Study On The Evolution Law Of Vanadium During The Interface Reaction Process Of Vanadium Slag Sodium Roasting

As an important nonferrous metal and strategic resource,vanadium is widely used in metallurgy,chemical and material industries because of its chemical activity and particular assist on improving performance.The major raw source of vanadium worldwide is from vanadium titano-magnetite ore that has proven tremendous reserves at Pan-Zhihua mineral region in China.At present,vanadium is extracted mainly from vanadium titano-magnetite ore by vanadium hydrometallurgy technology,by enriching into solid slag and then leaching with water after oxidative roast with sodium slats,so called sodium roasting-water extraction technique.This technique has been widely used in industrial production,but the current researches to evolution law of the V-containing phase in V-slag during the roasting process focus on the macro level phase changes,not directly on V itself.Consequently,micro level research to state of V including valence,distribution and phase is becoming increasingly essential to truly understand and control the sodium roasting-water extraction process.The atomic evolution law can be of a great significance to optimize operating conditions in order to raise vanadium recovery rate.The micro structure,phase compositions,mineralogical morphology and elements distribution on the surface of vanadium slag during the roasting process were thoroughly investigated by XRD and SEM/EDS analyses at micrometer level.The results indicate that olivine?Mg,Fe?SiO₄ decomposed in the first place while spinel stays intact during the initial stage of roasting.When roasting for 10 min,vanadium spinel begun to decompose and be oxidized,V and Fe atom transferred to new phase,Mg₂VO₄ was formed when V³⁺was been oxidized to V⁴⁺.When roasting for 15 min,V⁵⁺and O^2-started to form vanadate,and restrained the reaction between silicate mineral and Na₂CO₃.Vanadium atoms continuously gathered from the inside to the interface of the slag to be oxidized and form vanadate.When roasting for 50 min,in sipte of V,C,O and Na,no more other elments can be detected on the surface of the V slag.When roasting for 70 min,new shape of vanadate was formed.Then the whole surface of vanadium slag was carpeted with vanadate in forms of Na₄V₂O₇?Na₃VO₄ and NaV₃O₈ after roasting for 90 min.The final product NaAlSiO₄ of olivine and Na₂CO₃,which has high melting point,can contribute to the vanadium extraction process.The element of Cr can be detected after roasting for 90 min,which means that the roasting process finished at 90 min and there is no reason to extend roasting time.XPS results expressed the changing of valence state of vanadium on the interface of the vanadium slag during the roasting process.V³⁺was oxidized to V⁴⁺after roasting for 5 min,and then oxidized to V⁵⁺at 15 min.Finally all V existed in state of V⁵⁺after roasting for 90 min.So,during the roasting process,the total content of vanadium on the interface was increased,while the total content of V³⁺in solid state was continually decreased and V⁵⁺was continually increased.As for V⁴⁺,climbed up and then declined.The dissolved state of vanadium was investigated by MALDI-TOF-MS,the results confirmed the existence of vanadate on the interface of the V-slag,primarily in the state of VO₄^3-?V₂O₇^4-and V₃O₈^-.At different stage during the roasting process,different forms of vanadate were formed,and they can transform at different conditions.When roasting for 15 min and 30 min V exists in forms of V₂O₇^4-,and change to VO₄^3-after roasting for 50 min and 70 min,while after roasting for 90 min,V coexists in forms of VO₄^3-and V₃O₈^-.Furthermore,to monitor the speciation of vanadium in solution,in this study,speciation and determination of vanadium in solution has been studied.UV-Vis,ESI-MS and MALDI-TOF-MS enabled the identification of the 1:1 stoichiometry of[VO?EDTA?]^2-and[VO₂?EDTA?]^3-and their mass spectra showed these vanadium complexes coexisted in the solution in forms of[VO?EDTA?-H]^-,[VO₂?EDTA?-2H]^-,[VO?EDTA?]^2-and[VO₂?EDTA?-H]^2-.In addition,a novel method is described,which utilizes pre-column complex formation-high performance liquid chromatography with photodiode array detector to separate vanadyl V⁴⁺and vanadate V⁵⁺ions.Finally,different forms of vanadium-EDTA were separated by anion exchange chromatography with an eluent containing 30 mmol/L NaH₂PO₄,20 mmol/L EDTA and 3%ACN at pH6.0 within 8 minutes.