Study On Co-deposition And Cathodic Alloying Preparation Of Variant Valence Rare Earth (Yb?Tm?Nd) Alloy By Molten Salt Electrolysis

With the widely application of rare earth metals in various industries,the demand is continuously growing,which greatly promotes the development of rare earth preparation technology.In recent years,preparation of rare earth?RE?intermediate alloys by molten salt electrolysis has become a hot area of research and development.Because molten salt electrolysis has various advantages,such as short production cycle,no segregation,without burning,a large scale,without reducing agent,continuous production and more economical and convenient.In this paper,we investigated the alloying process of three variant valence REs?Yb?Tm?Nd?with Mg?Al?Zn in chloride molten salt systems and their reduction behavior on reactive Mg?Al?Zn cathodes.The feasibility of corresponding RE alloy preparation process in molten salt is studied.We obtained important achievements in the aspects of phase control and composition control of alloys.1.Anhydrous YbCl3 was obtained via solid�phase reaction from Yb2O3 with NH4Cl as a chlorination agent and KCl as an additive agent.The effect of temperature on the efficiency of chlorination was explored.On an inert electrode,the alloying processes of Yb???and Tm???ions with Mg???ions were investigated by cyclic voltammetry?square wave voltammetry?chronopotentiometry and open�circuit chronopotentiometry in LiCl�KCl melts.The feasibility of preparation of Mg�Li�RE alloys via co�reduction was demonstrated.RE alloy with different phases and RE�rich Mg�based alloy were prepared on W and Mg solid electrodes.The influence of MgCl2 and RECl3 concentrations on the alloy composition were investigated.The corrosion resistance and crystal lattice of Mg�Li alloy were improved after the addition of RE.Alloys were analyzed via XRD?SEM?OM?ICP.2.Anhydrous YbCl3 was obtained via solid�phase reaction from Yb2O3 with AlCl3 as an chlorination agent.The effect of temperature on the efficiency of chlorination was explored.Moreover,it is also feasible to chlorinate Yb2O3 into YbCl3 through liquid�phase reaction in LiCl�KCl melts with AlCl3 as an chlorinating agent.Yb???and Tm???ions can form various intermetallic compound with Al??? ions in LiCl�KCl melt on inert W and active Al electrodes.The co�reduction process was investigated by different electrochemical measurements.Al�based alloys with Al2Yb?Al3Yb?Al2Tm?Al3Tm and AlTm phases were prepared by galvanostatic and potentiotatic electrolysis.The influence of AlCl3 concentration on the alloy composition was investigated and the corresponding alloys were analyzed by XRD?SEM?OM?ICP.3.Electrochemical co�reduction behavior of Mg?Li?Al and Yb was investigated in LiCl�KCl�Yb2O3 melt containing different concentrations of AlCl3 and MgCl2 on W electrodes at 923 K.The formation of Mg�Al�Yb ternary intermetallics was confirmed in different melt systems containing Mg????Al???and Yb???ions.Mg�Li�Al�Yb alloys were directly prepared via potentiostatic and galvanostatic electrolysis on W electrodes from LiCl�KCl�Yb2O3�MgCl2�AlCl3 melt.XRD analysis reflects that the main intermetallic compound is Al2Yb in Mg�Li�Al�Yb alloys.The results of ICP indicated that the contents of Li and Al in Mg�Li�Al�Yb alloys can be adjusted by the changing of Mg Cl2 and AlCl3 concentrations.Therefore,quarternary Mg�Li�Al�Yb alloy with controllable composition can be prepared by co�reduction in molten salt.4.The electrochemical behavior of Tm???ions was investigated in LiCl�KCl melt on W and liquid Zn electrodes via cyclic voltammetry.The potential of reduction of Tm???on liquid Zn electrode is 0.9 V more positive than that at the solid W electrode.Then,the co�reduction of Tm???and Zn???ions on W electrode was studied by cyclic voltammetry and chronopotentiometry.The result show that five Zn�Tm intermetallic compounds were detected in the system.XRD result of deposit obtained by galvanostatic electrolysis in LiCl�KCl�ZnCl2�TmCl3 melt on W electrodes showed Tm2Zn17 phase.When galvanostatic electrolysis was carried out at liquid Zn electrode,to some extent,the applied current density affected the constitution of the deposit.At�200 mA cm^�2,the obtained deposit is composed of22.3 wt.%Tm and 77.7 wt.%Zn.In molten NaCl�KCl salt,the reduction of Nd???ions to Nd?0?metal occurred in a single reaction step on both W and liquid Al electrodes.Its reduction potential on liquid Al electrode is much more positive than that on the W electrode.The co�reduction of Al???and Nd???in NaCl�KCl�AlCl3�NdCl3 melt was studied on W electrode by cyclic voltammetry and open circuit chronopotentiometry.Five Al�Nd intermetallic compounds at about�1.44?�1.35?�1.27?�1.20 and�1.08 V were detected.Al�Nd alloys were prepared via intermittent galvanostatic electrolysis at 0.6A at liquid Al electrodes and the current efficiency was calculated.Nd?AlNd3?Al2Nd and AlNd phases were identified by XRD and Al11Nd3 phase was confirmed by EDS analysis in Al�Nd alloy.During electrolysis process,the disproportionation reaction of 2Nd???+Nd?3Nd???can be avoided.