Preparation of spherical oxalate particles of rare earths in emulsion liquid membrane system

Submicron-sized spherical oxalate particles of rare earths were prepared by using an emulsion liquid membrane (ELM, (water-in-oil-in-water (W/O/W) emulsion) system, consisting of Span 83 (sorbitan sesquioleate) as a surfactant and EHPNA (2-ethyl-hexylphosphonic acid mono-2-ethylhexyl ester) as an extractant (cation carrier). Rare-earth ions were extracted from the external water phase and stripped into the internal water phase to make submicron-sized oxalate particles. In the case of Ce, Pr, Nd, Sm, and Gd, well-defined spherical oxalate particles (0.2-0.6 mu m dia.) were obtained, following the formation of primary particles of about 20 nm. In contrast, less spherical or rather tabular particles were formed for La, Dy, and Y, possibly owing to their formation in the external phase following some breaking of the water-in-oil (W/O) emulsion drop. Oxalate particles of Yb were not obtained in this system. Effects of the infernal water droplet size, the rare-earth ion concentration in the external water phase, and the volume ratio of organic membrane phase to internal water phase of the W/O emulsion (O/A ratio), on the size of spherical particles, were investigated A simulation study based on the model of transport mechanism of rare-earth ions through the organic membrane phase revealed that the particle size was determined by the distribution of rare-earth ions into the internal water droplets, as confirmed by the experimental results. The control of the particle size was found to be feasible by control of the feed rare-earth concentration and size of the internal water droplets.