Project Title: FINE COAL CLEANING WITH AN AUTOMATED ENHANCED DENSE MEDIUM GRAVITY CIRCUIT

ICCI Project Manager: Dr. Ken Ho, Illinois Clean Coal Institute

The main objective of this project is to automate the operation of the dense medium Falcon concentrator to provide optimal yield and grade with varying feed characteristics.

The Jader coal contains 19.73% ash and 4.09% total sulfur, where as the American coal contains about 28.66% ash having 2.26% total sulfur. Water-injection cyclone experiments were carried out to remove the –325-mesh fraction from the feed coal. The most important parameters of the water-injection cyclone are vortex finder diameter, water-injection rate and spigot diameter for both the Jader and American coals. The truncated cone diameter is a significant parameter for Jader and American coals at 90% confidence level. It was observed that the diameter of the truncated cone should be equal to or more than vortex finder diameter for proper classification. The D₉₅ values are 75 (200 mesh) microns and 45 (325 mesh) microns respectively for with and without water injection. The percentage of –500 mesh and -325 mesh fractions reporting to underflow is 24% and 80% respectively in case of with water injection, where as it is 40% and 92% in case of without water injection.

The dense medium Falcon experiments show that the feed ash and underflow rate have the greatest effect on the product ash content. An increase in feed ash content from 12.77% to 18.44% increased the product ash from about 8% to 10.5% with a decrease in yield from about 92 % to 82%.

Magnetite recovery tests using a high intensity magnetic separator indicate an inability to achieve acceptable levels of separation in a once-flow through operation. However, a rougher-scavenger arrangement of magnetic separators resulted in a separation equivalent to 2.9 lb/ton of magnetite losses.

A semi-automatic control of the dense medium Falcon concentrator was successfully demonstrated. Online measurements of feed ash content and pulp density with the Amdel analyzer were used to maximize product mass yield at a targeted product ash content. This was accomplished using an online constrained optimization routine to control the bowl speed and underflow rate of the Falcon concentrator. An automatic control of the bowl speed was demonstrated while the underflow rate was controlled manually.

Background

The research conducted during the past two decades on processing of coal fines has changed the description of fine coal as an environmental liability to a material allowing possible economical gains. The development of “Enhanced Gravity Technique” is a product of the research conducted during this period aimed at improving the efficiency of fine particle processing. Research funded in large part by the state of Illinois has shown that enhanced gravity concentration has the ability to improve the rejection of both ash forming minerals and coal pyrite while maximizing the recovery of fine coal. The process efficiently treats particles of sizes of 1 mm, down to as small as 44 mm.

Based on the previous studies conducted at SIU using an enhanced gravity separator called the Falcon Concentrator, it has been observed that though the high centrifugal acceleration helps to achieve efficient separation of finer fraction (-100x325 mesh), it also leads to faster settling of coarser fraction (-28x100 mesh), thus reducing the recovery of coarser coal fraction. Hence, an innovative method was developed at SIUC with the funding provided by ICCI, to improve the separation of coarser coal particles. The method uses dense medium (DM) in the Falcon concentrator thus reducing the effect of size on separation of coal particles. Previous studies with the dense medium Falcon concentrator (DMFC) showed that the separation efficiency depends on the amount of minus 325-mesh present in the feed coal. Also, the efficiency of dense medium operation depends on the feed and media characteristics, which is widely observed in coal preparation plants. Hence, to address these problems studies will be conducted using water-injection cyclone to reduce the fines content in the feed coal and automation of the DM Falcon concentrator to provide an optimal yield and grade for varying feed characteristics.

