Investigate On Crystallization Kinetics Of Spinels And Selective Separating Vanadium And Phosphorus In Vanadium Slag With High CaO And P₂O₅ Content

Due to the excellent physical properties such as high tensile strength,hardness and fatigue resistance of vanadium alloys,vanadium plays an extraordinarily important role in steelmaking,chemical industry and aviation fields.At present,vanadium-containing hot metal oxidized in converter to enrich vanadium is the main vanadium extraction process.However,there are many issues in this process such as a heavy dephosphorization burden,difficult to produce low or ultra-low phosphorus steel.In order to solve these issues,CaO is added into the converter during the production of vanadium slag,then vanadium slag with high Ca O and P₂O₅ content is obtained.In this paper,the phase compositions of vanadium slag with high CaO content,the crystallization kinetics of spinels in the slags and the method for extracting vanadium selectively are investigated.It is of significance to achieve the goal of dephosphorization and vanadium extraction simultaneously in the converter.The phase compositions,mineralogical morphology and elements distribution in the vanadium slag with high CaO content were investigated by thermodynamic,XRD and SEM/EDS analyses.The results indicate that with the increase of CaO content in the slag,vanadium is always concentrated in the spinel phase.However,part of titanium is concentrated and formed into CaTiO3 from Fe₂TiO₄.Moreover,phosphorus is concentrated in C₃P and formed into nC₂S-C₃P solid solution.The effects of basicity and P₂O₅ content on the crystallization behavior of spinels in vanadium slag were investigated by theoretical modeling.The non-isothermal crystallization kinetics of spinels in the slag during cooling process from 1723 K to 1523 K were described by high temperature quenching-image analysis,and the statistical analyses based on CSD theory.The results indicate that the high basicity and low P₂O₅ content are beneficial for the growth of spinels.The growth mechanism is revealed to be controlled by interface reactions and diffusion at the cooling rates of 5 and 15 K/min,respectively,while the temperature decrease from 1673 to 1523.However,at higher temperatures?> 1673 K?,the growth of spinel crystals are both controlled by nucleation.Experimental results combining with theoretical reveal that low cooling rate benefits spinels growth,especially for the interval of 1200-1523 K.In order to achieve the selective separation of V and P in vanadium slag with high CaO and P₂O₅ content,and obtain high quality vanadium products.Two effective roasting-leaching processes were studied,i.e.sodium roasting-water leaching and dephosphorization-sodium roasting-water leaching,and the mechanisms of selective separation were discussed.The results indicate that the leaching rates of V and P in the vanadium slag with high CaO content are 90.82% and 5.64%,respectively,via sodium roasting-water leaching under the conditions of V/Na molar ratio of 0.2 and roasting temperature of 800?.The pretreatment of vanadium slag with high CaO and P₂O₅ content was carried out with weak acid buffer.The rate of dephosphorization is 86.37%.After pretreatment,vanadium slag was used to extract vanadium by sodium roasting-water leaching process.The leaching rate of V is 92.93%,and compared with the unpretreated vanadium slag,the V/P?mass ratio?was increased from 134.51 to 203.70.Under the experimental conditions,Na₂Ca?VO₃?₄,NaCaVO₄,NaV₃O₈ and NaVO₃ four water-solube vanadium-containing phases can be formed during sodium roasting process,and there is no insoluble calcium vanadate in roasted slag.Therefore,the two processes can achieve the purpose of V and P separation,vanadium slag dephosphorization pretreatment-sodium roasting-water leaching process can obtain higher purity of leaching solution,and then to produce the high grade vanadium products.