Phase Structure,Phase Equilibria And Thermodynamic Properties In Iron-vanadium Oxides

Vanadium is an important strategic resource and widely used in various industries such as steel,nonferrous metals,chemical engineering and aerospace.It is called "modern industrial monosodium glutamate".There are abundant vanadium-titanium magnetite resources in China.The improvement of vanadium extraction process and development of vanadium products are very important for the efficient utilization of vanadium resources.Converter V-recovering process is restricted by the vanadium slag.Therefore,the study of phase equilibria and thermodynamic properties of iron-vanadium oxides system(main component of vanadium slag)is helpful for thorough understanding of thermodynamic behavior of iron-vanadium oxides in converter,and thereby it has theoretical significance in the improvement of slag-making and enhancement of vanadium slag grade.It is also a basis to construct the thermodynamic database of multi-component vanadium slag system.Moreover,iron and vanadium with various valency states can form rich ternary iron-vanadium oxides FexVyOz.In order to understand various phenomena and characteristics of materials from micro to macro scales,it is necessary to carry out theoretical research from the level of electronic structure.This is helpful for developing the iron-vanadium oxide materials with better performances.Combining the experimental determinations,calculation of phase diagrams and first-principle calculations,a comprehensive and intensive study has been conducted on phase structure,phase equilibria and thermodynamic properties in iron-vanadium oxides.The practical needed phase diagram and thermodynamic data for vanadium extraction process and material design of iron-vanadium oxides are available in this dissertation.Meanwhile,the mechanism of successive structural transitions in FeV2O4 spinel as well as band structures and phonon spectra of FeVO4 and Fe2V4O13 semiconductors are clarified.The main conclusions are as follows:(1)Phase diagrams of the FexO-V2O3 system from 1273 to 1808 K and in the range of oxygen partial pressure from 10-15 to 10-4 atm are determined by high-temperature quenching method.With the increasing oxygen partial pressure or decreasing temperature,solubilities of iron oxide in(V2O3)s.s.and iron-vanadium spinel raise and solubilities of vanadium oxide in(FeO)s.s.and iron-vanadium spinel reduce.Iron-vanadium spinel with high melting point and FexO form eutectic mixture with low melting point.This reaction provides the thermodynamic basis for dissolution of iron-vanadium spinel in vanadium slag.According to the relationship between oxygen partial pressure and solid solubilities of(FeO)s.s.,(V2O3)s.s.and iron-vanadium spinel,the corresponding structural models are proposed as(Fe2+,Fe3+,V3+)xO,(V2+,V3+,V4+,Fe3+)2O3 and(Fe2+,Fe3+,V3+)[Fe2+,Fe3+,V3+,?]2O4.(2)The thermodynamic properties of iron-vanadium oxides are studied by experimental and theoretical methods.High-temperature heat capacities of FeVO4,Fe2V4O13,FeV2O4 and FeV3O8 are determined by differential scanning calorimetry.The anharmonicity caused by lattice vibrations contributes more to the heat capacities of FeV2O4 and FeV3O8.The standard enthalpies of formation for iron-vanadium oxides are obtained by first-principle calculations,and the mean absolute error is 0.026 eV·atom-1.The standard Gibbs energies of reaction for FeV2O4 at 1623 K and 1773 K are derived based on the oxygen partial pressures of Fe+FeV2O4+V2O3 three-phase equilibrium:-183.51±0.47 kJ·mol-1 and-174.81 ± 0.51 kJ·mol-1.The activities of Fe3O4 and FeV2O4 components in iron-vanadium spinel solid solution are obtained according to the oxygen partial pressures of Fe+ spinel two-phase equilibrium and cation distribution in iron-vanadium spinel,respectively.(3)Based on the method of calculation of phase diagrams,the thermodynamic database of Fe-V-O system over the whole composition range is developed.A set of self-consistent model parameters is obtained to describe the Gibbs energy of each phase.The calculated phase diagrams and thermodynamic properties are consistent with the experimental data.The varieties of phase equilibrium for iron-vanadium oxides with composition,temperature and oxygen partial pressure are revealed.(4)The orbital orderings of Fe2+ and V3+ ions causing the successive structure transitions in FeV2O4 are explained by first-principle calculations.At Fe sites,orbital states transform from dz2 to dx2-z2,to dx2-y2 and to dy2-z2.At V sites,one electron occupies the dxy,dxz,dxy and dyz orbitals,respectively.The second electron occupies the dxz±dyz,dxy±dyz,dxz±dyz and dxy±dxz orbitals,respectively,which alternate in the corresponding ab,ac,ab and be planes along the c,b,c and a axes,respectively.Orbital ordering is driven by the combination of Jahn-Teller and electron correlation effects.The effect of spin-orbital coupling on electronic structure and orbital ordering of FeV2O4 is negligible.(5)The electronic properties of FeVO4 and Fe2V4O13 semiconductors are clarified by first-principle calculations.Band structures indicate they are indirect band gap semiconductors and they possess the ability of photocatalytic water splitting into oxygen.Lacking of negative frequencies in phonon dispersions indicates they are dynamically stable.High-frequency vibrations are only dominated by V and O atoms due to the shorter V-O bond length.All Raman and infrared frequencies at ? point are assigned unambiguously and vibration types in different frequency ranges are established based on the atomic displacements.