Sample characterization

The Jader and American coal samples were collected after analyzing the washability data of various coals of Illinois. The samples were subjected to particle size analysis and washability tests and the resulting samples were analyzed for ash, total sulfur and calorific values. The Jader coal contains about 19% ash, having 4% total sulfur. The sample contains around 15% of –400-mesh fraction having an ash and total sulfur contents of about 43% and 6% respectively. The percentage of ash and total sulfur contents increases with decrease in size, indicating the necessity of using an enhanced gravity separator. The American coal contains about 29% ash having 2.26%total sulfur. The amount of –400 mesh fraction is about 28% having an ash content of about 69%.

The washability analysis data on a particle size fraction –16x325 mesh show that the near gravity material within the specific gravity rage of 1.5 to 1.8 is in between 1 to 3% for Jader coal and 0.3 to1.5% for American coal. This finding suggests that the particles are liberated and a moderately efficient gravity separator is sufficient to ensure a high level of combustible recovery. However, since about 35% of the material is between –48x325 mesh fraction it is essential to use an enhanced gravity technique to achieve the required combustible recovery. The theoretical product ash and yield that can be obtained from Jader and American coals respectively are 7% and 85%; and 5% and 90%.

Hydrocyclone without water injection

The hydrocyclone experiments based on the Box-Benkhen design without water injection showed that, for cut point the most important parameters are vortex finder diameter and percent solids. An increase in vortex finder diameter increases the cut point because of higher volumetric flow rate of solids to the overflow, which removes higher fraction of stratified material from the center of the hydrocyclone, thus increasing the cut point. The percent solids also affect the cut point, which increases with increase in percent solids. A feed containing higher percent solids, does not allow the conditions favorable to proper settling of particles, thus increasing the misplacement of course fraction to overflow. A lower percent solids level is required for a sharper cut point. As the feed pressure increases, the centrifugal force on the particle increases, thus forcing the particles to move along the wall of the cyclone, which evidently discharged as underflow. Hence, more particles will report to underflow with increase in pressure, which decreases the cut point.

Water injection cyclone

Statistically designed experiments were carried using the Plackett-Burman on the Jader and American coal samples to identify the important variables of the water-injection cyclone. The following parameters were used for the experimental campaign.

The most important parameters of the water-injection cyclone are vortex finder diameter, water-injection rate and spigot diameter for both Jader and American coals. The truncated cone is a significant parameter for Jader coal at 95% confidence level. Where as for American coal the cone is a significant parameter at 90% confidence level. The percent solids do not have much significant effect on the partition number. In normal operation of cyclone the spigot diameter does not have much effect on partition number. Usually, it is expected that the larger the spigot diameter, higher percentage of feed material report to underflow. The feed pressure significantly affects the partition number for the Jader coal, where as for the American coal it does not have much effect. The increase in feed pressure generally increases the centrifugal force, thus providing smaller D₅₀ values. Alias structures for the 8 run Plackett-Burman design show that main effect F (water-injection) is partially confounded with B (feed pressure) and C (vortex finder diameter) two-factor interaction. Based on the analysis of the Plackett-Burman design, further experiments as per Box-Benkhen design were conducted with the American coal varying vortex finder diameter, water-injection rate and feed pressure.

Based on the Palckett-Burman design tests, experiments were carried out varying Vortex finder diameter, feed pressure and water injection rate. During the experimentation it was observed that a large fraction of the coarse fraction was reporting to the overflow of the cyclone. It was concluded that the ratio of vortex finder diameter to truncated cone diameter is very important. It is preferable to have this ratio one or less than one. Hence, further experiments were carried out keeping the ratio of vortex finder diameter to truncated cone diameter as one, varying water injection rate. The D₉₅ values were 75 microns (200 mesh) and 45 microns (325 mesh) respectively for with and without water injection. The percentage of –500 mesh and -325 mesh fractions reporting to underflow is 24% and 80% respectively in case of with water injection, where as it is 40% and 92% in case of without water injection. The water injection slightly increases the cut point, while reducing the percent of fines in the underflow. This study clearly shows that the water injection at the apex of the cyclone reduces the fines content in the underflow.

