Elemental sulphur formation

Flower shaped crystalline deposit on the surface of the solid non-crystalline mass of platinum sulphide was probably due to the precipitation of elemental sulphur, which deposited as a floral growth on the non-crystalline platinum sulphide precipitate. Ultrasonic irradiation seemed to have broken tender sulphur flakes and cleaned the surface. The free sulphur, however, did not deposit further. This was probably due to the formation of other compounds of sulphur such as H2S, S02, etc. which could have been removed from the solution due to the phenomenon of degassing. 

Fuerstenau (1980) found that sulphide minerals are naturally floatable in the absence of oxygen. Yoon (1981) ever attributed the natural floatability of some sulphide minerals to their very low solubility. Finkelstein et al. (1975) considered that the natural floatability of sulphide minerals are due to the formation of elemental sulphur and related to the thickness of formation of elemental sulphur at the surface. Some authors reported that the hydrophobic entity in collectorless flotation of sulphide minerals were the metal-deficient poly sulphide (Buckley et al., 1985). No matter whichever mechanism, investigators increasingly concluded that most sulphide minerals are not naturally floatable and floated only under some suitable redox environment. Some authors considered that the natural floatability of sulphide minerals was restricted to some special sulphide minerals such as molybdenite, stibnite, orpiment etc. owing to the effects of crystal structure and the collectorless floatability of most sulphide minerals could be classified into self-induced and sulphur-induced floatability (Trahar, 1984 Heyes and Trahar, 1984 Hayes et al., 1987 Wang et al., 1991b, c Hu et al, 2000). 

The most controversial and contradicting problem is, perhaps, the natural and collectorless floatability of sulphide minerals. Gaudin (1957) classified the natural hydrophobicity of different minerals according to their crystal structure and showed that most sulphide minerals were naturally hydrophobic to some extent, which had been fiirther proved based on van der Waals theory by Chander (1988, 1999). Lepetic (1974) revealed the natural floatability of chalcopyrite in dry grinding. Finklestein (1975, 1977) demonstrated that orpiment, realgar and molybdenite were naturally floatable, and that pyrite and chalcopyrite had natural floatability at certain conditions due to the formation of surface elemental sulphur. Buckley and Woods (1990,1996) attributed the natural floatability of chalcopyrite... 

The reaction potential producing elemental sulphur are 0.24 V at pH = 6 based on the reaction (2-10), 0.28 V at pH =8 on the reaction (2-11), and 0.1 V at pH = 11 on die reaction (2-12) with lO" mol/L concentration of dissolved species. The reported flotation initial potential (see Table 2.1) are very close to these theoretical calculation values. The theoretical and experimental values in Table 2.1 indicate that the elemental sulphur might be responsible for the hydrophobicity of sulphide surfaces. At different pH media the formation of elemental sulphur occurs and hence the flotation behavior undergoes different processes. 

Figure 2.24 shows the voltammograms of pyrite electrode in neutral media. It follows that the potential range of flotation relates to reactions (2-24) and (2-25). The potential calculated in reaction (2-24) is 161- 180 mV, pyrite exhibit collectorless floatability due to the formation of elemental sulphur. 

Because arsenopyrite is floatable in acidic conditions and non-floatable in basic conditions (see Fig. 2.16), it seems reasonable to assume that reactions (2-63) or (2-28) and (2-29) are dominant oxidation in acidic solutions. Elemental sulphur is responsible for the hydrophobicity of arsenopyrite in acidic media. In alkaline solutions, reactions (2-64) and (2-65) may be dominant resulting in the formation of oxy-sulfur species and arsenate species with minor sulphur. 

There is one example of a CD process (for deposition of tin sulphides) in which elemental sulphur dissolved in a nonaqueous solvent is used as a source for S. Since this appears to be the only example in the literature for this type of film deposition, it will be discussed in Chapter 6 together with the relevant study on tin sulphides. However, there is no reason to believe that this process may not be applicable to other materials. Metal sulphides (and selenides) are known to form, as precipitates, by reacting certain metal salts with dissolved elemental chalcogen, although visible film formation seems to be limited, up to now, to this one example. 

Deposition of elemental sulphur formed from sulphate Essential collaboration of at least two different microbial species occurs in the transformation of sulphate to S° in salt domes or similar sedimentary formations (see Ivanov, 1968). This transformation is dependent on the interaction of a sulphate reducer like Desulfovibrio desulfuricans, which transforms sulphate to H2S in its anaerobic respiratory metabolism, and an H2S oxidizer like Thiobacillus thioparus, which, under conditions of limited O2 availability, transforms H2S to S° in its respiratory metabolism (van den Ende van Gemerden, 1993). The collaboration of these two physiological types of bacteria is obligatory in forming S° from sulphate because sulphate reducers cannot form S° from sulphate, even as a metabolic intermediate. It should be noted, however, that the sulphate reducers and H2S oxidizers are able to live completely independent of each other as long as the overall formation of S° from sulphate is not a requirement. 

Howarth R. W. and Jorgensen B. B. (1984) Formation of S-35-labeUed elemental sulphur and pyrite in coastal marine sediments (Limfjorden and Kysing Fjord Denmark) during short-term S-35 sulphate reduction measurements. Geochim. Cosmochim. Acta 48(9), 1807-1818. 

Thode-Anderson S. and Jorgensen B. B. (1989) Sulphate reduction and the formation of sulphur-35 labelled FeS, FeS2 and elemental Sulphur in coastal marine sediments. Limnol. Oceanogr. 34(5), 793-806. 

