Ultrafine Coal Research Articles

Micro-properties of coal aggregates: Implications on hyperbaric filtration performance for coal dewatering

The recovery of ultrafine coal can be enhanced significantly by flocculating the particles prior to dewatering. It is thus crucial to be able to quantify the effects of aggregated particles on the separation efficiency, of which they are still not fully understood. This study focused on the relationship between the micro-properties (size, structure, and strength) of flocculated coal particles and the characteristics of filter cakes obtained through hyperbaric filtration. Tools such as small angle light scattering and confocal scanning laser microscopy were used to determine the aggregate structure. The comparative bond strength of aggregates generated under different flocculation conditions was measured through a non-dimensional approach. The effects of these properties on specific cake resistance, saturation, and moisture content, as indicative of the filtration performance, were observed. It was found that flocs generated using a moderate amount of flocculant dosage (60–100 g of polymer/tonne of coal) for this ultrafine coal (97% < 35 μm) optimised the properties of dewatered filter cakes. Although cakes composed of larger and stronger aggregates with more flocculant were found to have higher porosity, the effects on specific cake resistance and cake saturation were detrimental, possibly due to the increasing proportion of intra-aggregate pores that were harder to dewater. The trend was less noticeable at low filtration pressures where most of the dewatering occurred within the larger capillaries of the inter-aggregate zones. In addition, the presence of polymer molecules was responsible in absorbing residual moisture that could not be removed through mechanical means. Through a series of tests using pre-formed filter cakes, the amount of moisture retained by the flocculant was quantified to be approximately 10.2 mg of water for every addition of 1 g of polymer/tonne of coal, calculated per 100 g of filter cake produced from hyperbaric filtration in this case.

Characterization of ultrafine coal fly ash particles by energy‐filtered TEM

In this study, energy-filtered transmission electron microscopy is demonstrated to be a valuable tool for characterizing ultrafine coal fly ash particles, especially those particles encapsulated in or associated with carbon. By examining a series of elemental maps (K-edge maps of C and O, and L-edge maps of Si, Al, Ti and Fe) recorded using the three-window method, considerable numbers of titanium and iron species with sizes from several nanometres to submicrometre were shown to be present, typically as oxides dispersed in the carbonaceous matrix. Crystalline phases, such as rutile and iron-rich oxide spinel, were also identified from electron diffraction patterns and high-resolution TEM images. Information about these ultrafine coal fly ash particles regarding their size, morphology, elemental composition and distribution, and crystalline phases, which has not been available previously in conventional ash studies, should be useful in toxicological studies and related environmental fields.

STEEL BELT FILTER: A SUITABLE TECHNOLOGY FOR DEWATERING FINE CLEAN COAL AND TAILINGS

ABSTRACT Many coal preparation plants in the United States continue to discard the minus 150 micron size fraction of their run-of-mine coal without any attempt to recover the clean coal content of this size fraction. This undesirable practice is partly caused by a variety of deficiencies with existing fine coal dewatering technologies, such as high moisture content of the product, loss of significant amounts of ultra-fine clean coal, high capital/operating costs, and operational unfriendliness. This study investigated the suitability of a newly developed fine particle dewatering technology, known as Steel Belt Filter (SBF), for dewatering both fine clean coal and fine tailings materials generated at coal preparation plants in the United States. The unique feature of this new technology is the combined use of mechanical pressure and suction force (vacuum), a combination which is known to provide excellent dewatering performance. A SBF prototype unit having a belt width of 0.6 m was tested at the Illinois Coal Development Park. A factorial experimental design using the response surface methodology was conducted to optimize the dewatering performance of the SBF prototype unit. Two different clean coal slurry samples having mean particle sizes of 400 micron and 50 micron and two fine coal tailings samples having mean particle sizes of 20 and 10 micron were dewatered using the SBF technology. A solid recovery of greater than 99% was achieved by using a suitable flocculant type and dosage for each sample. Surface moisture content of dewatered products varied from nearly 18.5% for coarser clean coal to 31% for finer clean coal and 18% for coarser tailings to 26% for finer tailings. A preliminary economic analysis conducted for a near-term commercialization of the SBF technology in the U.S. coal industry estimates the SBF dewatering cost for combined spiral + flotation product to be $1.09/ton of dry coal.

Ultrafine Coal Cleaning Using a Centrifugal Fluidized-Bed Separator

Ultrafine coal (nominal 150 × 44 µm) cleaning using a pilot-scale, centrifugal fluidized bed separator was investigated. The effects of the operating parameter values were evaluated and optimized to maximize mass recovery at a given product grade. Significant ash reductions were achieved from the cleaning of two coal sources having different cleaning characteristics. The concentrator provided a range of separation densities between 1.46 RD to 2.18 RD with probable error (EP) values of around 0.25. Rougher-scavenger circuit simulations indicate that the process efficiencies can be improved significantly while achieving low density separations. EP values ranging from 0.17 to 0.20 were realized, while the organic efficiency increased by 20 absolute percentage points.