Dense medium Falcon

Statistically designed experiments were carried using the Box-Benkhen design on the American coal to quantify the effects of the operating parameters associated with the dense medium-based Falcon using the following operating parameters.

The results of the dense medium Falcon tests show that feed ash and underflow rate have the greatest effect on product ash content. The curvilinear nature of the product ash curve suggests that the product ash is a strong quadratic function of underflow rate, which is supported by a relatively low t-statistic obtained for C² term. The increase in feed ash increases the product ash with decrease in yield. An increase in feed ash content from 12.77% to 18.44% increased the product ash from about 8% to 10.5% with a decrease in yield from about 92 % to 82%. Hence, the change in feed properties significantly affects the process performance. Therefore, it is necessary to automate the densese medium Falcon to maximize the product yield for a given product ash content for varying feed properties. The higher bowl speed increases the centrifugal force, thus providing accumulation of magnetite particle near the wall of bowl. This process increases the specific gravity of cut increasing the product ash and with a marginal increase in yield.

Magnetite Recovery

Magnetite recovery from the dense medium Falcon concentrator clean coal product was evaluated. Statistically designed experiments were conducted on a laboratory Eriez L-8 wet drum separator to evaluate the influence of flow rate, magnetic intensity, pulp density and magnetic loading on the separation performance. Experiments were conducted as a single stage operation as well as in a rougher-scavenger arrangement. The experimental results indicate that the magnetite recovery efficiency increases with a decrease in flow rate. The efficiency is seen to improve with an increase in magnetic efficiency. Pulp density and magnetite loading interactively influence the separation performance.

A single stage operation resulted in an inferior performance resulting in a magnetite loss of approximately 35 lb/ton (lbs of magnetite/ton of coal). This is a result of the fineness of the magnetite used in the dense medium falcon cleaning operation. However, a rougher-scavenger operation was able to achieve a separation performance equivalent to 3 lb/ton of magnetite loss.

Dense medium Falcon Automation

An Amdel Coal Slurry Analyzer (CSA) was installed at the high-bay facilities at SIU. The CSA was calibrated to measure the ash content and solids percent in a fine coal slurry. Since the CSA uses a metal scatter channel for measurement, it was possible to use the CSA to measure the ash and solids percent for a fine coal slurry containing magnetite. The measurement accuracy achieved for ash content was ± 1.25% and that for solids percent was ± 0.75%.

A control system was developed that responded to changes in feed quality by changing the operating parameters (bowl speed and underflow rate) of the Falcon concentrator to maintain the targeted product quality (ash content) while simultaneously maximizing the mass yield. Automatic control of the bowl speed was implemented while a manual control for the underflow rate was used. Manual control for the underflow rate was necessitated due to a lack of correlation between the actual underflow rate and the air pressure used to control the same. Also, measurement of the underflow rate was difficult due to the high density and flow characteristics of this stream.

The control system was demonstrated by disturbing the ash content of the Falcon concentrator while in operation. The Amdel analyzer, measuring the ash content every minute, detected the change in ash content and transmitted the reading to its control computer. The Falcon concentrator control computer polled the Amdel computer once every minute. Upon receipt of the changed ash content value by the Falcon concentrator computer, an optimization routine was automatically initiated that determined the bowl speed and underflow rate settings required to maximize the product yield while maintaining the product quality constraints for the observed change in the feed quality. The new setting for bowl speed was transmitted to the Falcon controller for automatic adjustment of the bowl speed. The new underflow rate setting was displayed on the computer screen prompting the operator to manually adjust the underflow rate.

A series of eight tests was carried out under artificially varied feed ash contents from 13% to 25%. The target ash content level was varied from 10% to 12%. The actual yield and ash contents as calculated from the feed, product and tailings assays matched very well with those predicted by the online control optimization routine.

