Palladium Recovery from Aqueous Solution By Electrodialysis
Authors: Zimmermann Pauline , Burheim Odne Stokke , Wilhelmsen Øivind , Deng Liyuan , Tekinalp Önder 
Published: July 2022

In this work, we investigate the potential of Electrodialysis to increase the palladium concentration stage in Pd recycling to ensure a maximum recovery rate. We further explore and discuss optimization of Electrodialysis process design.

PGMs are in high global demand, driven mainly by their use in autocatalysts. However, the European Commission classifies PGMs as Critical Raw Materials, and the primary production of PGMs is covered to a large extent by South Africa and Russia (ca. 85%). As a result, the PGM market is in continuous deficit. Furthermore, the recycling input rate of End-of-life products containing PGMs is estimated at 21% in Europe, which is largely insufficient to meet the EU demand. At the same time, recovery rates are low, and significant amounts of metals are lost in bi-products and residuals.

Palladium containing End-of-life products are typically leached in aqua regia (hydrochloric and nitric acid) before elemental separation. The separation methods are chemical intensive and do not allow for a 100 % recovery rate. However, with Pd being a critical raw material, recovering even very dilute concentrations is relevant.

We investigate the potential of Electrodialysis to increase the palladium concentration stage in industrial draw solution after the chemical separation of PGMs. The only consumable required by the process is electricity. We evaluate the viability of the process and investigate the most beneficial process conditions that ensure the highest recovery rate and energy efficiency.

In Electrodialysis, anion and cations-exchange membranes are stacked alternatingly between two electrodes. When a current is drawn, the cations will move towards the cathode, passing the cation-exchange membrane and vice versa for the anions. Since the membranes are selective for either anions or cations, they strongly retard the passage of the opposite ion. Therefore, cations and anions are accumulated in every second compartment, while every other chamber is desalinated.

In Electrodialysis, the current density is proportional to the sum of the ion fluxes and their charges. Due to boundary layer effects at the membrane surfaces causing depletion of ions, the resistance, and specific power consumption will increase with increasing current density. We investigate the optimal operating current for an industrial mixed-ion solution containing dilute concentrations of Pd (~1200 mg/L) by doing amperodynamic sweeps and recording current-voltage curves. We then test the desalination performance of the ED stack with two different commercially available anion-/cation-exchange membrane pairs. Membrane properties are a crucial factor for ED performance. To increase the concentration stage of a target ion (Pd), the ability of the membrane to exclude water from transporting through it is of paramount importance. Therefore, we compare two types of membranes with different hydrophobicity and discuss the influence of water transport on ion permselectivity. The ion permselectivity of the ED stack is evaluated based on irreversible thermodynamics, taking into account the transport numbers for ions and water. Mass transport of ions and water will depend both on membrane properties and process design.

The flow rate and the applied current density are the dominant performance parameters concerning the process design. The flow velocity is limited by the pressure drop through the electrochemical cell, while the current density determines the energy efficiency to a large extent. Therefore, we vary those two parameters to investigate their effect on the system performance.

Within three hours of Electrodialysis, we increased the Pd concentration in an industrial multi-component solution by more than 60 %. However, the decrease on the desalinated side was higher than the increase on the concentrated side, suggesting that some Pd is stuck in the membrane. We conclude that ED is a promising technology for Palladium recovery, justifying large room for future research and development of membranes and operational design.