Print9.4.2c Longwall Mining
Longwall mining applies to thin, bedded deposits, with uniform thickness and large horizontal extension. Typical deposits are coal seams, potash layers or conglomerates, trona, and gold reefs. Longwalls are found in nearly horizontal deposits of coal and trona, whereas in metal/nonmetal deposits, a steeper dip is tolerated. The difference is in the equipment that is used. Coal and trona are using massive suites of mechanized and semi-automated equipment that is unsuitable for use in greater than 5-10% grades. There are exceptions, but we’re not going to complicate this discussion with those.
Longwall mining takes its name from the characteristic long face or wall, which may be several hundred feet or more in length. The figure below illustrates this nicely. The ore is extracted in a slice along this long wall. The region adjacent to the face is kept open, i.e., free of obstructions, to allow space for miners and equipment. This region might extend 10’-20’ out from the wall. If we are looking at mining a gold reef, for example, a line of posts will be installed to support the roof or back and protect the active mining activity. At some distance back from the face, caving will be allowed to occur, and in most instances, this is necessary to relieve superimposed loads on the working face. If we are looking at a coal application, the process will be somewhat different. A significant percentage of the coal that is mined underground comes from longwall mines; and not just in the U.S. but around the world. As such, we’ll look more closely at longwall mining in coal.
Longwall Mining of Coal
Longwall mining of coal is a high production and high productivity method, employing sophisticated electrical, mechanical, and hydraulic systems, as well as computer-based monitoring and control systems. Most modern (coal) longwall faces are semi-automated. It is noteworthy that longwall operations in trona mines utilize essentially the same equipment and processes that are used in coal mine longwalls. As the long wall or face is mined and the roof supports advance forward with mining, the roof in the mined-out area will cave.
When the panel is initially mined, caving will be delayed. This is a worrisome period because all of the weight of the unsupported roof is transferred to the face and also the gateroad pillars. Sometimes caving may not start for 10 - 20 or more passes of the shearer. If the superimposed load becomes too great, the face and pillars can begin to crush. Thus, for longwall to work safely and productively, the cave must occur in a timely fashion. Once it has started to cave, it will generally continue with each pass of the shearer. Ground control experts will conduct a cavability analysis of the overburden before a decision is made to employ this mining method. This requirement for caving is the reason this method is classified in the caving class of methods.
Please be aware that the longwall panels within the coal deposit are created by room and pillar mining. Thus, many room and pillar (coal and trona) mines are also longwall mines, and in most of them, the room and pillar work is simply to develop the panels and the infrastructure to facilitate operation of the longwall. By that, I mean you need to have a well-developed ventilation, materials handling, and power systems to support a high production longwall. The room and pillar mining creates the mains, submains, and panel entries for these systems. Let’s look at a few figures. These won’t answer all of your questions right now, but these in combination with some videos that will come afterwards, should give you a good understanding of the method.
Let’s start with a plan view of a mine. This figure, below, shows a portion of the mains or submains and the longwall panels. Notice the three-entry gateroads that define the panels. One set of these will be known as the headgate entries and the other as the tailgate entries. The one of the longwall face will be known as the headgate and the other end of the face will be known as the tailgate. The entries adjacent to the mined out panel are the tailgate entries, and that defines the tailgate side of the panel. The longwall face is mined on retreat. That is to say, the gateroads are mined on advance, and then the longwall face retreats back to the submains or mains.
The equipment required for a longwall face is unique to this mining method. Let’s look at it in this figure below, and then when you see it in the videos, it will make more sense.
The armored face conveyor is a massive steel structure containing a chain conveyor. The shearer (or plow) rides on the AFC and cuts the coal. The cut coal falls into the AFC, and is transported to the headgate. At the headgate, the coal is crushed to a size suitable for transport on a conveyor belt and then fed at a controlled rate onto the belt. This panel belt feeds the outby belt system. It takes an enormous amount of power (1000s of hp) to operate the AFC, and there are drives at both the tail and headgates to power the chain conveyor. All of this equipment and the miners working at the face are protected by what has been called an umbrella of safety, i.e., the series of shields. As the coal is cut, the AFC snakes into place immediately adjacent to the face. On the right side of this figure, they are depicting this advance of the face, and you can see the shields that have moved into place.
