STABILITY OF RARE EARTH CARBONATES

Rare earth elements (REEs) are crucial for modern technology and are in high demand due to high prices and unstable supply chains. They are essential in green energy transition, consumer, and military goods, and have applications in energy, defense, electronics, and ceramics. Most REE are extracted from REE carbonates. To explore new sources of REE, recycling of Al-bearing waste sludges from aluminum production using supercritical CO₂ has been evaluated to be a promising technique for REEs extraction. To understand and model the fractionation and mobility between the REEs as well as optimize the extraction processes, it is therefore essential to fully constrain the REE - CO₂ - H₂O system thermodynamically. Surprisingly, the solid phases within this system are not well thermodynamically constrained.

Between ambient conditions to 250 °C, there are 5 solid REE carbonates that can form in the presence of water and carbonate (amorphous, lanthanite, tengerite, kozoite, and bastnaesite). Over 120 - 130 number of syntheses runs with variable conditions show a strong dependence of their formation on both thermodynamic as well as kinetic factors. To explore the thermodynamic constraints, enthalpies of formation are obtained from high-temperature oxide melt drop solution calorimetry and the entropies from the Quantum Design Physical Property Measurement System. Stability trends from both synthesis results as well as thermodynamic stability show a strong dependance of the carbonate phases on temperature and ionic radius of the REE. The lanthanite is constrained to low temperatures for light REEs while the tengerite is constrained to intermediate REEs. Amorphous carbonates form as an intermediate precipitate before the formation of crystalline products for the light REE while for heavy REEs they remain stable. Kozoite and bastnaesite form for almost all REE endmembers and become more stable at progressively increasing temperatures.