Basic Research On Direct Preparation Of Metals From Refractory Metal Oxysalts By Molten Salt Electrolysis

Refractory metals are basically produced by thermal reduction method,which is criticized for low efficiency,high pollution and long process.Molten salt electrolysis is the most promising technique to replace the thermal reduction method.However,for molten salt electrolysis,metal chlorides and oxides are usually chosen as the raw materials.Preparation of metal chlorides is a multi-step process with high pollution and strong corrosive chemicals evolution.Also,metal oxides are usually hardly soluble in molten salts,leading to unfavorable reduction kinetics.Selecting more favorable raw materials and developing green and high-efficienct molten salt electrolysis process are the ugent need for the refractory metals metallurgy.As ionic compounds,refractory metal oxysalts are the direct products during the mineral processing.Compareded with metal chlorides and oxides,metal oxysalts are easier accessible.In addition,many metal oxysalts can be dissolved in molten salts.By direct converting the refractory metal oxysalts to metals by molten salt electrolysis,the traditional long process can be largely shortened and many high-pullotion steps can be avoided,potentially providing a green and high-efficient process.Herein,feasibility was verified for direct converting typical refractory metal(V/Cr/Ti)oxyalts to metals by molten salt electrolysis.In addition,the reduction mechanism was studied in detail.Also,competing reduction reactions between CrO42-and CO32-was discussed at large during CrO42-electrolysis.Importantly,reasons leading to low current efficiency were investigated quantificationally.Finally,electrolysis was intensified by electrode structure optimization and selecting liquid Zn metal cathode.The original results are as following:(1)Feasibility was verified for direct converting typical refractory metal oxysalts to metals by molten salt electrolysis.Vanadium and chromium oxysalts are easily dissolved into the chloride molten salts while CaTiO3 is soluble in cryolite molten salts.In addition,the theoretical decomposition voltages for typical refractory metal oxysalts are all much lower than that of chlorides and flurides.Therefore,chlorides and cryolite molten salts are suitable molten electrolytes for vanadium/chromium oxysalts electrolysis and CaTiO3 electrolysis,respectively.Thermodynamic calculations indicated that the typical refractory metal oxysalts can be electro-reduced to metals by multi-steps.Soluble CaTiO3 can be also electrochemical reduced to Ti by the reduction process of TiO32-?TiO?Ti2O?Ti.Due to a large vapor pressure difference between Ti and Na3AlF6,the Na3AlF6 inclusions in the metal Ti was removed by evaporation at 1550 ?.(2)Reduction mechanism and controllable electrochemical conversion of NaVO3/Na2CrO4 was studied.In pure NaCl molten salt,NaVO3 and Na2CrO4 can be only electro-reduced to insoluble V2O3/Cr2O3.By introducing CaCl2,V2O3/Cr2O3 was converted to soluble CaV2O4/CaCr2O4,promoting the transfer of V3+/Cr3+ to the cathode surface.In addition,CaO was generated and electro-reduced to Ca.Resultantly,CaV2O4/CaCr2O4 was reduced to V/Cr by calcium-thermal reduction.In conclusion,NaVO3/Na2CrO4 was converted to metals by the synergistic effect of electrochemical and thermal reduction.Therefore,by adjusting the properties of intermediate products and the reduction ability of electrode surface,NaVO3/Na2CrO4 can be controllably electro-reduced to V2O3/Cr2O3 or metal V/Cr.(3)Reasons for carbon pollutions and low current efficiency were discussed during K2CrO4 electrolysis,and the electrolysis process was intensified by electrode structure optimization.CO32-formation was accelerated due to the O2-release during K2CrO4 electrolysis.Resultantly,competing reactions between CO32-and Cr42-reductions took place,leading to the high carbon contents in the obtained metal chromium.On the other hand,short-circuit current between anode and cathode was generated by a carbon layer on the molten salt surface,leading to a low current efficiency.By protecting the graphite anode with a porous MgO tube,the carbon layer was ellimilated and the current efficiency was increased to 88.4%.Correspondingly,the carbon content in the cathode product was decreased to 0.57%.(4)Depolarization and de-carbonization for K2CrO4 electrolysis was verified when the liquid Zn metal electrode was used.CrO42-reduction was promoted by formation of Zn-Cr alloy.In addition,carbon pollution was avoided due to an extremely low solubility of carbon in liquid Zn metal.More importantly,pure chromium metal can be crystallized from the Zn-Cr alloy and can be separated by super gravity.