Magnetite kinetics

Under these circumstances the use of calculated preexponentials in kinetic model development is likely to lead to significant errors, in addition to those inherent in the use of calculated heats of adsorption. These problems are reflected in the attempt to model magnetite kinetics using observed heats of adsorption and estimated frequency factors, which gave rise to a calculated reaction rate a factor of 10 too low. The development of mechanistically sound, kinetic models will therefore remain dependent on the direct determination of the heats of adsorption, activation energies, and frequency factors for the forseeable future. 

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. 

In general there does not appear to be any direct correlation between the rate of the chemical dissolution of oxides and the rate of scale removal, although most work on oxide dissolution has concentrated on magnetite. For example, Gorichev and co-workers have studied the kinetics and mechanisms of dissolution of magnetite in acids and found that it is faster in phosphoric acid than in hydrochloric, whereas scale removal is slower. Also, ferrous ions accelerate the dissolution of magnetite in sulphuric, phosphoric and hydrochloric acid , whereas the scale removal rate is reduced by the addition of ferrous ions. These observations appear to emphasise the importance of reductive dissolution and undermining in scale removal, as opposed to direct chemical dissolution. 

Huff, G. A., Jr., and Satterfield, C. N. 1984. Intrinsic kinetics of the Fischer-Tropsch synthesis on a reduced fused-magnetite catalyst. Ind. Eng. Chem. Process Des. Dev. 23 696-705. 

Magnetite that has 5 Fe values near zero might reflect formation during slow (equilibrium) reduction rates, whereas the lowest 5 Fe values might reflect rapid Fe(III) reduction and kinetic isotope fractionation (Fig. 19), or multiple reduction cycles. The correlation between Magnetite-siderite 8 Fe of magnetite observed in BIFs therefore probably reflects changes in... 

Mandemack KW, Bazylinski DA, Shanks III WC, Bullen TD (1999) Oxygen and iron isotope studies of magnetite produced by magnetotactic bacteria. Science 285 1892-189 Mann S, Sparks NHC, Couling SB, Larcombe MC, Franke RB (1989) Crystallochemical characterization of magnetite spinels prepared from aqueous solution. J Chem Soc Far Trans 85 3033-3044 Matthews A, Zhu XK, O Nions K (2001) Kinetic iron stable isotope fractionation between iron (-II) and (-III) complexes in solution. Earth Planet Sci Lett 192 81-92... 

Natarajan, K. A., Riemer, S. C., Iwasaki, I., 1984. Influence of pyrrhotite on the corrosive wear of grinding balls in magnetite ore grinding. Inter. J. Miner. Process, 13(1) 73-81 Nesbitt, H. W., Bancroft, G. M., Pratt, A. R., Scaini, M. J., 1998. Sulfur and iron surface states on fractured pyrite surfaces. American Mineralogist, 83 1067 - 1076 Neeraj, K. M., 2000. Kinetic studies of sulphide mineral oxidition and xanthate adsorption. Doctor thesis of Virginia Polytechnic Institute and State University. A Bell Howell Company UMI dissertation Services... 

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... 

Baumgartner, E. Blesa, M.A. Maroto, A.J.G. (1982) Kinetics of the dissolution of magnetite in thioglycolic acid solutions. J. Chem. Soc. Dalton Trans. 1649-1654... 

Gillot, B. Rousset, A. Dupre, G. (1978) Influence of crystallite size on the oxidation kinetics of magnetite. J. Solid State Chem. 25 263-271... 

Swaddle, T.W. Oltmann, P. (1980) Kinetics of the magnetite-maghemite-hematite transformation, with special reference to hydrothermal systems. Can. J. Chem. 58 1763-1772 Swallow, K.C. Hume, D.N. Morel, F.M.M. (1980) Sorption of copper and lead by hydrous ferric oxide. Environ. Sci. Tech. 14 1326-1331... 

The source of this discrepancy is unknown to us. Equation (349) is, undoubtedly, adequate for the description of the reaction kinetics on an iron-chromium oxide catalyst. The fact that in one of the works (124) magnetite without the addition of chromium oxide served as a catalyst can hardly be of consequence since a study of adsorption-chemical equilibrium (344) on an iron-chromium oxide catalyst (7% Cr203) (52) led to the value of the average energy of liberation of a surface oxygen atom that practically coincides with that found earlier (50) for an iron oxide catalyst with no chromium oxide. It may be suspected that in the first work (124) the catalyst was poisoned with sulfur of H2S that possibly was contained in unpurified C02... 

The kinetics of a similar oxidation reaction was studied by Pritchard and Dobson (236). These authors studied the oxidation between 450 and 560 K of a metallic-iron foil (0.02 mm thick electroplated with 1 mg cm 2 57Fe) by deoxygenated water. The resulting Mossbauer spectra (at room temperature) showed Fe304 to be the only detectable reaction product, and from the ratio of the Fe304 spectral area to that of metallic iron, the magnetite film thickness y can be calculated. Assuming that the rate law is of the form... 

Straub, D. W. Bums, R. G. (1992) A kinetic study of the conversion of hematite to magnetite with applications to the metastability of hematite on Venus. Lunar Planet. Sci., XXIII, 111-2. 

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