Problems Sulphide

If produced gas contains water vapour it may have to be dried (dehydrated). Water condensation in the process facilities can lead to hydrate formation and may cause corrosion (pipelines are particularly vulnerable) in the presence of carbon dioxide and hydrogen sulphide. Hydrates are formed by physical bonding between water and the lighter components in natural gas. They can plug pipes and process equipment. Charts such as the one below are available to predict when hydrate formation may become a problem. 

The most common contaminants in produced gas are carbon dioxide (COj) and hydrogen sulphide (HjS). Both can combine with free water to cause corrosion and H2S is extremely toxic even in very small amounts (less than 0.01% volume can be fatal if inhaled). Because of the equipment required, extraction is performed onshore whenever possible, and providing gas is dehydrated, most pipeline corrosion problems can be avoided. However, if third party pipelines are used it may be necessary to perform some extraction on site prior to evacuation to meet pipeline owner specifications. Extraction of CO2 and H2S is normally performed by absorption in contact towers like those used for dehydration, though other solvents are used instead of glycol. 

There aie a number of major indusuial problems in the operation of the steam reforming of metlrane. These include the formation of carbon on the surface of the catalyst, the sulphidation of the catalyst by the H2S impurity in commercial natural gas, and die decline of catalytic activity due to Ostwald ripening of the supported catalyst particles by migration of catalyst atoms from the smaller to tire larger particles, as the temperamre is increased. A consideration of tire thermodynamics of the principal reaction alone would suggest that the reaction shifts more favourably to the completion of the reaction as the temperature is increased. 

Most of the early work carried out in relation to these aspects of the problem used nitrates as the cracking environment where low-strength steels have been the objects of interest. Consequently most of what follows refers to cracking in boiling concentrated nitrate solutions except where otherwise stated. The medium and higher strength steels, such as involved in sour oil well equipment and other applications, are more frequently tested in chloride- or sulphide-containing environments related to service conditions, but the failure of these steels is dealt with elsewhere (see Section 8.4). 

Marine fouling leading to the local production of HjS increases crack growth rate, but what the effect is when combined with CP is uncertain. Some of the factors mentioned earlier in connection with other steel corrosion problems are important to sulphide stress-corrosion cracking, (SSCC), eg. compositions, particularly C which usefully can be reduced to below 0.05%, S, microstructure and segregation . Compositional homogenisation by heat treatment can be beneficial ", whilst the presence of Cu in the... 

Organic materials Corrosive vapours are sometimes emitted by organic materials used either in packaging or in the manufactured article, and may be troublesome in confined spaces. Some woods, particularly unseasoned oak and sweet chestnut, produce acetic acid (see Section 18.10), and certain polymers used in paints, adhesives and plastics may liberate such corrosive vapours as formic acid and hydrogen sulphide . It may be necessary to carry out exposure trials, particularly where materials capable of liberating formaldehyde or formic acid are involved. Most corrosion problems of this kind can be prevented by using desiccants, and in many cases they are confined to imperfectly cured materials. For an excellent review see Reference 9. 

The ER system has been used successfully in a range of industries for process plant monitoring. As ER can be applied in any liquid or gaseous environment the areas of application are considerable. However, there is a problem with ER if a conductive corrosion product is produced as is the case with sour crude oil or gas due to the deposition of iron sulphide. 

Loss of catalytic activity resulting from internal displacements is not usually a serious problem below temperatures of about 100 C. However, highly active R-groups, such as benzyl, methyl and allyl, undergo internal displacement more readily, particularly in the presence of strong nucleopfiles. For instance, the presence phenolates and thiolates may lead to the formation of benzyl alcohol, ethers, or sulphides from benzyl-substituted quaternary ammonium salts. 

The first indication of a possible connection between geological processes occurring at the boundaries between tectonic plates of the mid-oceanic ridges and the biogenesis problem was provided by J. B. Corliss (1981). He considered the hydrothermal conditions to be ideal reactors for abiotic synthesis these ideal conditions were the water temperature gradients, the pH, and the concentrations of solutes in the hot springs. The presence of certain minerals which could act as catalysts, such as montmorillonite, clay minerals, iron oxide, sulphides etc., was also very important. The initial model presented for the hydrothermal synthesis of biomolecules (Corliss, 1981) was modified, particularly by Russell (1989) and Wachtershauser (see Sect. 7.3). 

Post-precipitation involves the deposition of a sparingly soluble impurity of similar properties to the precipitate on the surface of that precipitate after it has been formed. It is particularly a problem where similar materials are being separated on the basis of their different rates of precipitation, e.g. calcium and magnesium oxalates or zinc and mercury sulphides. Copreci-... 

Literature on flotation of gold, PGMs, rare earths and various oxides is rather limited, compared to literature on treatment of sulphide-bearing ores. As mentioned earlier, the main problem arises from the presence of gangue minerals in the ore, which have flotation properties similar to those of valuable minerals. These minerals have a greater floatability than that of pyrochlore or columbite. In the beneficiation of oxide minerals, finding a selectivity solution is a major task. 

One of the major problems associated with beneficiation of PGM from sulphide-dominated deposits is the presence of hydrophobic gangues, such as talc, chlorites, carbonates and aluminosilicates. The concentrates produced in most of the Morensky Reef operations (South Africa) varies from 80 to 150 g/t of combined PGM, where most of the contaminants are silicates, aluminosilicates and talc (i.e. up to 60%). The major hydrophobic gangue depressants used are carboxymethyl cellulose (CMC) and different modifications of guar gums. 

Flotation of the lead oxide minerals is a difficult problem not least because there are no known direct acting collectors. Normally, during oxide lead flotation, a sulphidization method is used with xanthate as a collector. In the majority of cases, the ore is pretreated using a desliming process, especially if the ore contains clay and Fe-hydroxides. Another method includes the preconcentration using heavy liquid. 

A method [62] has been described for the determination of down to 2.5pg kg-1 alkylmercury compounds and inorganic mercury in river sediments. This method uses steam distillation to separate methylmercury in the distillate and inorganic mercury in the residue. The methylmercury is then determined by flameless atomic absorption spectrophotometry and the inorganic mercury by the same technique after wet digestion with nitric acid and potassium permanganate [63]. The well known adsorptive properties of clays for alkylmercury compounds does not cause a problem in the above method. The presence of humic acid in the sediment did not depress the recovery of alkylmercury compounds by more than 20%. In the presence of metallic sulphides in the sediment sample the recovery of alkylmercury compounds decreased when more than lmg of sulphur was present in the distillate. The addition of 4M hydrochloric acid, instead of 2M hydrochloric acid before distillation completely, eliminated this effect giving a recovery of 90-100%. 

Doctor et al. (2000) point out the technical problems in the transport infrastructure that could arise from impurities in the C02. Any transport system requires the C02 to be dried to prevent the formation of C02 hydrates. Considerable problems with the formation of iron sulphide in natural gas pipelines indicate that C02 also has to be cleaned of hydrogen sulphide content. 

Corrosion and erosion of the tubes immersed in the bed are at a low level, although there is evidence that the addition of limestone or dolomite causes some sulphide penetration. The chief operating problem is corrosion by chlorine. 

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... 

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