Evaluation of magnetic nanoparticles coated by oleic acid applied to solvent extraction processes

• Magnetic nanoparticles speed the phase disengagement of solvent extraction process.
• Phase disengagement was up to five times faster using magnetic nanoparticles.
• Bare and oleic acid coated nanoparticles were characterized.
• The presence of nanoparticles did not affect the cobalt extraction with Cyanex 272.
• Chemical stability of nanoparticles coated by oleic acid is limited to pH > 2.

In this study, a magnetic organic liquid called ferrofluid was used to improve the speed and efficiency of the phase disengagement of solvent extraction processes applied to hydrometallurgical systems. The ferrofluid consisted of magnetic nanoparticles (mean diameter of 10.2 nm), comprised of magnetite (Fe3O4) and maghemite (γ-Fe2O3), which were coated by oleic acid and dispersed in an organic diluent. The nanoparticles were characterized by applying various methods: transmission electron microscopy, X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, and thermogravimetric and magnetic analysis. The effect of the nanoparticles on the solvent extraction of cobalt sulfate with Cyanex 272 was investigated using ferrofluids at different concentrations (0, 10, 20, and 30 g/l of magnetic nanoparticles in commercial aliphatic kerosene Exxsol D80). It was observed that, under the conditions studied, the presence of nanoparticles does not interfere in the efficiency of metal extraction and stripping, and that the time necessary for phase disengagement can be up to five times faster. However, the ferrofluid containing nanoparticles coated solely by oleic acid is not stable when in contact with aqueous solutions at pH ⩽ 2.0. Limited stability restricts the application of the magnetic fluid used in the present study in solvent extraction processes that operate at a pH level beyond 2.