Magnetite conductivity

Magnetite, Fe304, is a crystalline phase that contains an equal number of Fe and Fe " ions. Its crystal structure is that of an inverse spinel with an oxygen anion lattice containing octahedral and tetrahedral sites that are occupied by metal ions. The Fe " ions occupy the tetrahedral sites and half of the octahedral sites, whereas Fe " is found exclusively on octahedral sites. In contrast to FeOOH phases, magnetite conducts electricity. In addition, it is magnetic. 

The disk filter is similar to the dmm in operation, but filtration is conducted using a series of large diameter filter disks that carry the filter medium on both sides of the disk. They are connected to the main horizontal shaft and partly immersed in the feed slurry. The central shaft is connected by a set of valves which serve to provide vacuum and air as in dmm filters. As the disk sections submerge during rotation, vacuum is appHed to form a cake on both sides of the disk. The cycle of operation is similar to that in a dmm filter. One unit can have as many as 12 disks of up to 5-m diameter. Disk filters, both compact and cost effective, are used extensively in the iron ore industry to dewater magnetite concentrates. 

Conductive films such as magnetite (Fe,0 ) or mill scale on steel, and conductive nonmetals such as carbon can function as cathodes when in contact with anodes... 

The principal cathodic reaction on the upper surface of the membrane is the reduction of Cu " that is formed by the reaction of Cu with dissolved oxygen in the water these Cu ions are provided partly from the diffusion through the pores in the oxide membrane from within the pit and partly from those produced by cathodic reduction (equation 1.154). Lucey s theory thus rejects the conventional large cathode small anode relationship that is invoked to explain localised attack, and this concept of an electronically conducting membrane has also been used by Evans to explain localised attack on steel due to a discontinuous film of magnetite. 

These tracer and tagged polymer products are very useful for conducting test work such as in the examples given above, but, similar to their application in cooling water, the monitoring system only measures the bulk-water concentration of polymer. This may have little or no relevance to metal surface chemistry and the protective integrity of the magnetite film. 

For example, assuming anhydrite-magnetite-calcite-pyrite-pyrrhotite buffers redox in sub-seafloor reaction zones and a pressure of 500 bars, dissolved H2Saq concentrations of 21 °N EPR fluid indicate a temperature of 370-385°C. However, the estimated temperatures are higher than those of the measurement. This difference could be explained by adiabatic ascension and probably conductive heat loss during ascension of hydrothermal solution from deeper parts where chemical compositions of hydrothermal solutions are buffered by these assemblages. 

Ceramic Materials An example of a sufficiently conductive metal oxide is magnetite Fe304, which has been used, for example, in the past as corrosion resistant anode material for industrial chlorine evolution (it can be smelted and casted at 1500 °C, but it is a very brittle material). 

Hematite, wiistite, maghemite and magnetite are semiconductors magnetite displays almost metallic properties. For a compound to be a semiconductor, the essential characteristic is that the separation between the valence band of orbitals and the conduction band is less than 5 eV this condition is met for the above oxides. In a semiconductor the Fermi level (i. e. the level below which all electron energy levels are filled) lies somewhere between the valence band and the conduction band. 

A recent review of the experimental situation has been given by Honig(1985). It is pointed out that the electrical properties, particularly near to the transition, are very sensitive to purity and specimen preparation, and that much of the extensive experimental work is therefore open to doubt. None the less, the broad features of the behaviour of this material are clear. The history of the so-called Verwey transition in this material goes back to 1926, when Parks and Kelly (1926) detected an anomalous peak near 120 K in the heat capacity of a natural crystal of magnetite. The first detailed investigations were those of Verwey and co-workers (Verwey 1939, Verwey and Haayman 1941, Verwey et al. 1947), who showed that there was a near discontinuity in the conductivity at about 160K. The conductivity as measured by Miles et al. (1957) is shown in Fig. 8.1. 

The Travancore monazite is first reduced to convert leucoxene (ilmenite) into magnetite, thus changing the magnetic behaviour of the ilmenite [139]. The conductivity of monazite is not affected by the reduction process. Flotation processes have also been adopted to dress the Travancore monazite. 

Some mixed-valence Fe2+-Fe3+ minerals, because they possess high electrical conductivities at elevated temperatures, may be important on Venus (Bums and Straub, 1992). As a result, magnetite, ilvaite, laihunite, oxyhomblendes... 

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