Magnetite metal substituted

Fig. 3.2 Fraction of various metals released versus Fe released during acid dissolution of synthetic metal-substituted magnetites (upper six plots Sidhu et al., 1978, with permission), goethites and hematites (lower plots Lim-Nunez dikes, 1987 with permission).

In the proposed two-step process, it is important to attain high efficiencies for conversion of coal to COx (CO -I- CO2) in the first-step reaction and then for conversion of CO2 to CO in the second-step reaction by an external heat input. From the thermodynamic conditions and the low cost, the redox pair of Fe304/a-Fe was one of the promising redox systems for the two-step process, but it still required the operating temperature above 1200°C[2]. It is well known that many kinds of metal ions can be incorporated into the spinel lattice structure of magnetite by replacing ferrous or ferric ions. There is the possibility that metal-substitution for Fe or Fe " in magnetite causes a phase transition to the metallic phase, which proceeds readily even at low temperatures and improves the conversion efficiencies of coal and CO2 to CO in the two-step process. 

Crucial to the understanding of metal-substituted magnetite and their properties is the determination of the site occupancy of the metals. The structure of magnetite is very well known. It is a cubic, inverse spinel with one quarter of the tetrahedral (Tj) and one half of the octahedral (0 j) sites filled by Fe. The formula for magnetite is (Fe )[Fe Fe ]04 where Fe ... 

Burnishing is the formation of black-brown oxide films on iron and its alloys by controlled oxidation of cleaned metal surfaces. These films are extremely complex and contain, in addition to maghemite and magnetite (or a substituted magnetite for Ni, Mo or Co alloys), various nitride phases - Fe4N, FeaN and FeN. The nitride phases are adjacent to the metal and the iron oxides are in the outer layers of the film (Gebhardt, 1973). 

When other metals (M) substitute for Fe in the structure of an Fe oxide the mole ratio of substitution is given by Mt/(Mt + Fet)(mol/mol), where Mt and Fet (t = total) are expressed in mol. Fe and other metals present at the surface of the iron oxide or in separate phases must be determined separately to correct the extent of substitution. Ferrihydrite as a separate phase can be selectively dissolved an with acid oxalate solution (see p. 50). This treatment also dissolves any separate Mn or Cr oxides. Alternatively, a short extraction (30 min, 25 °C) with 0.4 M HCl removes adsorbed surface species this method is useful if the solubility of the substituting ion in acid oxalate solution is not known or if the iron oxide under consideration (for example magnetite) is soluble in acid oxalate solution. The total Fet and Mt have then to be corrected for the oxalate soluble Fe and M. 

The other two materials which have become useful as anode materials are compounds lead dioxide and magnetite which are not oxidized any further under anodic conditions, indeed the stability of lead dioxide has been remarkable. Lead dioxide has been more widely used for anode construction than magnetite, its specific resistance is lower than many metals, it is hard and the oxygen overvoltage on it is the same as on platinum. Thus it is suitable as a substitute in many processes which initially required platinum anodes. The difficulty with both lead dioxide and magnetite anodes is their fabrication and this has delayed their development. 

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