Dissertations / Theses on the topic 'Coal dewatering'

Fine coal constitutes a relatively small portion of a product stream in a coal cleaning plant. However, its processing cost is approximately three times higher than the cost of processing coarse coal. Therefore, many coal companies chose to discard the fines to refuse ponds, causing a loss of profit and creating environmental concerns. This problem can be solved by developing more efficient fine coal dewatering processes, since bulk of the cost associated with processing fine coal is due to dewatering. For this reason, Virginia Tech has developed new chemicals that can increase the efficiency of mechanically dewatering coal fines. To determine the performance of the novel reagents on fine coal dewatering, laboratory vacuum filtration and centrifugation tests were conducted. The utilization of the novel dewatering aids in the dewatering systems decreased the final moisture contents of the filter cakes to sufficiently low values. There was approximately 50% reduction in the cake moisture of many coal samples with the usage of the novel dewatering aids. The tests were performed on various coal samples from different coal preparation plants. This gave the advantage of testing the novel dewatering aids at many different conditions since each sample had its own characteristics. The vacuum filtration tests were extensively used to compare the efficiency of each novel reagent in dewatering. The best performing dewatering aids were determined and they were further utilized to analyze the effects of operational variables, such as; drying cycle time, cake thickness, vacuum pressure level and slurry temperature on dewatering. A statistical analysis was also performed to observe the effect of each factor quantitatively. The analyses were very useful in terms of determining the synergistic effects of these factors in dewatering of fine coal. The centrifuge tests were conducted to examine the efficiency of the novel reagents in a different dewatering application. The experimental results showed a significant improvement in centrifuge dewatering with the usage of proper coal sample. The moisture contents of fairly thick cakes decreased down to 5-10%. This outcome was very satisfactory since most of the dewatering aids commonly used in the coal industry were observed to increase the final cake moisture in centrifuge dewatering instead of decreasing it. Finally, surface chemistry analyses were performed on the coal samples and slurries to analyze the changes in the chemistry of the dewatering system in the presence of the novel dewatering aids. It was observed that there was a favorable improvement in the system chemistry, which was helpful in terms of decreasing the cake moisture content. These observations were also consistent with the results of the dewatering tests. The combined effect of the novel additives in decreasing the surface tension of the slurry and increasing the contact angle of the coal surface at the same time was concluded to be the reason for their significant performance as dewatering aids.
Master of Science

The solid-solid separation processes employed by modern coal preparation plants require large amounts of process water that must be removed from the surfaces of particles using mechanical dewatering equipment. Unfortunately, the existing processes that are used to dewater fine particles are inefficient in terms of moisture reduction and/or solids recovery. Many coal preparation plants are forced to discard fine coal particles because of the inability of existing technologies to reduce the moisture content of this product to an acceptable level. In light of this problem, a new ultrafine dewatering process called hyperbaric filter centrifugation (HFC) has been developed. This novel method combines centrifugation and pressure filtration within a single process to substantially reduce moistures over what can be achieved using conventional dewatering systems. In the current study, steady-state and dynamic dewatering models were developed in order to be able to simulate the behavior of the HFC technology. The steady-state model, which was based on grain-size properties, used empirical expressions to predict product moistures. On the other hand, the dynamic model was based on fundamental theories of filtration and centrifugation. Although the dynamic model provided a better understanding of the working principles of the process, the steady-state grain model produced more accurate equilibrium moisture predictions. Therefore, the steady-state model was used to further investigate the effects of several parameters on cake moistures. As such, the steady-state model was useful for scale up and design purposes. The steady-state dewatering model was also used to perform an economical analysis of potential applications of the HFC technology. The model was used to investigate a variety of new circuit designs that have the potential to be commercially applied in the coal industry. The results clearly showed that this new technology would allow coal companies to process difficult-to-dewater ultrafines using the HFC process, while coarser solids would be more appropriately dewatered using conventional technologies such as vacuum filters or screenbowl centrifuges. This â split dewateringâ concept would provide substantially higher profitability due to lower moistures and higher recoveries of ultrafine solids than could be achieved using a single dewatering process. Laboratory- and pilot-scale versions of this technology has been constructed and tested at the facilities of Mining & Minerals Engineering Department of Virginia Tech. Results of this testing program showed that 30-50% lower moisture values than the ones obtained using conventional mechanical dewatering processes could be achieved with the HFC technology. Based on these promising results, a pilot-scale prototype unit, which was tested successfully at several commercial U.S. coal plants, was also constructed by Decanter Machine, Inc. Finally, the process of developing of this novel technology was successfully completed with the sale of the first full-scale commercial unit by Decanter Machine, Inc. to a major U.S. coal producer.
Ph. D.

