1、翻译原文An investigation into an improved method of fine coal dewateringM. le Roux*, Q.P. CampbellSchool of Chemical and Minerals Engineering, Potchefstroom University for CHE, Private Bag X6001, Potchefstroom 2520, South Africa，Received 18 June 2003; accepted 8 August 2003AbstractAn improved method of
2、dewatering fine coal ()500lm) by vacuum filtration was developed. Tests on a bench scale vacuum filter.showed that when the applied vacuum was interrupted during the dewatering cycle, a lower final moisture content could be achieved.There was also an increase in the dewatering rate, as well as a low
3、er breakthrough pressure. Further investigation showed that if the filter cake was intentionally damaged (causing for an increased air flow rate, and a decrease in applied vacuum), a product with lower final moisture and an increase in the dewatering rate was obtained. This led to the conclusion tha
4、t increased air flow through a filter cake was more advantageous than an increase in the applied vacuum.Keywords:Coal; Dewatering; Filtration1. IntroductionSouth Africa has large coal reserves, and is one of the worlds leading producers and exporters of coal. The country currently produces approxima
5、tely 290 million tons of ROM coal per annum. Of this, about 14% is thought to be finer than 500lm (defined as fine coal in this text) and 23% finer than 100 lm (ultra-fine coal)(Breed, 1992). The fines content in the ROM had been increasing in recent years partly due to the introduction of mechanise
6、d mining methods. Coal producers have a few options as to what to do with the fines. Initially, fines and ultra-fines were dumped with the discard streams or tailings ponds, but since the value of these fines had been recognised, it was either added to the product streams as is, or alternatively, up
7、graded and sold.The economic potential of coal fines has led to developments in fine coal beneficiation processes, like spirals and froth flotation. The major reason for not fully exploiting coal fines as an additional energy source is the high moisture levels found in the fine fractions of coal. By
8、 using an economical dewatering technique, it is possible for the coal industry to turn this previous liability into an asset. The financial advantage is obvious,while added benefits include a positive impact on the environment, and easier handling of the fine coal.2. BackgroundMoisture exists in co
9、al as different states. This description was quantified by Rong (1993) while defining the different types of moisture as follows: Surface or free moisture refers to the water on the surface of a coal particle. Hence, when a filter cake is formed, the free moisture will accumulate in the pores of the
10、 filter cake (inter-particle moisture). Most of this moisture can be removed from the filter cake by mechanical means like filtration and centrifuging. Inherent or capillary moisture is the moisture allocated within the pores of each individual particle. It is also commonly known as intra-particle m
11、oisture,and can only be removed by using thermal methods. Chemically bound moisture is found in the chemical structure of the ash fraction of the coal, as crystal water for example. This moisture cannot be removed except by pyrolysis, and was found to be in the order of 38% for bituminous coals (Bou
12、rgeois et al., 2000).Studies done on fine coal beneficiation plants showed it not to be uncommon for the coal have a final moisture percentage between 20% and 30% after filtration,depending on the method of dewatering. This problem deserves attention for various reasons. Tao et al. (2000) showed tha
13、t a 1% reduction in the moisture level of typical USA coal can lead to a 1.4% increase in the calorific value of the fuel available for combustion. In addition, environmental concerns and difficulty in handling are just two other problems that justify this research.3. Capillary pressure- or desatura
14、tion curvesThe dewatering (or de-liquoring) process of any filter cake is described by a two phase flow of liquid and air through the cake. Capillary pressure curves are used to quantify and describe this process (Condie and Veal,1998). These curves are obtained by performing filtration experiments
15、at certain applied pressure (or vacuum)differentials on a filter cake until no more filtrate flows from the cake. The tests are then repeated at increasing pressure or vacuum differentials, and the resulting capillary pressure curve (Fig. 1) characterises the dewatering behaviour of the system.The p
16、oint at which the saturation is at 100% (or S1), is the level where all the voids in the filter cake are completely filled with liquid prior to dewatering,without excess supernatant liquid present. The capillarystage is the first dewatering stage, where the applied pressure or vacuum is less than the threshold pressurePT(related to the capillary pressure of the largest pores).This results in no flow of liquid from the filter cake. The second, or funicular
