Oxidation Mechanism And Parameter Optimization Of Electrochemical Synthesis Of Sodium Chromate From Ferrochrome In Sodium Hydroxide Solution

As the basic intermediate material of chromium salt,sodium chromate is an important chemical material in the development of the national economy.Due to the serious pollution problems in traditional chromium salt production process,Qinghai Salt Lakes Institute of Chinese Academy of Sciences had developed a novel technology for the preparation of sodium chromate by electrochemical method in2013.Although this technology performs well at soft reaction conditions,controllable process,environmentally friendly process,etc.,the electrochemical oxidation of the main elements in ferrochrome is still unclear at present.Therefore,the electrochemical oxidation of metal chromium,metal iron and ferrochrome in Na OH aqueous solution were studied by cyclic voltammetry?CV?,linear polarization sweep voltammetry?LSV?,scanning electron microscope?SEM?,energy disperse spectroscopy?EDS?,X-ray diffraction?XRD?,X-ray photoelectron spectroscopy?XPS?,UV-visible spectroscopy?UV-Vis?and X-ray fluorescence?XRF?.On this basis,the optimization of process parameters and changes of working voltage were studied by single factor experimental system with the current efficiency of synthesis Na₂Cr O₄ as the reference index.The results are as follows:?1?The electrochemical oxidation of metal chromium electrode as working electrode in Na OH aqueous solution was studied.Metal chromium electrode easily underwent electrochemical oxidation to form Cr?OH?₃ and Na₂Cr O₄.At the same time,oxygen was deposited on the electrode surface.Metal chromium electrode had two electrochemical oxidation ways to form Na₂Cr O₄:firstly,Cr?0??Cr?VI?;next,Cr?0??Cr????Cr?VI?.The anodic polarization of metal chromium electrode has included activation.?2?The electrochemical oxidation of metal iron electrode as working electrode in Na OH aqueous solution was studied.Metal iron electrode was easily passivated to successively form unstable Fe₃O₄ film and stable Fe₂O₃ film.At this time,oxygen was deposited on the electrode surface.As the concentration of Na OH increased,Fe₂O₃film has been increasingly unstable due to pitting corrosion at more positive potential.?3?According to the results of the electrochemical oxidation of metal chromium electrode and metal iron electrode in Na OH aqueous solution,the electrochemical oxidation of ferrochrome electrode as working electrode in Na OH aqueous solution was studied.Ferrochrome electrode underwent electrochemical oxidation to form sodium chromate by Cr?0??Cr?VI?.As the concentration of Na OH increased,passive films were formed on the surface of the ferrochrome electrode,and the electrode was temporarily in the passivation state at negative potential due to the effect of iron element;passive films broke down,and the ferrochrome electrode underwent electrochemical oxidation again to form Na₂Cr O₄,Fe?OH?₃ and O₂ at more positive potential.The reaction conditions were found to be favorable for the formation of Na₂Cr O₄ through electrochemical tests:electrolyte temperature 65°C,Na OH concentration 1?10 mol·L^-1,anodic potential 0.6?2.0 V?vs.SCE?.?4?The effects of the initial Na OH concentration,current strength,electrolyte temperature and electrolyte circulation flow rate in the preparation of sodium chromate by ferrochrome electrochemical method were investigated by single factor experimental system.The optimized process conditions were as follows:initial Na OH concentration 8 mol·L^-1,current strengh 2.0 A,electrolyte temperature 65°C and electrolyte circulation flow rate 24 L·h^-1.Under these conditions,the average working voltage was 1.67 V and current efficiency of synthesis sodium chromate was 54.80%.These research results provide basis for understanding the electrochemical oxidation of ferrochrome in Na OH aqueous solution.Its help to optimize the process control index of the new technology for the preparation of sodium chromate by ferrochrome electrochemical method,and make a positive contribution to the promotion and application of the new process.