The costs of cleaning fine coal are substantially higher than those of cleaning coarse coal. Therefore, many many coal companies in the U.S. choose to discard fine coal (150 micron x 0) by means of 6-inch diameter hydrocyclones. The cyclone overflows are stored in fine coal impoundments, which create environmental concerns and represent loss of valuable national resources. The major component of the high costs of cleaning fine coal is associated with the difficulty in fine coal dewatering. Therefore, the availability of efficient of fine coal dewatering methods will greatly benefit companies. In the present study, three different novel dewatering aids have been tested. These include Reagents W (RW), Reagent U (RU), and Reagent V (RV). These reagents are designed to increase the contact angles of the coal samples to be dewatered, which should help decrease the Laplace pressure of the water trapped in filter cake and, hence, increase dewatering rate. They were tested on i) the fresh coal samples from Consolidation Coal Corporation's Buchanan Preparation Plant, ii) a composite drill core sample from the Smith Branch Impoundment, Pinnacle Mine Mining Company, and iii) a blend of coals from the Smith Branch Impoundment, thickener underflow, and thickener feed. The coal samples were used initially for laboratory-scale tests using a 2.5-inch diameter Buchner vacuum filter. The results showed that the use of the novel dewatering aids can reduce the cake moisture up to 50% over what can be achieved without using any dewatering aid. The use of the dewatering aids also increased the kinetics of dewatering by up to 6 times, as measured by cake formation times. On the basis of the laboratory test results, pilot-scale continuous vacuum filtration tests were conducted using a 2-feet diameter Peterson vacuum disc filter. The cake moistures obtained in the pilot-scale test work were similar to those obtained in the laboratory tests, while the fast dewatering kinetics observed in the laboratory tests was manifested as higher throughput. It was found that high-shear agitation is essential for achieving low cake moistures and high throughput.
Master of Science

A new processing technique, known as hydrophobic displacement, was explored as a means of simultaneously removing both mineral matter and surface moisture from coal in a single process. Previous thermodynamic analysis suggests that coal moisture will be spontaneously displaced by any oil with a contact angle greater than ninety degrees in water. Based on these results, six methods of hydrophobic displacement were evaluated: hand shaking, screening, air classification, centrifugation, filtration, and displacement. In the first five methods hydrophobic displacement took place during the cleaning stage. A recyclable non-polar liquid (i.e. pentane) was used to agglomerate coal fines followed by a physical separation step to remove the coal agglomerates from the mineral-laden slurry. Bench-scale tests were performed to identify the conditions required to create stable agglomerates. Only the last method, displacement, did not utilized agglomeration and performed hydrophobic displacement during dewatering, not cleaning. A procedure was also developed for determining moisture content from evaporation curves so that the contents of water and pentane remaining in a sample could be accurately distinguished.

Two primary coal samples were evaluated in the test program, i.e., dry pulverized 80 mesh x 0 clean coal and 100 mesh x 0 flotation feed. These samples were further screened or aged (oxidized) to provide additional test samples. The lowest moisture, 7.5%, was achieved with centrifugation of the pulverized 80 mesh x 0 clean coal sample. Centrifugation provided the most reliable separation method since it consistently produced low moisture, high combustible recoveries, and high ash rejections. Hand shaking produced the next lowest moisture at 16.2%; however, the low moistures were associated with a drop in combustible recovery. There was also a great deal of error in this process due to its arbitrary nature. Factors such as oxidation, size distribution, and contact angle hysteresis influenced the concentrate moistures, regardless of the method utilized.
Master of Science

In modern coal preparation plants, solid-solid and solid-liquid separation processes used to treat fine coal are least efficient and most costly operations. For example, field studies indicate that the froth flotation process, which is normally used to treat minus (-0.2 mm) fine coal, often recovers less than 65 to 70% of the organic matter in this size range. Fine coal separation processes are also inherently less effective in removing pyrite than that of coarse coal separations. Moreover, while fines may represent 10% or less of the total run-of-mine feed, this size fraction often contains one-third or more of the total moisture in the delivered product. In order to address these issues, several multistage coal processing circuits were set up and experimentally tested to demonstrate the potential improvements in fine coal upgrading that may be realistically achievable using an "optimized" fine coal processing flowsheet. On the basis of results obtained from this research, engineering criteria was also developed that may be used to identify optimum circuit configurations for the processing different fine coal streams.
In the current study, several fine coal cleaning alternatives were evaluated in laboratory, bench-scale and pilot-scale test programs. Fine coal processes compared in the first phase of this work included spirals, water-only cyclones, teeter-bed separators and froth flotation. The performance of each technology was compared based on separation efficiencies derived from combustible rejection versus ash rejection plots. The resulting data was used to identify size ranges most appropriate for the various alternative processes. As a follow-up to this effort, a second phase of pilot-scale and in-plant testing was conducted to identify new types of spiral circuit configurations that improve fine coal separations. The experimental data from this effort indicates that a four-stage spiral with second- and fourth-stage middlings recycle offered the best option for improved separation efficiency, clean coal yield and combustible recovery. The newly developed spiral circuitry was capable of increasing cumulative clean coal yield by 1.9 % at the same clean coal ash as compared to that of achieved using existing conventional compound spiral technology. Moreover, the experimental results also proved that slurry repluping after two turns is not effective in improving separation performance of spiral circuits.
The third phase of work conducted in this study focused on the development of methods for improving the partitioning of pyrite within fine coal circuits. The investigation, which included both laboratory and pilot-scale test programs, indicated that density-based separations are generally effective in reducing sulfur due to the large density difference between pyrite and coal. On the other hand, the data also showed that sulfur rejections obtained in froth flotation are often poor due to the natural floatability of pyrite. Unfortunately, engineering analyses showed that pyrite removal from the flotation feed using density separators would be impractical due to the large volumetric flow of slurry that would need to be treated. On the other hand, further analyses indicated that the preferential partitioning of pyrite to the underflow streams of classifying cyclones and fine wire sieves could be exploited to concentrate pyrite into low-volume secondary streams that could be treated in a cost effective manner to remove pyrite prior to flotation. Therefore, on the basis of results obtained from this experimental study, a combined flotation-spiral circuitry was developed for enhanced ash and sulfur rejections from fine coal circuits.
Finally, the fourth phase of work conducted as part of this investigation focused on evaluating a new mechanical, non-thermal dewatering process called Nano Drying Technology (NDT"). Experimental results obtained from bench-scale testing showed that the NDT" system can effectively dewater fine clean coal products from more than 30% surface moisture to single-digit moisture values. Test data obtained using a pilot-scale NDT" plant further validated this capability using a continuous prototype facility. It was also observed that, unlike existing fine coal dewatering processes, the performance of the NDT" system is not constrained by particle size.

