What Is A Drag Line?
A dragline system is an unusually large land machine that is used in civil engineering, strip mining, and excavation. The larger types of dragline excavators are used in strip mining operations to extract phosphate rock. These are among the largest types of mobile equipment and can weigh upwards of ten thousand tons.
The dragline's bucket system consists of a large bucket that is suspended from a boom. The bucket is moved by many cables, chains and ropes. The hoisting rope, which is powered by either a diesel or electric motor, will support the bucket and hoist coupler assembly from the boom. The drag cable on the assembly is used to draw the bucket assembly horizontally. Through skillful maneuvering of the hoist and drag cable, the bucket is controlled for many different types of operations.
In a typical excavation cycle, the bucket is positioned high above the material that is being stripped. The bucket is then lowered down and the drag cable is drawn so that the bucket is dragged along the ground surface, digging ever deeper, as the bucket draws near the main body of the dragline at end of the cycle. Using the hoist cable system, the bucket is then lifted. A swing operation is then performed using the main body of the dragline to pivot on the carrier axis in order to move the huge bucket. The drag cable is released at the proper time which will cause the bucket to tilt, causing the bucket to empty the load of material in the bucket, which is commonly known as a dump operation.
The limitations of dragline are the height and length of their boom, as this limits where the drag line can dump waste material. Being inherent with their construction, the drag line is most effective when excavating material below the level of their tracks such as strip mining. Draglines aren't suitable for loading piled up material. Despite their limitations and high capital cost, draglines remain very popular with phosphate rock mining industry, due to their very low waste removal cost, performance, and reliability.
Multiple Stripping Sequences
Draglines also use multiple stripping sequences or "methods". Each of these stripping sequences is very functional and remains in practice today.
The first is the side casting method which uses "offset" benches. This method involves throwing the overburden sideways onto water cannon blasted material, creating a "slurry mix" in order to move large amounts of material to the phosphate processing plant.
The second method is a key pass. This pass will cut a key at the toe of the new "highwall" and will also shift the bench further towards the low wall. This can also require a chopping pass if the wall is blocky. A chopping pass will involve the bucket being dropped down onto an angled "highwall" to strip the surface.
The next method is the slowest, known as the blocks pass. This method will however, move the most material. The blocks pass involves using the key to access the bottom of the material to lift it up to spoil or to an elevated bench level. If required, the final cut is a pull back, which pulls the material back further to the "lowwall" side.
The phosphate strip mining is accomplished by various methods of stripping the materials from the ground as illustrated above. The phosphate industry relies heavily on the dragline excavator to keep moving large amounts of overburden to mine the phosphate rock just beneath the surface.