Sulphur Chemistry

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Sulphur chemistry on metals has been intensively studied as being particularly important in different technologies among which is metallurgy and metal corrosion. An example is the study by Hamilton and woods (Hamilton and Woods, 1981) who studied the products of sulphur oxidation of pyrite and pyrrhotite. In the floatation of sulphide minerals and base metal oxides, alkali sulphides have been widely used for a long time (Buswell and Nicol, 2002) which lead to extensive study of sulphur chemistry. In natural gas and crude oil industries, the effects of corrosion that sulphide ions have on iron based alloys has been well known. This goes to show the importance of sulphur chemistry to metallurgical engineers.
Pyrite oxidizes to both sulphur and
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The implication of this is that Sulphur is stable in the presence of water and in acidic conditions free from oxidizing agents. In alkaline conditions, it is unstable and tends to disproportionate to give HS–, S2– (and polysulfides), SO42−, and other oxidation products. In industrial applications, these reactions are slow and take place only in hot and very alkaline media. Sulphur is usually a major product of sulphur mineral oxidation at pH of 4.6. Increase in pH to 9 accompanied by an increase in potential results in the formation of sulphate ions (Hamilton and Woods, 1981).

From thermodynamic considerations, sulphur is expected to be easily oxidized to sulphate or bisulphate over a wide range of conditions. These can be seen from the small region of sulphur stability as depicted in Figure 3.1 and Figure 3.2 below. With increased temperature, the sulphur stability regions are reduced even further supporting the hypothesis of easy oxidation of sulphur over a wide range of conditions (Eh and pH). However, because of kinetic constraints, the sulphur stability range is considerably larger than that depicted by thermodynamics (Peters,
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Elemental sulphur has been noted as the end product of pyrite oxidation in acid media (low pH), but it is unstable in alkaline solutions. Thiosulphate was detected as an intermediate in anodic dissolution of pyrite (Mishra and Osseo-Asare, 1988). Studies conducted by Mycroft during electrochemical oxidation of pyrite (Mycroft et al., 1990) showed the formation of sulphur and polysulphide in near neutral aqueous solutions. However, in alkaline solutions, the formation of sulphur and polysulphide is only detected at higher potentials. On the contrary, Hamilton and Woods (Hamilton and Woods, 1981) operating at pH 9.2 suggested formation of monolayer sulphur during pyrite oxidation a potential of about 0.0

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