The rationale for selecting a method for mining coal depends on such factors as topography, geometry of the coal seam, geology of the overlying rocks and environmental requirements or restraints. Overriding these, however, are the economic factors. They include: availability, quality and costs of the required work force (including the availability of trained supervisors and managers); adequacy of housing, feeding and recreational facilities for the workers (especially when the mine is located at a distance from a local community); availability of the necessary equipment and machinery and of workers trained to operate it; availability and costs of transportation for workers, necessary supplies, and for getting the coal to the user or purchaser; availability and the cost of the necessary capital to finance the operation (in local currency); and the market for the particular type of coal to be extracted (i.e., the price at which it may be sold). A major factor is the stripping ratio, that is, the amount of overburden material to be removed in proportion to the amount of coal that can be extracted; as this increases, the cost of mining becomes less attractive. An important factor, especially in surface mining, that, unfortunately, is often overlooked in the equation, is the cost of restoring the terrain and the environment when the mining operation is closed down.
Health and Safety
Another critical factor is the cost of protecting the health and safety of the miners. Unfortunately, particularly in small-scale operations, instead of being weighed in deciding whether or how the coal should be extracted, the necessary protective measures are often ignored or short-changed.
Actually, although there are always unsuspected hazards—they may come from the elements rather than the mining operations—any mining operation can be safe providing there is a commitment from all parties to a safe operation.
Surface Coal Mines
Surface mining of coal is performed by a variety of methods depending on the topography, the area in which the mining is being undertaken and environmental factors. All methods involve the removal of overburden material to allow for the extraction of the coal. While generally safer than underground mining, surface operations do have some specific hazards that must be addressed. Prominent among these is the use of heavy equipment which, in addition to accidents, may involve exposure to exhaust fumes, noise and contact with fuel, lubricants and solvents. Climatic conditions, such as heavy rain, snow and ice, poor visibility and excessive heat or cold may compound these hazards. When blasting is required to break up rock formations, special precautions in the storage, handling and use of explosives are required.
Surface operations require the use of huge waste dumps to store overburden products. Appropriate controls must be implemented to prevent dump failure and to protect the employees, the general public and the environment.
There is also a variety of methods for underground mining. Their common denominator is the creation of tunnels from the surface to the coal seam and the use of machines and/or explosives to extract the coal. In addition to the high frequency of accidents—coal mining ranks high on the list of hazardous workplaces wherever statistics are maintained—the potential for a major incident involving multiple loss of life is always present in underground operations. Two primary causes of such catastrophes are cave-ins due to faulty engineering of the tunnels and explosion and fire due to the accumulation of methane and/or flammable levels of airborne coal dust.
Methane is highly explosive in concentrations of 5 to 15% and has been the cause of numerous mining disasters. It is best controlled by providing adequate air flow to dilute the gas to a level that is below its explosive range and to exhaust it quickly from the workings. Methane levels must be continuously monitored and rules established to close down operations when its concentration reaches 1 to 1.5% and to evacuate the mine promptly if it reaches levels of 2 to 2.5%.
In addition to causing black lung disease (anthracosis) if inhaled by miners, coal dust is explosive when fine dust is mixed with air and ignited. Airborne coal dust can be controlled by water sprays and exhaust ventilation. It can be collected by filtering recirculating air or it can be neutralized by the addition of stone dust in sufficient quantities to render the coal dust/air mixture inert.