Evaporation-Assisted Magnetic Separation of Rare-Earth Ions in Aqueous Solutions

4 years ago

Evaporation-Assisted Magnetic Separation of Rare-Earth Ions in Aqueous Solutions

Barbara Fritzsche, Kerstin Eckert, Zhe Lei

This work aims to answer the question of why an enrichment of paramagnetic ions can be observed in a magnetic field gradient despite the presence of a counteracting Brownian motion. For that purpose, we study a rare-earth chloride (DyCl₃) solution in which weak evaporation is adjusted by means of small differences in the vapor pressure. The temporal evolution of the refractive index field of this solution, as a result of heat and mass transfer, is measured by means of a Mach–Zehnder interferometer. We develop a numerical algorithm that splits the refractive index field into two parts, one space-dependent and conservative and the other time-dependent and transient. By using this algorithm in conjunction with a numerical simulation of the temperature and concentration field, we are able to show that 90% of the refractive index in the evaporation-driven boundary layer is caused by an increase in the concentration of Dy(III) ions. A simplified analysis of the gravitational and magnetic forces, entering the Rayleigh number, leads to a diagram of the system’s instability. Accordingly, the enrichment layer of elevated Dy(III) concentration is placed in a spatial zone dominated by a field gradient force. This leads to the unconditional stability of this layer in the present configuration. The underlying mechanism is the levitation and reshaping of the evaporation-driven boundary layer by the magnetic field gradient.

Publisher URL: http://dx.doi.org/10.1021/acs.jpcc.7b07344

DOI: 10.1021/acs.jpcc.7b07344