G.G. Jang, H. Liang, A. Medley, P. Zhang, P. Paranthaman, C. Tsouris
Oak Ridge National Laboratory,
United States
Keywords: membrane solvent extraction, rare earths, phosphate process streams
Summary:
This study reports on the selective separation of rare earth elements (REEs) from phosphate process streams, including phosphoric acid (PA) sludge and phosphogypsum (PG), using a membrane solvent extraction (MSX) process. While MSX has been proven effective for highly concentrated REEs (e.g., ~43,000 ppm neodymium) found in acid leachates from scrap permanent magnets, its effectiveness for dilute REEs solutions has not been shown yet. PA sludge and PG solid particles contain 200 to 2,900 ppm of total REEs, including 15 different REEs in addition to uranium and thorium. Acid leaching, however, implemented to dissolve REEs, significantly dilutes the total REEs concentration to only ~200 ppm for PA sludge and ~10 ppm for PG. This low REEs concentration, in the presence of order-of-magnitude higher non-REE ions including P, Mg, Al, Fe, Ca, and undesirable radioactive species of uranium and thorium pose a challenge for effective separation using MSX. By modifying the diluent and the organic solvent, we have demonstrated that the neutral extractant N,N,N,N-tetraoctyl diglycolamide (TODGA) selectively separates the REEs from acid leachate, while rejecting uranium and thorium. The light REEs (e.g., cerium) concentration in the strip solution could be enriched by more than 2 times, and the heavy REEs were preferentially extracted, despite their low overall concentration. Based on these results, a process flow diagram is considered for the recovery of REEs and other valuable materials from phosphate industry streams. This work suggests that the MSX process is an economically viable option for the separation and recovery of REEs from low-concentration process streams, offering a significant advantage over regular solvent extraction processes.