Coal fines were suspended in water or in salt solutions for agglomeration with either heptane or paraffin oil. Coal recovery with a limited amount of oil was found to increase markedly as the salt concentration was raised. The increase in recovery appeared due to compression of the electrical double layer surrounding individual coal particles and oil droplets. The recovery was observed to depend on the pH of the suspension with the greatest recovery occurring at the isoelectric point. When a mixture of hydrophobic coal particles and kaolin particles was suspended in 1.5 M sodium chloride and agglomerated with paraffin oil at a level of 0.065 goil/gcoal, 95% of the coal was recovered and over 90% of the kaolin was rejected. This recovery was 265% greater than the recovery achieved by suspending the mixture in water and agglomerating it with the same amount of oil. These results show that the use of salt solutions in the oil agglomeration process can result in reduced oil consumption

A reaction scheme, developed at the Ames Laboratory, for the determination of the sulfur forms in coal takes advantage of the selective oxidizing power of perchloric acid. Sulfate, pyritic and organic sulfur are removed sequentially from a single sample of coal by solutions of perchloric acid boiling at 120, 155 and 205 °C, respectively, and converted to sulfate for subsequent turbidimetric measurement. Work in this paper focused on improving the selectivity by trying to remove pyrite at lower temperatures and improving the low sulfur recoveries observed for some coals. The results indicated that pyritic sulfur can be removed at lower temperatures and that possibly two different forms of organic sulfur can be delineated. After several potential explanations for the low sulfur recoveries had been explored, a standard additions technique, when applied to the turbidimetric measurement of sulfate, alleviated the problem of low sulfur recoveries.

Over 400 published papers, presentations at scientific meetings, and reports relating to the determination of sulfur and sulfur forms in coal-related materials have been accumulated, classified, and an evaluation made of their content.

Fine-size particles of coal and pyrite were suspended in aqueous solutions of various salts and agglomerated with heptane. The agglomerates were recovered by screening. For highly oleophilic Upper Freeport coal, the addition of a salt to the suspension improved the recovery, whereas for weakly oleophilic Illinois No. 6 coal or pyrite the addition of salt reduced the recovery. The improvement in recovery of Upper Freeport coal was independent of the type of salt, provided the ionic strength was held constant and complexation was avoided. On the other hand, the reduction in pyrite recovery was dependent on the type of salt with the effect increasing in the following order: NaCl < CaCl: < MgCl2. The agglomeration of both Upper Freeport coal and Illinois No. 6 coal was suppressed by adsorption of the metal hydroxy complexes of certain multivalent cations within specific and relatively narrow ranges of pH.

Precipitation of jarosite compounds to remove Na, Fe, and SO4(2-) from spent acid solutions from a chemical coal-cleaning process was studied in relation to reaction time and pH. Although Fe and SO4(2-) could be removed effectively from model solutions at pH values of 1.5-2.3, optimum Na removal was possible only within the narrow pH range of 1.4-1.6. Maximum precipitate yields were obtained within approximately 6 h at both 80 and 95-degrees-C, with at least 80% of the Fe and SO4(2-) and approximately 60-75% of the Na removed. An additional benefit of precipitation of jarosite compounds to remove impurities from spent acid streams is that these compounds have low solubilities in water and are therefore attractive from a waste disposal perspective. Based on this study, the feasibility of cleaning spent acid from a chemical coal-cleaning process by precipitating jarosite compounds has been demonstrated.

The association of mineral particles with the organic coal matrix is being quantitatively assessed using scanning electron microscope-based automated image analysis (SEM-AIA). Routine SEM analyses of mineral matter for particle size and mineral phase can now be supplemented by SEM-AIA results in which samples are also classified according to the degree of association of the mineral matter with the coal matrix. Such association can be measured either in terms of the mass fraction of the various minerals found in cross sections of the particles or in terms of the relative amount of mineral matter and coal present on the surface of the particles. These measurements can be related to the behavior of the coal during density-based and surface-based physical cleaning processes, respectively. Examples of such association measurements are included for samples of Upper Freeport and Indiana No. 3 coals. Variations in the coal-mineral association the two coals, and even for different minerals within the same coal, are compared to ash reduction and coal recovery during cleaning of these coals by various physical methods.