An investigation on dewatering kinetics of ultrafine coal

Ultrafine coal dewatering is of great importance to the coal industry due to its impacts on the handling and utilization characteristics of coal products. Commercially available filtration techniques are either ineffective or costly for dewatering of ultrafine coal to the desired moisture level of about 20%. Considerable efforts have been devoted to understanding the dewatering process and developing new technologies for applications in the coal industry. Most of the previous work on ultrafine coal filtration was focused on the final filter cake moisture and few studies have been conducted to investigate the filtration kinetics. The present investigation was undertaken to better understand the kinetics of vacuum filtration of ultrafine coal under various conditions. The filtrate weight was continuously monitored using a precise load cell during the entire filtration process. Operating parameters such as vacuum pressure and reagent conditioning time were examined for their impacts on filtration kinetics. Use of cationic and anionic flocculants showed significant improvement in filtration kinetics. Kinetic data obtained from the study were used to determine the fundamental parameters of filtration such as cake permeability, specific cake resistance, and filter medium resistance using the integrated form of the Darcy's law.

Prospects for use of micronized coal in power industry

Heat-and-power engineering is the basis for industrial development of developed countries and the main energy fuel for plants is coal. The main directions in improvement of coal energy technologies are related with better ignition of powered fuel and with gas and mazut substitution with coal powder. This paper considered the prospects of energy coal enrichment and the method for production of ultrafine coal with the average size of particles about 10-20 microns, and the existing machines for ultrafine coal production. This method increases substantially the velocity of ignition and combustion of pulverized coal flame. The changes of physical and chemical properties of coal after grinding were considered, the processes of ignition, combustion of micronized coal, spaying and stabilization of flame combustion were analyzed in this paper. The problem of ultrafine coal ignition were considered also.

Evaluation of a solid-bowl centrifuge for ultrafine size separations

Solid-bowl (decanter) centrifuges are used extensively to dewater slurries in a wide range of industries. However, the application of decanters as classification devices is not as common. In this study, the performance of a continuous, pilot-scale, solid-bowl centrifuge was evaluated when classifying an ultrafine coal slurry containing nearly 60% material less than 10 μm. The test conditions included centrifuge bowl and scroll speeds, volumetric feed rate, solids concentration and pond depth. Centrifuge performance was evaluated using size selectivity curves and the corresponding performance parameters, along with product yields and solids concentrations.

Effects of Vacuum Filtration Parameters on Ultrafine Coal Dewatering

Coal fines (-0.6 mm) are often cleaned using wet separation processes such as froth flotation. Dewatering of clean coal slurry is essential for marketing the product and reducing the transportation cost. Vacuum filtration is one of the most widely used processes for fine coal dewatering. However, the filter cake often contains more than 20% moisture demanded by the coal industry The present study was undertaken to better understand ultrafine coal (-0-15 mm) dewatering process using vacuum filtration by evaluating several important but often overlooked process parameters. A continuous filter leaf dewatering system and a small-scale drum filter were employed to investigate effects of cake formation time, cake drying time, flocculant conditioning time, feed size distribution, slurry agitation intensity etc. Results have shown that ultrafine coal dewatering performance evaluated by cake moisture, solids throughput etc., is strongly dependent on these parameters. Enhanced ultrafine coal dewatering can be achieved by optimizing these process variables.

00/01201 Efficiency promotion in separation of ultrafine coal

00/01201 Efficiency promotion in separation of ultrafine coal

00/00626 Adaptation of a briquetting machine for the dewatering and consolidation of fine coal

The Illinois Clean Coal Institute (ICCI) developed a new coal preparation method to recover the high heating value Illinois coal, containing high amounts of sulfur (in the form of pyrite). Unfortunately, the new cleaning method, consisting of fine grinding the coal to liberate mineral matter, such as the pyrite, produced large amounts of high moisture ultra-fine coal. One of the economically viable alternatives to dewater and recover the fine coal was to adapt a commercially available machine. A roll briquetting machine was chosen for the dual purpose of dewatering and consolidating fine coal. Preliminary results indicated that robust briquettes with low moisture content could be produced under the current configuration. However, two areas of concern were realized during the pilot-scale experiments. First, the arching and caking problems resulted in inconsistent feeding of material. Second, back drainage into the feed hopper resulted in varying the feed moisture. Therefore, to effectively utilize the briquetting machine to dewater fine coal, appropriate mechanical modifications were needed. Design changes were conducted on the feed area of the machine, which resulted in the elimination of the problems encountered. Briquettes with consistent low moisture content and adequate strength characteristics could now be produced under the new configurationmore » of the briquetting machine.« less

Enhanced ultrafine coal dewatering using flocculation filtration processes

Ultrafine coal (−150 μm) can be effectively cleaned using advanced separation techniques such as column flotation, however, dewatering it to below 20 percent moisture level using the conventional dewatering techniques is difficult. A comparative flocculation filtration study was performed for enhancing dewatering of ultrafine coal using vacuum, hyperbaric, and centrifugal filters. The cationic and anionic flocculants were added into the slurry individually or in combinations. Vacuum filtration results showed that use of flocculants increased filtration rate by several times and/or substantially reduced cake moisture. Combined use of anionic and cationic flocculants showed further improvement. Addition of flocculants significantly increased filtration rate of hyperbaric filtration and reduced cake moisture in centrifugal filtration. Anionic flocculant was more effective in enhancing fine coal dewatering than cationic flocculant in vacuum filtration while cationic flocculant was more effective in high shear centrifugal filtration. A new approach on using flocculants in vacuum filtration is proposed for enhanced fine coal dewatering.

