Owing to the high Na/V molar ratio of Na 3 VO 4 solution, traditional precipitation of vanadium will consume more H 2 SO 4 and discharge ammonia–nitrogen wastewater with more Na 2 SO 4 . In present work, a new process was proposed, in which CO 2 was employed to carbonate Na 3 VO 4 solution with simultaneous recovery of NaHCO 3 , followed by NH 4 VO 3 precipitation and solution recycling. Carbonation kinetics of Na 3 VO 4 solution under both atmospheric and high pressures were studied. Under atmospheric pressure, the solution pH first decreased rapidly, slowly, rapidly again with carbonation time, and finally stabilized at pH 7.3; the formation of CO 3 2− and its conversion to HCO 3 − mainly occurred and the CO 2 absorption rates of different vanadium concentrations were almost the same (about 0.026 mol·m −3 ·s −1 ) at pH of 9.0‒9.6. Under high pressure, the reaction rates at early stage increased with CO 2 pressure and vanadium concentration, and stirring speed had the most significant effect on reaction rate with the highest CO 2 absorption rate of 1.73 mol·m −3 ·s −1 at 550 rpm. The average CO 2 utilization efficiency was ~ 90% with NaHCO 3 precipitation of 42.3% at 70 g·L −1 V under 1 MPa of CO 2 pressure, while the average CO 2 utilization efficiency was ~ 35% under atmospheric pressure. Finally, the reaction rate equation of carbonation under high-pressure was deduced. Graphical Abstract
The material accumulation during roasting in the practical production causes the hypoxia of the internal materials, leading to the relatively lower vanadium recovery than that in theoretical studies. This work aims to solve the problem of material hypoxia through the Na 2 O 2 -briquetting roasting route. The vanadium slag and Na 2 O 2 powders were mixed, briquetted, and roasted at various parameters. The generated phases and their distributions were analyzed using X-ray diffraction, Fourier transform infrared spectroscopy, and electron probe microanalysis. The analyzing results showed that Fe 2 O 3 , Fe 3 O 4 , Mg 0.165 Mn 0.835 O, CaTiO 3 , and Na 3 VO 4 were generated during roasting. The solid–solid reaction between V 2 O 5 and Na 2 O was described using the vacancy-mediated diffusion mechanism. Na + (or V 5+ ), in the Na 2 O (or V 2 O 5 ) layer, jumped and diffused through the Na + (or V 5+ ) vacancies (in the Na 3 VO 4 layer) to the Na 3 VO 4 /V 2 O 5 (or Na 3 VO 4 /Na 2 O) interface. Na + (or V 5+ ) reacted with V 2 O 5 (or Na 2 O) and O 2 decomposed from Na 2 O 2 or transported from the air to generate Na 3 VO 4 . The roasted mixture was leached in an aqueous solution to study the effects of the roasting variables on the vanadium leaching recovery. The vanadium leaching recovery was very sensitive to the roasting variables, it showed very complicated relationships with the Na/V molar ratio, temperature, time, and briquetting pressure. The maximum vanadium leaching recovery of 95.57 pct was achieved when the Na/V molar ratio, temperature, time, and pressure were optimized as 3/1, 850 °C, 2.5 hours, and 5 MPa, respectively. The leaching solution containing vanadium was processed via precipitation and calcination to gather vanadium. The obtained compound was identified as V 2 O 5 with a purity of 96.84 pct, based on the high consistency of diffraction peaks between the present work and that of the joint committee on powder diffraction standards.
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