Ph. D.

In modern coal preparation plants, solid-solid and solid-liquid separation processes used to treat fine coal are least efficient and most costly operations. For example, field studies indicate that the froth flotation process, which is normally used to treat minus (-0.2 mm) fine coal, often recovers less than 65 to 70% of the organic matter in this size range. Fine coal separation processes are also inherently less effective in removing pyrite than that of coarse coal separations. Moreover, while fines may represent 10% or less of the total run-of-mine feed, this size fraction often contains one-third or more of the total moisture in the delivered product. In order to address these issues, several multistage coal processing circuits were set up and experimentally tested to demonstrate the potential improvements in fine coal upgrading that may be realistically achievable using an "optimized" fine coal processing flowsheet. On the basis of results obtained from this research, engineering criteria was also developed that may be used to identify optimum circuit configurations for the processing different fine coal streams. In the current study, several fine coal cleaning alternatives were evaluated in laboratory, bench-scale and pilot-scale test programs. Fine coal processes compared in the first phase of this work included spirals, water-only cyclones, teeter-bed separators and froth flotation. The performance of each technology was compared based on separation efficiencies derived from combustible rejection versus ash rejection plots. The resulting data was used to identify size ranges most appropriate for the various alternative processes. As a follow-up to this effort, a second phase of pilot-scale and in-plant testing was conducted to identify new types of spiral circuit configurations that improve fine coal separations. The experimental data from this effort indicates that a four-stage spiral with second- and fourth-stage middlings recycle offered the best option for improved separation efficiency, clean coal yield and combustible recovery. The newly developed spiral circuitry was capable of increasing cumulative clean coal yield by 1.9% at the same clean coal ash as compared to that of achieved using existing conventional compound spiral technology. Moreover, the experimental results also proved that slurry repluping after two turns is not effective in improving separation performance of spiral circuits. The third phase of work conducted in this study focused on the development of methods for improving the partitioning of pyrite within fine coal circuits. The investigation, which included both laboratory and pilot-scale test programs, indicated that density-based separations are generally effective in reducing sulfur due to the large density difference between pyrite and coal. On the other hand, the data also showed that sulfur rejections obtained in froth flotation are often poor due to the natural floatability of pyrite. Unfortunately, engineering analyses showed that pyrite removal from the flotation feed using density separators would be impractical due to the large volumetric flow of slurry that would need to be treated. On the other hand, further analyses indicated that the preferential partitioning of pyrite to the underflow streams of classifying cyclones and fine wire sieves could be exploited to concentrate pyrite into low-volume secondary streams that could be treated in a cost effective manner to remove pyrite prior to flotation. Therefore, on the basis of results obtained from this experimental study, a combined flotation-spiral circuitry was developed for enhanced ash and sulfur rejections from fine coal circuits. Finally, the fourth phase of work conducted as part of this investigation focused on evaluating a new mechanical, non-thermal dewatering process called Nano Drying Technology (NDT™). Experimental results obtained from bench-scale testing showed that the NDT™ system can effectively dewater fine clean coal products from more than 30% surface moisture to single-digit moisture values. Test data obtained using a pilot-scale NDT™ plant further validated this capability using a continuous prototype facility. It was also observed that, unlike existing fine coal dewatering processes, the performance of the NDT™ system is not constrained by particle size.
Ph. D.

Thesis (M.S.)--West Virginia University, 2006.
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The cleaning and dewatering of ultrafine (minus 44 micron) coal slurries is one of the biggest challenges faced by coal industry. Existing commercial technologies cannot produce sellable products from these ultrafine streams; therefore, the industry is forced to discard this potential energy resource to waste impoundments. This practice also has the potential to create an environmental hazard associated with blackwater pollution. To address these issues, researchers at Virginia Tech have worked over the past decade to develop a novel separation process that simultaneously removes both mineral matter and surface moisture from fine coal particles. The first stage of the process uses immiscible non-polar liquids, such as straight chain hydrocarbons, to selectively agglomerate fine coal particles in an aqueous medium. The agglomerates are then passed second stage of processing where mild agitation is used to disperse and fully engulf hydrophobic coal particles into the non-polar liquid and to simultaneously reject any residual water and associated hydrophillic minerals entrapped in the agglomerates. The non-polar liquid, which has a low heat of evaporation, is then recovered by evaporation/condensation and recycled back through the process. The research work described in this document focused on the engineering development of this innovative process using batch laboratory and continuous bench-scale systems. The resulting data was used to design a proof-of-concept (POC) pilot-scale plant that was constructed and successfully demonstrated using a variety of fine coal feedstocks.
Ph. D.

