Electrochemical Study On Iron (Nickel) Ions In Molten Slag

Electrolyzing molten oxide containing iron oxide to produce iron by using inert electrode is a potential short steelmaking process which can reduce or even eliminate the CO₂ emissions.Nickel,as an important alloying element in steel,can enhance the corrosion resistance ability of steel.In electrolysis process of the slag containing nickel and iron oxide,nickel contained steel?including stainless steel?can be obtained by further electro-deposition of nickel and iron.The technological parameters of electrolytic smelting for nickel-containing steel are closely related to the electrochemical behavior of nickel and iron ions in slag and the variations of electrochemical reaction parameters.At present,there are few reports on the electrochemical studies of slag with nickel ions or iron ions,but there has no report for the slag containing both of them.Also the measurements on the electrochemical parameters of iron ions in molten slag are few.The lack of high-temperature electrochemical studies on the slag is main due to poor stability of the cell container and the electrode?especially the reference electrode?.ZrO₂-based solid electrolyte has a strong slag resistance,high oxygen ion conductivity and low electrionic conductivity,so it has been widely used in metallurgical production and scientific research.In this research,a novel oxygen flow controlled electrolytic cell with Pt,O₂?air?|ZrO₂ as reference electrode was constructed integrally by magnesia partially stabilized zirconia solid electrolyte tube.Electrochemical behavior of Ni²⁺and Fe²⁺in high-temperature molten slag was studied by the combination of thermodynamic theory and some electrochemical techniques such as cyclic voltammetry?CV?,square wave voltammetry?SWV?,chronopotentiometry?CP?,chronoamperometry?CA?,potentiostatic electrolysis?PE?,Electrochemical Impedance Spectroscopy?EIS?and Tafel.The electrochemical parameters such as diffusion coefficient,exchange current density,electrode reaction rate constant were also been measured.At first,electrochemical behavior of coexisting Ni²⁺and Fe²⁺in SiO₂-CaO-MgO-Al₂O₃ molten slag was studied on iridium electrode at 1723 K by means of electrochemical techniques such as CV,SWV and PE.The results show that there is a weak interaction between FeO and NiO in the slag system,the nickel ions are in the presence of Ni²⁺and the iron ions are in the presence of Fe²⁺in the slag;When doing SWV analysis on the slag,the peak currents of coexisting electroactive ions(Ni²⁺,Fe²⁺)present different characteristics,and their peak currents increases with the increasing of frequency,but the peak potential does not change with the frequency in each suitable frequency range.The electrochemical reduction of Ni²⁺?Ni or Fe²⁺?Fe on the iridium electrode in the molten slag is a single two-electron transfer process,and the rate of the process is diffusion controlled,but the reduction of Fe²⁺is obviously affected by the reduction of Ni²⁺.The diffusion activation energy of Ni²⁺ions is 517.1±78.5 kJ·mol^-1 which obtained from the diffusion coefficients at different temperatures:1623 K,1673 K and 1723 K.The electrochemical behavior of Fe²⁺on Fe electrode at different concentrations of FeO and temperatures in SiO₂-CaO-MgO-Al₂O₃-FeO molten slag was investigated by EIS and Tafel.EIS was used to calculate the electrochemical parameters of charge transfer resistance,Warburg resistance,electric double layer capacitance of working electrode and diffusion coefficient of Fe²⁺in the molten slag.It is found that the electron transfer reaction is very fast and the transfer plays a leading role.With the increase of FeO concentration,the diffusion coefficient decreases.As the temperature increases,the diffusion coefficient increases.The diffusion activation energy of Fe²⁺ions is 204.3±16.0 kJ·mol^-1 which obtained from the diffusion coefficients at different temperatures.In addition,the exchange current density and charge transfer coefficient at different concentrations and temperatures are calculated by the Tafel curve,and the electrode reaction rate constant is 68.8±7.5 kJ·mol^-1 which obtained by further calculation.With the increases of FeO concentration and the temperature,the exchange current density increases,which proves that the reversibility of the reaction on Fe electrode can be improved.