Electrochemistry of Deposition of Rare Earth Metals and Their Alloys in
Molten Salts: An overview of molten-salt studies at Zhongshan University
School of Chem. and Chem. Eng., Zhongshan University, Guangzhou 510275,
P. R. China
The rare earth(RE) metals and their alloys have many functional
performances and have been widely applied in magnetic, optical,
superconducting etc. materials. We have studied the electroreduction of
La3+, Ce3+, Pr3+, Nd3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+,
Yb3+, Lu3+, Y3+ and electrolytic preparation of the rare earth alloys
with Fe, Co, Ni, Cu, Mg in high temperature molten salts (723-1123 K,
NaCl-KCl, KCl, LiCl-KCl, NaF-CaF2 ) since 1980. Recently, we develop the
research of electrodeposition rare earth-iron group alloy films in low
temperature molten salts (373-398 K, urea-NaBr-KBr, urea-NaCl,
1. Electroreduction of rare earth metallic ions in chloride melts
The electrode processes were studied by cyclic voltammetry,
convolution voltammetry, choronopotentiometry, potentiostatic
electrolysis and open circuit potential-time curve after electrolysis,
EDAX and XRD, Most of RE3+ reduced on Mo or W electrode are reversible
in one step. Reduction of Sm3+, Eu3+, Tm3+, Yb3+ is stepwise, i.e.
RE3+e=RE2+, RE2++2e=RE. The diffusion coefficients (D) of RE3+ in melt
were determined as10-6~10-5 cm2 s-1 and the diffusion activation
energies were calculated as about 40 kJ mol-1. When RE3+ are reduced on
Fe, Co, Ni, Cu electrodes, rare earth alloys formed first and then rare
earth metals are deposited.
2. Cathodic alloying of RE3+ reduced on Fe, Co, Ni, Cu electrodes
The rate determining step of cathodic alloying is the deposited RE
atoms diffuse into the cathode. The D of RE atoms in the alloy phases
were determined as 10-11~10-10 cm2 s-1 and the diffusion activation
energies were calculated as about 100 kJ mol-1 at 973-1193 K. The
formation free energies of intermetallic compounds were determined, eg
?Gf0,YCu = -63.1 kJ mol-1 (973 K). Many surface layers of rare earth
alloy were obtained by cathodic alloying.
3. Electrolytic preparation of rare earth alloys in high temperature
Most alloys were prepared by consumable cathode in NaCl-KCl melt.
The RE contents of the alloys attain 90 wt%. Both current
efficiencies(CE) and rare earth recoveries of alloys of La, Pr with Fe,
Co, Ni, Cu reach 85%-95%. For preparation of heavier rare earth alloys,
it is preferable to use fluoride melt, eg Y-Ba-Cu electrodeposited in
NaF-CaF2-YF3-Y2O3 melt (CE:~80%). Y?Mg obtained in YCl3-MgCl2-KCl by
electrolytic codeposition has higher Y content compared with that
obtained by liquid cathode, the former is~70wt%, the later is ~30 wt%.
4. Electroreduction of Fe2+, Co2+, Ni2+ and inductive codeposition
with RE3+ in urea melt
The electric conductivities of molten urea-alkali metal halides at
different temperatures were determined. The conductivities of these
melts are 15~20 mS cm-1 at 373 K.
Reduction of Fe2+, Co2+, Ni2+ to Fe, Co, Ni (M) is irreversible in
one step. The transfer coefficients of M2++2e=M and D of M2+ in melt
were determined as 0.3~0.5 and10-6 cm2 s-1.
The rare earth can be inductively codeposited with iron group metals
to form alloys. Over twenty RE-M alloy films were obtained. The RE
contents in the deposited films changed with cathode potential, current
density, molar ratio of RE3+/M2+ in melt and reach 90wt%. The deposited
films present an amorphous structure determined by XRD. The
crystallizing process was monitored by DSC. The crystallizing
temperature of Er-Co film was determined as 743 K.