Recovery of rare earth elements adsorbed on clay minerals: I. Desorption mechanism

The ongoing development of new, advanced technologies created increasing demands for rare earth elements (REE) in the international markets, with emphasis on identifying new resources to ensure adequate supply and access. The present study investigates the use of clay minerals as a source for extracting rare earth metals by leaching with sulfate and chloride salts. It was found that REE adsorbed on clays can be easily recovered via an ion-exchange mechanism during leaching with monovalent salt solutions under ambient conditions. The leaching efficiency of various salts at 0.5 M and 25 °C was investigated as a function of monovalent cation type (i.e. Li+, Na+, Cs+ and NH4+) and salt system (sulfates vs. chlorides). The initial concentration was based on a 3:1 stoichiometric ratio between all trivalent lanthanides in the clay and the exchange monovalent cation. Leaching efficiency (in terms of % REE extracted) decreased in the order Cs+ > NH4+ > Na+ > Li+, from 90% to ~ 60%, respectively, with sulfates exhibiting ~ 10% better extraction behavior than chlorides. Differences in rare earth metal desorption capability were explained in terms of differences in cation hydration energies: species with low hydration energy extract to a lesser degree compared to species with high hydration energy (i.e. higher affinity for water). Based on these findings, (NH4)2SO4 was identified as the lixiviant of choice for further studies.