Magnetite porous

The reduction of hematite with H2 at 387-610 °C has been followed in situ using TEM and an environmental cell (Rau et al., 1987). The reduction reaction started at nudeation sites on the edge of the sample and as the reaction proceeded, a particle showed four reaction zones consisting of umeacted hematite, lamellar magnetite, porous magnetite and finally porous iron (the temperature was too low for wiistite). 

Holding of the temperature between 400 and 575°C causes the iron particles to coagulate and the scale becomes further enriched in oxygen. Since wiistite is unstable below 575°C, scales produced at temperatures lower than this contain magnetite and haematite only. In addition, the scales are often cracked and porous. This is due to the difference in contraction... 

At lower pH values, hydrogen ions are discharged, whereas at higher pH values, the magnetite layer thickens coarsely, becomes porous, and may peptize. Hydrogen blistering also may occur. 

The direct reaction of ferric oxide particles with steel heat transfer surfaces (corrosion) also is possible, producing a thick, coarse, and porous deposit of magnetite. 

The eventual result is the production of a thick, porous film of coarse magnetite (ferroso-ferric oxide or ferrous-ferrite), as shown below. This corrosion product is of no operational benefit and further hinders efficient heat transfer. 

The minimum rate of boiler steel corrosion (i.e., the maximum development of dense, adherent, and protective magnetite) occurs at pH of 11 to 12. However, at significantly lower pH values (acid conditions) hydrogen ions are discharged (depolarization), the magnetite layer becomes porous, and corrosion rates increase. 

Coarse magnetic corrosion Fe3°4 NOTE Passivation is a form of corrosion, albeit the resulting magnetite is desirable. Thick, porous, bulk water or boiler surfaces deposit. Formed under both high and low pH and other adverse BW conditions. Of no practical passivation benefit. 

Caustic gouging corrosion caused by localized high concentrations of caustic developing within and under porous surface deposits dissolving the protective magnetite film forming ferrite and hypoferrite ions. 

Still following the macro-structural hypothesis which we favored at that time, we abondoned the idea of a specific favorable influence of flux promoters and assumed instead that the cause for the success of the magnetite experiment was the compact porous structure of the iron sponge which was formed in the test oven by the reduction of the Swedish ore. An apparent support of this idea was that contrary to the favorable action of the dense iron sponge obtained from magnetite, catalysts of a looser structure such as, e.g., iron asbestos preparations had always been particularly ineffective. 

Maghemite formed by oxidation of magnetite is non porous, whereas that obtained by dehydration of lepidocrocite is meso porous. 

Under conditions leading to a porous shell of magnetite, the kinetic curve displayed an induction period corresponding to formation of nuclei and the subsequent reaction followed the cube root law. Diffusion of the reducing gas to the reactant/ product interface took place readily with a porous product. Whether chemical or diffusion control predominated depended on reaction conditions. With small crystals... 

The principal iron oxides used in catalysis of industrial reactions are magnetite and hematite. Both are semiconductors and can catalyse oxidation/reduction reactions. Owing to their amphoteric properties, they can also be used as acid/base catalysts. The catalysts are used as finely divided powders or as porous solids with a high ratio of surface area to volume. Such catalysts must be durable with a life expectancy of some years. To achieve these requirements, the iron oxide is most frequently dis-... 

The protective oxide on ferritic steel consists of two layers, which are porous to carbon dioxide. The inner layer consists of crystallites of Cr and Si and the outer layer Fe304 (magnetite) formed by the reaction of metallic iron and C02 and the product CO giving elemental carbon. 

The reduction of oxidic catalyst is generally effected with synthesis gas. The magnetite is converted into a highly porous, high surface area, highly catalytically active form of a-iron. The promoters, with the exception of cobalt, are not reduced [33]. 

The subsequent reduction of the magnetite is of crucial importance to the quality of the catalyst. It is normally carried out with synthesis gas in the pressure reactor of the ammonia plant at not too high pressures (70 to 300 bar, depending on the plant type) and at temperatures between 350 and 400°C, whereupon highly porous a-iron is formed ... 

Fig. 3.1.4. Rock phosphate from Malpelo Island, D.F., Colombia. The porous rock is essentially a mixture of strengite and phosphosiderite, but contains relict oxides (titaniferous magnetite). Magnification 33x. Reproduced with permission (McConnell, 1943).

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