Selective sulphidation

Finkelstein, N. P., 1999. Addendum to The activation of sulphide minerals for flotation a review. Inter. J. Miner. Process, 55(4) 283 - 286 Fomasiero, D., Montalti, M., Ralston, J., 1995. Kinetics of adsorption of ethyl xanthate on pyrrhotite in situ UV and infiared spectroscopic studies. Langmuir, 11 467 - 478 Forssberg, K. S. E., Antti, B. M., Palsson, B., 1984. Computer-assisted calculations of thermodynamic equilibria in the chalcopyrite-ethyl xanthate system. In M. J. Jones and R. Oblatt (eds.). Reagents in the Minerals Industry. IMM, Rome, Italy, 251 - 264 Fuerstenau, M. C., Kuhn, M. C., Elgillani, D. A., 1968. The role of dixanthogen in xaomthate flotation ofpyrite. Trans. AIME, 241 437 Fuerstenau, M. C. and Sabacky, B. J., 1981. Inter. J. Miner. Process, 8 79 - 84 Fuerstenau, M. C., Misra, M., Palmer, B. R., Xanthate adsorption on selected sulphides in the presence of oxygen. Inter. J. Miner. Process... 

The beneficial effects of sulphur (a few p.p.m. in the feed) on Pt reforming catalysts (lower initial activity but enhanced lifetime and stability, less coking) were reviewed in the earlier Report and further examples have appeared. A selectively sulphided Pt(0.25)-Re(0.25)-Q(1.0)/Al2O3 catalyst with S/Re atomic ratio 0.93 was more active and longer-lived in the reforming... 

One final interesting point about sulphidation is that, because the free energies of formation of the sulphides of iron, nickel, cohalt, chromium, and alvuninium are quite similar, selective sulphidation of chromium and aluminium is not favoured as in the case of oxidation. Lai has reviewed the literature of the sulphidation of alloys and has made several generalizations as follows. 

Cie3] Cieslak, J., Dubiel, S.M., Further Support for the Inversion in the Selective Sulphidation of Fe-Cr Alloys , Solid State Commun., 105(4), 235-238 (1998) (Experimental, Phase Relations, Interface Phenomena, 4)... 

The direct reductive acid leaching process was developed by Hindustan Zinc Limited, Udaipur. Starch was used in the process as reductant during leaching. The separation process is based on selective sulphide precipitation followed by solvent extraction and electrolysis. The other two processes are based on ammonia leaching, developed at the Institute of Materials and Minerals Technology, Bhubaneshawar (IMMT-B) and National Metallurgical Laboratory,... 

It is convenient to include under Aromatic Amines the preparation of m-nitroaniline as an example of the selective reduction of one group in a polynitro compound. When wt-dinitrobenzene is allowed to react with sodium polysulphide (or ammonium sulphide) solution, only one of the nitro groups is reduced and m-nitroanUine results. Some sulphur separates, but the main reaction is represented by ... 

Crystallised sodium sulphide NajS,9HjO is very deliquescent, and only a sample which has been kept in a tightly-stoppered bottle should be used crystals as dry as possible should be selected. Altemativelj, an equivalent amount of analysed fused sodium sulphide may be employed this dis.solves somewhat more slowly in alcohol. 

General corrosion damage was the cause of failure of an A1 alloy welded pipe assembly in an aircraft bowser which was attacked by a deicing-fluid — water mixture at small weld defects . Selective attack has been reported in welded cupro-nickel subjected to estuarine and seawater environments . It was the consequence of the combination of alloy element segregation in the weld metal and the action of sulphate reducing bacteria (SRB). Sulphide-coated Cu-enriched areas were cathodic relative to the adjacent Ni-rich areas where, in the latter, the sulphides were being continuously removed by the turbulence. Sulphite ions seemed to act as a mild inhibitor. 

A highly selective and rapid oxidation of sulphides to sulphoxides occurs when hydrogen peroxide/selenium dioxide system is used32. The reaction takes place immediately upon addition of a solution of hydrogen peroxide and selenium dioxide to a solution of a sulphide in methanol at room temperature. Yields of sulphoxides (Table 2) are in the range between 80 and 95%. It is most probable that perseleninic acid 19 is the true oxidizing agent. 

A detailed study revealed that sulphides may react with nitric acid to give sulphoxides, sulphones and their nitro derivatives54. However, under suitable conditions the nitric acid oxidation of sulphides leads to a selective formation of sulphoxides. This is probably due to the formation of a sulphonium salt 30 which is resistant to further oxidation50 (equation 12). 

The selective oxidation of the sulphide grouping in the presence of the disulphide bond was observed when a methanolic solution of amide 46 was treated with an aqueous solution of sodium metaperiodate77 (equation 20). 

Alumina supported sodium metaperiodate, which can be prepared by soaking the inorganic support with a hot solution of sodium metaperiodate, was also found to be a very convenient reagent for the selective and clean oxidation of sulphides to sulphoxides79. The oxidation reaction may be simply carried out by vigorous stirring of this solid oxidant with the sulphide solution at room temperature. As may be expected for such a procedure, solvent plays an important role in this oxidation and ethanol (95%) was found to be... 

Silica gel supported sodium metaperiodate was used for the selective oxidation of dibenzyl sulphide80. Metaperiodate anion soaked on strongly basic-ion-exchange resins Amberlite IRA-904 or Amberlyst A-26 was found to be able to oxidize sulphides into the corresponding sulphoxides in 82-99% yield81. 

Chromic acid oxidation of sulphides to sulphoxides was reported in 1926124. However, this oxidation procedure is not selective and sulphone formation was observed125. When pyridine was used as a solvent the sulphone formation was strongly reduced126. 

Clean and selective oxidation of dibenzyl and dibutyl sulphides to the corresponding... 

Optically active hydroperoxides 244 were found285 to oxidize prochiral sulphides into the corresponding sulphoxides in higher optical yields (up to 27%) in comparison with those observed with peracids (equation 132). Moreover, the optical purity of the sulphoxides formed may be enhanced by addition of Ti(OPr-i)4. The oxidation of racemic 2-methyl-2,3-dihydrobenzothiophene 246 with these peroxides gave a mixture of cis and trans-sulphoxides 247 (equation 133). In all cases of the oxidation with the hydroperoxide alone the formation of the trans-isomer was strongly preferred and the e.e. value (up to 42%) of the cis-isomer was always higher than that of the trans-isomer. Moreover, the addition of Ti(OPr-i)4 furthermore promoted the selective formation of the frans-sulphoxide 247 and remarkably enhanced the e.e. value of both isomers. 

There are many transition metal ion oxidants used in organic chemistry for the interconversion of functional groups. Those which have been used for the preparation of sulphones from sulphoxides will be discussed below. It is very interesting to note that this type of oxidant often reacts more rapidly with sulphoxides than with sulphides and so sulphoxides may be selectively oxidized with transition metal ion oxidants in the presence of sulphides. This is in direct contrast to the oxidation of sulphides and sulphoxides with peracids and periodate, for example, where the rate of reaction of the sulphide is more than 100 times that for the corresponding sulphoxide. 

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