Drills

Drills are used to create a hole of a certain diameter and depth. Occasionally, the goal of drilling is to create an empty hole, but more often, the purpose of the hole is to accept explosives. The major components of a drill include the bit, which fragments the rock; a power source that transfers energy to the bit; and lengths of drill steel, sometimes called the drill string, that connect the bit to the drill rig proper. Drills vary by: the method of rock penetration, e.g., rotary or percussion; the location of the power source, which can be at the top of the drill string, e.g., top hammer, or at the bit, e.g., down-the-hole; primary method of powering the drill, e.g., diesel engine, electric motor, or compressed-air; and the method of mounting the drill rig, e.g., track-mounted or tire mounted.

Here, we have a track-mounted down-the-hole drill with an articulating boom to facilitate drilling holes at precise angles.

Figure 5.1.1: Track-mounted down-the-hole drill

Source: Atlas Copco

While the previous drill employs a down-the-hole hammer to apply energy to the bit, this one is a top-hammer drill, i.e., the energy source for the bit, at the top of the drill string. That means the “pounding and rotational” action has to be transmitted through the drill string to the bit. The biggest disadvantage of this approach is the loss in accuracy. The drill string tends to travel in a large helical track with the top hammer, and this causes the drill bit to “wander” off the desired location of the hole.

Figure 5.1.2: Top-hammer drill

Source: Atlas Copco

If these drills are going to be used underground where the headroom is limited, the mast is not as high, but otherwise the drill is similar. While the accuracy of hole location is most always important, in some underground applications, it is crucial. In those, a down-the-hole hammer will be used.

Vertical or inclined holes are commonly required in surface mining, and sometimes in underground mining. It is very likely that horizontal or nearly horizontal holes will be required in underground mining, as well as overhead vertical or overhead angled holes.

The dual boom jumbo drill is designed to drill horizontal or inclined holes at an angle off of the horizontal. The depth of these holes is typically limited by the application and is on the order of 15’.

Figure 5.1.3: Dual boom jumbo drill

Source: Atlas Copco

Ring or fan drilling and longhole drilling are characteristic of a few underground metal mining methods. The holes may be 150’ or long, and must be drilled to precise depths at the exact design angles. Drills to accommodate these requirements employ computer control to achieve the required accuracy, as do more and more jumbo and other drills being used in production operation. A typical drill is shown here. Note the remote operating station for the drill operator.

Figure 5.1.4: Computer controlled drill with remote operator

Source: Atlas Copco

This diagram illustrates the drilling pattern required in a sublevel stoping mine. Notice the layout of the holes. What do you think would happen if a hole was started, but the angle was off by a few degrees? Or what if a particular hole were drilled to 128’ instead of the designed length of 135’? Intuitively, I am sure you can imagine that it will affect the performance of the blasting, and you are correct. We’ll talk more about this when we cover blasting, but for now, please recognize that productivity, cost, and safety are adversely affected by less-than-optimal drilling practices.

Figure 5.1.5: Diagram of drilling pattern required in a sublevel stoping mine

Source: Atlas Copco

Finally, we should close our overview of drills with the handheld “jackleg” drill. In years past, these were used instead of jumbo drills in hardrock mining and for ground control applications in coal and metal/nonmetal mines. The size of hole depended on the cylinder bore of the drill, as they were powered by compressed air, as well as the size of the drill bit. Hole sizes ranged from approximately 1” to 4”. Depending on the size, and consequently what the drill was used for, they were given names such as drifters or stopers. They are only used in modern mines for very specialized functions, requiring a few holes on occasion, here or there. Unfortunately, you will find them in widespread use in the underground mines of lesser-developed countries. I say unfortunately because they are brutal to use. Although much of the weight, around 75 lbs., is supported on the jackleg, the miner has to apply the thrust, i.e., pushing the drill bit into the hole. That alone is hard work. They are very loud, there is bone-jarring vibration, and they can produce dust-laden air and that along with the oil mist from the compressed air creates a respiratory hazard. Teams of miners would stand all shift-long operating these drills. Often they were paid based on the production that they achieved, and 15 years ago in the western U.S., one of these miners could easily earn $60-80 thousand dollars per year… but they earned every penny of that!

Figure 5.1.6a and b: The jackleg drill

5.1.6a: J. Kohler, © Penn State University, is licensed under CC BY-NC-SA 4.0, and 5.1.6b: Atlast Copco

The practice of loading sticks of dynamite has all but disappeared from modern mining. Safer and more economic practices utilize explosives that are pumped, gravity-fed, or pneumatically blown into the hole. That’s not to say that we never use packaged material, because we do, and we’ll look at that when we discuss explosives and blasting practice. The vast majority of blasting, however, is accomplished with powders, gels, or emulsions that are bulk loaded.

