The washability data on coals of different origins have been analysed in terms of relation between the cumulative weight fraction of feed material floated and the recovery of non-ash and ash material in the float fraction at a given specific gravity. It has been noted that a unique recovery curve exists for both the recoveries of non-ash and ash material when plotted against the cumulative weight fraction of feed material floated. On the basis of this observation, a simple Index of Washability has been developed. An explicit mathematical expression has been derived to estimate the index value for a given coal.
Because of the importance of washability and sink-float analysis to the coal industry and the health hazards associated with organic liquids, considerable effort is being aimed at alternatives to the organic liquid method. To determine the partition curve of a gravity separation unit, density tracers may be used. These are plastic particles manufactured to precise density such as 0.005 SG units . These tracers are available in cubic shape from 1 to 64mm or as crusher particles to simulate real ore with sizes from 0.125 to 32mm or more. Density ranges are from 1.24 to 4.5 S.G. and can be colour coded or made magnetic or fluorescent for ease of recovery. A range of tracers of different density and size are added to the unit feed and retrieved from the floats and sinks fractions. The ratio of numbers in the floats and feed will give the partition coefficient.
For an alternative to the sink-float analysis, the Julius Kruttschnitt Mineral Research Centre (JKMRC) have developed an automatic gas pycnometer in which the dry density of individual particles is determined by separate mass and volume measurements . The instrument is capable of analysing 30 particles a minute. A sink-float data analysis requiring about 3000 particles can be obtained in 100 minutes.
The behavior of coal in density-based cleaning processes can be predicted from a series of float-sink (or washability) tests carried out on a crushed sample in the laboratory. The sample is placed in a heavy liquid of known density (e.g., 1.40gcm3) and the floating fraction skimmed off for weighing and analysis. The sinking fraction is then placed in a liquid of somewhat higher density and the process repeated, with further tests in still denser liquids until the desired range of separation densities has been covered (Table 6).
Data from float-sink testing are used to determine the yield and quality of both clean coal and refuse fractions produced by separation in coal cleaning equipment, such as a dense medium bath, adjusted to split the stream at a particular density value. Such data may also give the density required in the separating medium, as well as the yield of product, the yield of refuse, and the quality of refuse that will be obtained from the material tested, if clean coal of a particular quality is produced. Floatsink testing is invaluable in the design of coal preparation plants, the development of marketing strategies, and the estimation of marketable coal reserves for a given coal deposit.
The behavior of coal in density-based cleaning processes can be predicted from a series of float-sink (or washability) tests carried out on a crushed sample in the laboratory. The sample is placed in a heavy liquid of known density (e.g., 1.40g/cm3) and the floating fraction skimmed off for weighing and analysis. The sinking fraction is then placed in a liquid of somewhat higher density and the process repeated, with further tests in still denser liquids until the desired range of separation densities has been covered (Table VI).
Data from float-sink testing are used to determine the yield and quality of both clean coal and refuse fractions produced by separation in coal cleaning equipment, such as a dense medium bath, adjusted to split the stream at a particular density value. Such data may also give the density required in the separating medium, as well as the yield of product, the yield of refuse, and the quality of refuse that will be obtained from the material tested if clean coal of a particular quality is produced. Floatsink testing is invaluable in the design of coal preparation plants, the development of marketing strategies, and, of course, the estimation of marketable coal reserves for a given coal deposit.
The yield and quality of the clean-coal product from an industrial coal preparation plant and the theoretical yield and quality determined from washability curves are known to be different. In the ideal cleaning process, all coal particles lower in density than the density of separation would be recovered in the clean product, while all material of greater density would be rejected as refuse. Under these conditions the product yield and quality from the actual concentration process and the yield and quality expected from the washability curves would be identical.
The performance of separators is, however, never ideal. As a result, some coal particles of lower than the separation density report to rejects, and some high-ash particles of higher than the separation density report to clean coal. These are referred to as misplaced material.
