Comparison of liquid membrane processes for metal separations: Permeability, stability, and selectivity

A comparison of bulk, supported, and contained liquid membranes (BLM, SLM, and CLM) for metal separations with respect to permeability, stability, and selectivity is presented. The overall mass-transfer coefficient of magnitude order of 10(-6) and 10(-7) ms(-1) was typically found for SLM BLM and CLM, respectively. The SLM has the highest organic utilizing efficiency and the poorest membrane stability. Osmotic pressure across the membrane proves to be one of the major causes of SLM instability, and membrane pore elongation due to morphology changes reduces the SLM lifetime. The separation efficiency of liquid membranes can be enhanced by minimizing the organic inventory and maximizing the contact area between the aqueous phase and the organic membrane phase. However, this needs to be balanced against the improvement in stability of various liquid membrane configurations, which can be achieved by employing a significant organic inventory. The selectivity of liquid membranes mainly depends on the extractant used in the membrane phase and is similar to that of one-stage solvent extraction. The paper concludes with a technical comparison of the various liquid-membrane techniques.