Applied Fundamental Research On Vanadium Extraction From The Vanadium-bearing Steel Slag By Alkaline Hydrothermal Oxidation Method

Vanadium-bearing steel slag(VBSS)is a typical bulk solid waste in the steel industry,and it could be used as a secondary vanadium extraction resource to achieve its high value-added utilization owing to the relatively high grade of V₂O₅.However,the high content of Ca and Fe,as well as the complex occurrence state of V,makes vanadium extraction from the VBSS very difficult.The current processes for recovering vanadium from the VBSS have some defects including high energy consumption,serious pollution,and difficulty in large scale application.In this thesis,the mineral structure and the vanadium occurrence state of the VBSS were deeply studied,and an efficient and clean alkaline hydrothermal oxidation method was proposed to extract the vanadium.The method could not only achieve the selective extraction of vanadium,but also generate no wastewater and aggressive gases,and the tailings was convenient to utilize due to its low sodium content.The thesis achieved the following innovative results:(1)The mineral phase and vanadium existence state of the VBSS were studied in detail,and it was found that 90.3%of the vanadium in the VBSS existed in the form of high valence(+4 and+5),while 9.7%was trivalent.High-valent vanadium was mainly in the tricalcium silicate phase,which could be regarded as Ca₆(Si O₅,PO₄,VO₄),and trivalent vanadium existed in the iron spinel(Fe O·V₂O₃).Thermodynamic studies have shown that it was feasible to recover vanadium from the VBSS by alkaline hydrothermal method.(2)The alkaline hydrothermal process of the VBSS was systematically studied,and the effects of key factors on the leaching results were investigated.The vanadium leaching rate could attain 68.4%under optimal conditions.The kinetic study showed that the alkaline hydrothermal process of the VBSS was controlled by the internal diffusion step and the interface reaction,while the former is the dominant factor.The apparent reaction activation energy of the hydrothermal process was 26.67 k J/mol.(3)According to the valence of vanadium in the VBSS,an alkaline hydrothermal oxidation process of the VBSS was proposed.The effects of different factors on the vanadium leaching rate were studied systematically and the optimal conditions were determined as reaction temperature of 200°C,Na OH concentration of 50 wt%,oxygen partial pressure of 0.2 MPa,stirring speed of 500 rpm and liquid-solid ratio 4:1,and the leaching rate of vanadium could reach 88.1%,which was 19.7%higher than that of alkaline hydrothermal process.The kinetic study indicated that the alkaline hydrothermal oxidation process was controlled by the internal diffusion step,and the apparent activation energy was 20.46 k J/mol.(4)The mechanisms of both alkaline hydrothermal processes were studied in detail,and the two methods were comprehensively compared.It was found that the alkaline hydrothermal oxidation method could recover the trivalent vanadium and improve the leaching rate significantly.In addition,the alkaline hydrothermal oxidation process had low production cost and could control the sodium content in the tailings by adjusting its mineralogical phase.It is an efficient and clean method for vanadium extraction from the VBSS and has broad application prospects.