Coal particles of density well below the density of separation and mineral particles of density well above the density of separation report to their proper products: clean coal and refuse. But as the density of separation is approached, the proportion of the misplaced material reporting to an improper product increases rapidly.
Tromp, in a study of jig washing, observed that the displacement of migrating particles was a normal or near-normal frequency (gaussian curve), and from this observation the partition curve (distribution, Tromp curve) in the form of an ogive was evolved.
The partition curve, the solid line in Fig. 22a, illustrates the ideal separation case (Ep=0), and the broken-line curve represents the performance of a true separating device. The shaded areas represent the misplaced material. The curves are plotted according to European convention and represent the percent of feed reporting to reject. In American practice, Tromp curves usually give the percent of feed reporting to washed coal.
The Tromp curve from the mathematical point of view is a cumulative distribution curve and as such can be linearized on probability graph paper. Such anamorphosis is produced by plotting the partition coefficients on a probability scale versus specific gravity on a linear scale (Fig. 22b) for dense-media separation, and versus log (1) for jigs.
To determine the partition curve for a cleaning operation, one needs the yield of clean coal from this operation and the results of floatsink tests for both productsthat is, for the clean coal and the refuse. Such data allow the reconstituted feed to be calculated, and from this can be found the partition coefficients, which give the percentage of each density fraction reporting to reject. As seen in Fig. 22b, the particles with densities below 504Ep and the particles of density above 50+4Ep report entirely to their proper products. The density fractions within 504Ep are misplaced. Material of density very close to 50 (near-density material) is misplaced the most. As postulated by Tromp, 37.5% of fractions within 5025 and 7550 are misplaced (this corresponds to 50Ep), and this percentage falls off drastically with the distance of the actual density fraction from the 50 density.
Figure 23 shows partition curves for the major U.S. coal-cleaning devices. As seen, the sharpness of separation in dense-media separators is much better than in jigs or water-only hydrocyclones. Figure 24 shows Ep values plotted versus the size of treated particles for various cleaning devices. Ep values for the dense-medium bath and dense-medium cyclone are in the range of 0.020.04 for particles larger than 5mm; Ep values of jigs and concentrating tables are in the range of 0.080.15; and for water-only cyclones, Ep values exceed 0.2. As seen, in all cases the efficiency of separation as given by Ep values decreases sharply for finer particles.
FIGURE 23. Performance of gravity separators. , hydrocyclones, 14 in. 200 mesh; , air tables, 2 in.200 mesh; , jigs: Baum, 6 in.14 in.; Batac 34 in.28 mesh; , concentrating tables, 34 in.200 mesh; , dense-medium separators: cyclone, 34 in.28 mesh; vessel, 6 in.14 in. [From Killmeyer, R. P. Performance characteristics of coal-washing equipment: Baum and Batac jigs, U.S. Department of Energy, RI-PMTC-9(80).]
FIGURE 24. Probable error Ep vs mean size of the treated coal for dense-media baths (DMB), dense-media cyclones (DMC), jigs, concentrating tables, and water-only cyclones (WOC). [After Mikhail, M. W., Picard, J. L., and Humeniuk, O. E. (1982). Performance evaluation of gravity separators in Canadian washeries. Paper presented at 2nd Technical Conference on Western Canadian Coals, Edmonton, June 35.].
Conventional float and sink methods for the derivation of the partition curve are expensive and time-consuming. In the diamond and iron ore industries, the density tracer technique has been developed to evaluate the separation efficiency. This technique has also been adopted for studies of coal separation.
The tracers, usually plastic cubes prepared to known specific gravities rendered identifiable through color coding, are introduced into a separating device, and on recovery from the product and reject streams, are sorted into the appropriate specific gravity fractions and counted. This allows the points for the partition curve to be calculated. The technique, as described above, was adopted at the Julius Kruttschnitt Mineral Research Centre, Brisbane, while tracers made from plasticmetal composites that can be detected with metal detectors mounted over conveyer belts, known as the Sentrex system, were developed in the United States.
