EGEE 101
Energy and the Environment

Underground Mining

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 Dr. Mathews in a miners outfit

The picture to the left is a dirty, smelly Dr. Mathews as happy as a pig in swill! It was taken three decades ago when I was just starting my Ph.D. studies here at Penn State. Thinner and braver in those days, I would venture into mines looking for coalified trees. Six years later I had a doctorate in Fuel Science - "Following the changes in the constitution of rapidly heated bituminous vitrinites." Ahhh, the good old days!

Mining has always been hard work, but without the availability of modern machinery, lots of manpower was necessary.

For an in-depth and very interesting look into the jobs of miners, and the methods they use to do their jobs, check out this United Mine Workers Association link.

2 drawings. One with a vertical entrance from the ground into the earth and through the side of a mountain.
For underground coal mining in some cases they are accessing from above via a shaft, in other cases, you can enter via the side of a mountain to chase the seam(s). Once underground, there are two underground mining approaches: room and pillar mining, and longwall mining.

Room and Pillar Mining

The following video from Catapillar Mining provides a good overview of room and pillar mining.

Click here for a transcript of the Principles of Room & Pillar Mining video.

In room and pillar mining, the coal seam is mined in a checkerboard style, leaving pillars of coal to support the roof, which allows for instant coal access with a relatively low invest compared to longwall mining, however, only utilizes the coal reserves between 50% and 75%. It is a mining method of its own right, as well as a supporting technology to develop roadways in order to prepare the coal face for a long wall operation.

In room and pillar mining, continuity is the key to profit, from the continuous miner to the continuous flow of material. The coal is cut by a continuous miner, which delivers the product to haulers. They bring the product to feeder breaker units that prepare and deliver it under the belt system.

While feeder breakers are moved only occasionally, the other equipment is in constant motion, so maneuverability, cableless operation, and maximum load capacities are vital. With a full range of battery or diesel powered vehicles, Caterpillar has an answer to every challenge in room and pillar mining. It all starts at the coalface with the right cutting technology.

Today's continuous miners are designed to cut at highest efficiency while keeping dust levels to a minimum with water sprays and dust collectors. They are available for operations from as low as 70 centimeters up to a maximum of 5 meters. From production to the delivery point, it's just a question of volume and velocity.

That's easy physics for our range of utility vehicles. Being equipped with rubber tires and an industry leading capacity, they keep the circulation of product and material at a healthy and profitable level. The roof bolter follows production on its tail to create a safe mining environment. Driving the bolts into the roof in a safe and efficient way is most important in this real hands-on job.

Therefore, ergonomic controls and easy material access are one of the most important features in our roof bolters. The Scoops Multipurpose Contoured Bucket will carry equipment, serve as a multi-tool, or clean roadways and feeder sections. Extended battery life and a dual motor option make it versatile yet powerful. The scoop has often been called the miners Swiss army knife. As a matter of fact, it is a real workhorse too.

The image below provides a birds-eye view. The pillars are a bit large but you get a general idea. The continuous miner (the machine with the big drum and all of the teeth) extracts the coal, it is loaded directly or picked up by the coal hauler that takes the coal to the conveyor system for transport out of the mine. The roof bolter adds metal rods to the mine ceiling so the rocks don't fall (the roof bolts are needed for safety). Obviously, the pillars are coal and so we leave behind 30% or more of the coal.

diagram of a room pillar mine. It has square 'pillars' of coal with 'road' around them where the mining equipment works.
Room and pillar mining
Credit: Advanced Mining Solutions
coal on a conveyor belt headed out of the mine
Conveyor system carrying the coal out of the mine.

Mining anthracite is quite different from bituminous coal (where the seams are mostly horizontal). Anthracite seams can be at a 60° pitch so slightly different techniques are used either mining the seam from above or below. 

Longwall Mining

A longwall miner. Described  in text below.
A longwall miner
Credit: Consol

The productivity in underground coal mining has dramatically increased due to mechanization. Watch this video of a longwall miner that can run the length of a football (American) field or longer (3:25 minutes).

Click here for a transcript of the Principles of Longwall Mining video.

Whenever mining coal underground, longwall 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 longwall panel would typically be 3 to 4 kilometers long and 250 to 400 meters wide.

Four seam heights of up to 1.8 meters or 71 inches, the plow is the cutting tool of choice. It travels fast, with speeds of up to 3.6 meters per second along the coal face pulled by a plow chain that transmits a force of up to 2 times 800 kilowatts. The plow is cutting coal at predefined depth up to 25 centimeters or 10 inches.

In seams from 2.3 meters to 6 meters and above, the shear has proven to be the most efficient cutting technology. It travels typically with a speed of 16 meters per minute, while normally cutting 100 centimeters with its powerful cutting drums, generating a production capacity of 5,000 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 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 and abrasion, that the AFC is called the backbone of the longwall.

Roof supports are vital, not only for a constant production and advance of the longwall 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 a seam heights from 0.8 meters to 7.5 meters.

Our roof supports have been tested to advance 60 kilometers and more underground before a complete overhaul is necessary.

Integrated automation has become unrenouceable in longwall mining. Constant optimization of the entire longwall system, control of thousands of kilowatts of power in motors and drives, as well as a few 100,000 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 longwall system, achieving maximum productivity and availability. Automation systems also keep the miners away from hazardous areas, increasing safety standards even further.

What is the role of the shearer, roof supports, face conveyor, and gob? The machinery behind the operator holds up the roof and then the whole device walks forward leaving the roof to fall safely behind the work area. Thus, the extraction is much higher and more of the coal is removed by this technique. Safety has improved in the mines but one of the reasons for lower loss of life is the reduction in the number of miners because of productivity enhancements and fewer miners.

Mine Safety

Roof falls are a major danger in coal mines. The roof is bolted together with meter-long bolts but the roof has to be checked anyway. How the roof "sounds" is a good indication of the stability. If you happen to have rats handy, and they leave the mine, it is a good idea to follow, as their hearing can often pick up the sound of the roof straining when it's not audible to humans.

Black Lung

The small coal dust particles (and silica particles) enter the lungs and stay there. Long-term exposure produces a debilitating disease and often premature death. In the early years of coal mining, a miner was an old man at age 44 and the life span was not expected to be much longer.

Photograph of a healthy lung in comparison to one showing black lung (discolored, and shrunken) from the National Institute of Occupational Health and Safety
Here is the comparison of a healthy lung and a black lung from a coal miner. 
Credit: National Institute of Occupational Health and Safety
coal dust on a white background
Coal dust

One of the major dangers to coal miners had nothing to do with physical injury (although that is highly possible, especially in the "good old days"), but rather with lung damage. The medical community calls the infliction pneumoconiosis but the miners know it as black lung. All coal mining methods will produce some dust but underground mining methods produce more and the dust (small coal particles - smaller than 100 microns) in an enclosed space. 

Many miners still do not wear masks to filter out some of the particles but the technology is much better at controlling dust, water sprays help to reduce the problem as well as air handling systems. Fewer miners in the mine also help to reduce exposure, as does the remote control machinery that allows the miner to operate the continuous miner of a roof bolter from several meters away. Historically, this was a massive problem, resulting in the premature death of perhaps 100,000 miners. Modern miners can still "get" black lung disease and it is increasing in some locations.

More information if you're interested in learning more about these hazards is available from the United Mine Workers Association's Black Lung webpage.