Reduction of magnetite

Aluminothermic reduction is one among the few pyrometallurgical processes where the actual process closely follows the route theoretically predicted from thermodynamic data. This characteristic, coupled with the simplicity associated with the whole process, makes it well suited for demonstration experiments. The reduction of magnetite by aluminum is a suitable example in this context. 

The rate controlling step was considered to be the reduction of magnetite to iron. During the reduction of hematite to magnetite there is an overall increase in volume due to dilation parallel to the c axis of hematite the dilation behaviour is dependent upon the reduction temperature (Husslage et al. 1999). 

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... 

Kostka, J.E. Nealson, K.H. (1995) Dissolution and reduction of magnetite by bacteria. Environ. Sci. Techn. 29 2535-2540 Kostov, I. (1968) Mineralogy. Oliver Boyd, Edinburgh, London, 587 p. 

Ammonia synthesis catalysts have traditionally been based on iron and have been made by the reduction of magnetite (Fe304). The difference between different commercially available products lies in optimized levels of metal oxide promoters that are included within the magnetite structure. These metal oxides promote activity and improve the thermal stability of the catalyst. Typical promoters are alumina (AI2O3X potassium oxide (K2O), and calcium oxide (CaO). The interactions between the many components in the catalyst can radically affect 1) the initial reducibility, 2) the level of catalyst activity that is achieved, 3) the long-term catalyst performance and 4) the long-term catalyst stability204. 

L. Takacs, Reduction of Magnetite by Aluminium A Displacement Reaction Induced by Mechanical Alloying, Mater. Lett., 1992, 13, 119. 

In a batch system the reduction of magnetite becomes thermodynamically favorable at temperatures far above 2,200 K. Experimentally, reduction was demonstrated in an inert atmosphere at ca. 300 K above the melting point of 1,811 K. The low surface area of the resulting product, however, makes mechanical activation of the reduced phase mandatory in order to achieve high reaction rates and productivity and therefore appears as a major obstacle for successful implementation [12]. 

Therefore in the region of pressures not exceeding 6-8 kbar dissociation proceeds according to reaction (4.25), and subsequently the independent process of reduction of magnetite to fayahte takes place. 

For industrial catalysts made by careful reduction of magnetite fused with nonreducible oxide promoters the important role of the (111) face seems to be confirmed [154]. However, the question whether the active industrial catalyst exposes mostly (111) faces remains unresolved. If not, further improvements of the catalyst are at least theoretically possible [155]. A critical evaluation of our present knowledge of the reaction mechanism was recently made by Schlogl [156]. 

Orientation is, therefore, quite important for catalytic praxis. It may well be assumed that empirically found preferred methods for preparing active catalysts are often those methods leading to the most active orientation. Westrik and Zwietering 18) proved that the iron catalyst for anunonia synthesis, prepared by a careful slow reduction of magnetite is well oriented in the [111] direction. 

At what temperatures is the following reaction, the reduction of magnetite by graphite to elemental iron, spontaneous ... 

In order to demonstrate the principles of the formation of multiphase non-porous reduction layers, we shall now discuss the reduction of magnetite to iron. Even though the limiting case of non-porous reduced layers is only observed in exceptional cases, this discussion will nevertheless serve to illustrate the essential points, such as the coupling of fluxes at the phase boundaries. 

Fig. 9-12. Reaction scheme for the reduction of magnetite to metallic iron with the formation of a compact product.

The parabolic rate constant for the growth of the wustite layer during the reduction of magnetite... 

The reduction of magnetite to iron with the formation of a porous metal layer can also be treated quantitatively in an analogous manner, as long as gas diffusion in the pores of the metal, and not a phase boundary reaction, is rate-determining. In this case, one needs only express the oxygen flux in eqs. (9-32) and (9-33) in terms of the appropriate effective diffusion coefficients for gas transport in porous layers. 

That is, the 70%-80% of metallic iron of the waste catalyst can be used as a reducing agent for the reduction of magnetite to FeO, and Fej xO-based catalysts can be obtained. At the same time, all the promoters such as AI2O3, K2O, CaO, CoO, etc in the waste catalyst enter into the fresh catalyst (the amount of promoters in fresh catalyst should include that from the waste catalyst), so that all of the components of the waste catalyst are utihzed effectively. Based on this idea, the... 

Fe-FeO, Fe0-Fe304 is in a state of direct contact with each other during the reduction of magnetite while the outer layer of wiistite interacts with the reductant gas. For example,... 

Many researchers verified that the strong effect of water vapor in gas phases cannot be ruled out during the reduction of magnetite at low temperatures, which can also be interpreted as the impact of the oxygen activity on nucleation and growth of metal. It has been identified that the critical value of a-Fe activity is not more than 1.015. In this case, if the activity of oxygen in gas phase is not near equilibrium, the supersaturation of the metal over phase boundary does not affect the reaction kinetics. It is believed that the critical supersaturation which is necessary for the formation of nuclei should be decreased when temperature increases. Therefore, the effect of nucleation on reduction rate is also reduced with increasing temperature. 

The apparent isotropic particle size of the iron particles was estimated from the full width at half maximum (FWHM) using the Scherrer equation to be ca. 80 nm as shown in Fig. 7.35. Whether the iron nuclei are formed by thermal decomposition of wiistite or by reduction of magnetite can be decided from the observation that ammonia iron exhibits a strong texture. Microscopic studies showed the iron particles to be textured in the [111] direction. Unfortunately, the Fe (111) line is not detectable for symmetry reasons. But the texture is characterized by smaller (hOO)/ hkO) ratios than calculated from the structure factors of isotropic a-iron. The fact that the (200) (110) ratio of the initially formed detectable iron particles is identical with the theoretical value of 0.2 unambiguously indicates the formation of... 

A few processes with reactive solids are listed in Table 8.1. Oxidation of zinc ore, that is, zinc sulfide, is a process in which the size of the reactive solid particle remains approximately the same zinc sulfide is oxidated to zinc oxide. Similar reactions occur, for example, in the oxidation process of pyrite to hematite. Reduction processes of metallic oxides with hydrogen in the production of pure metals are also processes in which the size of the solid phase remains unchanged. Another example of a reduction process is the reduction of magnetite to metallic iron. 

The active catalyst for the ammonia synthesis is alpha iron with small amounts of oxidic additives.. .. The quality of the final catalyst is crucially influenced by the activation process which is the reduction of magnetite to metallic iron. It is important to minimise the concentration of the reaction product water which is a catalyst poison. ... 

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