This next figure illustrates more completely the relationship of the longwall panel to the overburden, the gateroads, and the longwall face itself.
Next, let’s take a look at some videos, each of which is less than five minutes in length. I think these videos are helpful for the details that they show. There are four of them, and they are addressing the same basic topic. However, in each one you can see certain important details more clearly than in the other videos. I suggest that you watch all of them twice, and don’t hesitate to pause them and look more closely at the image. In so doing, you’ll develop a more complete understanding of the equipment and the process.
Whenever mining coal underground, long wall mining is a highly productive efficient and safe way of doing it. The coal seam is mined cut by cut with a plow or a shear until a complete panel is mined out. Such a long wall panel will typically be three to four kilometers long and 250 to 400 metres wide. For seam heights of up to 1.8 metres or 71 inches, the plow is the cutting tool of choice. It travels fast with speeds up to 3.6 m/s along the coalface pulled by a plow chain. The chain transmits a of force of up to 2 times 800 kilowatts. The plow is cutting coal at a predefined depth up to 25 centimeters or 10 inches. For inseams from 2.3 meters to 6 meters and above, the shear has proven to be the most efficient cutting technology. It travels typically with the speed of 16 meters per minute while normally cutting 100 centimeters with its powerful cutting drums generating a production capacity of 5000 tons per hour. Constant loading of the face conveyor which transports the coal to the crusher and the belt is the goal to fulfill this task. The armored face conveyor can be equipped with three drive systems each holding a power of up to 1,800 kilowatts. This power package enables them to handle extreme peak loads. The AFC provides a rail on which the cutting device travels, but it's not only for this, but also for the capability to handle all the strains especially on the chain and the constant wear an abrasion. The AFC is called the backbone. The long wall roof supports are vital not only for constant production and advance of the long wall, but for the safety of the miners with a bearing force of up to 1,750 tons each they can handle even severe roof and floor conditions. They are available for seam heights from 0.8 meters to 7.5 meters. Our roof supports have been tested to advanced 60 kilometers and more underground before a complete overhaul is necessary. Integrated automation has become unrenouncible and long wall mining. Constant optimization of the entire long wall system control of thousands of kilowatts of power at motors and drives as well as a few hundred thousand tons of combined yielding capacity of the roof supports surmounts human abilities. A network of intelligent control units collects and shares data thereby optimizing the entire long wall system achieving maximum productivity and availability. Automation systems also keep the miners away from hazardous areas increasing safety standards even further you.
Low seems medium seems high productivity. You'll make the most of every type of coal with the world's most advanced low seem long wall system.
The flower is a simple and robust tool. The intelligent control and a powerful camp longwall system, allow it to cut up to 3,500 tons per hour. No other mining system can do that in low or medium scenes. The plow cuts fast. A GH 1600 is pulled along the face by 2 times 800 kilowatts at up to 3.6 m/s regardless of the type of coal or even rock. The plow cuts faults. It cuts and produces even in geological faults up to seem height. The plow cuts low. With the cat baseplate plow, you will reach where other mining systems can't. Even in seems down to 0.8 meters, you will make the most out of your valuable deposit.
The plow cuts costs. It is the most cost-effective solution for hard coal mining in seams up to 1.8 meters. CAT Automation and CAT drive systems are the brains and the power behind every successful plowing system. We offer the industry's most advanced remote control system so you can operate the plow long wall without an operator at the face.
The plow chain is vital for your sustained productivity, so we protect it. Overload drives from Caterpillar limit the power within the optimum range, protecting the chain that sudden stops or loads.
Horizon control. That is how your plow long wall will follow the coal seam up or down cutting as little rock as possible. That is unique in the industry. It gives you full control even when traversing geological faults.
