While the dragline does the majority of the excavation work at the mine, additional excavators are needed. Several track-mounted excavators are used to load coal and to strip overburden in areas difficult for the dragline to maneuver. The large shovels have buckets capable of approximately 26 cubic-yards and can load the 150-ton trucks in approximately four passes.
Trucks and other equipment support operations
A fleet of eleven trucks is used to haul coal and overburden (dirt, topsoil, gravel and rocks). During 1995, the Caterpillar 785 haul truck was added to the UCM fleet. The truck has a capacity of 150 tons, more than 50% greater than the previous 95-ton Dresser HaulPaks. Local modifications are made to the 150 ton truck after delivery from the manufacturer. UCM customizes the truck bed by adding an additional 16 inches in height and 24 inches in width of the bed. The additional height and width allows a larger volume of coal (which is lighter than overburden) to be hauled. UCM currently operates a fleet of eight Caterpillar 785 (150 ton) and three Caterpillar 777 (100 ton) trucks. Road graders operate during day and night shifts to keep the haul roads smooth for the larger trucks. Two water trucks provide dust control on the roads during the summer months. Additionally, a high-pressure nozzle mounted on the water trucks serves as ready reaction firefighting equipment, should it be needed.