Surface phenomena in air-assisted solvent extraction

Abstract

Air-assisted solvent extraction (AASX) which is based on the coating of organic extractants on the air bubbles was developed to recover valuable metals from dilute Cu solutions. The effect of the chemical composition of aqueous and organic phases was investigated on the interfacial tension of aqueous/organic/air phases and related thermodynamic coefficients. Results were used to analyze the behaviour of phases and process efficiency in a continuous-mode AASX pilot plant. Interfacial data indicated that a suitable organic coating occurred in the presence of Flomin-F742, Nasfroth-240, and pine oil frothers with positive spreading coefficients. It was found that organic phase recycling in the AASX column was decreased from 86% to 13% by reducing the bubble size from 659 μm to 366 μm, respectively. By the addition of silicone oil dosage from 0 to 75 mg/L to the organic phase, Cu recovery was enhanced from 44% to 85% mainly as a consequence of decreasing the A/O phase ratio and increasing the coating degree of air bubbles by the organic extractant. An intense drop of organic phase recycling (from 70 to 30%) was observed at the silicone oil dosage beyond the critical micelle concentration of the organic phase, which was accompanied by producing a wide size range of air bubbles. It can be concluded that spreading the organic phase on the air bubbles and having a narrow size range of bubbles to overcome the upward velocity of bubbles to the downward velocity of the aqueous phase are the main issues for achieving a good efficiency in the AASX process. Results indicated that AASX method could widen the application of solvent extraction for the recovery of valuable metals from aqueous solutions.