in the minerals. In summary, the bulk of the sulfur (organic and/or inorganic) present in coal has the potential to occur as gaseous combustion products.
Rank has been assumed to have an effect on the extent of hydrophobic character of coal (Speight, 2013). However, recent work on the prediction of coal hydrophobicity indicates that this property correlates better with the moisture content than with the carbon content and better with the moisture/carbon molar ratio than with the hydrogen/ carbon or oxygen/carbon atomic ratios (Labuschagne et al., 1988). Thus, it appears that there is a relationship between the hydrophobicity of coal and the moisture content. But there are differences in the behavior of coals and the differences are usually referred to as wettability, which can be quantified by measurement of the contact angle of the solid with water.
3.5.2 Methods
Methods employed for the removal of impurities from coal depend on the physical differences between the impurity and the coal; one such example is the difference in specific gravity.
Coal preparation plants generally use gravity process equipment to separate the refuse from the coal. Most of the extraneous impurities mined with coal are much heavier than the coal itself – coal has a specific gravity between 1.35 and 1.5, while the refuse rock has a specific gravity on the order of 2.1 to 2.3 – and separation can be effected by immersing the run-of-mine coal in a fluid having a specific gravity greater than that of the coal but less than that of the impurity (heavy media process). This allows the coal to float and the heavy waste material to sink and the two products are collected separately (Figure 3.2) (Couch, 1991).
Figure 3.2 A dense-medium separation method (Speight, 2013).
3.5.2.1 Dense Media Washing
The dense media washing process (heavy media washing process) is the most popular method of cleaning coarse sizes and jig plants are probably the second most common method used for coarse coal.
In a heavy-media washing plant, all the cleaning is done by flotation in a medium of selected specific gravity, maintained by a dispersion of finely ground magnetite in water. The incoming raw coal is separated at 1/4 inch on an inclined screen. The oversize material (overs) proceeds to a flat pre-wet screen, where the fine dust particles are sprayed off from the +1/4-inch coal. This increment is discharged into a heavy-medium vessel or bath, where the refuse is separated from the coal. The refuse is discharged to a “refuse rinse” screen, where it is dewatered. The use of magnetite has also been investigated in cyclone cleaning of coal (Klima et al., 1990).
The freed medium is divided into two parts, one returning directly to circulation via the heavy-medium sump and the other pumped to magnetite recovery. The refuse is discharged from the screen for disposal. The coal is discharged from the washer to a coal-rinse screen, where the coal is dewatered and the medium is treated as from the refuse screen. The clean coal is then centrifuged, crushed, and loaded. The fine coal (less than 1/4 inch) from the raw coal screens is combined with magnetite and water and pumped to a heavy-media vessel in that the magnetite is finer and the effective specific gravity is different.
The refuse is dewatered and the medium is recovered, as in the coarse coal selection. The coal is discharged over a sieve bend and then proceeds to a centrifuge for final dewatering prior to transfer to a thermal dryer or to loading.
Sand processes employ suspensions, often unstable, of sand in liquids whose effectiveness can be maintained only by high rates of agitation and recirculation. Specific gravity separations up to 1.90 are obtainable, but specific gravities of 1.45 to 1.60 are more commonly employed for bituminous coals.
The most successful commercial application of this process has been the Chance sand process (Figure 3.3) which is one of the most widely adopted heavy-medium cleaning processes and consists of a large, inverted conical vessel in which sand is maintained in suspension in an upward current of water. The density of the fluid can be varied by increasing or decreasing the amount of sand held in suspension.
Figure 3.3 The chance sand flotation process (Speight, 2013).
Some heavy-medium flotation plants use finely crushed barium sulfate or magnetite in suspension in water. In some processes, termed dry, or pneumatic, cleaning air is used as the separating medium. Because densities are additive properties, the specific gravity of a suspension may be calculated from the concentration of solids in unit weight of suspension and the true density of the solids are known. However, the effective specific gravity of a suspension is strongly dependent on the stability of the suspension, which is, in turn, dependent on the fineness of the suspended particles.
3.5.2.2 Pneumatic Cleaning
Pneumatic cleaning devices, or air tables, are applied to the small fractions (less than 3/8 inches). In these devices, currents of air flow upward through a perforated bottom plate over which a layer of coal passes. The extreme fines are entrapped in the air and must be recaptured by cyclones and bag filters for return without quality improvement. As the coal reaches the end of the tables, the bottom layer is heavy (high-ash) material, a center layer is medium-weight coal and bone (high-ash), and the top layer is coal (low-ash). The middle layer must be incorporated with the refuse (and rewashed) or with the coal.
The efficiency of these devices is poor. Their ability to remove ash is limited to 2 to 3%, regardless of how much is present. These devices represent the lowest capital investment of all cleaning devices, and they entail no problems of water supply and disposal.
The incoming coal must be screened, and, because feed to the tables must be dry, thermal drying of the raw feed is required at some plants. The thermal dryers, in turn, require cyclones and scrubbers for control of particulate emissions.
3.5.2.3 Jig-Table Washing
Jig-table washing plants are so named because jigs are used to clean the >0.25 inch increment and Diester tables (oscillating table-sized sluices with a flat, riffled surface, approximately 12 feet square, which oscillates perpendicular to the riffles, in the direction of the flow of coal) are used to clean the <0.25 inch increment. Froth cells and/or thermal dryers may be used in conjunction with this equipment.
The raw coal, restricted to sizes smaller than eight inches, is separated on a wet screen (usually 0.25 inch mesh). The large-sized increment goes into the jig; the remaining coal is sent to a separate cleaning circuit. The coal is dewatered on screens and in centrifuges, crushed to the desired size, and loaded. The jig makes the equivalent gravity separation on the principles of settling in rising and falling currents. The small-sized coal (less than 1/4 inch) is combined with the proper amount of water and distributed to the tables, where the refuse is separated from the coal. The refuse is dewatered on a screen and discarded. The cleaned coal is dewatered on a sieve bend (a stationary gravity screen), where the extreme fines are removed and discharged into a centrifuge for final dewatering and removal of the fines. The cleaned coal (+28 meshes) is then loaded or conveyed to a thermal dryer where the heavy rejects are discharged off one end of the discharge side of the table. The light coal is discharged from the opposite end, and the middlings are distributed between.