A new dewatering technology, called low temperature drying, has been developed to remove water from ultrafine (minus 325 mesh) coal particles. The process subjects partially dewatered solids to intense mechanical shearing in the presence of unsaturated air. Theoretical analysis of the thermodynamic properties of water indicates residual surface moisture should spontaneously evaporate under these conditions. This is contingent on the large surface area of these fine particles being adequately exposed to an unsaturated stream of air. To demonstrate this process, three dispersion methods were selected for bench-scale testing; the static breaker, air jet conveyor, and centrifugal fan. Each of these devices was chosen for its ability to fully disperse and pneumatically convey the feed cake. The moisture content of the feed cake, and the temperature and relative humidity of the process air were the key parameters that most significantly affected dryer performance. Of the three methods tested, the centrifugal fan produced the best results. The fan was capable of handling feeds as wet as 21.5% and consistently dried the coal fines below 2% moisture. The cost of the air and heat required to provide good drying performance was modeled to explore the practicality of the drying process. Modeling was accomplished by modifying equations developed for thermal dryers. The modeling results indicate, if good exposure of the fine particle surface area is achieved, dryers operating with either heated or unheated (ambient) air can be used for drying ultrafine coal.
Master of Science

A vast majority of coal and mineral cleaning and upgrading processes involve the addition of water. The water allows the movement of particles throughout the processing plant and the upgrading of the material. When the process is complete the finished product must be dewatered. This is due to storage concerns, in which the water takes up a majority of the space, and high transportation costs, in which no compensation is obtained from the buyer for the shipment of the liquid. Dewatering is accomplished by many devices, with the two most common pieces of equipment being the screen-bowl centrifuge and disk filter. This thesis tests and compares the effect of reagents on dewatering using the screen-bowl centrifuge and disk filter. Coal was obtained from the Upper Banner, Pittsburgh No. 8, Taggart, and Dorchester seams, crushed and ground to the desired size, and run through the dewatering circuits. The results showed that the moisture content of the product can be greatly reduced in the disk filter while being only slightly reduced in the screen-bowl centrifuge. It was also shown that the recovery can be slightly increased in the screen-bowl centrifuge. Overall, with the addition of reagents, the disk filter outperformed the centrifuge in both recovery and moisture content. A model was also developed for the screen-bowl centrifuge. The results from the screen-bowl tests helped in the development of this model. This model can be used to predict the moisture content of the product, the recovery, particle size distribution of the effluent and particle size distribution of the product. The model also predicted how the product moisture and recovery were affected by changing the feed flow rate, feed percent solids, centrifuge speed, and particle size distribution.
Master of Science

Coal preparation plants use large quantities of water for cleaning processes. Upon cleaning, the spent water must be removed such that the final product moisture level meets market constraints. However, removal of free water from the surface of fine particles is difficult and costly, and often the results are less than desirable. Fine particles inherently have very large surface areas, and hence retain large amounts of water. Increased amounts of fines also cause denser particle packing, which creates relatively small capillaries in filter cakes and, thus, cause slower dewatering kinetics. As a result, dewatering costs for fine particles are much higher than for dewatering coarse particles. Considering the technical and economic issues associated with dewatering coal and mineral fines, an extensive matrix of laboratory- and pilot-scale dewatering tests have been conducted to evaluate the use of novel dewatering aids. The reagents are designed to lower the surface tension of water, increase the hydrophobicity of the particles to be dewatered, and increase the capillary radius by hydrophobic coagulation. All of these are designed to lower the moisture of the filter cakes produced in mechanical dewatering processes. Laboratory-scale dewatering tests confirmed that using the novel dewatering aids can lower the final cake moisture of coal by 20-50%, while increasing the dewatering kinetics. Several on-site, pilot-scale tests were conducted to demonstrate that the process of using the novel dewatering aids can be scaled. Based on the laboratory- and pilot-scale tests conducted, a scale-up model for the process of using the novel dewatering aids has been developed. It can predict the final cake moistures as a function of vacuum pressure, filtration time and specific cake weight. The model can be useful for the scale-up of vacuum disc filters (VDF) and horizontal belt filters (HBF). Simulation results indicate that dewatering aids can be very effective, especially when used in conjunction with HBF due to its ability to control cake thickness and drying cycle time independently. In light of the promising laboratory- and pilot-scale test results, an industrial demonstration of the novel dewatering aids has been conducted at the Smith Branch impoundment site, which contains 2.9 million tons of recoverable coal. When the reagent was used for dewatering flotation products using a VDF, the moisture content was reduced from 26 to 20% at 0.5 lb/ton of reagent addition and to 17.5% at 1 lb/ton. The use of the dewatering aid also improved the kinetics of dewatering, increased the throughput, and reduced the power consumption of vacuum pumps by 30%. The novel dewatering aids were also tested successfully for dewatering of kaolin clays. In this case, the mineral was treated with a cationic surfactant before adding the dewatering aids. This two-step hydrophobization process was able to reduce the cake moisture and also increase the throughput.
Ph. D.

