Natural magnetite

Custom-ground natural magnetite is available in many size ranges. Table 19-13 gives atypical specification sheet. Cost is based on truckload quantities in 45.4-kg (100-lb) paper bags, FOB Frazer, Pennsylvania. 

TABLE 19-13 Typical Size Distribution of Ground Natural Magnetite ... 

Sidhu et ak (1981 a) compared the oxidation upon heating, of natural and synthetic magnetites. The coarse, natural magnetites were much more resistent to oxidation and higher temperatures or longer times were needed for it to take place. Hematite was the only oxidation product, which is in agreement with the results quoted above. 

Fig. 14.25 Change in degree of oxidation and unit cell size of synthetic and natural magnetites with time of heating. (The time scale is in minutes for synthetic and in hours for natural magnetites) (Sidhu et al., 1981 a with permission).

Vertical shaft furnaces (see Fig. 6.2) are used to harden natural magnetite pellets. Pellets are fed into the top of the shaft, which is rectangular in cross section, and descend against an upward flow of hot air. Typical operating data are given in Table 6.3. 

The raw materials — usually, natural magnetite, lime, potash, and alumina — must, as far as possible, be free of catalyst poisons (see Section 3.6.1.5). Many ores have too high a content of free or bound silica, which can be lowered with magnetic separators [344], Melting is accomplished in electrical resistance or induction furnaces (arc furnaces in the past) operating at 1600-2000 °C. The walls of these furnaces should... 

Flint M. E. (1984) The structure of magnetite two annealed natural magnetites Fe3 005O4 and Fe2.96Mgo 04O4. Acta Cryst. C40, I49I-I493. 

Milonjic, S.K., Kopecni, M.M., and Ilic, Z.E., The point of zero charge and adsorption properties of natural magnetite, J. Radioanal. Chem., 78, 15, 1983. 

Milonjic, S.K., Ruvarac, A.L., and Susie. M.V.. Adsorption of hydrogen and hydroxide ions at the natural magnetite-aqueous electrolyte interface. Bull. Soc. Chim. Beograd, 43, 207, 1978. 

NH3 and Derivatives. As part of a systematic study of properties and functions of catalysts for ammonia synthesis, correlations have been sought between the promoter addition procedure and the physicochemical properties and activity of catalysts obtained by the fusion of natural magnetite. The results show that the manner of addition of the promoter affects, to a large extent, the investigated properties and catalyst activity. 

Figure 1. Aqueous Fe(II) release from natural magnetite (A) and ilmenite (B) as functions of time and pH at 25°C (adapted from ref. 16).

Electrochemical Studies. The mechanisms and kinetics associated with the decoupled reactions (equations 1 and 2) have been investigated by dynamic polarization of electrodes constructed from natural magnetite and ilmenite samples (16). Examples of resulting potentiodynamic scans are shown in Figure 2. The log of the absolute current density generated is plotted as a function of potential applied to the mineral electrodes relative to a calomel electrode. Increases in electrode current densities above approximately 1000 mV and below -200 mV corresponded to the respective oxidation and reduction of H2O to form O2 and H2. 

During their initial studies about the airmionia synthesis catalysts, Bosch et alA found that the catalysts obtained from the reduction of natural magnetite are better than the catalysts from other iron compoimds. Almquist et alA studied the relation between the activity of iron catalyst and the oxidization degree before its reduction and found that those catalysts, of which the ratio of Fe +/Fe + is closer to be 0.5 and compositions closer to magnetite, has the highest activity. Bridger et alA further studied the fused iron catalysts promoted with binary promoters AI2O3-K2O,... 

Among all the materials involved in the preparation of the catalysts, the content of the refined magnetite is between 70% and 95%, which is also the major source of the silicon dioxide and harmful impurities such as sulfur. Therefore, the quality of magnetite must be strictly controlled. The chemical composition of natural magnetite is listed in Table 4.1. 

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