REEs recovery from molten salt electrolytic slag:Challenges and opportunities for environmentally friendly techniques

Due to the unique physical and chemical properties,rare earth elements(REEs)play a significant role in the high-tech field.In the past few decades,the rare earth reserve in China has been gradually decreasing and more pressure has been exerted on the global rare earth supply for the increasing demand of REEs,which indicates that it is essential to recycle secondary resources to meet the rare earth demand.As for rare earth molten salt electrolytic slag(REMES),although its high rare earth content has potential huge economic value,its high fluorine content of approximately 10 wt%-20 wt%can pollute the environment.Three methods are used to treat REMES.Hydro metallurgical and pyro-hydrometallurgical methods have gotten a big success for solving most of the hydrometallurgical problems,while some problems,like long route and waste water,need to be solved.Vacuum distillation is a new and promising method with a short process due to its harmlessness and high efficiency,but has shortcomings such as high energy consumption and material adaptability.This review presents these above three treatment methods,and the challenges and chances of using the recovery technique of REMES in an environmentally friendly way.

Effect of roasting activation of rare earth molten salt slag on extraction of rare earth,lithium and fluorine

A new process was proposed to extract rare earth elements(REEs),Li and F from electrolytic slag of rare earth molten salt by synergistic roasting and acid leaching.Firstly,the thermodynamic analysis of roasting reaction was carried out,then the effects of roasting factors on leaching REEs,Li and F in slag were investigated.In additions,the mineral phase and morphology of molten salt slag,roasting slag and acid leaching slag were characterized,and the migration mechanism of REES,Li and F minerals in roasting and leaching process was analyzed.The results show that the synergistic roasting and activation of molten salt slag by CaO and Al_(2)(SO_(4))_(3)are thermodynamically feasible.The optimum roasting conditions are as follows:molten salt slag of 20 g,Al_(2)(SO_(4))_(3)of 31.25 g and CaO of 6.25 g,roasting temperature of 1173.15 K and reaction time of 2 h,under this condition,the leaching rates of Nd,Pr,Gd,Li and F are 92.47%,91.56%,91.08%,96.69%and 96.8%,respectively.X-ray powder diffraction(XRD)and scanning electron microscopy-energy dispersive X-ray spectroscopy(SEM-EDS)analysis show that the rare earth fluoride(REF3)in molten salt slag transforms into soluble rare earth oxide(REO)after roasting and activation.After leaching,the leaching residue is mainly strip CaSO4,indicating that REES,Li and F can be fully extracted from molten salt slag.

Electrochemical Preparation of La-Ni Alloy Film in Acetamide-Urea-NaBr Melting System

The reduction of Ni(Ⅱ) is an irreversible reaction and La(Ⅲ) cannot be reduced to La directly but be co-deposited inductively in the present of Ni(Ⅱ) in the Acetamide-Urea-NaBr molten salt electrolyte at 353 K. The uncrystallized alloy film of La-Ni is obtained by potentiostatic electrolysis, and the amount of La grows with increasing cathodic overpotential, molar ratios of La(Ⅲ) to Ni(Ⅱ) and the electrolysis time. The maximum amount of La in alloy film reaches to 78.81% (mass fraction) in present study.

Electrochemical performance of NiCl_(2) with Br-free molten salt electrolyte in high power thermal batteries

NiCl_(2) with high theoretical voltage and thermal decomposition temperature attracts much attention as cathode material for thermal batteries with the requirement of high power density, high energy density and long work time. Unfortunately, the practical utilization of thermal batteries with NiCl_(2) cathode is limited by their poor electrochemical performance under large current, even with the conventional Li F-Li Cl-Li Br all-lithium molten salt electrolyte which proposes ultrahigh lithium ion conductivity. In this work, an unexpected ionic exchange reaction between NiCl_(2) and Li Br in Li F-Li Cl-Li Br was found, which would be the main reason for the poor electrochemical behavior of thermal batteries with NiCl_(2) cathode and Li F-Li Cl-Li Br molten salt. On this basis, Li F-Li Cl-Li_(2)SO_(4), another all-lithium molten salt free of Li Br, was investigated as the new electrolyte for NiCl_(2) cathode. For the single cell of Li(Si)/Li F-Li Cl-Li_(2)SO_(4)/NiCl_(2), a discharge capacity of 377 mA h g^(-1)(with a cut-off voltage of 1.2 V) was achieved with large current density(500 mA cm^(-2)) applied at 520℃, which is almost twice of that of Li(Si)/Li F-Li Cl-Li Br/NiCl_(2)(190 mA h g^(-1)) at the same conditions.

Electrochemical properties of Ca-Pb electrode for calcium-based liquid metal batteries

The Ca-Pb electrode couple is considered to be one of the least expensive(~36$/(k W·h))among various optional materials for liquid-metal batteries(LMBs).The electrochemical properties of Ca-Pb alloy in a Ca|Li Cl-Na Cl-Ca Cl_(2)|Pb cell were investigated in this paper.The electrode potential maintained a linear relationship in the current density range of 50-200 m A·cm^-2,which indicates that the alloying and dealloying processes of Ca with Pb attained rapid charge transfer and mass transport in the interface between the liquid electrode and electrolyte.The Ca-Pb electrode exhibited remarkable properties with a high discharge voltage of 0.6 V,a small self-discharge current density(<2 m A·cm^-2 at 600℃),and a high coulombic efficiency(>98.84%).The postmortem analysis showed that intermetallics Ca Pb3 and Ca Pb were uniformly distributed in the electrode with different molar fractions of Ca,which indicates that the nucleation of solid intermetallics did not hinder the diffusion of Ca in the electrode.This investigation on Ca-Pb electrode sheds light on the further research and the design of electrodes for Ca-based LMBs.

Instability of lithium bis(fluorosulfonyl)imide(LiFSI)–potassium bis(fluorosulfonyl)imide(KFSI) system with LiCoO_2 at high voltage

The cycling performance, impedance variation, and cathode surface evolution of the Li/LiCoO2 cell using Li FSI–KFSI molten salt electrolyte are reported. It is found that this battery shows poor cycling performance, with capacity retention of only about 67% after 20 cycles. It is essential to understand the origin of the instability. It is noticed that the polarization voltage and the impedance of the cell both increase slowly upon cycling. The structure and the properties of the pristine and the cycled LiCoO2 cathodes are investigated by x-ray diffraction（XRD）, scanning electron microscopy（SEM）, Raman spectroscopy, x-ray photoelectron spectroscopy（XPS）, and transmission electron microscopy（TEM）. It is found that the LiCoO2 particles are corroded by this molten salt electrolyte, and the decomposition by-product covers the surface of the LiCoO2 cathode after 20 cycles. Therefore, the surface side reaction explains the instability of the molten salt electrolyte with LiCoO2.