The overall goal of the proposed project is to develop an advanced automated dense medium fine coal cleaning technology that provides a significant improvement in the separation efficiency currently achieved from the treatment of fine coal, while maintaining the economics of the process. In addition to providing a higher product quality while maximizing energy recovery, the process must have a relatively high capacity and be amenable to on-line control for plant optimization purposes. A successful test program to develop the heavy media application with the Falcon concentrator for fine coal processing will meet the aforementioned project goal. The specific project objectives are:

· Evaluate and optimize the performance of the dense medium Falcon operation for coals of different characteristics;

· Development of a control strategy to adjust the DMFC parameters to varying operating conditions to maximize the separation efficiency.

· Study the effect of magnetite medium properties on the performance of the dense medium Falcon operation.

· Perform a preliminary evaluation of media loss and develop appropriate schemes for a magnetite recovery circuit.

Coal samples from the fine coal circuit were collected in 55gallon drums. The samples were homogenized and a representative sample was prepared for each coal for characterization. The representative sample was characterized on particle size-by-size and density-by-density. The magnetite required for the test work in this investigation was obtained from a commercial producer. Upon arrival, the particle size distribution of the magnetite was evaluated.

Task 2. Evaluation of Water-injection cyclone

The object of this task is to remove –325-mesh fraction from the feed slurry coal using a water-injection cyclone. The important variables were identified and optimized for maximum removal of –325-mesh fraction.

In this task, the effects of the operating parameters associated with the dense medium-based Falcon were quantified using a statistically designed experimental program on one of the coal slurry samples prepared in Task 1.

Empirical relationships were developed using the experimental data for providing maximum yield over a range of ash were quantified. These relations were used for calculation of optimum values for process control.

The aim of this task was to develop a control strategy for the Falcon Concentrator, using an on-line coal slurry analyzer to adjust its design parameters suitably to varying feed characteristics. A semi-automatic control was successfully implemented and demonstrated to effectively achieve optimal separation efficiency for the Falcon concentrator under varying feed qualities.

Task 6. Evaluation of a Media Recovery Circuit

The aim of this task was to develop an efficient magnetic recovery circuit using Magnetic Separators. To evaluate the effect of media and contaminant loading on magnetic separation, a series of laboratory tests were conducted. Based on these results, a rougher-scavenger circuit configuration was tested and found to provide adequate magnetite recovery performance.

Due to the increased mechanization of underground coal mining, the proportion of fine coal reporting to the coal processing plants has increased from approximately 5% to about 20% of the overall run-of-mine production. It has also been a trend in the industry to increase the top size being fed to the fine coal cleaning circuit from about 28 mesh to 10 mesh in order to reduce the load on the intermediate circuits or to allow processing of the entire +10 mesh fraction using a single unit operation. However, this trend will ultimately result in a higher fraction of the feed coal reporting to the fines circuit, which has used relatively inefficient separation techniques in the past. Hence, some coal processing plants prefer to dispose the entire fine coal fraction.

Southern Illinois University has been actively engaged in evaluating and optimizing the performance of enhanced gravity separation technologies with the help of research funding from the Illinois Clean Coal Institute (ICCI), Illinois coal companies and equipment manufacturers. Based on initial laboratory and pilot-plant studies, successful in-plant trials of the advanced fine coal cleaning technologies were conducted. The in-plant trials have shown a significant improvement in the fine coal cleaning circuit performance by using advanced clean coal technologies. The ability of dense medium falcon to achieve efficient low gravity cut-points has been successfully demonstrated with the help of funding support provided by ICCI.

This pilot-scale study of the DMFS revealed that the separation efficiency is very sensitive to the presence of the percentage of –325-mesh size fraction in the feed. The most practical method for the removal of –325-mesh size fraction is to utilize hydrocyclone. However, the conventional cyclones do not provide better classification efficiency due to the bypass of feed material to the underflow. At least 10% to 25% of very fine particles report to underflow. Hydraulic water addition through the apex, to displace the underflow liquid has been used to increase the sharpness of separation. Hence, one of the main aims of this project was to study the effect of water-injection in the removal of fine particle.

