Abstract
Quantifying mass transfer mechanisms governing rare earth extraction from aqueous solutions is critical for process design and scale-up. In this work, droplet microfluidics is combined with liquid–liquid extraction to systematically analyze these mechanisms. The extraction of dysprosium (III) ions (1 – 10 mM, pH 1) from aqueous droplets into an organic phase (Cyanex® 572, Span 80, Shellsol® D70) is used as a model system. Water-in-oil (W/O) droplets are generated via co-flow in a modular 3D-printed microfluidic device. Increasing fluid velocity (9.55 – 76.39 mm·s⁻¹) reduces droplet diameter (0.90 – 0.52 mm) and enhances overall mass transfer coefficients (0.008 – 0.282 s⁻¹).A mass balance-based model is employed to quantify the coupled effects of fluid velocity, interfacial area and Dy³⁺ concentration within individual droplets. The resulting data leads to a Sherwood-based correlation that captures the system behaviour under conditions of interfacial saturation and reduced effective diffusivity, enabling accurate prediction of the overall mass transfer coefficient with a relative root mean square error (RRMSE) below 15.40%.This study introduces a quantitative framework to tune mass transfer in co-flow droplet microfluidic systems and, for the first time, resolves the coupled impact of hydrodynamics and solute concentration in rare earth extraction at the microscale.