By S. Keles, G. Luttrell and R.H. Yoon; and T. Estes and W. Schultz
The dewatering of fine coal is widely considered to be the most difficult and costly step in coal preparation. The cost of dewatering is a strong function of particle size and increases sharply for particles finer than 28 mesh. In fact, many coal producers often find it to be more economical to discard fine coal provided it constitutes only a small fraction of the overall product stream (Leonard, 1991). As a result, approximately 2 billion tons of fine coal has been discarded in abandoned ponds in the U.S., and 500 to 800 million tons are in active ponds (Orr, 2002). On a yearly basis, U.S. coal producers discard approximately 30 to 40 million tons of fresh fine coal to ponds. This represents a loss of valuable natural resources, a loss of profit for coal producers, and the creation of potential environmental concerns related to waste storage.
In the past, thermal drying was the only practicable method of drying fine coal to below 10% moisture by weight (Osborne, 1988). Unfortunately, thermal dryers are capital intensive and difficult to justify in the current coal market. This is particularly the case with pond fines recovery projects, whose life spans can be only a few years. Furthermore, new air quality standards can make it difficult to obtain permits to install thermal dryers in many states. An attractive alternative to thermal drying would be to use mechanical dewatering systems; however, existing processes are currently incapable of generating very low moisture contents when used to treat fine coals. Therefore, new dewatering technologies are needed to overcome this limitation.
In light of the problems associated with fine coal dewatering, Virginia Tech researchers have teamed with Decanter Machine to develop a hyperbaric filter centrifuge that uses a combination of gas pressure and centrifugal force to increase the driving force for dewatering (Yoon and Asmatulu, 2002). Data collected to date suggest this process can significantly reduce the moisture of fine coal by 30%-50% compared to existing dewatering processes.
Substantial improvements in moisture removal can be expected through the use of a hyperbaric (air pressurized) filter centrifuge. To demonstrate the potential of this approach, several test runs were conducted with the batch laboratory-scale test unit. The test unit consisted of a rotating filter drum that was perforated and lined with a suitable filter cloth (or mesh).
During testing, feed slurry was placed within the drum. The slurry formed a compact filter cake along the wall of the filter chamber upon application of the centrifugal force. Compressed air was then injected into the rotating assembly to further increase the pressure drop across the filter cake. The combined action of the centrifugal force and air pressure made it possible to achieve a high rate of dewatering and low equilibrium cake moisture.
The laboratory tests were conducted using fine coal slurry samples that had been thickened to 60%-70% solids by means of sedimentation or prefiltration. All of the tests were conducted at a 15-mm cake thickness using minus 28 mesh coal. As shown in Table 1, the first series of tests were performed under a centrifugal force of 2,000 G without injection of compressed air. The moisture contents obtained using centrifugation alone were in the range of 24.4% to 21% at 30-120 sec of centrifugation time. The next series of tests were carried out at 15 psi of pressure, but without centrifugation. In this case, the cake moistures ranged from 23.8% to 27.5% depending on the drying times employed. In the third series of tests, both air pressure (15 psi) and centrifugal force (2,000 G) were used. The cake moistures obtained by the combined action of these forces were in the range of 14.2% to 10.6%. At a common drying cycle time of 60 sec, the pressure filter, filter centrifuge and hyperbaric centrifuge produced filter cakes with moisture contents of 25.8%, 22.6% and 12.9%, respectively. These results show the addition of compressed air greatly increased the moisture reduction, which can be attributed to the increased pressure applied to the dewatered cake.
Table 2 shows the results of another series of laboratory-scale tests conducted on a very fine (22 micron median) coal sample from a column flotation/pond recovery plant. Because of the extreme fineness of this sample, two series of tests were performed using (i) a sample of the as-received froth product and (ii) a sample of partially deslimed froth product. The deslimed product was prepared by removing half of the minus 325 mesh material from the sample by wet sieving. The tests were conducted at 2,500 G using a constant 0.6 inch cake thickness for the as-received product and a 0.4 inch cake thickness for the partially deslimed product. As expected, the removal of moisture improved when either the spin time or air pressure was increased