Another problem faced by today’s coal preparation plants is the wide fluctuations encountered in the feed coal properties in terms of the ash content as well as the washability characteristics. The fluctuation in the plant feed ash content is caused by the out-of-seam dilution during the mining operation, while the change in the washability characteristics occurs typically in preparation plants treating feeds from multiple sources. Therefore, processes that can effectively handle wide variations in feed characteristics and provide efficient separations at high mass yield are required to maximize mine profitability. Thus, it is imperative to use an automation and control strategy that can respond quickly to any change in the feed characteristics. Therefore, the main aim of this study was to develop an efficient enhanced gravity separation circuit addressing the important problems faced in the fine coal processing.

EXPERIMENTAL PROCEDURES

Coal samples from the feed to the cyclone were collected from the Jader and Galatia coal preparation plants. The composite feeds to the cyclone were collected in 55 gallon barrels and 15-20 barrels of sample were collected from each plant.

Sample Preparation

The samples contained a significant portion of +16 mesh fraction. As this study is directed towards fine coal cleaning, the +16 mesh fraction was removed from both the samples using wet screening.

Since the samples were collected during a period of time, it was necessary to homogenize the samples to obtain a representative sample for the experimental study. In this investigation sample preparation was accomplished by mixing the material into a slurry state by the addition of water. For this purpose, a large 2000-gallon mixing tank was employed along with a 5 hp pump to extract the slurry through an underflow discharge point. To ensure representative splitting of the sample, the pump discharge was cycled around different barrels, which were protected with a liner, using an equal time interval for each barrel.

Representative Sampling

Representative samples (around 5 gallons) were withdrawn from each barrel and were further reduced using a rotary slurry splitter. The representative samples obtained from the slurry splitter were used to verify an equal split of sample in each barrel and for all the sample characterization including gravity washability and release analysis tests. The remaining sample from the rotary splitter was returned to the respective barrel of origin.

The average ash content is 19.73% and 29.64% with a standard deviation of 1.10 and 1.63 between samples respectively for Jader and American coals. Therefore, all barrels were assured to posses similar properties and hence a representative bulk sample was prepared by combining the same amount of material from each barrel. The sample was subjected to particle size analysis using the wet sieving technique.

The water-injection cyclone test equipment requires a unique method to enable collection of the overflow and underflow products of the cyclone. When there is no injection of water to the cyclone, the underflow and overflow of the cyclone can be recirculated. However, upon injection of water to the cyclone, the overflow and underflow have to be redirected to a separate vessel to avoid the dilution of the feed to the cyclone and also be able to collect representative samples.

Hence, to achieve the above-mentioned goal a special sample collection method has been devised. The sampler was fabricated using metal sheets, which diverts the underflow and overflow of the water-injection cyclone to sampling containers. It is also possible to collect both streams simultaneously for a given period of time. The water-injection equipment also contain two flow meters to measure feed flow rate and injected water flow rate to the cyclone.

Experiments with the Water-injection Cyclone

The water-injection cyclone experiments with Jader and American coal samples were conducted using the Krebs–Cyclowash. The schematic diagram of the experimental setup is shown in Figure 1. The feed coal to the cyclone (-16 mesh) was prepared by screening coal samples to remove +16 mesh fraction. The cyclone was fitted with the required diameters of vortex finder, spigot and truncated cone. Prior to each test, the feed sump was filled with a measured quantity of water and the required quantity of coal sample. The slurry in the feed sump was kept in suspension using stirrers and a re-circulation circuit. After measuring the feed density, the desired feed pressure was set by adjusting the by-pass valve. Required amount of water is injected to the cyclone. While injecting water, the cyclone overflow and underflow were redirected to another tank to avoid dilution of the feed sample. The feed, cyclone underflow and overflow were collected simultaneously, using the fabricated sample cutter. The samples collected were weighed to find out the percent solids and were subjected to detailed wet sieve analysis. The samples from the wet screen analysis were weighed and analyzed for ash and total sulfur contents using ASTM procedures.

