Iron oxide , magnetite, crystal

HTS catalyst consists mainly of magnetite crystals stabilized using chromium oxide. Phosphoms, arsenic, and sulfur are poisons to the catalyst. Low reformer steam to carbon ratios give rise to conditions favoring the formation of iron carbides which catalyze the synthesis of hydrocarbons by the Fisher-Tropsch reaction. Modified iron and iron-free HTS catalysts have been developed to avoid these problems (49,50) and allow operation at steam to carbon ratios as low as 2.7. Kinetic and equiUbrium data for the water gas shift reaction are available in reference 51. 

Internal surfaces of all tubes were severely attacked (Fig. 4.29). A brown deposit layer consisting of magnetite, iron oxide hydroxide, and silica covered all surfaces. Deposition was thicker and more tenacious along the bottom of tubes. These deposits had a distinct greenish-blue cast caused by copper corrosion products beneath the deposit. Underlying corrosion products were ruby-red cuprous oxide crystals (Fig. 4.29). Areas not covered with deposits suffered only superficial attack, but below deposits wastage was severe. 

The structures of iron oxides have been determined principally by single crystal X-ray diffraction or neutron diffraction with supplementary information coming from infrared spectroscopy, electron diffraction and high resolution electron microscopy. A few years after the first successful application of X-ray diffraction to crystal structure determination, this technique was used to establish the major features of the structures of magnetite (Bragg, 1915 Nishikawa, 1915) and hematite (Bragg Bragg, 1918). 

Hydrothermal processes, i. e. the heating of suspensions of ferrihydrite in alkaline media under pressure, have been used to produce large platy crystals of hematite. This process gives vell formed crystals, but is expensive. The crystals can be reduced to produce isomorphous magnetite plates. Flame hydrolysis involves burning Fe " chloride at 400-800 °C to iron oxide. Owing to the many technical difficulties associated with this process, it is not commercially important. 

Murad, E. Schwertmann, U. (1984) The influence of crystallinity on the Mdssbauer spectrum of lepidocrocite. Min. Mag. 48 507-511 Murad, E. Schwertmann, U. (1986) Influence of Al substitution and crystal size on the room-temperature Mdssbauer spectrum of hematite. Clays Clay Min. 34 1-6 Murad, E. Schwertmann, U. (1988) Iron oxide mineralogy of some deep-sea ferromanganese crusts. Am. Min. 73 1395-1400 Murad, E. Schwertmann, U. (1993) Temporal stability of a fine-grained magnetite. Clays Clay Min. 41 111-113... 

Feldspars may be refractory as well or crystallized from partial melts. Whereas potassium feldspars are found to be mostly refractory, anorthite rich plagioclase may be newly formed. Pyroxene and spinel were identified in varying amounts but no olivine could be detected. Metals and iron oxide minerals (magnetite-hematite) are always present in BA, but have not been quantified yet. In Table 4 the amount and the ranges of measured mineral contents are summarized. 

Mechanism of the Promoter Effect. The action of the so-called structural promoters (stabilizers), such as A1203, is closely associated with their solubilities in the iron oxide matrix of the unreduced catalyst or with the capability of the regular crystallizing magnetite to form solid solutions with iron - aluminum spinels [33], [289]-[291]. The solid solutions of Fe304 and the spinel FeAl204 have a miscibility gap below 850 °C... 

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