Magnetophoretic Separation and Recovery of Metal Ions for Electronic Waste Recycling

There is increasing need for more efficient methods to recycle electronic waste. Millions of tonnes of electronic waste are currently generated per year, and this rate is projected to increase rapidly. An important component of this process is separation and recovery of critical metal species. In solution, mixtures of metal ions may be separated magnetophoretically based on differences in their magnetic susceptibility and their hydrodynamic drag. However, the magnetophoretic transport of these metal ion mixtures is highly sensitive to collective electrostatic, magnetostatic, and hydrodynamic interactions among the ions. There are currently no predictive models for describing these effects on magnetophoretic transport, which hinders our ability to design efficient metal recovery processes. To address this challenge, we investigate the effects of ion-ion correlations and hydrodynamic interactions on the magnetophoretic transport of metal ions mixtures using Brownian dynamics simulations. We quantify the relative magnetophoretic separation velocities for mixtures of paramagnetic, diamagnetic, and nonmagnetic ions immersed in an external magnetic field as a function of ionic concentrations and field strength. We observe significant hydrodynamic coupling among different metal species in mixtures that cannot be understood in terms of linear combinations of single-ion solutions. Nonmagnetic metal ions, which normally have no phoretic motion in a magnetic field, move rapidly down a magnetic field gradient when mixed with paramagnetic metal ions due to the hydrodynamic backflow generated by phoretic motion of the latter up a magnetic field gradient. This effect greatly enhances the relative separation velocity between the two species. We also observe that hydrodynamics interactions induce aggregation of like-ions that enhance transport and aid in recovery. Our understanding of the dynamics of metal ions will inform the design of improved metal recovery processes in the lab.