Magnetic Separation

The magnetic separation experiments were carried out the University of Kentucky. The magnetic separator used to evaluate magnetite recovery in this investigation was an Eriez L-8 laboratory wet drum unit. The unit was operated in a continuous mode at flow rates up to 8 gallons/minute. The unit was operated in a co-current arrangement where the slurry flow through the separator is in the same direction as the magnetic drum rotation. This arrangement ensures maximum magnetite recovery. The drum was 12 inches in diameter and 7.5 inches wide and had a rotational speed of 50 rpm. An electromagnetic element was used which allowed adjustment of the magnetic field intensity through control of the applied D-C voltage.

The magnetite-coal feed slurry was placed in a 30-gallon sump. A recirculation pump was used to agitate the slurry and maintain a suspension. Representative samples of the feed were taken from the recirculation line. A peristaltic pump was used to feed the magnetic separator from a location close to the pump discharge. The feed to the magnetic separator (magnetite + coal) was prescreened at 65 mesh for removal of the coarse magnetite which could have led to destabilization of the magnetic suspension.

A set of 10 statistically designed experiments were conducted to study the influence of feed volumetric flow rate, magnetic field intensity and feed pulp density on the magnetite recovery performance. In addition 4 experiments were conducted to evaluate the response of the separator to varying levels of magnetite loading. One additional test was conducted to evaluate the performance in a rougher – scavenger arrangement.

For each experiment, the magnetic concentrate and non-magnetic stream was analyzed for magnetics content using the Davis Tube.

Automation and Control

The automation of the Falcon concentrator requires continuous analysis of feed or product streams. The developed algorithm to control the critical operating parameters of the Falcon concentrator to achieve the desired product quality and quantity uses this information. An AMDEL on-line coal slurry analyzer (CSA), which was developed in Australia, was used at SIU for online measurement of the fine coal feed slurry ash and solids content. The CSA was used on both the water-only and magnetite containing slurries. Calibration equations were developed for both these cases over varying ranges of ash contents and solids contents. Calibration equations were developed using the following parameters.

A Levenberg-Marquardt gradient search algorithm was implemented to perform a constrained optimization routine in realtime, which identified the operating conditions (Bowl speed and underflow rate) that provided the maximum product yield for any given product quality constraint.

A set of eight experiments was conducted varying the feed ash content going to the Falcon concentrator while keeping the feed solids percent constant. The change in feed ash content was achieved by addition of another high ash content sample to the Falcon feed sump while continuously operating the Falcon. The change in ash content was detected by the CSA and transmitted to the control computers. The optimization routine was initiated in realtime, which, in turn, automatically controlled the bowl speed. The computer simultaneously prompted the operator to manually adjust the underflow rate as and when needed.

RESULTS AND DISCUSSION

Selection of Sample

The efficiency of water-only process reduces with increase in near gravity material and the amount of fines present in the coal sample. The enhanced dense medium Falcon concentrator is more efficient than the water-only Falcon concentrator. Hence, it is necessary to select coal samples, which are difficult to clean using water-only process, to take advantage of the enhanced dense medium Falcon concentrator. The previous washability data of different coals were analyzed to select coal samples for the present work.

The washability data for coals from Galatia, Marissa, Wabash and Jader Fuel (» -16 mesh) were analyzed to select two coal samples for the current work. The ROM coal of Galatia mine contains about 14% (by wt.) of -16-mesh coal, having an ash content of 26%. The near gravity material (NGM) in the region 1.4 to 1.8 densities (gm/cm³) is about 3%. The presence of lower amount of NGM indicates that a moderately efficient process may be used to clean the coal. However, the presence of high amount of fines indicates that the coal requires an efficient gravity separator.

