Solid Solution Of Magnetite Research Articles

The Ideal and Regular Mechanisms of Hematite Reduction Reactions

AbstractThe thermodynamic data of iron oxides reduction reactions from the most recognized thermodynamic database (NIST‐JANAF) show remarkable difference between stoichiometric and nonstoichiometric iron oxides. Iron oxides reduction equilibria in both CO‐CO2 and H2‐H2O atmospheres are calculated with Dieckmann's and Weiss’ defect models. Relevant literatures are investigated and reduction experiments are carried out to complement and interpret the newly calculated equilibrium diagram. The results suggest a conjecture of two routes for hematite reduction reactions. With the ideal and widely accepted mechanism, hematite is reduced to magnetite and then to iron below 576 °C, while the reduction route follows the sequence of Fe2O3 → Fe3−δO4 → FexO → Fe above 576 °C. With the regular but always unrecognized mechanism, the reduction process of hematite experiences Fe3O4, FeO, and Fe step by step in all possible temperature above 156 °C. In the regular mechanism, sufficient scattered impurities occupy crystal interstices of magnetite and prevent the newly produced FeO unit cell from dissolving into the solid solution of magnetite, and then the FeO will accumulate above 156 °C. Actually the regular mechanism is hard to realize and usually confused with the ideal mechanism. The presence of the regular mechanism is proved by experimental phenomena of the drop of eutectoid temperature.

Reduction Process of Iron Catalyst Precursors for Ammonia Synthesis Doped with Lithium Oxide

Iron catalyst precursors promoted with oxides of calcium, aluminum, and lithium were prepared by a fusion method. Using XRD analysis, it was found that catalyst precursors had a magnetite structure. Lithium oxide, which is dependent on the Fe2+/Fe3+ molar ratio in a catalyst, was built into the magnetite structure as a solid solution and/or formed a separate Li2Fe3O4 phase. Lithium oxide forming the solid solution in magnetite accelerated the magnetite phase reduction. However, it was observed that magnetite, in the presence of lithium oxide, was not reduced to iron directly, but to a transient phase, Li2O·xFeO, where x < 3 was formed, which meant that the reduction to iron was much slower. Activity of the catalysts promoted with lithium oxide increased, while the degree of reduction increased.

Effect of TiO2‐content on Reduction of Iron Ore Agglomerates

The effect of titanium oxide on iron ore agglomerates is studied by the use of test sinter, test pellets and synthetic briquettes under laboratory conditions. Titanium favours secondary hematite rather than magnetite, which is the main phase in the sinter of Rautaruukki's Raahe plant. Additionally, the effects of sinter RDI and pellet LTD on the blast furnace process are evaluated using the test results of basket trials in LKAB's Experimental Blast Furnace. The effect of titanium in synthetic hematite is studied as hematite is reduced to magnetite in the RDI test. This occurrence causes deterioration in burden permeability. Synthetic titanium-bearing iron oxides under controlled conditions are investigated at the University of Oulu. The effect of TiO2, in solid solution in magnetite, on the magnetite to hematite oxidation is studied separately in order to simulate the final stage of the sintering process. In other experiments, hematite samples doped with various contents of TiO2 are studied using thermogravimetry under a controlled gas atmosphere (CO/CO2/H2/N2). The TiO2 content of hematite has a clear effect on reduction degradation. Also increasing content of TiO2 in solid solution in magnetite radically accelerates the oxidation rate. In the pilot tests, TiO2 content has a similar negative effect on the reduction strength of both sinter and pellets

STRUCTURE AND HIGH MAGNETIZATION OF RAPIDLY QUENCHED ZINC FERRITE

Rapidly quenched zinc iron oxides with nominal compositions of xZnO·(100−x)Fe 2O 3 where x=50, 55, 60, 65 and 70 (mol%) were prepared by using a twin-roller quenching method. X-ray diffraction analyses indicate that the most predominant phases precipitated in all the as-quenched specimens are zinc ferrites with spinel structure. The lattice parameters of rapidly quenched zinc ferrites are slightly smaller than that of normal spinel ZnFe 2O 4. The magnetization of as-quenched 60ZnO·40Fe 2O 3 is 23.8 emu g −1 at room temperature when the external magnetic field is 10 kOe. This value is much higher than the magnetization of ZnFe 2O 4 prepared by the solid state reaction. We suggest that some of the Fe 3+ ions occupy tetrahedral sites as well as octahedral sites. Some of Zn 2+ ions also occupy octahedral sites in the present rapidly quenched zinc ferrites. This leads to the high magnetization caused by strong superexchange interaction between octahedral and tetrahedral Fe 3+ ions, as proposed for ultrafine particles of ZnFe 2O 4 which show high magnetization even at room temperature. Zn K-edge extended X-ray absorption fine structure (EXAFS) measurements support that there exist six-coordinated Zn 2+ ions in the present rapidly quenched zinc ferrites. In addition, a solid solution of magnetite which is suggested by the lattice parameters of as-quenched specimens contributes to the high magnetization as well.

