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.
Highlights
The recovery of platinum group metal (PGM) from hydrochloric acid (HCl) solutions has been widely studied because PGMs in spent catalysts can be leached in HCl medium[16,17]
Narita et al reported that designed compounds, tertiary amines containing two or three N-disubstituted amide groups, act as Rh (III) extracting agents, and these nitrogen compounds were successfully used for the separation of Rh (III) from Pd (II) and Pt (IV)[22,23]
Since Rh (III) forms chloro-complex anions in HCl, we expected that m-phenylene diamine-containing precipitant (m-PDA) forms ion-pairs with chloro-complex anions of Rh (III) to recover Rh (III) as a precipitate
Summary
The recovery of PGMs from HCl solutions has been widely studied because PGMs in spent catalysts can be leached in HCl medium[16,17]. Precipitation from HCl solutions containing Pd (II), Pt (IV) and Rh (III). The separation of Rh (III) required two steps: (1) extraction of Pd (II), Pt (IV) and Rh (III) from an HCl solution into an organic phase and (2) back-extraction of Rh (III) into a concentrated HCl solution. Tertiary amines and organophosphines act as extractants for Rh (III) from SnCl2-containing HCl solutions, co-extraction of Pt (IV) cannot be suppressed. Since Rh (III) in HCl solutions is nearly inert for ion-pair formation as mentioned above, only Rh (III) can be eluted from the anion exchange resins despite the adsorption of Pd (II) and Pt (IV) on the resins. Rh recovery methods up to this point are based on the inert nature of Rh (III) towards coordination and ion pair formation. We have reported that the m-phenylene diamine-containing precipitant (m-PDA) can form ion-pairs with [PdCl4]2− and [PtCl6]2− in 0.1 M HCl32. We present a procedure for the selective and preferential precipitation of Rh (III) from a mixture of Pd (II), Pt (IV) and Rh (III) in HCl, which relies on the use of m-PDA
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