The ROM coal from Marissa mine contains about 42% (by wt.) of –16-mesh fraction having an ash content of 25%. The NMG in the density range 1.4 to 1.8 is about 4 to 6%, requiring an efficient separator for the cleaning of this coal. In the case of Wabash coal, the ROM contains about 10% (by wt.) -16-mesh fraction having an ash content of about 7%. The NGM in the density range 1.4 to 1.8 is around 1%. Therefore, a moderately efficient separator may be utilized for the cleaning this coal. The washability data on composite Jader Fuel feed to cyclone (seams 2+3) indicates that the –16 mesh sample contains about 21% ash. The NGM in the density range 1.4 to 1.8 is about 4 to 17%. Hence, this coal requires an efficient separator for cleaning.

The above analyses show that coal samples from Galatia, Marissa and Jader Fuel mines are good candidates for use in this study. Of these, samples were collected from Jader and Galatia coal preparation plants.

Characterization of Jader and American Samples

Bulk sample obtained for Jader feed to cyclone was subjected to particle size analysis using the wet sieving technique. The material collected in each particle size fraction was subsequently analyzed for ash, total sulphur and calorific value contents. For the calorific value determination, a LECO AC-350 Model BTU analyzer was used. The reference samples used were the LECO Series 501 bituminous coal samples, which have been standardized in accordance with ASTM D-3286 isoperibol calorimetry. The results are presented in Tables 1 and 2.

Size fraction (mesh)	Weight (%)	Ash (%)	Total Sulfur (%)
+16	2.36	9.48	3.33
16x28	20.05	9.85	2.90
28x48	25.17	10.07	3.01
48x65	10.92	11.68	3.30
65x100	9.51	14.52	3.43
100x200	10.49	25.26	4.58
200x325	4.78	44.22	7.60
325x400	1.58	46.34	7.96
400x500	2.20	44.43	10.50
-500	12.93	42.93	5.41
TOTAL	100.00	19.42	4.00

Size fraction (mesh)	Weight (%)	Ash (%)	Total Sulfur (%)
+16	0.16	5.14	1.34
16x28	14.04	4.66	1.45
28x48	19.64	4.83	1.51
48x65	9.34	6.21	1.70
65x100	8.92	6.96	1.72
100x200	11.90	15.25	2.15
200x325	5.47	57.17	2.55
325x400	2.28	65.40	2.86
400x500	3.50	68.74	2.98
-500	24.74	68.72	2.95
TOTAL	100.00	28.66	2.26

The Jader coal contains about 19% ash, having 4% total sulfur. The sample contains around 15% of –400-mesh fraction having an ash and total sulfur contents of about 43% and 6% respectively. The percentage of ash and total sulfur contents increases with decrease in size, indicating the necessity of using an enhanced gravity separator. The American coal contains about 29% ash having 2.26%total sulfur. The amount of –400 mesh fraction is about 28% having an ash content of about 69%.

Washability analysis was conducted on -16x325 size fraction of the Jader and American coal samples. The analysis was conducted using the ASTM static bath procedure and lithium metatungstate solution as the medium. Each density fraction was rinsed, dried and prepared for ash and total sulfur analyses. The results of the washability tests regarding respective size fractions are presented in Table 3 and 4.

Specific gravity fraction	Individual			Cumulative
Specific gravity fraction	Weight ( %)	Ash (%)	Combustible Recovery (%)	Weight (%)	Float Ash (%)	Combustible Recovery (%)
1.3	62.55	5.45	68.87	62.55	5.45	68.88
1.3X1.4	17.09	9.34	18.05	79.64	6.28	86.93
1.4X1.5	5.88	16.09	5.75	85.52	6.96	92.68
1.5X1.6	2.94	21.74	2.68	88.46	7.45	95.35
1.6X1.7	1.20	26.83	1.02	89.66	7.71	96.37