Preparation of rare-earth-metal oxalate spherical particles in emulsion liquid membrane system using alkylphosphinic acid as cation carrier

Oxalate particles of rare-earth-metal elements 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 bis(1,1,3,3-tetramethylbutyl)phosphinic acid as an extractant (cation earlier). Rare-earth-metal ions were extracted from the external water phase and stripped into the internal water phase to make oxalate particles. Well-defined spherical oxalate particles, smaller than 0.5 mu m in diameter, were obtained for lighter and heavier rare-earth-metals such as Pr, Sm, Eu, Gd, Dy, Y, and Yb, following the formation of primary particles of about 20 nm in size. The present result differs from that obtained using EHPNA (2-ethylhexylphosphonic acid mono-2-ethylhexyl ester) as an extractant, where no submicrometersized spherical particles are obtained for the heavier rare-earth metals such as Dy, Y, and Yb. The smaller extracting ability of alkylphosphinic acid eases the stripping of rare-earth-metal ions by oxalic acid into the internal phase. Effects of the internal water droplet size, the rare-earth-metal 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. The control of the particle size was found to be feasible by control of the feed rare-earth-metal concentration and the size of the internal water droplets.