Developing sustainable processes to minimize greenhouse gases is an ongoing effort in various industries around the world. Wind energy or electrical vehicles are two prominent technologies driving the green transition. Neodymium-iron-boron (NdFeB) permanent magnets with a rare earth element (REE) content of around 30 wt.% are typically used within electric motors. Large wind turbines can contain more than a ton of rare earths. The above mentioned factors are causing an increasing demand for these metals whilst industrial nations being heavily dependent on producing countries in Asia. Developing new recycling methods to recover REE from scrap materials (termed as long-loop recycling processes) is an internationally growing topic.

A pyrometallurgical recycling process for waste NdFeB permanent magnets named as slag extraction method is discussed in detail here. Recently, it was found that selective oxidation of REE by metal oxides is promising to recover REE in a slag phase while iron, boron, alloying elements (e.g., cobalt) or coating elements (e.g., nickel) are efficiently collected in an iron phase. This efficient separation of impurities leads to a concentration of REE in a single-stage process providing a significant advantage over direct chemical leaching of permanent magnets. Moreover, the remaining iron phase contains >3 wt.% cobalt and can be utilized to further extract other valuable metals.

In this approach boron oxide was added as flux and ferric oxide as oxidant to produce a slag with concentrations >85 wt.% RE₂O₃. Beside sintered also polymer-bonded NdFeB magnets were investigated in this study which are typically not treatable by direct leaching due to high organic content. Different crucible materials such as graphite, clay-graphite or alumina were tested. Experiments were performed at temperatures between 1300 °C and 1500 °C under inert argon atmosphere. High REE extraction rates of >99 % could be achieved at 1400 °C and 2 h dwell time. In the slag phase, the concentration of impurities such as iron, nickel or cobalt were <1 wt.% detected by ICP-OES and SEM-EDX. The use of clay-graphite crucibles leads to Al and Si contaminations in the slag phase which were avoided in pure graphite crucibles. The developed process can be used as pre-concentration step prior to hydrometallurgical refining of REE. To further optimize the process parameters and provide a scalable technology, kinetic studies are currently conducted.