Explosives Loaders

Figure 5.1.7: Explosive truck for surface mining

Credit: J. Kohler, © Penn State University, is licensed under CC BY-NC-SA 4.0

The truck below is used in surface mining. It carries the materials to mix the explosive at the hole, and the equipment to place the explosive into the hole. Such an arrangement can be found in some underground mines as well. In this example, the explosive is being pumped into the hole.

Figure 5.1.8: An example of gravity flow, in which the explosive is fed augered into the hole.

Credit: J. Kohler, © Penn State University, is licensed under CC BY-NC-SA 4.0

In a mine in which the height of the active mining face is on the order of 15’ to 60’ feet, you will see equipment like this. The boom allows the blaster, standing in the basket, to reach each hole, insert a hose into the hole, and then load the explosive. In this picture, you can see the yellow tank that contains the blasting agent. There is a second tank on the other side, which is obscured from view in this photo. These tanks are known colloquially as “powder monkeys.”

Figure 5.1.9: Explosive loading machine. Note the blaster standing in the basket.

Credit: J. Kohler, © Penn State University, is licensed under CC BY-NC-SA 4.0

Rippers

Rippers are dozers that have been equipped with one or more large drag bits. These are pulled through the ore as the dozer moves forward. Typically, they break up the top 6” – 18”, depending on the mechanical properties of the ore. Rippers are not very common because they are suited to few deposits. Occasionally, I’ve seen them used in soft high calcium limestone and in coal, but there are other examples. In the picture here, this ripper (seen at the back of the image) has three large drag bits.

Figure 5.1.10: Ripper with drag bits

Source: Stock image by Worradirek, shutterstock.com

You’ll recall that the production cycle for harder ores is drill-blast-load-haul; while for softer ores, it is excavate-load-haul. I am emphasizing load because I want to talk about the equipment used for loading. In harder ores, the material is broken free of the orebody by drilling and blasting. Once it is freed, it usually must be loaded into something so that it can be hauled out of the mine. In softer ores, if a ripper is used, the broken ore will require loading. In softer ores, it is often practicable to free them from the orebody by digging or excavation, without the need to drill and blast. Once freed, the ore is usually loaded into something so that it can be hauled out of the mine. By the way, I am using the word something to describe the haulage out of the mine. I’ll be more specific shortly when we look at haulage.

The reason for this quick review of the difference in production cycles is because the equipment used in loading harder ores is sometimes used for both excavating and loading softer ores. I want you to be aware of this now so that you are not confused when we encounter this with certain pieces of loading equipment.

Draglines

Draglines have the ability to excavate huge quantities of material and then to place that material at quite a distance from the dragline itself. Here are some of the more important characteristics of draglines.

• The drag bucket is filled by pulling (dragging) the bucket towards the dragline.
• The dragline can only dig at a level below the level at which the dragline is sitting.
• The drag bucket is outfitted with “teeth” to aid excavation, but even so, it is only effective in softer materials or materials that have been blasted.
• The size of the dragline is based on the capacity of the drag bucket. A 15 yd dragline has a bucket capacity of 15 yd³. Typically, draglines are in the range of 60 – 150 yd. The largest one ever built had a bucket capacity of 220 yd³. It could fill the bucket, swing it to the dump point nearly 300’ from the machine, dump the bucket, and return to begin the cycle over again in 45 seconds!
• The reach of the dragline determines the distance from the machine that material can be dumped. The maximum reach is approximately 450’.
• The weight of the dragline is so great, approaching 8000 tons in the larger units, that crawler tracks are used on only the very smallest draglines, and instead, they sit on two large “shoes” to distribute the weight. The shoes are attached to powered eccentric cams that allow the dragline to walk as mining advances.

Figure 5.1.11: Dragline

source: CAT dragline product page

Draglines are commonly used to remove the overburden, also known as stripping, in open cast mining. In many cases, the overburden is drilled and blasted to facilitate removal by the dragline. Occasionally, the dragline will be used to remove the ore as well.

Shovels

Shovels have the ability to load large quantities of material, but less than draglines. A major difference between a shovel and dragline is that a shovel loads material at the same level as the shovel is sitting up to the height of the shovel’s boom. Here are some additional characteristics of shovels.

• The shovel’s dipper is filled by pushing the dipper into a bank of material. This action is known as crowding.
• The dipper is outfitted with “teeth” to aid excavation, but even so, it is only effective in softer materials or materials that have been blasted.
• The size of the shovel is based on the capacity of the dipper. A 12 yd shovel has a dipper capacity of 12 yd³. Typically, shovels are in the range of 10 – 90 yd.
• The digging height of the shovel is determined the size of the boom.
• Shovels are mounted on large crawler tracks, to facilitate movement into the bank, known as propelling; as well as to allow the shovel to move to the working faces or banks.

