| Vanadium is an important alloying element that is used in the steel industry, chemical industry, battery and drug domain due to its excellent properties. With the ceaseless production and application of vanadium, increasing attention is being paid to the exploitation of vanadium secondary resources and vanadium contamination. Vanadium is a multiple valence element and exists in the+2,+3,+4 and+5 oxidation states in the nature, in which the+4 and +5 oxidation states possess a higher toxicity. It is found that the toxicity of vanadium compounds is not only affected by their total concent, but also affected by their combination properties and combined forms. The combined states in aqueous solution are mainly dependent on pH value and potential, which can represent in the Pourbaix diagrams (potential-pH) expediently. Meanwhile, the Pourbaix diagrams for V-H2O system can be used to analyze the extraction of vanadium by hydrometallurgy, predict the vanadium anticorrosive property, guide the selection of ion exchange resin in the effluent treatment.On the basis of describing the development in researching vanadium and application of Pourbaix diagrams, the potential-pH diagrams for the vanadium-water and vanadium-sulphur-water systems were depicted and the thermodynamical properties was analyzed. Meanwhile, this work studied the dissolution kinetic of vanadium trioxide, which is one of the main states in the vanadium-bearing secondary resources.The thermodynamics of vanadium are calculated from the most recent critically assessed reviews, which publish standard Gibbs energies of formation for the various species and phases considered. The Pourbaix diagrams for vanadium systems at 25℃show that:1) Vanadium mainly exists as mononuclear ions at low concentrations in nature waters and polyvanadates replace the mononuclear oxyanions as the preponderant species in more concentrated solutions.2) Comparison of the potential-pH diagrams for V-S-H2O with simple V-H2O system displays that SO42- and HSO4- ions obviously enlarge the regions of stability of V3+, VO2+ and VO2+ ions, forming VSO4+, VOSO4(aq) and VO2SO4-. The predominance areas for vanadium sulfates (VSO4+, VOSO4(aq), VO2SO4-) increase with the decrease on the vanadium activities.3) The activity-pH diagrams for each of the stable vanadium oxidation states indicate that an increase on the activity of sulfuric acid from 0.1 to 1.0 results in an increase on the predominance areas for vanadium sulfates and decrease on the stability areas for solid phases, such as V2O3, V2O4 and V2O5, etc.The Pourbaix diagrams for the V-H2O system at elevated temperature indicated that:1) The Pourbaix diagrams for vanadium with the activities of 100,10-2, 10-4 and 10-6 show that the corrosion area enlarges over the whole pH range with decreasing the soluble vanadium activity. Vanadium pentoxide (V2O5) disappears lower than the activity of 10-3.17 and 10-3.51 at 25℃and 150℃, respectively.2) The Pourbaix diagrams for the V-H2O system at 25-150℃with the activity of 100 indicate that the stability regions of vanadium oxides extend with the elevation of the temperature and the vanadium stability region is independent of temperatures. Vanadium represents a good corrosion resistance with this activity at low temperature.3) The Corrosion-Immunity-Passivation diagrams (10-6 activity of all dissolved vanadium species) show that the vanadium will not be corroded at lower potential. The corrosion domain minishes slightly and the stability region of vanadium is hardly changed with rising the temperature. At this condition, the immunity/passivation performance of vanadium is infirm due to the dissolution of vanadium pentoxide (V2O5) at low activity.Simultaneously, the kinetics of dissolution of vanadium trioxide in sulphuric acid-oxygen media was presented. The various parameters considered in this work were stirring speed, concentration of sulphuric acid, temperature, partial pressures of oxygen and particle size. The results showed that the negative effect of boundary layers was eliminated when stirring speed exceeded 800 r/min. The dissolution rate of vanadium trioxide increased with temperature and partial pressures of oxygen, but decreased with increasing particle size. No effect of concentration of sulphuric acid was observed on the conversion. The dissolution kinetics was controlled by the chemical reaction at the surface with the estimated activation energy of 43.46 kJ/mol. |
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