Transformation of magnetite

Microbial transformation of magnetite to hematite. Geochim. Cosmochim. Acta 61 3341-3348... 

Welo, L.A. Baudisch, O. (1925) The two-stage transformation of magnetite into hematite. Phil. Mag. 50 399-408 Wen, L. Melendres, C.A. (1998) On the mechanism of hematite disposition on a metal surface under boiling conditions. Colloids Surfaces A 132 315-319 West, J.M. (1980) Basic corrosion and oxidation. Ellis Horwood, Chichester, 247 pp. 

Welo, L. and Baudisch, O. (1925) The two-stage transformation of magnetite into hematite. Phil. Mag. 50 399-408. 

Okamura, T. (1931), Transformation of magnetite at low temperature. Scientific Reports, Tohoku University, 21, 231-41. 

A fracture of particles was found not only in thermal decomposition of siderite, but also in the transformation of magnetite to wustite, and iron grain size grew along with roasting time. 

In the reaction blends within the temperatures range of 90-750 Xr dehydration and dehydroxylation of serpentines chrysotile and antigorite (Fig. 8, curve 1), decomposition of olivine, and the transformation of magnetite (see Table 3) were found to proceed according to the reactions (7)-(10). 

Duncan and Frankenthal report on the effect of pH on the corrosion rate of gold in sulphate solutions in terms of the polarization curves. It was found that the rate of anodic dissolution is independent of pH in such solutions and that the rate controlling mechanism for anodic film formation and oxygen evolution are the same. For the open circuit behaviour of ferric oxide films on a gold substrate in sodium chloride solutions containing low iron concentration it is found that the film oxide is readily transformed to a lower oxidation state with a Fe /Fe ratio corresponding to that of magnetite . 

In addition to oxalate, malonate and citrate accelerate the dissolution of iron oxides in the presence of Fe (Sulzberger et al., 1989). Fe " also promotes the dissolution of magnetite in sulphuric acid (Bruyere Blesa, 1985). Small amounts of Fe in solution speed up the transformation of ferrihydrite to goethite at 50 °C (see Fig. 14.24) by promoting the dissolution of ferrihydrite (Fischer, 1972). Adsorption... 

In moderately alkaline solutions (pH >8) oxidation of Fe" solutions proceeds via Fe(OH)2 and usually yields magnetite (David Welch, 1956 Sidhu et al., 1977). Under these conditions the solubility product of magnetite is exceeded so the mixed oxide is more stable than the pure Fe " oxides (see Chap. 8). Tamaura et al. (1981) monitored the transformation of Fe(OH)2 at pH 11 and 65 °C. Initially both goethite... 

In the dry state magnetite is readily oxidized to maghemite by air. Ultrafme crystals of magnetite change (over years) from black to the brown of maghemite even at room temperature (Murad Schwertmann, 1993). At temperatures >300°C, the transformation proceeds further to hematite (see section 14.2.7). 

A feature of this transformation is the influence of magnetite crystal size on the nature of the reaction products (Feitknecht, 1964 Gallagher et ak, 1968 Gillot et ak, 1978). At 200-250 °C, crystals smaller than 300 run transformed via the mixed phase to maghemite which in turn transformed to hematite at temperatures above 500 °C. 

In the presence of Fe" ions, lepidocrocite transforms into magnetite in alkaline media (Tamaura et al., 1983). The transformation which takes place at room temperature is stoichiometric, i. e. 

Blesa, M.A. Maroto, A.J.G. (1986) Dissolution of metal oxides. J. chim. phys. 83 757—764 Blesa, M.A. Matijevic, E. (1989) Phase transformation of iron oxides, oxyhydroxides, and hydrous oxides in aqueous media. Adv. Colloid Interface Sci. 29 173-221 Blesa, M.A. Borghi, E.B. Maroto, A.J.G. Re-gazzoni, A.E. (1984) Adsorption of EDTA and iron-EDTA complexes on magnetite and the mechanism of dissolution of magnetite by EDTA. J. Colloid Interface Sci. 98 295-305 Blesa, M.A. Larotonda, R.M. Maroto, A.J.G. Regazzoni, A.E. (1982) Behaviour of cobalt(l 1) in aqueous suspensions of magnetite. Colloid Surf. 5 197-208... 

Blesa, M. (1994) Morphological properties of a-FeOOH, y-FeOOH and Fej04 obtained by oxidation of aqueous Fe(II) solutions. J. Colloid Interface Sd. 165 244-252 Dong, H. Fredrickson, J.K. Kennedy, D.W. Za-chara, J.M. Kukkadapu, R.K. Onstott,T.C. (2000) Mineral transformations associated with the microbial reduction of magnetite. Chem. Geol. 169 299-318 Donnay J.D.H. Marker, D. (1935) A new law... 

Ishikawa.T Nakasaki, H. Yasukawa, A. Kandori, K. Seto, M. (1999) Influences of Co ", Cu " and Cr " ions on the formation of magnetite. Corr. Sci. 41 1665-1680 Ishikawa.T Nitta, S. Kondo, S. (1986) Four-ier-transformation infrared spectroscopy of colloidal a-, P- and y-ferric oxide hydroxides. 

Mendelovici, E. Yariv, S. (1980) Infrared study of the thermal transformation of goethite to magnetite in alkali-iodide discs. Thermo-chim. Acta 36 25-38... 

The CO that is consumed in the process, besides forming hydrocarbons and C02 also caused the transformation of hematite to magnetite and subsequently the magnetite to iron carbide. Therefore, a better measure of Fischer-Tropsch synthesis (FTS) activity is the rate of CH4 formation plotted in Figure 28.1(b). It is seen that activation at 543 K makes the... 

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