Electrodialysis for Removal of Impurities in Silver Electrowinning

In this work, we investigate the potential of electrodialysis to separate copper from silver electrowinning solution. We further explore and discuss optimization of the electrodialysis process design. Ag is in a high global demand, driven mainly by its use in electronics, jewelry and as investment. The recycling potential for Ag is large and will increase in the coming years due to Ag usage in solar panels. One route of recycling Ag from End-of-life products is batch-electrowinning. In preparation, the products are melted and reshaped into scrape plates which are anodically dissolved in nitric acid solution. Typically, silver is found in alloys with less noble metals like copper, zinc, and iron. While silver is deposited on the stainless-steel cathode surface, the impurities accumulate in the electrolyte. To maintain a high purity of the recycled silver, the contaminated nitric acid solution must be replaced regularly. We investigate the potential of implementing a continues electrodialysis stage parallel to the electrowinning bath to remove copper and other divalent cations from the solution. In electrodialysis, anion- and cations-exchange membranes are stacked alternatingly between two electrodes. When a current is drawn, the cations 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. By using monovalent-selective ion-exchange membranes, separation between monovalent (f. ex., Ag + ) and multivalent ions (f. ex., Cu 2+ , Zn 2+ , Fe 2+ , Fe 3+ ) can be achieved [1]. If the concentration of impurities can be held at a constant level, the nitric acid solution can be recycled continuously rather than replaced as a batch. The only consumable required by the process is electricity. We evaluate the viability of the process and investigate the influence of pH on the process efficiency. We determined the optimal operating current for separating Cu 2+ from Ag + in an equimolar concentration of 10 mM by doing amperodynamic sweeps and recording current-voltage curves. We then tested the desalination performance of an electrodialysis stack with different commercially available anion-/cation-exchange membrane pairs. The single membrane area was 36 cm 2 , and 9 membranes were used in total (five anion-exchange membranes and four monovalent-selective cation-exchange membranes). The Cu 2+ and Ag + concentrations in the diluate and concentrate compartments were determined by atomic mass spectrometry, and the membranes were analyzed with scanning electrode microscopy to detect adsorbed silver. Since scaling and fouling are dominant drawbacks in industrial applications of electrodialysis, we studied the influence of pH on the adsorption of silver in the membrane. Commercial monovalent-selective cation-exchange membranes by Neosepta showed high selectivity for silver over copper. The limiting current density was 10.4 A/m 2 for neutral 10 mM AgNO 3 solution at a flow rate of 100 mL/min and increased by almost 10 times when we adjusted the pH to 1 using nitric acid. In neutral solution, silver was recovered to almost 100 % after two hours of electrodialysis at 10 mA, while copper leakage was 7.5 %. At pH 1, silver recovery and copper leakage were 83 % and 9 %, respectively, after two hours electrodialysis at 100 mA. Consequently, the percentage silver adsorbed in the membranes were close to 0 % in neutral solution and 17 % at pH 1. The work input was significantly higher at pH 1 due to electric current consumed by mass transport of protons. As a next step, we investigate the long-term effect on the membranes with regards to Ag and Cu adsorption, which could be stabilizing at a certain concentration of Ag and Cu or increasing over time. We conclude that electrodialysis is a promising technology for removal of impurities in Ag electrowinning from End-of-life products, justifying large room for future research on development of membranes and operational design. [1] Zimmermann, P., Tekinalp, Ö., Deng, L., Forsberg, K., Wilhelmsen, Ø., Burheim, O. (2020). Electrodialysis in Hydrometallurgical Processes. In: Azimi, G., Forsberg, K., Ouchi, T., Kim, H., Alam, S., Baba, A. (eds) Rare Metal Technology 2020. The Minerals, Metals & Materials Series. Springer, Cham.