Coal Preparation

The purpose of coal preparation is to improve the quality of the coal to make it suitable for a specific purpose by [20,21]:

• Conventional cleaning;

Run-of-mine (ROM) coal generally falls into two major groups: that from underground mining and that from strip mining. Underground mining tends to produce a finer product than strip mining; however, both products are further crushed to produce the desired size for a coal cleaning process or directly for utilization (e.g., combustion or gasification) [20]. In addition, increased mechanization in the underground mining industry has decreased selectivity and increased the volume of refuse [21]. Equipment such as continuous miners or longwall shearers often takes roof and floor rock in addition to the coal. Also, equipment currently used to mine and transport coal produces more fine coal particles than did earlier equipment [21].

End-use facilities such as power plants are designed for optimal combustion to burn a coal with a specific composition of ash, sulfur, energy, and, sometimes, volatile matter content. These requirements are becoming more difficult to meet with ROM coal or coal from one specific source because of several factors: coal variability (e.g., ash and sulfur contents) within the coal seam, considerable variation in underground mined coal quality due to the inclusion of roof and floor layers, the need for reduced sulfur content due to limitations on sulfur dioxide emissions from power plants, and the declining quality of coal being mined in the eastern United States as higher quality reserves have been depleted. Consequently, techniques have been implemented to upgrade the coal quality, and the need for advanced coal cleaning processes for both coarse coal and fine coal has grown [21].

In the past, coal was predominantly cleaned by dry methods, which have been abandoned in recent years in favor of wet cleaning processes due to several factors: particle size requirements (grinding the coal to finer sizes liberates more ash from the coal); dust emissions (which can be controlled by the use of water in underground mines); transportation issues; health, safety, and noise impacts; and the better performance of wet processes for coal cleaning [21]. A coal preparation plant separates the material it receives into a product stream and a reject stream, which may be further divided into coarse and fine refuse streams. Depending on the source, 20 to 50% of the material delivered to a coal preparation plant may be rejected [20]. One of the reject streams is a slurry, a blend of water, coal fines, silt, sand, and clay particles, which is most commonly disposed of in an impoundment [21].

The coal cleaning processes predominantly in use now include dense medium separation, hydraulic separation, froth flotation, and agglomeration [20]. Dense medium separations include those coal preparation processes that clean raw coal, coarse or fine, by immersing it in a fluid having a density intermediate between clean coal and the rejects. Because there is a general correlation between ash content and specific gravity, it is possible to achieve the required degree of removal of impurities from raw coal by regulating the specific gravity of the separating fluid, which can be organic liquids, dissolved salts in water, aerated solids, or suspensions of fine solids suspended in water [20]. Hydraulic separation jigging is a process of coarse particle stratification in which the particle rearrangement results from an alternative expansion and compaction of a bed of particles by pulsating flow. The rearrangement results in layers of particles that are arranged by increasing density from top to bottom of the bed, with the coal near the top. Hydraulic concentration of fine coal is performed using wet concentrating tables, cyclones, launders, feldspar jigs, and hydrorotators. These processes depend on the physical characteristics—size, shape, and density—of particles suspended in a liquid medium to effect a concentration of desired quality [20]. Froth flotation is a chemical process that depends on the selective adhesion of some solids (i.e., fine coal) to air and the simultaneous adhesion of other particles (i.e., refuse) to water. A separation of coal from coal waste then occurs as finely disseminated air bubbles are passed through a feed coal slurry. Agglomeration works on a similar principal as froth flotation (i.e., differences in the surface properties of coal and inorganic matter), and fine coal particles in a suspension can be readily be agglomerated by the addition of a bridging liquid (many different oils), under agitation, and then recovered while the inorganic constituents remain in the aqueous suspension and are rejected.

These processes, briefly described here, impact the environment in several ways, including making it necessary to deal with the contamination aspects of fine coal cleaning and "blackwater" disposal, air contamination, refuse disposal and control, and operator health and safety.

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