CAT line paths do their job in the majority of the most productive long walls in the world. CAT line pan wear parts can easily be replaced which lowers your cost of ownership in the long run. And that also goes for cat flower roof supports. They're flexible enough to follow low seems and undulations but will bear enormous loads at every position and step. You'll make the most of every type of coal with a world's most advanced low seem long wall system.
Wherever you break ground, we're there.
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Down through the centuries coal has been one of the primary energy sources used to fuel the world's industrial operations. As industrialization and the demand for coal increased, better mining methods were needed.
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This increasing demand for coal led to greater technical advances in mechanized mining methods.
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Potentially the most productive and safest method of mining being used today is the long wall system.
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Long wall mining is relatively new to most american coal operators in contrast to other major coal producing countries which mine practically all their coal by this method. The long wall method is believed to have originated in england near the end of the 17th century and has since become the most widely accepted mining method there and throughout all europe. Basic long wall mining equipment consists of either a planer or plow type cutting machine that slices off the coal as it is pulled along the face. Or a shearer type cutter that travels along the face cutting the coal as it goes. Coal cut by the shearer falls onto a mobile face conveyor called the pan line.
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Hydraulic chucks or shields support the roof and are individually advanced as the coal is cut.
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The long wall mining system differs from conventional mining in that the coal seam is removed in one operation, by means of a long working face or wall, from which the name long wall is derived. The coal is mined in a continuous line, cutting across a coal face which may be several hundred feet in length.
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There are two basic systems of long wall mining, the advancing, and the retreating system. In the long wall advancing system of mining the coal is extracted from the face starting from the shaft and advancing through the coal deposit toward a boundary or other limit lines. In the long wall retreating system, narrow developing headings are driven through to the boundary or limit line of the coal seam and then the coal is extracted by long wall faces retreating back in the direction of the shaft. As the mining machine makes a pass across the face, cutting a width of coal the pan conveyor and roof supports are advanced immediately behind it toward the new face. As this happens a void is left behind the supports, the roof is allowed to collapse or cave in, filling this void. The fallen material is called gob.
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A double ranging drum shearer is shown in action here. It is especially effective in high seams of coal and where the coal bed varies in thickness and composition. The spiral drums are equipped with an internal water spray system as well as externally mounted sprays to more effectively control dust.
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The broken coal that falls from the face, is picked up by an armored flexible conveyor, or heavy scraper chain conveyor, that also serves as a track for the shearer. This conveyor carries the coal along the face to the head gate entry and dumps it onto a gate chain conveyor, or stage motor, which then feeds it to the entry belt conveyor.
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From here the coal will be delivered inthe mine cars, or conveyors, which will transport it to the surface. A very important part of the long wall mining system is the powered self-advancing roof supports. The most common type in present use are the chuck roof supports. The chuck system shows much improvement over conventional roof control methods, but a number of disabling injuries have been caused by loose rock falling between the canopies striking the worker unexpectedly. In many long wall operations today, chucks are being replaced by a newer shield type support this assembly of steel hydraulic supports is moved forward as an integral unit by means of a hydraulic ram coupled to the face conveyor. Proper alignment of long wall support and conveyor is important. Failure to keep the distance between the support tip and coal face to a minimum can cause bad roof conditions to occur in the face area.
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The shield type of self-advancing roof support provides an added safety factor, in that it isolates the worker from the gob.
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There is less dust or rock falling down between the supports and because they are fully enclosed they prevent virtually any spalling, rupturing, or caving of the gob into the working area. Improved shield supports provide better overall protection from fall of ground and offer other important safety features not provided by earlier conventional chucks. For example, rocks falling from the roof, as shields advanced, are deflected by the canopy or gob shield. The travel way under the shields will also have fewer obstructions and since the overhead canopies are of solid construction, with no gap between adjacent shields, the worker is safer and feels more secure. Long wall mining methods usually create more dust than other methods and methane emission rates are increased. Special attention to the application of water and ventilation is required to control these hazards. Current methods employed to control coal mine dust and methane, include the use of cowls to help confine the dust. Proper positioning of water sprays and proper application of a sufficient quantity of air. Air can be directed up one entry, across the face, and out the return entry, or through the gob into the bleeder system. Long wall miners have methane monitors mounted on the machine itself, or at the tailgate, to warn the operator or shut it down if methane emissions reach a dangerously high level. Also the operator is required by law to make gas tests periodically at intervals not to exceed 20 minutes.
