One of the oldest forms of coarse coal cleaning is jigging. Jigging is the process of particle stratification due to alternate expansion and compaction of a bed of particles by a vertical pulsating fluid flow. A single jigging vessel can treat large particles (up to 200 mm) at capacities approaching 680 tonnes (750 tons) per hour. Common versions include the baum and batac designs. The baum jig, which is most common, uses a
U-shaped vessel that is open on one size and closed on the other (Fig. 9). Water is pulsed through the particle bed atop the perforated plate by sequentially injecting and releasing air in and out of the enclosed chamber. A batac jig is a variation of this design that uses an air chamber below the perforated plant to trap air and create the pulsing action. Jig installations require specific conditions in order to be effective and are significantly less efficient than competitive heavy media processes. As a result, the use of jigs for coal cleaning has declined rapidly. However, jigs can treat a wider size range than single heavy media processes and provide an obvious advantage in applications where capital is limited.
The most common process for upgrading coarse coal (> 12.5 mm) is the heavy media bath. A common type of heavy media bath is the Daniels vessel (Fig. 10). This unit consists of a large open vessel through which an aqueous suspension of finely pulverized magnetite (SG = 5.2) is circulated. The magnetite creates a suspension with an artificial density that is between that of pure coal and pure rock. Lighter coal particles introduced
into media float to the surface of the vessel where they are transported by the overflowing media to a collection screen. Waste rock, which is much denser, sinks to the bottom of the vessel where it is collected by a series of mechanical scrapers (called flights). The flights travel across the bottom of the vessel and eventually drag the waste rock over a discharge lip at one end of the separator. The unit is highly flexible, since the quality of the clean coal product can be readily adjusted by varying the density of the media from 1.3 to 1.7 SG by controlling the amount of magnetite introduced into the suspension. Sharp separations are possible even in the presence of large amounts of middlings, since SG can be controlled very precisely (as close as 0.005 SG units in some cases) using online instrumentation. A wide variety of different designs of heavy media baths exist, with the major differences being the mechanisms used to discharge the clean coal and waste rock products. Common examples include heavy media cones, drums, and several variations of deep and shallow baths. Drums, usually Wemco or Teska, are widely
used in South Africa and to a certain extent in Australia. This is where coals are difficult to wash and very high efficiency levels are required.
The primary disadvantage of a heavy media process is the ancillary equipment that must be installed to support the operation of the separator (Fig. 11). A typical circuit requires that the clean coal and refuse products pass over drain-and-rinse screens to wash the media from the surfaces of the products and to dewater the particles. The media that is recovered from the drain section is circulated back to the separator by means of a pump, while media from the rinse section is diluted by spray water and must be concentrated before being fed back to the separator. Fortunately, magnetite is magnetic and can be readily recovered from a dilute suspension using magnetic separators. A rotating drum magnetic separator is commonly used for this purpose (Fig. 12). These devices are highly efficient and generally recover >99.9% of the magnetite to the magnetic product. For cost reasons, losses of magnetite must be kept as low as possible. Typical losses fall in the range of about 0.45 kg of magnetite loss per tonne (1 lb per ton) of feed coal treated. A portion of the circulating media from the drain section may also be diverted (bled) to the magnetic separator so that the buildup
of contaminants such as ultrafine clay can be prevented. Such a build increases the viscosity of the circulating media and adversely impacts the separation efficiency.
Heavy media processes make use of a variety of instrumentation to monitor and control the performance of the separator. In most cases, this consists of a gamma (nuclear) density gauge or other device that continuously monitors the specific gravity of the circulating media. Automated controls are used to add water or magnetite so as to maintain a constant density in the circulating media.