Electrochemical Enhanced Recovery of Precious Metals from Electronic Waste

The fast technology development of electrical and electronic devices is leaving behind and increasing amount of electronic waste (E-waste). If improperly disposed, this material can generate a devastating environmental impact by means of the leaching of brominate flame retardants and various heavy metals present [1]. Nevertheless, the increasing amount of E-waste present a significant potential for the recovery and recycling of base and precious metals, whose concentrations can exceed those found on mineral sources [2]. While some material is being recycled, it is generally being performed using low-tech methods leading to ecological damage and health issues for workers and local communities [3]. This recycling targets recovery of gold, the most valuable element while ignoring other metals such as Ag, Pd, Cu, and Sn. This work is seeking to recover the bulk of metals in E-waste using an electrochemically driven mediated process. The goal is to leaves a material that is ready for efficient precious metal recovery.    

In order to dissolve base metals such as Cu and Sn, chemical dissolution methods consume significant quantities of acids which add to process costs. An alternate method of dissolving metals involves using a chemical mediator to attack the base metals. The mediator can be turned over electrochemically to limit the amount of chemicals added and the metals dissolved depend on the oxidizer generated in the process. While the use of  a strong oxidizer will dissolve all metals from the material [4], the approach taken in this work uses a weaker oxidizer that attacks the base metals (Cu, Sn, Zn, Ni) in the non ferromagnetic fraction of mobile electronic devices (steel fragments are magnetically separated prior to processing). This approach has been taken as the remaining small concentrations of precious metals can be extracted efficiently with minimum chemical consumption.

In this work, the recovery of base metals has been performed using the Fe^+3/+2 mediator couple as the oxidant. Work has progressed from pure metals to recovery from milled recycled phone materials. Figure 1 shows the outlet composition of an experiment where a Fe⁺³ solution was continuously fed to an extraction reactor containing milled phone material while collecting the outflow. The extraction order of metals follows the galvanic series where Sn is observed initially and Cu appears only after most of the Sn is removed. This suggests that selective recovery could be achieved. The process has been developed using an electrochemical cell, operating in series with an extraction reactor containing phone material. The oxidant generated at the anode is flowed through the reactor and returned to the cathode side of the cell for recovery at the cathode. The process has achieved over 90% recovery of metals.  

Figure 1. Fraction of metal extracted from milled cell phone material through a continuous feeding of 0.2 M Fe₂(SO₄)₃ in 1 M H₂SO₄. 

References

[1] B. Ghosh, M.K. Ghosh, P. Parhi, P.S. Mukherjee, B.K. Mishra, Waste Printed Circuit Boards Recycling: An Extensive Assessment of Current Status, J. Cleaner Prod.

[2] A. Akcil, C. Erust, C.S. Gahan, M. Ozgun, M. Sahin, A. Tuncuk, Precious metal recovery from waste printed circuit boards using cyanide and non-cyanide lixiviants – A review, Waste Manage.

[3] K. Huang, J. Guo, Z. Xu, Recycling of waste printed circuit boards: A review of current technologies and treatment status in China, J. Hazard. Mater., 164 (2009) 399-408.

[4] N. Brandon, G. Kelsall, T. Müller, R. Olijve, M. Schmidt, Q. Yin, Metal recovery from electronic scrap by leaching and electrowinning, in:  The Electrochemical Society Proceedings Series, PV, 2001, pp. 323-338.