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Coal preparation capabilities and coal sizing

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Coal preparation capabilities and coal sizing

July 7, 2023 nflg 0 Comments

Coal Preparation Process Description

As it leaves the mine, coal varies widely in size, ash content, moisture content, and sulfur content. These are the characteristics that can be controlled by preparation. Sizes range upward to that of foreign materials, such as a chunk of rock that has fallen from the mine roof or a metal tie; large pieces of coal from a very hard seam are sometimes included.

Ash content ranges from three to sixty percent at different mines. Most of the ash is introduced for the roof or bottom of the mine or from partings (small seams of slate) in the coal seam. This ash, called extraneous ash, is heavier than 1.80 specific gravity. The remaining ash is inherent in the coal. The density of coal increases with the amount of ash present.

The moisture content of the coal is also of two types. The surface moisture, that which was introduced after the coal was broken loose from the seam, is the easier to remove. This moisture is introduced by exposure to air, wet mining conditions, rainfall (in stockpiles), and water sprays. The remaining moisture, called “bed”, “cellular”, or “inherent” moisture, can be removed only by coking or combustion. This moisture was included during formation of the coal.

Foreign materials are introduced into the coal during the mining process, the most common being roof bolts, ties, car wheels, timber, shot wires, and cutting bits. Sulfur in coal occurs as sulfates, organic sulfur, and pyrites (sulfides of iron). The sulfates usually are present in small quantities and are not considered a problem. Organic sulfur is bound molecularly into the coal and is not removable by typical coal preparation processes.

Pyrites generally are present in the form of modules or may be more intimately mixed with the coal. Coal preparation plants remove only a portion of the pyritic sulfur; therefore the degree of sulfur reduction depends on the percentage of pyrites in the coal, the degree to which this is intimately mixed with the coal, and the extent of coal preparation. All of the materials described above are combined with the coal to form the ROM feed. Coal, as referred to above, denotes the portion of the feed that is desired for utilization.

Capabilities of Coal Preparation

Coal preparation processes can improve the ROM coal to meet market demands, as limited by the inherent characteristics of a given coal. The top size of the ROM can be reduced to any size specified, although control of the varying size increments can be poor, depending on the amount of crushing required. No practical technology is known for increasing the sizes of coal as mined.

Although inherent moisture cannot be changed, the surface moisture can be reduced to any level that is economically practicable. Considerations include the possibility of reexposure to moisture during shipment and subsequent storage and the fact that intense thermal drying increases ideal conditions for readsorption of moisture.

The free sulfur in the coal is subject to removal only by chemical treatment, which is not a coal preparation process, or by combustion. The reason that the pyrites can be partially removed in washing processes is that they are heavy enough to be removed with the ash. The processes can remove only 30 to 60 percent of the pyrites, however, because some pyrites are not broken free of the coal and are present in a given piece in a quantity too small to increase its weight enough to be rejected. Foreign metals can be removed easily. Most wood fragments can be removed, although a few small pieces of wood cause no particular harm because they are combustible.

Application of Cleaning Processes to Size Increments

Different types of mechanical cleaning apparatus are required for cleaning of coals in different size ranges. Coal larger than eight inches is usually crushed to a smaller size; when lump coal is required, the large fraction is cleaned by slate pickers.

Coal Sizing

The ROM coal is first exposed to a high-intensity magnet, usually suspended over the incoming belt conveyor which pulls the iron impurities out of the coal. This magnet sometimes follows the breaker, but always precedes a screen -crusher.

The coal then goes to the breaker, which is a large cylindrical shell with interior lifting blades; the shell is perforated with holes (two to eight inches in diameter) to permit passage of small material. The breaker rotates on a horizontal axis, receiving material in one end, tumbling it as it passes through the holes in the shell, and permitting the hard, large, unbroken material to pass out the rear of the machine. The small material (four inches) goes to the cleaning plant, and the large rejected material falls into a bin to be hauled away.

Various types of crushers are available for coal crushing. The hammermill, shown in Figure 3-2, and the rotary breaker, shown in Figure 3-3, are most commonly used.

An alternate flow directs the ROM coal to a scalping screen, from which the oversize material (+ four inches) falls to a crusher, where it is reduced to four inches and is recombined with the screen underflow for transportation to the cleaning plant. This system is used more than the breaker but is somewhat vulnerable to large pieces which pass through the crusher and must be removed in a later process. The crusher most commonly used for this purpose is a heavy-duty single roll with tramp iron protection.

Double rolls are more difficult to maintain in this heavy service, are more expensive, and offer no particular advantage. Slow-speed hammermills or impactors are more difficult to maintain, and jaw crushers have not been required.

The raw coal is sometimes stored, prior to washing, to allow optimum scheduling of mine and plant operations. Open storage is the most common; silos are also used.

At mines using unit train shipment, prepared coal is stored to accumulate enough to fill a train. For this purpose, silos are used most often to prevent accumulation of moisture and exposure to wind. Some open storage is also practiced. At other mines, cars or barges are loaded directly as the coal is processed, received, and shipped each day.

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