Because few rare earth minerals were found in the18th century, only a small amount of water-insoluble oxides could be obtained by chemical methods. Historically, this oxide was customarily called "soil", hence the name rare earth.
Vacuum reduction:
Electrolysis can only prepare general industrial grade rare earth metals. If the metal with low impurity and high purity is to be prepared, it is generally prepared by vacuum thermal reduction. Generally, rare earth oxides are first made into rare earth fluorides, and then the crude metal is obtained by reduction with metal calcium in a vacuum induction furnace, and then the purified metal is obtained by remelting and distillation. All single rare earth metals can be produced by this method, but samarium, europium, ytterbium and thulium cannot be produced by this method.
Refer to the above content: Baidu Encyclopedia-Rare Earth