Recovery of Palladium Metal with High Purity by Selective Reduction from Inorganic Acid Leaching Solutions of Cemented Zinc

Abstract

A small amount of Pd(II) present in spent electroplating solutions is concentrated by cementation. In this work, a process was developed to recover pure palladium metal by selective reduction from the hydrochloric and the sulfuric acid leaching solutions of cemented zinc. For this purpose, hydrazine and ascorbic acid were employed as reducing agents and the effect of several variables such as the acidity of the leaching solutions, molar ratio of reducing agents to Pd(II), reaction time and temperature was investigated. Since neither of the reducing agents can reduce Pd(II) from strong acidic solutions, it was necessary to decrease the acidity of the leaching solutions by dilution with distilled water. Our results indicated that it was possible to recover Pd metal with high purity by selective reduction after diluting the hydrochloric and sulfuric acid leaching solutions of cemented zinc with distilled water. When the molar ratios of the reducing agents to Pd(II) were the same, the reduction percentage of Pd(II) from the sulfuric acid solution was higher than that from the hydrochloric acid solution, due to the much higher complex formation constant of PdCl42- than Pd(SO4)22-. The reduction of Pd(II) by hydrazine occurred within 15 mins at room temperature, while a higher temperature (60 oC) and longer reaction time (1h) were necessary to reduce Pd(II) with ascorbic acid. The reduction of Pd(II) from sulfuric acid solution was more efficient than from hydrochloric acid solution. It was possible to recover Pd metal with high purity from the leaching solution of cemented zinc using the reduction method.