Foam flotation of rare earth elements by conventional and green surfactants

Abstract

Foam (ion and precipitate) flotation is a promising green method of recovery of rare earth elements (REE) from leachates of the primary and secondary resources. However, the capabilities of foam flotation in solutions simulating real leachates are currently poorly understood, especially for green surfactants as collectors. In this work, we report the results of foam flotation of La(III), Ce(III), Gd(III), and Yb(III) ions individually, as a group, and as a group with gangue ions (Al(III), Zn(II), Ca(II), and Mg(II)). These REE represent both light and heavy REE. As the collectors, we test two quintessential synthetic and green anionic surfactants—sodium dodecyl sulfate (SDS) and mono-rhamnolipid, respectively. We find that the floatability of REE with SDS depends considerably on the composition of the REE solution and demonstrate that the recovery kinetics can be used as additional control of the REE separation in the group flotation with gangue ions. Another important finding is that mono-rhamnolipid at a concentration as low as 10 μM can recover up to 100% REE in the group flotation against gangues, except for Al(III) and Zn(II). At pH 9, mono-rhamnolipid selectively removes REE in the group flotation with gangue cations at a surfactant:total cation (excluding Na+) ratio as low as 1:13. We offer an interpretation of the effects observed. Overall, our results demonstrate that foam flotation is capable of selectively recovering an individual REE from a mixture of several REE with gangue ions, provided that the flotation conditions such as surfactant structure, surfactant concentration, solution pH, and flotation time are properly selected. These results are an important milestone toward the commercialization of foam flotation for the recovery and purification of REE.