Calcium sulfate can be decomposed at a high temperature (e.g., 1100°C) in the presence of reducing agents such as carbon monoxide and hydrogen. However, conditions must be carefully controlled to avoid over-reduction and the formation of calcium sulfide. The problem of over-reduction can be avoided by using a fluidized bed reactor in which the gas phase is alternated in a periodic manner between reducing conditions and oxidizing conditions. This method was demonstrated with a bench-scale fluidized bed reactor which was fed continuously with granulated waste gypsum and a mixture of air and natural gas. The ratio of air to natural gas was varied in a periodic manner to provide alternating reducing and oxidizing conditions. Calcium sulfate was largely converted into the desired products while producing very little calcium sulfide. Operating performance was found to depend on cycle time, air to fuel ratio, temperature, and feed rate.

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 turbidmetric method was used to investigate the kinetics of a gas-promoted process for agglomerating an aqueous suspension of coal particles using either heptane or hexadecane as an agglomerant. Suspensions of either Pittsburgh No. S seam coal or Upper Freeport seam coal were mixed with measured quantities of air and an agglomerant in a closed cylindrical tank fitted with baffles and a controllable speed agitator. Agglomeration was monitored by observing the change in turbidity of the suspension. Initial particle concentration, agglomerant dosage, amount of air, and agitator speed were varied among runs. The experimental data were correlated and analyzed by employing a semiempirical rate equation which gave consistent results. During any given run the agglomeration rate (— dN/dt) was proportional to the particle number concentration (N) raised to a power between l.O and 1.3. The rate increased with increasing amounts of either air or agglomerant and with increasing agitator speed. The rate was also greater for Upper Freeport coal than for Pittsburgh coal, apparently because of the greater hydrophobicity of the former.

The suction potential method for measuring the three-phase contact angle was modified and used to determine the oil-water-solid contact angle for various samples of coal. The method involves measuring the pressure changes required to draw an oil/water interface and then an oil/air interface into a packed bed of solid particles. Before the latest modification, the method was very time consuming and difficult to control. These problems were overcome by modifying the apparatus and measuring technique. Values of the contact angle determined with the new technique were very similar and as reproducible as those determined with the older method. Also, measurements of the contact angle for a series of oxidized coal samples were consistent with other surface property measurements. In addition, for this series of samples, the recovery of coal from an aqueous suspension by oil agglomeration was found to correlate very well with the contact angle.

A group of bifunctional organosulfur compounds were tested as pyrite depressants for improving the separation of coal and pyrite by selective oil agglomeration. Preliminary screening tests of potential depressants were carried out with pyrite which had been sulfurized by treatment with a freshly acidified solution of sodium sulfide to increase its response to oil agglomeration. High-grade mineral pyrite was used for most of these tests, but pyrite extracted from coal was used for some tests. When heptane was used as an agglomerant, the following organic thiols were found to suppress the agglomeration of sulfurized pyrite: thioglycolic acid, thiolactic acid, mercaptosuccinic acid, 3-mercaplopropionic acid, and 2-mercaptoetbanesulfonic acid. Each of these materials contains both a sulfhydryl group (-SH) and a hydrophilic group. The materials were also shown to improve the separation of artificial mixtures of Illinois No. 6 coal and sulfurized pyrite in other tests involving agglomeration with heptane. However, the best separation was achieved when thioglycolic acid was employed.

A number of potential pyrite depressants were identified for use in coal beneficiation methods involving agglomeration of aqueous suspensions of fine particles with oil. Potential depressants were screened by subjecting sulfurized particles of mineral pyrite to agglomeration with heptane after application of a given material. Sulfurized pyrite was utilized because it was readily agglomerated. The following materials were found to suppress the agglomeration of sulfurized pyrite: ferric chloride, potassium monopersulfate, hydrogen peroxide, ferrous sulfate, sodium dithionite, sodium thiosulfate, sodium sulfide, titanous chloride, pyrogallol, quebracho, and milk whey. Ferric chloride was shown to improve greatly the separation of an artificial mixture of Upper Freeport coal and sulfurized pyrite by agglomeration with heptane.

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

Many coals exhibit a certain degree of native hydrophobicity. The more hydrophobic coals (the higher-rank coals) are easily beneficiated by froth flotation or oil agglomeration, while the more hydrophilic coals (the lower-rank coals) are floated or agglomerated with difficulty. Coals of different ranks and often even of the same rank sometimes differ greatly in hydrophobicity as measured by contact angle or natural floatability. Although the degree of hydrophobicity of a coal is related to its rank and has been correlated with other surface properties of the coal , the known information is still not sufficient to allow a good estimation to be made of the hydrophobicity of a given coal and does not explain the variation of coal hydrophobicity as a function of rank. A statistical analysis of previously published data, as well as newly acquired data, shows that coal hydrophobicity correlates better with moisture content than with carbon content, and better with the moisture/carbon molar ratio than with the hydrogen/carbon or oxygen/carbon atomic ratios. These findings indicate that there is a strong association between hydrophobicity and coal moisture content.