The urgent problem of supplying our nation's increasing fuel needs has resulted in a search for high productivity methods of mining. Long-wall mining may be one of the solutions. A growing number of coal mines in the united states today have operating long walls and many of them report that the system has not only improved productivity but has also helped reduce the accident frequency rate.
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Despite the fact that long wall mining is potentially the safest technique available, in actual practice up to now, its overall safety effectiveness has not been appreciably better than other mining methods. Therefore the hazards involved in long wall mining must be identified and eliminated or guarded against if the workmen are to be effectively protected and the safety of the mining operation improved. To help prevent accidents from roof falls, shields are equipped with hydraulic controls placed safely beneath the support. This allows the miner to advance the unit while remaining under the protection of the shield at all times. When compared with conventional chalk systems of roof support, workmen are much safer with a long wall mining system using shields as the primary means of support, but experience has shown that there are still hazards involved with the use of shields and that the potential for an accident is always present. For instance there's the possibility of pinch points causing foot accidents. These, and other accidents, sometimes occur during the advancing of supports or when moving the long wall equipment to another panel. Some typical ones are hydraulic hoses can suddenly break. Jacks can pull loose and fall on a worker. Ropes and chains and come-alongs can break, and workers can get muscle strains, pinched fingers, and be hit by falling crib blocks while constructing cribbing. Moving supplies across the long wall face is difficult due to limited space between the hydraulic legs and the pan line. Disabling back injuries, twisted knees, and muscle strains due to tripping, slipping, and falling frequently occur on long wall mining sections. This emphasizes a need for greater alertness on the part of minors to prevent these injuries.
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Clean up and repair activities along the pan line can also involve mine workers in hazardous situations.
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The worker may be subjected to possible falls of the face or roof from between the tip of the canopy and the face. After the long wall miner has passed, roof hazards are often present in front of the supports before they have been moved up. For this reason the roof support should be advanced as soon as possible to keep newly exposed unsupported roof to a minimum. Proper safety procedures must be taken to control the roof when working on the pan line. Additional support may be required such as hinged face shields, or if the equipment does not have hinged face shields, hydraulic props may be used.
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Installation of additional roof supportis often necessary at the gate entry. Cribbing, roof bolts, or hydraulic props, such as these can be used. This extra support helps protect miners who have to work in this belt head area from fall of ground accidents. Accidents can happen during repair and maintenance activities. The equipment on long walls is massive and space is usually limited especially between the conveyor and shields.
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Workmen may be struck by hoses or machinery and they may bump into or be caught between the equipment.
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The danger of working with, and around, electrical equipment, especially in cramped underground quarters, is another hazard that workmen must be aware of. Recent accident statistics show that well over half of the injuries on long wall sections occurred to minors with less than two years experience at their regular jobs. Greater emphasis must be put on the miner's training before he starts his regular job to better prepare him to do his job, not only effectively and efficiently, but in the safest way possible. Safety improvements are continually being made in long wall mining equipment. Permissible fluorescent lights that mount on the shield supports have been developed to provide illumination for the entire length of the line. Improved communication systems have made it easier, more efficient, and safer for the long wall mining crew to operate, or work together. [worker] Stand clear of the face conveyor stand clear.
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The system can also be used as a warning device to let everyone in the long wall mining section know when the chain conveyor or shearer is going to start up. This is vitally important for the safety of the crew members.
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As our country's need for coal continues to grow, the trend to use long wall mining methods continues to find increasing application in our coal mining industry. Long wall mining offers the advantages of higher production and efficiency and the potential to provide our nation's miners with a safer work environment.
