High sulphidizing atmosphere

High temperature corrosion of Inconel-600 tube used as a furnace accessory has been reported by Krishna and Sidhu (2001). The corrosion of the tube was found to be due to severe oxidizing and sulphidizing atmospheres generated by interdiffusion of base metal constituents and sulphur through the microchannels. 

Sulphates, which form part of the ash from the combustion of many fuels, are not harmful to high-alloy steels, but can become so if reduction to sulphide occurs. This leads to the formation of low melting point oxide-sulphide mixtures and to sulphide penetration of the metal. Such reduction is particularly easy if the sulphate can form a mixture of low melting point with some other substance. Reduction can be brought about by bad combustion, as demonstrated by Sykes and Shirley , and it is obviously important to avoid contact with inefficiently burnt fuels when sulphate deposits may be present. Reduction can also be brought about in atmospheres other than reducing ones and the presence of chlorides or vanadium pentoxide has been shown to be sufficient to initiate the reaction. It has also been shown that it can be initiated by prior cathodic polarisation in fused sodium sulphate. The effect of even small amounts of chloride on oxidation in the presence of sulphate is illustrated in Fig. 7.33 . 

The deposit resists atmospheric tarnish even in the presence of high pollution by sulphur dioxide (in contrast to nickel) and hydrogen sulphide, and coatings exposed to the outdoor atmosphere remain bright indefinitely, sometimes taking on a slightly more pink colour as the oxide film thickens. 

It must be appreciated that at high temperatures platinum permits the flame gases to diffuse through it, and this may cause the reduction of some substances not otherwise affected. Hence if a covered crucible is heated by a gas flame there is a reducing atmosphere in the crucible in an open crucible diffusion into the air is so rapid that this effect is not appreciable. Thus if iron(III) oxide is heated in a covered crucible, it is partly reduced to metallic iron, which alloys with the platinum sodium sulphate is similarly partly reduced to the sulphide. It is, advisable, therefore, in the ignition of iron compounds or sulphates to place the crucible in a slanting position with free access of air. 

During prebiotic times, water-soluble ferrous iron was present and was used in the first stage of life, while copper was in the water-insoluble Cu(I) state, as highly insoluble sulphides. About 109 years ago the metabolism of a primitive prokaryote (cyanobacteria) led to the evolution of dioxygen into the Earth s atmosphere. A... 

This technique uses a nitrogen atmosphere in grinding and flotation to retard oxidation of reactive sulphides, and has been successfully applied on carbonaceous ores from Nevada (USA). The effectiveness of the method depends on (a) the amount of carbonaceous gangue present in the ore, and (b) the amount and type of clay. Ores that are high in carbon or contain high clay content (or both) are not amenable for nitrogen atmosphere flotation. 

Traces of pentathionic acid are also stated to be found in the condensed liquid from the interaction of steam and sulphur vapour at a red heat,2 and in a mixture of sulphur and water after exposure to atmospheric oxidation.3 In the former ease the pentathionic acid probably results after the high temperature reaction by the interaction of sulphur dioxide and hydrogen sulphide in the condensate. 

Rock samples, especially those containing sulphides, should be crushed in a percussion mortar and then ground to about 20 mm in an agate mortar. High-energy crushing and grinding techniques are to be avoided and particular care should be taken to minimise contamination of sulphides by atmospheric Hg and cross-contamination of unmineralised samples by sulphides. 

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