Abstract
A novel solvent-extraction (SX) process was developed for the concentration and separation of rare earth elements (REEs) from the pregnant leach solution (PLS) generated via sulfuric-acid heap leaching of the Round Top ore. Prior to SX, the PLS underwent a proprietary pretreatment designed to remove tetravalent species primarily hafnium, zirconium, thorium, and uranium which are known to poison the organic phase. As part of this pretreatment, ferric ions present in the PLS were reduced to ferrous using metallic iron, mitigating the risk of Fe³⁺-induced extraction inefficiencies and organic poisoning issues. The extraction system utilized Ionquest 801 diluted in Orfom SX-80 to achieve the bulk separation of heavy rare earth elements (HREEs; terbium and heavier) from the light rare earth elements (LREEs; gadolinium and lighter). Although the PLS contained a relatively low total rare-earth oxide concentration (TREO ≈ 1 g/L), the application of a high-acid stripping regime enabled upgrading of the strip liquor (pregnant strip solution) to approximately 15 g/L TREO. This also allowed for reducing the volumetric throughput for the downstream separation circuits. To facilitate downstream individual-element separations, the SX circuit incorporated a media-exchange mechanism in the strip section, ensuring that the final preg product was delivered directly in nitrate media. Bench-scale shakeout tests were used to establish equilibrium behavior, which combined with a steady-state stagewise mass-balance model guided the determination of stage counts and operating conditions for extraction, scrubbing, and stripping. The flowsheet was subsequently validated in a continuous pilot-scale campaign, during which the production of a 15 g/L TREO preg solution was successfully demonstrated. Because of the large volumetric throughput associated with this front-end circuit, baseline separation was not the objective; accordingly, a modest co-extraction of gadolinium into the preg stream was observed. This was acceptable, as final purification was deferred to downstream separation circuits where the volumetric load is substantially reduced and tighter separation efficiencies can be achieved.