Pickering emulsions used as an efficient separation process for rare earth minerals

Emulsions stabilized by solids present many opportunities as new mineral separation processes. Their effectiveness is based on the fundamental mechanism of particle affinity: certain mineral particles preferentially attach to either the oil or water phase, which helps stabilize the emulsion. In this context, wettability refers to how easily a liquid spreads across or adheres to a solid surface. It is quantified by the contact angle, which is the angle formed between a droplet of liquid and the surface of a solid (angle>90° is more hydrophobic; contact angle < 90° expresses hydrophilicity). Quantifying wettability for fine particles is challenging due to their non-planar shapes and rough surfaces. To overcome this issue, we developed an innovative technique for measuring these properties. Our study addresses the challenge of selectively separating rare earth element (REE) minerals, such as bastnaesite and monazite, from gangue minerals like dolomite and calcite. We use a solid stabilized emulsification (SSE) process. In this process, particles with subtle differences in physicochemical surface properties—especially wettability as indicated by their contact angle with water—are separated. The valuable REE, which have higher contact angles and are thus more hydrophobic, preferentially attach to oil droplets. In contrast, the more hydrophilic carbonate minerals (contact < 48°) remain in the aqueous phase. Monazite and bastnaesite demonstrated a strong affinity for the oil, resulting in a 68% REE recovery and an enrichment ratio of 2.9 in a single-stage process. These results demonstrate that SSE is a promising approach for the beneficiation of fine REE.