Comparison of solvent extraction and LEM extraction in static mixer for recovery of Mo(VI) from dilute aqueous solution

• Static mixer exhibits superior performance in LEM-mode than regular extraction mode.
• Continuous LEM extraction is attractive for metal recovery from lean solutions.
• Once-through approach is better than recycling mode in continuous LEM process.
• Driving force for LEM extraction shows reversal beyond certain height of extractor.
• Higher mass transfer rate (lower HTU) in LEM as compared to solvent extraction.

In this paper, the liquid emulsion membrane extraction of molybdenum(VI) has been conducted in static mixer. The performance in LEM mode is compared to a conventional solvent extraction mode. The rate of extraction in LEM mode is found to be significantly higher than conventional solvent extraction. Study reveals that the optimum residence time, height of the static mixer and optimum surfactant concentration are the deciding parameters of the performance of static mixer. The effect of various parameters such as superficial velocity of aqueous phase and organic phase, residence time, initial Mo(VI) concentration in feed phase, height of column, and surfactant concentration on rate of extraction has been investigated. A mathematical model is developed to predict volumetric mass transfer coefficient (kLa) in a static mixer. The kLa value is reduced from 0.045 s−1 to 0.015 s−1 when height of the mixer was increased from 31 cm to 93 cm respectively under the same hydrodynamic conditions. This anomalous decrease of kLa with increase in height is due to acid extraction by TBP which reduces driving force for extraction of Mo(VI). HTU for solvent extraction and LEM extraction are estimated as 0.67 m and 0.15 m respectively which indicates less mass transfer resistance in LEM.