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I want to make one correction to the videos. Caterpillar or CAT as it is known, is a global manufacturer of excellent mining equipment. One important detail of their second video is not quite correct, however. All longwalls in the U.S. use shearers, not plows, including the longwalls operating in thin seams. The reasons for this are not important here, but please remember that shearers, not plows are applied through the U.S coal fields. In MNG 410, we take a more detailed look at longwall mines, including the conditions that favor the use of the plow.
I imagine that you have a reasonable understanding of modern longwall mining after studying the figures and the videos. Allow me to summarize the process and add a few additional details of interest.
Development of the panels is done with the room and pillar method. It generally will take at least two continuous mining sections to develop the longwall panels. From a mine planning perspective, the goal is to ensure that you have panels developed and ready to go. Often, the mine will have spare longwall equipment, and they will partially set up a longwall face on the next panel, so that minimal time will be lost in moving to the next panel and commencing with longwall production. This requires that panel development activities stay far enough ahead of the longwalls to ensure that the next panel can be set up before mining of the previous panel is completed. However, it is important that panel development does not get too far ahead, which would result in developed panels sitting idle for several months. This would represent a poor use of resources, but more importantly, the roof rock often begins to deteriorate when exposed to the moist mine atmosphere. This could result in roof problems before you have begun to mine in those panels.
Services to the longwall face will be placed in the headgate entries. This includes the panel belt, the staged loader and crusher, the hydraulic pumps for the shields, the electrical power centers, the computer control boxes, and the refuge chamber.
The tailgate is under additional roof stresses, and requires additional roof support, such as concrete pillars, timbered crib structures, and so on.
The number of gate roads is typically three, which is a legal requirement in the U.S. Under severe ground pressures, fewer gate entries are advantageous. In the western U.S., a few mines have received special permission to have only two gate roads. Outside of the U.S., you will find single entry gate roads. A reduction in the number of gateroads does create safety hazards that must be managed.
Panel Development
- Develop the gate roads (headgate and tailgate).
- Connect the headgate and tailgate.
- Develop (mine) the setup rooms – the entries connecting the head and tail gate, are mined to specific dimensions and in a pattern to facilitate setting up the long face of equipment.
- Develop (mine) the recovery rooms – once the longwall has been mined, the longwall face of equipment must be moved to the next panel. Given the size of the equipment, additional openings are mined to facilitate the removal of the face equipment. Finish the setup of the equipment on the new panel.
Production Cycle
The basic production cycle is straightforward, as you no doubt saw in the videos. The cutter, whether a plow or shearer, mines along the width of the face. As the cutter moves, the shields advance forward. As they advance forward, the AFC is pushed to the face; and as such, it is ready for the next cut. A few details that are not apparent would include:
- Mining along the face can always be done in the same direction, e.g., headgate to tailgate, which is known as unidirectional cutting, or it can be done in both directions, known as bidirectional cutting.
- Beginning the cutting cycle at either gate is a multi-step process, requiring that the shearer cut into the gateroad, and then gradually cut into the face over a few hundred feet to reach its full cutting depth, returning back to the gate, and then commencing with a full web cut along the full face.
- The shields are lowered to move forward, and when in the new position, the canopy is extended. Usually, the shields will push into the roof momentarily to aid in the formation of a crack in the roof, which facilitates the caving action. The roof typically does not cave immediately after the shield advance, but once caving has begun, the roof will cave reasonably close behind the shields.
Closing Comments
Modern longwall mining systems represent the highest level of technology and engineering achievement in mining method design. The design of the individual components has pushed the envelope on electrical, mechanical, and hydraulic component design. The productivity of these systems is unrivaled, and the raw tonnages achievable per shift are staggering compared to what was state-of-the-art 20 years ago. As cutting technology advances to allow continuous cutting of harder materials, you will see these systems applied in other commodities… but only if what is true?
This brings us to the end of our discussion of the supported and caving classes of underground mining methods.