Solvent extraction of Pt, Ru, and Ir using Cyanex 923 in chloride media to develop a recycling route for spent polymer electrolyte membrane (PEM) electrolyzers

Abstract

The development of an efficient recycling route for spent polymer electrolyte membrane (PEM) electrolyzers is essential for the recovery of platinum-group metals (PGMs). Among the other refining processes, solvent extraction is a highly selective technique that can provide high-purity products. Moreover, the extraction behavior of Pt, Ru, and Ir from the PEM electrocatalysts is not studied extensively. In this work, the effect of extraction conditions for the efficient recovery of PGMs from model solutions was investigated. To determine the efficiency of extraction and stripping processes, metal contents in the feed solution, raffinate, and loaded strip solutions were quantified using inductively coupled plasma optical emission spectroscopy (ICP-OES). For achieving high separation efficiencies for Pt and Ru from Ir, the following conditions were found to be optimal: 100 mg/L of each PGM in 2 M HCl, 15 vol.-% Cyanex 923 in diesel, an organic to aqueous volume ratio (O:A) of 1:1, 30 min of stirring time, a temperature of 25 °C, and 400 rpm stirring rate. A short mixing time of 5 min resulted in a high separation factor of Pt/Ir. Stepwise recovery of PGMs from the organic phase was studied. The three-step stripping strategy was proposed to extract PGMs: 1) the separation of Pt and Ir in the aqueous phase using water, 2) the stripping of Ru using a mixture of ascorbic acid and HCl, and 3) the effective stripping of the remaining Pt using water. Furthermore, the separation of PGMs from the leach solution of the spent PEM electrocatalyst with other metal ions (Sb and Sn) was investigated.