Magnetically Assisted Separation of Weakly Magnetic Metal Ions in Porous Media.Part 1: Experiments

TL;DR

This study addresses magnetophoretic transport of weakly magnetic metal ions in porous media under a non-uniform magnetic field. Using MnCl$_2$ (paramagnetic) and ZnCl$_2$ (diamagnetic) dissolved in silica gel beds with varied pore sizes, the authors quantify near-field enrichment and depletion for single ions and in binary mixtures, demonstrating that magnetic field-induced clustering enhances transport when Brownian forces dominate for individual ions. The results show that, under the tested conditions, single-ion magnetophoresis is enabled by cluster formation, with larger pore sizes increasing mobility via reduced drag, and that in mixtures both ions can migrate toward high-field regions due to mixed clusters. A set of dimensionless analyses (magnetic Péclet number, magnetic coupling $\Gamma$, aggregation $N^*$, electrostatic $\Xi$, and Manning $\xi$ parameters) suggests clustering and inter-ion interactions drive the observed phenomena; Part II will develop a multi-physics framework to quantitatively interpret the data and guide design of magnetically assisted separation for e-waste streams.

Abstract

We report experiments on the magnetophoresis of paramagnetic (MnCl2) and diamagnetic (ZnCl2) metal ions in porous media under the influence of a non-uniform magnetic field generated by a permanent magnet. Experiments were carried out in a range of initial ion concentrations (1-100 mM), porous media particle sizes (63 um and 500 um), and varying mixture ratios of metal ion concentrations. For single-ion magnetophoresis, paramagnetic MnCl2 migrated toward the magnet surface, with an enrichment of approximately 2-4 percent near regions of high magnetic field. Conversely, diamagnetic ZnCl2 moved away from regions of highest magnetic field gradients, with depletion levels of 0.5-1.8 percent relative to the initial concentration. Our results demonstrate that magnetophoresis is directly proportional to porous media particle size, increasing with larger particle sizes, a trend attributed to the reduced drag forces experienced by the ions in media with larger particles. Interestingly, in binary mixtures, both MnCl2 and ZnCl2 migrated toward regions of highest magnetic field, contrary to their individual behaviors. The magnetophoretic effect of MnCl2 was diminished with increasing concentrations of ZnCl2, indicating interactions between the two ions. These findings suggest that both metal ions undergo field-induced cluster formation, with cluster sizes in the micrometer range, in both single and binary ion systems. In binary mixtures, the two ions appear to interact, potentially forming mixed clusters containing both MnCl2 and ZnCl2.

Figures (8)

• Figure 1: (a) a 2D snapshot showing the silica gel containing cell on the permanent magnet. The standard calibration curve for MnCl$_{2}$ (circle) and ZnCl$_{2}$ (square) metal ions obtained using (b) UV spectrophotometer, and (c) the ICP-OES.
• Figure 2: (a) Magnetic flux density (primary y-axis) as a function of location along the magnet surface at z = 0, and magnetic field gradients (secondary y-axis) at z = 0 along the magnet surface. The 2D map of magnetic flux density (b), and magnetic field gradients (c), in the measuring cell used for magnetophoresis study.
• Figure 3: Temporal evolution of paramagnetic MnCl$_{2}$ salt concentration (circle) in polydisperse silica gel induced by a NdFeB permanent magnet ($\mathbf{B}$ = 0.74 T, $(\mathbf{B} \cdot \nabla) \mathbf{B}$$|_{max}$ = 100 T$^{2}$/m) for initial concentrations of (a) 1 mM (b) 10 mM, and (c) 100 mM. Markers are presented as follows: blue is far section, gray is near section, and triangle denotes the control experiments with $\mathbf{B}$ = 0.
• Figure 4: Temporal evolution of diamagnetic ZnCl$_{2}$ salt concentration (squares) in polydisperse silica gel induced by a NdFeB permanent magnet ($\mathbf{B}$ = 0.74 T, $(\mathbf{B} \cdot \nabla) \mathbf{B}$$|_{max}$ = 100 T$^{2}$/m) for initial concentrations of (a) 1 mM (b) 10 mM, and (c) 100 mM. Markers are presented as follows: blue is far section, gray is near section, and triangle denotes the control experiments with $\mathbf{B}$ = 0.
• Figure 5: Temporal evolution of concentration for paramagnetic MnCl$_{2}$ salt in 500 $\mu$m (a-c) and 63 $\mu$m (d-f) silica gels induced by a NdFeB permanent magnet ($\mathbf{B}$ = 0.74 T, $(\mathbf{B} \cdot \nabla) \mathbf{B}$$|_{max}$ = 100 T$^{2}$/m). Here blue, gray circles and triangles correspond to far section, near section, and the control experiments.