Dewatering coal, and especially fine coal (-600μm), is a significant problem in the preparation of coal. The final moisture level of fine coal can be anything up to 30% by weight, depending on the type of dewatering equipment used. Moisture in coal can cause many problems, for example by increasing the transportation costs, as well as decreasing the calorific value of the coal. In industry today there is a need for a dewatering technique that will produce a drier final product. It was found that an interruption in the application of vacuum during a single dewatering cycle yielded a filter cake with a lower final moisture content. It was also demonstrated that the rate at which the coal is being dewatered is much higher than during continuous vacuum application. A further study of this phenomenon showed a twofold time dependency involving both the duration of the vacuum break, and the instant it is introduced in the dewatering cycle. An optimum was found at about 29s time duration and an introduction time of 30s, after the start of the cycle. The possibilities of diffusion and cake structural changes were investigated. For the diffusion tests, repeated interruptions of the vacuum were performed during a single dewatering cycle. Although the kinetics agreed with what was expected, the final moisture content was not as low as that found for the optimum single break test. The compressibility of a coal filter cake was one of the structural changes investigated, the other being an increase in area and, thus, airflow through the cake. Coal filter cakes were shown to be largely incompressible. It was, however, shown that an increase in area, and thus an increase in the airflow through the cake, gave excellent results. An increased area resulted in a much lower final moisture content as well as an increase in the dewatering rate. The addition of a cake surface cutter to a standard vacuum belt filter will make the application of these finfings relatively easy to industry.
Thesis (M.Ing. (Chemical Engineering))--Potchefstroom University for Christian Higher Education, 2003.

This objective of this project was to develop a new dewatering device that could produce a lower moisture content and better fine particle recovery than current technology. To meet this goal, a hyperbaric horizontal belt filter was designed and constructed over the course of 18 months. Once built, the filter was then thoroughly tested to determine operational capabilities. The test data showed that the lowest moisture content that could be achieved with a coarse feed (minus 1 mm screen-bowl centrifuge feed) was 8.8%. This value could be further reduced to 8.2% and capacity increased with the use of dewatering aids. When testing with a fine feed (minus 0.15 mm column product feed), the lowest moisture content was 35% without chemicals and 29% with chemicals. A 50/50 mixture by volume of coarse and fine feeds was artificially created and provided a moisture of 10.8%, which was reduced using reagents to 8.4%. The machine provided a very high recovery rate for all feed materials. Of the coal input, no less than 94% of it reported to the dry product. The pressure used to dewater the coal was the controlling factor for the air consumption of the unit. The data from these tests suggest that a full size production unit is feasible, although the power requirements for gas compression would be high.
Master of Science

The effects of coal bed methane (CBM) development on the quantity and quality of groundwater in the vicinity of the City of Merritt, British Columbia were assessed through a modeling study. The impacts of coal seam dewatering for C B M at a pilot scale and at a regional scale are assessed here using a series of groundwater flow models. Two potential pathways were identified that could hydraulically connect a dewatered coal seam and the aquifer: faults within the Tertiary rock and coal seam subcrops. A pilot scale model included coal seam subcrops along the unconformity between the Tertiary rocks and the Quaternary sediments and examined their potential response to coal seam dewatering. Using estimates of hydraulic conductivity (K) and subcrop exposure, the rate at which groundwater enters the subcrops ranges from approximately 7500 m³/ day for a high hydraulic conductivity scenario to approximately 70 m³/day for a low hydraulic conductivity scenario. For the medium hydraulic conductivity scenario the groundwater loss was 725 m³/day. Under a modified scenario where dewatering takes place only in relatively continuous coal seams and relatively far from subcrops, the loss was approximately 45 m³/day. The regional scale model assessed the role of a fault that extends from the southwest to the northeast through the region. For a thick, high hydraulic conductivity fault, the estimated loss was approximately 1430 m³/day whereas for a narrow, medium hydraulic conductivity fault the estimated loss was 83.2 m³/ day. Based on the results of this study, if coal seam dewatering takes place in areas relatively unaffected by faults, subcrops or other potentially high hydraulic conductivity features, the risk towards the City of Merritt's groundwater supply are likely to be low. However, as the city continues to develop and the groundwater demands increase, there is inherently greater risk to the groundwater supply.

