Coal Pyrite Research Articles

Sulphur in coal by programmed-temperature oxidation

A controlled-atmosphere, programmed-temperature, oxidation apparatus has been constructed and used to characterize organic-sulphur distribution in coals, partially desulphurized coals, and model systems. Samples were diluted with WO 3 and heated, in a stream of Ar containing 10% O 2, at a programmed-temperature increase of 3 °C min −1. Concentrations of SO 2, CO 2, CO, O 2 and H 2O in the effluent gas were continuously measured until the runs were completed at 1000 °C. Evolution patterns produced by the oxidation products characterize the coal and, upon integration, provide total CHS analyses consistent with classical coal analysis methods. The SO 2 derived from coal pyrite evolves with a maximum at about 430 °C, and that from the organic portion of coal produces principal evolution maxima at ≈ 320 °C (attributed to non-aromatic coal structures) and at ≈ 480 °C (attributed to aromatic structures). During oxidation the H C ratio decreases with increasing temperature to ≈ 0.6–0.7 at 400 °C. The n.m.r. spectrum of residue isolated after oxidation up to 400 °C shows a sharp decrease in aliphatic carbon compared with the original coal.

The electrochemical behaviour of coal pyrite: 2. Effects of coal oxidation products

The physico-chemical mechanism of coal desulphurization by aqueous leaching is reported. The electrochemical behaviour of coal pyrite in the presence of oxalic (H 2C 2O 4) and succinic (H 6C 4O 4) acids was investigated for two coal pyrite samples from Pennsylvania State, USA. A general trend observed in the polarization curves is that the rest potentials become nobler, i.e. attain more positive values in the presence of organic acids. From analysis of polarization results it can be concluded that oxalic acid and iron form a strong non-porous film, probably β-FeC 2O 4.2H 2O, on the pyrite surface which may be responsible for the observed decrease in the anodic current and could reduce the surface reactivity of coal pyrite during a coal desulphurization process.

Size and maceral association of pyrite in Illinois coals and their float-sink fractions

The amount of pyrite (FeS 2) removed by physical cleaning varies with differences in the amount of pyrite enclosed within minerals and of free pyrite in feed coals. A microscopic procedure for characterizing the size and maceral association of pyrite grains was developed and evaluate by testing three coals and their washed products. The results yield an index to the cleanability of pyrite. The index is dependent upon particle size and has intermediate values for feed coals, lower values for cleaned fractions, and higher values for refuse fractions; furthermore, it correlates with pyritic sulfur content. In the coals examined, the summed percentage of grain diameters of pyrite enclosed in vitrinite, liptinite, and bi- and trimacerite provides a quantitative measure of the proportion of early diagenetic deposition of pyrite.

Sulphur in peat and coal

Abstract Coal sulphur content is a major consideration in coal marketing. To predict sulphur content and distribution in a coal basin, a sulphur geochemical model needs to be developed. A series of questions are posed regarding coal sulphur distribution, forms and origins. Salient aspects of literature on coal sulphur content and distribution are reviewed to establish a framework for developing a sulphur geochemical model. Some coal sulphur originates from peat-forming processes. Sulphate, ferrous iron, and microorganisms are key ingredients for the origins of coal pyrite and organic sulphur. Hydrogen sulphide is an important intermediate for pyrite and organic sulphur formation in peats. Sulphur isotope values for peat sulphate, plant sulphur, peat pyrite and peat organic sulphur corroborate the hypothesis that microorganisms reduce sulphate to hydrogen sulphide, which in turn reacts with available ferrous iron or organic matter to produce pyrite and organic sulphur respectively.

A new approach to the determination of pyrite in coals by Mössbauer spectroscopy

A new approach to the quantitative determination of pyrite in coals by Mössbauer spectroscopy is described. The method uses a duplex absorber consisting of the coal sample and an iron foil. The use of iron as an internal standard, together with a calibration graph, should allow reproducible Mössbauer spectroscopic determination of the pyrite content of coals to be made in different laboratories. Good agreement is obtained between pyrite values obtained by the Mössbauer spectroscopy and conventional methods.

The electrochemical behaviour of coal pyrite 1. Effects of mineral source and composition

The dissolution in perchloric acid of four coal pyrites, from different sources and of various compositions, was studied using electrochemical measurements. The processes taking place on the coal pyrite surfaces are limited by surface reaction rather than by aqueous diffusion. The mode of reaction at the coal pyrite surface also depends on the composition of the coal pyrite. It could be concluded from the results that pyrite removal from coal is possible by aqueous leaching in acidic conditions.

