Air oxidation of iron

In pocket plate cells, the active materials are a mixture of finely powdered metallic iron and Fe304. The preparation of this mixture varies from manufacturer to manufacturer, but generally involves a final process in which controlled air oxidation of iron powder or reduction of Fe304 with hydrogen is used to form the appropriate composition. Additives such as cadmium, cadmium oxide or graphite are commonly included to improve the capacity retention and electronic conductance. The performance of the electrode is improved by the addition of up to 0.5% of FeS the mechanism of the sulphide involvement is not well understood. If sulphide is lost by oxidation after prolonged use, small amounts of soluble sulphide may be added to the electrolyte,... 

From the above equation we see that the air oxidation of iron is favored in acidic solution lower pH values would have more positive Eam- Increased levels of carbon dioxide and water generate carbonic acid that lowers the pH of the medium, which makes the corrosion process more favorable. 

A number of workers (8, 13, 14, 15, 20, 21, 22, 23, 33, 34, 35) have reported studies of the rates of air oxidation of iron (II) to iron (III) in aqueous solution and have discussed the mechanistic implications of the kinetic studies. Only Pound (28) has reported the results of a preliminary study of the reaction in nonaqueous solvents. We have investigated the reaction in organic solvents because the faster rates are more convenient to follow and because a wider range of organic cosubstrates can be included. The ultimate aim of this study is to increase our understanding of the various catalytic and inhibitory roles played by metallic compounds in autoxidations. 

Air oxidation of dyestuff waste streams has been accompHshed using cobalt phthalocyanine sulfonate catalysts (176). Aluminum has been colored with copper phthalocyanine sulfonate (177,178). Iron phthalocyanine can be used as a drier in wood oil and linseed oil paints (179). 

The result is that the oxidation of iron in aerated water (rusting) goes on at a rate which is millions of times faster than that in dry air. Because of the importance of (c), wet oxidation is a particular problem with metals. 

A number of water-soluble calcium salts provide convenient vehicles for the administration of therapeutic anions. Probably the most widely encountered is the acetylsalicylate of soluble aspirin (patented 1935) the urea complex of calcium acetylsalicylate (water-solubility 231 gdm-3 at 310K, pH 4.8 (717)) is also widely used. Other examples include calcium bromide and bromolactobionate (sedatives), calcium 2-hydroxy-3-mercapto-l-propanesulfonateaurate(III) (chrysanol, antiarthritic), and calcium Af-carbamoylaspartate (tranquillizer). Calcium iron(II) citrate has been used to remedy iron deficiency - it has the advantage of being stable to air oxidation of the iron(II). The cyclamate anion is certainly not therapeutic, but is relevant here. 

The production of copper from sulphide minerals is accomplished with a preliminary partial roast of the sulphides before reaction with air in the liquid state, known as mattes, to form copper metal (conversion). The principal sources of copper are minerals such as chalcopyrite, CuFeS2 and bomite Cu3FeS3, and hence the conversion process must accomplish the preferential oxidation of iron, in the form of FeO, before the copper metal appears. As mentioned before, the FeO-Si02 liquid system is practically Raoultian, and so it is relatively easy to calculate the amount of iron oxidation which can be carried out to form this liquid slag as a function of the FeO/Si02 ratio before copper oxidation occurs. The liquid slag has a maximum mole fraction of FeO at the matte blowing temperatures of about 0.3, at solid silica saturation. 

These materials are very easily autoxidised and often have a low autoignition temperature. It is reported that many of the less volatile liquid aldehydes will eventually inflame if left exposed to air on an absorbent surface. The mechanism is undoubtedly similar to that giving rise to easy ignition in the air-oxidation of acetaldehyde and propionaldehyde initial formation of a peroxy-acid which catalyses the further oxidation[l]. Autoignition temperatures of lower aldehydes are much reduced by pressure, but appear to depend little on oxygen content. The effect is worst in the presence of free liquid, in which initial oxidation appears to occur, possibly catalysed by iron, followed by ignition of the vapour phase [2], An acetaldehyde/rust mix exploded at room temperature on increasing the air pressure to 7 bar. 

For oxidation of iron to occur at high temperatures, the oxygen partial pressure must be above that of the dissociation pressure of the appropriate corrosion products. For example, at ca. 700 °C, an oxygen partial pressure of greater than 10 Pa is required for wiistite to form. In air, of course, this condition is readily satisfied, at least initially. As oxidation continues and the film thickens and becomes coherent, an oxygen gradient across the film is established and the composition of the corrosion layer changes. 

Kiyama, M. Akita, T. Shimizu, S. Okuda,Y. Takada,T. (1972) Conditions favourable for the formation of y-FeOOH by aerial oxidation in an acidic suspension of iron metal powder. Bull Chem. Soc. Japan 45 3422—3426 Kiyama, M. Shamoto, S. Horushi, N. Okuda, Y. Takada,T. (1986) Growth of needle-like a-FeO(OH) particles by air oxidation of aqueous suspensions containing Fe(ll) predpi-tates. Bull. Inst..Chem. Res. Kyoto Univ. 46 150-156... 

Studies on the oxidation of iron(II) ions during formation of Fe304 and a-FeOOH by air oxidation of Fe(OF[)2 suspensions. J.C.S. Dalton, 1807-1811... 

Also, the tetroxide may be produced by partial oxidation of iron or iron(II) sulfate by heating under limited amount of air. Another method of production involves heating iron metal with steam ... 

The oxidation of iron by (dry) air at ambient temperature proceeds according to the inverse logarithmic rate law, which may be written... 

Compare and contrast the mechanisms of oxidation of iron by dry air ( scaling see Chapter 5) and by aqueous environments. 

Acrolein and Acrylic Acid. Acrolein and acrylic acid are manufactured by the direct catalytic air oxidation of propylene. In a related process called ammoxida-tion, heterogeneous oxidation of propylene by oxygen in the presence of ammonia yields acrylonitrile (see Section 9.5.3). Similar catalysts based mainly on metal oxides of Mo and Sb are used in all three transformations. A wide array of single-phase systems such as bismuth molybdate or uranyl antimonate and multicomponent catalysts, such as iron oxide-antimony oxide or bismuth oxide-molybdenum oxide with other metal ions (Ce, Co, Ni), may be employed.939 The first commercial process to produce acrolein through the oxidation of propylene, however, was developed by Shell applying cuprous oxide on Si-C catalyst in the presence of I2 promoter. 

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