Selective and Preferential Separation of Rhodium (III) from Palladium (II) and Platinum (IV) Using a m-Phenylene Diamine-Containing Precipitant

Abstract

Although Rh is an industrially important and the most expensive platinum group metal (PGM), the selective and preferential separation of Rh from PGM mixtures still remains as a big challenge. In this work, the separation of Rh (III) from Pd (II) and Pt (IV) in a hydrochloric acid (HCl) solution was studied using a m-phenylene diamine-containing precipitant (m-PDA). At high HCl concentrations (6.0–8.0 M), most of the Rh (III) (>90%) was precipitated, and Pd (II) and Pt (IV) were hardly precipitated (<5%). On the other hand, over 85% of Pd (II) and Pt (IV) precipitated along with small amount of Rh (III) (<25%) at low HCl concentrations (1.0–2.0 M). As a consequence, m-PDA enabled selective and preferential precipitation of Rh (III) at high HCl concentrations. XPS and TG analyses revealed that the Rh-containing precipitate is an ion-pair complex composed of one [RhCl6]3− anion and three m-PDA cations. The Rh desorption from the precipitate as well as the recovery of m-PDA was successfully achieved using an NH4OH solution. This method is a promising practical approach to Rh recovery.