Mining methods and terminology

a graphic illustrates the underground mining methods and surface mining methods

Underground Mining
Underground modes of access include drift, slope, and shaft mining, and actual mining methods include longwall and room and pillar mining. Drift mines enter horizontally into the side of a hill and mine the coal within the hill. Slope mines usually begin in a valley bottom, and a tunnel slopes down to the coal to be mined. Shaft mines are the deepest mines; a vertical shaft with an elevator is made from the surface down to the coal
In room and pillar mining, the most common type of underground coal mining, coal seams are mined by a "continuous miner" that cuts a network of "rooms" into the seam. As the rooms are cut, the continuous miner simultaneously loads the coal onto a shuttle or ram car where it will eventually be placed on a conveyor belt that will move it to the surface. "Pillars" composed of coal are left behind to support the roof of the mine. Each "room" alternates with a "pillar" of greater width for support. Using this mining method normally results in a reduction in recovery of as much as 60 percent because of coal being left in the ground as pillars. As mining continues, roof bolts are placed in the ceiling to avoid ceiling collapse. Under special circumstances, pillars may sometimes be removed or "pulled" toward the end of mining in a process called "retreat mining." Removing support during retreat mining can lead to roof falls, so the pillars are removed in the opposite direction from which the mine advanced: hence the term "retreat mining."

Longwall mining is another type of underground mining. Mechanized shearers are used to cut and remove the coal at the face of the mine. After the coal is removed, it drops onto a chain conveyor, which moves it to a second conveyor that will ultimately take the coal to the surface. Temporary hydraulic-powered roof supports hold up the roof as the extraction process proceeds. This method of mining has proven to be more efficient than room and pillar mining, with a recovery rate of nearly 75 percent, but the equipment is more expensive than conventional room and pillar equipment, and cannot be used in all geological circumstances. As mining continues, roof bolts are placed in the ceiling to avoid ceiling collapse. In longwall mining, only the main tunnels are bolted. Most of the longwall panel is allowed to collapse behind the shields (which hold the roof as coal is excavated).

Surface or Opencast Mining
Surface-mining methods include area, contour, mountaintop removal, and auger mining. Area mines are surface mines that remove shallow coal over a broad area where the land is fairly flat. Huge dragline shovels commonly remove rocks overlying the coal (called overburden). After the coal has been removed, the rock is placed back into the pit. Contour mines are surface mines that mine coal in steep, hilly, or mountainous terrain. A wedge of overburden is removed along the coal outcrop on the side of a hill, forming a bench at the level of the coal. After the coal is removed, the overburden is placed back on the bench to return the hill to its natural slope. Mountaintop removal mines are special area mines used where several thick coal seams occur near the top of a mountain. Large quantities of overburden are removed from the top of the mountains, and this material is used to fill in valleys next to the mine. Augur mines are operated on surface-mine benches (before they are covered up); the coal in the side of the hill that can't be reached by contour mining is drilled (or augured) out

Variables Influencing Mining Methods
Variables Influencing Mining Methods
When selecting a mining method, it is important to maintain flexibility within the method, in case something goes wrong. Any mining layout must establish the following facilities. Creation of the above facilities is dependent on the following variables:
Physical Properties of the mined rocks and ore. The stress/strength relationships of rocks and ore determine the horizontal and vertical stope dimensions which will stand without support. This in turn establishes the appropriateness of one of the three basic stoping methods ...
  • Naturally supported
  • Artificially supported
  • Unsupported (caving)
Orebody Dimensions - The strike length, width (footwall to hanging wall) and height of the orebody determine the maximum spans that are possible, which in some cases will exceed the permissible spans. For greater orebody widths, longitudinal stopes will give way to transverse stopes. Longitudinal stopes constitute mining along strike and transverse stopes are perpendicular to strike. Stope and pillar dimensions must be established.
Orebody Orientation - the dip of the orebody determines the part gravity will play in the mining operation. In this course, the following orebody classifications are used ...
  • Flat Dip: 0-20o
  • Medium Dip: 20-50o
  • Steep Dip: 50-90o
Ore Grade - The grade of the orebody influences considerations of selective or mass production methods and thus the need and method for pillar recovery.
Structural Features - Major faults, dykes, and rock mass jointing influence the location and size of stopes and pillars.
Depth of Operation - This is a factor in so far as stress increases with depth ... in addition, hoisting heights, and hence costs, also increase.
Proximity of Other Orebodies - Orebody proximity determines the method of mining one orebody or lens with respect to an adjacent ore block. It also determines extraction sequences, location of access, pillar sizes, etc., and whether mining progresses from hanging wall to footwall or footwall to hanging wall.
Ground Surface Effects ... Considerations with respect to restriction of subsidence, e.g. where there are lakes, towns, etc. above; the requirements of land reclamation and waste and tailings disposal; and the control of effluents in surface water discharge may all impact on the choice of mining method. The climate and topography must also be considered.
Labour and Supplies - Cheap, locally available, labour does not necessarily mean low costs. Instead, mining methods must generally be modified for different parts of the world in accordance with the competency of available labour. The presence of skilled labour but in short supply influences the choice of methods and equipment. Local availability of supporting materials such as timber, fill, tailings, cement, etc. also influence mining practice.
Capital Available ... Costs in mining are of two types, capital and operating. If the capital for certain desirable expenditures is not available, operating costs are likely to be higher. For example, the open stoping method in some cases ties up less capital than shrinkage stoping. Some methods at a higher operating cost may require less development expenditure. In the overall picture retreating methods may be more economical than advancing methods. In each case a capital outlay might be recoverable through lower operating costs when sufficient tonnage is in sight. The judicious outlay of capital is always desirable. The difficulty arises in defining judicious in terms of developed and prospective ore at various stages in the life of a mine.
Physical Location - The location and jurisdiction of the orebody (political, provincial, federal, foreign) must be taken into account as well as ownership (park, reservation, private, common property) or boundary with another mining operation.

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