The preparation of rare earth phosphate fine particles in an emulsion liquid membrane system

Phosphate particles of the selected rare earth metals have been prepared, using an emulsion liquid membrane [ELM, water-in-oil-in-water (W/O/W) emulsion] system, consisting of Span 83 (sorbitan sesquioleate) as surfactant and EHPNA (2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester) as extractant (cation carrier). The rare earth ions are extracted from the external water phase of the ELM system and are stripped into the internal water phase, containing phosphoric acid solution, to form the corresponding rare earth phosphate particles. The morphology and size of the particles are well-controlled by using the ELM system, and submicrometer-sized spherical phosphate particles are obtained, while micron-sized and irregular-shaped phosphate particles are obtained in homogeneous aqueous solutions. The resulting submicrometer- sized spherical particles were characterized by scanning electron microscopy (SEM), powder X-ray diffraction (XRD), and thermal analysis (TG-DTA). The characterization of La-based particles revealed that the fine particles obtained were of hexagonal structure, but their crystalline structure was changed to a monoclinic structure by calcination at 1373 K. Composite LaPO4Ce3+,Tb3+ phosphor particles were then prepared by transporting La3+,Ce3+, and Tb3+ ions simultaneously, with the resulting particles showing good photoluminescence at 545 nm. In the ELM system, the size of the particles is able to be controlled in the range of 0.28 to 0.65 mum by changing the concentration of Span 83, whereas that in the homogeneous system is about 0.7 mum.