(531b) In LLE, Inorganic Extractants' Behavior with Specific Inorganic Solutes Compared with Critical Fluctuation Phenomena

Liquid-liquid extraction (LLE) is a widely used separation process for recycling technologically important metals, including critical materials. In LLE, dissolved aqueous metal ions are transferred to an immiscible liquid phases, typically a nonpolar organic phase. The extractant molecule in the organic phase facilitates transports of metal ions by selective binding of targeted species. One limitation of this process is third phase formation, which occurs after extraction of sufficient polar species into the nonpolar organic phase. As chemical processing equipment is designed for two phases, this deleterious phase transition limits the amount of polar species than can be extracted. Aggregation phenomena in the organic phase has traditioanlly been linked to this phase transition, although the mechanism is not clear. Often, the organic phase is modeled as composed of water-in-oil reverse micelles. As we discuss in the presentation, we find that the structure in LLE organic phases across a wide range of compositions and conditions are consistent with critical fluctuations, where the critical point corresponds to the highest temperature at which third phase formation can occur. We present our work on mapping organic phase structure across a wide range of extractant (a malonamide, DMDOHEMA) and aqueous nitric acid concentrations. Using small angle x-ray scattering, we study how the extraction of nitric acid and water by a malonamide extractant induces critical fluctuations, and show how it depends significnatly on the extractant concentration. Using the scaling laws of critical phenomena, the fluctuation length scale's dependence on temperature is measured, showing universal 3D Ising-like behavior. Then, using x-ray photon correlation spectroscopy, we measure the dynamics of the critical fluctuations as we cool the organic phase and approach the critical point. Overall, our results show that the theory of critical phenomena provide a robust, quantitative description of LLE organic phases over a wide range of compositions. By understanding the mechanism of structuring in the organic phase, we will be able to design more phase transition-resistant LLE processses.