Effect of alkali metal nitrates on palladium dissolution in nitric acid solutions

Hydrometallurgical unit operations are typically used to recover palladium (Pd) from its ores and secondary resources with high selectivity owing to their low energy consumption, cost effectiveness, and volume flexibility. Herein, diluted HNO₃ solutions with added nitrate salts were used to dissolve Pd powders. Moreover, solutions of nitrates with same valency cations (such as HNO₃, LiNO₃, NaNO₃, KNO₃, CsNO₃, and NH₄NO₃) and same period cations (such as NaNO₃, Mg(NO₃)₂, and Al(NO₃)₃) were used to reveal the involved beneficial effects of the nitrates on the Pd dissolution in an environment friendly way with low acidity of the solutions. Among all added alkali metal nitrates, LiNO₃ resulted in the highest Pd dissolution efficiency, which was attributed to the higher dissociation constant of LiNO₃, resulting in a higher concentration of free nitrate and hence a higher oxidation potential of the overall system. The dissolution process was systematically investigated to determine the optimal temperature (353 K), LiNO₃ and HNO₃ concentrations (6 and 1 mol L⁻¹, respectively), stirring speed (500 rpm), and reaction time (5 h). These optimal conditions yielded a dissolution efficiency of 99.6%. Notably, as the reaction proceeded, the Pd powder surfaces corroded to form numerous holes, indicating that internal diffusion control also affected Pd dissolution.