Applied Fundamental Research On The Electrochemical Enhanced Extraction Of Vanadium And Chromium From The Vanadium Slag Using The Sub-molten Salt Method

Vanadium is one of the national strategic metals,and is widely used in iron&steel making,chemical industry,electronics and painting fields,due to its excellent mechanical,chemical,electrochemical,catalytic and other properties.Sodium salt roasting is the most representative vanadium extraction method.Although it is mature and is widely used,the roasting technology has obvious disadvantages in overall resource&energy efficiency and environmental pollution.These problems were effectively solved by using the sub-molten salt methods,in which the vanadium slag was treated in highly concentrated alkaline solutions,under oxidative conditions using air,oxygen or nitrate as the oxidant.Furthermore,vanadium and chromium could be extracted synchronously in the sub-molten salt methods,and due to the medium circulation and integrated treatment techniques,almost no waste would be discharged in the whole process.While new problems were found that huge energy need to be consumed for evaporating and concentrating the recycle alkaline solutions,also the investment on the anticorrosion of reactor and other equipment should not be ignored,due to the high corrosive reaction medium adopted.In order to solve the above mentioned problems,a new method was proposed for electrochemical oxidation of vanadium slag in NaOH solutions under much more moderate conditions,and the thermodynamic feasibility,micro and macroscopic mechanisms of the electrochemical reactions,the parameters determination of the electrolysis,process simulation,clean production index evaluation were systematically investigated,and the main process is as follows:(1)In this paper,a new electrolysis method was proposed to extract V and Cr simultaneously from the vanadium slag in NaOH sub-molten salt medium at a relative low temperature.The thermodynamics of all the possible reactions were calculated,and it was found out that the low-valence metal oxides,such as FeO,V2O3,and Cr2O3,could be oxidized to vanadate,chromate,ferrite or iron oxide via directly electrochemical oxidation or chemical oxidation caused by the dissolved oxygen or the reactive oxygen species(ROS)in the medium.(2)The mechanisms of the electrochemical oxidation of vanadium slag were investigated.The results show that when the potential is lower than the oxygen evolution potential,the low-valence metal oxides can be oxidized directly on the surface of the anode.With the increase in the potential,the hydroxide ions near the anode would lose electrons and be oxidized to stable ROS or micron-sized oxygen bubbles,and could oxidize the above mentioned low-valence metal oxides effectively.The ratios of the directly electrochemical oxidation and the indirectly electrochemical oxidation are 1:3 for V and 3:7 for Cr.(3)The key factors influencing the electro-extraction of V and Cr from the vanadium slag were determined by using the orthogonal as well as the single-factor experiments,and the optimal electrolysis conditions were determined to be:temperature of 120 ℃,NaOH concentration of 40 wt.%,liquid-to-ore mass ratio of 10:1,stirring speed of 750 rpm,anodic current density of 750 A/m2,mineral particle size less than 200 mesh.After reacting for 6 h,the yields of V and Cr could reach 93.71 and 87.52%,respectively.On this basis,a hybrid electrolysis method was proposed by combining microporous gas oxidation and electrolysis with low electrochemical filed,to solve the high cell voltage and high electric consumption problems found in the electrolysis method.Using the newly optimized method,after reacting for 4 h,the yields of V and Cr could reach 92.44 and 85.92%,respectively.(4)The phases and morphologies of the reacted residues were traced,and it was found that the electrochemical oxidation of the vanadium slag was greatly controlled by its structure.At the initial stage,the silica coating would be decomposed by reacting with NaOH medium,and a small amount of spinels was oxidized under the control of interfacial chemical reaction rate;as reactions go on,a porous solid product layer was gradually formed by the adhered iron oxide and some other Ca/Si/Al components,and the diffusivity of the reaction component would deteriorate drastically,and thereby the rate-determine step of the whole extraction process would shift from interfacial chemical reaction rate to a joint control of interfacial chemical reaction rate and the diffusion rate in the product layer.(5)A technological process for the electrolysis of vanadium slag in NaOH sub-molten salt medium was proposed.And the material&energy balance was examined using the METSim and HSC Chemistry softwares.The calculation results demonstrate that the newly developed method can achieve the same or even better yields with much less consumption of raw materials as well as energy.After a comprehensive assessment,the electrolysis of vanadium slag in NaOH sub-molten salt medium is in advanced level with a clean production index of 96.49.