Successful dewatering and filtration of coal fines remain the major obstacles in preventing the extensive re-use of large reserves of high calorific quality coal fines as an additional energy source in South Africa. The high levels of final moisture in coal fines make it uneconomical to transport, handle and use. The industry is rapidly reaching the limit of current technology of mechanical dewatering; this limit is defined by fundamental coal properties, like amongst others, particle size, porosity and mineral content. This thesis describes research investigating a shift in approach from high vacuum or pressure systems, to high air flow systems. Results from various projects at laboratory scale showed that it was possible to decrease the fine coal filter cake moisture to as low as 15%. This was obtained by allowing air to flow freely through a filter cake, even at ambient temperatures, and replacing the necessity for high applied vacuum levels. There was also an increase in the dewatering rate, as well as a lower breakthrough pressure. Such an approach can utilise existing equipment with minor modifications. Other investigations showed that forced air-drying, both at ambient and elevated temperatures, could be used to overcome this mechanical limit. Again, an increased air flow rate at ambient pressure was used. Using air drying, moisture levels down to zero were possible. These investigations led to the conclusion that increased air flow through a fine coal cake was more advantageous than an increase in the applied vacuum, or a longer dewatering time. This new approach to lowering the final moisture content in coal fines is crucial in any advancement of the use of this largely untapped energy source.
Thesis (Ph.D. (Chemical Engineering))--North-West University, Potchefstroom Campus, 2006.

A simple thermodynamic analysis suggests that oil can spontaneously displace water from coal's surface if the coal particle has a water contact angle greater than 90°. However, the clean coal products obtained from laboratory-scale dewatering-by-displacement (DbD) test work assayed moistures substantially higher than expected. These high moisture contents were attributed to the formation of water-in-oil emulsions stabilized by coal particles. Four different approaches were taken to overcome this problem and obtain low-moisture agglomeration products. These included separating the water droplets by screening, breaking emulsions with ultrasonic energy, breaking agglomerates with ultrasonic energy, and breaking agglomerates using vibrating mesh plates. On the basis of the laboratory test work, a semi-continuous test circuit was built and tested using an ultrasonic vibrator to break the water-in-oil emulsions. The most promising results were obtained agglomerates were broken using the ultrasonic probe and the vibrating mesh plates. Tests conducted on flotation feed from the Kingston coal preparation plant gave a clean coal product containing 1% by weigh of moisture with a 94% combustible recovery. The separation efficiency of 93% is substantially higher than results achievable using froth flotation. When agglomerates formed from thermal coal from the Bailey coal preparation plant were broken using either ultrasonic energy or vibrating mesh plates, the obtained results were very similar: clean coal products assayed less than 5% moisture with separation efficiencies of 86% in average.
Master of Science

In the present work, novel dewatering aids and a novel centrifuge configuration were developed and applied for the purpose of dewatering fine particles. Three different types dewatering reagents were tested in different filtration and centrifugation units. These chemicals included low-HLB surfactants, naturally occurring lipids, and modified lipids. Most of these reagents are insoluble in water; therefore, they were used in solutions of appropriate solvents, such as light hydrocarbon oils and short-chain alcohols. The role of these reagents was to increase the hydrophobicity of the coal and selected mineral particles (chalcopyrite, sphalerite, galena, talc, clay, phosphate, PCC and silica) for the dewatering. In the presence of these reagents, the water contact angles on the coal samples were increased up to 90o. According to the Laplace equation, an increase in contact angle with the surfactant addition should decrease the capillary pressure in a filter cake, which should in turn increase the rate of dewatering and help reduce the cake moisture. The use of the novel dewatering aids causes a decrease in the surface tension of water and an increase in the porosity of the cake, both of which also contribute to improved dewatering. A series of batch-scale dewatering tests were conducted on a variety of the coal and mineral samples using the novel dewatering aids. The results obtained with a Buchner funnel and air pressure filters showed that cake moistures could be reduced substantially, the extent of which depends on the particle size, cake thickness, drying time, reagent dosage, conditioning time, reagent type, sample aging, water chemistry, etc. It was determined that use of the novel dewatering aids could reduce the cake formation time by a significant degree due to the increased kinetics of dewatering. At the same time, the use of the dewatering aids reduced the cake moistures by allowing the water trapped in smaller capillaries of the filter cake. It was found that final cake moistures could be reduced by 50% of what can be normally achieved without using the reagents. However, the moisture reduction becomes difficult with increasing cake thickness. This problem can be minimized by applying a mechanical vibration to the cake, spraying a short-chain alcohol on the cake and by adding a small amount of an appropriate coagulant, such as alum and CaCl2 to the coal and mineral slurries. The novel dewatering aids were also tested using several different continuous filters, including a drum filter, disc filter and horizontal belt filter (HBF). The results obtained with these continuous filtration devices were consistent with those obtained from the batch filters. Depending on the coal and mineral samples and the type of the reagent, 40 to 60% reductions in moisture were readily achieved. When using vacuum disc filters, the cake thickness increased substantially in the presence of the novel dewatering aids, which could be attributed to the increased kinetics of dewatering. A dual vacuum system was developed in the present work in order to be able to control the cake thickness, which was necessary to achieve lower cake moistures. It was based on using a lower vacuum pressure during the cake formation time, while a full vacuum pressure was used during the drying cycle time. Thus, use of the dual vacuum system allowed the disc filter to be used in conjunction with the novel dewatering aids. Its performance was similar to that of HBF, which is designed to control cake thickness and cake formation time independently. The effectiveness of using the novel dewatering aids were also tested in a full-continuous pilot plant, in which coal samples were cleaned by a flotation column before the flotation product was subjected to the disc filter. The tests were conducted with and without using novel dewatering aids. These results were consistent with those obtained from the laboratory and batch-scale tests. The novel centrifuge developed in the present work was a unit, which combined a gravity force and air pressure. The new centrifuge was based on increasing the pressure drop across the filter cake formed on the surface of the medium (centrifuge wall). This provision made it possible to take advantage of Darcy s law and improve the removal of capillary water, which should help lower the cake moisture. A series of tests were conducted on several fine coal and mineral particles and obtained more than 50% moisture reduction even at very fine particle size (2 mm x 0). Based on the test results obtained in the present work, two proof-of-concept (POC) plants have been designed. The first was for the recovery of cyclone overflows that are currently being discarded in Virginia, and the other was for the recovery of fines from a pond in southern West Virginia. The former was designed based on the results of the plant tests conducted in the present work. Cost vs. benefit analyses were conducted on the two POC plants. The results showed very favorable internal rates of return when using the novel dewatering aids. Surface chemistry studies were conducted on the coal samples based on the results obtained in the present investigation. These consisted mainly of the surface characterization of the coal samples (surface mineral composition, surface area, zeta potential, x-ray photoelectron microscopy (XPS)), acid-base interactions of the solids and liquids, dewatering kinetic tests, contact angle measurements of the coal samples and surface force measurements using AFM. In addition, carbon coating on a silica plate using palsed laser deposition (PLD) and Langmuir-Blodgett (LB) film deposition tests were conducted on the sample to better understand the surfactant adsorption and dewatering processes. The test results showed that the moisture reductions on the fine particles agree well with the surface chemistry results.
Ph. D.