Chemical desulfurization of west Kentucky coal using air and steam

This paper presents the results of a detailed study on air-steam desulfurization of high volatile bituminous coal from West Kentucky that contains very fine pyrite (<10 ..mu..m). In this, study, temperature, coal treatment time, and the steam-to-air ratio were found to be important process variables, while particle sizes within the range of 1.4 mm to 150 ..mu..m were found to have no significant effect on desulfurization. An optimum steam-to-air mass ratio was found for maximum desulfurization as a result of two opposing mechanisms. While desulfurization was found to increase with oxygen concentration (which increases as the steam-to-air ratio decreased) at a low steam-to-air ratio, the rate of oxygen diffusion into the reaction sites descreases because of the reduced superficial gas velocity. This study also concludes that the use of steam together with air provides not only the advantage of increasing superficial gas velocity expected of the inert diluting gas but also the advantages of minimizing coal oxidation and improving coal desulfurization in a manner not obtainable with inert diluting gas such as nitrogen. Furthermore, through the air-steam treatment, the pyrite in coal was converted into magnetic iron compounds, iron sulfates, and pyrrhotites at first, and iron oxides (..gamma..-hematite and magnetite)more » in the end. This effect has provided additional desulfurization in subsequent magnetic separation.« less

Pyrite/marcasite size, form, and microlithotype association in western Kentucky prepared coals

Pyrite/marcasite size category, form, and microlithotype association were examined in the feed, clean, and refuse coals from 14 western Kentucky preparation plants. The feed coals are high in sulfur and a relatively large amount of pyritic sulfur remains in the clean products. Pyrite in the clean coals was generally less than 10 micrometers in diameter. Pyrite forms concentrated in the clean coals were frequently framboidal and euhedral types associated with vitrite and clarite. The refuse concentrated the coarser massive and cleat forms associated with carbominerite.

Biological removal of pyritic sulfur from coal by the thermophilic organism Sulfolobus acidocaldarius

More than 90% of initial pyritic sulfur was removed from bituminous coal samples (containing 2.1% pyritic sulfur) using the thermophilic organism Sulfolobus acidocaldarius. Microbial desulfurization rate was improved nearly ten fold by adjusting the N/P and N/Mg ratios in the nutrient medium. Environmental conditions were optimized. The optimal values of temperature and pH were 70 degrees C and 1.5, respectively. The influence of certain process variables (such as coal pulp density, particle size, and initial cell number density) on the rate of pyritic sulfur removal were determined. A pulp density of 20%, particle size of D (p) < 48 microm, and an initial cell number density of 10(12) cells/g pyrite in coal were found to be optimal. The carbon dioxide enriched air did not improve the rate of pyritic sulfur removal compared to pure air at 10% pulp density of coal samples containing 2.1% pyritic sulfur. The kinetics of microbial leaching of pyritic sulfur from coal was investigated. The rate of leaching was found to be first order with respect to pyritic sulfur concentration in the reaction medium.

Characterization of the mineralogy and microchemistry of fly ash

The microstructures of coal fly ash particles were studied by transmission electron microscopy. Some particles are glassy and microscopically homogeneous; however, most particles examined contained one or more crystalline phases. Frequently, particles contain crystallites of a spinel compound in a glass matrix. It is believed that these crystallites nucleate homogeneously during cooling. Particles containing a greater fraction of crystalline material often have a dendritic structure. In principle, it should be possible to calculate solidification rates based on crystallite size or dendrite arm spacings. The distribution of trace elements was studied by thermogravimetric analysis and by studying their partitioning between fly ash and bottom ash. Trace element partitioning was then related to the distribution of particle types observed optically. This analysis indicated that only a few elements are present as volatile species. Many important trace elements appear to be associated with a particular particle type, namely, the ferrospheres. These ash particles have been shown to be the oxidized remains of framboidal pyrite in the feed coal.

