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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.

There is growing interest in the recovery of pure palladium (Pd) compound from cemented Pd from spent electroplating solutions due to increasing demand for Pd, and there is considerable motivation to develop an efficient recovery process. In conventional methods of leaching Pd metal, HCl solutions containing oxidizing agents are employed. In this work, ethylene glycol (EG) replaced water as a diluent in the HCl solution containing oxidizing agents such as H2O2, NaClO and NaClO3. Leaching of the Pd metal and the subsequent precipitation of the Pd(IV) compound was investigated. Among the three kinds of oxidizing agent, NaClO3 showed the best Pd metal leaching efficiency from a metallic mixture of Pd and Zn from HCl in EG solution. The role of EG in enhancing the leaching and precipitation is discussed in light of its interaction with other components in the solutions. To precipitate the Pd(IV) compound with NH4Cl, it is important that the dissolved Pd ion exists as Pd(IV). Therefore, NaClO3 was again added to the leaching solution to verify the oxidation of Pd(II) to Pd(IV). Extra pure ammonium hexachloropalladate ((NH4)2PdCl6) was recovered by precipitation from the leaching solution of the cemented Pd under the optimum conditions. Compared to conventional HCl systems, the present process shows some advantages, by reducing both chemical dosage and energy consumption. The results confirm a process can be developed for the recovery of the Pd(IV) compound from spent electroplating solutions containing a small amount of Pd(II).

The demand for palladium (Pd) has increased and this has made it necessary to recover pure Pd from diverse secondary resources. To recover the small amount of Pd(II) present in spent electroplating solutions, Pd(II) is concentrated by cementation with zinc metal. In this work, a hydrometallurgical process consisting of leaching and solvent extraction was developed to recover Pd from the cemented Pd. The extraction behavior of Pd(II) and Zn(II) from synthetic HCl solution by commercial extractants such as LIX 63, Cyanex301, DOS, TBP, Alamin 308, Alamine 336 and Aliquat 336 was investigated by varying HCl concentration. The results showed that LIX 63, Cyanex 301 and DOS selectively extracted Pd(II) over Zn(II). These three extractants were employed to separate Pd(II) and Zn(II) from actual leaching solutions with cemented Pd using a mixture of 7.0 M HCl and 0.5% M H2O2 with 2 g/L pulp density at 60oC for 120 min. Cyanex 301 and DOS selectively extracted Pd(II) over Zn(II) from the leaching solution, while a small amount of Zn(II) was co-extracted with Pd(II) by LIX 63. A two stage counter-current extraction of actual leaching solution with LIX 63 resulted in the co-extraction of Zn(II) with Pd(II). The complete stripping of Pd(II) from the loaded Cyanex301 was achieved using either diluted aqua regia or a mixed solution of 5 M HCl and 0.5% NaClO at an A/O ratio of ten. The hydrometallurgical process is proposed to recover a pure Pd(II) solution from cemented Pd.

Spent electroplating solutions contain a small amount of Pd(II). This Pd(II) can be concentrated by cementation with zinc metal powder. In order to recover pure palladium, a hydrometallurgical process consisting of leaching followed by solvent extraction was developed in this work. First, suitable extractants for the selective extraction of Pd(II) over Zn(II) were selected from synthetic sulfuric acid solutions. Among cationic, solvating and basic extractants employed in this work, Cyanex 301, LIX 63 and Aliquat 336 showed selectivity for Pd(II) over Zn(II) from 5 M sulfuric acid solutions. Stripping results for the loaded phases of these three extractants showed that HCl was effective at stripping Pd(II) from the loaded LIX 63 and Aliquat 336, while a mixture of inorganic acid and NaClO as an oxidizing agent was necessary to strip Pd(II) from the loaded Cyanex 301. The cemented Pd was completely dissolved using 5 M sulfuric acid solution at 25 oC, and the concentration of Pd(II) and Zn(II) in the real leaching solution was 1.926 and 0.074 g/L, respectively. Application of the optimum conditions for the selective extraction of Pd(II) over Zn(II) to the real sulfuric acid leaching solution of cemented Pd indicated that only LIX 63 and Cyanex 301 could separate Pd(II) from Zn(II). The stripping efficiency of Pd(II) by a mixture of 5 M HCl and 3% NaClO was much higher from the loaded LIX 63 than from the loaded Cyanex 301. Considering the extraction and stripping characteristics of Pd(II), LIX 63 was selected as an optimum extractant for the separation of Pd(II) from the real sulfuric acid leaching solution of cemented Pd.

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.