Solvent Extraction Kinetics

Extraction Velocity

Solute transfer speed depends on the hydrodynamic conditions of the agitation and the surface area generated by the dispersion. High shear zones increase the rate by creating smaller droplets which expand the interfacial contact area. These droplets facilitate the rapid movement of metal ions into the organic carrier.

The mass transfer coefficient decreases as the system approaches saturation because the chemical potential gradient between the phases diminishes.

Mass Resistance

Diffusion through the stagnant liquid films surrounding each droplet dictates the speed of the separation process. Molecules overcome the energy barrier of the interface through chemical reactions that form transient complexes between the solute and the extractant. Impurities that accumulate at the interface often create a physical barrier which slows the exchange.

Temperature spikes alter the viscosity of the organic phase and improve the internal diffusion rates of the solute.

Operational Efficiency

Throughput capacity in a continuous plant relies on the ability of the chosen chemistry to reach equilibrium before the mixture exits the tank. Incomplete separation occurs if the flow rate forces the mixture out of the vessel before the reaction finishes. Designers adjust the power input of impellers to balance the energy cost of creating droplets against the yield gains of faster kinetics.

Higher extraction rates permit smaller equipment footprints by reducing the required volume for a given production output.