Coal, pyrite, and graphite particles were agglomerated individually with heptane to obtain a more complete picture of the interacting effects of several parameters on particle recovery. The parameters included particle hydrophobicity, heptane dosage, and the ionic strength of the suspending medium. The relative hydrophobicity of the different solids was determined by measuring the induction time, or time required for attachment of one or more particles to a small gas bubble when brought into contact with it. Increasing the ionic strength of the suspending medium increased both the apparent hydrophobicity and recovery of highly hydrophobic coals and graphite, whereas it decreased both the hydrophobicity and recovery of weakly hydrophobic materials. The hydrophobicity of various materials was also evaluated by measuring the heat of immersion of the materials in water and sodium chloride solutions with a microcalorimeter. Results seemed to correlate with both oil agglomeration recovery and induction time measurements. The application of various pyrite depressants such as Ca(OH)/sub 2/ or the salts of magnesium, copper, and aluminum to the cleaning of Upper Freeport coal by oil agglomeration showed that both the sulfur and ash contents of the coal were reduced significantly by the depressants. Results indicated that the successful application ofmore » such depressants will depend on several critical factors including the type of coal, depressant concentration, pH, oil dosage, and coal particle size. 4 refs., 16 figs., 3 tabs.« less

Many coals exhibit a certain degree of native hydrophobicity. The more hydrophobic coals (the higher rank coals) can be beneficiated by froth flotation or oil agglomeration, while the more hydrophilic coals (the lower rank coals) are difficult to float or agglomerate. Coals of different rank and sometimes even of the same rank differ greatly in hydrophobicity as measured by contact angle or natural floatability. Although the degree of hydrophobicity of a coal is related to its rank and the floatability of coal has been correlated with various indicators of hydrophobicity, the characterization of coal hydrophobicity is incomplete and its variation with rank has not been completely accounted for. A statistical analysis of previously published experimental data showed that coal hydrophobicity correlates better with coal moisture content than with carbon content.

The overall objective is to determine the basic mechanisms which underlie a number of selective coalescence or oil agglomeration processes that have been proposed for beneficiating fine-size coal. Recent work has shown that the oil agglomeration recovery of less hydrophobic Illinois No. 6 coal is reduced by an increase on ionic strength of the suspending medium. Also the oil agglomeration recovery of iron pyrite is reduced by an increase in ionic strength and the reduction is affected by the cations present. Thus, pyrite recovery is affected increasingly by Na/sup +/, Ca/sup + +/, and Mg/sup + +/ ions in that order. To study the effect of mild surface oxidation on the oil agglomeration of Pittsburgh seam coal, a freshly ground sample of the coal was exposed to air at room temperature for one week. When agglomerated with heptane, the recovery from neutral or acidic suspensions was similar to that of untreated coal, but the recovery from basic suspensions was lower than that of untreated coal. The total sulfur content of the agglomerated product was significantly lower for the treated coal. The use of magnesium salts as selective depressants for pyrite was investigated and found to be very effective in themore » separation of a synthetic mixture of hydrophobic Upper Freeport coal and pyrite by oil agglomeration. However, when a magnesium salt was applied to the separation of a synthetic mixture of the less hydrophobic Illinois No. 6 coal and pyrite, both coal and pyrite were depressed. 3 refs., 12 figs., 4 tabs.« less

Measurement of the zeta potential of fresh Illinois No. 6 coal and similar coal which had been mildly oxidized by exposure to air at room temperature for one week showed only slight differences in the electrokinetic properties of the two materials. However, when the two materials were agglomerated with heptane, the recovery of the oxidized coal was significantly lower than the recovery of the unoxidized coal. On the other hand, the sulfur and ash contents of the product were the same for both materials. The recovery of the unoxidized coal with a small amount of heptane (5 w/w %) was improved significantly by traces of sodium oleate. For pure mineral pyrite, the state of oxidation of the surface was shown to greatly affect both the electrokinetic properties and oil agglomeration characteristics of the material. Various methods of oxidation, including prolonged exposure to air at room temperature, seemed to generate both hydrophilic and hydrophobic oxidation products on the surface of the pyrite. The hydrophilic products were partially removed by hot water and more completely removed by boiling hydrochloric acid, whereas the hydrophobic product was removed by acetone. This evidence suggests that the hydrophilic products included various species such as ferric oxidemore » and ferric sulfate while the hydrophobic product was probably elemental sulfur. The acid-cleaned pyrite responded readily to oil agglomeration whereas the acetone-cleaned pyrite did not. Recovery of the acid-cleaned material was suppressed by adding salt to the agglomeration system, while recovery of oxidized pyrite was greatly enhanced by adding traces of either sodium laurate or sodium palmitate to the system. 4 refs., 13 figs., 4 tabs.« less

As the removal of sulfur from coal prior to combustion acquires more importance in order to meet evermore stringent antipollution regulations, research on the development of methods for the cleaning of coal continues to expand. Reviews are available which describe the various methods for desulfurizing coal (1, 2, 3). The sulfur content in coal is usually a few per cent, but it can range from less than 0.5 per cent to as much as 8 per cent or more. Much of the sulfur is inorganic in nature, occurring in discrete mineral phases; the inorganic sulfur is mostly pyrite with small amounts of sulfates such as gypsum. Part of the sulfur in coal is termed organic sulfur, being intimately bound to the organic coal matrix. The chemical nature of this organic sulfur is not well established. During the desulfurization of coal, some of the coarse inorganic sulfur components can be removed

Click to increase image sizeClick to decrease image size Additional informationNotes on contributorsT.D. WHEELOCKTelephone: (515) 294-5226