A spectroscopic study212 of solutions of alkali-metal polysulphides in DMF has shown the blue coloration to be due to the formation of the trisulphur radical anion S3. This result is contrary to that previously published (W. Giggenbach, J. C. S. Dalton, 1973, 729), which identified the species as the supersulphide ion S2. Further information on the formation of S3 in solutions of elemental sulphur in HMPA was also obtained, and it is now thought that the elemental sulphur is reduced by impurities present in the solvent, probably by dimethylamine. The reaction of piperidyl-lithium with S8 in HMPA was studied in order to determine the stoicheiometry and the nature of the final products. The stoicheiometry of the reaction was found to be ... 

Some preliminary results on the electrochemical behaviour of elemental sulphur (and selenium) in AlCla-NaCl melts have been reported. The results for both elements were difficult to interpret, due in part to the low solubility of both elements in the melt and the modification of the electrode surfaces by the products of electrode reactions. In basic melts it was suggested that an cation is formed, in agreement with spectral studies. In acidic melts the results were more complex, and no exact conclusions could be drawn as to the nature of the species. A spectrophotometric study of the reactions of sulphur, selenium, and tellurium with aqueous solutions of NaOH at 150 °C (S, Se) and 300 °C (Te) has been carried out. From the experimental data the formation of the chain-like structure of polychalcogenide compounds was presumed. The spectra confirmed previous results concerning the mechanism of interaction of elemental chalcogens with aqueous NaOH solutions. 

Isolation of the hydrocarbons from other lipids The total lipid extract may be subjected to removal of elemental sulphur by passage through an activated copper column (Blumer, 1957) and then to chromatographic separation on adsorbent columns or thin layer plates. For column chromatography, silic el is used with a short alumina bed on the top of the silic el. Both adsorbents should be partially deactivated by the addition of water (2—5%) to prevent the formation of artifacts (Blumer, 1970). Elution with a non-polar solvent such as hexane or pentane and subsequently with mixtures of non-polar and polar solvents, e.g. benzene and methanol, permits the isolation of several fractions containing saturated, unsaturated, aromatic hydrocarbons and more polar compounds (methyl esters, alcohols, acids, phenols and heterocyclic compounds). The interference from esters encountered in the isolation of aromatic hydrocarbons can be avoided prior to separation by saponification of the esters of fatty acids, which are easily removed. 

In an attempt to simulate petroleum formation under mild conditions (T < 150° C) De Roo and Hodgson (1978) reacted ethylbenzene and sulphur in the presence of oxygen and water and identified a number of S-compounds, among which substituted thiophenes seemed to be the major products (Fig. 7). Baker (1973), however, found only three poorly defined organosulphur compounds on reaction of Beaufort Sea sediments with elemental sulphur, when studying whether hydrocarbons could be generated from the humic material present in the area. 

The reaction of vinylacetylene with sodium sulphide and sodium hydrosulphide in DMSO-sodium or potassium hydroxide and water has been studied in detail. The products formed were (17), (18), and (19). Conditions for selectively obtaining (17) or (19) have been worked out, while (18) was only formed in 25% yield. Interestingly, thiophen is the main product from the reaction of vinylacetylene with sulphide ions generated from elemental sulphur. The formation of thiophen appears not to be caused by oxidation of (19) by elemental sulphur. ... 

The cyclic dinucleotide phosphorothioate (79) has been prepared" stereoselectively and in good yield from the 3 -hydrogen phosphonate of the dinucleoside phosphorothioate (80) by standard activation with pivaloyl chloride, followed by sulphurisation with elemental sulphur. Interestingly the second stereocentre is formed with complete Rp stereoselectivity. This is attributed to the reduced conformational freedom associated with the formation of the cycle, since thiooxidation of H-phosphonate intermediates is known to be stereoselective. ... 

Reaction of 1,2-benzoquinone with reduced elemental sulphur (H2S ) gives a low yield of the pentathiabenzocycloheptene-l,2-diol 149. A mechanism of formation, summarised in Scheme 37, has been proposed (07JCX72951). The reaction is of some interest because similar cytotoxic natural products contain dopamine fragments and may be derived from the corresponding orf/zo-quinone. 

The spectral maxima at 320, 390, and 590 nm were found to correspond to absorptions by the ions S , S , and S7, respectively. Direct observations (resonance Raman, i.r., and e.s.r. spectra) and indirect measurements (visible— u.v. spectra, conductivity, and magnetic susceptibility) strongly suggest that the intensely blue species formed by alkali-metal polysulphides or elemental sulphur in hexamethylphosphoramide can be attributed to the Sj radical anion. Cryoscopic, conductance, and magnetic measurements euid i.r. and u.v. spectral data have been used to study the cations formed in solutions of sulphur in disulphuric acid. The blue coloration was shown to be due to the formation of the Si" " ion and the colourless solutions were found to contain the S ion. The solid compounds S4S30ioand 8383010, formed by the action of SO3 on elemental sulphur in 8O2 media at low temperatures, have been isolated and characterized. 

A paramagnetic ion attributed to S20 has been observed by e.p.r. spectra when H2S and SO2 are allowed to react on MgO at 25 °C. The proposed mechanism for the formation of S20 involves the reaction of elemental sulphur, as S2, with oxide ions of the MgO surface. The photoelectron spectra of H2C=S=0 ° and HN=S=0 have been recorded and the ionization energies of X=S=0 derivatives compared. ... 

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