From the float and sink data of the two size fractions, 7525mm and 250.5mm of a raw coal sample, washability curves are drawn as in Fig. 7.21. From the curves, the results are read and recorded in Table 7.7 where it was found that the yield of clean coal was maximum when the specific gravity cuts of the two fractions are same, and also the direct ash of both is the same. The direct ash at the particular specific gravity was read from characteristic curves and yield gravity curves. The combined yield of clean coal at 17% ash was 65.5% when the two fractions are washed at sp.gr. cuts of 1.43 and 1.52 and the direct ash are 22.5% and 31%, respectively. Yet at the same specific gravity (sp.gr.) of cut (1.48), the corresponding yield becomes 70% with the direct ash of 27% for both the fractions.
In the Indian washeries, composite washing of the coal is done at different sp.gr. of cut. Since this practice cannot produce the maximum yield of clean coal at a particular ash content, the coals should be washed by the same sp.gr. of cut for proper optimisation of the plant, which in turn will reduce losses in the beneficiation process.
Release analysis is carried out in the laboratory to establish the best possible separation results achievable in flotation practice. It is similar to float and sink tests for generation of conventional washability data. Release analysis provides a measure of estimating the maximum achievable performance and the best possible selectivity which can be used for plant design. It serves as a benchmark for comparison between practical and laboratory results. As shown in Fig. 5.31, the process of conducting a release analysis consists of separating the different floating coals in different stages (fast-floating to slow-floating) to generate grade recovery data. Froth products are progressively refloated to collect only the particles that are fully hydrophobic and of best quality. The entrained solids are removed from the final froth products during each stage of the flotation process. Fig 5.32 shows the results of release analysis.
The size distribution of total sulphur in the raw coal crushed to 13mm is shown in Fig. 10.4. It can be seen that the majority of sulphur concentrates at above 1mm. The results of washability data are indicated in Fig. 10.5. It gives the sulphur content of that raw coal size for each density fraction. The fractional sulphur content is at a peak at 1.5 relative density (RD). It can be also seen from the same figure that the cumulative sulphur content is reduced to some extent at cut-off points below 1.3 RD. But when the cut-off points are raised, the cumulative sulphur does not reduce appreciably.
The three forms of sulphur: pyritic, sulphate and organic have been presented in Fig. 10.6, where three typical samples have been considered. It confirms that at RD as low as 1.3, there is some reduction of sulphur in cleans, whereas at 1.5 RD, there is only a marginal reduction.
The results of forth flotation tests for size fraction (<0.5mm) are indicated in Fig. 10.7. There is no elimination of sulphur in the flotation concentrate. It may yield better results if size is reduced to separate sulphur from coal by freeing locked particles.
As raw coal is composed of different noncoaly substances, it should be crushed to liberate the coaly matter. After crushing, the real separation can be done resulting in two or three products, depending upon the washability characteristics as well as the desired specifications of the products (Kumar, 1985). Let us first take the example of two-product separation, where products are represented by A and B and the raw coal is AB. In Fig. 3.4A, AB is fed to the system and the products A and B are produced. The system may represent the method of separation right from screening to flotation. Similarly, if ABC represents raw coal and A, B and C are the products, then separation takes place in two stages as shown in Fig. 3.4B, with each stage of separation being similar, as in Fig. 3.4A. Three-product separation can take place in one system incorporating the two stages as in the case of a three-product jig or a three-product heavy medium cyclone, as shown in Fig. 3.4C. In the system described in Fig. 3.4D, some external force is required for the separation. Here, the difference in colour of the two substances is made use of either by hand picking (manual) or a radiometric sorter (automatic). In hand picking, a human does the job of detecting, comparing, sorting and throwing out undesirable materials, whereas an automatic sorter signals the presence of objects that are then removed by an external device fitted with the system.
