Leaching of a Mixture of Palladium and Zinc Metal Using Hydrochloric and Sulfuric Acid Solutions

Abstract

Spent electroplating solutions contain small amounts of Pd(II). Cementation of Pd(II) with zinc metal powder is practiced to concentrate the Pd. Dissolution of the cemented Pd metal is necessary to recover pure Pd metal or compounds. In this work, the leaching behavior of Pd metal in inorganic acid solutions (hydrochloric and sulfuric acid) in the presence of an oxidizing agent such as H2O2, NaClO3, or NaClO was investigated. To determine the optimum conditions for Pd leaching, experiments were conducted by adjusting the concentration of the acids and oxidizing agents, reaction temperature and time, and pulp density. Complete leaching of Pd was possible using a hydrochloric acid solution with 3 kinds of oxidizing agents, whereas only NaClO was effective in the leaching with sulfuric acid solution. The chloride ions of HCl and NaClO3/NaClO play an important role in enhancing the leaching of Pd, due to the formation of Pd chlorocomplexes and by decreasing the reduction potential of Pd(II). Compared to H2O2, NaClO3 and NaClO showed some merits as oxidizing agents in terms of reaction time, temperature and low acid concentration. Based on the optimum leaching conditions of single Pd metal, the conditions for the complete leaching of Pd and Zn from the metallic mixtures were obtained.

Recovery of Pure Pd(II) from Spent Electroplating Solutions by Solvent Extraction with Ionic Liquids from Sulfuric Acid Leaching Solution of Cemented Pd

“…Compared to pure sulfuric acid solution, the extraction of Zn(II) is enhanced in the real leaching solution owing to the formation of Zn(II)-chloro complexes [34,42]. According to our results on the leaching of Pd and Zn metal, Pd metal would not be dissolved in the sulfuric acid solution when hydrogen peroxide is employed as an oxidizing agent [2], and thus the use of NaClO is necessary.…”

“…In our previous work, the concentration of both Pd(II) and Zn(II) in the sulfuric acid leaching solution was the same (1000 mg/L) [2]. However, the concentration of these two ions in the leaching solution depends on the size, composition and pulp density of the cemented Pd in the leaching.…”

“…The cemented Pd was obtained from the spent electroplating solutions by cementation with Zn metal powder. The cemented Pd was washed with diluted sulfuric acid solution and then the residues were dissolved at the optimum leaching condition [2], namely, a mixture of 5 M H 2 SO 4 and 1.5% NaClO, pulp density: 2 g/L, 1 h, and 25 • C. At this condition, the Pd and Zn metal in the cemented Pd were completely dissolved. The concentrations of Pd(II) and Zn(II) in the real leaching solution were 1926 and 74 mg/L, respectively.…”

“…Palladium electrodeposition has some advantages, such as easy control, versatile operating conditions, good throwing power and crack-free smooth and adherent coatings with high purity, and thus is employed in the manufacture of electronic devices and advanced materials [1,2]. The concentration of Pd(II) in the spent electroplating solutions depends on the electroplating conditions.…”

“…In practice, there are many kinds of ions in the diverse spent electroplating solutions, and the concentration of Pd(II) in these solutions is not high. In order to recover Pd(II) in the spent electroplating solutions, cementation with zinc (Zn) metal powder is generally practiced in order to concentrate Pd(II) [2]. In cementation, zinc metal powders with fine size are employed for the cementation kinetics.…”