Coal is a solid that, if we are to use it, must be extracted from the earth. This is coal mining, going into the earth to remove coal for our consumption. The basic steps of mining and processing coal are described below.
There are two general methods of coal mining: surface mining and underground mining.
Generally called surface mining, the industry also calls it "opencast" or "open cut mining," while others may refer to it as "strip mining." In this type of mining, workers use explosives and heavy earth moving equipment, such as power shovels and draglines, to break up and scoop off the layers of soil and rock (overburden) covering the coal seam. Once exposed, the coal seam is systematically mined in strips. It is broken up using drills and explosives, and then smaller shovels lift the coal from the ground and load it into trucks or onto conveyors for transport to a coal preparation plant or directly to where it will be used.
Mountaintop removal is a variant of strip mining technology commonly used in West Virginia and eastern Kentucky where local topography provides adjacent valleys which can be used as repositories for overburden. In this type of mining, bulldozers are used first to remove all topsoil and vegetation from the mountaintop. Then explosives are used to break up the bedrock above the coal. Huge draglines (the bucket can hold 15-20 pickup trucks) then remove the overburden and dump the waste rock ("spoil") into the adjacent valleys. Then the coal seam is blasted and front end loaders scoop up the coal and load it into the huge dump trucks that carry the coal to the coal preparation plant. The video below provides a pretty dramatic birds-eye view of a mountaintop removal operation, including the overburden and the coal seams below it. It is also quite clear what the mountains used to look like, as evidenced by the scenery in the background. Please watch the following (3:18) video which shows the process described above.
Surface mining works only when the coal seam is near the surface. It is, however, usually more cost-effective than underground mining and requires fewer workers to produce the same quantity of coal. And the industry reports that 90% or more of the coal is recovered, a higher proportion than from underground mining. Recall also that from a previous lesson that the EROI of surface-mined coal is higher than underground mines.
The optional (6:43) video below from PBS provides a good sense of the scale of the largest mine in the U.S., the Black Thunder surface mine in the Powder River Basin in Wyoming.
When the coal deposit lies deep below the surface of the earth, underground mining is used. Miners dig tunnels deep into the earth near the place where the coal is located. The tunnels may be vertical, horizontal, or sloping. Once deep enough, the tunnels interconnect with a network of passageways going in many directions. Entries allow fresh air into the mine and give miners and equipment access to reach the ore and carry it out. The coal extraction is done by either a room-and-pillar method or longwall mining.
When the room-and-pillar method is used, miners cutting a network of 'rooms' into the coal seam and leaving behind 'pillars' of coal to support the roof of the mine. Working from the tunnel entrance to the edge of the mine property, they remove sections of the coal while leaving columns of coal in place to help support the ceiling. This process is then reversed, and the remainder of the ore is extracted, as the miners work their way back out.
In the case of longwall mining, the area being mined is covered with hydraulically-powered self-advancing roof supports that temporarily hold up the roof while the coal is extracted. After the coal is removed, the roof is allowed to collapse. This method requires careful planning and appropriate geological conditions. Carl Hoffman in Popular Mechanics offers this vivid description of longwall mining,
From an elevator-like entrance shaft deep underground, continuous miners—cutting machines on wheels—bore passages on both sides of seams of coal up to a quarter mile wide and a mile or more long. At the mine face, a massive shearer on self-advancing ceiling supports known as 'shields' slides back and forth across the face like a giant cheese grater. Water sprays constantly against the coal face to dampen coal dust. After each pass, the whole apparatus, as wide as 1600 feet, lurches forward, letting the area behind the shields collapse. A conveyor belt catches the coal, moves it to another belt running along the side passages, and takes it to the surface, often several miles away. When a panel of coal is mined out, the longwall machine is moved to the next one. Over time, mines become enormous labyrinths of passages, and it can take miners a half hour or more to travel miles to the mine face in low-slung vehicles called mantrips.
To Watch Now
Please watch the following (3:30) video:
Coal Preparation ("Washing")
Once the coal is removed from the mine, it is taken to a coal preparation plant where the raw "run-of-mine" coal is processed to separate the coal from undesirable waste rock and minerals. The finer waste from this process (including silt, dust, water, bits of coal, and clay) is discharged as a thick slurry into a man-made impoundment. This structure is used to confine refuse or slurry, along with any chemicals used to wash and treat the coal at the coal preparation plant. Coarser waste from the preparation process, rock, is dumped back into the pit once mining has ceased or is used in the construction of an impoundment dam.
To Watch Now
Please watch the following (5:43) video: