Process And Mechanism Study On Selective Vanadium Extraction From Shale Based On Oxalic Acid Leaching

China is a major producer and consumer of vanadium?V?products,and nearly85-90%of the world's V products are used in the steel metallurgy industry.The strategic goal of"Made in China 2025"has an urgent need to improve the quality and efficiency of V industry.As the key material for high-end manufacturing,such as energy and special steel,the V demand continues to grow.V-bearing shale?VS?is a kind of superior V resource with enormous reserves in China,the V extraction technology and theoretical research of VS have been widely studied.V in VS mainly exists as V?III?in the form of isomorphism in the lattice of mica mineral,roasting and acid leaching method are usually adopted to realize high efficient V extraction.The selectivity for V of sulfuric acid leaching process is poor,the leachate usually contains multi-impurities,which have negative effect on V purification-enrichment and high-purity V product preparation processes.This study carried out a systematic study on the technology and mechanism of V extraction from VS under oxalic acid system,realizing source separation of V from Fe impurity.The main research contents and conclusions are as follows:?1?Process on selective V extraction from VS of oxalic acid systemThe direct acid leaching?DAL?process can obtain the technique datas as follows:V leaching efficiency is 71.5%,the Fe leaching efficiency is 3.4%,and the V/Fe separation coefficient is 0.0476.Under the same leaching conditions,the leaching efficiencies of V and Fe for sulfuric acid leaching system are respectively 74.1%and13.0%,and the V/Fe separation coefficient is 0.1754.Compared with sulfuric acid leaching system,oxalic acid leaching system has better V extraction selectivity.The source separation of V from Fe in raw VS can be realized during leaching process.The roasting-acid leaching?RAL?process can obtain the technique datas as follows:the leaching efficiencies of V and Fe are respectively 86.8%and 35.2%,and the Fe/V concentration ratio is 3.29.The Fe impurity in the leachate can be deeply removed using the reduction precipitation?RP?process,a 92.6%removal efficiency of Fe as FeC₂O₄·2H₂O precipitation with a 2.2%loss ratio of V and a 0.24 concentration ratio of Fe/V can be realized under the conditions:Fe/Fe???molar ratio of 1.25,reaction time of 150 min and reaction temperature of 25°C.The average particle size D50 is 5.41?m and the purity is 98.5%for FeC₂O₄·2H₂O.Therefore,the RAL-RP process realizes high efficiency V extraction and high value utilization of Fe.?2?Dissolution and migration mechanism of V in VS during oxalic acid leachingThe muscovite in the residues for raw VS and roasted VS are scattered with obvious corrosion holes.The characteristic diffraction peaks of muscovite disappear,and the Al-O and Si-O bonds in its crystal lattice shift and disappear,indicating that the structure of V-bearing mica is effectively destroyed and the V is released.The dissolution behaviors of muscovite and V oxides are analyzed theoretically by using Factsage thermodynamic calculation software.The results show that the formation reaction for muscovite as AlC₂O₄⁺,Al?C₂O₄?^2-and Al?C₂O₄?₃^3-is spontaneous,and the V oxides can spontaneously dissolve into VOC₂O₄ and VO?C₂O₄?₂^2-.The mechanism of dissolution and migration of V during oxalic acid leaching is revealed as follows:under the attack of H⁺and HF,the lattice structure of muscovite is destroyed and the V activity is enhanced,and the V forms stable VOC₂O₄ and VO?C₂O₄?₂^2-complex molecule and ion into the leachate.?3?Mechanism of Selective V extraction from VS using DAL processThe thermodynamic analysis shows that pyrite can form stable ferrous oxalate during oxalic acid leaching;the quantum chemical analysis shows that H₂C₂O₄,HC₂O₄^-and C₂O₄^2-can stably adsorb on the surface of pyrite?100?,and the stability of adsorption configuration from large to small is C₂O₄^2->HC₂O₄^->H₂C₂O₄.The stable adsorption configuration of C₂O₄^2-on the surface of pyrite?100?is characterized by two O atoms in C₂O₄^2-bonding with two Fe atoms on the surface of pyrite?100?respectively,forming a seven-membered ring and an eight-membered ring structure;the stable annular structure and Fe-O bond reduce the activity of Fe in pyrite.The SEM-EDS analysis shows that the pyrite surface in leaching residue is coated with ferrous oxalate precipitation.The result reveals the mechanism of selective V extraction from VS using DAL process in oxalic acid system as follows:H₂C₂O₄,HC₂O₄^-and C₂O₄^2-adsorb on pyrite?100?surface and bond with Fe,generating ferrous oxalate passive film coated on pyrite surface.The passive film inhibits the pyrite dissolution,realizing source separation of V from Fe impurity during leaching.?4?Mechanism of selective V extraction from VS using RAL-RP processIn the case of oxalate ion and fluorine ion complexation system,solid phase transformation and dissolution reaction for hematite co-exist.In the leaching process without oxalic acid,Fe₂O₃ and HF can react to form FeF₃ solid phase.However,in the leaching process with oxalic acid,no FeF₃ solid phase was detected.Thermodynamic analysis shows that the transformation of Fe₂O₃ to FeF₃,Fe₂O₃ to FeC₂O₄⁺,Fe?C₂O₄?₂^-,Fe?C₂O₄?₃^3-,and FeF₃ to Fe?C₂O₄?₂^-,and Fe?C₂O₄?₃^3-can occur spontaneously during leaching,indicating that the solid phase transformation reaction of Fe₂O₃?FeF₃ and the solid phase dissolution reaction of Fe₂O₃/FeF₃?FeC₂O₄⁺/Fe?C₂O₄?₂^-/Fe?C₂O₄?₃^3-exist in the acid leaching process.The solution chemistry research on the leachate shows that V,Al,Mg,Na,K,P and F are mainly in the form of complexing ions and molecules,in which Fe???is mainly in the form of Fe²⁺,FeC₂O₄ and FeH₂PO₄⁺,and Fe??? is mainly in the form of FeC₂O₄⁺,Fe?C₂O₄?₂^-and Fe?C₂O₄?₃^3-.The thermomechanical analysis shows that the mechanism of RP process is as follows:the iron powder can adequately reduce Fe???to Fe???,and iron powder reacts with H⁺forming Fe???.The Fe???ion concentration increased rapidly,accelerating the crystal nucleation rate,and FeC₂O₄·2H₂O precipitates gradually.The Fe in the leachate can be selectively recycled,realizing selective separation of V from Fe impurity.