Effective dewatering and environmental program poised to have a significant impact on the feasibility of saprolite mining operations. It is therefore necessary to strike a balance between an effective dewatering program and sound environmental policy. Using assessments such as rainfall, climate studies, groundwater flow, and aquifer characterizations, the Separi coal dewatering program includes the construction of water channels, flood protection levees, water wells, and placing various environmental monitoring sites. The construction of water channels and flood protection levees has reduced the water runoff that entered the mining area by approximately 75%. For a six-month testing period, the average pumping rate of the dewatering well was 24.78 m³/day. These pumping rates were determined to result in groundwater level that would generally be 10 meters below the lowest mining benches at all times. Ten meters is the recommended single bench height based on the slope stability analysis. After six months of dewatering, the groundwater level was lowered 10.88 meters, permitting the mining project may begin its mining operation to commence. A re-design of maximum pit slope angle is indicated in this research. During the testing period, the environmental management plan did not show any negative impacts of dewatering programs on surface and groundwater resources. The monitoring sites all yield acceptable range of water quality parameters, such as Electrical Conductivity (EC), Total Suspended Solids (TSS), Total Dissolved Solids (TDS), and pH value. The company continues to monitor the water resources to maintain acceptable water quality in the study areas.

Student Number : 0418764K - MSc (Eng) dissertation - School of Civil and Environmental Engineering - Faculty of Engineering and the Built Environment
Increasing quantities of finer wastes often contain reactive sulphide minerals and high water contents that pose stability and environmental concerns. This study investigates how electrokinetic process can be improved, to make it more viable towards dewatering finer coal slurries. In the electrokinetic process, a direct current induces the movement of water out of a porous material. A wooden test box was filled up to two-thirds with fine coal slurries. Electrokinetic Geotextiles (EKGs) and brass were used as electrodes. The conducting wires were attached to each electrode and connected to a DC source to form an electro-osmosis cell. Current was passed through the cell and water moved to the cathode where it was withdrawn. The dewatering efficiencies ranged from 13.13 to 109.84 ml/Ah. The energy consumptions ranged from 5.23 to 14.03 kWh/m3 and are in line with those recorded by Johns (2005). Conductivity and pH measurements were taken. EKGs performed better than brass electrodes.

A dissertation submitted to the Faculty of Engineering, University o£ the Witwatersrand, Johannesburg, in fulfilment of the requirement for the degree of Master of Science in Engineering 1995
MT2017

M.Sc. (Eng.), Faculty of Engineering and the Built Environment, University of the Witwatersrand, 2012
The aim of this research project was to test the response of ultra fine coal sourced at Klipspruit Colliery to froth flotation and the response of the froth flotation products to dewatering using two different types of filter presses, namely the Tecnicas Hidraulicas (TH) and the Ishigakhi presses. During test work, some difficulty was experienced with coarse material feeding the froth flotation pilot plant. This led to pilot plant modifications. Further process complexities necessitated laboratory scale flotation test work on the Klipspruit coal to be carried out. The results for both the laboratory scale and pilot plant test work for froth flotation indicated that froth flotation as applied to the Klipspruit fines was not economically feasible because neither the required quality of the product (calorific value of 27.80 MJ/kg) nor the product yield of 50% could be achieved when subjected to a primary and secondary stage of froth flotation. The coarse material, which fed the pilot plant and the Ishigakhi filter press, gave low moisture values (12.3%) not typical of ultra fine coal moisture values. However when fed with very fine particle size distributions, prior test work with the Ishigakhi showed that moisture values below 20% could be achieved. The moisture values obtained for very fine particles using the TH filter press on product thickener underflow material sourced at Goedehoop colliery reached values below 20%. Thus both of the two dewatering options, i.e. the Ishigakhi filter press equipment or TH filter press equipment for the ultra fine coal dewatering, can be utilized. Since the filter rate is the determining factor specifying filter press size, it was determined that a larger TH filter area is required in 1 comparison with the Ishigakhi press. Based upon the pilot and laboratory scale test work undertaken and the assessment of the results, it appears that both dewatering options could be successfully employed on a technical basis for the dewatering of coal flotation products, tailings and the arising raw ultra fine fraction. Froth flotation for Klipspruit ultra fine coal was deemed unfeasible for both pilot plant and laboratory scale tests conducted. For this reason a capital expenditure for the construction of a froth flotation plant at the Western Coal Complex Phola plant was not considered feasible since Klipspruit coal forms part of the feed that will feed the Phola plant. In conclusion, following dewatering using either the TH filter press or the Ishigakhi filter press, it was established that both froth flotation concentrate and unbeneficiated ultra fines gave acceptable total moisture results (below 20%). These dewatered raw ultra fines may therefore be blended into inland product as thermal coal to be utilised by Eskom for power generation. Based upon this premise, it is estimated that profits of 76.5 million Rand could be generated by blending Klipspruit ultra fine coal into thermal coal production at the new Phola plant.

