REE metals are be made using two different processes, depending upon the metal desired, the relative cost of the process, and the amount of metal required.
These two processes are termed “electrowinning” and “metallothermic reduction”, with metallothermic reduction having two process variations. Electrowinning is a continuous process and metallothermic reduction is a batch process.
The reason for choosing one process over the other depends in part upon the vapor pressure of the molten metal, i.e. the molten REE metal’s unwillingness to remain in a hot vessel and escaping as a metal gas, potentially lowering recovery. These REEs are Sm, Eu, Tm and Yb, and must be processed metallothermically with vacuum and cold vessel containment. The others are processed primarily with electrowinning, but small amounts can also be produced metallothermically.
REE Metals in the Baotou REE Museum
In this process, an electrolytic cell utilizes a carbon anode and a Fe cathode. The electrolyte filling the cell is a mix of alkali fluorides, alkaline earth fluorides, and for Nd metal production, neodymium fluorides. The electrolyte is molten at approximately 1000° C, and neodymium oxide is fed into the molten mixture. The currents between the electrodes are extremely high—1000s of amps—and Nd metal is produced at the cathode, dripping down and forming a molten layer at the bottom of the cell. It is tapped periodically. The amount of Nd metal produced is about 100 kg per day per cell. Due to the use of a Fe cathode, the Nd metal produced is actually Nd/Fe alloy, and well suited for the next step in magnet manufacture. In terms of tonnage, Nd for magnets is the key REE produced by this process. Other non-volatile REEs, REE blends, and REE/Fe alloys can be produced.
In this process, either La or Ca metal can be used, in a sacrificial manner, to make the other REE metals.
Lanthanum metal reactions
Due to the high vapor pressure of Sm, Eu, Tm and Yb, they are produced with sacrificial lanthanum metal. Lanthanum metal is used here, as it has a low vapor pressure, does not contaminate the product, and has sufficient thermodynamics to produce the other metals.
In this process, for example to make Sm metal, La metal chips are mixed with samarium oxide powder, and placed into an inert crucible. This crucible is placed into a radio frequency (RF) furnace, and the system is placed under vacuum. The RF furnace heats the charge to about 1200° C, causing the samarium oxide to transform into the metal, and the lanthanum metal back to the oxide or equivalent. Due to its high vapor pressure, the Sm metal is vaporized and collects upon a water-cooled crucible lid. After all of the samarium oxide has been transformed to the metal, the RF furnace is turned off, and the system opened up. The Sm metal product is scraped off of the crucible lid.
Calcium metal reactions
This process requires the starting material of the REE metal desired to exist as a fluoride. The REE fluoride, when reacted with Ca metal, will produce the REE metal and calcium fluoride in an exchange type reaction. This is a batch process, run under vacuum, heated with a RF furnace, and best used for small production runs.