Microfluidic Solvent Extraction of Rare Earth Elements

"This paper reports solvent extraction of rare earth elements (REEs) carried out using a microfluidic device and considers the REE loading limit of the organic phase under laminar flow. The microfluidic device contacts aqueous and organic phase streams for a precisely defined contact time (subsecond resolution) and then separates them for offline analysis. Experiments are reported examining the effect of contact time on extraction. We focus on three REEs: one ‘light’ (Nd), one ‘mid’ (Dy), and one ‘heavy’ (Yb) REE, which are extracted using Cyanex® 572. It appears that loading limits may be locally exceeded in the organic phase near to the liquid-liquid interface, where third phase deposits are observed. Third phase formation was also observed close to the liquid-liquid interface in two-dimensional confinement, showing similar third phase adsorption on glass plates. It is also shown that, under microfluidic flow, a typical first ‘split’ between heavy REEs and light REEs can be carried out without exceeding loading limits.INTRODUCTIONMicrofluidic solvent extraction has emerged as a useful tool for studying extraction of metals in a mineral processing context (Ciceri, Mason, Harvie, Perera, & Stevens, 2013; Ciceri, Perera, & Stevens, 2011; Kriel et al., 2015; Mason, Ciceri, Harvie, Perera, & Stevens, 2013; Priest, Zhou, Klink, Sedev, & Ralston, 2012; Priest et al., 2011). The main advantages are the extremely well-defined and tunable flow behaviour, large surface-to-volume ratios, short contact times, and minimisation of sample and reagent amounts. Two main flow configurations have been studied to date: droplet-based and co-flowing streambased extraction. In the former, droplets are formed at a microchannel junction (usually a T-junction or flow-focusing junction) and flow downstream as plugs or ‘free’ droplets. In our case, stream-based extraction, the chip is a plate (often glass) with embedded microchannels (i.e. with cross-sections of tens of micrometres) that carry the aqueous and organic phases in contact without forming droplets. Thus, these chips play the role of a mixer in a conventional mixer-settler and phase separation takes place by branching the streams or collection in a vessel at the chip outlet. Stream-based extraction requires stable streams that are usually supported by a particular channel geometry (e.g. guide structure or small ‘pillars’) or surface chemistry, and flow parallel bounded by the liquid-liquid interface(Aota, Nonaka, Hibara, & Kitamori, 2006; Hibara et al., 2005; Hibara et al., 2002). Phases are disengaged downstream at a Y-junction, which is a qualitative difference between microfluidic and bulk extractions."