Pure Magnetite

Fig. 11-3. X-ray diffraetograms of (from top to bottom) pure magnetite Al-sub-stituted magnetite with 7 mol% Al maghemite produced by heating pure magnetite at 250 °C. Note the peak shift relative to pure magnetite due to AI substitution (middle) and to oxidation to maghemite (bottom).

In the course of the spontaneous oxidation of magnetite to y — Fe-jOa, the magnetic moment of the particles changes linearly with the Fe +/Fe3+ ratio between that of pure magnetite and pure y — Fe203. 

Fid. 18. Debye-Sclienvr diagram. of iron Fiscliur-Trop.sch catalysts reduced fresh catalyst, used catalyst, used catalyst, pure iron, pure magnetite. 

The HogenSs process is widely used for the production of reduced metal powders [32.24]. In the production of iron powder, for example, pure magnetite ores are comminuted, screened and purified using magnetic separators. They are then mixed with a blend of coke and quicklime, and heated in a silicon carbide lined retort at 1050 to 1200 °C for 24 to 40 hours. The iron oxide is reduced by the coke to iron and the quicklime reacts with sulfur, silica and alumina in the ore and coke, forming a slag. 

If the reaction involves pure hematite Fe203 and pure magnetite Fe304, then = 1 and Xp o, = U so = 1 and ap, o, = 1- Therefore... 

A = Mg, Fe, NT, Co, and Zn and trivalent cations B = Al, Fe, Cr, and V L Hence two types of spinel structure must be distinguished normal spinels withx = 0, meaning that all the divalent cations occupy tetrahedral sites, and inverse spinels with x = 1. Of these, rods of pure magnetite (Fe OJ or its doped form" obtained by casting molten iron oxides have been used as industrial anodes since 1870. Apart from magnetite and ferrites, today other classes of spinels have been investigated such as cobaltites and chromites. Due to their better electrocatalytic properties and fewer health and safety issues, cobaltites (e.g., MCo O with M= Mg, Cu, and Zn) are now preferred and are the only ones being developed. 

Table 7.20 Comparison of the reflection intensities (%) of pure magnetite and...

Miller index Pure magnetite Magnetite precursor Miller index Piu-e magnetite Magnetite precursor... 

At low temperatures, the spectrum of pure magnetite is very complex (Fig. 3.14a) and may be described by at least five subspectra [118]. This is due to the 3d electron localization below the so-called Verwey transition at about 125 K leading to discrete Fe " and Fe " spectral contributions of the B sites. However, this transition temperature is lowered in the case of substitution or partial oxidation [119-121]. This is illustrated in Fig. 3.14b where oxidized magnetite (Fe2.94404)... 

The velocity vim s of pure magnetite particles with their magnetic polarization IIT in a magnetic field gradient grad HIA m is given in a first approximation [18] ... 

Another way of following the activation kinetics is to monitor the weight loss of the sample, which for complete reduction would be 27.3 wt% in the case of pure magnetite. The experiments need to be carried out with great care since the gas flow pattern, which is a crucial parameter in the kinetics of a gas-solid reaction, will be different for a catalyst placed in a fixed bed reactor and a catalyst suspended on a weighing pan. Much of the kinetic data available on activation, and which will be discussed below, were obtained by monitoring the weight loss. 

It is impossible to describe the activation process with a simple kinetic model using only one rate-determining step such as the chemical reaction itself or hydrogen diffusion. Furthermore, it has been shown that gaseous diffusion did not influence the parameters. From the particle-size dependence of the parameters, it was found that diffusion within the particle plays a dominant role in contrast with the case of pure magnetite, it was not possible to distinguish between surface diffusion and bulk diffusion. 

In both samples, the iron-to-oxygen ratio is lower than the calculated value for pure magnetite (nominally 0.75, with an experimental value for a powder sample of 0.70). The chemical state of much of the excess oxygen was found to be adsorbed water. Desorption of the water layer at ca 650 K caused an increase in the iron-to-oxygen ratio. Pretreatment of the catalyst in the high-pressure cell... 

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