Kinetic study of uranium (VI) extraction with tributyl-phosphate in a stratified flow microchannel

Solvent extraction kinetics are of considerable importance in process and apparatus design, especially when the mass transfer is coupled with a chemical reaction. In such circumstances, the mass transfer can either be diffusion limited, reaction limited, or both in a mixed regime. Conventional extraction techniques lead to acquire an overall mass transfer coefficient for fast chemical systems. The latter brings together diffusional and reactional parts that are difficult to distinguish independently. This problem is addressed using a high velocity parallel flow microfluidic setup for the investigation of the extraction kinetics of the fast chemical system U(VI):HNO3/TBP: TPH. Experimental data were analyzed using a combination of overall mass transfer resistance calculations and advection-diffusion-reaction models. Results indicate that a chemically limited regime was obtained at a high initial concentration (119 g.l- 1) of uranium (VI) due to Marangoni convections. Whereas, for low initial concentrations (10-30 g.l- 1) and short residence times in the microchannel (<= 13 ms), both diffusion and reaction impact the transfer rate. The obtained value of the apparent rate constant under current operating conditions ([HNO3] = 3 M, % TBP = 30 % (v/v)) was of (6.5 +/- 1.7) x 10-4 m.s- 1. This value is much higher than what conventional extraction techniques predicted, indicating the better mass transfer efficiency in the microfluidic setup due to the decrease of the diffusional limitation.