Fluidised bed drying is currently receiving much attention as a dewatering option after the beneficiation of fine coal (defined in this study as between 1mm and 2mm particles). The aim of this study was to investigate the removal of moisture from fine coal by using air at relatively low temperatures of between 25°C and 60°C within a controlled environment by lowering of the relative humidity of air. The first part of the experimental work was completed in a controlled climate chamber with the coal samples in a static non-fluidised state. Drying in the second part was carried out using a fluidised bed with conditioned air as the fluidising medium. Introduction of airflow to the system led to a lower moisture content in the coal samples and it also proved to have the ability to increase the drying rate. It was determined that the airflow had the ability to remove more free moisture from the filter cake. In addition more inherent moisture could also be removed by using upward flowing air, resulting in a lower equilibrium moisture content. It was proven that the airflow rate and relative humidity of the drying air contributed to faster drying rates. The effect of temperature was not as significant as expected, but higher temperatures did increase the drying rate at higher airflow and lower humidity conditions. The larger surface areas of particles create surface and capillary forces that prevent the moisture from leaving the finer coal particles. It was found that the rate of drying is independent of the moisture content in the coal sample. Just in terms of the fastest drying time and drying rate in the fluidised bed, it was concluded that the most efficient conditions is airflow above minimum fluidisation point causing vigorous mixing and maximum contact with the drying air. In addition to the high airflow it was concluded that 30% relative humidity and 55°C resulted in the fastest drying time. All the drying processes at all the airflow rates, temperature and relative humidity conditions were energy efficient. This process was shown to be energy positive, resulting in an overall energy gain. The overall energy consumption for the fluidised bed is lower than for all the dryer systems compared to and it compared favourably with other thermal drying technologies. It was therefore shown that this is a viable technology for the dewatering of fine coal.
MIng (Chemical Engineering), North-West University, Potchefstroom Campus, 2014

碩士
國立臺灣大學
環境工程學研究所
87
In the sludge treatment, dewatering process is an important unit. No matter how the sludge is disposed, reused, incinerated, or landfilled, volume reduction and the water content lowering of sludge should be done at first. Thus, the research concentrated on the efficiency improving of sludge dewaterability by using the fly ash from the coal-fired power plant as filter aid in the dewatering process of sludge treatment. The objective of this study was to reduce the treatment and disposal loads of sludge in both wastewater treatment plant and water treatment plant. The scope of this research included PolyAluminum chloride (PAC) sludge, alum sludge, and food-processing biological sludge. Capillary suction time (CST), specific resistance to filtration, and coefficient of compressibility were adopted as the evaluating factors of sludge dewatering efficiency. According to the experimental result, the most suitable chemical conditioners are all cationic polyacylamides (PC-325C). When the fly ash is employed as filter aid, the optimal addition dose to PAC sludge, alum sludge, and food-processing biological sludge are 8%, 4% and 8% respectively. When the fly ash is used as the sole conditioner, it is not so pronounced in improving the sludge dewaterability. But, dual conditioning process using a physical conditioner (fly ash) with a chemical conditioner (cationic polyacylamides PC-325C) can greatly improve the dewatering characteristics of sludge. Through the skeleton building by the fly ash, the sludge and polyacylamide form a sludge-polymer matrix which has been beneficial in increasing the porosity, rigidity, and incompressibility of sludges. As a result, the efficiency of sludge dewatering is greatly improved, the optimal dosage for PAC sludge, alum sludge, and food-processing sludge are 25 mg/L of PC-325C with 2% fly ash, 15 mg/L of PC-325C with 2% fly ash, and 30mg/L of PC-325C with 6% fly ash respectively. Besides, a good regression linear relationship (R2 > 0.7) exist between the specific resistance to filtration, coefficient of compressibility and the capillary suction time can be applied in evaluating the dewatering characteristics of PAC, alum and food-processing sludges. Keyword: fly ash, sludge dewatering, sludge conditioning, dual conditioning, capillary suction time, specific resistance to filtration, coefficient of compressibility.