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Size reduction (sometimes called pretreatment) is, simply, breaking, crushing, and screening of the run-of-mine coal in order to provide a uniform raw coal feed of predetermined top size thereby minimizing the production of material of ultrafine size by excessive crushing or handling.

Size reduction of coal plays a major role in enabling run-of-mine coal to be utilized to the fullest possible extent for power generation, production of coke, as well as other uses such as the production of synthetic fuels (Bevan, 1981). Run-of-mine coal is the as-received coal from the mine, whether the mining process is stripping, auger mining, continuous mining, short- or longwall mining, or any other method currently practiced (Speight, 2013 and references cited therein).

Most conventional coal cleaning facilities utilize gravity methods for the coarser size fractions and surface treatment methods for the finest particle sizes (Riley and Firth, 1993). The selection of equipment, especially for the finer sizes, depends on the mining method, coal hardness, and size distribution and amounts thereof.

The first operations performed on run-of-mine coal are removal of tramp iron and reduction of size to permit mechanical processing. The run-of-mine 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.

Most commercial circuits utilize dense media vessels of jigs for the coarsest size usually +3/8”, dense media cyclones, concentrating tables or jigs for the 3/8” x 28 mesh size, water-only cyclones, or spirals and sometimes flotation for the 28 x 100 mesh size and flotation for the –100 mesh.

Since the mining processes differ in operation and since size reduction actually begins at the coal face (i.e., during mining), the mined coal will exhibit different characteristics. In fact, the mining process has a direct bearing on the size and on the size consistency of the coal. Thus, prior to final utilization of the coal, some degree of size reduction, or size control, is usually required. The number of stages in the size reduction process depends upon the specific utilization of the coal as well as the condition of the coal.

For example, coal which is destined for power generation may undergo size reduction to produce a product with a top size of 4 inch (1 mm). On the other hand, the size of the coal needed for a coking operation is coarse and the number of stages of size reduction involved in preparing a coal feed for a coking is somewhat less than required to prepare coal as the feedstock for power generation utilization.

Coal is reduced in size by crushing and pulverizing. Pre-crushed coal can be economical for smaller units, especially those which are stoker fired. In a coal handling system, crushing is limited to a top size of 6 or 4 mm. The devices most commonly used for crushing are the rotary breaker, the roll crusher, and the hammer mill.

It is necessary to screen the coal before crushing, so that only oversized coal is fed to the crusher. This helps to reduce power consumption in the crusher. Recommended practices in coal crushing are (i) incorporation of a screen to separate fines and small particles to avoid extra fine generation in crushing, and (ii) incorporation of a magnetic separator to separate iron pieces in coal, which may damage the crusher.

The fines in coal produced during the crushing (sizing) operation can present problems in combustion on account of segregation effects. Segregation of fines from larger coal pieces can be reduced to a great extent by conditioning coal with water. Water helps fine particles to stick to the bigger lumps due to surface tension of the moisture, thus stopping fines from falling through grate bars or being carried away by the furnace draft. While tempering the coal, care should be taken to ensure that moisture addition is uniform and preferably done in a moving or falling stream of coal. If the percentage of fines in the coal is high, wetting of coal can decrease the percentage of unburned carbon and the excess air level required to be supplied for combustion. In cases where the sized coal has an excessive amount of fine coal, blending with predominantly lump coal (depending upon the coal-fired system) may be an option. This may thus help to limit the extent of fines in coal being fired to not more than a specified maximum. In fact, blending of coals of different quality may also help to supply a uniform coal feed to the boiler (Chapters 7, 8).

The size of a coal particle is defined in terms of a surface opening through which the particle will barely pass or will not pass at all. In sizing a material, the individual particles are presented to the surface openings numerous times. However, the size of a particle mut be recognized because a three-dimensional particle is presented to a two-dimensional opening. In the case of a narrow, elongated particle, the particle will be sized according to the orientation of the particle with respect to the surface opening (ASTM D431) defines the top size or upper limit of a quantity of material as the smallest sieve (smallest opening) upon which is retained a total of less than 5% of the sample. In practice, the term nominal top size is used extensively when describing the output range of a size reduction device. The nominal top size of a quantity of material is the smallest sieve (smallest opening) upon which is retained a total of 5 to 20% of the sample.

Reduction ratio is the ratio between the feed top size and the product top size, or the ratio between the feed and product sizes at which a specific percentage of the material passes. For size reduction units that produce a product top size larger than 1 mm, size reduction ratios are normally of the order of 8-to-1 while for units where the product top size is smaller than 1 mm, size reduction ratios can range from 200-to-1 and higher.

The top size upper limit of the crushed (sized) material is defined by the smallest sieve opening upon which is retained a total of less than 5% of the sample (ASTM D431). The term nominal top size is used to describe the product of a size reduction operation and is the smallest sieve opening upon which is retained a total of 5 to 20% of the sample. The reduction ratio is the ratio between the feed top size and the product top size or the ratio between the feed and product sizes at which a determined percentage of the material passes. The relative ease with which a coal can be comminuted (reduced in top size) by mechanical action affects the design of a size reduction unit or operation. The term used to refer to this relative ease is the grind ability index of the coal.

The specific energy for size reduction is proportional to the grindability index of the coal (Hardgrove grindability index, ASTM D409) and is also a function of the reduction ratio. Thus, the energy required for the size reduction of coal increases with increasing throughput as well as with the reduction ratio. For a “standard” throughput of coal, the energy required varies with the reduction ratio and the relationship between the specific energy and the grindability is dependent upon the type of device (Speight, 2013).

Moisture in coal is not just an issue in terms of whether the coal is dry or not and whether the transportation costs warrant coal drying. Indeed, moisture content is a factor that must be taken into account when considering the energy requirements of a size reduction unit. Excessive moisture, but more particularly excessive surface moisture, can cause a lesser efficiency in fines removal (due to fines agglomeration) and the formation of emulsions can also be a problem in the selective agglomeration process (Bensley et al., 1977).

In general, five types of equipment are used for the size reduction of run-of-mine coal: (i) rotary breakers, (ii) roll crushers, (iii) hammer mills, also called ring mills, (iv) impactors, and (v) tumblers. At best, the crushing operation produces the desired (sized) product in a single stage. However, there are cases where the size reduction of coal entails multiple stage size reduction units. The final stage of coal pretreatment is to screen it into size fractions convenient for handling by the various process streams.