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Rare Earth Element reCYCLing with Low harmful Emissions

Final Report Summary - REE-CYCLE (Rare Earth Element reCYCLing with Low harmful Emissions)

The aim of the rare earth extraction with low harmful emission “REE-CYCLE” project was to allow development of new methods for recycling rare earths from permanent magnets and other electronic wastes. This required parallel development of methods based on milli-fluidics to evaluate the yield and selectivity of recycling processes based liquid-liquid extraction methods as well as theoretical predictive models of chemical activity driving towards phase transfer. Real-time measurement of activities able to design rare earth separation and purification with low amounts of toxic effluents in realistic conditions for an environmentally and economically acceptable conditions.
Success relies on the triple alliance of the knowledge of the molecular forces driving reversible transfer at meso-scale and colloidal scale, predictive modelling of phase stability and reference chemical potentials as well as on-line determination of the chemical activities of species involved, including the solvent.

On the experimental side, an original device able to measure a few percent of variation of solvent and water activity when coupled to a milli-fluidic device was built, patented and openly published with collaboration of the University of Ulm. The principle of measurement and ultra-sensible measurement of the partition coefficients of rare earth was optimized with determination of the free energies of transfer involved with an unprecedented precision( (< 2 kj/mole /rare earth nitrate salt).
The intensification by ultra-sound, predicted in the sixties, was measured for the first time in kHz and MHz range and shown to be only intensification by a factor of 2-3, un-sufficient to be used on large scale in pertraction devices at reasonable cost and safety.
On the theoretical side, multiple-equilibria multi-parametric approximations were abandoned towards a general approach estimating all the terms competing with the free energy of complexation by the water-in-oil supramolecular aggregates driving the extraction.
The first ever theory based on frrst-principles of complex fluid thermodynamics of the liquid-liquid extraction in a brine versus reverse-micelle equilibrium was published in the case of uncharged extractant . The real case containing mixed extractants in synergy used industrially is planned to be submitted for publication early 2019. As a result of this study, it is likely that variation of 0.6 units of pH in a pilot plant can be envisaged safely at industrial scale; this means a reduction by a factor of four of toxic effluents for a given amount or rare erath extrcated , specially if addition of hydrotropes or solvent modifier agents are used to avoid formation of emulsified “third phase” that stop operation of any extracting device in industrial use.
Finally, initially unplanned experiments and results were even richer in break-though: the thermodynamic origin of the Ultra flexible microemulsions (UFME) obtained using in conjunction ethanol as a hydrotropic co-solvent and a typical solvent “modifier” such as octanol was discovered in a collaboration with Regensburg during the ERC project with consequences well beyond the field of separation and recycling.

Last but not least, an original confocal Fabry-Perot device focused by a millimetre droplet of extracting phase placed in a brine reservoir has been shown to detect nanometre resonant surface waves associated to ion transfer and interphase formation: this progress by three orders of magnitude in experimental sensitivity while studying ion transfer between phases will certainly be the source of immense progress of understanding coupling between the colloidal and hydrodynamic scales in the whole technology of recycling.

The general predictive theory, including equations, approximations and organigram is available openly to any industry lab for planning laboratory or pre-pilot scale. The general approach determining free energy of transfer is the common ground of projects in the SCARCE CEA/NTU laboratory that was opened in by the co-PI in September 2018 in Singapore, as a direct outcome of the success in the methodology developed y the ERC" "REE_CYCLE" to improve recycling processes and lowering effluent quantities.