The Electrochemical Behaviours Of Nd Ions In Chloride Melts

Neodymium is a crucial rare earth element and its alloys have been wildly used in many industries.Among many neodymium alloys production processes,molten salt electrolysis is the research focal point.In this work,the transforming relationship between different electrodes reference systems and standard chloride electrode reference system was obtained in chloride melts.Based on standard chlorine electrode reference system,the reduction potentials and mechanisms of Nd³⁺on inert electrode（W）,active electrode（Cu）and liquid electrode（Sn）were investigated by different electrochemical methods.The standard formation Gibbs free energies of the intermetallic compounds were also determined.Besides,the feasibility of selective separation of Ndfrom Cu₆Ndalloy by molten salt electrolysis was explored.The main conclusions are shown as blew:（1）.In the chloride melt,the conversion relationship of the potentials between different electrodes reference systems(E_Nn+_/N)and standard chlorine electrode（SCl E）reference system(φ_Nn+/N)was obtained by direct and indirect methods.The direct method utilized the potential of SCl E verse different reference electrode(E_Cl2/Cl–)and the conversion relationship was determined:φ_Nn+/N=E_Nn+_/N-E_Cl2/Cl–.The equilibrium potential of metallic ions verse SCl E(E_Mm+_/M)and the decomposition voltage of the chloride melt(φ_Mm+/M)was used in the indirect method and the conversion relationship was obtained:φ_Nn+/N=E_Nn+_/N+φ_Mm+/M–E_Mm+_/M.In the LiCl-KCl melt at 400 ℃,the equilibrium potential of Li⁺/Liwas determined by steady polarization curves and potential errors of Li⁺/Liduring the other electrochemical measurements were evaluated.Combined with the theoretical decomposition potential of LiCl,the relationship of potential between 1 mol%AgCl/Agreference system and standard chlorine electrode reference system was obtained by indirect method:φ_Nn+/N=_Nn+_/N–1.13.Compared with the potential of AgCl/Agobtained by direct and indirect methods,the potential obtained from indirect method was more accurate and reliable.Based on the indirect method,the SCl E reference system was established in different literature, which realized potentials conversion and the reference system unification.（2）The reduction of Nd³⁺in LiCl-KCl-NdCl₃ melt on the W electrode was two-step process at 400 ℃.The reduction potentials of Nd³⁺/Nd²⁺and Nd²⁺/Ndwere–3.1and–3.3 V（vs.Cl₂/Cl^–）,respectively.These reactions were diffusion-controlled and quasi-reversible and the W electrode was an inert electrode for the reduction process of Nd³⁺.The diffusion coefficient of Nd³⁺in LiCl-KCl-0.4 mol%NdCl₃ melt at 400 ℃ was calculated from CV and SWV curves and determined to be 1.08×10^–5 cm²·s^–1.The activity coefficient of Nd³⁺in LiCl-KCl-0.4 mol%NdCl₃ melt determined by OCP and SWV curves was 0.654.With the effect of depolarization,the Nd³⁺was reduced at–2.8V（vs.Cl₂/Cl^–）with one-step to the Cu-Ndintermetallic compounds on the Cu electrode.The Cu electrode was an active electrode for the reduction process of Nd³⁺.With the potential negatively shift,the ratio of Nd/Cu in Cu-Ndintermetallic compounds was increased.（3）.Liions were reduced into[Li]_Sn on the liquid Sn electrode at–2.7 V（vs.Cl₂/Cl^–）in LiCl-KCl melt.With the potential negatively moving,six different Li-Sn intermetallic compounds were formed and their standard formation Gibbs free energies were also calculated by corresponding equilibrium potentials.Nd³⁺was reduced into[Nd]_Sn on the liquid Sn electrode at–2.5 V（vs.Cl₂/Cl^–）with one-step process in LiCl-KCl-NdCl₃ melt,which was a diffusion-controlled and quasi-reversible reaction.With the electrode potential negatively moving,the Nd-Sn intermetallic compounds,NdSn₃and Nd₂Sn₅,were formed at–2.63 and–2.75 V（vs.Cl₂/Cl^–）on the liquid Sn electrode,and their standard formation Gibbs free energies were also obtained,respectively.Considering the reduction potential of Li⁺was positive than that of Nd₂Sn₅ on liquid Sn electrode,it was hard to produce the Nd-Sn intermetallic compounds with high ratio of Nd/Sn in the LiCl-KCl-NdCl₃ melt.After electrolysis at–2.74 V（vs.Cl₂/Cl^–）on liquid Sn electrode for 5 h,NdSn₃ was obtained on the Sn electrode.The loose structure of NdSn₃ was benfint for the diffusion of Nd³⁺in the melt moving to the surface of liquid Sn electrode,which was helpful for the continuous reduction of Nd³⁺on the liquid Sn electrode.（4）.The Nd³⁺was reduced to metallic Ndat–3.4 V（vs.Cl₂/Cl^–）with one-step process in the NaCl-KCl-NdCl₃ melt at 750 ℃ and the diffusion coefficient of Nd³⁺was 1.62×10^–5 cm²·s^–1.The oxidization potential of Ndand Cu in the Cu₆Ndwere–2.9and–2.2 V（vs.Cl₂/Cl^–）,respectively.The cell voltage range for the selectively separation of Ndfrom the Cu₆Ndintermetallic compound was 0.3～1.1 V.After electrolysis at 0.6 V（cell voltage）for 4 h,the compact copper-riched layer for the thickness of 100μm was left on the surface of the Cu₆Ndanode.The metallic Ndwas deposited on the Mo cathode and the Fe-Ndalloy was formed on the Fe cathode.With the current density of 0.1 A·cm^–2,the cell voltage would increase to 1.1 V,which lead the oxidization of Cu in the Cu₆Ndanode.The copper-riched layer on the surface of the anode would block the contact between electrolyte and Cu₆Ndand lead the“passivation”of the anode,which cause the difficulty for the selectively separation of Ndand Cu in Cu₆Nd.