| Converter vanadium slag is the most important vanadium resource for vanadium pentoxide production. The traditional process for vanadium extraction from converter slag is sodium roasting followed by water leaching. However this process is falling into disuse due to the serious environmental pollution and low vanadium recovery. Some new processes for vanadium extraction from vanadium slag, like sub-molten salt extracting vanadium, direct acid leaching and calcium roasting, which obtain more satisfactory vanadium recovery than sodium roasting, are not mature and still under developed. On the basis of the previous research, a big breakthrough is obtained in purification of acidic vanadium solution with chemical precipitation separation method and then a whole cleaner process for vanadium pentoxide preparation from converter vanadium slag is developed in this paper. This process is comprised of six units, calcium roasting, acid leaching, purification of vanadium bearing solution, vanadium precipitation with ammonium salt, manganese recovery and, recycle of mother liquor and reduction of tailings. Each of these units is systematically studied.The mineralogy of raw vanadium slag is studied to determine the chemical composition, the phases composition, the occurrences of V, Fe, Ca, Si, Mn and other major elements, and the mineral grain size. The influence of vanadium slag structure on the vanadium extraction is explored, which provides theoretical basis for the roasting and leaching experiments.Reactions occurring in the process of vanadium slag roasting and their reaction limits are evaluated through thermodynamic analysis. The relationships between roasting conditions and vanadium leaching rate are discovered, and the influences of roasting conditions on the changes in mineral phases and vanadium occurrence are investigated. The results show that oxidation of fayalite happens at temperature ranging from 400 to 550 ?, and the vanadium spinel starts to be oxidized at temperature higher than 600 ?. The oxidation and calcification reactions of vanadium proceed simultaneously at 600-850 ?. The starting temperature for heating which is lower than 500? has little effect on the leaching rate of vanadium while the heating rate has great effect on the vanadium leaching rate. Lowering the heating rate from 4 to 2 ?min-1 changes the optimum roasting temperature from 800 ? to 850 ? with an increase of about 7% in vanadium leaching rate. Increasing the amount of CaO can promote manganese vanadate, ferric vanadate, and magnesium vanadate to transform into calcium vanadate, and consequently vanadium leaching rate is increased due to the relatively higher solubility of Ca2V2O7. The vanadium leaching rate is up to 93.31% under the optimum conditions.TG-DSC is used to study the isothermal and non-isothermal oxidation kinetics of vanadium slag. Isothermal oxidation kinetics tests show that oxidation of vanadium slag is controlled by the interfacial reaction in the temperature range of 400-500 ?, while the oxidation process is handled by gas diffusion through the solid product layer within 650-850?. Nonisothermal oxidation process of vanadium spinel in 657-914? and 691-954? at heating rates of 5?·min-1 and 10? min-1 respectively, are controlled by an third order chemical reaction while lowering the heating rate to 3 ? min-1 the oxidations of vanadium spinel and augite overlap within 608-959? and the oxidation controlling step is a 3/2 order chemical reaction. When particle sizes of vanadium slag are+250 ?m and 120-250 ?m, the oxidation process is controlled by gas diffusion while the controlling step is changed to chemical reaction as the particle size is decreased.Dilute sulfuric acid leaching is employed to extract vanadium from the roasted slag. The thermodynamic analyses for reactions in the leaching process are conducted, and the E-pH diagrams for V-H2O Fe-H2O, Al-H2O, Si-H2O and Ca, Mg, Mn-H20 systems at 25,55 and 100? are drawn, respectively. Thermodynamic analyses show that apart from Fe2O3, Fe2TiO5 and CaCrO4, other minerals contained in the roasted slag are readily to react with sulfuric acid thermodynamically in the standard state. Therefore, Ca, Mn, Mg easily transfer into leaching solution, while concentrations of Fe, Al, Si in the leaching solution are closely related to the aqueous acidity. The vanadium leaching rate reaches over 91% under the optimized conditions, meanwhile, the leaching rates of impurities such as Ca, Mn, Mg, Al, Si, P reach 10% to 30%.Since the weak selectivity of acid, concentrations of Ca, Mn, Mg, Al and Si are higher in vanadium bearing solution, which in turn affect the vanadium precipitated rate and the quality of final product. According to the analyses for occurrence of impurity ions and vanadium in leaching solution, neutralization separation is applied to remove Si and Al. The removal of Al is higher than 99% and that of Si is around 55% when the solution pH value is adjusted to 4.5, but when the pH value is greater than 5.0, loss of vanadium increases dramatically. A composite agent of MnNH3F3 is adopted to remove Ca, Mg and Al and Si are precipitated simultaneously through adjusting the solution pH in the process of removing Ca and Mg. Eventually, removals of Ca, Mg, Al are all higher than 95% and loss of V is less than 5%. Adding polyacrylamide flocculant can accelerate the settling of impurity precipitations and decrease the adsorption of V on the impurity precipitations. Diatomaceous earth can improve the filtration performance of the slurry in purification process. The experimental results for removing Mn2+ from acidic solution with ammonium persulfate show that using oxidation precipitation method to remove Mn2+ from vanadium leaching solution is not feasible because the strong adsorbability of the resultant of new eco-hydrous manganese dioxide results in a large loss of vanadium.Mn in solution is not removed before vanadium precipitation step and ammonium salt is added to precipitate vanadium since this method can alleviate the impact of Mn on the quality of final product compared with the hydrolysis precipitation. The effects of operational parameters in ammonium salt precipitating vanadium process on vanadium precipitated and the quality of vanadium pentoxide are studied. The research results indicate that lowering the aqueous pH leads to a higher content of Si in product and improving adding amount of ammonium salt reduces the Mn content in vanadium pentoxide. Vanadium precipitated is over 96%and the obtained product meets the national standard of 98 grade vanadium pentoxide on the basis of the optimal operational parameters obtained from the conditional experiment. The vanadium precipitation process can be better described by Avrami crystallization kinetic equation and the corresponding apparent activation energy Ea is 93.233 kJ·mol-1 and the pre-exponential factor A is 9.14×1011 min-1. Vanadium precipitation and vanadium pentoxide are characterized by XRD, SEM-EDS, TG and laser particle size analyzer. The precipitated product is hydrated hexamer of ammonium vanadate (NH4)2V6O16·1.5H2O existing in spherical and cylindrical structure with average particle diameter of 43.9 um. The powder vanadium pentoxide is obtained after calcination of ammonium poly-vanadate. It is present in cylindrical shape with average grain size of 1.25?m and specific surface area of 6.95m2·g-1. The main impurity in the product is manganese in occurrence of metavanadate(MnV2O6).The concentration of Mn2+ in the mother liquor after vanadium precipitation is high, so oxalic acid is used to recover Mn2+ from the acidic solution with recovery of 94.33% and the resultant is MnC2O4 with purity higher than 98.5%. Therefore, the residual Mn2+ in mother liquor is 557 mg·L-1. After recovering manganese the mother liquor is returned to acid leaching step to extract vanadium. The results show that the recycle numbers of mother liquor have no effect on the vanadium leaching rate, while impurities of P, Si, Cr and Ti significantly impact the quality of final product when the mother liquor is recycled for three times. Powder CaO can reduce the concentrations of these ions effectively and make the solution continue to be recycled.In the whole process from roasting to obtaining V2O5, the total vanadium recovery is more than 83% and the tailings of vanadium extraction perform good reduction property since little Na is contained. |
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