Recovery of palladium from aqueous solution using surfactant mediated precipitation process

Abstract

In recent years, the demand for precious metals has increased in most emerging countries. The precious metals are mainly used not only in jewellery but also in electrical devices, medical instruments and as catalysts. They can be primarily recovered not only from the ores but also secondary sources like electronic wastes and spent catalysts. This research aimed to develop a method for the recovery of palladium (Pd) from synthetic and real Pd solutions using surfactant mediated precipitation process. The surfactant used was cetyltrimethylammonium bromide (CTAB), while the leaching agents used were hydrochloric acid (HCl), nitric acid (HNO3): sulphuric acid (H2SO4), hydrochloric acid (HCl): hydrogen peroxide (H2O2) and aqua regia. The real Pd solution was prepared using selected leaching agents and spent catalyst. The effect of process parameters such as leaching agent types, CTAB concentrations, Pd concentrations, temperature, and contact time on Pd recovery efficiency were studied. Comparative study between CTAB and other reducing agent (i.e. formic acid) was also carried out. The concentration of Pd was determined using atomic absorption spectroscopy (AAS), while the CTAB concentration was determined by the two-phase titration method. The experimental results show that the Pd recovery efficiency strongly depends on the CTAB and Pd concentrations. The increase of temperature resulted in lowering the Pd recovery efficiency. The precipitation process was relatively fast (less than 3 minutes). The Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) analyses showed changes in CTAB functional groups after the precipitation process. The stoichiometry reaction between Pd and CTAB as given by the molar ratio of Pd to CTAB was 2. The results from the leaching process of spent catalyst using several leaching agents (i.e. HCl, HNO3:H2SO4, aqua regia) show that HCl as the leaching agent gave high selectivity towards Pd compared to other metals present in the spent catalyst. It was found that 85% Pd could be recovered from the real Pd solution (i.e. spent catalyst leaching solution) using 1 mM CTAB.