Palladium stripping rates in PGM refining

This study investigates the stripping (back-extraction) of palladium from the perspective of mixer–settler hydrodynamics. A hydroxyoxime extractant and hydrochloric acid medium are used. Firstly, the palladium stripping rate (chemistry) is determined using a vigorously stirred laboratory unit cell. This increases with operating temperature and stripping acid concentration. At optimal chemical conditions (25 °C, 6 N HCl), the stripping rate is sufficiently fast for equilibrium to be reached within 5–10 minutes. Secondly, the palladium stripping rate (hydrodynamics) is determined using a pilot-scale mixer–settler. The mixer–settler is operated with a 4-bladed radial disc and a 6-bladed Rushton turbine impeller, over a range of impeller speeds corresponding to power intensities of 2 to 5 kW/m3. The palladium stripping rate increases with increasing impeller speed for both the radial disc and Rushton turbine impellers. The overall mass transfer coefficient also increases with increasing power intensity, up to power intensities 100% higher than those used in typical plant stripping mixer vessels. When benchmarked against the power intensity, not much difference between the two impellers is noted. The study concludes that the stripping of palladium by solvent extraction as used in the PGM industry can be improved by operating at higher power intensities.