Comparison of Separation of Palladium(II) and Platinum(IV) from Hydrochloric Acid Solution by Selective Precipitation Between Sodium Iodide and Sodium Sulfide

Leaching of secondary resources containing palladium and platinum metals with HCl solution in presence of oxidizing agents results in solution containing PdCl42− and PtCl62−. According to hard soft acid base principle, PdCl42− is a soft acid compared to PtCl62−. By utilizing this difference, the possibility of separating Pd(II) and Pt(IV) from HCl solution by selective precipitation of Pd(II) with a soft base was investigated. For this purpose, NaI and Na2S were employed as precipitants and the precipitation behavior of both metal ions was evaluated as a function of the dosage of the precipitants, HCl concentration, the concentration ratio of the two metal ions, reaction time and temperature. In the HCl concentration ranges to 7 mol/L, Pd(II) was completely precipitated by both NaI and Na2S as long as the molar ratio of the precipitant to Pd(II) was larger than 3. The precipitation percentage of Pt(IV) in the presence of excess precipitants was decreased as HCl concentration increased to 3 mol/L. Optimum conditions were obtained from the results to separate Pd(II) from Pt(IV) by selective precipitation. The purity of PdI2 obtained at the optimum condition was 99.9%, while that of PdS was 99.6%. Owing to thermal stability of solid PdS, Pd(II) was completely precipitated by Na2S at 80°C, while the precipitation percentage of Pd(II) by NaI decreased rapidly at 70°C. Treatment of synthetic leaching solution of spent catalysts with NaI and Na2S at the optimum conditions verified that Pd(II) can be separated from Pt(IV) by selective precipitation.