Pyrophanite-ilmenite solid solution in magnetite of the Beemerville nepheline syenite, Sussex County, New Jersey

Pyrophanite-ilmenite solid solution in magnetite of the Beemerville nepheline syenite, Sussex County, New Jersey

Mineralogical contraints on Curie isotherms in deep crustal magnetic anomalies

Ferrimagnetic minerals above their respective Curie temperatures are paramagnetic. Deep crustal magnetic anomalies are constrained, therefore, to a depth detection limit which is defined by the Curie isotherm. Although magnetite, with a Curie temperature of 580°C, is widely employed to map Curie isothermal surfaces it is a relatively rare mineral as stoichiometric Fe3O4. Solid solution of magnetite towards antiferromagnetic ulvöspinel (Fe2TiO4) reduces the Curie temperature to a minimum of −153°C and a high order of ferrimagnetic behavior can only be accomplished in plutonic rocks under the most exacting conditions of equilibrium exsolution and oxidation. Magnetic anomaly models need not be constrained by the 580°C limit because the range in temperatures may vary between 300°C if maghemite (αFe2O3)dominates and 680°C if hematite (γFe2O3) is dominant. Because many magnetic anomaly interpretations require Curie isothermal depths in excess of 580°C, and because these may extend to the lower crust‐upper mantle boundary, it is proposed that these anomalies may arise from partially serpentinized ultramafic bodies which have metal alloys as the prime magnetic source material with Curie temperatures in the range 620°‐1100°C.

Magnetic and optical properties of thin films in the system 1-xFe<inf>3</inf>O<inf>4</inf>·xFe<inf>8/3</inf>O<inf>4</inf>

The magnetic and optical properties of thin films prepared by chemical vapor deposition in the system (1-x)Fe 3 O 4 .xFe 8/3 O 4 are a strong function of x. In particular, the coercive field increases significantly with x. Considering the solid solution of magnetite and λ-Fe 2 O 3 as a defect vacancy form of magnetite, Fe3+[Fe3+ 1+2δ Fe2+ 1-3δ box$^b$ δ ]O 4 where δ = x/3, it is suggested that the increase in H c with x is due to a Fe2+- box$^b$ interaction producing a preferred alignment of the vacancy on an octahedral site, which in turn would produce an additional uniaxial anisotropy superimposed on the cubic anisotropy. This interpretation is supported by the observed variation of H c with time for compositions within the system. The optical absorption coefficient at 0.63 μm and the optical Faraday effect also are functions of the value of x. The decrease in α with x is associated with the decrease in Fe2+content, while the decrease in θ is associated with the near equality of the opposing contribution to the rotation of the Fe3+ions on the octahedral and tetrahedral sites, respectively. Thin films of compositions in the above system have been deposited on glass disks suitable for disk-file applications. Typical recording outputs are shown.

Omission solid solution in magnetite: Kenotetrahedral magnetite

Omission solid solution in magnetite: Kenotetrahedral magnetite

Omission solid solution in magnetite: Kenotetrahedral magnetite*

Magnetite and sulfur, when heated together for 1 to 30 days days in silica tubes, where vapor is always present, react to form pyrite and iron-deficient magnetite (Fe3–vO4, 0 < v ≤ 1/3) when S/Fe3O4 ≤ 2/3 (at. ratio). For 2/3 < S/Fe3O4 ≤ 7/4, FeSO4 occurs as an additional phase. In this magnetite a measurable fraction of the vacancies is found to occupy tetrahedral sites in the inverse-spinel structure, thus forming “kenotetrahedral magnetite”. Sharpness of powder lines, absence of superstructure reflections, and difference in cell dimension distinguish this magnetite from the previously known “keno-octahedral” type, in which vacancies occur exclusively on octahedral sites. Total number of vacancies and number of tetrahedral vacancies are given on a nomogram as functions of density and cell dimension. Kenotetrahedral magnetite does not take measurable amounts of sulfur in solid solution. It forms when stoichiometric Fe3O4 is heated in a flow of pure oxygen. Silica-tube experiments conducted over extended periods of time and collapsible-gold-tube experiments at 2 kbar show that “kenotetrahedral magnetite” is metastable. The reaction 3Fe3O4 + 2S → 4Fe2O3 + FeS2 proceeds through the formation of metastable Fe3–vO4 and apparently a stage involving a metastable type of hematite before stable hematite is produced.

Tabular formations in titanomagnetite

A visually homogeneous intrusive titanium magnetite is shown microscopically to consist of two phases: a) solid solution of magnetite in ulvospinel and b) solid solution of ulvospinel in magnetite; the phase ratio is about 1 : 2. On heating in air two new phases are produced in titanium magnetite, as oriented plates forming the mineral's characteristic reticulate microstructure: ilmenite and hematite. Ilmenite disappears on further heating and the end-phase becomes a solid solution of ilmenite in hematite. It is believed that natural oxidation in depths also yields to ilmenite plates in titanium magnetite, if ulvospinel is present. Oriented platelets of hematite may be similarly produced in TiO2 -free magnetite and in magnesium ferrite, although MgO retards their development in the latter. -- V. P. Sokoloff.