Rates of Iron Sulfide Oxidation in Coal Spoil Suspensions

AbstractRates of pyritic coal spoil oxidation were studied in water suspensions of each of two particle‐size fractions (&lt; 0.5 mm and 0.5–2 mm). The treatments on each of the size fractions included liming with reagent grade CaCO3 at rates of 0, 14.4, and 28.8 g kg−1. In another treatment, toluene was added to unlimed suspensions to inhibit microbial growth.The greatest rate of salt formation (taken as a measure of pyrite oxidation) was associated with the larger size fraction. Toluene suppressed the oxidation‐dissolution rate, suggesting a microbial role in pyrite oxidation in these systems. The application of CaCO3 at 14.4 g kg−1 spoil to the 0.5‐ to 2.0‐mm size fraction enhanced the oxidation‐dissolution process during the initial phase of the reaction (0–100 min). When the larger size fraction was amended with CaCO3 at a rate of 28.8 g kg−1, three rates of oxidation‐dissolution were apparent. The first rate was attributed to dissolution of indigenous sulfate salts. The second rate was attributed to oxidation of a more reactive pyrite fraction than that described by the third rate (slowest rate). This last rate was attributed to a nonmicrobial oxidation pathway due to the higher pH values observed in the suspension.

Oxidation of pyrite in coal to magnetite

When bituminous coal is heated in an inert atmosphere (He) containing small amounts of oxygen at 393–455 °C, pyrite (FeS 2) in coal is partially converted to magnetite (Fe 30 4). The maximum amount of Fe 30 4 formed during the time of heating corresponds to 5–20% of the total pyrite present, depending on the coal sample. The magnetite forms as an outer crust on the pyrite grains. The fact that the magnetic properties of the pyrite grains are substantially increased by the magnetite crust suggests that pyrite can be separated from coal by use of a low magnetic field. In a laboratory test, 75% removal is obtained by means of a 500 Oe magnet on three samples, and 60% on a fourth sample.

Macronutrient and Boron Ratios in Tall Fescue: Relationship to Yields on Pyritic Coal Wastes

AbstractA previous greenhouse study had demonstrated that yield of tall fescue (Festuca arundinacea Schreb.) grown directly on a pyritic waste was not significantly different from yield on a soil despite low tissue concentrations of P and K in the waste‐grown plants. In an attempt to explain this result, and to determine if this phenomenon was typical of such wastes, a greenhouse study was carried out with pyritic wastes from five disposal sites for coal‐cleaning refuse in southern lllinois. The wastes and an agricultural soil (Elliott silt loam: fine, illitic, mesic Aquic Arguidolls) were treated with limestone (to pH 6.5) and fertilizers. Yields of 8‐week‐old shoots of tall rescue grown on the wastes were significantly lower than yield of plants grown on the soil when the Ca/B ratio in the shoots was outside a range of 1.7 to 2.8 (where Ca/B is the ratio of log10 concentrations of Ca and B expressed as microgram‐atoms per gram dry matter). Within this range, yields were not significantly different from yield on the soil despite significantly lower concentrations of P and K in the shoots of the waste‐grown plants. It is suggested that elemental interactions should not be ignored in reclamation of waste sites, particularly where “trace” element concentrations are either higher or lower than concentrations in typical soils. Results also suggest a P‐ and K‐sparing effect of elevated B concentrations in tall rescue.

Rapid Mössbauer-based methods for the determination of iron-bearing phases

The variation of Mössbauer γ-ray resonant absorption with the velocity amplitude of a vibrating γ-ray source is examined theoretically in connection with Mössbauer-based methods for rapid, nondestructive determination of iron-bearing phases in different materials. The theoretical analysis can be used to simulate such methods, which are based on the difference in the Mössbauer resonant absorption by a sample with the γ-ray source stationary and in rapid oscillation. Methods for the determination of pyrite in coal and metallic forms of iron in steels and slags are examined both theoretically and experimentally to illustrate the advantages and limitations of such rapid Mössbauer-based methods.

Coadsorption Phenomena in the Separation of Pyrite from Coal by Reserve Flotation

