Reduction iron particle size

Iron particle size from X-ray line broadening and percentage-reduction/CO-chemisorption ... 

Anderson et al. (1964) studied fused iron catalysts which had either been reduced or reduced and nitrided prior to use in fixed-bed reactors, determining reaction kinetics and the effects of the extent of reduction and particle size on catalyst activity. Particle sizes ranged from 42—60 mesh to 4—6 mesh. The catalyst activity increased with smaller particle size until the diameter reached about 0.3 mm for the most active catalysts tested. Catalyst particles were modeled as an active layer of catalyst surrounding an inert core, with the depth of the active layer governed by the reduction temperature. Their calculations allowed them to estimate the effective reactant dif-fusivity, and they were also able to quantify the depth of the active layer of catalyst. Variations in catalyst activity were attributed to the diffusion of reactant through a wax-filled pore and the depth of the active layer. 

Suppose you prepared an iron oxide catalyst supported on an alumina support. Your aim is to use the catalyst in the metallic form, but you want to keep the iron particles as small as possible, with a degree of reduction of at least 50%. Hence, you need to know the particle size of the iron oxide in the unreduced catalyst, as well as the size of the iron particles and their degree of reduction in the metallic state. Refer to Chapters 4 and 5 to devise a strategy to obtain this information. (Unfortunately for you, it appears that electron microscopy and X-ray diffraction do not provide useful data on the unreduced catalyst.)... 

As already indicated, the difficulty of reducing supported iron in hydrogen is well-known [6,8,11]. It probably arises from a combination of causes, the two most important of which are a strong interaction with the support [6,8] and reoxidation or inhibition by water vapour in the pores of the oxide [14]. With MgO as support, there is undoubtedly a strong tendency for iron, especially at the Fe2+ stage of reduction, to be present at least in part as FeO-MgO (Fe Mgi.jjO) solid solution [6,8]. This need not be deleterious to the ultimate formation of finely-divided iron, provided the method of preparation has led to a solid solution in which the Fe2+ ions are well-distributed. The iron particles are limited in size... 

In 2000, Sun and co-workers succeeded in synthesis of monodispersed Fe/Pt nanoparticles by the reduction of platinum acetylacetonate and decomposition of Fe(CO)5 in the presence of oleic acid and oleylamine stabilizers [18]. The Fe/Pt nanoparticle composition is readily controlled, and the size is tunable from 3 to 10 nm in diameter with a standard deviation of less than 5%. For practical use, we developed the novel symthetic method of FePt nanoparticles by the polyol reduction of platinum acetylacetonate (Pt(acac)2) and iron acetylacetonate (Fe(acac)3) in the presence of oleic acid and oleylamine stabilizers in di- -octylether [19,20]. The Fe contents in FePt nanoparticles can be tuned from 23 to 67atomic%, and the particle sizes are not significantly affected by the compositions, retaining to be 3.1 nm with a very narrow size distribution, as shown in Figure 6. 

SPIREX A DR process for making iron powder or hot briquetted iron from iron ore fines. Three stages are used. The first is a circulating fluidized bed preheater whose turbulent conditions reduce the particle size of the ore. The second and third stages achieve the reduction in fluidized beds, fed by reducing gases from a MIDREX reformer. Developed by Midrex Direct Reduction Corporation and Kobe Steel. A demonstration plant was scheduled to be built at the Kobe Steel plant in Venezuela in 1997. 

The usual techniques for the determination of particle sizes of catalysts are electron microscopy, chemisorption, XRD line broadening or profile analysis and magnetic measurements. The advantage of using Mossbauer spectroscopy for this purpose is that one simultaneously characterizes the state of the catalyst. As the state of supported iron catalysts depends often on subtleties in the reduction, the simultaneous determination of particle size and degree of reduction as in the studies of Fig. 5.10 is an important advantage of Mossbauer spectroscopy. 

The solubility and the hydrolysis constants enable the concentration of iron that will be in equilibrium with an iron oxide to be calculated. This value may be underestimated if solubility is enhanced by other processes such as complexation and reduction. Solubility is also influenced by ionic strength, temperature, particle size and by crystal defects in the oxide. In alkaline media, the solubility of Fe oxides increases with rising temperature, whereas in acidic media, the reverse occurs. Blesa et al., (1994) calculated log Kso values for Fe oxides over the temperature range 25-300 °C from the free energies of formation for hematite, log iCso fell from 0.44 at 25 °C to -10.62 at300°C. 

Monodispersed, 2- to 6-nm-dia- Particles, formed from their metal salts by meter nickel boride, cobalt boride, NaBH4 reduction, were used as catalysts, nickel-cobalt boride, and iron Micellar core size controlled particle sizes... 

The iron used may be either iron powder or iron filings, but it should pass a 100-mesh sieve in order to give invariable results. If iron particles of larger size are used, the reduction may be incomplete although complete reduction was obtained with a sample of iron powder contaminated with filings. 

From these results, it is concluded that, in a fully reduced catalyst, FeAl204 is not present furthermore, the aluminum inside the iron particle is present as a phase that does not contain iron (e.g., A1203), and this phase must be clustered as inclusions 3 nm in size. These inclusions may well account for the strain observed by Hosemann et al. From the Mossbauer effect investigation then, the process schematically shown in Fig. 17 was suggested for the reduction of a singly promoted iron synthetic ammonia catalyst. Finally, these inclusions and their associated strain fields provide another mechanism for textural promoting (131). 

It was shown by these authors that the amount of nitrogen present during pretreatment of a catalyst affects the ultimate activity for ammonia synthesis (206). Specifically, it was found that treating H2-reduced small particles with ammonia at 670 K, followed by re-reduction of the catalyst with a H2 N2 gas mixture, gave rise to an increase in the catalytic activity compared to the activity measured after H2 reduction alone. However, when the catalyst in this high-activity state was further treated with H2 alone at 670 K, the catalytic activity was found to decrease to that value observed before the above ammonia treatment. Subsequent ammonia treatment returned the catalyst to its high-activity state. No such effects were observed for metallic-iron particles greater than 10 nm in size. 

Another approach receiving increased interest is direct reduction of iron ore. In this process, coal is added directly to the smelting vessel and is the source of the reducing gases and thermal energy. Thus, easily crushed and handled, inexpensive noncoking coals of high calorific value can be used. Particle size requirements are variable, but less than 1 mm is most often used and, in some cases, there is an effort to restrict moisture content to below 6 percent. 

Electron paramagnetic resonance experiments also support the existence of ferromagnetic iron particles. X-ray powder diffraction experiments, electron microscopy, and surface analysis measurements show the existence of both metals on the surface before and after reduction. We know the particle size again is quite large, although there is a wide digtribution in these samples, ranging from 30 A to about 150 A. 

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