Coal Mining: Basic Overview

Coal Mining: Basic Overview

This article relates to The Secret Wisdom of the Earth

According to the World Coal Association, the global annual haul for hard coal is over 6000 million tons, with the top five producers being China, the United States, India, Australia and South Africa.

Coal mining is usually broken up into two categories: Surface (also known as opencast) and underground. The latter currently accounts for a greater percentage of the world's coal production, but its popularity depends on the country; in the United States, for example, surface mining is the method more widely used.

The choice of method largely depends on the geology of the coal deposit. Surface mining, used when when the ore is near the surface, can recover nearly 90% of the coal in a deposit. In this method, the strata covering the coal (known as the overburden) is broken up by explosives and hauled away. Once the seam is exposed, it's drilled and fractured so that the coal ore can be loaded on to large trucks or conveyor belts for transportation.

Surface mining can be subdivided into several types:

• Strip mining
  Used for large deposits of coal very near the surface. The deposits are extracted in long parallel strips (hence the name), with the overburden being used to fill in the previously dug strip. Mines that are located on mostly flat terrain use a technique called area stripping, while on hillier ground contour stripping is employed, where the cuts follow the contour of the ground.
• Auger mining
  Following contour stripping of a hillside, an auger (drill) is often used to dig a horizontal or near-horizontal hole into the coal bed in order to recover more of the coal deposit.
• Open pit mining
  Just as it sounds, an enormous pit is dug into the ground from which the coal is excavated.
• Mountaintop removal
  The top of a hill or mountain is exploded to reach the coal and the overburden is used to fill in nearby valleys. This is perhaps the most controversial surface mining technique, as it alters the topology of the area dramatically.

Underground mining is the type of operation most think of when the subject of mining is broached. Heavy equipment drills holes in the ground to reach deep deposits and laborers work underground to extract the coal deposit. Most of the heavy work these days is done by machines.

There are two general types of underground mining techniques:

• Room and Pillar
  A network of rooms is cut into the coal seam, leaving behind rock pillars to support the dig. This technique, often aided by large machinery such as a "continuous miner," (shown in picture) can recover only up to 40-60% of the coal deposit. Once the maximum deposits have been extracted, the pillars are removed in a process known as retreat mining - collapsing the mine as each pillar is removed. This is a very dangerous process which, according to the Mine Safety and Health Administration, historically accounts for 10% of the industry's production but 25% of American coal mining deaths caused by roof and wall failures.
• Longwall Mining
  Full extraction of a section of the seam using mechanical shearers that move back and forth across a panel of coal. Hydraulic roof supports known as shields are used to temporarily hold up the roof while the cut coal falls onto a conveyor belt for removal, after which the roof is allowed to collapse behind the dig using retreat mining. Up to 75% of the deposit can be removed using this method.

Both these types can be established by using either a drift (which is a tunnel dug horizontally); a slope (dug from the surface on an angle down to the deposit); or a shaft (drilled vertically and employing an elevator to reach the lowest levels).

In some countries including the United States, coal mining companies are required to restore areas they've disturbed during the mining operation. These lands are replanted and re-purposed for cropland, wildlife habitat, industrial use or recreation.

Picture of Wyoming surface mine by Focal Point
Picture of Continuous Miner from Wikipedia

Filed under Nature and the Environment

This "beyond the book article" relates to The Secret Wisdom of the Earth. It originally ran in July 2015 and has been updated for the January 2016 paperback edition. Go to magazine.