The reverse flotation of pyrite from coal was studied to determine surface chemical aspects of this system. Included in the research was determination of the effect of dextrin on the adsorption of xanthate by pyrite, measurement of rest (mixed) potentials for crystal- and coal-pyrite electrodes and evaluation of the bench scale flotation response with respect to the understanding of the adsorption reactions. Although little dextrin is adsorbed by crystal pyrite in the absence of xanthate, significant adsorption of dextrin in the presence of xanthate occurs. These results further support the general hypothesis that dextrin adsorption involves a hydrophobic bonding mechanism, In this system hydrophobic bonding is thought to occur at the surface between the non-polar portion of the dextrin and the dixanthogen oil which forms on the crystal pyrite surface. On the other hand, the slightly hydrophobic character of coal pyrite (pyrite associated with locked coal) exhibits a higher adsorption potential for dextrin in the absence of xanthate. The coal impurities presumably provide hydrophobic sites for the adsorption process. As with crystal pyrite, the dextrin adsorption density is increased further when potassium amyl xanthate is added. Most importantly, the coadsorption of dextrin and xanthate does not affect the flotability of pyrite in the range of concentrations tested. From these results it was inferred that the dixanthogen either wets surface dextrin molecules or that the dixanthogen forms in surface patches, providing the necessary hydrophobic character to permit xanthate flotation of pyrite even in the presence of coadsorbed dextrin. Unlike crystal pyrite electrodes, the rest (mixed) potential of a locked coal-pyrite electrode was not a strong function of either the xanthate or dextrin concentration. In fact the rest (mixed) potential of the locked-electrode was very close to the potential of crystal pyrite in the absence of xanthate (0·42 V at pH 7). It seems that the coal portion of the locked electrode may act as the anodic half cell which determines the electrode potential. In any event, coal pyrite exhibits the necessary electrochemical characteristics for the discharge of xanthate to dixanthogen. Bench scale flotation of pyrite from a ROM Illinois No.6 coal was studied by factorially designed experiments. The separation effectiveness was evaluated as a function of xanthate addition, dextrin addition, and acidity by surface response methodology. The pyrite flotation behavior was explained to some extent in terms of the coadsorption of collector and depressant.

A Kinetic study of leaching of coal pyrite with nitric acid

The leaching of coal pyrite with nitric acid has been investigated. The temperature ranged from 313 to 363 K, and the concentration of nitric acid was varied from 0.154 to 1.54 mol/l. A coal sample of 50 grams was leached in a reactor containing 500 ml of solution in an open system. It was observed that the leaching reaction could remove 47 pct of the pyrite sulfur in seven minutes and 88 pct in 30 minutes at 343 K with 1.54 mol/l of nitric acid. The reaction order with respect to hydrogen and nitrate ion activity was found to be first order. The activation energy for the initial stage of the reaction was determined to be 14.7 K cal/mol (61.5 kJ/mol). A mathematical model was developed on the basis of mixed kinetics (reaction zone model) to explain the leaching rates. Good agreement between experimental rate data and predicted rate curves by the developed model was obtained. Ultimate analysis was used to determine the extent of nitration of the leached coal. This nitration was found to be insensitive to the reaction temperature and acidity of the solution.

Erratum: Sulfur Diagenesis in Everglades Peat and Origin of Pyrite in Coal

In the article "Sulfur diagenesis in Everglades peat and origin of pyrite in coal" by Z. S. Altschuler et al. (15 July, p. 221) the equation at the bottom of the middle column on page 221 had a misprint; it should have read Fe 2+ + S 2- x + HS - → FeS 2 + S 2- x -1 + H +

Erratum: Sulfur Diagenesis in Everglades Peat and Origin of Pyrite in Coal

Erratum: Sulfur Diagenesis in Everglades Peat and Origin of Pyrite in Coal

Pyrite decomposition in fresh Blacksville No. 2 coal: Effect of additives

The Mössbauer effect has been used to study, under liquefaction conditions, the effect of additives (S, FeS 2, Fe 7S 8 and Fe) on the transformation of pyrite in fresh Blacksville No. 2 coal. It was observed that the addition of iron sulphides enhanced the conversion of the inherent pyrite in coal, an effect attributed to a combined cracking activity of the pyrrhotites and H 2S on the less reactive organic covering of some of the inherent coal pyrite. The addition of sulphur produced a higher partial pressure of H 2S; total conversion of pyrite to pyrrhotite was not observed. The stoichiometry of the pyrrhotites present in the residue of the experiment with sulphur showed a smaller atomic percentage of iron than in the experiment without additives, this change in stoichiometry being attributed to the higher partial pressure of H 2S in the reactor. When metallic iron was added, the partial pressure of H 2S in the reactor decreased (iron acts as a scavenger) and troilite was also formed.

ChemInform Abstract: A METHOD FOR THE DETERMINATION OF PYRITE IN COAL

AbstractEs wird eine schnelle einfache Methode zur Bestimmung des Pyrits in Kohle vorgestellt, die auf einer Zweistufenextraktion des Fe aus der Kohle basiert.