Applied Fundamental Research On The Novel Process Of Vanadium Extraction From Vanadium Slag Via Hydrothermal Leaching With Ammonium Salt

As an important rare metal, vanadium has excellent performance and wide applications.China has abundant reserves of vanadium and bacomes one of the largest producers and consumers of vanadium products in the world. Vanadium slag, which is obtained through the iron-making process of vanadium titano-magnetite, is the main raw materials for manufacturing vanadium products including vanadium oxide. In order to resolve the problems of long production routes from vanadium slag to vanadium oxide, large amount of waste water to be treated, large amount of cheap sodium sulfate by-products to be discharged, and environmental threats from the final residues, the hydrothermal leaching of vanadium slag with high chromium contents by ammonium salts was employed in this work. The process operation parameters for selective oxidation and hydrothermal leaching of vanadium slag were optimized, the macro kinetics of the hydrothermal leaching process were investigated, and the phase diagrams of the NH4HCO3-MH4VO3-H2 O system were measured for optimizing the cool crystallization process of NH4VO3 intermediate product.In this thesis, the following innovative progresses were achieved:1) The selective oxidation process of vanadium slag was investigated by XRD, SEM,EDS and ICP-OES. It was shown that: the vanadium slag was consisted of olivine phases and spinel phases, and the low calence vanadium can be mostly oxidized under the roasting temperature of 900℃, the roasting time of 1 h, and the thickness of vanadium slag under 4 mm.2) A novel process was developed for the hydrothermal leaching of vanadium from vanadium slag using NH4HCO3, and the effects of stirring speed, reaction temperature,concentration and particle size were studied. The kinetic analysis results show that the leaching process of vanadium slag in NH4HCO3 solution can be well interpreted by the shrinking core model under external diffusion control. With the increasing of liquid-solid ratio, stirring speed, reaction temperature, and solution concentration and the decreasing of the particle size of the vanadium slag, the leaching rate of vanadium was increased.Finally, the apparent activation energy for the hydrothermal leaching process of vanadium slag was calculated as 18.0 kJ/mol, and the pre-exponential factor was calculated as 0.713min-1.3) Based on the studies of reaction kinetics, single factor experiments were applied tooptimize the reaction process. The optimal process parameters are found as ammonium bicarbonate as the leaching agent, the liquid-solid ratio of 5, the reaction temperature of150 ℃, the NH4HCO3 solution concentration of 500 g/L, the reaction time of 3 h, the stirring speed of 400 r/min, and the paticle size of 50 μm. Leaching rate of vanadium from vanadium slag can be achieved as high as 93.39%.4) The phase equilibria data for the NH4HCO4-NH4VO3-H2 O ternary system at 40℃and 50℃ were measured. And the phase diagrams were constructed. Furthermore, the different crystallization areas in the phase diagrams are discussed in detail. Due to the small solubility of NH4VO3, the crystallization area of NH4VO3 is very small, but comparatively, higher temperature can increase the crystallization area of NH4VO3,therefore, the cooling crystallization process should be controlled at a relatively high final temperature.