Molten caustic leaching is an advanced chemical coal-cleaning process which results in the removal of over 90% of the sulfur and ash from coal. One of the steps in this process is the water washing of caustic-leached coals to remove unreacted caustic and impurities released by reactions with the molten caustic. A countercurrent procedure, designed for efficient washing with minimal water consumption, has been evaluated in the present work. A Pittsburgh No. 8 coal was leached with a one-to-one mixture of molten sodium and potassium hydroxides, and the resulting coal-caustic cake was washed using this countercurrent procedure. The countercurrent washing did result in recovery of caustic at predicted concentrations, and a relatively ash-free and sulfur-free coal was the final product. However, significant problems occurred during the countercurrent washing, all of which could be linked with the formation of a massive precipitate of carbonates from the alkaline process streams. The mass of the precipitate retained fluids and thus led to far lower than predicted recoveries of caustic solutions. The precipitate also caused a significant decrease in filtration rates.

The surface of high-grade mineral pyrite from Huanzala, Peru, appeared to oxidize rapidly on exposure to the atmosphere and the oxidized surface was hydrophilic. When fine particles of this material were suspended in water and treated with small amounts of heptane under rigorous agitation, there was only limited tendency for the particles to agglomerate. However, if the pyrite particles, were first treated with an acidified solution of sodium sulfide, the particles became highly oleophilic and they were readily agglomerated by heptane. Compact spherical agglomerates were produced with relatively small amounts of heptane. The oleophilicity appeared due to a coating of elemental sulfur on the surface of the pyrite. The coating was fairly durable as the aggromerability of the particles declined only slightly when the material was stored as a dry powder for up to three weeks. The agglomerability of the coated pyrite declined somewhat in suspensions having a high pH. Also agglomeration of the material was suppressed by sodium sulfide, especially at high pH.

ADVERTISEMENT RETURN TO ISSUEPREVArticleAutomated image analysis of minerals and their association with organic components in bituminous coalsWarren E. Straszheim and R. MarkuszewskiCite this: Energy Fuels 1990, 4, 6, 748–754Publication Date (Print):November 1, 1990Publication History Published online1 May 2002Published inissue 1 November 1990https://pubs.acs.org/doi/10.1021/ef00024a023https://doi.org/10.1021/ef00024a023research-articleACS PublicationsRequest reuse permissionsArticle Views66Altmetric-Citations23LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose Get e-Alerts

Techniques employing scanning electron microscopy-based automated image analysis (SEM-AIA) are emerging as useful tools for the characterization of coal and its associated mineral matter to be used in conjunction with advanced coal conversion and coal cleaning processes. New methodology and applications have been developed at the Ames Laboratory and Iowa State University to study the nature and amount of mineral phases, their particle size distributions, and their degree of association with the organic coal matrix. Since the mineral content and composition can vary widely from coal to coal, the detailed characterization provided by these SEM-AIA techniques is quite helpful for the planning, design, testing, and evaluation of coal beneficiation and conversion processes. The methodology of SEM-AIA is described along with several illustrative applications related to coal beneficiation.

Leaching coal with molten sodium hydroxide at 370–390°C converts most of the sulfur and mineral components of the coal into soluble species. The unreacted caustic and soluble components are then separated from the cleaned coal by a series of washing and filtration steps. A laboratory-scale simulation of a 6-stage countercurrent washing and filtration procedure was performed on Illinois No. 6 and Kentucky No. 11 coal samples that had been leached with molten sodium hydroxide. The mass of filter cakes and filtrates during each wash cycle and the concentrations of all major components of the caustic solutions were determined in each process stream. The countercurrent washing procedure resulted in a relatively clean coal and a final filtrate with a caustic concentration close to the desired 50%. However, after several coals had been processed, filtration rates decreased markedly and the mass of filtrate recovered also decreased. This was due to a build-up of a precipitate, consisting predominantly of Na2CO3, on filter cakes in the middle stages of the countercurrent process step. Process modifications to avoid this build-up are proposed.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTRole of induction time and other properties in the recovery of coal from aqueous suspensions by agglomeration with heptaneC. W. Fan, Y. C. Hu, R. Markuszewski, and T. D. WheelockCite this: Energy Fuels 1989, 3, 3, 376–381Publication Date (Print):May 1, 1989Publication History Published online1 May 2002Published inissue 1 May 1989https://pubs.acs.org/doi/10.1021/ef00015a021https://doi.org/10.1021/ef00015a021research-articleACS PublicationsRequest reuse permissionsArticle Views87Altmetric-Citations26LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose Get e-Alerts