The level of capacity utilisation of different equipment in the washery is one of the leading factors contributing to the cost of washed coal. The effective capacity as well as efficiency of equipment depends upon coal characteristics like size and washability. The efficiency of screening is dependent on the coal feed, moisture, fines and clays. In the case of dense-medium (DM) separation, the capacity to treat coal increases with the increase in mean particle size of coal as shown in Fig. 19.4A. In many designs of open-type baths, the 100% sinks in the feed can be extracted, thus the floats capacity is the limiting capacity of the equipment. However, the separating efficiency of the DM bath slightly decreases with a decrease in the mean particle size, as shown by Ep in Fig. 19.4B. The dense medium cyclones are used in a small capacity. The limiting capacity in this case is regulated by the quantity of sinks present in the feed. Although the optimum percentage of sinks allowed in the feed is 60%, any increase over 40% reduces the cyclone feed capacity. The cyclone capacity is dependent on the cyclone diameter. To take care of extra sinks, the cyclone diameter is increased, as in Fig. 19.4C.
Figure 19.4. (A) Effect of particle size on float capacity in a DM bath. (B) Dense medium open bath, variation of Epm with mean particle size. (C) DM cyclone, variation of sinks capacity with diameter.
The grading of noncoking coal is discussed with historical reference to gain more clarity on present conditions and scenarios. It is explained why washing is necessary in order to highlight the ill effects of ash content in power plants and how washed coal can be beneficial. The washability data are examined and a suitable flow scheme developed. The potential of desulphurisation is been studied. The present system of washing is described and grouped into three levels. Both existing and future washeries are examined. Of three coking coal washeries converted to noncoking coal, Kargali washery stands out as a unique example because of its several modification schemes. India is poised for a massive growth of coal washeries. Dovetailing a washery with an existing coal-handling plant can help in this growth, among other numerous benefits. Moreover, the use of coal and its effect in the cement industry are described.
An index based on a graphical approach is developed to compare the amenability to washing of several coals and evaluate coal washability characteristics. A curve labeled the CM-curve, which may be viewed as the complement of M-curve, is introduced. For each coal sample the associated M-curve and CM-curve are normalized, using the overall ash content of the coal sample, such that different coals can be evaluated using the same base. At a particular cumulative mass the abscissa distance between the two normalized curves has a maximum value that can be used as an index for evaluating washing characteristics and comparing amenability to washing of several coals. The index varies between zero and one. The larger the index the easier the coal is to wash. The proposed index is simpler and superior to existing indices. Practical data are used to illustrate the index developed.
A new coal washability index, termed as near gravity material index (NGMI), has been developed based on the sink-float data. This new index incorporates the effect of ash distribution in the near gravity material at various specific gravities of separation. The optimum specific gravity of separation of a particular coal may be determined by plotting NGMI as a function of various specific gravities of separation or the desired clean coal ash content. The concept and the method of calculation of this index have been discussed in detail. Using some published float-sink analytical data, a quick comparison of the washability characteristics of several different coals is demonstrated utilizing the NGMI curves for these coals.
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Independent SGS test results are recognized globally and comply with all applicable international standards. Data from your float-sink/washability tests provide you with the information that you need to ensure:
Washability characteristics are determined from float/sink analysis of core samples, as mined samples, or from preparation plant feed samples. During the test, your sample is characterized and quantified as low-density clean coal, high-density reject or intermediate-density middlings.
Our float/sink analysis can be performed on samples ranging in size from bulk washability samples (over 1 ton) to bench-scale size samples. We conduct the tests over a range of densities (from 1.3 specific gravity up to 2.25 specific gravity) and a variety of sizes (from coarse to fine coal).
We also perform froth recovery testing on finer sizes. If your coal contains a high percentage of middlings, we can perform the crushing studies required to determine if you can obtain additional yield by liberating coal from the middlings through crushing.
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