(423e) Continuous-Flow Centrifugal Solid-Liquid Separation for the Recovery of Rare-Earth Elements Containing Particles from Phosphoric Acid Sludge

Phosphoric acid sludge contains concentrated phosphoric acid (up to 54% P₂O₅) and solid precipitates, including rare earth elements (REEs) at concentrations near 2,200 ppm. Low cost recovery of valuable P₂O₅ and simultaneous separation of solid particles from the phosphoric acid sludge could be an economically feasible approach to recovering REEs, while increasing the production of phosphoric acid. The phosphoric acid sludge, however, is a complicated fluid stream that cannot be simply separated by traditional separation technologies such as batch centrifugation, decantation, settling, or filtration because of its high viscosity and high solids content (e.g., 30–40%). Guided by a force balance model, a highly efficient solid/liquid separation method was demonstrated in this study, using a continuous-flow centrifugal contactor. A high shear is typically used to increase the contact surface between two liquid phases, thus enhancing the interphase mass transfer rate, and a centrifugal force is exerted on the fluids, inducing phase separation at the exit. By introducing the sludge directly through the phase separation region, we demonstrated sludge recovery and studied the effects of process parameters on solid particle capture with the goal to optimize the separation process. Three contactors in series yielded 94% solid recovery and 98% recovery of the liquid component. After saturation, the slurry was withdrawn and the accumulated solids in the contactor were flushed out. Recovered solid particles contained 1,500–2,895 ppm of REEs, which can be recovered after further processing.