Effect of various gases on the column flotation of fine coal

Column flotation has proven to be the most effective technique for the flotation and recovery of ultrafine coal. The commercial application of columns in the coal industry is steadily increasing (Parekh et al., 1990; Yoon et al., 1992). For the flotation of coal or mineral air is used for generating bubbles. Using a conventional flotation cell, Miller and Mishra (1985) compared froth flotation behavior of fine coal using air, nitrogen and carbon dioxide gas. They concluded that carbon dioxide gas provided a higher rate of recovery and a lower ash content. The objective of the present study was to investigate the effect of various gases such as compressed air, nitrogen, argon, and carbon dioxide on the column flotation of fine coal.

97/02611 Surface chemical control of ultra-fine coal to improve dewatering

97/02611 Surface chemical control of ultra-fine coal to improve dewatering

97/01641 Preparation of ultrafine and ultraclean coal

Preparation of ultraclean and ultrafine coal has been conducted in the authors laboratory. The coal particle size could be reduced to about 1 {micro}m by using a special micron grit mill after a bowl mill. The optimum conditions for desulfurization and demineralization of Datong coal with caustic leaching were established, and it was found that the particle size has significant effects on the deep cleaning of coals. Then the coal with particle size of less than 2--3 {micro}m could be produced with the ash and sulfur content of less than 0.2% and 0.08% respectively.

Pelletization studies of ultra-fine clean coal

Handling of fine coal is an importance issue for coal as well as the utility industry. Reconstitution in the form of a pellet or briquette would be desirable if it could be done economically. This paper evaluates the effectiveness of three binders e.g., asphalt-emulsion, corn starch and Brewex, in forming pellets of ultra-fine clean coal. It was fond that asphalt emulsion and corn starch were not effective binders for ultra-fine clean coal, however, Brewex provided excellent quality of pellets, which exceeded all the minimum quality requirements of coal pellets.

Dry coal cleaning process for high-quality coal.

In this study, a dry coal cleaning process was developed for cleaning fine and ultrafine coal to obtain a high-quality coal. The design criterion was determined, and the operational conditions were formulated. In addition, the deash and desulfurization performances for this process were examined.A classification column was used for fine coal down to 45 microns, and a centrifugal classifier was also used for cleaning ultrafine coal particles of less than 38 microns. Some species of coal were cleaned into a high-quality coal that contained less than 1% ash content by the fluidized classification column. For finely ground coal, ash was concentrated in the ultrafine coal fraction. Pyritic sulfur was also removed to some extent in this process for fine coal particles. For most coals, the ash content could be reduced by half and the thermal recovery was up to 90%. The method of comminution was also found to play an important role in the coal cleaning process.

Coal Dewatering: Australian R&amp;D Trends

Coal (and tailings) dewatering is a major RD judicious selection, modification and application of dewatering technology; and control of the process in respect of feed material, operational parameters, and product characteristics. For these areas, this paper analyses R&D needs, summarises recent R&D results, and discusses the on-going research in the context of the Australian coal industry situation, including the critical economic constraints. Both centrifugal and filtration technologies and applications to coarse, small, fine and ultrafine coal and also tailings, are covered. The overall analyses are distilled into a summary Table of current status and R&D trends for five distinct categories of coal suspensions.

Surface Chemical Control of Ultra-Fine Coal to Improve Dewatering

Lowering of filter cake moisture of ultra-fine (d50 = 25 µm) coal to a 20 percent level using the conventional vacuum filtration could be achieved using a combination of metal ions and surfactants. Pretreating coat slurry with about 0.2 Kg/ton of a surfactant and 50 ppm of copper or 10 ppm of aluminum ions provided filter cake containing 22.8 percent moisture. The lowering of moisture is explained due to formation of hydrophobic micro-aggregates which facilitates dewatering.

Beneficiation of ultrafine coal slurries

Beneficiation of ultrafine coal slurries

The inherent heterogeneity within the vitrinite maceral group

Research has been directed towards the characterization of ultra-fine coal. Equipment and procedures have been developed which permit full reflectance characterization on vitrinite particles as small as 1.5 μm diameter using rotational polarization techniques. The application of this technology towards the evaluation of vitrinite macerals separated using density gradient centrifugation has provided insights into the inherent heterogeneity of the vitrinite maceral group. It is concluded that much of the scatter in reflectance values obtained for a given coal is due to the presence of remnant cell structure. This structure cannot only be observed through etching, but can be directly measured using reflectance. Density gradient centrifugation can be used to separate and concentrate these remanent structures in high volatile bituminous vitrinite, provided the sample has been crushed to 5 μm or less. Within this rank range there is a strong trend towards increased reflectance with increasing vitrinite density.