Figure 5.1.12: A shovel in action

source: direct industry.com

Large shovels are used for overburden removal, and in this application are known as stripping shovels. Perhaps more commonly, they are used to load ore into trucks. The relationship of key shovel parameters is illustrated in this diagram. Also, I’d like you to take notice of the wire ropes that are used to control the boom and the dipper. This is the original configuration for shovels. Over the past three decades, a modification of the shovel, using hydraulics cylinders instead of wire ropes, has become increasingly popular. Let’s take a look at it next.

Figure 5.1.13: Diagram of an Electric Mining Shovel

source: Joy/Komatsu

Hydraulic Excavators

This class of loader had its origin with machines that looked like this. These excavators had the advantage of being able to excavate or load below or above the level at which the machine is sitting. They also have the ability to be rather selective in what they remove.

Figure 5.1.14: A traditional excavator

source: Volvo Construction Equipment.com

These excavators have gotten much larger over the years and have buckets of 40 yd³ or more, and a reach of 60’ or more.

Figure 5.1.15: A modern excavator

source: Joy/Komatsu

The wire ropes on electric shovels have been replaced with hydraulics, and this has led to hydraulic excavators that look like this, and are often called hydraulic shovels. By replacing the wire ropes, additional degrees of freedom can be incorporated into the machine, resulting in better performance.

Figure 5.1.16: A hydraulic excavator

source: Joy/Komatsu

Wheeled loaders are mobile and maneuverable, and commonly found in surface and underground mining applications. The bucket typically ranges in size from a few yd³ to 10yd³. The high-lift linkage allows loading of the largest haul trucks available. These loaders are capable of digging soft and unconsolidated materials, but are most commonly used for loading material that has been blasted. Chains are often put on the tires to increase the traction available, which allows the bucket to push more efficiently into piles of broken rock. In everyday use, they are often called loaders, rather than wheeled loaders.

Figure 5.1.17: Wheeled loader

source: CAT.com

Here we can see a very common use of the wheeled loader.

Figure 5.1.18: Wheeled loader in action

source: CAT.com

Next, let’s take a look at some loaders that are used exclusively underground. The amount of clearance available underground can be quite restrictive. While some underground mines will have openings that are approaching 100’ in height, less than 10’ is much more common. The equipment designed to operate in these confined spaces has to be designed quite differently to fit into these spaces and still be capable of doing something useful.

Scoops and LHDs (Load-Haul-Dump)

There are some differences between scoops and LHDs. But for the purpose of this discussion on loaders, we are not going to differentiate between the two other than to note that scoops are more likely to be used strictly for loading ore into an underground mine truck in metal mining. Also, they are often used for moving supplies around in underground coal mines.

These machines are designed with a low profile to function in confined spaces. Moreover, they are often articulated, i.e., the machine is split into two parts and connected with an articulating joint. This allows the machine to turn and maneuver in tighter spaces. The scoop loader shown here illustrates the concept quite nicely. At less than 3-1/2’ wide, 6’ high, and 16’ long, it would easily fit inside of the bucket of many surface loaders! The bucket capacity of these machines ranges from just under 1 yd³ to 4 or 5 yd³.

Figure 5.1.19: An articulated scoop loader

source: Heavy Equipment Solutions.com

Here is a picture of a typical LHD, which stands for Load-Haul-Dump. These machines are commonly used in underground metal mining, and they push into a pile of broken ore (load) and then they haul (transport) their load to a dump point. The haul distances are relatively short – typically less than a 1000’, although this can vary. This LHD is available with bucket capacities ranging from 10 to 15 yd³.

When we talk about the underground metal mining methods, you will see the importance of LHDs in those operations.

Figure 5.1.20: A typical LHD, or Load-Haul-Dump

source: www.CAT.com

This brief introduction to loaders is by no means exhaustive. There are still some legacy devices in use – overshot loaders and slushers, for example, in metal mining. However, here, we have covered the devices that account for virtually all tonnage produced in modern mining systems. This also brings to a close our discussion not only loaders but also ore winning, more or less. Wait a minute, more or less? Well mostly more… however, we have not talked about an important class of ore winning machines known as continuous miners, road headers, and shearers. We’ll do that in the next lesson. For now, we’re now ready to move into materials handling – the last unit operation in the production cycle of drill-blast-load-haul for hard rock or excavate-load-haul for soft rock.